10th Alpine workshop "CorseAlp2011"
CorseAlp 2011
10 16 April 2011
Saint Florent (Corsica)
ABSTRACT VOLUME
COORDINATORS: Malavieille J. (CNRS Geosciences Montpellier) Molli G. (Università di Pisa)
ORGANIZING COMMITEE: Ottaviani Spella M. M., Ferrandini M., Ferrandini J., Ciancaleoni L. & Pereira E. (Université de Corse) Vitale Brovarone A. (Università di Torino)
SCIENTIFIC COMMITEE: Beyssac O. (CNRS Paris) Beltrando M. (Università di Torino) Faccenna C. (Università di Roma 3) Gueyedan F. (Université de Rennes) Lagabrielle Y. (CNRS Geosciences Montpellier) Malusà M. (Università di Milano Bicocca) Rossetti F. (Università di Roma 3)
Edited by: Bernardo Carmina – Università di Pisa
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Abstracts list
1. Alania V.: Architecture of Kura Foreland Fold and Thrust Belt, Central Caucasus 2. Andersen T.B., Austrheim H.O., Timm J., Medvedev S., Deseta N. & Ashwal L.D.: Inside Intermediate depth foci Earthquakes: What can we Learn from Combining Field based Observational Studies with Numerical Models 3. Angiboust S., Agard P., Langdon R., Waters D. & Chopin C.: Exhumation of Large Volumes of Oceanic Lithosphere During Subduction: Examples from the Western Alps 4. Argnani A.: Plate Motion and the Evolution of Alpine Corsica and Northern Apennines 5. Balen D.: Evolution of Columnar Joint Polygonal Patterns at Rupnica Geosite (Papuk Geopark, Croatia) 6. Battaglia S., Ellero A., Leoni L., Catanzariti R., Cerrina Feroni A., Bertok C., d’Atri A., Martire L., Perotti E. & Piana F.: Illite and chlorite “cristallinity” as constraints for thermal history of the Brianconnais Dauphinois boundary in the Western Ligurian Alps 7. Bauve V., Rolland Y., Sanchez G., Giannerini G., Schreiber D., Corsini M., Perez J. L. & Romagny A.: Characterization of Active Fault Systems in Slow Orogenic Domains, Example of the Alpine Front in SE France and Insights for Geological Hazard 8. Bellanger M., Augier R., Baudin T., Bellahsen N., Beyssac O., Gumiaux C. & Jolivet L.: 3D Geodynamic Evolution of a Section Between Grenoble and Saint Jean de Maurienne (External Western Alps) 9. Beltrán Triviño A., Winkler W. & von Quadt A.: Evaluating the Alpine History by Detrital Zircon U Pb Dating/Geochemistry and Provenance Analyses 10. Beltrando M., Manatschal G. & Compagnoni R.: How Complex is Orogeny? Recognizing Distal Rifted Margins in High Pressure Alpine Type Orogenic Belts 11. Bertok C. , d’Atri A. , Martire L, , Perotti E, & Piana F,: Transfer Tectonics along the Briançonnais Dauphinois Boundary (Western Ligurian Alps) 1: Stratigraphic Evidence of Jurassic Eocene Synsedimentary Tectonics 12. Bertrand A., Garcia S., Rosenberg C., Gueydan F. & Fugenschuh B.: Cooling Pattern, Fault slip Analysis, and 2D Thermal Modelling in the Tauern Window 13. Beyssac O.: Evaluating a new geothermometer for low grade metamorphic rocks (200 350°C) based on Raman spectroscopy of carbonaceous material 14. Brandano M., Lustrino M., Ronca S. & Tomassetti L.: The impact of volcanism on the sedimentary record of Central Western Mediterranean 15. Brogi A. , Giorgetti G. & Liotta D.: Indication of HP LT Metamorphic Conditions in the Elba Island (Northern Apennines,Italy) 16. Cao H.: Resolution of the Genesis of Isoclinal Folds in Multiply Deformed Basement Terrains 17. Cardello G.L., Almqvist B., Hirt A, & Mancktelow N.: Magnetic Anisotropy Related to Stretching Parallel to the Alps in the Rawil Axial Depression (SW Switzerland) 18. Carlini M., Artoni A., Bernini M., Torelli L., Camurri F. & Vescovi P.: Vertical Movements and Emplacement of Ligurian and Subligurian Units Constrained by a new Dataset of low Temperature Thermal and Thermochronological Analyses Along a Regional Cross Section Across the Northern Apennines (Italy) 19. Carminati E., Lustrino M., Cuffaro M. & Doglioni C.: Cenozoic geodynamic and magmatological evolution of central western Mediterranean 20. Cascella A., Puccinelli A., & Perilli N.: New stratigraphic constrains of the para autocthonous Caporalino Sant’Angelo Unit belonging to the “Alpine Corsica” (Corte Ponte Leccia area, Corsica) 21. Cenki Tok B., Darling J., Rolland Y., Rossi M. & Engi M.: Allanite Behavior in Upper Mid Crustal Shear Zones: Can we Date Deformation Directly? Example from the Mont Blanc Massif and Implications on Its Alpine Evolution 22. Champagnac J. D., Molnar P., Christian Sue C. & Herman F.: Tectonics, Climate, and Mountain Topography
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23. Ciancaleoni L.: Heterogeneous Deformation of Granitoids at Plate Boundaries During the Late Continental Collision in the Insubric Area (Central Alps) 24. Clemenzi L., Perrin C., Malavieille J. & Molli G.: Complex Underplating and Kinematics of Folding in Accretionary Wedges: Analogue Models and Case Studies 25. Clerc C., Yves L. , Chopin C., Vauchez A., de Saint Blanquat M., Lahfid A. & Labaume P.: Extreme Crustal Thinning during Cretaceous Rifting in the Pyrenean Realm: Field evidence from the North Pyrenean Zone 26. Deák Kövér S., Milovsky R. & Fodor L.: Cretaceous nappe stacking in the Inner Western Carpathians – p T conditions from basal rauhwacke studies 27. Egli D. & Mancktelow N.: Neogene Tectonics of the Mont Blanc Area and Implications for Young Exhumation 28. Elfert S., Reiter W. & Spiegel C.: Timing and Evolution of Neogene Updoming of the Lepontine Dome. Constraints through new Fission Track and (U Th Sm)/He Data on Apatites 29. Favaro S., Hawemann F., Gipper P., Scharf A., Handy M.R. & Schmid, S.M.: Stucture and Kinematics in the Sonnblick Area (Eastern Tauern Window Austria) 30. Fodor L., Horváth F., Márton E. , Ustaszewski K. & Dombrádi E.: Constraints for the Extrusion Tectonics and Back arc Extension in the Pannonian Basin: a State of the art 31. Frasca G., Beltrando M. & Compagnoni R.: Field Mapping of the Valaisan Domain in the Breuil Valley (La Thuile, Aosta, Italy): evidence for the preservation of rift related mantle windows 32. Galster F., Cavargna Sani M., Epard J. L. & Masson H.: New Stratigraphic data in and Around the Adula Nappe, and Consequences for the Tectonics and the Paleogeography of the Central Alps 33. Galvez M.E., Martinez I., Beyssac O., Benzerara K., Chopin C. & Malvoisin B.: Formation of Graphitic Carbon Through Calcite Reduction in Eclogites From Corsica (France) 34. Garcia S.: Active Lateral Escape in the Eastern Alps 35. Garzanti E., Andò S., Limoncelli M., Malusà M., Resentini A. & Vezzoli G.: From Sink to Source: Looking at Alpine Geology Through the Garbage Can 36. Guillot F., Dupuis M. & Averbuch O.: The Briançonnais Fan Structure, Revisiting the Models 37. Herman F., Fox M., Reverman R., Champagnac J. D., Sternai P. & Willett S.: Quantifiying exhumation rates in the Alps using low temperature thermochronology 38. Herwartz D., Nagel T. J., Münker C., Scherer E. E. & Froitzheim N.: Lu Hf Geochronometry of two Garnet Generations Within one Eclogite Sample From Trescolmen (Adula Nappe) 39. Hunziker D., Burg J. P., Caddick M., Reusser E. & Omrani J.: The Influence of Ferric/Ferrous Iron Ratios in Bulk and Mineral Chemistry on Calculation of Metamorphic Conditions: Application to the N Makran Blueschists, SE Ian 40. Keil M. & Franz F.: The Miocene Enns Valley basin (Austria) and significance of Neogene faults 41. Kirst F., Froitzheim N., Nagel T. & Pleuger J.: Deformation along the Combin Fault and the Dent Blanche Basal Thrust 42. Krenn K., Kurz W., Fritz H. & Hoinkes G.: Evolution of monometamorphosed Austroalpine units (Schneeberg and Radenthein Complex) with implications for Eoalpine tectonics of the Eastern Alps 43. Krische O., Kurz W. & Gawlick H. J.: Prägosauic Bloc Re Arrangement as Recorded From Late Jurassic to Early Cretaceous Facies Belts in the Central Northern Calcareous Alps (Salzburg, Austria) 44. Kurz W., Fritz H., Mair A. & Pfingstl S.: The Rannach Formation – A Permian Trough Within the Lower Central Austroalpine Realm 45. Kurz W., Wölfler A. & Genser J.: Polyphase activity along the Lavanttal Fault Zone (Eastern Alps): constraints from 40Ar/39Ar white mica, zircon and apatite fission track, and apatite (U Th)/He dating 46. Lacassin R, & Armijo R.: Subduction Orogens, a new Perspective From the Andes 47. Larroque C. & Sage F.: Active Deformation of the Northern Ligurian Margin (Western Mediterranean): faulting along inherited structures ?
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48. Larroque C., Scotti O. & Ioualalen M.: Reappraisal of the 1887 Ligurian Earthquake and Tsunami (Western Mediterranean): Consequences for the Regional Seismic and Tsunami Hazards 49. Limoncelli M., Malusà M.G., Carter A., Guillot F. & Garzanti E.: Short Term Erosion Patterns in the Alps Constrained by Downstream Changes of Detrital Zircon U Pb Ages (Po River Basin) 50. Lustrino M., Svend Duggen S. & Rosenberg C.L.: The Central Western Mediterranean: Anomalous igneous activity in an anomalous collisional tectonic setting 51. Maggi M., Ranalli G., Rossetti F. & Theye T.: Intensive Fluid Rock Interaction and Rheology of the East Tenda Shear Zone (Haute Corse, France) 52. Maggi M., Rossetti F. & Tecce F.: Fluid Rock Interaction During the Schistes Lustrés Nappe Exhumation: an Integrated Structural, Petrological, Fluid Inclusions Study of the Erbalunga Shear Zone (Haute Corse, France) 53. Maggi M., Rossetti F., Theye T., Andersen T., Corfu F. & Faccenna C.: Sodic Pyroxene Bearing Phyllonites From the East Tenda Shear Zone: Constraining P T Conditions and Timing of the Ligurian Piemontese Ocean Overthrusting Onto the Variscan Corsica 54. Malusà M.G., Andò S., Resentini A. , Vezzoli G., Barbarano M., Botti A., Locatelli M. & Ragazzo P.: Provenance analysis of synorogenic sandstones from northern Corsica 55. Malusà M.G., Villa I.M., Vezzoli G. & Garzanti E.: Are detrital geochronology and stratigraphy necessarily in conflict? The Bregaglia story 56. Manatschal G., Mohn G., Masini E. & Beltrando M.: The Importance of Rift Inheritance in the Evolution of the Alpine Orogen 57. Mancktelow N. & Pleuger J.: The Periadriatic Fault east of Val d’Ossola 58. Manzotti P., von Niederhaeusern B., Darling J., Zucali M., Rubatto D. & Engi M.: Permian HT Metamorphism in Austroalpine Units of the Internal Western Alps 59. Manzotti P., Zucali M., Darling J., Rubatto D. & Engi M.: Basement Cover Relations in the Dent Blanche Klippe: Strong Strain Partitioning Helps Preserve a Complex Polyphase Evolution 60. Masini E., Manatschal G. & Mohn G.: The Tectono sedimentary Signature of Distal Rifted Margins: the Example of the Adriatic Margin Preserved in the Lower Austroalpine Nappes in SE Switzerland 61. Mattioli M., Lustrino M., Ronca S. & Bianchini G.: Subduction unrelated subduction related magmatism in Northern Apennines. The Early Oligocene Aveto Petrignacola volcaniclastic succession 62. Milovský R., Prokešová R. & Plašienka D.: Did the Metamorphic Fluid Flow Trigger Thin skinned Thrusting? 63. Mohn G. , Manatschal G. ,Masini E. , Beltrando M. & Müntener O.: Rift Related Inheritance in Alpine Type Orogens: Evidence From the Austroalpine Nappes 64. Montanini A., Tribuzio R. & Thirlwall M.: Multi stage evolution of the External Liguride mantle section (Northern Apennine ophiolites) 65. Musumeci G. Mazzarini F. & Sani F.: Late Neogene Tectonics in the Northern Tyrrhenian Sea Northern Apennines System: a Clue from new Seismic and Structural Data 66. Nocquet J. M.: Present Day Kinematics of the Mediterranean: a Synthesis 67. Ortner H., Fügenschuh B., Zerlauth M. & von Hagke C.: Geometry, Sequence and Amount of Thrusting in the Subalpine Molasse of Austria and Bavaria 68. Ottaviani Spella M.M. & Berlinghi A.: Metamorphic Zonations in the Corsican Batholith (France) and Relationships With the Post Hercynian Events in the Mediterranean area 69. Palotai M. & Csontos L.: Palaeogene Fold and Thrust Belt Beneath a Neogene Strike Slip Zone in Central Hungary 70. Piana F. , Bertok C., d’Atri A., Martire L. & E. Perotti: Transfer Tectonics along the Briançonnais Dauphinois Boundary (Western Ligurian Alps) 2 : the “Limone Viozene Zone” 71. Piccardo G.B.: Structural Evolution of the Lithospheric Mantle during Continental Rifting: insights from Ligurian Ophiolitic Peridotites 72. Pleuger J., Mancktelow N. & Spikings R.: The Canavese Fault
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73. Pomella H. & Fügenschuh B.: The Giudicarie Fault System in the light of Zircon Fission Track Data 74. Prosser G., Caggianelli A., Altenberger U. & Spiess R.: Pseudotachylytes Generated in Lower Crustal Levels: Constraints for Numerical Modelling 75. Ranalli G.: Melt percolation and softening of the continental lithospheric mantle: a prelude to sea floor spreading 76. Regis D., Engi M., Rubatto D., Darling J. & Zucali M.: Constraints on Scales and Rates of Tectonic Mobility Within Continental Collision Zones: Case Study From the Sesia Zone (Western Alps, NW Italy) 77. Reiter W., Elfert S. & Spiegel C.: Plio Pleistocene Changes in Drainage Evolution and Implications for the Denudation History of the Central European Alps 78. Resentini A., Malusà M.G. & Garzanti E.: Petrographic and Fission Track Analyses on Modern Rriver Sands from the Western Alps: Insights on Short term Erosion Distribution 79. Reverman R.L., Fellin M.G., Herman F. & Willett S.D.: Miocene Exhumation of the Southern Alps: New Constraints from Low Temperature Thermochronology 80. Rosenberg C., Bousquet R. &Garcia S.: On the Relationship Between Shortening, Exhumation and Orogen Width During Collision in the Central and Eastern Alps 81. Rossi M. , Rolland Y. & Chauvel C.: Fe Mg metasomatism in the Mont Blanc Massif 82. Ruiz Constán A., Galindo Zaldívar J., Pedrera A. & Marin Lechado C.: Stress Distribution at the Northwestern and Central Betic Cordillera Alboran Sea: the Transition From Subduction to Continental Collision 83. Sandmann S., Herwartz D., Kirst F., Froitzheim N., Nagel T. & Münker C.: Lu Hf Garnet Ages from the Southeastern Part of the Pohorje Mountains, Eastern Alps, Slovenia 84. Sanfilippo A. & Tribuzio R.: Lower crust generation in the Jurassic Ligurian Piedmontese Basin (Pineto gabbroic complex, Corsica) 85. Sanfilippo A. & Tribuzio R.: Melt transport and deformation history in a “non volcanic” ophiolitic section (Northern Apennine, Italy): implications for crustal accretion at slow spreading settings 86. Scharf A., Favaro S., Handy M.R. & Schmid S.M.: Exhumation Related Structures at the Eastern Margin of the Tauern Window (Eastern Alps, Austria) 87. Scheiber T., Pfiffner O. A. & Schreurs G.: The Fate of a Composite Permian Intrusion During Alpine Collision: the Rofna Porphyry Complex of the Suretta Nappe (E Switzerland) 88. Schenker F.L., Reusser E., Caddick M.J., Madonna C., Grassi D. & Burg J. P.: Kinematic and Metamorphic Evolution of the Lukmanier Pass Region (Switzerland): Extension Related History? 89. Schmid S.M., Bernoulli D., Fügenschuh B., Matenco L., Oberhänsli R., Schefer S. & Ustaszewski K.: New Tectonic map of the Alpine collision zone between Eastern Alps and Western Turkey 90. Schmid S.M., Scharf A., Handy M.R., Rosenberg C., Favaro S. & Bertrand A.: New Tectonic map and Cross Sections of the Tauern Window (Eastern Alps, Austria) 91. Schneider J., Ganino C., Fernandez L., Rolland Y., Bosch D. & Sanchez G.: An Element Mobility and Geochronological Investigation of a Mid crustal Alpine Shear Zone – La Valetta Shear Zone (Argentera Mercantour) 92. Schneider S., Bertrand A.S., Favaro S., Garcia S., Hammerschmidt K., Handy M.R., Rosenberg C.L., Scharf A. & Schmid S.M.: Erosive Denudation vs. Extensional Unroofing; A Discussion on Exhumation of the Tauern Window 93. Schulz M., Pomella H., Zerlauth M., Ortner H. & Fügenschuh B.: The Eastern and Western Alps boundary in Vorarlberg (western Austria) 94. Schuster R, Koller F., Gawlick H. J., Frank W. & Lein R.: Relation of the Tethys ocean to the Cretaceous (Eoalpine) subduction in the Alps: data, framework and ideas 95. Silva S., Duarte H., Terrinha P., Matias L. & NEAREST Working group: New insights into the seismotectonics of the Gulf of Cadiz area based on instrumental seismicity and seismic reflection profiles reinterpretation: preliminary results
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96. Spiess R., Caggianelli A., Prosser G. & Lucchiari A.: Melt Present Deformation of the Mafic Lower Crust Exposed in Calabria 97. Sternai P., Herman F., Willett S.D., Champagnac J. D. & Salcher B.: Reconstruction of the pre Glacial Alpine Topography to Address the Glacially Induced Modifications 98. Turco E., Schettino A., Pierantoni P. P. & Macchiavelli C.: Kinematic Evolution of the Corsica Triple junction 99. Ustaszewski K., Herak M., Tomljenović B., Herak D. & Matej S.: Neotectonics of the Dinarides – Pannonian Basin Transition Area and possible sources of the ML 6.5 Banja Luka Earthquake of 1969 100. Vergés J. & Fernàndez M.: Tethys Atlantic Interaction Along the Boundaries of the Iberian Plate: Pyrenees and Betics Mountain Belts 101. Vitale S., Iannace A., Mazzoli S., Molli G., Reddy S. & Jourdan F.: Along strike variability in structural style, exhumation history and overall nappe architecture of distal Adria derived units: northern Calabria vs. Alpi Apuane 102. Vitale Brovarone A., Beltrando M., Malavieille J., Giuntoli F., Tondella E., Groppo C., Beyssac O. & Compagnoni R.: Inherited Ocean Continent Transition zones in deeply subducted terranes: Insights from Alpine Corsica 103. Vitale Brovarone A., Beyssac O., Malavieille J., Molli G., Beltrando M. & Compagnoni R.: What happens during subduction? A perspective from the architecture of Alpine Corsica 104. Vitale Brovarone A., Groppo C., Hetényi G., Compagnoni R. & Malavieille J.: Role of Seafloor Processes During Subduction: heterogeneous densification of subducting oceanic crust and Coexistence of Lawsonite Eclogite and Blueschist From Alpine Corsica 105. von Hagke C., Oncken O., Stöckli D.F. & Ortner H.: How much material was eroded in the Northern Alpine Foreland Basin and why and when did it happen? 106. von Niederhaeusern B., Cenki Tok B., Darling & Engi M.: Geochronological Constraints on the pre Alpine Evolution of the II DK (‘Seconda Zona Diorito Kinzigitica’, SW Sesia Zone, Italy) 107. Vouillamoz N., Sue C. & Champagnac J. D.: 3D Modeling of the Western Alpine Arc 108. Wolff R., Dunkl I., Kiesselbach G., Siegesmund S., & Eynatten H.v.: Refined Exhumation History of the Ivrea Verbano Zone, Southern Alps 109. Wölfler A. , Kurz W., Fritz H. & Stüwe K.: Is Lateral Extrusion in the Eastern Alps Really Lateral? 110. Yang J., Zhiqin Xu Z. & Robinson P.T.: HP UHP metamorphic belts in the Eastern Tethyan Orogenic System in China 111. Zhihui C. , Zhiqin X. , Zhemin T. & Bizhu H.: Structure and Evolution of the early Paleozoic orogenic belt on the northeast Margin of Tarim Basin 112. Zibra I.: Late Palaeozoic Deformation of Post Variscan Lower Crust: Shear Zone Widening due to Strain Localization During Retrograde Shearing
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Architecture of Kura Foreland Fold and Thrust Belt, Central Caucasus
Victor Alania 1, 2
(1) Frontera Eastern Georgia, Tbilisi, Georgia (2) Institute of Geophysics, Tbilisi, Georgia
Kura foreland fold and thrust belt which developed formerly as a foreland basin (Oligocene Miocene) is located between Greater Caucasus and Lesser Caucasus fold and thrust belts. Seismic reflection profiles show that the Kura foreland fold and thrust belt of eastern Caucasus are an active thin skinned fold and thrust belt and they preserve growth strata that record the tectonic and stratigraphic evolution. Analysis of growth strata in seismic section from Georgian Territory documents that evolution of deformation has been continuing during the last ~ 5 4.5 Ma together with the thrust system kinematics.
The growth stratigraphy (Plio Pleistocene) consists of shallow marine and continental sediments. The Kura fold and thrust belt can be divided into two structural zones: forethrust and backthrust. The structure of the S vergent forethrust zone is interpreted as due to the southward thrusting of Greater Caucasus wedge and includes fault bend folds, fault propagation folds and duplexes. South of Baida Chalari and South Taribani area is developed backthrust zone.
The S vergent backthrust zone (Akstafa area Azerbaijan Territory) is characterized by fault propagation and fault bend folding and consists of a ~ 25 km wide zone. The kinematic evolution of the backthrust zone with respect to the northward propagating Lesser Caucasus thrust wedge (or duplexes). N vergent thrust wedge (or duplexes) is consists by Mesozoic Paleogene strata. The style of deformation within the back thrust system includes fault propagation and fault bend folds with amount of the deformation decreasing to the south. The Maikopian (Oligocene Lower Miocene) strata are identified as the main detachment horizons for this backthrust zone.
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Inside Intermediate-depth-foci Earthquakes: What can we Learn from Combining Field-based
Observational Studies with Numerical Models
Torgeir B. Andersen 1, Håkon O. Austrheim 1, Timm John 1, 2 , Sergei Medvedev 1, Natalie Deseta 3 & Lewis D. Ashwal 3
(1) PGP, University of Oslo, Norway (2) University of Münster, Germany (3) University of Witwatersrand, South Africa
High pressure and low temperature (HP LT) metamorphic terranes in the Caledonides and Alpine Corsica host some of the best examples of intermediate depth (>60 km) co seismic fault rocks in the world. These pseudotachylites (PSTs) quenched at eclogite and blueschist facies conditions during continental collision and subduction, respectively. Globally, such rocks are rare, but in the localities of Norway and Corsica HP LT PSTs are numerous. The mechanical consequences of eclogitization can be observed in the field in several localities in the Caledonides. Here, protoliths of the Proterozoic basement underwent partial to complete eclogitization during the Caledonian collision (~430 400 Ma). Eclogitization commenced in localized zones of combined fluid infiltration and deformation. Accumulated stress was in parts released abruptly by co seismic faulting, which are recorded by PSTs. In the same localities and at the same HP LT conditions, localized deformation also produced ductile shear zones. We assume that the co seismic faulting and shearing events may represent ancient equivalents of earthquakes and creep to those presently recorded in the lowermost crust below the Himalayas.
The HP LT rocks in Corsica formed by the Early Alpine subduction, which presumably started within the narrow deep ‘Ligurian’ basins. These basins had formed by large magnitude continental extension exhuming continental mantle lithosphere and by local oceanic spreading in the Jurassic. Blueschist facies PSTs in gabbro and peridotite at Chima di Gratera and a new lawsonite eclogite locality at Défilé de Lancone of Cape Corse are probably both related to these early stages of Alpine convergence. The geotherm during formation of the lw eclogites was very cold, close to the “forbidden zone” (~5 0C/km) with a temperature of only ~350 0C at 2.4 GPa. By analogy with modern subduction, the HP LT rocks of Corsica were strongly affected by co seismic deformation evinced by numerous PSTs. We have identified hundreds of small (mm) and a number of large (5 15cm) PST injection and fault veins. Most of the PST veins occur near the palaeo Moho along the gabbro peridotite contact at Chima di Gratera. The thermal energy required to form PST can be used to determine the minimum stress drop during seismic faulting. We have shown that even very small earthquakes (cm displacement) produced PSTs and thus the energy involved is large and so is the stress drop. Because the seismic energy is very small compared to heating, we obtain much larger (order of magnitude) stress drops than estimates based on seismology.
These stress drops can be used to constrain numerical models. Our combined approach suggests a mechanism, which is an alternative to the commonly used dehydration embrittlement for intermediate to deep earthquakes. This mechanism is referred to as “self localizing thermal runaway” and is more favourable than ‘Byerlee law’ faulting since it requires lower failure stresses for a number of conditions in a subducting lithosphere. Formation of PST as fault rock along intermediate and deep foci earthquakes is probably the rule rather than the exception.
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Exhumation of Large Volumes of Oceanic Lithosphere During Subduction:
Examples from the Western Alps
Samuel Angiboust 1, Philippe Agard 1, Ryan Langdon 2, Dave Waters 2 & Christian Chopin 3
(1) Institut des Sciences de la Terre de Paris, Université Pierre et Marie Curie, Paris, France (2) Earth Sciences Department, University of Oxford, Oxford, UK (3) Lab. De Géologie, Ecole Normale Supérieure, Paris, France
The Western Alps provide key insights into the behaviour of a portion of oceanic lithosphere during subduction and exhumation. The Zermatt Saas (ZS) and Monviso eclogitized ophiolitic nappes (sandwiched between the Dora Maira and Grand Paradiso internal crystalline massifs and the upper, non eclogitic Combin and Queyras Liguro Piemontese units) provide a record of the detachment and subsequent exhumation of very large volumes of oceanic material along the subduction interface. We herein present a comparison of their structure, P T paths and discuss potential mechanisms for detachment from the downgoing slab and exhumation processes.
ZS unit is internally made up by a series of several tectonic slices detached from the slab at ca. 80 km deep (550°C, 24 kbar; Angiboust et al., 2009 ). It is proposed that a particularly pervasive hydration of this portion of the Tethyan ocean floor led to the crystallization of lighter eclogite facies assemblages, thus facilitating the later detachment of this relatively continuous portion of slab (~70km across; Angiboust & Agard, 2010 ).
Detailed petrological and structural analysis on the Monviso ophiolite revealed the presence of relatively continuous boudins showing homogeneous P T conditions (530 560°C; 25 27 kbar). These observations contrast with the common view that the Monviso ophiolite corresponds to a (chaotic) subduction melange. Moreover, we emphasize that many similarities exist between the Monviso and ZS ophiolites (200 km apart) in terms of parageneses, P T time conditions and overall structure. We propose a similar mechanism for the detachment and stacking of these two ophiolitic domains in a partially serpentinized subduction channel, which could also apply to the other large pieces of oceanic lithosphere found in the Western Alps.
REFERENCES Angiboust, S., Agard, P., Jolivet L., Beyssac, O., 2009, The Zermatt Saas ophiolite: the largest (60 km wide) and deepest (c. 70 80 km) continuous slice of oceanic lithosphere detached from a subduction zone?, Terra Nova , 21: 171 180. Angiboust, S. & Agard, P., 2010, Initial water budget: The key to detaching large volumes of eclogitized oceanic crust along the subduction channel?, Lithos , 120: 453 474.
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Plate Motion and the Evolution of Alpine Corsica and Northern Apennines
Andrea Argnani
ISMAR CNR, Bologna, Italy
The west verging mountain belt of Alpine Corsica is opposing the east verging Northern Apennines. These two orogenic belts are currently separated by the Northern Tyrrhenian Sea, which is less than 200 km wide. The geology of this region is remarkably puzzling as the opposite structural polarity observed in Alpine Corsica and Northern Apennines has to be explained and related to the evolution of the Alps. These issues have been addressed by several authors, who mostly focussed on the relationships between Corsica and the Northern Apennines. Interpretations concerning Corsica and the Northern Apennines can be broadly framed within two groups. In the first group the initial eastward (European) subduction was followed by a flip of polarity, leading to the westward (Apennine) subduction of Adria. In contrast, a single west dipping subduction, active since Late Cretaceous is required by the interpretations of the second group. However, models adopting either an original W dipping subduction or preferring a flip in the polariy of subduction, from E dipping to W dipping, present major inconsistencies at regional scale. It appears that the main inconsistencies encountered by both models are often due to the two dimensional approach in the presented reconstructions, that cannot fully account for the geological observables.
Models implying only a west dipping subduction face major difficulties in explaining the westward emplacement of high pressure/low temperature ophiolites and European crystalline basement in Corsica. Moreover, this group of models moves further to the north the problem of the connection with the E SE dipping Alpine subduction in the Western Alps. Models implying east dipping subduction followed by a flip in polarity require either the occurrence of a colliding microcontinent or the initiation of the Apennine as a backthrust of the Alpine subduction. Both instances imply the breakoff of the subducted Alpine slab. In the former view the colliding microcontinent (Nebbio) is possibly part of a supposedly larger Alkapeca continental domain. However, very little remains of this Nebbio microcontinent can be found, and its occurrence poses several problems in the subsequent evolution. On the other hand, the assumption of the Apennines initiating as an Alpine backthrust in the Oligocene doesn’t seem compatible with the early Oligocene calcalkaline volcanism in Provence and Sardinia, and with the fact that the activity at the thrust front of Alpine Corsica ended by late Eocene. Finally, many models predict a continental collision that is accomplished between Late Eocene and Late Oligocene, independently from the initial polarity of subduction. This interpretation implies that the post Oligocene evolution of the Northern Apennines occurred by continental subduction, and contrasts with the opening of the Balearic backarc basin behind the rotating Corsica Sardinia microcontinent, which requires the subduction of an oceanic lithosphere. A new proposal attempting to solve the long debated issue of the polarity of subduction in the Corsica Northern Apennine system is presented, taking into account the major control played by plate motion. In fact, large scale horizontal plate motions, in the order of several hundreds of kms, occurred from Eocene to Oligocene in the central Mediterranean.
The Late Cretaceous to Present Day kinematics of the Central Mediterranean has been reconstructed using the magnetic anomalies in the Atlantic Ocean to constrain the relative positions of Africa and Eurasia. A solid connection between Africa and Adria has also been assumed, as indicated by palaeomagnetic studies. A wide oceanic embayment to the west of the Adriatic Promontory has been reconstructed by using the distribution of Oligocene to Present calcalkaline volcanism and backarc extension in the Balearic and Tyrrhenian basins. It follows that the continental collision that gave rise to the Alps s.s. could not continue SW ward of the Adriatic Promontory. The flip of subduction polarity that can be currently observed, going from the Alps, where Africa is overriding Europe, to the Apennines, where the opposite occurs, is taken as an original feature since the beginning of convergence. This setting closely recalls the reconstruction of the western Pacific, where oceanic subduction with opposite polarity coxisted laterally in the Eurasian plate and Philippine Sea plate before the occurrence of arc/continent collision in Taiwan. Kinematic reconstructions allow the point along the plate boundary where the flip of polarity occurs to be tracked back in time. Following the N ward motion of the colliding Adriatic Promontory, the point of polarity flip moved along the plate boundary from Late Cretaceous to Eocene. As a result, areas that previously experienced continental collision were subsequently affected by oceanic subduction. This sequence of events led to the collapse of the Alpine belt of Corsica and to the opening of the Balearic backarc basin above a retreating oceanic subduction. A similar kinematic evolution is currently ongoing in Taiwan. The kinematic reconstructions presented here suggest that a flip in subduction polarity did occur along the Corsica Northern Apennines transect, but this flip was simply due to the 3D nature of plate motions; it can be explained once the relative Africa Europe convergence is taken into account, and does not require the collision of a microcontinent. Finally, the Northern Tyrrhenian basin opened when delamination affected the Adriatic continental margin, following the consumption of oceanic lithosphere at the end of Corsica Sardinia rotation.
4 10th Alpine workshop "CorseAlp2011"
Evolution of Columnar Joint Polygonal Patterns at Rupnica Geosite (Papuk Geopark, Croatia)
Dražen Balen
Department of Geology, Faculty of Science, University of Zagreb, Croatia
The Rupnica locality is situated in the NW part of the Nature Park Papuk (Croatia), near the small town of Voćin. It is the first proclaimed “Geological Monument of Nature” in the Croatia, protected since October 14 th , 1948. Geological importance of this site lies in an exceptional geomorphological appearance of the columnar jointing in volcanic rocks (albite rhyolite comprising quartz, albite and mica with subordinate alkali pyroxene, magnetite, apatite, amphibole, chlorite, illite, smectite and calcite). In the year 2007 Papuk became the first area in Croatia to be awarded with a Geopark status and became a member of the European Geopark Network under protection of UNESCO which strengthens position of Rupnica as one of the Papuk’s key localities. Past volcanic activity in the area produced heterogeneous volcanic complex with extraordinary, well exposed columnar jointing formed by cooling of volcanic rock and form 4 6 sided polygons i.e. columns.
Fundamentally, simple and universal laws of physics govern the behavior of cracks and development of columnar joints in volcanic rocks, but also in analogous materials like mud, ice and starch slurry. Simplified, the fast cooling rate near or close to surface conditions will cause the formation of cracks which will reorganize themselves in the form of “T junctions”, producing mostly four sided columns, while on greater depths and with slow cooling rates cracks will propagate into depth, reorganizing themselves into “Y junctions”, in ideal case giving six sided columns.
Analysis performed on the Rupnica geosite (Table 1) shows statistical values for propagating joints counted through three campaigns and formation of n sided columns. The values are expressed as total number of columns, average number (N) of column sides and n gonality (X 6, X 5, X 4) indices, later represent a measure of discrepancy in the number of joints between the real sample and an ideal (i.e. theoretical=1) case.
Table 1. Total number of columns and distribution of the polygon sides/bounded cracks at the Rupnica geosite and corresponding statistical parameters (N=average, X 6, X 5, X 4 =heksa-, penta-, tetra-gonality indices).
Rupnica Total N X6 X5 X4 2008 347 4.64 1.48 0.84 1.00 2009 509 4.53 1.46 0.92 0.95 2010 771 4.64 1.47 0.79 0.95
Four sided (0.95 1.00) and five sided (0.84 0.92) columns show the greatest resemblance to the idealized case(s). Such results are in good agreement with results expected for shallow placed and relatively rapidly cooled volcanic mass. If such measurements are correlated with other features visible at or near the Rupnica locality, like relatively narrow columns, tightly packed striae (propagating cracks inside columns), curved sets of columns and occurrence of pyroclastic material in the vicinity, than we can depict the Rupnica columnar jointing as the upper part of the volcanic body. Consequently, analysis of cracking pattern responsible for the origin of columnar joints at the Rupnica geosite shows that this genetic process is related to a shallow, fast cooling environment.
5 10th Alpine workshop "CorseAlp2011"
Illite and chlorite “cristallinity” as constraints for thermal history of the Brianconnais-Dauphinois
boundary in the Western Ligurian Alps
Stefano Battaglia 1, Alessandro Ellero 1, Leonardo Leoni 2, Rita Catanzariti 1, Andrea Cerrina Feroni 1, Carlo Bertok 3, Anna d’Atri 3, Luca Martire 3, Elena Perotti 3 & Fabrizio Piana 4
(1) CNR Istituto di Geoscienze e Georisorse, Pisa, Italy (2) Dipartimento di Scienze della Terra, Università di Pisa, Italy (3) Dipartimento di Scienze della Terra, Università di Torino, Italy (4) CNR Istituto di Geoscienze e Georisorse, Torino, Italy
In the frame of the reconstruction of the Meso Cenozoic evolution of the junction between the Western Ligurian Alps and Maritime Alps and the kinematic role of this zone as regional scale transfer (“Ligurian transfer”, see Bertok et al. and Piana et al., thys symposium) analyses of the “cristallinity” of illite and of chlorite have been performed in order to constrain the thermal history and crustal paths of Briançonnais, Provençal Dauphinois and Ligurian units involved in the strike slip fault zones that affected this sector of the Alpine chain and in particular of the Limone Viozene Zone (Piana et al., 2009).
The study of mineralogical parameters points to very low grade conditions for both the Briançonnais and Provençal Dauphinois units, with an approximate value of metamorphic temperature between 240 260°C, characteristic of a middle anchizonal grade. The Ligurian flysch units overthrust at the top of the thrust sequence of the Western Ligurian Alps (S.Remo M.Saccarello unit), during the latest stages of the Alpine evolution, underwent only diagenetic conditions (temperature between 150 180°C).
Conversely, all the units involved in the major transpressive shear zones developed along the B/D boundary, comprehending also slices of Ligurian flysch, record higher temperatures, between 250 and 290°C, suggesting that the shear stress played an important role in rock heating. These data confirm the interpretation that, in the Western Ligurian Alps, the Brianconnais and Provençal Dauphinois domains are parts of a single foreland fold and thust belt that experienced the same thermal history and similar crustal paths. It can be also inferred that the Ligurian units involved within the main tectonic structures developed along the B/D boundary share the same metamorphic grade as the Briançonnais and Provençal Dauphinois units. Therefore, slices of Ligurian unit were involved since the beginning of the Alpine tectogenesis: during the Eocene, when they were re sedimented in the foreland basin succession (Perotti et al., 2011) up to the Oligocene Early Miocene, when they were sheared within the LIVZ.
Conversely, the Ligurian units that did not follow these paths, such as the S.Remo M.Saccarello unit that are presently placed at the top of the thrust sheet sequence and bounded at the base by a first order thrust flat zone, show a very different thermal history. Illite and chlorite crystallinity data thus indicate for this unit a shallower crustal path, in agreement with a late tectonic emplacement, as also strongly suggested by map evidence and structural observations.
REFERENCES Perotti E., Bertok C., d’Atri A., Martire L., Piana F. & Catanzariti R. (2011) Eocene mélange in West Ligurian Alps as the result of tectonically induced sedimentary processes. Tectonophysics, submitted. Piana F., Musso A., Bertok C., d’Atri A., Martire L., Perotti E., Varrone D., Martinotti G. (2009) New data on post Eocene tectonic evolution of the External Brianconnais (Western Ligurian Alps). Italian Journal Geosci., (Boll.Soc.Geol.It.), Vol. 128, No. 2, pp. 353 366.
6 10th Alpine workshop "CorseAlp2011"
Characterization of Active Fault Systems in Slow Orogenic Domains, Example of the Alpine Front in
SE France and Insights for Geological Hazard
Victorien Bauve 1, Yann Rolland 1, Guillaume Sanchez 1, Gérard Giannerini 1, Dimitri Schreiber 1, Michel Corsini 1, Jean Louis Perez 2 & Adrien Romagny 2
(1) GEOAZUR, UMR 6526, Université de Nice Sophia Antipolis, Nice, France (2) Laboratoire de Nice CETE Méditerranée, Nice, France
Seismic hazard assessment of active faults in slow orogenic domains is a challenging issue. In this paper we present a multi disciplinary approach based on Digital Elevation Model (DEM), 3D geological modelling, fracture analysis and strain analysis of pebbles in a recent Pliocene molasse cross cut by a slow active fault, the Donaréo and Saint Blaise Aspremont fault system. DEM shows a topographic disturbance emphasized by slope gradients and the drainage system, which is ascribed to the active fault trace. Fracturation analysis evidences two fault corridors mainly oriented N150°E and N20°E. Paleo stress analysis provides similar orientations as data derived from focal mechanisms of current regional seismicity, with main stress σ1 oriented N20°E and (σ2 σ3)/(σ1 σ3) ratio of 0.31, emphasized by σ2 versus σ3 permutations in agreement with ongoing strike slip deformation at least since the early Pliocene.
Discontinuous fracturation and comparison with seismic monitoring on similar active fault zones in the region suggest that shallow seismicity may be expressed by low magnitude (Mw < 4) seismic swarms. Deformation of pebbles occurs mainly by pressure dissolution. Pebbles show a bimodal distribution, explained by parallelisation with the two fault strands, which suggests rare seismic unloading and long lasting creeping processes. Geometrical 3D analysis shows the formation and migration of a Plio Quaternary basin about 500 metres East of the main fault system together with folding and tilting of the post Messinian Pliocene molasse.
These observations prove that the faults remain active from the Pliocene to the Quaternary, and most likely to Present. However, the faults minimum slip rate estimates of about 0.02 mm.a 1 vertical and 0.03 mm.a 1 35 horizontal are unlikely to produce any significant high magnitude earthquakes, but rather swarm like seismicity with long temporal recurrence.
7 10th Alpine workshop "CorseAlp2011"
3D Geodynamic Evolution of a Section Between Grenoble and Saint Jean de Maurienne
(External Western Alps)
Mathieu Bellanger 1,2 , Romain Augier 2, Thierry Baudin 1, Nicolas Bellahsen 3, Olivier Beyssac 4, Charles Gumiaux 2 & Laurent Jolivet 2
(1) BRGM Service Géologie 3 avenue Claude Guillemin BP 36009 45060 Orléans Cedex 2 France (2) ISTO CNRS, Université d'Orléans, 1A rue de la Férollerie, 45071 Orleans Cedex 2, France (3) iSTeP, Université PM Curie, UMR 7193, 4 place Jussieu, 75252 Paris Cedex 5, France (4) iMPMC, Université PM Curie, UMR 7590, 4 place Jussieu, 75252 Paris Cedex 5, France
From rifting to continental collision, the Alps are the result of a succession of tectonic events whose impact is variable both in space and time. Particularly, the respective contributions of N S and E W shortening events to the present day structure are difficult to unravel without a quantitative 3D model of the belt. This project aims at constructing a 3D model of key areas of the French Italian Alps in order to decipher the amount of shortening due to both stages of shortening and relate to them geodynamics of the Alps Apennine junction. Areas that have been chosen offer several opportunities such as: (1) the steep topographic gradients that permit a structural analysis in 3 dimensions. (2) the ante rift Triassic deposits that are supposed to have been deposited over a peneplain. Attached to the basement through most of the tectonic evolution, this level is a good marker of the metric to kilometric deformations after the rifting. (3) The metapelitic in the Liassic sediments that allows to estimate Pressure Temperature time deformation (P T t d) conditions, essential for geodynamics reconstructions.
With the perspective to constrain the multiphased character of the shortening proposed in the litterature namely a N S ante to syn Priabonian followed by an E W phase post Priabonian and to quantify the respective contributions of these two phases, an identification of structures was carried out in the field. The analysis of basement cover interface and ductile deformation above and below shows that the intense deformation of the overlying Lias is accommodated in the basement by a limited number of reverse shear zones who reactivates only very partially the normal faults inherited from the rifting and lead to an apparent ‘folding’ of the basement.
The kinematics of these shear zones is mainly East to West. P T estimates (multi equilibrium methods and RSCM) suggest that deformation occurred in the greenschist facies conditions (3 4 kbar and 270 310°C). In addition, the 3D model allows the visualization of structures in their entirety. A point of interest is to avoid bias of quantification during restoration caused by the orientation of cross sections. It is then possible to quantify the amount of shortening of each phase (N S and E W) in space.
8 10th Alpine workshop "CorseAlp2011"
Evaluating the Alpine History by Detrital Zircon U-Pb Dating/Geochemistry and Provenance Analyses
Alejandro Beltrán Triviño 1, Wilfried Winkler 1 & Albrecht von Quadt 2
(1) Geological Institute, ETH Zurich, Switzerland (2) Institute for Geochemistry and Petrology, ETH Zurich, Switzerland
The Alpine orogenic cycle is recorded in sandstone formations related to rifting, drifting, convergence and collision. Provenance analyses allow reconstructing the rock types in the source terranes of the deposits, and in turn to locate the various basins within the Alpine plate tectonic environment. Although standard provenance analysis (sandstone framework grains, heavy minerals) has provided and complemented comprehensive tectono stratigraphic models of the Alpine Tethys, several palaeogeographic and tectonic affiliations still are ambiguous because the complexity of the nappe tectonics has veiled the original basin source relationships. Furthermore, the standard provenance analyses have reached their limits of resolution and hardly can provide more details in order to refine palaeogeographic models of the Alpine Tethys. New concepts and analytical methods are required for higher resolution provenance analysis. In situ dating and geochemical characterization of detrital zircons add knowledge of the chronostratigraphic framework in the related source terranes. In the present work we show first results from U/Pb dating of detrital zircons from several palaeogeographic domains. We distinguish formations as traditionally have been considered to be sediment supplied from basement rocks present either within the northern or the southern margin of the Alpine Tethys. Furthermore, the analysis of the Hf isotopes ratios ( 176 Hf/ 177 Hf) allow to identify the nature of the magmatic source rocks with respect to their derivation from depleted mantle, recycled continental crust or mixtures.
Within different Late Cretaceous Prealpine flysches (South Penninic) , presumably sourced in the southern Apulian margin of the Alpine Tethys, four age groups are common: (1) scattered Meso proterozoic and older zircons, (2) a clear cluster of Neo proterozoic to Silurian ones, which correlates with the Pan African orogenic cycle (650 450 Ma), (3) a prominent Carboniferous to Permian population, which can be correlated with the Variscan orogenic cycle, and (4) a less pronounced population of Triassic to Early Jurassic zircons (with exception of Mocasusa/Rodomont Flysch) attributable to the Alpine Tethys rifting. North Penninic units , presumably sourced in Eurasian plate basement, generally show less variable age populations. The Jurassic sedimentary basement and the overlying Late Cretaceous Niesen Flysch show similar Palaeozoic age distributions, except for the presence of more Neo /Meso proterozoic zircons in the latter. The two youngest formations of the Northpenninic Prättigau Flysch mirror a population of Late Variscan age and an irrregular pattern of Devonian to Neo proterozoic zircons.
Two rifting related sandstones from the Aar and Gotthard nappes show well distinct zircon age patterns. In the Middle Triassic sandstone of the Aar Nappe Palaeozoic (Caledonian and early Variscan) zircons prevail; in the Gotthard Nappe Liassic cover, in addition, strong recycling of earlier Proterozoic orogenic events is observed. The preliminary Hf isotopes analyses show a mantle crust mixture as origin of the detrital zircons found in the studied sedimentary units. The Hf isotopes and correlating U Pb ages indicate that the Late Variscan, post collisional, extensional magmatism, had mixed mantle and crust sources.
9 10th Alpine workshop "CorseAlp2011"
How Complex is Orogeny?
Recognizing Distal Rifted Margins in High-Pressure Alpine-Type Orogenic Belts
Marco Beltrando 1, Gianreto Manatschal 2 & Roberto Compagnoni 3
(1) Consiglio Nazionale delle Ricerche Istituto di Geoscienze e Georisorse, Torino, Italy (2) CGS EOST, Université Louis Pasteur, Strasbourg, France (3) Dipartimento di Scienze Mineralogiche e Petrologiche, Università di Torino, Torino, Italy
Typically, the most highly metamorphosed/deformed sections of Alpine type metamorphic belts consist of ophiolitic units variably associated with meta sediments and continental basement units ranging in thickness from a few meters to 2 4 kilometers. In the Western Alps, these rock units are interpreted as slices of sediments and crust scraped off the Piemonte Ligurian oceanic lithosphere and the Adriatic and European continental margins. However, it is still unclear to what extent the complex architecture of the Western Alps is a result of orogen dynamics as opposed to being controlled by the pre Alpine geometry of the margins that were sampled during orogeny. The solution to this debate hinges on the ability to provide firm constraints on the pre Alpine architecture of the margins that were later involved in the formation of the Alps. Indeed, a detailed knowledge of the relative position and lithostratigraphy of the different units that are now stacked in the mountain belt is necessary to understand the sequence of processes that led to tectonic burial, sampling and exhumation of specific parts of the rifted margins.
The recent recognition of lithostratigraphic associations typical of Ocean Continent Transition Zones preserved in the (U)HP Piemonte Units (Beltrando et al., 2010) and in the HP Valasian Domain of the Western Alps (Frasca et al., this congress) indicates that the juxtaposition between typical ‘oceanic’ and ‘continental’ basement rocks may also be established during lithospheric thinning related to Jurassic rifting. The study presented here aims at showing that rift related structures and lithostratigraphic associations typical of distal continental margins and OCT’s are regionally preserved also in other parts of the Western Alps and that much of the apparent complexity of the orogenic belt is inherited from its pre Alpine history. In the External Sesia Zone, ‘Gneiss Minuti type’ Palaeozoic continental basement is regionally overlain by a thin layer of Mn bearing quartzite, which is interpreted as the metamorphic equivalent of early post rift Mn rich radiolarian cherts of Middle Jurassic age. A typical gneiss micaschist layered sequence up to 20 meter thick, lacking any evidence of pre Alpine metamorphism, is often found between the Palaeozoic continental basement and the Mn bearing quartzite. A similar lithostratigraphic association, with Palaeozoic continental basement overlain by syn rift clastic sediments and post rift Mn bearing cherts and calcschists, is typically found in distal thinned margins, such as those preserved in the Err nappe in the Eastern Alps. In those circumstances, continental basement is exposed at the seafloor during rifting at the footwall of low angle detachment faults.
A similar lithostratigraphic association, consisting of layered gneisses micaschists, local mafic layers and Mn bearing quartzites is also found folded together with the ultramafics of the northern Lanzo massif, in the southern Sesia Zone, near the town of Lanzo. Slivers of Palaeozoic continental basement, locally preserving evidence of pre Alpine metamorphism, are scattered along the ultramafics/meta sediments interface. Structural and petrographic evidence indicate that ultramafics, continental basement and metasediments shared the same Alpine tectonometamorphic history. We conclude that the External Sesia Zone, in its southern part, consists of extensional allochthons of continental basement and abundant syn rift clastics that were deposited on the exhumed ultramafics of the northern Lanzo massif along the distal Adriatic margin.
Therefore, features characteristic of OCT’s and distal continental margins can still be recognized regionally in the External Sesia Zone despite pervasive deformation and metamorphism during the Alpine orogeny. These findings indicate that the Sesia Zone consists of continental basement that was exhumed at the floor of the Western Tethys during Jurassic rifting. Extreme thinning locally led to the formation of allochthons of continental basement resting upon exhumed ultramafics. A relatively proximal position within the extended Adriatic margin is suggested by the abundance of syn rift continent derived clastics. Our studies indicate that the apparent complexity of the axial zone of the Western Alps, with the frequent juxtaposition of continental basement and exhumed ultramafics or syn to post rift sedimentary sequences, is largely inherited from the rifting evolution of the Tethyan margins. Orogenic and subduction dynamics resulted in tectonic sampling of relatively large slivers of crust/lithosphere, ranging in thickness from 500 m to 2 4 km. Such units underwent internal deformation that did not alter their Jurassic lithostratigraphy significantly, while boundary shear zones (Insubric Line, Gressoney Shear Zone, Basal Brianconnais Thrust, etc.) accommodated most of the finite strain.
REFERENCES Beltrando M., Rubatto, D., Manatschal, G. (2010), From passive margins to orogens: The link between Ocean Continent Transition zones and (Ultra )High Pressure metamorphism, Geology 38 (6), 559 562, doi:10.1130/G30768.1 Frasca, G., Beltrando, M., Chiari, M., Compagnoni, R. (this conference), Architecture of the Internal Valaisan units in the Petit St. Bernard area (Western Alps), Conference abstract volume
10 10th Alpine workshop "CorseAlp2011"
Transfer Tectonics along the Briançonnais-Dauphinois Boundary (Western Ligurian Alps) -1:
Stratigraphic Evidence of Jurassic-Eocene Synsedimentary Tectonics
Carlo Bertok 1, Anna d’Atri 1, Luca Martire 1, Elena Perotti 1 & Fabrizio Piana 2
(1) Dipartimento di Scienze della Terra, Università di Torino, Italy (2) CNR Istituto di Geoscienze e Georisorse, Torino, Italy
The Cretaceous to Eocene diachronous closure of the Liguria Piemonte ocean and the following Oligocene onward oblique collision of the European and Adria continental plates led to the formation of a continuous orogenic chain, characterized by a northern Alpine branch with a regional vergence to the west, and a southern Apennine branch with a regional vergence to the east. This necessarily implied the presence of a regional transfer zone accomodating, for several tens of millions years, the displacement between the two opposite verging branches and controlling the evolution of the interposed “Tertiary Piemonte Basin”.
A western segment of this articulated transfer zone (here labelled as “Ligurian transfer”) is assumed to have controlled the evolution of the junction zone between the western Ligurian Alps and the southern sector of the Western Alps, where its activity could have contributed to the local tight curvature of the chain, the sharp southward decrease of metamorphic grade and the uplifting of the Argentera Massif. Therefore, a regional East West sinistral strike slip zone, active since the Oligocene, was already documented since the 80’s (Stura couloir: Ricou, 1981; Lefevre, 1983), while Molli et al. (2010) claimed the existence of a wide corridor in the Western Ligurian Alps, in which several en echelon kinematic transfer zones allowed since Early Oligocene an indipendent kinematic of Ligurian Alps with respect to the Maritime Cottian Alps, with sinistral regional main transfer in the Oligocene followed by dextral movements in Late Miocene Pliocene up to now. A major structure of the “Ligurian transfer” is the Limone Viozene deformation zone (LIVZ, Piana et al., 2009), a ESE WNW transpressive shear zone that runs for several tens kilometers along the boundary between the Ligurian Briançonnais and Provençal Dauphinois domains. Its activity is documented since the first Alpine tectonic stage of the external Ligurian Alps, although stratigraphic and sedimentologic evidence points to synsedimentary tectonics along a LIVZ precursor zone since the Middle Late Jurassic. These topics will be discussed in two separate contributions: the first one regards the Stratigraphic evidence of Jurassic-Eocene synsedimentary tectonics along the boundary between the Ligurian Briançonnais and Provençal Dauphinois domains, while the second contribution focuses on the LIVZ structural setting and Oligocene Miocene tectonic evolution of the Argentera Maritime Ligurian Alps junction zone. In this contribution a large set of stratigraphic and sedimentologic data from the transition zone between the Dauphinois and Ligurian Briançonnais domain in the Western Ligurian Alps will be illustrated and discussed, testifying for the fundamental role of a strike slip tectonics during the whole Meso Cenozoic evolution of the sedimentary basins on this portion of the European continental margin. The most striking evidence is represented by:
condensed stratigraphic successions characterized by strong lateral changes of facies and thicknesses, unconformities corresponding to wide gaps, and occurrence of Eocene conglomerate layers with basement clasts, which clearly point to fault bounded, narrow basins with a complex Meso Cenozoic subsidence history; angular unconformities, seismites and fault related neptunian dyke networks within the Middle Upper Jurassic succession; Cretaceous paleo escarpments related to km scale high angle paleofaults with displacements of several hundreds metres; dolomitized fault and fracture zones within the Triassic Jurassic carbonate formations, representing the product of Cretaceous hydrothermal dolomitization events; a regional unconformity corresponding to a hiatus spanning more than 20 My (Maastrichtian early Bartonian), bearing evidence of subaerial exposure; Upper Eocene Lower Oligocene chaotic complexes including formational, extraformational and exotic blocks and olistholits, leading to infer a Late Eocene, early Alpine, tectonic juxtaposition of slices pertaining to different paleogeographic domains.
REFERENCES Lefèvre, R. 1983. La cicatrice de Preit: une discontinuité structurale majeure au sein de la zone briançonnaise entre Acceglio et l’Argentera (Alpes Cottiennes méridionales). Compte Rendu Académie Science Paris 296, 1551 1554. Molli, G., Crispini, L., Malusà M., Mosca, P., Piana, F., Federico L. (2010) Geology of the Western Alps Northern Apennine junction area: a regional review. Journal of the Virtual Explorer, Electronic Edition, ISSN 1441 8142, volume 36, paper 10. In: (Eds.) M. Beltrando, A. Peccerillo, M. Mattei, S. Conticelli, C.Doglioni, The Geology of Italy: tectonics and life along plate margins. Piana F., Musso A., Bertok C., d’Atri A., Martire L., Perotti E., Varrone D., Martinotti G. (2009) New data on post Eocene tectonic evolution of the External Brianconnais (Western Ligurian Alps). Italian Journal Geosci., (Boll.Soc.Geol.It.), Vol. 128, No. 2, pp. 353 366. Ricou, L. E. (1981). Glissement senestre des nappes penniques le long de la bordure nord de l’Argentera: Son role dans le jeu de l’arc alpin. Compte Rendu Académie Science Paris, 292, 1305 1308.
11 10th Alpine workshop "CorseAlp2011"
Cooling Pattern, Fault-slip Analysis, and 2D Thermal Modelling in the Tauern Window
Audrey Bertrand 1, Sebastian Garcia 1, Claudio Rosenberg 1, Frederic Gueydan 2 & Bernhard Fugenschuh 3
audrey.bertrand@fu berlin.de
(1) Institut für Geologische Wissenschaften, Freie Universität Berlin, Berlin, Germany (2) Géosciences Rennes, UMR6118/CNRS, Université de Rennes 1, Rennes, France (3) Institute of Geology and Paleontology, University of Innsbruck, Innsbruck, Austria
The Tauern Window (TW) exposes Penninic basement and its cover units, forming a thermal and structural dome in front of the Dolomite indenter. Tertiary cooling and exhumation in the Eastern Alps were preferentially localized within this window. The internal structure of the dome is dominated by upright folds of large amplitude (> 15 km) in the west and more open ones in the east. The Brenner and Katschberg normal fault systems form the western and the eastern margins of the TW, respectively. Therefore, both extensional unroofing and folding associated with erosion contributed to the exhumation of the window. The relative contribution of these two processes are discussed in this work by investigating the cooling pattern derived from apatite (AFT) and zircon fission tracks (ZFT) ages, the paleo stress fields, and by modelling the thermal evolution of the TW.
The compilation of new and published AFT and ZFT ages indicates a concentric younging of both the apatite and zircon ages toward the core of the TW, in its western part, where ages vary from 17±1 Ma (ZFT) and 11±1 Ma (AFT) along the northern and southern boundaries to 13±1 Ma (ZFT) and 5±1 Ma (ATF) in the core. In the eastern part of the TW the AFT and ZFT ages show a large plateau shaped area of 17±1 Ma (ZFT) and 10±2 Ma (AFT). As the isochrones follow the map trace of the axial planes of the tight upright folds in the west of the TW and of the open ones in the east, they are inferred to be related to upright folding within the TW. This suggests that orogen prependicular folding and erosion were substantially contributing to the exhumation of the TW.
In order to test this hypothesis and to relate the low temperature geochronology to the brittle deformation, we performed a systematic study of brittle structures and their associated paleostress fields over the entire TW. In contrast to our expectations, only few reverse faults have been found in the investigated area. Indeed, the core of the TW is principally affected by strike slip faulting passing into extensional faults close to the Brenner and the Katschberg normal fault systems. The direction of the minimal stress ( σ3) is ESE WNW to ENE WSW for the strike slip regimes in the core of the TW. Along the Brenner and Katschberg normal fault systems, σ3 is oriented ESE WNW and SE NW, respectively. The lack of evidence for brittle reverse faulting compatible with upright folding most likely indicates that brittle strike slip and extensional structures postdate the reverse ones of more ductile character. Rocks affected by the inverse regime in the brittle domain have been eroded since. Assuming that the brittle ductile transition temperature is around 250°C, the maximum age of the brittle structures is given by the ZFT ages, whose closure temperature is ~240°C±10°C. The collected brittle data are therefore inferred to be younger than 17±1 Ma along the south western and north western boundaries, younger than 13±1 Ma in the core of the western part and younger than 17±1 Ma in the eastern part of the TW. This implies that no pure shortening affected the western and the eastern TW since at least 13±1 Myrs and 17±1 Myrs, respectively. As a consequence the youngest exhumation of the TW is only due to extensional denudation along the Brenner and the Katschberg normal fault systems and is partly accommodated by strike slip faulting in the core of the TW.
The lack of a reverse brittle regime precludes thickening and erosion for the recent exhumation. On the other hand extensional exhumation does not explain the above described, concentric pattern of cooling. Therefore, we postulate that the cooling is inherited from previous folding of the window. We test this idea with a set of 2D thermal models to investigate the influence of thermal inheritance from subduction, nappe stacking and nappe folding during cooling of the TW. 2D thermal numerical models are compared to the cooling patterns derived from AFT and ZFT ages in the TW and surrounding areas. The results show that the cooling history of the dome is highly dependent on the initial conditions. For rocks subducted to a depth of 60 km and incorporated in a crust reaching a thickness of 50 km by nappe stacking and folding, ca. 20 Myrs are necessary to attain thermal re equilibration of the isotherms. This suggests that the cooling pattern of the TW may at first order be inherited from deformation of the isotherms during nappe stacking and folding. This is consistent with the structural interpretation above, suggesting that brittle extension and strike slip deformation followed a previous deformation dominated by upright folding.
12 10th Alpine workshop "CorseAlp2011"
Evaluating a new geothermometer for low-grade metamorphic rocks (200-350°C)
based on Raman spectroscopy of carbonaceous material
Olivier Beyssac
CNRS IMPMC Paris, France
Quantifying temperature for advanced diagenesis to low grade metamorphism is critical to understand the formation and evolution of sedimentary basins and fold and thrust belts, or more generally, of the shallow part of the terrestrial crust. Numerous tools exist from thermochronology to isotopic (C, O) thermometry or thermo chemistry of phyllosilicates. Each of these methods has specific contingences in terms of temperature range, mineralogy or methodological applicability.
Among these methods, some largely used in the petroleum industry are based on the evolution of carbonaceous material (CM) with temperature and provides a maturity index or, even, quantitative thermal constraints (vitrinite reflectance, Rock Eval). Here, the potentiality of Raman spectroscopy for characterizing quantitatively the structural and chemical evolution of CM in the range 200 350°C is investigated.
A set of samples from various geological settings has been selected to discuss the respective roles of temperature, geothermal gradient, lithology or organic precursor. The evolution of the Raman spectra will be systematically described and quantitative parameters extracted after appropriate fitting will be compared to the temperature history of the rocks. Altogether, the Raman spectroscopy of CM (RSCM) may provide a new, efficient, reliable and simple geothermometer for quantifying the peak metamorphic temperature of sediments in the range 200 350°C. Some application of this new geothermometer will be discussed.
13 10th Alpine workshop "CorseAlp2011"
The impact of volcanism on the sedimentary record of Central-Western Mediterranean
Marco Brandano 1,2 , Michele Lustrino 1,2 , Sara Ronca 1 & Laura Tomassetti 1
(1) Dipartimento di Scienze della Terra, Università degli Studi di Roma “La Sapienza”, Roma, Italy (2) Istituto di Geologia Ambientale e Geoingegneria (IGAG) CNR, Monterotondo Stazione, Roma, Italy
During Late Oligocene Middle Miocene (Chattian to Langhian) the Sardinia Corsica block first rifted and then drifted with a counter clockwise rotation from the main European plate. More or less during the same interval also the Balearic Rise rotated clockwise, leading to the embryon of the Central Western Mediterranean. The two basins formed as consequence of these blocks rotation (Ligurian Provençal and Valencian, respectively) are believed to be back arc basins related to the W NW directed Apennines Maghrebides subduction system. Associated with such major tectonic movements and lithospheric stretching of the Southern European paleo margin, a huge subaerial and submarine volcanic activity developed along the two conjugate margins (Corsica Sardinia and Provence plus Balearic Islands and SE Spain) as well as within the subsiding basins.
This igneous activity may have exerted two main effects on the sedimentary sequences of Central Western Mediterranean area:
1) a primary impact on basin physiography and stratigraphy, flooding them with thick volcanic and volcaniclastic products. Obviously, also the terrigenous fraction in siliciclastic carbonate mixed sequences is strongly influenced by the volcanic activity;
2) a secondary impact on geochemical composition of seawater.
Examples of primary impact are represented by the filling N S trending Fossa Sarda Graben by volcanic products that controlled the facies distribution of Burdigalian coastal and carbonate platform systems. During the same time interval, the Apennine deep water successions are characterized by several pyroclastic horizons (e.g. Bisciaro Fm, Schlier Fm, Northern Italy). The secondary impact is produced by repeated volcanic CO 2 pulses that may have caused repeated decreases in pH and carbonate ion concentrations in surface waters. CO 2 increase causes a reduction in seawater carbonate saturation, favouring siliceous production in acid seawaters (e.g., Guadagnolo Fm, Bisciaro Fm, Schlier Fm) and affecting the organic and inorganic carbon cycle recorded by δ 13 C curve The detailed review of sedimentological and volcanic record plus new data from the circum Italian area, allow us to better constrain the effects of endogenous forces in shaping Earth surface processes.
14 10th Alpine workshop "CorseAlp2011"
Indication of HP-LT Metamorphic Conditions in the Elba Island (Northern Apennines,Italy)
1 1 2 Andrea Brogi , Giovanna Giorgetti & Domenico Liotta
(1) Dipartimento di Scienze della Terra, Università di Siena, Italy (2) Dipartimento di Geologia e Geofisica, Università di Bari, Italy
The collisional tectonics which affected the Northern Apennines is defined by the stacking of the tectonic units deriving from the related Jurassic Oligocene palaeogeographic domains and by the HP LT metamorphic conditions reached during the collisional events; mineralogical parageneses indicating pressure and temperature values of about 1 1.5GPa and 300° 400°C have been widely recognized in southern Tuscany and in the Tuscan Archipelago. An exception to this context is represented by the Elba Island where apparently contrasting features are displayed. In fact, although the tectonic pile deriving from the collision is clearly defined by the overlap of oceanic units onto continental units and by out of sequence thrusts, HP LT conditions were never constrained, accounting for the overprint of HT LP metamorphism deriving from emplacement and cooling of the Elba Island Late Miocene Pliocene plutons. In order to contribute to fill this gap, quartzites from the Zuccale area (Barabarca Fm, Trias), have been analyzed through scanning electron microscopy (SEM) coupled with analytical energy dispersive system (EDS).
They are mainly comprised of quartz, muscovite, chlorite, K feldspar, Fe and Ti oxides, calcite, apatite, ±albite, ±kaolinite. Quartz grains are strongly deformed and aligned along the main schistosity; they often show sigmoid shape and phyllosilicates form in quartz pressure shadows. Newly formed phyllosilicates are also aligned along the main schistosity. Two compositionally different chlorites, one Fe rich (xMg=0.46) and one Mg rich (xMg=0.61) chlorite, coexist in one sample where they form spot shaped intergrowths of tens of micron in size. Muscovite shows a quite low phengitic substitution with Si=3.1 atom per formula unit (a.p.f.u.).
Kaolinite forms as a secondary phase on pristine micas. The metamorphic assemblage indicates that quartzites underwent a low grade metamorphic event. The pressure conditions of this event are scarcely constrained; the presence of K feldspar in association with muscovite with Si=3.1 a.p.f.u. indicates a possible minimum pressure of 1GPa for temperature of ca. 300°C. These conditions are in agreement with the documented HP LT estimates for the Triassic metasediments cropping out in Tuscan Archipelago and southern Tuscany (i.e. the inner part of the northern Apennines).
15 10th Alpine workshop "CorseAlp2011"
Resolution of the Genesis of Isoclinal Folds in Multiply Deformed Basement Terrains
Hui Cao 1, 2
(1) Key Laboratory of Continental Dynamics of the Ministry of Land and Resources, Institute of Geology, Chinese Academy of Geological Sciences, Beijing, P. R. China (2) School of Earth and Environmental Sciences, James Cook University, Townsville, Australia
Whether isoclinal folds originate as antiforms or synforms in steeply dipping multiply deformed high metamorphic grade gneisses and schists is commonly difficult to ascertain because sub vertical stretching and high strain rotates fold axes towards the vertical. A succession of five FIAs (foliation intersection/inflection axes preserved within porphyroblasts) trending SSW NNE, SW NE, W E, NW SE and NNW SSE in cordierite schist units interlayered with gneisses in the Arkansas River region of South Central Colorado reveal the timing and origin this fold. The asymmetry of curvature of the extremities of foliations preserved as inclusion trails within porphyroblasts, from gentle to steep pitches in sections near orthogonal to the associated FIA trend, reveal that a steeply plunging isoclinal fold, which was originally interpreted to be a synform, formed as an antiform (Fig.1). It developed during the first stages of porphyroblast growth. This approach can be applied to all similar rocks and can be used to resolve stratigraphic successions in rocks containing no younging criteria.
a b
c d
Fig. 1. a & b. Schematic diagrams of changes (lower 2 porphyroblasts) in porphyroblast inclusion trail asymmetry that would accompany the development of an antiform (a) and a synform (b). c & d. Histograms for each limb of the fold showing the asymmetry of inclusion trails changing from gentle to steep pitches preserved within porphyroblasts.
16 10th Alpine workshop "CorseAlp2011"
Magnetic Anisotropy Related to Stretching Parallel to the Alps in the Rawil Axial Depression
(SW Switzerland)
Giovanni Luca Cardello, Bjarne Almqvist, Ann Hirt & Neil Mancktelow
ETH Zürich Geologisches Institut, Zürich, Switzerland
Structural field analysis of faults and veins in the SW Helvetic nappes indicates that a progressive counter clockwise rotation of the stretching direction took place during post nappe deformation in the Neogene. Initial post nappe stretching is approximately orogen parallel and defined by the orientation of extensional veins and oblique faulting oriented WSW ENE, with evidence for both ductile and brittle deformation. Subsequently the stretching direction rotates towards an orientation approximately perpendicular to the orogen, associated with mainly brittle deformation, which would be consistent with exhumation and cooling.
We have used Anisotropy Magnetic Susceptibility (AMS) to investigate whether the magnetic fabric records only part of, or the complete, stretching history during exhumation of the Rawil depression. Representative samples were collected from all nappes currently exposed in the Rawil Depression. In the western and central part of the Rawil Depression, the AMS confidence ellipses show a sub horizontal magnetic lineation (k1) directed WSW ENE, with a mean dip direction to the ENE. The great circle resulting from the plot of the mean values of k1 and k2 fits well with extension parallel to the regional fold trend. Our results therefore indicate that the magnetic fabric records only the initial stage of stretching associated with ductile brittle extension, i.e., stretching approximately parallel to the Alps. Assuming that the temperature was high enough for new minerals to grow or recrystallize during deformation, the magnetic fabric must have formed prior to or during formation of the Rawil depression.
Because the magnetic lineation is parallel to the post nappe orogen parallel stretching direction, we interpret this as additional evidence for the post nappe formation of the Rawil depression, rather than it having developed as a depression between antiformal nappe stack culminations developed in the Aar Gastern and Mont Blanc Aiguilles Rouges massifs. The last event of stretching is not recorded in the magnetic fabric of any sample, although this late orogen perpendicular stretching is clearly reflected in late movements recorded on faults and in veins. The relative uplift of the Aar Gastern and Mont Blanc Aiguilles Rouges massifs and the formation of the Rawil depression therefore occurred under cooler deformation conditions (with brittle faulting, veining, and meteoric water flow in densely fractured zones), which did not further affect the magnetic fabric.
17 10th Alpine workshop "CorseAlp2011"
Vertical Movements and Emplacement of Ligurian and Subligurian Units Constrained by a new Dataset of low-Temperature Thermal and Thermochronological Analyses Along a Regional Cross Section Across the Northern Apennines (Italy) Mirko Carlini, Andrea Artoni, Massimo Bernini, Luigi Torelli, Francesca Camurri & Paolo Vescovi
Dipartimento di Scienze della Terra Università degli Studi di Parma, Italy
Low temperature thermal and thermochronological analyses have been widely employed to constrain vertical movements and erosion experienced by cropping out rocks presently exposed in orogenic belts; these types of analyses give new insights on the long term evolution of the orogens. Despite of the many published works on this subject, few studied cases integrate thermal, thermochronological and well constrained geological data within the same orogen; this would further improve the knowledge about the maximum burial and uplift and the tectonic evolution of mountain chains over long time periods. It is our aim to carry out this integrated approach in the Northern Apennines which offer a large amount of published data regarding apatite (U Th)/He, apatite fission track (AFT), zircon (U Th)/He, zircon fission track, vitrinite reflectance and clay mineral analyses (Kübler index and illite content in I/S); these data can also be integrated with well established tectonic and geological constraints and, in the present work, with a new cross section based on deep seismic reflection profile, which is the only regional scale profile existing in the study area. In order to pursue this aim a regional scale resource of maximum burial data, exhumation ages and geological data for the Northern Apennines of Italy has been compiled on a GIS platform, over an area defined by coordinates 44°57’N 8°48’E to the NW and 44°48’N 11°59’E to the SE; the collected data are projected on the new available deep cross section roughly extending from La Spezia to Parma (about 44°10’N 9°35’E to the SW, 44°47’N 10°21’E to the NE). Integrating the great amount of data and analytical methods implies many drawbacks and assumptions of which the most critical one is to assume a geothermal gradient, used to convert temperature values in depth values, invariable through space and time and fixed to 30°C/km, as the mean Earth’s crust geothermal gradient. Despite the drawbacks and assumptions, in the study area it is possible to distinguish five main sectors which show noticeable differences in maximum recorded load and/or uplift timing. These five sectors correspond to the different outcropping tectonic units (mainly Oligocene Miocene foredeep units underthrusted to a Jurassic to Paleogene fossil accretionary wedge constituted by Ligurian and Subligurian Units).
A first overview of both maximum burial and uplift ages recorded by the aforementioned units highlights major differences between the overthrusted (Ligurian and Subligurian) units and the foredeep units. Ligurian and Subligurian Units show maximum temperatures of 180°C 200°C (corresponding to a maximum load of ~ 6 7 km) in the inner southwestern portion of the chain (sectors 1 and 3), decreasing down to <130°C (maximum load ~ 3 4 km) in the external northeastern portion (sector 5). This trend, already recognized on a regional scale in published works, is also supported by AFT data that show, from SW to NE, progressively fully reset and partially reset fission tracks. AFT ages are available only for the Ligurian Units outcropping in the inner sectors and they span from ~ 17.5 Ma to ~ 7.6 Ma. Concerning the foredeep units (Macigno formation and Pracchiola Unit), they show maximum temperatures spanning from ~ 170°C up to ~ 250°C (maximum load ~ 6 8 km) and much younger AFT (~ 4.6 Ma) (sector 2) and apatite U/Th(He) (~ 2.7 Ma) ages (sector 4).
Nonetheless, in these latter units, the thermochronological and thermal data do not have a unique and easily explicable interpretation, mainly because most of them lack detailed information about the techniques, the analytical procedures and the geological reference frame followed in producing the data themselves. In order to overcome these problems a new sampling and new field surveys are planned. The simple and rough analysis of the compiled dataset coupled with the regional cross section reveals that the Ligurian nappe might have had a wedge shape geometry tapering towards NE (decreasing temperature/maximum load from sector 1 to sector 4). Nowadays, on the contrary, the Ligurian wedge shows its maximum thickness (~ 4 km) very close to the external margin of the Apennine chain (sector 5), where it lies upon the Serravallian foredeep and has an estimated maximum palaeo thickness of ~ 6 7 km (the preserved ~ 4 km plus ~ 2 3 km deduced from the missing load recorded by thermal data). This value of palaeo thickness is well comparable to the one recorded at the base of the Ligurian nappe in the most inner portion of the chain (see above, sector 1); herein, the nappe lays on top of the late Oligocene early Miocene foredeep units and, nowadays, preserved a thickness of ~ 1 km.
Considering the ages coming out from the compiled dataset it is clear that the emplacement timing of the Ligurian nappe, progressively younger to the northeast (post Aquitanian in sectors 1 4, post Serravallian in sector 5), fits well the uplift ages of the Ligurian nappe in the internal sector (~ 17.5 Ma i.e. post Aquitanian to ~ 7.6 Ma i.e. post Serravallian) as well as the later uplift of the deeper Aquitanian foredeep units (~ 4.7 Ma). Therefore, we suggest that the uplift of the inner sectors of the Apennines could be considered one of the major contributors in triggering and enhancing the advancement and the thrusting of the Ligurian and Subligurian Units over the more external Serravallian foredeep, as recognized in other area of the Northern Apennines and outlined in models of orogenic collapse. The new thermal and thermochronological data coming out from the planned new sampling campaign should further constrain the emplacement mode of the Ligurian units as well as the amount and the rates of vertical movements occurred in this portion of the Northern Apennines.
18 10th Alpine workshop "CorseAlp2011"
Cenozoic geodynamic and magmatological evolution of central-western Mediterranean
Eugenio Carminati 1,2 , Michele Lustrino 1,2 , Marco Cuffaro 2 & Carlo Doglioni 1,2
(1) Dipartimento di Scienze della Terra, Università degli Studi di Roma La Sapienza, Roma, Italy (2) Istituto di Geologia Ambientale e Geoingegneria IGAG, CNR, Roma, Italy
We present a tectonic reconstruction of the central western Mediterranean during the last 50 Myr. The fragmented Mesozoic and early Cenozoic paleogeography between Africa and Eurasia constrained the evolution and polarity of subduction zones. Both oceanic and continental lithosphere was consumed along the plate boundaries, followed by diachronous continent continent collision in several sectors. The subduction hinge of the slabs was converging toward the upper plate in the Alps Betics and Dinarides, forming a double vergent orogen and two foredeeps with low subsidence rates.
On the other hand, in the Apennines Maghrebides and Carpathians cases, the hinge was rapidly diverging relative to the upper plate, forming a single vergent accretionary prism, a single fast subsiding foredeep, and the retreating lithosphere deficit was compensated by the asthenosphere and the opening of back arc basins. The two end members mimic the asymmetries that can be recognized along world's subduction zones as a function of their direction with respect to the westward drift of the lithosphere along the "tectonic equator" of Crespi et al. (2007, GJI).
In our reconstruction, the thrusts related to W directed subduction systems of the Apennines Maghrebides and Carpathians nucleated along the retro belt of the Alpine and Dinarides subduction zones, respectively. The variegated magmatism confirms a strong heterogeneity of the Mediterranean upper mantle and of the subducting plates. In the Apennines Maghrebides and Carpathians systems the subduction related igneous activity (mostly medium to high K calcalkaline melts) are commonly followed in time by mildly sodic alkaline and tholeiitic melts.
19 10th Alpine workshop "CorseAlp2011"
New stratigraphic constrains of the para-autocthonous Caporalino-Sant’Angelo Unit belonging to the
“Alpine Corsica” (Corte-Ponte Leccia area, Corsica)
Cascella Antonio 1, Puccinelli Alberto 2 & Perilli Nicola 2
[email protected] & [email protected]
(1) Istituto Nazionale di Geofisica e Vulcanologia, Pisa, Italy (2) Dipartimento di Scienze della Terra, Università di Pisa, Pisa, Italy
The results of a stratigraphic and biostratigraphic study carried out on the Caporalino S. Angelo (Alpine) Unit outcropping in the Omessa area (NE Corsica), where the units from the “Alpine” Corsica overlies the “Hercynian” Corsica, are here presented. The new geological mapping of the Corte Ponte Leccia area, the lithostratigraphy and biostratigraphy of three key composite sections, allowed us to invalided the previous tectono sedimentary models that considered the Caporalino Sant’Angelo Unit a continuous stratigraphic succession sedimented in a distensive semigraben basin from Permian/Jurassic to Eocene time interval.
The Caporalino Sant’Angelo Unit is a clastic succession composed of eight levels and three main clastic intervals. The lower clastic interval consists of a fining upwards siliciclastic sequence with breccias, conglomerates and sandstone (with basement clasts).
The coarse grained middle clastic interval (with basement clasts and limestone clasts) also includes the huge olistoliths of Caporalino Limestones. The upper clastic interval is a monotonous succession of thin bedded sandstones with pelitic interbeds and rare intercalations of conglomerates. According to the recovered microfossil assemblages (calcareous nannofossils and foraminifera), the studied succession is Early to Middle Eocene in age. In our opinion, the Caporalino Sant’Angelo Unit is an Eocene clastic wedgde accumulated in a basin located between the European foreland and the deforming accretionary wedge, and receiving the (1) siliciclastic sedimentary input (mainly) from the European foreland and the (2) carbonatic input (mainly) from other nearby units belonging to the “Alpine” Corsica, which were progressively involved into the thrust and fold Alpine belt.
20 10th Alpine workshop "CorseAlp2011"
Allanite Behavior in Upper-Mid Crustal Shear Zones: Can we Date Deformation Directly?
Example from the Mont Blanc Massif and Implications on Its Alpine Evolution
Bénédicte Cenki Tok 1,2 , James Darling 1, Yann Rolland 3, Magali Rossi 4 & Martin Engi 1
benedicte.cenki [email protected] montp2.fr
(1) Institute of Geological Sciences, University of Bern, Baltzerstrasse 3, Bern, Switzerland (2) Géosciences Montpellier, Université de Montpellier 2, Montpellier, France (3) Géoazur UMR6526, Université de Nice Sophia Antipolis, Parc Valrose, Nice, France (4) Edytem, UMR 5204, Technolac, Le Bourget du Lac, France
Allanite is a promising petrochronometer: it may yield ages that can be correlated with physico chemical conditions, 3+ 2+ based on petrological observations (e.g. Gregory et al., 2007). Allanite (CaREE(Al,Fe )2(Mg,Fe )Si 3O12 (OH)) is an epidote group mineral that can incorporate LREE, Th and U. Understanding of the petrology and geochemistry of allanite has improved greatly (e.g. Wing et al., 2003, Janots et al., 2008), but little is known so far regarding the effects of deformation and recrystallization on allanite U Th Pb systematics. A first attempt to test whether allanite can be used to infer the age of mylonitisation revealed that allanite can be remarkably resistant to deformation (Cenki Tok et al., 2011). At Mt Mucrone (Italy), allanite located in an eclogite facies shear zone of Alpine age shows exclusively Permian ages. In that case, it appeared that deformation alone is not the major factor controlling allanite metamorphic recrystallization and isotopic resetting.
The present study reports U Th Pb in situ ages from allanite from a greenschist facies shear zone from the Mont Blanc Massif. Field, petrological and geochemical observations indicate that ductile deformation in this chlorite and phlogopite bearing shear zone (ca. 100 m in width) was accompanied by intense fluid percolation (Rossi et al., this meeting). Allanite is present as euhedral grains homogeneous in major and REE elements composition. Allanite appears to be in compositional and textural equilibrium with the mylonitic assemblage (chlorite, phlogopite, K feldspar, albite, quartz). Th Pb Laser Ablation (LA) ICPMS dates yield an age population with a weighted mean of 29.0 ± 1.8 Ma (2σ). These ages were corrected for inherited common Pb, using the Pb isotopic composition measured on the associated chlorite and albite crystals. In this example, allanite does record the age of mylonitisation and fluid influx. The age of 29 Ma obtained for allanite is the first radiometric age available for these chlorite and phlogopite bearing shear zones in the Mont Blanc Massif. In low strain domains crosscutting adular bearing veins can be observed. These were dated by 40 Ar 39 Ar methods at ca. 14 Ma. Outside the shear zone, high strain domains contain phengite ± greenschist facies minerals (Rolland et al. 2008). 40 Ar 39 Ar ages on these phengite bearing shear zones located in an adjacent structural domain with no evidence of large fluid influx are younger in age (16 Ma; Rolland et al., 2008). The diachronism revealed by the two studies highlights a succession of events unrecognized in the Mont Blanc Massif so far: i) Ductile deformation and fluid percolation at ca. 29 Ma; this may be linked to the activation of the Penninic front as revealed more to the South in the Pelvoux Massif (Simon Labric et al. 2009); ii) reactivation of the shear zones at 16 14 Ma, with the onset of exhumation. We conclude that allanite may be a useful chronometer in highly deformed rocks provided a robust petrological, macro and microtextural context has been worked out, so as to help in interpreting these ages.
REFERENCES Cenki Tok, B., Oliot, E., Rubatto, D., Berger, A., Engi, M., Janots, E., Thomsen, T.B., Manzotti, P., Regis, D., Spandler, C., Robyr, M., Goncalves, P., 2011. Preservation of Permian allanite within an Alpine eclogite facies shear zone at Mt Mucrone, Italy: Mechanical and chemical behavior of allanite during mylonitisation. Lithos, in press. Gregory, C.J., Rubatto, D., Allen, C.M., Williams, I.S., Hermann, J. & Ireland, T., 2007. Allanite micro geochronology: A LA ICP MS and SHRIMP U Th Pb study. Chemical Geology, 15, 162 182. Janots, E., Engi, M., Berger, A., Allaz, J., Schwarz, J. O., Spandler, C., 2008. Prograde metamorphic sequence of REE minerals in pelitic rocks of the Central Alps: implications for allanite monazite xenotime phase relations from 250 to 610°C. Journal of Metamorphic Geology 26(5), 509 526. Rolland, Y., Rossi, M., Cox, S.F., Corsini, M., Mancktelow, N., Pennacchioni, G., Fornari, M., Boullier, A.M., 2008. 40 Ar 39 Ar dating of synkinematic white mica: insights from fluid rock reaction in low grade shear zones (Mont Blanc Massif) and constraints on timing of deformation in the NW external Alps. Wibberley, C.A.J., Kurtz, W., Imber, J., Holdsworth, R.E. & Collettini, C. (eds). The Internal Structure of Fault Zones: Implications for Mechanical and Fluid Flow Properties. 299, 1 23. Rossi, M., Rolland, Y., Chauvel, C., 2011. Fe Mg metasomatism in the Mont Blanc Massif. Corsealp meeting 2011. Simon Labric, T. Rolland Y., Dumont, T. Heymes, C. Authemayou, M. Corsini, M. Fornari. 2009. 40 Ar/ 39 Ar dating of Penninic Front tectonic displacement (W Alps) during the Lower Oligocene (31 34 Ma). Terra Nova, 21, 127 136. Wing, B.A., Ferry, J.M., Harrison, T.M., 2003. Prograde destruction and formation of monazite and allanite during contact and regional metamorphism of pelites: petrology and geochronology. Contributions to Mineralogy and Petrology 145(2), 228 250.
21 10th Alpine workshop "CorseAlp2011"
Tectonics, Climate, and Mountain Topography
Jean Daniel Champagnac 1, Peter Molnar 2, Christian Sue 3 & Frédéric Herman 1
(1) ETH Zürich, Switzerland (2) CIRES, University of Colorado at Boulder, Colorado, USA (3) Université de Franche Comté Besançon, France
The topography of mountain ranges results from both tectonic processes that can elevate rock and erosive processes, which depend strongly on climate. Understanding how tectonics and climate affect topography remains convoluted because of the interactions and feedbacks between them, and the various factors that may affect erosion rates.
By regressing independent external variables that simply describe the climate and tectonic forcing against measures of topography and relief, we quantify the relative importance of these factors in shaping the observed landscape of 69 mountain ranges worldwide. Climate variables include latitude (as a surrogate for the potential of glaciation) and mean annual precipitation. Tectonics is represented by shortening rates across each range. Several topographic variables are used to describe the landscape, including mean and maximum elevations and relief calculated over different length scales. We show that shortening rates account for only small portions of the variance in most measures of topography and relief (i.e. with low correlations and large scatter), but mean and maximum elevations of mountain ranges are largely explained the interaction of climate (negative correlation) and tectonics (positive correlation). Relief is insensitive to mean annual precipitation, but does depend on latitude. This is especially clear for small scale (1 km) relief, which we infer to reflect the importance of glacial erosion. Larger scale (~10 km) relief, however, correlates positively with tectonic shortening rate. Moreover, the ratio between small scale and large scale relief, as well as the relative relief (the relief normalized to the mean range elevation) depends most clearly on latitude (strong positive correlation). Therefore, the location of a mountain range on earth, not just tectonic forcing, appears to be a key factor to determine its shape. In any case, tectonics and climate as quantified here can account for approximately half of the variance in any measure of topography.
As argued by Molnar [2009], the landscape reflects an equilibrium (or a transient state toward a new equilibrium) between processes that raise or lower it, tectonics and climate (through erosion). The present study quantifies a plausible set of parameters of such equilibrium that are not biased by theory, but rather are based on data extracted from natural examples. Despite a large scatter inherent to the kinds of data used, we show that some good correlations exist between independent variables, which therefore justify a posteriori the quality of the database. The following points are of particular importance:
1) Elevations of ranges directly reflect the interaction between tectonics, which thickens the crust (hence increases the elevation), and climate (through erosion), which thins the crust (hence decreases the elevation). This statement may seem trivial, but this is a far as we know the first demonstration of this over such a large natural database. 2) Relief is not sensitive to mean annual precipitation amounts, probably because fluvial erosion is much less efficient in creating steep slopes and high relief than is glacial erosion. Relief scales primarily with shortening rates and latitude (hence glacial erosion). Relief defined by averaging over large is more sensitive to tectonic forcing than is relief measured on short distance scales, which is the more sensitive to latitude. This can be caricatured by imagining relief in high latitude mountain ranges to result largely from glacial excavation of the landscape. 3) The location of a mountain range on earth appears to be a key factor to determine its shape. This is especially so for small scale relief and for the relative relief (the amount of relief with respect to the volume of the range). Similarly, latitude plays a crucial role in shaping the scale length of the relief.
22 10th Alpine workshop "CorseAlp2011"
Heterogeneous Deformation of Granitoids at Plate Boundaries
During the Late Continental Collision in the Insubric Area (Central Alps)
Laurent Ciancaleoni
ciancaleoni@univ corse.fr
Université de Corse Pasquale Paoli, Laboratoire des Sciences de la Terre, Corte, France
How the Tertiary convergence between the Adriatic subplate and the stable European foreland has been accomodated by deformation along the Insubric Line (Periadriatic Fault System) in the Eastern Central Alps remains a matter of debate. This study provides new insights into the kinematics and timing of deformation along the plate boundaries, during the late Oligocene and Neogene.
Granitic rocks initially have a bulk homogeneous and "isotropic" texture and represent good analogues for the study of the rheological behaviour of the continental crust. When deformation occurs, these granitic rocks commonly undergo heterogeneous strain reflected by conjugated patterns of anastomozing shear zones in the ductile field, and fault populations in the brittle field. Such heterogeneous strain in granites provide useful paleostrain/stress indications to constrain the bulk kinematics during the continental deformation. On the one hand, shear zone patterns are described as slip surfaces that accommodate most of the bulk deformation, and therefore they have been used as reliable and large scale shear criteria and strain markers (e.g. Gapais et al., 1987). On the other hand, fault slip analysis inversion methods are used to derive principal stress and strain axes orientation (e.g. Angelier, 1984). In this study, methods of fault slip analysis are applied to brittle ductile deformation fields. The resolved incremental principal strain axes (P and T axes) are compared to the finite strain axes deduced from classical strain analyses (via schistosity stretching lineation couples measured in low strain domains).
This study is integrated in a wide thematic project focused on the use of shear zone and fault patterns in metagranites to understand the deformation and kinematics in different tectonic units along the NFP20 East seismic profile of the Alps (e.g. Marquer, 1991; Marquer et al., 1996; Ciancaleoni and Marquer, 2006). These studies show that shear zone pattern studies in metagranites are an important tool: (1) to interpret the bulk alpine kinematics at large scale in different tectonic units; (2) to understand the mechanical behaviour of the continental crust, which is mainly controlled by heterogeneous deformation at different stages of the mountain building processes (subduction/collision). In this study, the brittle ductile heterogeneous deformation of granitic rocks at mid crustal levels in the vicinity of the Insubric Line is investigated and compared: in the 30 Ma old Sondrio intrusion (Giger, 1991), which occurs immediately north of the Insubric Line, in the mylonitised Upper Austroalpine series; in the Dazio metagranite, which occurs immediately south of the Insubric Line, in the South Alpine basement rocks. The symmetrical/asymmetrical patterns versus schistosity, lineation and P T strain axes are used as large scale kinematic indicators and discussed in terms of coaxiality/non coaxiality of the deformation.
REFERENCES Angelier, J. (1984). Tectonic analysis of fault data sets. Journal of Geophysical Research B: Solid Earth, 89 , 5835 5848. Ciancaleoni, L., and Marquer, D. (2006). Syn extension leucogranite deformation during convergence in the Eastern Central Alps: example of the Novate intrusion, Terra Nova, 18(3), 170 180. Gapais, D., Bale, P., Choukroune, P., Cobbold, P., Mahdjoub, Y. and Marquer, D. (1987). Bulk kinematics from shear zone patterns: some field examples. J. Struct. Geol. , 9(5/6), 635 646. Giger, M. (1991). Geochronologische und petrographische studien an Geröllen und Sedimenten der Gonfolite Lombarda Gruppe (Südschweiz und Norditalien) und hir Vergleich mit dem alpinen Hinterland. Bern, Universität Bern: 227 p. Marquer, D. (1991). Structures et cinématique des déformations alpines dans le granite de Truzzo (Nappe de Tambo: Alpes centrales suisses). Eclogae geol. Helv. , 84 (1), 107 123. Marquer, D., Challandes, N. and Baudin, T. (1996). Shear zone patterns and strain partitioning at the scale of a Penninic nappe: the Suretta nappe (Eastern Swiss Alps). J. Struct. Geol. , 18(6), 753 764.
23 10th Alpine workshop "CorseAlp2011"
Complex Underplating and Kinematics of Folding in Accretionary Wedges:
Analogue Models and Case Studies
Luca Clemenzi 1,2 , Clément Perrin 3,4 , Jacques Malavieille 4 & Giancarlo Molli 2
[email protected] & [email protected]
(1) Dipartimento di Scienze della Terra, Università degli Studi di Parma, Parma, Italy (2) Dipartimento di Scienze della Terra, Università di Pisa, Pisa, Italy (3) Géoazur Nice Sophia Antipolis, Valbonne, France (4) Géosciences Montpellier, Université Montpellier 2, Montpellier cedex 5, France
During the last thirty years, many experimental studies allowed to better understand the main tectonic processes involved in the development of accretionary wedges. Using this approach, the effects of numerous parameters on geometry, deformation mechanisms and faults kinematics have been investigated. More recently, the impact of surface processes has been shown to be major in the development orogenic wedges. Indeed erosion, and sedimentation influence wedge’s dynamics through surface and deep mass transfers (e.g. Davis et al., 1983; Mugnier et al., 1997; Konstantinovskaia & Malavieille, 2005; Cruz et al., 2008). New analogue experiments have been performed at Géosciences Montpellier Laboratory to study the evolution of eroded thrust wedges taking into account specific inherited structures of the downgoing plate. A classical sandbox set up has been improved to allow large shortening and to account for a more realistic geometry of lower plate structures. Thin layers of weak materials are interbedded in the subducting materials to favor strain partitioning while thin layers of “plastic” materials (e.g. plastiline) are used to study the development of folding in the wedge. Basic experiments are presented first, to outline the effects of the new boundary conditions imposed. Different positions of the weak layers are tested to unravel which one favor strain partitioning in the wedge. Weak layers induces duplexing and subsequent underplating in the inner part of the wedge. For large shortening, a cyclical behavior of underplating is observed, leading to the development of two adjacent antiformal stacks. The influence of erosion on exhumation processes is also analyzed by comparing models submitted to or not submitted to erosion. Secondly,we present the results of models built to study the development of kilometric scale recumbent folds, in the upper structural levels of foreland mountain chains. Observation of large folds with horizontal inverted limbs are common in the non metamorphic domains of the European Hercynian chain, but their conditions of development is not well known and is difficult to explain with the classical concepts of folding. The inclusion of a plastic layer in the deforming sequence combined with the presence of a weak layer (décollement) in the lower sequence (figure). The presence of the weak layer favor strain partitioning: the deformation of the plastic layer induces the development of overturned folds in the superficial domain after, while an antiformal stack of underplated units develops at the base of the prism. The progressive growth of the duplex structure leads to the passive deformation of the upper units, situated above the décollement layer. Stacking of tectonic units in the domain of underplating induces forward tilting of the recumbent folds, whereas erosion avoid important burial of folded structures, leading to the development of a recumbent folds domain. Results of experiments are used to better understand deformation in several case studies where complex underplating structures and kilometric scale recumbent folds in superficial units are observed: the Montagne Noire (southern part of Massif Central, France), the northern Apennine (Italy) and the Galice belt (Spain).
Figure - The succes sive development of two domains of basal accretion allows tilting of upper series and the rotation of the folds flanks to the horizontal (Digital dial: shortening in cm)
REFERENCES
Cruz, L., Teyssier, T., Perg, L., Take, A., Fayon, A. (2008). Deformation, exhumation, and topography of experimental doubly vergent orogenic wedges subjected to asymmetric erosion. Journal of Structural Geology , 30, 98–115 Davis, D., Suppe, J. And Dahlen, F. (1983), «Mechanics of fold and thrust belts and accretionary wedges», Journal of Geophysical Research, volume 88 (B2), p. 1153–1172. Konstantinovskaia, E., & Malavieille, J. (2005). Erosion and exhumation in accretionary orogens: Experimental and geological approaches. Geochemistry, Geophysics, Geosystems , Vol. 6, N°2, 25 pp, Q02006, doi: 10.1029/2004GC000794, ISSN: 1525 2027 Mugnier, J.L., Baby, P., Colletta, B., Vinour, P., Bale, P., Leturmy, P. (1997). Thrust geometry controlled by erosion and sedimentation: A view from analogue models. Geology. 25, N°5, 427–430.
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Extreme Crustal Thinning during Cretaceous Rifting in the Pyrenean Realm:
Field evidence from the North Pyrenean Zone
Camille Clerc 1,2 , Yves Lagabrielle 2, Christian Chopin 1, Alain Vauchez 1, Michel de Saint Blanquat 3, Abdeltif Lahfid 4 & Pierre Labaume 1
(1) Laboratoire de Géologie, CNRS UMR 8538, Ecole Normale Supérieure, Paris, France (2) Géosciences Montpellier, CNRS UMR 5243, Université de Montpellier 2, Montpellier, France (3) LMTG, CNRS UMR 5563, Université Paul Sabatier, Toulouse, France (4) BRGM / LMA / MIN, Orléans, France
Fragments of sub continental mantle can be found all along the Pyrenean mountain belt. These fragments range from a few meters to a square kilometer in size, and are concentrated in a narrow band of Mesozoic sediments parallel to the North Pyrenean Fault, which is known as the major tectonic boundary between the Eurasian and Iberian plates. In the light of the recent reinterpretation of some North Pyrenean peridotites as remnants of subcontinental mantle exhumed in the vicinity of an Albian aborted rift, we propose a new detailed examination of the North Pyrenean metamorphic basins. Focusing on the Aulus (Haute Ariège, France) and the Ballongue (Haute Garonne) basins, we present the results of a comprehensive mapping of the relationships between the peridotites, the surrounding meta sediments and the Paleozoic and older basement rocks.
We show that uplift of the ultramafic and lower crust rocks is linked to motions along low angle faults, which present an evolution from hot and ductile to cold and brittle deformation mode, characteristic of extensional detachment. We emphasize the importance of tectonic decoupling between the Mesozoic sedimentary cover and the Paleozoic basement, which leads to the juxtaposition of metamorphosed and deformed Mesozoic sediments directly on the foot wall of the low angle fault controlling mantle exhumation. Exhumation processes enventually ended with the sedimentary reworking of both the deformed Mesozoic metasediments and the exhumed ultramafic rocks.
Paleotemperatures obtained with Raman Spectrometry of Carbonaceous Material (RSCM) geothermometry reveal strong anomalies in the Mesozoic sediments, related, at a regional scale, to the proximity of mantle rocks and lower crust units as already suggested further east in the Boucheville Synform and the Pays de Sault. Furthermore, clearly marked Crystal Preferred Orientation (CPO) revealed by Electron Backscattered Diffraction (EBSD) analyses in the marbles of the North Pyrenean Zone indicate intracrystalline slip on the basal plane of calcite. According to existing experiments on synthetic and natural marbles, basal slip is likely to occur at high temperatures of deformation estimated between 300 and 600°C. We are hence able to confirm that i) the marbles observed in the North Pyrenean basins underwent a synmetamorphic mylonitic ductile deformation, ii) this deformation took place at relatively high temperature which can be reasonably estimated to be higher than 300°C.
REFERENCES Lagabrielle, Y. & Bodinier, J. Submarine reworking of exhumed subcontinental mantle rocks: field evidence from the Lherz peridotites, French Pyrenees. Terra Nova 20, 11 21 (2008). Lagabrielle, Y., Labaume, P. & de Saint Blanquat, M. Mantle exhumation, crustal denudation, and gravity tectonics during Cretaceous rifting in the Pyrenean realm (SW Europe): Insights from the geological setting of the lherzolite bodies. Tectonics 29, (2010). Jammes, S., Manatschal, G., Lavier, L. & Masini, E. Tectonosedimentary evolution related to extreme crustal thinning ahead of a propagating ocean: Example of the western Pyrenees. Tectonics 28, (2009). Golberg, J.M. & Leyreloup, A.F. High temperature low pressure Cretaceous metamorphism related to crustal thinning (Eastern North Pyrenean Zone, France). Contributions to Mineralogy and Petrology 104, 194 207 (1990). Saint Blanquat (de), M. Recherche sur la tectonique distensive contemporaine du métamorphisme d'âge crétacé dans le massif Nord pyrénéen du Saint Barthélémy et sa bordure. (1985).
25 10th Alpine workshop "CorseAlp2011"
Cretaceous nappe stacking in the Inner Western Carpathians - p-T conditions from basal rauhwacke studies 1 2 3 Szilvia Deák Kövér , Rastislav Milovsky & László Fodor [email protected]
(1) Geological Research Group of the Hungarian Academy of Sciences, Budapest, Hungary (2) Geological Institute, Slovak Academy of Sciences, Banská Bystrica, Slovakia (3) Geological Institute of Hungary, Budapest, Hungary
It has long been recognized that fluids play a key role in rock deformation and thereby influence tectonic processes. Fluid overpressure may dramatically lower friction during thrusting and decrease the tensile strength of rocks. This obvious relation between fluids and deformation motivated our studies of basal rauhwackes. These basal cataclasites, occuring on thrusting planes of thin skinned nappes in Inner Western Carpathians (Silica, Meliata, Torna, Bodva nappes) are carbonatic polymict breccias containing newly formed quartz, albite, K white mica, dravitic tourmaline, apatite and pyrite. The investigated nappes are supposed to derive from the Northern margin of the Neotethys Ocean (Kovács 1988) and built up by Triassic and/or Jurassic sedimentary rocks. Silica and most probably Bodva were sheared off their pre Alpine basement along Upper Permian evaporitic horizon. They form thin, maximum 2 km thick nappe sheets, and didn’t suffer stronger than deep diagenetic alteration (Árkai & Kovács 1986). While the other two nappes (Meliata, Torna) show signs of low grade metamorphism (300 350 °C, 2.5 3.5 kbar, 120 140 Ma) connected to the closure of the Neotethys Ocean (Árkai & Kovács 1986, Kövér et al. 2009). Present day nappe order is (bottom to top): Bodva Torna Meliata Silica. Most nappe emplacements resulted in formation of basal tectonic breccias (rauhwacke). Their carbonate textures and syn deformational minerals were investigated by means of fluid inclusions (FI) microthermometry and microbarometry. FI in authigenic quartz contain samples of synkinematic fluids that lubricated the thrusting planes during the nappe emplacement. The primary FI were enclosed in crystal during its growth, while secondary FI were trapped along crack planes. Both genetic types have very similar properties and refer to the same process. Trapped fluids are hypersaline brines of the system H 2O NaCl KCl CaCl 2 MgCl 2 CO 2, with salinity of 28 42 wt.% NaCl eq. and KCl contents up to 14 wt.%. In one sample from the base of Silica nappe, native sulfur inclusions coexist with brine FI in heterogenous population, involving pure sulfur inclusions, pure brine FI and mixed composition FI with brine and accidentally trapped sulfur. They imply existence of heterogenous mixture of two immiscible liquids on the nappe base, with molten sulfur “bubbles” intermixed with brine. Most likely mechanism to involve sulfur into circulating brine is thermochemical reduction of sulphates of the evaporites at the base of the Silica Nappe. Homogenization temperatures (dissolution of halite crystal) and corresponding pressures are considered as lowest limit rather than true trapping conditions, and they vary depending on the nappe contacts. The lowest values derive from a second order nappe contact (Bodva (normally lowermost) on Meliata (normally upper): 220 260 °C and 0.3 1.0 kbar. The supposed first order contacts (Silica on Meliata, Meliata on Torna) resulted in higher values and much larger spans: 200 320 °C and 2.0 3.6 kbar. The source of hot fluids can be easily explained: we suggest their source in the Meliata nappe, where the peak metamorphism (300 350 °C) correlates well with the “peak” temperature of our fluids (320 °C). The low pressure values in case of the Bodva on Meliata contact strengthen our previous structural interpretation being a second order contact. But part of the pressure data is quite enigmatic. FI from basal Silica rauhwackes refers to peak pressure of 3 3.5 kbar, while the supposed overall thickness of this nappe does not exceed 2 3 km. Metamorphic petrologic data from Silica nappe show no higher than diagenetic alteration even in lower levels of the nappe (Árkai & Kovács 1986), suggesting that it always occupied an upper position in the nappe stack, thus excluding deeper burial. However, in convergent tectonic settings, fluid pressure may considerably exceed the lithostatic pressure, due to contribution of horizontal compressional stresses to the rock strength. This effect is particularly relevant to the upper crust, where rocks are thought to be deformed brittly. In these domains the maximum sustainable fluid pressure gradient may be as much as twice the lithostatic gradient (Petrini & Podladchikov 2000). But considering the 3 km nappe thickness and a density of 2700 kg/m 3, we still yield maximum pressure of 1.6 kbar. What is wrong? Were the minerals pre kinematic? Is Silica thicker than everybody expects? Is it possible to achieve even more overpressure under compression? Trends and patterns in p T data for particular samples/localities strongly resemble those observed in Krížna nappe (Milovský et al., this volume) and in previously studied Muráň nappe (Milovský et al. 2003): the data fields are strongly elongated roughly along isochore of H2O NaCl system with similar salt concentration as in studied brines. We suggest that lowest pT data represent “ambient” diagenetic conditions of pore fluids in basal formations. The “peak” p T values roughly correlate with metamorphic grade of Meliata nappe, which presumably fed the overlying nappe base with its metamorphic fluids. Mixing of these hot overpressured fluids (rich in silica, alumina and alkalies) with cool formation fluids (in equilibrium with carbonates and evaporites) caused coeval precipitation of silicates (with prograde solubility) and calcite and anhydrite (retrograde solubility). Thermal overpressuring of basal fluid likely triggered thrust detachments on trespassing the shear strength of basal rocks. Acknowledgements: This research was supported by the grant No. APVV-SK-HU-0015-08 of the Slovak Research and Development Agency, and the Operational Programme Research and Development through the project “Centre of Excellence for Integrated Research of the Earth's Geosphere” (ITMS: 26220120064), co-financed by ERDF.
REFERENCES Árkai P. & Kovács S. (1986) Diagenesis and regional metamorphism of Aggtelek Rudabánya Mountains (Northern Hungary). Acta Geol. Hung. 29: 349 373. Kövér Sz., Fodor L., Judik K., Németh T., Balogh K., Kovács S. (2009) Deformation history and nappe stacking in Rudabánya Hills (Inner Western Carpathians) unravelled by structural geological, metamorphic petrological and geochronological studies of Jurassic sediments Geodinamica Acta 22/1 3, 3 29 Milovský R., Hurai V., Plašienka D. & Biroň A. (2003): Hydrotectonic regime at soles of overthrust sheets: textural and fluid inclusion evidence from basal cataclasites of the Muráň nappe (Western Carpathians, Slovakia). Geodinamica Acta 16, 1 20 Petrini K. & Podladchikov Yu. (2000) Lithospheric pressure depth relationship in compressive regions of thickened crust. J. Metamorphic Geol., 18, 67 77 26 10th Alpine workshop "CorseAlp2011"
Neogene Tectonics of the Mont Blanc Area and Implications for Young Exhumation
Daniel Egli & Neil Mancktelow
Geological Institute, ETH Zürich, Sonneggstrasse 5, CH 8092 Zürich
The Mont Blanc massif constitutes part of the chain of External Crystalline Massifs belonging to the Helvetic domain of the Alps. It has a young exhumation history, as indicated by apatite fission track ages ranging down to 1.4 Ma (e.g. Seward and Mancktelow, 1994). Various models have been proposed to explain this young exhumation but there is only very limited information on the structural evolution of the area. This study investigates the Cenozoic structural evolution of the Mont Blanc region and considers the implications of this structural history on possible exhumation processes. The study area reaches from Martigny (Switzerland) in the north to the area of Beaufort (France) in the south and places particular emphasis on the Helvetic cover units and the three main tectonic structures framing the Mont Blanc basement massif: (1) the Chamonix line, (2) the Mont Blanc back thrust, and (3) the Pennine Frontal Thrust.
The Alpine deformation of the area is dominated by NW directed thrusting that formed the Helvetic fold and thrust belt, with the Mont Blanc massif forming the crystalline core of the lowermost Morcles nappe. The post nappe formation history shows widely distributed dextral transpressive movements on both the internal and external sides of the massif and nearly horizontal stretching lineations, slickenfibres and striae that can be observed throughout the whole field area.
On the SE side in the Swiss and Italian Val Ferret, transcurrent movements occurred under conditions ranging from the ductile field (Mont Chétif basement slice, Col de la Seigne) to the brittle field. On the NW side, in the Chamonix valley, dextral shearing is restricted to a rather narrow zone with brittle ductile transitional behaviour in the vicinity of the basement cover contact. Field constraints indicate that Late Neogene reactivation of the Frontal Pennine Thrust with a normal sense of movement, as described by earlier studies (e.g. Seward and Mancktelow, 1994), does not play an important role in the exhumation of the Mont Blanc massif. The Mont Blanc back thrust, which separates the Mont Blanc basement from the cover units over a distance of a few kilometres north and south of Courmayeur, is a rather local feature and therefore is unlikely to be responsible for the exhumation of the entire Mont Blanc massif. Abundant NNE SSW trending dextral shear zones and faults imply an overall NE SW directed compressional stress field following the earlier NW SE compression, with this shortening impinging on the Mont Blanc massif in a restraining bend scenario.
However, the most recent uplift of the Mont Blanc appears to be more widely distributed and is not directly related to discrete structures bounding the massif itself.
REFERENCES Seward, D., and Mancktelow, N.S., 1994, Neogene kinematics of the central and western Alps: Evidence from fission track dating, Geology, p. 803 806.
27 10th Alpine workshop "CorseAlp2011"
Timing and Evolution of Neogene Updoming of the Lepontine Dome. Constraints through new
Fission Track and (U-Th-Sm)/He Data on Apatites
Simon Elfert, Wolfgang Reiter & Cornelia Spiegel
simon.elfert@uni bremen.de
Geodynamics of the Polar Regions, University of Bremen, Germany
The Lepontine Alps experienced pressure dominated metamorphism related to the subduction of the European plate beneath the Adriatic plate. Regionally they show a later thermal overprint due to Barrow type metamorphism. While the metamorphic evolution is relatively well constrained, detailed studies of the Neogene exhumation history of the Lepontine Dome are still lacking.
Low temperature thermochronology, sensitive to the movements of the uppermost crust, can provide the knowledge of the latest stage exhumation patterns. We present new high resolution apatite fission track (AFT) and (U Th Sm)/He (AHe) data along the Maggia valley and the Ticino valley. Horizontal profiles (± 250 m) along the valleys combined with vertical profiles allow a detailed reconstruction of the spatial and temporal evolution. AFT ages of the horizontal profiles concur with the metamorphic patterns, decreasing towards the centre of the Lepontine Dome and increasing towards the Insubric Line and the Penninic Frontal Thrust (PFT). The association of the youngest AFT ages with the most deeply exhumed rocks suggests that the Neogene exhumation of the Lepontine Dome was mostly controlled by tectonic processes. If the AFT age patterns are indeed tectonically controlled, updoming in the Lepontine area lasted until latest Miocene, which is much longer than previously assumed. The vertical profiles show more complex patterns. In general cooling ages near the valley heads are younger than the those of the profiles near the valley outlets. This confirms the assumption that fast cooling first occurred in the south near the Insubric Line and proceeded northwards, which reflects a twofold process.
Furthermore, our results provide insights into the latest stage movements of the PFT. Crossing the fault, AFT ages shift distinctively. This points to a late vertical movement along the PFT after 6 Ma. In contrast, AHe ages along the same horizontal level (480 m a.s.l.) across the Lepontine Dome show no distinct trends. Accordingly they are not related to tectonic updoming, which indicates that erosion regime and controlling mechanisms have changed after 5 Ma.
28 10th Alpine workshop "CorseAlp2011"
Stucture and Kinematics in the Sonnblick Area (Eastern Tauern Window - Austria)
Silvia Favaro 1 , Friedrich Hawemann 1, Peter Gipper 1, Andreas Scharf 1, Mark Handy 1 & Stefan Schmid 1, 2
[email protected] berlin.de
(1) Freie Universität Berlin, Germany (2) ETH Zürich, Switzerland
In the eastern Tauern Window (Sonnblick area, Fig. 1a), the following sequence of nappes is exposed, from top to bottom (Fig. 1b): (1) Austroalpine units (AA); (2) Glockner (GN, derived from the Valais branch of Alpine Tethys); (3) Rote Wand Seidlwinkl (RS) isoclinal fold nappe with normal and inverted stratigraphic sequences of the most distal European margin (Kurz & Neubauer 1998); (4) GN, refolded around (3) ; and (5) Sonnblick basement and cover (SO). The SO unit is the highest nappe of the Venediger Duplex (VND) that derives from the European continental margin. The contact of the GN with the RS is a D1 thrust that was subsequently folded into an isoclinal D2 antiform, which we tentatively interpret as a crustal scale, N facing sheath fold. We further observe that the GN underlying the RS is in thrust contact with the SO, but that in the Mallnitz area (Fig. 1a and 1c) the RS is missing between the GN and its D2 folded D1 thrust contact with the VNC. There, this folded contact is cut by D3 thrusts and further deformed by the D4 Hochalm Dome. We note that this nappe stack differs from that found along the Katschberg normal fault and in the western Tauern Window where there is no RS sheath fold and the RS lies in direct thrust contact on top of the VND. Based on existing and own maps (1:10000), we discern the following deformation events: (D1) thrusting of the GN onto the RS; (D2) isoclinal folding of the D1 detached RS and overlying GN brings the GN into the inverted limb of an D2 antiform whose core comprises the RS unit; (D3) formation of the crustal scale Venediger Duplex whereby the previously stacked and folded RS GN composite unit overlies the roof thrust of this duplex structure; (D4) doming (Sonnblick, Hochalm Domes), tight folding (Mallnitz Synform) and sinistral shearing as part of the Katschberg Shear Zone system (Scharf et al., this vol.); (D5) dextral brittle strike slip faulting along the Mölltal Line overprints D4 structures, but does not extend NW ward into the central Tauern window. D1 D2 are inferred to have occurred during Eocene subduction and exhumation of European margin and adjacent GN, based on a correlation of D1 & D2 schistosities and stretching lineations with late Eocene (~42 32 Ma) HP mineral assemblages in the RS (Ratschbacher et al. 2004). D3 is attributed to incipient collision of Adriatic margin (upper plate) and previously accreted RS and GN with the European margin (lower plate). D4 doming and lateral E W extension accommodated by the Katschberg Shear System is partly a response of the thick orogenic wedge to Neogene indentation of the Adriatic plate. This induced cooling and exhumation between 28 25 and 17 Ma (Inger & Cliff, 1994; Dunkl et al. 2003) and was associated with a significant contribution of erosional unroofing.
Fig.1b
Fig.1c
Fig.1c Fig.1a
Fig.1: Simplified tectonic map (after Schmid et al., this vol.) showing traces of profiles in Fig.1b and Fig.1c. Units: VND = Venediger Duplex, HA = Hochalm Nappe, RS = Rote Wand- Seidwinkl Nappe, GN = Glockner Nappe, MA = Matrei Unit, AA = Austroalpine Nappes. In the square is highlighted the Mallnitz Area.
REFERENCES Kurz, W., Neubauer F., Genser J. and Dachs E., 1998. Alpine geodynamic evolution of passive and active continental margin sequences in the Tauern Window (eastern Alps, Austria, Italy): a review. Geol. Rundsch. Volume 87, Number 2, 225 242. Dunkl, I., Frisch W., and Grundmann G., 2003. Zircon fission track thermochronology of the southeastern part of the Tauern Window and the adjacent Austroalpine margin, Eastern Alps. Eclogae Geol. Helv., 96, 209 217. Inger, S., Cliff R. A., 1994. Timing of metamorphism in the Tauern Window, Eastern Alps: Rb Sr ages and fabric formation. J. metamorphic Geol., 12, 695 707. Ratschbacher, L., Frisch, W., Neubauer, F., Schmid, S.M. & Neugebauer, J., 1989. Extension in compressional orogenic belts: The eastern Alps. Geology, 17 , 404 407.
29 10th Alpine workshop "CorseAlp2011"
Constraints for the Extrusion Tectonics and Back-arc Extension
in the Pannonian Basin: a State of the art
László Fodor 1,2 , Frank Horváth 3, Emő Márton 4, Kamil Ustaszewski 5 & Endre Dombrádi 3
(1) Geological Institute of Hungary, 1143 Budapest Stefánia 14, Hungary (2) Eötvös University, Dept. of Regional Geology Budapest, Hungary (3) Eötvös University, Dept. of Geophysics, Budapest, Hungary (4) Eötvös Lóránd Geophysical Institute, Budapest, Hungary (5) Lithosphere Dynamics, GFZ German Research Centre for Geosciences, Potsdam, Germany
Recent advances in various fields of geosciences led to a better understanding the structural evolution of the Pannonian Basin and surrounding orogenic belt. In the presentation we review constraints which are important for the timing and kinematics of the two distinct crustal wedges (Alcapa and Tisza Dacia units) which were emplaced into the Carpathian embayment.
1) Dextral slip along the Periadriatic line (PL) and its eastern continuation, the Mid Hungarian Shear Zone (MHZ) at the southern margin of the Alcapa unit started in the late Early Oligocene. Basin stratigraphy indicates subsidence in the middle Oligocene (ca. 30 28 Ma) and widespread magmatism along the PL MHZ and also in the Sava zone of the Dinarides. Paleogeographic connection between the Hungarian and Slovenian Paleogene basins do not allow major separation before ca. 30Ma. Thus ~30 Ma can be considered as the onset of strike slip deformation. Structural data suggest transpressional deformation with compression perpendicular or slightly oblique to basin axis (N S in original position).
2) Major extension (~22 16 Ma) in the Eastern Alps resulted in the exhumation of mid crustal rocks units in the Tauern window. Along the western and southern boundary of the Pannonian basin a similar process led to the formation of the Rechnitz window, the Pohorje massif and several metamorphic complexes in the Sava zone. As an expression of extension, dextral slip continued along the PL MHZ up to the onset of late Early Miocene sedimentation (19 Ma). Extensional collapse of the Pannonian basin proper (syn rift sedimentation) took place in the 19 to 11.5 Ma time interval.
3) The most essential element of this extensional collapse were the coeval opposed rotations of the Alcapa (counterclockwise) and Tisza Dacia (clockwise) units of up to 80 90°. This rotation is extremely well constrained in the Alcapa unit, where it occurred between 18.5 and 14.5 Ma. Restoration of Alcapa and Tisza Dacia units and reconstruction of the paleostress fields suggest a quite homogeneous, originally E W oriented extensional features, which developed further during rotation. This suggests that rotational deformation was driven by the same mechanism of extension.
4) Rotations were not homogenous in amount within the two major blocks, but seem to increase eastward. The locations of the largest rotations seem to migrate eastward; a young rotation of 14(?) 12 Ma occurred only in the eastern part of the blocks, while the western and central parts did not show rotation. These differential rotations were accommodated by opening of deep grabens, also pointing to connection of extension and rotations.
5) The rotational deformation modified the geometry of the extruded blocks and juxtaposed Alcapa, and the Tisza Dacia units. Their further history during the 11.5 and 5 Ma time interval (post rift phase) was characterised by minor extension and major subsidence and sedimentary upfill.
6) The earliest phase of extension was associated with magmatism. Voluminous rhyolites and andesites of the central Pannonian basin are partly pre , partly syn rotational (~20 18.5 and ~17.5 14Ma, respectively). All volcanic rocks suffered brittle extensional deformation. The granodiorite of the Pohorje (18,6 Ma) suffered ductile to brittle extensional deformation corresponding to extrusion depth. This timing and the geochemical data indicate that magma generation was connected with crustal extension and melting. Subsequent calc alkaline magmatism has continued after major rotations (after 14 Ma) until recent times in the south eastern region of the Tisza Dacia unit.
7) The 5 0 Ma time interval of the Pannonian basin is characterised by a transition from extensional to compressional stress field, evidenced in the compressional or transpressional inversion of numerous normal faults. Counterclockwise rotations could associate with this deformation.
30 10th Alpine workshop "CorseAlp2011"
Field Mapping of the Valaisan Domain in the Breuil Valley (La Thuile, Aosta, Italy):
evidence for the preservation of rift-related mantle windows
Gianluca Frasca, Marco Beltrando & Roberto Compagnoni
Dipartimento di Scienze Mineralogiche e Petrologiche, Università degli Studi di Torino, Italy
Tectonic Units belonging to the Valaisan Domain in the Breuil Valley (La Thuile, Aosta, Italy) have been re mapped in detail and are subject of preliminary stratigraphic, structural and petrographic investigations, with the main aim of reconstructing the pre Alpine evolution of the area.
The nappe stack in the Breuil Valley comprises the Versoyen Petit Saint Bernard Unit (V PSB; Masson et al., 2008) and the newly defined Pointe Rousse Tormottaz Unit (PR T). The Pointe Rousse Tormottaz Unit is composed of serpentinized subcontinental mantle in contact with slivers of continental basement and polymictic breccias, passing upward to radiolarian schists and calcschists (Aroley Marmontains St. Christophe lithostratigraphic units). The slivers of continental basement consist largely of meta granitoids and of minor mafic rocks similar to those found in the typical Versoyen rocks. Intrusive relationships between the meta granites and the host mafics are locally observed. Cataclasites and gouges, developed at the expenses of granitic rocks and ultramafics, are commonly found along the interface between the mantle and crustal rocks, but no evidence of brittle deformation is found in the overlying meta sedimentary sequence. The slivers of continental basement are laterally discontinuous, being locally replaced by a meta breccia with clasts of granitic rocks, reddish micaschists, graphitic schists and rare greenstone pebbles. The radiolarian schists and the calcschists are found ubiquitously throughout the area. These observations are interpreted to indicate that ultramafics and continental basement were juxtaposed during Mesozoic rifting, prior to the deposition of the overlying sedimentary cover at the bottom of the Valaisan Basin. The juxtaposition was accommodated by the activity of low angle detachment faults. The post tectonic topography exerted a strong control on the deposition of the polymictic breccias, before the entire lithostratigraphic sequence was sealed by the radiolarian schists and the thick calcschist sequence.
The Versoyen Petit Saint Bernard Unit consists mainly of the typical “Versoyen” lithological association, with mafic sills intruded in black schists, and of the Petit Saint Bernard rocks, made of micaschists, carbonatic schists and impure marble with belemnites, where an ammonite has been found during this study. A discontinuous and laterally heterogenous body of breccias (particularly well developed in the Collet des Rousses area) is found along their contact. This meta breccia, which is also found to the NE of Pic de Lancebranlette and to the NE of the Glacier Septentrional de l’Hermite, is characterized by poorly rounded pebbles of marble, greenstone, dolomitic schist and graphitic schist in a carbonatic and prasinitic matrix. The Pointe Rousse Tormottaz and the Versoyen Petit Saint Bernard units are separated by a tectonic contact that post dates early folding (F e) in both units. The axial planar schistosity to F e, in the PR T unit, wraps around aggregates of white mica, chlorite and chlorithoid forming pseudomorphs over an earlier mineral phase (possibly carpholite), suggesting that all deformation events observable in the field and thin section post date the Pressure peak. Mineral stretching lineations defined by pseudomorphs on glaucophane and chlorithoid, related to the phase F e, usually plunge to SSE. Early isoclinals folds are also found in the V PSB unit. Significantly, the axial planes of F e in both V PSB and PR T units are truncated by the contact between the two units. Both units are then affected by large scale recumbent folds (F r), with axis plunging mostly to the SSW. This folding event is locally associated with a sub millimetric spaced crenulation cleavage, dominant in the schists, dipping towards SE, developed under greenschist facies conditions.
Both F e and F r are affected by large scale doming, spectacularly visible in the Tormotta M.Ouille Pointe de Fornet area. This late deformation is accommodated by two sets of folds with sub horizontal axes oriented alternatively at N50 and N140. Folding is associated with a sub vertical cm spaced axial plane crenulation cleavage. The Tormotta M.Ouille Pointe de Fornet dome is bounded by high angle normal faults oriented almost N S. A second generation of high angle faults is oriented N120 in the high Breuil valley.
Our study leads to the following conclusions: (1) the so called ‘Internal Valaisan Domain’ (e.g. Loprieno et al., in press) should not be considered as a coherent tectonometamorphic unit, but rather as an assembly of two independent units (V PSB and PR T) with specific lithostratigraphic associations and deformation histories; (2) the Pointe Rousse Tormottaz unit samples the most distal part of the European margin, where crustal excision led to the exhumation of mantle rocks in the Mesozoic; (3) intrusive relationships between Pointe Rousse type metagranitoids and Versoyen type rocks (mafic sills and graphitic schists), preserved in the Pointe Rousse extensional allochthons, demonstrates that, at least in part, these mafic rocks are older than ca. 270 Ma (age of Pointe Rousse granitoid emplacement from Beltrando et al., 2007).
REFERENCES Beltrando M., Rubatto D., Compagnoni R. & Lister G. (2007) Was the Valaisan basin floored by oceanic crust? Evidence of Permian magmatism in the Versoyen Unit (Valaisan domain, NW Alps). Ofioliti, 32, 85 99. Loprieno A., Bousquet R., Bucher S., Ceriani S., Dalla Torre Fh., Fügenschuh B. & Schmid S.M. (in press) The Valais units in Savoy (France): a key area for understanding the palaeogeography and the tectonic evolution of the Western Alps. Int J Earth Sci (Geol Rundsch). Masson H., Bussy F., Eichenberger M., Giroud N., Meilhac C & Presniakov S. (2008) Early Carboniferous age of the Versoyen ophiolites and consequences: non existence of a Valais ocean” (Lower Penninic, western Alps). Bull. Soc. Géol. Fr., 4, 337 355.
31 10th Alpine workshop "CorseAlp2011"
New Stratigraphic data in and Around the Adula Nappe, and Consequences for the Tectonics and the
Paleogeography of the Central Alps
Federico Galster 1,2 , Mattia Cavargna Sani 1, Jean Luc Epard 1 & Henri Masson 1
(1) Institut de Géologie et Paléontologie, Université de Lausanne, Switzerland (2) Institut de Mineralogie et Géochimie, Université de Lausanne, Switzerland
New data from the Penninic nappes at the northeastern edge of the Lepontine dome (Adula and surroundings) modify considerably our ideas on their tectonic relationships and on the paleogeographic evolution of the Central Alps (Cavargna Sani et al. 2010; Galster et al. 2010). The newly defined Luzzone Terri (L T) nappe, which extends along the front of the Adula nappe, is formed by a Jurassic series of Helvetic type (similar to the sedimentary cover of the nearby Gotthard massif), superposed on a Permo Triassic substratum which displays typical characteristics of the Briançonnais domain (see Figure). The gneissic core of the L T nappe shows definite analogies with the Permian of the “Zone Houillère” (external Briançonnais) in Valais. The Triassic part of the L T nappe contains all the stratigraphic units that characterize the Briançonnais Triassic platform (e.g. Mégard Galli & Baud 1977), however in a more reduced and confined form, a fact that suggests that during the Triassic the L T nappe was located on the external border of the Briançonnais basin. On the contrary, the Liassic part of the L T nappe has no Briançonnais or Subbriançonnais affinity, but mimic the well known Gotthard trilogy (Stgir, Inferno and Coroi Formations). This stratigraphic superposition of a Jurassic series of Helvetic type on a typically Briançonnais Permo Triassic substratum is unique in the Alps. It establishes a link between the Briançonnais and Helvetic domains and demonstrates their paleogeographic proximity during late Paleozoic and early Mesozoic times. This proximity is in line with paleogeographic reconstitutions recently proposed by several authors in the Western and Central Alps (e.g. Masson et al. 2008; Mohn et al. 2010; Bussien et al. in press). In the Adula nappe, the presence of bands of Mesozoic metasediments that look intermingled with the Paleozoic basement and the heterogeneous composition of this basement suggested to some authors that it could be a tectonic “mélange”. Our work reveals no “mélange” character; it rather shows the coherent internal structure of this nappe. The sediments are in stratigraphic contact with their basement. We propose that the “internes Mesozoikum” can be explained by the reactivation and inversion of Jurassic normal faults during the late Cretaceous and Tertiary convergence. The existence of normal faults in the Adula is supported by the discovery of synsedimentary post Triassic breccias (Cavargna Sani et al. 2010). The heterogeneity of the basement results from a long and complex Paleozoic history, notably form the intrusion of late Variscan granitoids (e.g. the Zevreila orthogneiss: ~294 Ma) into various early Variscan and older rock bodies. The Adula Triassic sequence has no affinity with the Briançonnais Triassic. This demonstrates the ultra Adula origin of the Luzzone Terri nappe. This nappe must have been translated over the Adula during the subduction of the latter. The Adula Triassic displays analogies with the Triassic of several other Lower Penninic nappes and we suggest that it could represent a transition from the Briançonnais to the Helvetic Triassic domains.
Stratigraphic column of the Luzzone-Terri nappe, compared with those of the Gotthard sedimentary cover and of the Briançonnais Permo-Triassic domain (after Galster et al. 2010).
32 10th Alpine workshop "CorseAlp2011"
Formation of Graphitic Carbon Through Calcite Reduction in Eclogites From Corsica (France)
Matthieu E. Galvez 1,3 , Isabelle Martinez 2, Olivier Beyssac 1, Karim Benzerara 1, Christian Chopin 3 & Benjamin Malvoisin 3
(1) MPMC, CNRS UMR 7590,Université Paris 6, Paris, France (2) Laboratoire de Géochimie des Isotopes Stables, IPGP, Paris, France (3) Laboratoire de Géologie, Ecole Normale Supérieure, Ulm, Paris France
The geochemistry of organic carbon in subduction zones may be strongly affected by mineral equilibria. We study here the geochemistry of carbon in siliceous marbles at the direct contact with serpentinites in the Alpine eclogitic meta ophiolithic units of northern Corsica (France). We have applied a set of spectroscopic (RAMAN) and isotopic technics to provide a description of both the organic and carbonate components of the rocks across a reaction front where the equilibrium CaCO 3+SiO 2=CaSiO 3+C+O2 is evidenced.
The continuous reaction zone is composed by a centimeter thick pale nephrite layer at the contact with the serpentinites, followed by a thin wollastonite layer and a 5 to 20 cm thick dark zone composed of wollastonite, carbonaceous material (CM), quartz but no carbonates. There is a sharp (<0.5cm) transition to the overlying original metasediment composed of calcite+quartz which is significantly less rich in CM. Raman spectroscopy shows that CM is much more graphitic in the reaction zone than in the original rock. Significant isotopic differences are observed apart the reaction front with δ13 C (CM) and δ 13 C (calcite) around 16‰ and 1.3‰ respectively in the original rock far from the reaction zone, whereas δ 13 C (CM) is around 1‰ in the reaction zone.
We interpret the graphitic CM in the reaction zone as formed from the destabilization and reduction of calcite due to the diffusion of reducing fluids from the underlying serpentinite unit. Mass transfer calculation supports this hypothesis and shows that a complete reduction of carbonates might have occurred. We show that the combined study of isotopic geochemistry and structure of inorganic and organic carbon in metamorphic rocks might be suited to reveal with great fidelity redox gradients in subduction zones.
33 10th Alpine workshop "CorseAlp2011"
Active Lateral Escape in the Eastern Alps
Sebastian Garcia
[email protected] berlin.de
Department of Geology, Freie Universität, Berlin, Germany
The eastern part of the Alps is the locus of important active deformation, as illustrated by the 1976 Friuli seismic sequence (Mw 6.4), which results from the counter clockwise motion of the Adriatic block with respect to the European plate. The few mm/yr (less than 5 mm/yr) of N S to NNW SSE convergence are interpreted to be partitioned into thrusting along the eastern Southern Alps and strike slip faulting in the Eastern Alps, connected to lateral escape, or to be completely absorbed within a narrow zone of the eastern Southern Alps, without any or only little residual strain transferred to the north.
In order to test the validity of these two interpretations and to define the kinematics of the main blocks, two N S striking and one E W striking cross sections through the Eastern part of the Alps, combining recently published GPS velocities [Caporali et al., 2009] with seismic and topographic data, were constructed. A first N S cross section, from the Northern Calcareous Alps and Tauern Window to the Southern Alps (Venetian Friuli Plain), shows that most of the seismic activity is accommodated by the N dipping thrusts of the eastern Southern Alps. The intense deformation associated to this seismic activity is also illustrated by the northward decrease of the northward velocity of GPS stations located in the Southern Alps, relative to stable Eurasia, North of the Periadriatic Line, in the Tauern Window, the GPS stations still move slowly northwards. In both blocks, no eastward displacement is detectable. North of the SEMP fault, in the Northern Calcareous Alps, a weak component of NE displacement is highlighted by the GPS data. The low seismic activity observed along the Inntal Fault and in the whole Northern Calcareous Alps may be related to the latter displacement. The second N S cross section runs from the Eastern Alps (Northern Calcareous Alps and Gurktal block) to the Istria Peninsula, passing through the Dinarides. Active shortening in the Dinarides is well illustrated by the important seismic activity, mostly located along the Dinaric strike slip fault system, and by the northward decrease of the northward movement of the GPS stations, relative to stable Europe. In the same cross section, a dextral movement is clearly visible along the Sava Fault, an E W strike slip structure running parallel and south of the Periadriatic Line. This observation is based on the westward movement of the stations located in the Dinarides, and the eastward movement of those located north of the Sava Fault. Because the GPS stations north of the Sava Fault also move slowly northward, the Gurktal block moves towards the north east. The Northern Calcareous Alps, north of the SEMP fault, also move towards the NE relatively to stable Eurasia. The third cross section strikes E W and goes from the Eastern Alps (Tauern Window and Gurktal Alps) to the western part of the Pannonian Basin (Styrian Basin). All along this cross section, the GPS stations move northward with no clear difference between the different blocks. This is not always the case when looking at the eastward velocity of the GPS stations. East of the Katschberg normal fault (eastern limit of the Tauern Window), the velocities increase eastward along the Gurktal block and the Styrian Basin to reach a maximum of 2 mm/yr at the eastern end of the cross section. This indicates extension across these two blocks and consequently their lateral escape towards the NE. The low seismic activity in the Gurktal block and the Styrian Basin, excepted for the scarce seismic activity along the main faults of this area (e.g., the Katschberg normal fault, the Mölltal, the Mur Mürz and the Lavanttal Labot strike slip faults), may reflect the low rate of lateral escape of these two blocks.
In summary, even if most of the shortening is accommodated within the Southern Alps and within the Dinarides, some strain is transferred to the north, resulting in small amounts of shortening in all the Eastern Alps. In the Gurktal block and in the Styrian Basin, this northward movement is combined with an eastward directed one, resulting in their NE directed lateral escape. The southern and western limits of this extruded wedge correspond to the Sava dextral fault and the Katschberg normal fault, respectively. To the north, this wedge is bounded by the Northern Calcareous Alps that, in their whole, are slowly escaping to the NE too.
REFERENCES Caporali, A., C. Aichhorn, M. Barlik, M. Becker, I. Fejes, L. Gerhatova, D. Ghitau, G. Grenerczy, J. Hefty, S. Krauss, D. Medak, G. Milev, M. Mojzes, M. Mulic, A. Nardo, P. Pesec, T. Rus, J. Simek, J. Sledzinski, M. Solari, G. Stangl, B. Stopar, F. Vespe, and G. Virag, Surface kinematics in the Alpine Carpathian Dinaric and Balkan region inferred from a new multi network GPS combination solution, Tectonophysics, 474, 295 321, 2009.
34 10th Alpine workshop "CorseAlp2011"
From Sink to Source: Looking at Alpine Geology Through the Garbage Can
Eduardo Garzanti, Sergio Andò, Marta Limoncelli, Marco Malusà, Alberto Resentini & Giovanni Vezzoli
Laboratory for Provenance studies, Earth Science and Geotechnology Department, Milano Bicocca University, Milano, Italy
Garbology, the science of finding stuff out about someone or something by digging through their trash, was invented in September 1970 by Alan Jules Weberman, a gadfly journalist who tried to unveil private secrets of Bob Dylan and his wife, the ex bunny girl Sarah Lowndes, by searching the trash outside their Greenwich Village home in New York.
Garbological applications to the Earth Sciences (“provenance analysis”) provide unique information on the hidden secrets of orogenic belts by scrutinizing the detritus they produce (“waste” in old geomorphological literature). This daring and exciting task requires painstaking dedication. First, we need to determine end member signatures of “waste” shed by the diverse tectonic units incorporated in the mountain belt. This can be done only in modern settings and climatic conditions that grant minimum chemical weathering. Distortions of the original compositional signals by hydrodynamic processes need to be carefully checked and corrected for. Next, by combining the end member signals as thus determined using forward modelling, we get a grasp of compositions expected to be found in the orogen’s garbage cans (“sedimentary basins”). The comparison between the expected and the observed may eventually lead us to decrypt the garbological record preserved in stratigraphic successions, and to unravel events the traces of which have long been erased by erosion from bedrock exposed in the thrust belt.
Simple statistical procedures allow us to interpret with a good amount of confidence information stored in recent deposits of the Pleistocene, but inferences become more and more uncertain when we try to extrapolate our knowledge of the present backwards in time to reconstruct sceneries of the remote past. By carefully checking diverse alternative hypotheses, however, garbologists may be of help to serious bedrock geologists to reconstruct with unsuspected precision the orogenic history of the Alps since the very beginning of convergence between Africa Adria and Europe.
REFERENCES Garzanti E., Ando’ S., Vezzoli G., 2006, The continental crust as a source of sand (Southern Alps cross section, northern Italy). J. Geol., 114, 533 554. Garzanti E. and Malusà M.G., 2008, The Oligocene Alps: domal unroofing and drainage development during early orogenic growth. Earth Planet. Sci. Lett., 268, 487 500. Garzanti, E., Resentini, A., Vezzoli, G., Andò, S., Malusà, M.G., Padoan, M., Paparella, P., 2010. Detrital fingerprints of fossil continental subduction zones (Axial Belt Provenance, European Alps). J. Geol., 118, 341 362. Garzanti, E., Vezzoli, G., Andò, S., 2011. Paleogeographic and paleodrainage changes during Pleistocene glaciations (Po Plain, Northern Italy). Earth Sci. Rev., 105, 25 48. Vezzoli, G. and Garzanti, E., 2009, Tracking paleodrainage in Pleistocene foreland basins. J. Geol., 117, 445 454. Weberman, A.J., 1980. My Life in Garbology. Stonehill Press.
35 10th Alpine workshop "CorseAlp2011"
Quantifiying exhumation rates in the Alps using low temperature thermochronology
Frédéric Herman, Matthew Fox, Rebecca Reverman, Jean Daniel Champagnac, Pietro Sternai & Sean Willett
ETH Zürich, Earth Science Department, Switzerland
A major constraint on exhumation rates across an orogenic belt comes from low temperature thermochronometry data. In well studied mountain belts like the Alps, ~1000 thermochronometric data have been published so far. We will present in this talk how such data can be used at both regional and local scales, as well as from tens of millions of years to hundreds of thousand years In particular, we will discuss the effects Quaternary Glaciations had on erosion rates and the implications it has on our understanding of glacial erosion processes.
For the regional scale problem, we have recently developed a method of statistical inversion, similar to seismic tomography, to extract exhumation rates that are variable in both space and time (Fox et al., in prep). The method is based on a parameterization that has discrete timesteps, but imposes only a statistical correlation structure in space. Results are in the form of spatial maps of exhumation rate, valid over a specific time range, along with a statistical measure of resolution and variance reduction. Using a suite of ~1000 fission track and (U Th)/He ages from zircon and apatite collected in the entire alpine belt, we investigate the ability of these data to resolve exhumation rates over the last 30 Myrs. High pulses of exhumation seen over long wavelengths provide evidence for either erosionally controlled exhumation, or changes within the underlying mantle. We also observe a shift in high exhumation from the southern Alps towards the north with time. In addition, the data seem to support an increase in average erosion rate towards the present day, though this signal is not present everywhere.
We will then show two local studies, in the southern Alps and northwestern Alps. First, we concentrate on the Adamello Massif (Reverman et al., in prep). Low temperature thermochronometers used here (i.e., apatite (U Th Sm)/He (AHe) dating and apatite fission track (AFT) dating) reveal fast cooling of the low elevation samples, as recorded by their AHe ages, indicating that at least 1.5km of rock was exhumed rapidly around 8 Ma. The magnitude and timing of this event constrains a period of transpressional activity along the South Giudicarie Line, further supported by AHe and AFT ages from a sample located between the S. Giudicarie Line and Mt. Sabion Line. We will then focus on the Valais. Recent thermochronometric studies (Valla et al. subm) suggest an increase of erosion rates associated with relief creation within the last million years, which is most likely related to the intensification of glaciations. These results are confirmed by the use of OSL thermochronology, a new very low temperature thermochronometer that relies on the thermally activated nature of optically stimulated luminescence (OSL) in quartz or feldspar (Herman et al., 2010). It is characterized by a closure temperature of approximately 30 0C. Samples collected in the Mattertal (between Zermatt and Visp) reveal that erosion could have locally been as high as 4 mm/a within the last 100 kyrs.
36 10th Alpine workshop "CorseAlp2011"
The Briançonnais Fan Structure, Revisiting the Models
François Guillot 1,2 , Matthieu Dupuis 1 & Olivier Averbuch 1,2
francois.guillot@univ lille1.fr
(1) Univ. Lille Nord de France, France (2) CNRS FRE3298 Géosystèmes SN5, Villeneuve d'Ascq Cedex, France
Known for more than a century (Bertrand 1894), the Briançonnais Fan Structure or "Eventail briançonnais" of the W Alpine arc is the transition from internal ward dip at the Penninic Front to external ward dip in more internal zones, namely internal Briançonnais, Gran Paradiso or Monte Rosa, and Piemont calcschists. In the Valais and Lepontine regions, Argand (1911, 1912 a to e) related such structure to a process called by him "Unterschiebung" that was later translated into French, by A. Amstutz, creating the word of “subduction”, referring to a general lithospheric process of deep underthrusting (White et al. 1970). This term was later mainly restricted to the burial of oceanic crust (and more recently, of continental crust) slabs into the mantle. Considering the Briançonnais fan structure, this process involved the underthrusting of the Internal Crystalline Monte Rosa + Insubrian block below the Briançonnais + Dent Blanche recumbent folds, primitively external verging but secondarily refolded backwards; Argand in his papers considered such mechanism to result from a "late deepening of the tangential displacement” that would have produced a 25 to 30 km underthrusting.
Following this view, most French authors since the 1950s, based on the study of the Briançon Queyras region in the southern part of the W Alpine arc, privileged a "rétrocharriage" (backthrusting) event as a secondary episode of deformation although Fabre et al. (1982) depicted early top to the E and late top to the W motions just to the N of Briançon. More recent works in Savoy and Valle d'Aosta (Platt et al. 1989; Ganne et al. 2005; Bucher et al. 2004) favour a vertical shortening due to the self collapse of an accretion prism. The idea of an early subduction toward the European plate was also episodically emitted for the mainland Alpine chain (Caby 1996) as well as for its Corsican segment (Orsini et al. 1980; Rossi et al. 2001).
All along the W Alpine arc, internal ward from the fan axis, the next thrusts are characterized by subtractive metamorphic gaps: higher pressure units lie at their footwall. Dent Blanche / Combin; Combin / Zermatt; GSB / Zermatt in Valsavaranche; Briançon / Ruitor; W Vanoise / S Vanoise; Briançon proper / Piemont; Piemont / Viso / Dora Maira. This pattern exists also in Corsica. The Ganne et al.'s model relates W dips to an accidental and local mechanism. Models by Schmid et al. (2004) consider a systematical internal ward downwarping of externally dipping unit boundaries, as soon as they pass under the visible level. Such options lead to solutions that may look far from the thermodynamics Principle of Minimum Energy. You have to (1) create eclogitic units by subduction (2) thrust them over less metamorphic, colder and lower pressure units following a postulated "normal order of nappes" (3) refold the whole pile to an inverted position along a 500 km long arc, with the HP unit coming back under LP ones.
Field exceptions to the above models deserve further examination. In Vanoise, a number of places show a late, top to the W thrusting event associated to pseudotachylite formation along thrust planes that cut through earlier, E verging folds having a W dipping axial plane cleavage (near La Plagne ski resort: Guillot 1987). Nearby, in the Briançonnais fan between Petit Saint Bernard and Ruitor, we are presently re evaluating the microstructures in order to assess the succession of deformation episodes, trying to ignore any a priori model for the fan genesis. Thanks to oriented samples of metaconglomerate and metapelite formations of the Louie Blanche unit, that here constitute the axis of the fan, we chose to study thin sections cut vertically at right angle from the main cleavages, i.e. along a NW SE section. Some local and preliminary results are, at variance from previous descriptions (Platt et al. 1989; Bucher et al. 2004), a top to the SE late episode of thrusting with a strain slip cleavage cutting through the main, earlier foliation.
REFERENCES Argand E (1911) Bulletin de la Société Vaudoise de Sciences Naturelles XLVII(174): XXXIII XXXVI Argand E (1912 a,b,c,d,e) Bulletin de la Société Vaudoise de Sciences Naturelles XLVIII(176): XIII XVI, XX XXII, XXIV XXVII, XXVIII XXXII, XXXVI XL Bertrand M (1894) Bulletin de la Société géologique de France 3: 69 118 Bucher S, Ulardic C, Bousquet R, Ceriani S, Fügenschuh B, Gouffon Y, Schmid SM (2004) Eclogae geologicae Helvetiae 97(3): 321 345 Caby R (1996) Eclogae geologicae Helvetiae 89(1): 229 267 Fabre R, Gidon M, Tricart P (1982) Géologie Alpine 58: 31 52 Ganne J, Bertrand J M, Fudral S (2005) Journal of Structural Geology 27(3): 553 570 Guillot F (1987) Thèse Doct. Univ. Lille: 280 p Orsini JB, Coulon C, Cocozza T (1980) Excursion 38. Géologie Alpine 56: 169 202 Platt JP, Lister GS, Cunningham P, Weston P, Peel F, Baudin T, Dondey H (1989) Geological Society, London, Special Publications 45: 135 152 Rossi P, Durand Delga M, Lahondère J C, Lahondère D (2001) Carte géol. Fr. (1/50 000) [Santo Pietro di Tenda], Bur. Rech. Géol. Min. eds, Orléans 1106: 224 p Schmid SM, Fügenschuh B, Kissling E, Schuster R (2004) Eclogae geologicae Helvetiae 97(1): 93 117 White DA, Roeder DH, Nelson TH, Crowell JC (1970) Geological Society of America Bulletin 81(11): 3431 3432
37 10th Alpine workshop "CorseAlp2011"
Lu-Hf Geochronometry of two Garnet Generations Within one Eclogite Sample
From Trescolmen (Adula Nappe)
Daniel Herwartz 1, Thorsten J. Nagel 1, Carsten Münker 2, Erik E. Scherer 3 & Nikolaus Froitzheim 1
d.herwartz@uni bonn.de
(1) Rheinische Friedrich Wilhelms Universität Bonn, Steinmann Institut, Bonn, Germany (2) Universität zu Köln, Institut für Geologie und Mineralogie, Köln, Germany (3) Westfähliche Wilhelms Universität Münster, Institut für Mineralogie, Münster, Germany
The Adula Nappe (Central Alps) is a classic high pressure unit and represents the former margin of the European Continent. The nappe dominantly consists of felsic ortho and paragneisses and mica schists, but it also contains mafic and ultramafic rock types such as eclogite, amphibolite, and peridotite. With some exceptions, the high pressure conditions are observed only within the mafic rock types, and preserved peak pressure conditions increase from north to south. These observations have led to contrasting views concerning the tectonometamorphic history of the Adula Nappe. Some authors favor a model where the high pressure rocks represent exotic fragments that have been intercalated with the felsic country rocks (devoid of a high pressure history) at mid crustal levels, forming a megascale tectonic mélange (e.g., Trommsdorff, 1990). Others interpret the north to south pressure gradient to indicate continuous post peak pressure deformation of a coherent Adula Nappe (Nagel 2008). The mélange models are seemingly supported by contrasting isotopic ages in the Adula Nappe. Whereas mostly Eocene ages are observed within the southern part of the nappe (Sm Nd, Lu Hf, U Pb), Paleozoic U Pb zircon ages are dominant in the central and northern part of the Adula Nappe. Eocene Rb Sr and K Ar ages in this part of the nappe remain controversial. They have either been attributed to the Barrovian, HT LP amphibolite facies overprint, which has affected the Adula at ca. 33 32 Ma, or interpreted to reflect Alpine high pressure conditions.
Here we present Lu Hf ages of two distinct garnet populations (Fig. 1, simplified after Herwartz et al., 2011), both occurring in one single eclogite sample from the locality of Trescolmen (Central Adula Nappe). Scarce, large (3 4 mm), dark garnet grains were separated from small (100 300 m), pale, euhedral grains by hand picking under a binocular, before Lu Hf analysis by MC ICPMS. Because the physical separation of the two garnet generations was imperfect, the resulting ages strictly represent minimum and maximum figures, respectively. The minimum age of 332.7 Ma provides evidence , that our sample had been subducted in the Variscan orogeny and subsequently became part of the continental crust. Later, at ca. 38 Ma (maximum age) during the Alpine orogeny, the sample was subducted again, together with the crustal rocks of the European continental margin. Hence, we conclude that the Adula Nappe was subducted and exhumed as one coherent unit in the Alpine orogeny. This view is corroborated by the following observations: (1) Our sample happened to be overprinted in such a way that most of the sample has equilibrated to Alpine conditions, with only few relict Variscan garnet grains surviving the Alpine event. Hence, we infer that PT conditions at Trescolmen represent Alpine and not Variscan conditions. This strengthens the case for an exclusively Alpine pressure gradient throughout the Adula Nappe. (2) All Alpine high pressure ages observed in the Adula Nappe (Sm Nd, Lu Hf, U Pb, K Ar, Rb Sr) consistently fall in a narrow Eocene age range (35 40 Ma), implying a consistent tectonometamorphic history.
REFERENCES Trommsdorff V. (1990) Metamorphism and tectonics in the Central Alps: The Alpine lithospheric mélange of Cima Lunga and Adula. Mem. Soc. Geol. Ital. 45, 39 49. Nagel T. J. (2008) Tertiary subduction, collision and exhumation recorded in the Adula Nappe, central Alps. Geol. Soc. Spec. Publ. Lond. 298, 365 392. Herwartz D., Nagel T.J., Münker, C., Scherer E.E. and Froitzheim N. (2011) Tracing two orogenic cycles in one eclogite sample by Lu Hf chronometry. Nature Geoscience doi:10.1038NGO1060.
38 10th Alpine workshop "CorseAlp2011"
The Influence of Ferric/Ferrous Iron Ratios in Bulk and Mineral Chemistry on Calculation of
Metamorphic Conditions: Application to the N-Makran Blueschists, SE Ian
Hunziker Daniela 1, Burg Jean Pierre 1, Caddick Mark 2, Reusser Eric 2 & Omrani Jafar 3
(1) Geological Institute, Structural Geology and Tectonics, ETH Zurich, Switzerland (2) Institute for Mineralogy and Petrology, ETH Zurich, Switzerland (3) Geological Survey of Iran, Tehran, Iran
Blueschists are the main source of information concerning the thermo mechanical processes along subduction zones. In particular, numerical models and thermobarometric recalculations are used to infer burial depth and exhumation pressure temperature paths. Most of the current thermodynamic solution models of minerals stable in high pressure/low temperature systems, such as sodic amphiboles and lawsonite, assume ferric/ferrous iron ratios based on charge balance calculations.
The actual values are poorly known, although they seem to have an important influence on mineral stability fields. Since the metamorphic blueschist facies is defined by the occurrence of sodic amphiboles, it is crucial to understand the conditions required to crystallize these minerals. Colorimetric titration was used for ferric/ferrous iron ratios in bulk compositions, Moessbauer spectroscopy in mineral standards, TEM (EELS) and micro XAS for the application on amphiboles formed at different pressure and temperature conditions. Implementing measured instead of modeled iron values in the bulk composition of the has a significant effect on pressure/temperature recalculations. The analyses of the Makran blueschists show that ferric iron values are generally estimated too low which can lead to errors of more than 10km depth and 250°C.
First results on the micro XAS measurements are compared to the other techniques. The consequences for pressure temperature conditions of the Makran and other blueschists under investigation will be discussed.
39 10th Alpine workshop "CorseAlp2011"
The Miocene Enns Valley basin (Austria) and significance of Neogene faults
Melanie Keil & Franz Neubauer
Dept. Geography and Geology, University of Salzburg, Salzburg, Austria
The Miocene Enns Valley basin initiated along the ENE trending Salzach Enns Mariazell Puchberg (SEMP) fault. This fault separates the exhumed and uplifted Hohe and Niedere Tauern blocks from the Miocene basin fill. The Miocene Enns Valley basin occurs in a number of dispersed exposures along the northern valley margin and is nearly exclusively sourced from the southern Ennstal Quartzphyllite, Wölz Micaschist and Schladming/Bösenstein complexes. We also recognized a very specific contributor, the Hochgrößen serpentinite massif.
The Enns Valley basin fill is confined and disrupted along its northern margin by the North Enns Valley (NEV) fault, a hitherto unidentified fault, which separates the Miocene Enns Valley basin from the uplifted Northern Calcareous Alps. The NEV fault postdates the deposition of the Miocene Enns Valley basin fill and likely extends to the WSW into the Mandling fault. If this interpretation is correct, then a ca. 20 km dextral offset and ca. 1 – 1.2 km northern block up displacement occurred along this fault, mostly during the late Miocene/early Pliocene inversion during E W shortening as postulated by previous models.
Dextral displacement along the NEV fault could also explain the Weyer Arc, a specific feature within the eastern Northern Calcareous Alps. This arc could be explained by accomodating a dextral displacement at the eastern termination of the NEV fault by counterclockwise rotation.
40 10th Alpine workshop "CorseAlp2011"
Deformation along the Combin Fault and the Dent Blanche Basal Thrust
Frederik Kirst 1, Nikolaus Froitzheim 1, Thorsten Nagel 1 & Jan Pleuger 2
fredster@uni bonn.de
(1) Steinmann Institut, Universität Bonn, Bonn, Germany (2) Geologisches Institut, ETH Zürich, Zürich, Switzerland
The Combin Fault (CF) and the Dent Blanche Basal Thrust (DBBT) are two major tectonic contacts in the Swiss Italian Western Alps. Both experienced a complex multistage evolution of compressional as well as extensional deformation phases. The evolution of the CF separating the Combin zone from the underlying Zermatt Saas zone is still enigmatic since opposing shear senses have been reported for different areas within the Combin zone (Ring, 1995; Wheeler et al., 2001). It represents a major break in peak pressure (higher P below, lower P above) and has been interpreted as a 100 km offset normal fault (Wheeler et al. 2001). The Combin zone comprises the South Penninic Tsaté nappe and the Permian Triassic Cimes Blanches and Frilihorn nappes of yet unknown origin. The DBBT separates the Dent Blanche nappe in the hanging wall from the Combin zone in the footwall and is often regarded as a major top to the NW thrust (e.g. Steck et al., 1999) but also top to the SE movement has been reported (Wust & Silverberg, 1989; Lebit et al., 2002).
Mylonites along the DBBT display synkinematic microstructures mostly resulting from persistent NW directed shearing: Top to the NW transport of the Dent Blanche nappe led to the formation of a mylonitic foliation under greenschist facies conditions. Locally, this foliation has been overprinted by small scale isoclinal folds and shear bands also indicating top to the NW transport. Mostly along the NW boundary of the Dent Blanche nappe, subsequent NW SE shortening led to formation of a non penetrative crenulation, meso scale folds and deformation of the contact itself. Along the SE border, late tight and upright folds within the Combin zone just below the DBBT with SW plunging fold axes are attributed to the same event and are cut off by the DBBT showing that it has been reactivated subsequently. Locally abundant top to the SE shear bands within Combin calcschists along the NW front of the Dent Blanche nappe and top to the NW shear bands on its southeastern side reflect a period of NW SE extension leading to a graben like position of the Dent Blanche nappe. Tight N vergent folds within metasediments of the Frilihorn nappe around Zermatt probably document an early stage of N directed thrusting before incorporation into surrounding calcschists since the axial plane foliation of the folds only affects the Frilihorn metasediments and is discordant to the foliation of the surrounding calcschists. However, this foliation has also been modified by later top to the NW shearing as documented by small scale shear bands. Some of the structures developed in the Zermatt Saas zone cannot be traced into the hanging wall of the CF. This applies e.g. to small scale shear bands within high pressure schists just below the CF showing a top to the SW sense of shear and NE vergent folds within metabasites folding a greenschist facies foliation. There, the CF seems to be a late feature resulting from N directed shearing as indicated by top to the N shear bands within metasediments of the Cimes Blanche nappe just above the CF.
The observed structures show that deformation along the CF and the DBBT has been dominated by NW directed shearing. Top to the NW movement mostly occurred during an early phase as a result of subduction, accretion and nappe stacking but also during later stages as documented by relatively young structures that are cut off by the contacts. NW SE shortening resulted in the formation of upright folds. NW SE extension (at 34 to 30 Ma?) appears to postdate this folding. Conjugate southeast dipping and northwest dipping shear zones formed during this stage, rather than a continuous top to the SE “Combin normal fault”. So far the structural studies along the CF and the DBBT suggest that the Combin zone and the Dent Blanche nappe partly shared a common tectonic history but they also show differences due to intermittent reactivation of the DBBT and the CF and localised deformation within the Combin zone. In contrast, the deformational history of the Zermatt Saas zone seems to be disconnected from the evolution of the overlying Combin zone and Dent Blanche nappe until a late stage of deformation.
REFERENCES Lebit, H., Klaper, E.M. & Lüneburg, C.M. (2002): Fold controlled quartz textures in the Pennine Mischabel backfold near Zermatt, Switzerland. Tectonophysics 359, 1 28. Ring, U. (1995): Horizontal contraction or horizontal extension? Heterogeneous Late Eocene and Early Oligocene general shearing during blueschist and greenschist facies metamorphism at the Pennine Austroalpine boundary in the Western Alps. Geol. Rundsch. 84, 843 859. Steck, A., Bigioggero, B., Dal Piaz, G.V., Escher, A., Martinetti, G. & Masson, H. (1999) : Carte tectonique des Alpes de Suisse occidentale et des régions avoisinantes, 1:100000, Carte spéc. n. 123 (4 maps). Serv. Hydrol. Géol. Nat., Bern. Wheeler, J., Reddy, S.M. & Cliff, R.A. (2001): Kinematic linkage between internal zone extension and shortening in more external units in the NW Alps. J. Geol. Soc. (London) 158, 439 443. Wust, G.H. & Silverberg, D.S. (1989): Northern Combin zone complex Dent Blanche nappe contact: extension within the convergent Alpine belt. Schweiz. Mineral. Petrogr. Mitt. 69, 251 259.
41 10th Alpine workshop "CorseAlp2011"
Evolution of monometamorphosed Austroalpine units (Schneeberg- and Radenthein Complex)
with implications for Eoalpine tectonics of the Eastern Alps
Kurt Krenn, Walter Kurz, Harald Fritz & Georg Hoinkes
kurt.krenn@uni graz.at
Karl Franzens University of Graz, Institute of Earth Sciences, Graz, Austria
Monometamorphic metasedimentary units of an assumed Mesozoic origin (Schneeberg and Radenthein Complex) experienced coherent deformation and metamorphism during Late Cretaceous times. Both complexes are part of the Eoalpine high pressure wedge that formed an intracontinental suture and occur between the polymetamorphosed Ötztal Bundschuh Nappe on top and the Texel Millstatt Complex at the base. During Eoalpine tectonics the Schneeberg and Radenthein Complex were dipping towards south and experienced a common tectonometamorphic history from ca. 115 m.y. until unroofing of the Tauern Window during Miocene times. This evolution can be subdivided into four distinct phases.
Deformation stage D1 is characterized by WNW directed shearing at amphibolites facies temperature conditions (550 600°C) and related to the initial exhumation of the high pressure wedge. Deformation stages D2 and D3 are largely coaxial and evolved during greenschist facies temperature conditions (ca. 450 to ≥550°C). These stages are related to advanced exhumation and associated with large scale folding of the high pressure wedge and the lowermost parts of the Ötztal Bundschuh Nappe above. Exhumation is linked with the formation of a prominent ESE directed normal fault system along the SAM Line located in higher structural levels above the high pressure wedge and during Cretaceous time.
Deformation stage D4 during Oligocene to Miocene times accounted for tilting of individual basement blocks along large scale strike slip shear zones. This tilting phase at the northern margin of the Southern Alps resulted in the inversion of the tectonostratigraphic sequences west and east of the Tauern Window.
42 10th Alpine workshop "CorseAlp2011"
Prägosauic Bloc Re-Arrangement as Recorded From Late Jurassic to Early Cretaceous Facies Belts
in the Central Northern Calcareous Alps (Salzburg, Austria)
Oliver Krische 1, Walter Kurz 2 & Hans Jürgen Gawlick 1
(1) University of Leoben, Department for Applied Geosciences and Geophysics, Leoben, Austria (2) Karl Franzens University of Graz, Institute of Earth Sciences, Graz, Austria
The geodynamic evolution of the Northern Calcareous Alps, as a part of the Upper Austroalpine Unit in the Eastern Alps, is nowadays a quite well understood process. The onset of the sedimentary sequence starts with a Late Permian to Early Jurassic passive margin. It changed in the Middle Jurassic to a convergent regime with ophiolite obduction and the formation of trench like basins with Wildflysch deposits in front of propagating nappes. The Early Cretaceous is a time of relative tectonic quiescence with the upfilling of the Middle to Late Jurassic trench like basins.
The next sedimentary cycle starts in the Late Cretaceous with Gosauic sediments, which overlie unconformably a not well understood Mid Cretaceous tectonic event with a re arrangement of different facies zones. But what had happened between the end of sedimentation in the late Early Cretaceous and the new Gosauic cycle in early Late Cretaceous times? To manage this question not only structural data but also stratigraphic and microfacies information are needed and brought together. The investigated area of the Weitenau syncline, north of the Lammer valley in the Salzburg Northern Calcareous Alps, is known as a typical Late Jurassic to Early Cretaceous basin fill. New detailed mapping and investigation of profiles and structures in combination with microfacies analysis and biostratigraphical data give now a different picture of the formerly as uniform interpreted basin fill. At the moment on the base of the new data five units can be determined. They differ in lithology, sedimentary environment, palaeogeographic provenance and age. The Lammer basin fill (Callovian Oxfordian) to the south is until now well described and is not reinvestigated within this study.
The footwall of the Sandlingalm nappe with its Bajocian to Oxfordian basin fill was thrusted in Late Jurassic times onto the Lammer Basin and is also described in modern literature. Interestingly in the hanging wall of the Sandlingalm nappe some exotic blocks occur in the Early Kimmeridgian Haselgebirge Mélange at the Mount Hochreith area. In the eastern and middle part of the Weitenau area two down faulted blocks of the Trattberg Rise occur nearby. They differ from each other in their latest Jurassic to Early Cretaceous sedimentary cover, showing that the Trattberg Rise was again dismembered, after the Tithonian break down, later as Early Aptian. On the northern edge the block puzzle of the Weitenau area is separated from another down faulted remnant of the Trattberg Rise (as a part of the Osterhorn Block) by a fault, most probably a strike slip fault. Direct dating of the tectonic lines is more or less unpossible. The only chance is to do it in an indirect way by dating and facies analysis of the different sedimentary successions of the distinct blocks. From the sedimentary point of view the tectonic movements of the unequal blocks must be younger than Early Aptian, because the sedimentation ends at this time, and older than the onset of Gosauic sedimentation in the Turonian east of the Weitenau area, e.g. near Abtenau, Rigaus, Gosau and elsewhere. Gosauic sediments overlie unconformably the different blocks.
With the two fixed ages of Early Aptian (filling of the Early Cretaceous remnant basins) and Turonian (start of Gosauic sedimentation) we see a disintegration of the older orogenic belt during Late Aptian to Cenomanian times, most probably by strike slip tectonics. This would also be the best explanation for the morphological deep level of the highest Sandlingalm nappe and its overlying sedimentary cover, which occurs today in a pull apart position. This Mid Cretaceous block movements of the upper, brittle crust may be comparable with ductile deformation of the same age in deeper crustal horizons, e.g. in the Austroalpine crystalline. With the combination of the different mentioned methods it is possible to get an idea what had happened in the Northern Calcareous Alps during this time of non sedimentation, when tectonic forces disintegrated the former facies belts and nappe stack, forming a pre Gosauic block puzzle, which was again disintegrated during Oligocene thrusting and Miocene strike slip faulting.
43 10th Alpine workshop "CorseAlp2011"
The Rannach Formation - A Permian Trough Within the Lower Central Austroalpine Realm
Walter Kurz, Harald Fritz, Agnes Mair & Stefan Pfingstl
walter.kurz@uni graz.at
Karl Franzens University of Graz, Institute of Earth Sciences, Graz, Austria
Permian to early Mesozoic sedimentary units, known as Central Austroalpine Mesozoic (CAM), are widely dispersed within the Lower Central Austroalpine realm due to Eo Alpine stacking and subsequent extension. Prominent examples for CAM are the Brenner Mesozoic west and the Stangalm, Thörl and Rannach Permo Mesozoics to the east of the Tauern Window, respectively. Along the eastern margin of the Eastern Alps those units have been summarized to as “Semmering Quartite” or “Alpine Verrucano”. By contrast to Permo Mesozoic units deposited on the Meliata margin and now exposed within Upper Central Austroalpine these units are dominated by clastic sequences with limited stratigraphic extend (Permian to Triassic, locally LowerJurassic). Most of the clastic Permian sequences have minor thicknesses between few meters (Brenner, Stangalm units) to at maximum 100 meters (Thörl unit). An exception is the Rannach Formation with up to 1000 meters of clastic sediments deposited on the Seckau basement. Hence we consider this unit as possible Permian though that developed above the Permian extension zone.
Several profiles across the Rannach Formation show a single general fining upwards sequence with 120 150 meters basal conglomerates followed by 600 700 meters sandstone and up to 300 meters fine grained clastics (phyllites) on top. Carbonate beds as developed elsewhere (e.g., Stangalm unit) are nearly absent. Provenience studies and relictic sedimentary structures suggest deposition of mature sediments eroded from an intra continental source related to basement, possibly starting with alluvial fan deposits grading upwards into lower energy fluvial systems. Comparable Permian sedimentary troughs are absent in the Eastern Alps but are known from Southalpine domains (Orobic Basins, Collio Trough, Bolzano Basin).
Structures recorded in the sediments display Alpine deformation, either thrust related or extension related. Strain derived from conglomerates is almost purely flattening strain with ratios of maximum to minimum principal strain axes between two and three. A slight gradient with diminishing strain towards higher units is observed. Quartz c axes patterns display low vorticity and NW SE coaxial stretch. These data much better fit a scenario of upper Cretaceous (Gosau) extension than thrusting. This is further supported by relatively high temperature deformation at high thermal gradient as suggested from fluid inclusion data (icochore 500°/ 4 kbar) and prevailing high temperature quartz patterns.
All in all two extensional scenarios are recorded in the Rannach Formation: Permian extension and through formation above a zone of crustal thinning from the sedimentary record and Intra Gosauic extension within an Alpine thinning zone from the structural record during Late Cretaceous times.
44 10th Alpine workshop "CorseAlp2011"
Polyphase activity along the Lavanttal Fault Zone (Eastern Alps): constraints from 40 Ar/ 39 Ar white
mica, zircon and apatite fission track, and apatite (U-Th)/He dating
Walter Kurz 1, Andreas Wölfler 1 & Johann Genser 2
walter.kurz@uni graz.at
(1) Karl Franzens University of Graz, Institute of Earth Sciences, Graz, Austria (2) Fachbereich Geographie und Geologie, Universität Salzburg, Salzburg, Austria
The Lavanttal Fault Zone (LFZ) is generally described to be related to Miocene orogen parallel escape tectonics in the Eastern Alps. By applying distinct thermochronological methods with retention zones ranging from ~450° to ~40°C we reveal the thermochronological evolution of the LFZ and the adjacent Koralm Complex (Eastern Alps). 40 Ar/ 39 Ar dating on white mica, zircon and apatite fission track, and apatite (U Th)/He thermochronology were carried out on host rocks and fault related rocks (cataclasites and fault gouges) directly adjacent to the unfaulted protolith. Sample material was taken from three drill cores transecting the LFZ.
Argon release spectra in cataclastic shear zones show in parts highly reduced incremental ages, indicating lattice distortion during cataclastic shearing or hydrothermal alteration. Integrated plateau ages from fault rocks are ~10 Ma younger (~73 Ma) than protolith ages. Zircon fission track ages within fault gouges (~65 Ma) are slightly reduced compared to the ages from host rocks and fission tracks show reduced lengths.
The combination with already published apatite fission track and apatite (U Th)/He ages from fault rocks of the same fault zone, allows the reconstruction of distinct faulting events along the LFZ from Late Cretaceous to Miocene and Pliocene times. Contemporaneous to faulting the Koralm complex experienced accelerated cooling in Late Miocene times.
45 10th Alpine workshop "CorseAlp2011"
Subduction Orogens, a new Perspective From the Andes
Robin Lacassin & Rolando Armijo
Equipe de Tectonique Mécanique de la Lithosphère, Institut de Physique du Globe de Paris, Sorbonne Paris Cité, Paris, France
The Andean orogeny is considered the paradigm for mountain belts associated with subduction plate boundaries. Yet, no mechanical model can explain satisfactorily the Andean mountain building process as a result of forces applied at its nearby Subduction Margin, along the western flank of the South America continent. In other words, the tectonic implications of the Chilean type subduction proposed by Uyeda and Kanamori (1979) are not fully understood. Contrary to earlier views, the Andes mountain belt appears to have a doubly vergent structure defined by two distinct orogenic thrust boundaries at the East and West Andean Thrusts. While the well known East Andean Thrust (in the back arc region) coincides with the basal thrust of the Back Thrust Margin over the eastern foreland (the South America continent), the generally disregarded and much less known West Andean Thrust (WAT, in the fore arc region) is located at significant distance from the basal mega thrust of the Subduction Margin and is synthetic to it. There is a ~200 km wide western foreland, the Marginal (or Coastal) Block, separating the WAT from the subduction zone. Consequently a mechanical partitioning occurs across the fore arc, between the subduction interface, a mega thrust that is responsible of significant short term strains and the occurrence of repeated large earthquakes, and the WAT, which appears fundamental in regard to processes associated with the Andean orogeny.
Some observations are now available to describe this partitioning (Armijo et al., 2010a, 2010b). A key tectonic section of the Andes is analyzed at latitude 33.5°S, where the belt is in an early stage of its evolution, aiming at resolving the primary architecture of the orogen. We focus on the active fault propagation fold system in the Andean cover behind the San Ramón Fault, a thrust ramp which is critical for the seismic hazard in the city of Santiago and crucial to decipher the structure of the WAT. The WAT steps down eastwards from the San Ramón Fault, crossing 12 km of Andean cover to root beneath the Frontal Cordillera basement anticline, a range ~5 km high and >700 km long. We propose a first order tectonic model of the Andes involving an embryonic intra continental subduction of the Marginal Block consistent with geological and geophysical observations. The stage of primary westward vergence with dominance of the WAT at 33.5°S is evolving into a doubly vergent configuration. Motion on the WAT thickens the crust and build the andean relief, thus forming a crustal scale backstop able to drive the eastward propagation of the backthrust front. A growth model for the WAT Altiplano similar to the Himalaya Tibet is deduced. We suggest the intra continental subduction at the WAT is a mechanical substitute of a collision zone, rendering the Andean orogeny paradigm obsolete. We consider likely that similar processes were at the begining of most andean type orogens: S and N American cordilleran belts or subduction margins later involved in collision belts such as the Alps or South Tibet.
REFERENCES Armijo et al. The West Andean Thrust, the San Ramón Fault, and the seismic hazard for Santiago, Chile. Tectonics, vol. 29, TC2007, doi:10.1029/2008TC002427, 2010a. Armijo et al. Reply to the comment by R. A. Astini and F. M. Dávila on “The West Andean Thrust, the San Ramón Fault, and the seismic hazard for Santiago, Chile”. Tectonics, vol 29, TC4010, doi:10.1029/2010TC002692, 2010b. Uyeda et Kanamori. Back arc opening and the mode of subduction. Journal of Geophysical Research, vol. 84, pp. 1049 1061, 1979.
46 10th Alpine workshop "CorseAlp2011"
Active Deformation of the Northern Ligurian Margin (Western Mediterranean): faulting along inherited
structures ?
Christophe Larroque 1 & Françoise Sage 2
(1) UMR GéoAzur 6526, Université de Nice Sophia Antipolis, CNRS OCA, Valbonne, France (2) UMR GéoAzur 6526, UPMC OCA, Villefranche sur Mer, France
The MALISAR geophysical surveys reveal recent and active deformation of the northern Ligurian margin: a set of N60°E scarps that are oblique to the margin is seen at the foot of the continental slope. These correspond to cumulated reverse strike slip faulting that is consistent with the present day kinematics of earthquakes. At the north western margin, the Marcel Fault appears at the surface as a 10 km long scarp trending N60°E. Several earthquakes of moderate magnitude (3.8 Structural sketch of the northern Ligurian faults system. At the foot of the northern Ligurian margin, several faults oblique to the margin direction, run 80- km long from Nice (7°15’E) to Savona (8°30’E), from Larroque et al., 2010 REFERENCES Larroque C., Mercier de Lépinay B. and S. Migeon (2010). Morphotectonic and fault earthquake relationships along the northern Ligurian margin (Western Mediterranean) based on high resolution multibeam bathymetry and multichannel seismic reflection profiles. Marine Geophysical Researches, doi:10.1007/s11001 010 9108 7 47 10th Alpine workshop "CorseAlp2011" Reappraisal of the 1887 Ligurian Earthquake and Tsunami (Western Mediterranean): Consequences for the Regional Seismic and Tsunami Hazards Christophe Larroque 1, Oona Scotti 2 & Mansour Ioualalen 3 (1) UMR GéoAzur 6526, Université de Nice Sophia Antipolis, CNRS OCA, Valbonne, France (2) Institut de Recherches et de Sûreté Nucléaire, Fontenay aux Roses, Cedex, France (3) UMR GéoAzur 6526, IRD OCA, Villefranche sur Mer, France Early in the morning, of February 23, 1887 a major damaging earthquake hit the towns along the Italian and French Riviera. The earthquake was followed by a tsunami wave with a maximum runup of 2 m near Imperia. At least 600 hundred people died, mainly due to building collapses. The “Ligurian earthquake” occurred at the junction between the Southern French Italian Alps and the Ligurian Basin. For such historical event, the epicentre and the equivalent magnitude are difficult to characterize with a high degree of precision, and the tectonic fault responsible for the earthquake remains unknown to this day. The recent MALISAR marine geophysical survey allowed identifying a set of N60°E recent scarps at the foot of the northern Ligurian margin, revealing a large system of active faults. The scarps correspond to cumulative reverse strike slip faulting consistent with the present day kinematics of earthquakes. We propose that the rupture of some segments belonging to this 80 km long Imperia Faults network is the source of the 1887 Ligurian earthquake. We investigate the seismological data from the historical databases SISFRANCE 08 and DBMI 04 using several models of intensity attenuation and crustal structures. For focal depth of 10 and 20 km, all results are consistent with the location offshore, indicating an epicentre around 43.70° 43.78°N and 7.81° 8.07°E with a magnitude M w in the range of 6.3 7.5 and a preferred magnitude of 6.5 6.9. We proposed 15 source earthquakes scenarii for a shallow strong earthquake occurring on the northern Ligurian margin. These scenarii have been tested with hydrographic data (tsunami modelling and historical observations). The main material used here is the tide gauge record at Genoa harbour. Multiple basic statistical tools have been computed for both the tide gauge record and the simulated tsunami at this location along with a simple harmonic analysis (wave spectrum characteristics). This effort indicates that the scenario of a reverse right lateral faulting along a N60°E striking and 70°N dipping fault plane with a magnitude M w of 6.9 is the best candidate that could explain the known characteristics of the induced tsunami. Distribution of the maxima of wave height for an earthquake of Mw 6.9 occuring along the Imperia Fault network (reverse – strike slip faulting with 70°N dip and 74° rake). The bathymetry is also represented at 50 m iso-levels (the 50 m isobath is bolded). The withe values are the local wave height maxima. FC are focusing areas located above submarine spurs. REFERENCES Larroque C., Mercier de Lépinay B. and S. Migeon (2010). Morphotectonic and fault earthquake relationships along the northern Ligurian margin (Western Mediterranean) based on high resolution multibeam bathymetry and multichannel seismic reflection profiles. Marine Geophysical Researches, doi:10.1007/s11001 010 9108 7 48 10th Alpine workshop "CorseAlp2011" Short Term Erosion Patterns in the Alps Constrained by Downstream Changes of Detrital Zircon U- Pb Ages (Po River Basin) Marta Limoncelli 1, Marco G. Malusà 1, Andy Carter 2, François Guillot 3 & Eduardo Garzanti 1 (1) Laboratory for Provenance Studies, Earth Science and Geotechnology Department, Milano Bicocca University, Milano, Italy (2) School of Earth Sciences, Birkbeck College, University of London, UK (3) Université Lille 1, Villeneuve d'Ascq Cedex, France Provenance studies based on detrital geochronological techniques are useful to investigate the present day pattern of erosion in mountain belts. In particular, LA ICPMS U Pb geochronology allows rapid generation of large datasets, and laser offers high spatial resolution for grains with complex age structure. The aim of this work is to investigate the downstream changes of detrital zircon U Pb ages in the Po drainage, in order to constrain the short term (10e2 10e4 a) erosion pattern in the Alps Apennines orogenic couple. We collected modern sands in selected tributaries draining the major tectonic units exposed in the orogen, as well as along the Po River main trunk and in different sites of the Po delta. Zircon concentrates were separated according to specific quantitative procedures coupled with grain size and grain density analyses, which provide an evaluation of zircon content in source rocks, of hydraulic sorting effects, and related intersample/intrasample zircon variability (Garzanti et al., 2009). Our methodological approach allows performing sediment budgets and mineral content in diverse sediment sources (mineral fertility Malusà & Garzanti, 2011). Such analyses based on detrital geochronology can be a key point to constrain the short term erosion pattern and discriminate between potential sources the fastest eroding area. Geochronological analyses performed on single mineral grains show that all samples from the Po delta yield similar grain age distributions, and include grain age populations that are rather constant both in age and size. Peaks in the distribution can be traced upstream up to their source. Major peaks are found at ca. 30 Ma, ca. 280 Ma, and 400 500 Ma. The geochronological signature of the Po delta sands is essentially displayed already upstream of the confluence of the Northern Apennine rivers, possibly suggesting lower short term erosion rates in the Northern Apennine than in the Western and Central Alps. Age peaks typical of the eastern Lepontine dome also characterize the Apenninic tributaries, reflecting Oligo Miocene sedimentary transfer from the Central Alps to the Adriatic foredeep, and next tectonic accretion of turbidite wedges to the Apennine belt. U Pb analyses were complemented by zircon typology for the whole dataset, integrated by SEM observations on selected samples. Major typological groups can be traced from entry points in the Po plain down to the Po delta. Euhedral zircons are largely shed from the External Massifs and Periadriatic plutons. Zircon grains showing chemical abrasion, possibly acquired under high grade metamorphic conditions, were detected in tributaries draining the Lepontine area. REFERENCES Malusà M. G., and Garzanti E., 2011. Detrital geochronology, mineral fertility and short term erosion patterns. Geophysical Research Abstracts, v. 13, EGU2011 10381. Garzanti, E., Andò, S., Vezzoli, G., 2009. Grain size dependence of sediment composition and environmental bias in provenance studies. Earth and Planetary Science Letters, v. 277, 422 432. 49 10th Alpine workshop "CorseAlp2011" The Central-Western Mediterranean: Anomalous igneous activity in an anomalous collisional tectonic setting Michele Lustrino 1,2 , Svend Duggen 3,4 & Claudio L. Rosenberg 5 (1) Dipartimento di Scienze della Terra, Università degli Studi di Roma La Sapienza, Roma, Italy (2) Istituto di Geologia Ambientale e Geoingegneria (IGAG) CNR, c/o Dipartimento di Scienze della Terra, Università degli Studi di Roma La Sapienza, Italy (3) IFM GEOMAR, Leibniz Institute of Marine Sciences, Research Division Dynamics of the Ocean Floor, Kiel, Germany (4) A. P. Møller Skolen, Upper Secondary School and Sixth Form College of the Danish National Minority in Germany, Schleswig, Germany (5) Institut für Geologie, Freie Universität Berlin, Berlin, Germany The central western Mediterranean area is a key region for understanding the complex interaction between igneous activity and tectonics. Plutonic rocks occur almost exclusively in the Eocene Oligocene Periadriatic Province of the Alps while relatively minor plutonic bodies (mostly Miocene in age) crop out in N Morocco, S Spain and N Algeria. Igneous activity is otherwise confined to lava flows and dykes accompanied by relatively greater volumes of pyroclastic (often ignimbritic) products. Overall, the igneous activity spanned a wide temporal range, from middle Eocene (such as the Periadriatic Province) to the present (as in the Neapolitan of southern Italy). The magmatic products are mostly SiO 2 oversaturated, showing calcalkaline to high K calcalkaline affinity, except in some areas (as in peninsular Italy) where potassic to ultrapotassic compositions prevail. The ultrapotassic magmas (which include leucitites to leucite phonolites) are dominantly SiO 2 undersaturated. Aalthough rare, SiO 2 saturated (i.e., leucite free lamproites) appear over much of this region, examples being in the Betics (southeast Spain), the northwest Alps, northeast Corsica (France), Tuscany (northwest Italy), southeast Tyrrhenian Sea (Cornacya Seamount) and possibly in the Tell region (northeast Algeria). Excepted for the Alpine case, subduction related igneous activity is strictly linked to the formation of the Mediterranean Sea. This Sea, at least in its central and western sectors, is made up of several young (<30 Ma) V shaped back arc basins plus several dispersed continental fragments, originally in crustal continuity with the European plate (Sardinia, Corsica, Balearic Islands, Kabylies, Calabria, Peloritani Mountains). The bulk of igneous activity in the central western Mediterranean is believed to have tapped mantle ‘wedge’ regions, metasomatized by pressure related dehydration of the subducting slabs. The presence of subduction related igneous rocks with a wide range of chemical composition has been related to the interplay of several factors among which the pre metasomatic composition of the mantle wedges (i.e., fertile vs. refractory mineralogy), the composition of the subducting plate (i.e., the type and amount of sediment cover and the alteration state of the crust), the variable thermo baric conditions of magma formation, coupled with variable molar concentrations of CO 2 and H 2O in the fluid phase released by the subducting plates are the most important. Compared to classic collisional settings, the central western Mediterranean area shows a range of unusual geological and magmatological features. These include: a) the rapid formation of extensional basins in an overall compressional setting related to Africa Europe convergence; b) centrifugal wave of both compressive and extensional tectonics starting from a ‘pivotal’ region around the Gulf of Lyon; c) the development of concomitant Cenozoic subduction zones with different subduction and tectonic transport directions; d) subduction ‘inversion’ events (e.g., currently along the Maghrebian coast and in northern Sicily, previously at the southern paleo European margin); e) a repeated temporal pattern whereby subduction related magmatic activity gives way to magmas of intra plate geochemical type; f) the late stage appearance of magmas with collision related ‘exotic’ (potassic to ultrapotassic) compositions, generally absent from simple subduction settings; g) the relative scarcity of typical calcalkaline magmas along the Italian peninsula; h) the absence of igneous activity where it might well be expected (e.g., above the hanging wall of the Late Cretaceous Eocene Adria Europe subduction system in the Alps); i) voluminous production of subduction related magmas coeval with extensional tectonic régimes (e.g., during Oligo Miocene Sardinian Trough formation). To summarize, these salient central western Mediterranean features, characterizing a late stage of the classic ‘Wilson Cycle’ offer a ‘template’ for interpreting magmatic compositions in analogous settings elsewhere. 50 10th Alpine workshop "CorseAlp2011" Intensive Fluid-Rock Interaction and Rheology of the East Tenda Shear Zone (Haute Corse, France) Matteo Maggi 1, Giorgio Ranalli 2, Federico Rossetti 1 & Thomas Theye 3 (1) Dipartimento di Scienze Geologiche, Università degli Studi Roma Tre, Italy (2) Department of Earth Sciences, Carleton University, Ottawa, Canada (3) Institut für Mineralogie und Kristallchemie, Universität Stuttgart, Germany The East Tenda Shear Zone (ETSZ) is the major Alpine tectonic boundary marking the overthrusting of the oceanic derived Schistes Lustrés nappe onto the Hercynian crystalline basement of western Corsica. In this work we present new structural and geochemical investigations along a transect ranging from the undeformed protolith (PR) to the contact with the Schistes Lustrés. The results are used to construct a rheological model for the ETSZ. Shear deformation within the ETSZ is heterogeneously distributed with high strain domains (shear zones, SZ) wrapping sigmoid shaped low strain domains (massive lenses, ML). Locally, mica rich mylonites occur (phyllonites). The main foliation is concordant with that in the overlying Schistes Lustrés, strikes NW SE, and is dominantly shallow dipping to NE. The ML mineralogy consists of an assemblage made of quartz, phengite and (relict) feldspar (epidote, Fe oxides, zircon and allanite as accessory phases). The SZ mineralogy is invariably dominated by highly celadonitic (Si 4+ = 3.5 3.7 a.p.f.u.) phengite (40 ± 10 vol%) and modally abundant quartz (35 ± 5 vol%), albite (15 ± 5 vol%) epidote (<5 vol%) and microcline (10 ± 5 vol%). Locally, Na amphibole (10 20 vol%) also occurs in the SZ assemblage to form thin (up to 1 m thick) dark mylonitic levels. Stretching lineations strike WSW ENE to E W and consist of quartz phengite albite in ML and of Na amphibole quartz albite phengite in SZ. Deformation is progressive and evolves from ductile to semibrittle conditions. Sense of shear is predominantly top to the SW and is locally reworked in the phyllonites with top to the NE sense of shear. Whole rock geochemistry suggests an increasing chemical alteration moving from the undeformed rocks to ML and SZ. In particular, Ca ++ is progressively leached while Na + and K + contents systematically increase as deformation proceeds. Destabilization of Ca bearing phases, such as plagioclase and epidote, and neoblastesis of feldspars (albite and microcline) is consistently observed in the more evolved shear zones. These observations indicate that progressive shear deformation was governed by intensive fluid rock interaction characterized by increasingly higher fluid/rock ratios. The effect of chemical alteration of the host rock by fluids on the rheology of the ETSZ has been estimated taking into account the modal composition and the fabric of the main lithotypes (PR, ML, SZ, phyllonites). Flow laws are obtained using an averaging procedure based on weighted averages of single phase rheology. These flow laws are used to infer strain rates, construct deformation maps, and estimate the depth of the brittle ductile transition for each lithotype during progressive deformation. The combined effects of the feldspar to mica reaction and the development of a strong planar fabric induce weakening and strain localization along the shear zones. Fluid channelling along these shear zones enhances dominance of Na and K over Ca and, particularly, albite and microcline neoblastesis. The latter, in turn, generates strain hardening. Among the possible consequences of such feedback processes between strain localization and fluid rock interaction are episodes of transient rheology. The main result of our observations and rheological estimates is that reworking during top to the E regional extension occurred only in the uppermost part of the deforming crustal section and localized within the weaker phyllonite levels. 51 10th Alpine workshop "CorseAlp2011" Fluid-Rock Interaction During the Schistes Lustrés Nappe Exhumation: an Integrated Structural, Petrological, Fluid Inclusions Study of the Erbalunga Shear Zone (Haute Corse, France) Matteo Maggi 1, Federico Rossetti 1 & Francesca Tecce 2 (1) Dipartimento di Scienze Geologiche, Università degli Studi Roma Tre, Roma, Italy (2) IGAG Istituto di Geologia e Geoingegneria Ambientale Centro Nazionale delle Ricerche, Roma, Italy In this work we present structural, petrological and fluid inclusion studies performed in a major retrogressive shear zone (the Erbalunga shear zone), which occurs within the HP/LT domain of the Schistes Lustrés Nappe of eastern Alpine Corsica. This shear zone is part of the post orogenic network of shear zones that favoured the exhumation of the HP core of Alpine Corsica (Daniel et al., 1996) during Late Oligocene/Early Miocene times (Brunet et al., 2000). The shear zone is characterised by a progressive ductile to brittle top to the E shearing, starting at greenschist facies conditions (ca. 600 MPa, 400 450 °C). Evidence for vigorous fluid flow through the shear zone is documented by widespread quartz and quartz calcite vein segregations, which accompanied the progressive evolution of shearing. Textural characteristics of three main generations of veins record the incremental evolution of the shear zone tracing the continuum transition from ductile to brittle dominated deformation environments. Regardless, of the vein generation, fluid inclusions hosted in quartz grains hosted within the three different sets of veins document a low salinity (<5% NaCl eq.) fluid circulation. Fluid trapping occurred under pore pressure conditions fluctuating from lithostatic to hydrostatic values, as also attested by the crack sealing textures preserved in most of the veins. The findings of this study suggest that the main source of fluid was of meteoric origin and argue for fluid percolation and infiltration at the brittle ductile depths. Such a fluid supply cause the availability of a higher amount of fluids in the deforming rock volume, working against ductile deformation and tendency to pore space reduction by recovery during progressive deformation. This impose definition of the (i) mechanism through which superficial fluids infiltrate the mid lower crust; and (ii) the modes (fracturing vs . ductile creep) of creation and maintenance of the structural permeability moving from the brittle to the ductile crust. The cyclical release of seismic energy in the brittle crust (Sibson et al., 1975) is a feasible scenario to frame the downward fluid flow in the ductile crust along pathways maintained by large scale fault zones active in the extending upper crust. REFERENCES Brunet, C., Monie´, P., Jolivet, L., Cadet, J.P., 2000. Migration of compression and extension in the Thyrrhenian Sea, insights from 40Ar/39Ar ages on micas along a transect from Corsica to Tuscany. Tectonophysics 321; Daniel, J.M., Jolivet, L., Goffe´, B., Poinsott, C., 1996. Crustal scale strain partitioning: footwall deformation below the Alpine Oligo Miocene detachment of Corsica. Journal of Structural Geology 18, 1841e1859. Sibson, R.,H., Moore, J. & Rankin, A.H., 1975. Seismic pumping a hydrothermal fluid transport mechanism. J. Geol. Soc., London, 131, 653 669. 52 10th Alpine workshop "CorseAlp2011" Sodic Pyroxene Bearing Phyllonites From the East Tenda Shear Zone: Constraining P-T Conditions and Timing of the Ligurian-Piemontese Ocean Overthrusting Onto the Variscan Corsica Matteo Maggi 1, Federico Rossetti 1, Thomas Theye 2, Torgeir Andersen 3, Fernando Corfu 3 & Claudio Faccenna 1 (1) Dipartimento di Scienze Geologiche, Università degli Studi Roma Tre, Italy (2)Institut für Mineralogie und Kristallchemie, Universität Stuttgart, Germany (3) Institutt for Geofag and PGP, Universitetet i Oslo, Norway The East Tenda Shear Zone (ETSZ) is the deformation zone that marks the Alpine overthrusting of the Ligurian Piemontese oceanic domain (Schistes Lustrés unit) onto the Variscan Corsica. Due to its regional significance, many papers have dealt with the tectono metamorphic evolution of the ETSZ, documenting structures and metamorphic gradients associated with the Alpine reworking of the Variscan basement. Nevertheless, the metamorphic gradients and timing of the Alpine evolution all along the ETSZ are still poorly constrained, hampering a full characterisation of the Alpine evolution in the region. Here we present the first report of a sodic pyroxene (acmite) and rutile bearing assemblage found within a cm thick phyllonitic shear zone that cuts through the gneissic lithologies of the ETSZ. The gneissic host rocks consist of a low variance assemblage made of phengite feldspar (albite and microcline) quartz ± epidote developed during a prominent top to the W sense of shear. The modal composition of the sodic pyroxene rutile phyllonite consists of phengite (>60% vol.), acmite (20 25% vol.), rutile (2 5 % vol.), quartz and albite + microcline + epidote + relic zircon (10 % vol.) as accessory phases. Microprobe data and BSE images indicate a rather homogeneous composition both for the phengite (3.6 3.7 Si 4+ a.p.f.u.) and acmite (average composition Acm 72 Jd 17 Di 9Hed 1Opx0.05 ) grains. The acmite crystals host inclusions of Na amphibole and are rimmed by retrogressive biotite. On the other hand, phengite from the host rocks of the phyllonite shows a strong core to rim chemical zonation (Si 4+ content ranging from 3.2 in the rim to 3.6 a.p.f.u., respectively). Forward modelling of the shear zone assemblages in the Na 2O CaO K2O FeO MgO Al 2O3 SiO 2 H2O TiO 2 chemical system constrains the Alpine metamorphic climax within the blueschist facies field, with maximum pressure of about 1.2 GPa and temperature lower than 450 °C. Exhumation occurred along a cooling P-T path, as imposed by the calculated Si 4+ isopleths for phengite. A U Pb TIMS study on syn kinematic rutile yields a 3D array with an age of 48 ± 18 Ma (MSWD 7.3) and coexisting acmite phengite and ox/sulph coatings have an intercept at 54 ± 8 Ma (MSWD = 48). The scatter of the arrays is considered to reflect secondary disturbances of the system. Our data indicate that the Variscan basement of Corsica was undergoing blueschist metamorphism in the Early Middle Eocene during the westward directed overthrusting of the Schistes Lustrés nappe onto the European foreland. These new metamorphic and geochronological data provides the first documentation on the onset of the Alpine reworking of Variscan Corsica, and may thus contribute to refine the regional reconstructions of the Alpine orogeny in the Central Mediterranean. 53 10th Alpine workshop "CorseAlp2011" Provenance analysis of synorogenic sandstones from northern Corsica Marco G. Malusà, Sergio Andò, Alberto Resentini, Giovanni Vezzoli, Marta Barbarano, Alessandro Botti, Michele Locatelli & Paola Ragazzo Laboratory for Provenance Studies, University of Milano Bicocca, Mlano, Italy Sedimentary sequences provide an unambiguous documentation of orogen exhumation and accurate constraints on topographic growth through time. In northern Corsica, siliciclastic sedimentary rocks coeval with Alpine orogeny are (i) preserved on the European foreland on top of Variscan basement, (ii) detached from their basement and now accreted in the prowedge, or (iii) folded unconformably on top of Alpine metamorphic units. In order to assess the provenance of detritus during the early stages of orogeny, we collected thirteen sandstone samples from major sedimentary units along a traverse from the Tenda massif to the European foreland. Samples include Eocene synorogenic sediments folded on top of the Tenda massif, Cretaceous to Eocene sediments and metasediments within the Balagne Nappe, and slightly deformed turbidites of the European foreland. They were prepared for a range of laboratory analyses, including bulk petrography, heavy minerals and fission track dating. Bulk petrography analysis (Gazzi Dickinson point counting method) included assessment of metamorphic rank of rock fragments. Heavy mineral analyses (area counting method) was coupled with a characterization of weathering features, and was integrated by Raman spectroscopy to quantify zircons metamictization. Sample preparation for fission track analysis was carried out according to the external detector method. Preliminary results show that bulk composition of synorogenic sandstones is homogeneous, and invariably dominated by quartz, feldspars (mainly plagioclase) and felsic volcanic grains with minor metasedimentary rock fragments. Heavy mineral suites are extremely poor to poor (heavy mineral concentration on bulk sample from 0.01% to 1%). They include apatite, zircon, tourmaline, rutile, epidote, titanite and garnet, in order of abundance. Apatite rims are commonly corroded and show evident etch pits; corroded amphiboles were recorded in a few samples. These observations suggest significant dissolution during diagenesis to low grade metamorphism. Petrographic data are consistent with sediment sourced from the European foreland, with minor supply from the growing Alpine wedge. This may indicate that the axial belt was topographically depressed during the early stages of orogeny, and became emerged and a major source of detritus only during later stages, after that nappe stacking in the accretionary wedge was largely completed. 54 10th Alpine workshop "CorseAlp2011" Are detrital geochronology and stratigraphy necessarily in conflict? The Bregaglia story Marco G. Malusà 1, Igor M. Villa 1,2 , Giovanni Vezzoli 1 & Eduardo Garzanti 1 (1) Università di Milano Bicocca, Milano, Italy (2) Institut für Geologie, Bern, Switzerland Tectonic reconstructions and quantitative models of landscape evolution are increasingly based on detailed analysis of detrital systems. Since the definition of closure temperature in the 1960s, mineral ages of low temperature geochronometers are traditionally interpreted as the result of cooling induced by erosion. Such an approach can lead to infer paradoxically high erosion rates that conflict with compelling geological evidence from sediment thickness in basins. This questions tectonic and landscape models that solely interpret mineral ages as due to cooling during exhumation. To address the paradox, we model the mechanical erosion of an unroofing magmatic complex and the resulting accumulation and burial of the eroded units in reverse order in the basin. Detrital mineral ages follow a regular pattern downsection. Some minerals, e.g. zircons by U Pb, date the crystallization of the magma, yielding constant ages along the stratigraphic column in the sedimentary basin; this we refer to as “stationary age peak”. Some other mineral ages, such as e.g. apatite fission track ages, may vary smoothly with depth, defining a “moving age peak”. This is the only possible effect of undisturbed cooling during overburden removal, and can therefore be used to calculate an erosion rate. Our model was tested on the Bregaglia (Bergell) pluton and on the sedimentary succession derived from its erosion, the Gonfolite Group. It resolves a long standing paradox in quantitative modelling of erosion sedimentation, namely the scarcity of sediment during apparently fast erosion. Starved basins are the observational baseline, and modelling must be tuned to include a correct analysis of detrital mineral geochronology in order to reconcile perceived discrepancies between stratigraphical and geochronological information. Our data demonstrate that volcanoes were active on top of the growing Oligocene Alps. We define rigorous criteria for detrital mineral geochronology that are applicable to any geological setting, providing fundamental interpretive keys to solve complex puzzles and apparent paradoxes in geological reconstructions. REFERENCES Malusà et al. (2011) Earth Planet. Sci. Lett. 301, 324 336. 55 10th Alpine workshop "CorseAlp2011" The Importance of Rift Inheritance in the Evolution of the Alpine Orogen Gianreto Manatschal 1, Geoffroy Mohn 1, Emmanuel Masini 1 & Marco Beltrando 2 [email protected] strasbg.fr (1) IPGS EOST, CNRS UdS, Strasbourg, France (2) Dipartimento di Scienze Mineralogiche e Petrologiche, Università di Torino, Torino, Italy The Alps in Western Europe is one of the best studied collisional orogens in the world. However, despite more than one century of geological investigations, its detailed orogenic evolution remains a matter of debate. In particular the paleogeographic restoration of the Alpine Tethys domain and the nature, width and age of this domain forming the internal parts of the orogen are still debated. In our presentation we will fist review the newest results of the study of the southern North Atlantic and the Pyrenees and discuss how these results may impact the plate kinematic reconstructions of the Alpine domain. In a second part, we will present the structural and stratigraphic evolution of the Alpine Tethys domain and will discuss if this domain developed into a stable mid ocean ridge, comparable to that of present day oceans. In a last part of the presentation, we will address the question of how far the rift inheritance may have controlled the structures and final architecture of the orogen, explain the intimate link between ophiolites and remnants of thinned continental crust and the strong segmentation and diachronous evolution of the mountain belt. The observation that the Alps in Western Europe developed from a complex paleogeographic domain and represent an orogenic collage in which coeval lateral domains evolved at different times may lead to the question if classical “ridge” and “subduction” models can really explain the Alpine system in Western Europe. Although Alpine geologists were among the first who realized that the mountains are the descendents of “oceans” the fact that they didn’t discover plate tectonics may be related to the fact that within the Alpine domain the plate tectonic paradigm may not be applied in the same way like in the Atlantic or Pacific oceans. At present many of the newest discoveries, such as the discovery of exhumed continental mantle and hyper extended crust associated with large sag basins within continental lithosphere, have not yet been fully integrated in the understanding of collisional orogens. It is important to understand how these hyper extended rift systems may control the structural and rheological evolution of collisional orogens and if the resulting orogen may be typified as the classical Alpine type orogen that we can observe in Western Europe. 56 10th Alpine workshop "CorseAlp2011" The Periadriatic Fault east of Val d’Ossola Neil Mancktelow & Jan Pleuger Geological Institute, ETH Zurich, Switzerland The Periadriatic Fault (PF) is the most prominent late fault in the Alpine chain and is exposed over some 700 km strike length from west of Torino in NW Italy to Slovenia. Here we consider the extension of the fault east of Val d’Ossola, as a complement to the presentation of Pleuger et al. (this volume) on the Canavese Fault to the SW. The PF crosscuts both nappe contacts and earlier shear zones at a low angle and these pre existing structures are bent asymptotically into near parallelism with the PF. As a result, it is not straightforward to distinguish between passively rotated or reactivated earlier structures and shearing related to activity on the PF itself. The PF generally consists of two distinct zones of localized shear and strong foliation development (cf. Schmid et al. 1987), a northern mylonite zone (NMZ) and a southern mylonite or ultracataclasite zone (SMZ), separated by a relatively discrete surface (the Periadriatic “Line”: PL). The NMZ and SMZ are developed exclusively from the respective northern or southern block protoliths: there is intense mixing of units within the zones, due to the strong shearing, isoclinal folding (with axes parallel to the stretching direction) and boudinage of layering, but not across the PL itself. Movement on the PF is generally oblique dextral plus north side up, quite consistently in the NMZ but often with a distinct change in the predominant movement direction across the PL into the SMZ, where the vertical component is typically smaller (more dextral strike slip) and locally reversed (i.e. south side up). The SMZ commonly contains sheared and boudinaged lenses of the Mesozoic sedimentary cover of the Southern Alps interleaved with upper crustal, mainly granitic, basement with a (very) low grade of metamorphism. In the area of the Lepontine Metamorphic Dome (i.e., from Val d’Ossola to Valtellina), there is a marked jump in peak metamorphic grade across the PL, with the higher peak metamorphic grade on the northern side reflecting the relative exhumation of this block. In the SMZ, shearing is prograde, in the NMZ it is retrograde, with shearing progressively localizing toward the PL as the northern block exhumed and cooled. As a result, the PF shear overprint is more gradual in the NMZ; in the SMZ the “fabric boundary” to little deformed southern block protolith is typically quite distinct and may be marked by a narrow zone of low grade intensified shearing, ultracataclasites and pseudotachylyte. West of Locarno, it is also the boundary between the mylonitic upper crustal granitic basement and low grade sedimentary lenses (referred to as the “Canavese Zone” west of Val d’Ossola) and the lower crustal Ivrea Zone. Three phases of folding of the PF mylonitic foliation Sm can be distinguished throughout the zones. The first is isoclinal with fold axes parallel to the stretching lineation and with a (weaker) new mylonitic foliation axial plane; these folds clearly developed during ongoing shearing and mylonitization. The later two phases are more open and with more upright axial planes. The fold axes and axial planes of the two phases make a large angle to each another, so that the interference pattern has a Type 1 dome and basin geometry. The more pervasively developed (and probably older phase) makes a small to moderate angle in an anticlockwise sense with the trend of the PF. Folds in Sm, both in the NMZ and SMZ, have a general vergence indicating an antiform to S or SE. In the Arcegno to Finero region, the PL is also folded (e.g., Steck and Tièche 1976). However, in Arcegno, the folds in Sm are obliquely transected by younger shearing and faulting localized on the SMZ southern boundary to the little deformed Ivrea Zone. To the east of Locarno, the PL is marked by a steep foliated cataclasite and gouge zone ca. 1 2 m thick that appears to be younger and shows no evidence of being folded. West of Locarno, this late brittle fault could have its continuation as either the Centovalli Fault or the southern boundary to the SMZ. Dextral Riedel faults with a (W)NW (E)SE strike are both transected by the PL and locally offset it. Sinistral antithetic Riedel faults with a NNE SSW strike are regionally developed but generally less obvious: the major exception is the important Guidicarie Fault in the Eastern Alps, which offsets the PL by some 15 20 km. Syn magmatic shearing of the Bergell tonalite, Prasenella tonalite, and many smaller dykes establishes that the PF was already active around 32 Ma. Early Miocene pseudotachylyte (Müller et al. 2001) and fault gouge (Zwingmann and Mancktelow 2004) ages indicate a later period of enhanced activity, synchronous with regional orogen parallel extension and major activity on the Simplon and Brenner low angle normal faults, around 20 17 Ma. In Val d’Ossola, zircon fission track ages of 9 10 Ma do not show any change across the PF (Keller et al. 2005) and the PF is not currently seismically active. REFERENCES Keller, L.M., Hess, M., Fugenschuh, B., Schmid, S.M., 2005. Eclog. Geol. Helv. 98, 19 49. Müller, W., Prosser, G., Mancktelow, N.S., Villa, I.M., Kelley, S.P., Viola, G., Oberli, F., 2001. Int. J. Earth Sci. 90, 623 653. Pleuger, J., Mancktelow, N., Spikings, R. (this volume). Schmid, S.M., Zingg, A., Handy, M., 1987. Tectonophysics 135, 47 66. Steck, A., Tièche, J.C., 1976. Schweiz. Mineral. Petrogr. Mitt. 56, 501 512. Zwingmann, H., Mancktelow, N., 2004. Earth Planet. Sci. Lett. 223, 415 425. 57 10th Alpine workshop "CorseAlp2011" Permian HT Metamorphism in Austroalpine Units of the Internal Western Alps Paola Manzotti 1, Brigitte von Niederhaeusern 1, James Darling 1, Michele Zucali 2, Daniela Rubatto 3 & Martin Engi 1 (1) University of Bern, Bern, Switzerland (2) Dipartimento di Scienze della Terra “Ardito Desio”, Università degli studi di Milano, Milano, Italy (3) The Australian National University, Mills road, Canberra, Australia The Internal Western Alps consist predominantly of continental terranes, which were assembled during the oblique Alpine convergence of S Europe and the thinned Apulian margin. Terranes derived from Apulia are termed Austroalpine units; these are well exposed in the Sesia Zone, the Dent Blanche nappe, and a few smaller tectonic outliers that may derive from the Apulian OCT (ocean continent transition). Basement rocks are dominant in all of these units, and Permian intrusive masses are widespread. The present study aims to characterize specific metamorphic stages, by combining detailed microstructural and petrologic analysis of select samples with U Th Pb dating of zircon, allanite, monazite and titanite. In the basement units, Permian and Carboniferous ages dominate. Examples presented here are chosen to document regional similarities, from SW Sesia to the Dent Blanche, i.e. all across the well established Austroalpine belt. In the Dent Blanche nappe (polymetamorphic Valpelline basement) granulite facies assemblages are well developed in paragneisses (grt+sil+bt+pl+qtz) associated with large migmatite domains. In this unit, chemical (EMP) Th U Pb dating of monazite yields ages from 280 to 350 Ma: these ages indicate Variscan metamorphism (~350 Ma) prior to the Permian high temperature evolution also marked by Permian intrusives (e.g. Matterhorn and Collon Gabbros). Tightly folded meta cherts and carbonates are amalgamated in the Arolla basement, e.g. in the Cuney and Becca di Salé areas. These metasediments contain pre Alpine relic mineral assemblages, developed under amphibolite facies conditions (cal+qtz+grt+di+ttn+aln in siliceous marbles; qtz+grt+mn cum+mag+aln+ap+zrn in meta cherts). SHRIMP data from titanite in siliceous marbles reveal two generations at 280±6 Ma and 260±3 Ma, whereas allanite ages from meta cherts are around ~290 Ma. In the SW Sesia zone (Valle di Ribordone), the Seconda Zona Diorito Kinzigitica (II DK) surfaces in lenses along the boundary between the Gneiss Minuti (GM) and the Eclogitic Micaschist Complex (EMC). We performed U Th Pb dating on zircon in a metapelite from a II DK lens situated between Orco and Soana valley. Metamorphic zircon rims that can be clearly related to HT metamorphic conditions (Ti in zircon thermometry: 800 850°C, trace element patterns indicating the presence of plagioclase and garnet), yield uniform LA ICP MS U Pb ages of 270 ± 3 Ma . Zircon ages from an eclogitized amphibolite within the EMC near Pont Canavese yield LA ICP MS U Pb ages of 276 ± 6 Ma. Textures of the dated zircons indicate that they crystallized at HT conditions. Trace element analysis is expected to further constrain the growth conditions of these grains. While a definitive interpretation of these isolated age data is certainly premature, the majority of our analysis from several localities dates Permian metamorphism, which is remarkable, and demands a discussion in the regional framework. Permian magmatism is well established in many parts of the Alps, with mafic intrusives (mostly gabbros) and more evolved types (e. g. Mucrone granodiorite, Arolla granite) occurring in the internal Western Alps. However, the widespread and diverse settings of the dated HT metamorphism is unlikely to represent contact metamorphism. Regional metamorphism in the Permian is now well established in the Eastern Alps (Schuster and Stüwe, 2008). In the Western Alps, Permian metamorphism has been suggested (e.g. Lardeaux and Spalla, 1991), but without reliable age data, its effects are difficult to discern in Variscan and older rocks, which generally reached upper amphibolite facies to migmatitic grade in the corresponding Austroalpine units. Certainly the model of Permian rifiting producing a high heat flow regime that caused large scale magmatic underplating and LP/HT metamorphism, as advocated by Vavra et al. (1996), Hermann and Rubatto (2003) and Schuster and Stüwe (2008), deserves further attention and scrutiny in the Western Alps. REFERENCES Lardeaux J.M., Spalla M.I., 1991. From granulites to eclogites in the Sesia Zone (Italian Western Alps): a record of the opening and closure of the Piedmonte ocean. Journal of metamorphic geology, 9, 35 59. Vavra G., Gebauer D., Schmidt R., Compston W., 1996. Multiple zircon growth and recrystallization during polyphase Late Carboniferous to Triassic metamorphism in granulites of the Ivrea Zone (Southern Alps): an ion microprobe (SHRIMP) study. Contributions to Mineralogy and Petrology, 122, 337 358 Hermann J., Rubatto D., (2003). Relating zircon and monazite domains to garnet growth zones: age and duration of granulite facies metamorphism in the Val Malenco lower crust. Journal of metamorphic geology, 21, 833 852 Schuster R. & Stüwe K., 2008. Permian metamorphic event in the Alps. Geology, 36(8): 603 606. 58 10th Alpine workshop "CorseAlp2011" Basement-Cover Relations in the Dent Blanche Klippe: Strong Strain Partitioning Helps Preserve a Complex Polyphase Evolution Paola Manzotti 1, Michele Zucali 2, James Darling 1, Daniela Rubatto 3 & Martin Engi 1 (1) University of Bern, Bern, Switzerland (2) Dipartimento di Scienze della Terra “Ardito Desio”, Università degli studi di Milano, Milano, Italy (3) The Australian National University, Canberra, Australia The Dent Blanche s.l. klippe is a large thrust sheet of the Austroalpine system (Western Italian Alps), now part of the axial Alpine chain. A km scale ductile shear zone within the Dent Blanche s.l. klippe separates two tectonic slices: the Dent Blanche s.s. and the Mont Mary thrust sheets. Both these two elements classically comprise the Valpelline and the Arolla units. The former consists of metapelites, mafic and carbonate rocks with a dominant metamorphic imprint under amphibolite to granulite facies conditions of pre Alpine age. The Arolla unit is mainly composed of Permian intrusives (granite, diorite, and gabbro), all metamorphosed and deformed to orthogneisses or schists during the Alpine evolution. Remnants of a sedimentary cover (Roisan Zone) of unknown origin are discontinuously exposed along the kilometric shear zone. The cover, classically considered monometamorphic, is dismembered into metre to hectometre scale bands and pods of dolomite marble, calcschists and quartz micaschists and it is amalgamated with the polydeformed Arolla basement. Due to high strain, geometric relationships within the lithostratigraphic sequence of the cover appear progressively and tectonically modified, and the successions of sedimentary layers are severely altered. Transposition leads to the development of a new orientation of “bedding” and pervasive tectonic contacts. Within the shear zone we discovered that some of the metasediments (i.e. calcite marble and meta chert), classically considered monometamorphic, are pre Alpine and preserve relic mineral assemblages developed under pre Alpine amphibolite facies conditions. SHRIMP and LA ICP MS U Pb analyses of titanite, allanite and zircon confirm a Permian age. Along the ductile shear zone the Arolla basement, mono and poly metamorphic sediments show a complex tectono metamoprhic history with four Alpine stages of evolution. The pre Alpine history, recognized in the Arolla basement and in the pre Alpine tectonic slices, is related to Permo Triassic lithospheric thinning and pervasive intrusion of magmas. The Alpine evolution initially developed under blueschist facies condition. This was followed by strong greenschist facies retrogression, largely overprinting the blueschist assemblages. In the area studied, only sparse evidence remains of a relatively HP imprint (in Arolla basement and in pre alpine metasediments); particularly in shear zones at the basement cover contact. The tectonic incorporation of pre Alpine tectonic slices and of a monometamorphic cover (Roisan Zone) in the polydeformed basement of the Arolla unit occurred prior to or coeval with the blueschist stage metamorphism, as shown by abundant structural evidence and petrographic relations. This complex pre Alpine and Alpine history is well preserved thanks to strong strain partitioning, which allows the evolution of local structures and assemblages to be correlated. This study shows how slices of metasediments of different age and origin, reflect a tectonic amalgamation with basement slices during the evolution of an orogenic belt. 59 10th Alpine workshop "CorseAlp2011" The Tectono-sedimentary Signature of Distal Rifted Margins: the Example of the Adriatic Margin Preserved in the Lower Austroalpine Nappes in SE Switzerland Emmanuel Masini Gianreto Manatschal & Geoffroy Mohn [email protected] strasbg.fr EOST/IPGS, Strasbourg, France Alpine geologists significantly contributed to the understanding of the tectonic and sedimentary processes leading to “Atlantic type” rifted margins. Major contributions were the discovery and first description of fault bounded rift basins. More recently the discovery of low angle detachment faults in distal domains of both present day and Alpine margins have shown that the continental lithosphere can be extremely extended before the onset of oceanic accretion. This architectural change from high angle to low angle faulting reflects a temporal and spatial change in the mechanical response of the continental lithosphere to extension. One of the main consequences of the onset of low angle detachment faulting is the development of supra detachment basins from which quasi nothing is known considering their tectonic, stratigraphic and sedimentologic evolution. In this study we present the tectono sedimentary evolution of a supra detachment basin belonging to the Adriatic distal rifted margin exposed in the Err nappe in SE Switzerland. Based on detailed mapping of the syn tectonic sedimentary sequence and its relation to a well preserved rift related detachment system it is possible to understand the structural and sedimentary elements that define the tectono sedimentary signature of a supra detachment basin. The restoration of the pre Alpine structures show that the detachment system is overlain by extensional allochthons consisting of upper crustal rocks and pre rift sediments. The extensional allochthons are up to several kilometres long and several hundreds of meters thick and compartmentalize the distal margin into sub basins, one of which is the Samedan Basin. This sub basin can be restored over a minimum distance of 11 km along and about 4 km perpendicular to the basin axis. Its sedimentary infill can be subdivided into 3 facies tracts corresponding to evolutionary steps of the basin. The two first tracts, the basal and intermediate tracts record the onset of opening and widening of the basin due to active exhumation at the basin floor. In contrast, the top facies tract (3) records the migration of deformation further outboard with a drastic retrogradation of the turbiditic system. The sediments show an evolution from (1) locally derived, poorly sorted gravity flow deposits that show a progressive change from hanging wall to footwall derived lithologies (tract 1) to (2) turbidity current deposits sourced by tectonic driven processes (tract 2) to (3) a starvation of the basin after migration of the deformation further outboard. Thus, the syn tectonic sedimentary record documents a thinning and fining upward sequence related to the back stepping of the sediment source, a progressive starvation of the sedimentary system and migration of deformation resulting in exhumation and progressive delamination of the thinned crust during final rifting. This study provides not only new constrains for assessing the tectono sedimentary processes in distal rifted margins during final rifting but suggests also that supra detachment basins of distal domains can present a specific tectono sedimentary signature. Another well preserved example is the Apto Albian Mauléon rift basin in the Western Pyrenees, were similar relationships between syn tectonic sediments and a detachment system can be mapped over an hyper extended continental crust. The existence of comparable facies in much more deformed Alpine units in more internal parts of the Alps (e.g. Sesia, Canavese, Dent Blanche, Rochebrune ) leads to the question if these settings may have undergone the same rift evolution and may be interpreted in the same way like the much better preserved Samedan and Mauléon basins. More detailed mapping of these tectono sedimentary systems may not only help to better understand 60 10th Alpine workshop "CorseAlp2011" Subduction-unrelated subduction-related magmatism in Northern Apennines. The Early Oligocene Aveto-Petrignacola volcaniclastic succession Michele Mattioli, Michele Lustrino, Sara Ronca & Gianluca Bianchini Dipartimento di Scienze della Terra, Università degli Studi di Roma “La Sapienza”, Roma, Italy During the Early Oligocene, the ~800 m thick Aveto Petrignacola volcaniclastic Formation (APF) was deposited in a relatively short time (~32 29 Ma) in the Northern Apennines. The APF volcanic rocks are mostly basaltic andesites, andesites and dacites, with minor rhyolites and gabbroic compositions found as pebbles ~0.001 0.5 m 3 in size (with average diameter ranging from ~5 to 30 cm). This volcaniclastic succession is interpreted as the product of subaerial effusive to explosive arc magmatism and closely associated turbidite sedimentation. Petrographic, mineral chemical, major and trace element content, as well as Sr Nd Pb isotopic ratios are all compatible with a calc alkaline magmatism generated as consequence of the metasomatic modifications related to the subduction of oceanic lithosphere. The APF volcanic rocks are a puzzle in the geodynamic evolution of the Italian area. This Formation is now part of the Apennine thrust system, developed as consequence of the West directed subduction system that involved recycling of the oceanic lithosphere of the Mesogean/Liguride ocean(s) beneath the southern paleo continental margin of Europe. Such a subduction system was associated with production of abundant calc alkaline/arc tholeiitic magmatism starting from Late Eocene/Early Oligocene in SE Spain (Malaga arc tholeiitic dykes), Sardinia (e.g., Calabona microdiorite) and SE France (Esterel microdiorite). During the Middle Late Eocene, the Alpine Tethys subduction system had already ceased, followed by Adria Europe continental collision. In other words, during the emplacement of the APF subduction related volcanic rocks, the Alpine subduction system was already shut down, whereas the Apennines subduction system was fully developing. Notwithstanding this, we propose that the APF volcanic rocks are genetically connected to the Alpine subduction system rather than the Apennines. We propose that the APF volcanic rocks are generated by a mantle source modified by the Alpine slab detached during Middle Late Eocene, wandering beneath the Adriatic lithosphere after the closure of the Piedmontese (Liguride) Ocean. The effects of the “zombie” slab would be a metasomatic modification of the original peridotitic matrix, allowing hydratation with the consequent formation of amphibole. Local effects of stress release in the Apennines foreland and in the Alps hinterland would have played an important role to trigger magmatism. Paradoxically, the subduction related igneous activity in the APF area developed several Myrs after the cessation of oceanic lithosphere subduction in an area that, few Myr later, would have been involved in a completely different orogenesis (Apennines tectonics). In summary, the APF volcano (now completely eroded) would have been generated as consequence of Alpine tectonics, but would have been involved in the Apennines thrusts soon after its formation. According to this view, the APF and the Mortara volcanoes, the latter now buried beneath the Po Plain, would have the same origin. The formation of the Po Plain (representing the foreland of the Apennines and the retro foreland of the Alps back thrusts would have prevented the Mortara volcano from being eroded, leaving it in its original position on the north western margin of Adria. 61 10th Alpine workshop "CorseAlp2011" Did the Metamorphic Fluid Flow Trigger Thin-skinned Thrusting? Rastislav Milovský 1, Roberta Prokešová 2 & Dušan Plašienka 3 (1) Geological Institute, Slovak Academy of Sciences, Banská Bystrica, Slovakia (2) Research Institute of Matej Bel University, Banská Bystrica, Slovakia (3) Department of Geology and Palaeontology, Comenius University, Bratislava, Slovakia Western Carpathian alpine tectonics is characterized by thick north vergent basement involved thrusts (Tatricum, Veporicum, Gemericum) overridden by a stack of thin skinned nappes (Fatricum, Hronicum, Silicicum etc. units). The latter are thin (up to ca. 5km) sheets of various sedimentary lithologies and of various rheostratigraphic properties. Krížna nappe of the Fatricum unit is uniquely preserved in whole its transverse, from root area in the S to the front in the N. Its variety of tectonic styles with typical polarity follows from its lithologic richness: Krížna nappe is typical rheological multilayer, with several rigid members and several weak horizons, the weakest being principal detachment plane. Our study focuses on polymict carbonatic cataclasites occuring on the basal thrusting plane in southern (rear) part of Krížna nappe, where it overlies the anchizonally metamorphosed Tatricum sediments. Ubiquitous chaotic hydraulic fracturing as well as our p T data imply their extreme weakness and we have evidences, that they represent remnants of “fluidized pillow”, that acted as transport medium of the nappe body. Basal breccias contain fragments of both substrate and nappe lithologies, authigenic quartz, albite, pyrite, dravite and apatite, cemented by newly precipitated calcite. The newly formed minerals crystallized synkinematically and they enclose abundant crystals of anhydrite and fluid inclusions (FI), which were investigated by means of microthermometry to constrain chemical properties of included fluids and trapping p T conditions at time of thrusting. We studied both primary and secondary inclusions, as they behave very similarly and presumably refer to the same tectonic process. Temperatures of phase transitions in fluid inclusions containing brine, vapour and halite, indicate presence of NaCl KCl CaCl 2 MgCl 2 in brine with overall salinity of 29 to 42 wt.% of NaCl equivalents. Final homogenization temperatures by halite dissolution and corresponding pressures (equations of Brown and Lamb 1989) are stretched over unusually large pT field between 193 and 344°C and 0.25 to 4.03 kbar (Fig.1) The lowest limit seems to correlate well with diagenetic imprint in Krížna nappe, while the highest extremities are highly anomalous under a rock pile of maximum thickness of 3 4 km and it may reflect the low grade metamorphic conditions in underlying Tatricum cover and basement unit. The whole datafield is distinctly elongated in direction roughly parallel to isochores of H 2O NaCl system, and so are the datafields of individual localities and even FI populations. Here, two trends were discerned: A) “transverse” nearly isochore parallel trend, and B) “vertical” nearly isothermic trend. As we do not have time series of p T data (the FI are measured in isolated quartz grains without known growth succession), we only hypothesize about possible courses of these two trends. We propose following scenario: ascending hot and pressurized metamorphic fluids from Tatricum basement were chanellized by shear zones and they epizodically invaded basal formations, where porous fluids circulated at ambient conditions. Mixing of these fluids, their overheating and thermal pressuring effectively weakened basal rocks, and possibly triggered thrusting events when trespassing rock strength. Then the transverse trend would be nearly isochoric, prograde mixing/thermal pressuring, the peak value represents maximum sustainable fluid pressure and vertical trend with downwoard arrow means adiabatic decompression, following fluid leak and collapse after shear failure. Textures of polyphase brecciation suggest, that the basement fluid injections may have been looped into recurrent pulses, triggering partial thrust displacements. Fig. 1 Pressure calculated at temperatures of total homogenization according to equations of Brown and Lamb (1989) for NaCl-H2O system. Dashed curves are isochores in one-phase liquid field. Shaded area - overall datafield for all studied samples, points - example of two prevailing p-T trends in FI populations: open symbols - „transverse“ trend, solid symbols - „vertical“ trend. Acknowledgements: This research was supported by the grants No. APVV-SK-HU-0015-08 and APVV-51-008305 of the Slovak Research and Development Agency, and the Operational Programme Research and Development through the project “Centre of Excellence for Integrated Research of the Earth's Geosphere” (ITMS: 26220120064), co-financed by ERDF. REFERENCES Brown P. E and Lamb W. M. 1989: P V T properties of fluids in the system H2O±CO2±NaCl: New graphical presentations and implications for fluid inclusion studies. Geochim. Cosmochim. Acta 53, 1209 1221. 62 10th Alpine workshop "CorseAlp2011" Rift-Related Inheritance in Alpine Type Orogens: Evidence From the Austroalpine Nappes Geoffroy Mohn 1, Gianreto Manatschal 1, Emmanuel Masini 1, Marco Beltrando 2 & Othmar Müntener 3 [email protected] strasbg.fr (1) IPGS EOST, Université de Strasbourg, Strasbourg, France (2) Dipartimento di scienze Mineralogiche e Petrologiche, Università di Torino, Torino, Italy (3) Institute of Mineralogy and Geochemistry, University of Lausanne, Lausanne, Switzerland Studies conducted in many present day rifted margins revealed a more complex and diverse crustal architecture than previously thought. Refraction seismic data show that crustal thickness decreases dramatically along the so called “necking zone”, which marks the transition from ”original” crustal thickness (± 30 km) at the proximal margin to hyper extended crust (≤ 10 km) at the most distal parts. This complex crustal architecture inherited from the rifting phase will play a major role during reactivation of the passive rifted margin in compressional belts. In the Alps, remnants of the ancient proximal and distal rifted margins are well described. However relics of the necking zones remain still unidentified. The Austroalpine and Upper Penninic nappes in SE Switzerland and N Italy preserve the remnants of a complete section across the former Adriatic rifted margin. In our study, we focused on the Bernina and Campo Grosina nappes located in between remnants of the distal margin (Lower Austroalpine) and proximal margin (Upper Austroalpine). Based on mapping combined with structural and thermochronological methods, we show that the Campo/Grosina nappes can be interpreted as remnants of a former necking zone that comprised pre rift upper and middle crust associated with Jurassic high strain shear zones (e.g. Eita shear zone, Grosina detachment). The Bernina nappe represents a relic of the Adriatic distal margin, characterized by the occurrence of Jurassic low angle detachment faulting associated with extensional allochthons overlain by deep water syn to post rift sediments. The better understanding of the pre Alpine geometry enable us to propose a tectonic evolution of the Austroalpine nappes in SE Switzerland and N Italy that links inherited rift related structures with present day Alpine structures (Fig. 1). Our results show that most of the late Cretaceous Alpine thrust faults result from the reactivation of rift related Jurassic structures. Three main nappe systems can be defined in the Austroalpine system (Fig. 1): (1) The Upper Austroalpine nappe system derived from the proximal margin; (2) the Middle Austroalpine nappe system derived from the necking zone; and (3) the Lower Austroalpine nappe system derived from the distal margin. We interpret the Middle Austroalpine Campo Grosina nappes as an antiformal stack controlled by the geometry of the former necking zone and the Lower Austroalpine Bernina nappe as a synform (Fig. 1). This geometry may explain the relative good preservation of the Mesozoic sediments in the Lower Austroalpine nappes. Our results highlight that many of the complexities of the Austroalpine nappes are resulting from the strong structural inheritance of the Adriatic rifted margin. On the scale of the Alpine Tethys margin, we suggest that the necking zones of the former rifted margins may have acted as buttress during Alpine collision. Consequently we propose that the necking zones of the former rifted margins may represent major Alpine tectonic boundaries separating external (weakly reactivated) from internal part of the Alps (subducted or/and incorporated into the orogenic prism). Figure 1: Tectonic, large scale restoration of the Austroalpine and Upper Penninic nappes in SE-Switzerland and N-Italy 63 10th Alpine workshop "CorseAlp2011" Multi-stage evolution of the External Liguride mantle section (Northern Apennine ophiolites) Alessandra Montanini 1, Riccardo Tribuzio & Matthew F. Thirlwall 3 (1) Dipartimento di Scienze della Terra, Università di Parma, Italy (2) Dipartimento di Scienze della Terra, Università di Pavia, Italy (3) Department of Geology, Royal Holloway, University of London, UK The External Liguride mantle bodies represent subcontinental litosphere exhumed in Jurassic times at an ocean continent transition during the opening of the Ligurian Tethys basin (Rampone et al., 1995; Montanini et al., 2006). They are associated with ophiolitic rocks formed in this pericontinental setting. The mantle exhumation occurred through high temperature ductile deformations developed along hectometre size shear zones, followed by a polyphase brittle deformation under decreasing temperature conditions, coupled with hydration and formation of serpentine carbonate cataclasites (Montanini et al., 2006).The mantle rocks include (i) porphyroclastic spinel tectonites with limited re equilibration in the plagioclase facies; (ii) plagioclase facies peridotite mylonites with spinel tectonite relics, (iii) coarse granular peridotites enriched in plagioclase. The spinel facies assemblage of the peridotites is characterized by Na 2O and Al 2O3 rich clinopyroxene with slight LREE depletion, spinel with low Cr# and disseminated Ti rich amphibole. The peridotites (i) locally preserve highly depleted Nd Sr isotopes coupled with Proterozoic Os model ages (Rampone et al., 1995; Snow et al., 2000). The peridotite plagioclase mylonites (ii) are characterized by Fe rich garnet clinopyroxenites, sometimes graphite bearing (Montanini et al., 2010) and opx rich websterites with textural relics of Mg rich garnet. The garnet clinopyroxenite layers record an early equilibration stage at deep lithospheric levels (T~1150°C and P~2.8 GPa). Major and trace element composition of minerals and bulk rocks indicate that the garnet clinopyroxenites are not simple HP cumulates from liquids of a peridotite source. They were likely derived from melting and redistribution of melts of recycled gabbroic protholiths in the garnet stability field. Nd Hf isotope data suggest that crustal recycling may be related either to (1) post Variscan crustal delamination or (2) to Mesozoic rifting inception and asthenospheric ascent which triggered melting of mafic heterogenities. The websterite origin could be attributed to peridotite replacement by olivine undersaturated melts derived from recycled mafic rocks and/or to magmatic replacement of pre existing garnet clinopyroxenite layers by asthenospheric melts under spinel facies conditions. A later infiltration of silica undersaturated (alkaline) melts under spinel facies conditions is testified in both websterites and peridotites by opx consuming reactions associated with crystallization of Ti rich amphibole + olivine ± clinopyroxene ± Ni rich sulphides. The plagioclase enriched peridotites (iii) are characterized by large plagioclase patches replacing spinel, Cpx porphyroclasts replaced by orthopyroxene + plagioclase, and gabbroic microgranular pods as a result of percolation and impregnation by MORB type melts in the plagioclase stability field. They were eventually intruded by MORB type gabbroic rocks and basalts (Montanini et al., 2008). Clinopyroxenes from these peridotites have convex upward REE patterns, Eu Sr negative anomalies and significant REE and Zr enrichment with respect to the porphyroclastic clinopyroxenes of rocks unaffected by impregnation. Their geochemical features were most likely due to interaction with low degree fractional melts of a MORB type source (Piccardo et al., 2004). As a whole, the External Liguride peridotites were considered to represent relatively fertile, MORB type mantle accreted to the lithosphere in Proterozoic times. However, the widespread occurrence of different types of pyroxenites, the diffuse amphibole formation and the evidence of interaction with asthenospheric melts during the shallow evolution, suggest that fertile nature of the External Liguride mantle bodies may be the result of multiple refertilization events under decreasing P T conditions, during and/or before their emplacement, as recently proposed for other orogenic peridotite massifs (Le Roux et al. 2007; Soustelle et al. 2009). REFERENCES Le Roux V., Bodinier J.L., Tommasi A., Alard O., Dautria J.M., Vauchez A., & Riches A.J.V. (2007): Earth Planet. Sci. Lett., 259, 599 612. Montanini A., Tribuzio, R. & Anczkiewicz, R. (2006): J. Petrol., 47, 1943 1971. Montanini A., Tribuzio, R. & Vernia L. (2008): Lithos 101, 453 479. Montanini A., Tribuzio, R. & Bersani D. (2010): Geol. Soc. London, Spec. Publ. 337, 87 105. Rampone, E., Hoffmann, A.W., Piccardo G.B., Vannucci, R., Bottazzi P. & Ottolini L. (1995): J. Petrol., 36, 81 105. Piccardo G.B., Müntener O., Zanetti A. & Pettke, T. (2004): Int. Geol. Review, 46, 1119 1159. Snow J.E., Schmidt G. & Rampone E. (2000): Earth Planet. Sci. Lett., 175, 119 132. Soustelle V., Tommasi A., Bodinier J. L., Garrido C. J. & Vauchez A. (2009): J. Petrol., 50, 1235 1266. 64 10th Alpine workshop "CorseAlp2011" Late Neogene Tectonics in the Northern Tyrrhenian Sea-Northern Apennines System: a Clue from new Seismic and Structural Data Giovanni Musumeci 1, Francesco Mazzarini 2 & Federico Sani 3 (1) Dipartimento Scienze della Terra, Università di Pisa, Italy (2) Istituto Nazionale Geofisica e Vulcanologia, Pisa, Italy (3) Dipartimento Scienze della Terra, Università di Firenze, Firenze, Italy Many reconstruction of the northern Apennines Neogene history base on the interpretation of northern Tyrrhenian Sea as back arc basin developed in an extensional setting due to the slab roll back of the westward subducted Adria lithosphere. This postulate is mainly rooted in the assumption that (i) late Miocene Pliocene sedimentary basins are syn rift sequences and (ii) eastward migration of sedimentary basins and magmatism coeval with advancing thrusts at the front of the chain (Padan Adriatic area). A revision of geological and geophysical data in the northern Tyrrhenian Sea highlight some critical points: (i) late Miocene Pliocene sedimentary successions show flat lying attitude and very small thickness (few milliseconds on seismic lines). These features are not consistent with their interpretation as syn rift deposits; (ii) low angle structures interpreted as detachment faults are shallow crustal level faults that mainly accommodate eastward horizontal displacements and their footwall rocks are not deep units exhumed by fault activity. (iii) high angle angle normal faults have offsets on the order few hundred meters (< 500m) that do not account for large amount of extension; New geophysical data based on reprocessing and re interpretation of the seismic line CROPM12A, reveal that throughout the northern Tyrrhenian Sea (from Corsica basin to Punta Ala basin) the tectonic structures always dip toward west and correspond to thrust planes and thrust anticlines, affecting both the acoustic basement (northern Apennines tectonic units) and the Late Tortonian Early Pliocene sedimentary basins. These latter, always located in synforms, show growth structures that feature progressive unconformities with migration of the basin depocentres and can be regarded as thrust top basins in the hanging wall of basement thrusts. Moreover, large magmatic bodies, imaged in seismic lines as isotropic bodies, are located at the core of large scale antiforms or in the footwall of the thrust planes. New geological data derive from metamorphic units in the southeastern Elba island, few kilometres north of the seismic line, where high to medium grade hornfels rocks (Porto Azzurro pluton contact aureole, 6.2 Ma) and late Miocene magmatic sills are both folded by a N S striking antiform (Calamita anticline; Cruden et al., 2009). This structure, cored by high grade hornfels rocks, is a large scale asymmetric fold overturned toward the east with a gently to moderately dipping backlimb and a steeply dipping forelimb. Westward dipping ductile and brittle reverse shear zones widely exploited by synkinematic magmatic sills correspond to forelimb and backlimb thrusts that led to eastward stacking of the contact aureole and inversion of metamorphic zoneography (Mazzarini et al., 2010 , Musumeci and Vaselli, 2010). Thus, the large scale Calamita anticline and reverse shear zones represents the along strike onshore continuation of the thrust anticlines recognised along the seismic line. These new geophysical and geological data suggest for the northern Tyrrhenian Sea northern Apennine system a late Miocene early Pliocene compressive setting, in marked contrast with the extensional setting in the southern Tyrrhenian Sea. In this scenario, out of sequence thrusts in the inner (Tyrrhenian) side of the northern Apennines provided efficient pathways for the emplacement of late Miocene early Pliocene intrusive bodies. The northern Tyrrhenian Sea therefore corresponds to a sector in which thinning of the lithospheric mantle and asthenosphere upwelling (40 km) were coeval with shortening in the upper crust (depth 6 7 km). This configuration requires a mechanical decoupling of a stronger upper crust from a weaker, thermally softened lower crust and upper mantle, thereby allowing the upper crust to sustain compressive stresses. Thermal anomaly due to the asthenospheric wedge results in a modification of lithospheric mantle lower crust rheology (thermal softening) thus favouring the development of magmatism whose crustal and mantle derived products were emplaced at upper crustal level along the active tectonic structures. On these basis we highlights the complexity of the Apennine lithospheric structure in the northern and southern regions of the Tyrrhenian Sea, that cannot be explained by simple models of westward subduction along a continuous slab. REFERENCES Cruden A.R., Mazzarini F., Bunger A.P., Musumeci G., 2009, Eos Trans. AGU, 90(52), Fall Meet. Suppl., Abstract T13A p. 1843. Musumeci, G., Vaselli, L., 2010. Società Geologica Italiana, 85° Congresso nazionale, Rendiconti online, 11, 342. Mazzarini, F., Musumeci, G., Cruden, A.R., 2010. LASI IV Conference, Moab and Mount Hillers, Utah (USA), Abstract, 114 115. 65 10th Alpine workshop "CorseAlp2011" Present Day Kinematics of the Mediterranean: a Synthesis Jean Mathieu Nocquet GéoAzur CNRS OCA IRD University of Nice, France Spatial Geodesy now enables us to monitor the present day Earth’s surface at the sub millimeter per year level accuracy in a global frame. In the Mediterranean, recent development of continuously recording GPS networks (CGPS) and GPS surveys (SGPS) carried out since the 90’s now provide new pictures of the current crustal deformation of the plate boundary zone separating the Nubia plate and the Eurasia plate, enabling us to discuss how the convergence between the Nubia and the Eurasia plates is accommodated across the Mediterranean region. Latest studies of the convergence between the Nubia plate towards Europe (e.g. Nocquet et al., 2006) agree with an Euler pole located off shore west of Equatorial Africa. Such an Euler pole predicts a convergence rate of 6 mm/yr in a ~N25°W direction in the eastern Mediterranean (longitude 34°E), which decreases slowly and becomes more oblique westward. Geodetic based estimates show more oblique and slower convergence rate than estimates derived from magnetic anomalies and plate circuit closure, which average the motion over the last 3.16 Ma (DeMets et al., 2010), therefore suggesting a slight recent change in the Nubia/Eurasia convergence. While the Nubia/Eurasia relative motion is taken up by a narrow zone (< 50 km) in the Atlantic ocean from the Azores triple point to the Gibraltar strait, both the distribution of seismicity and the geodetic velocity field indicate that the area where the relative motion is taken up becomes spread over hundreds of kilometers in the Mediterranean, where continental lithosphere is involved. Moreover, deformation regimes observed in the Mediterranean cannot usually be simply related with the Nubia/Eurasia convergence. In westernmost Mediterranean, the deforming zone comprises an area including the Betics mountains range in southern Spain, and the Rif mountain range in northern Morocco. Although the Nubia/Eurasia convergence is predicted to occur at 4 4.5 mm/yr in a N65°W direction at the longitude of Morocco, the GPS velocity field reveals a consistent 2 4 mm/yr southwestward motion of of the Rif region wrt Nubia (Fadil et al., 2006) also indicating an on going opening of the Alboran sea. The observed motion is unlikely to be produced by plate interaction and is therefore rather controlled by deep processes interacting with the lithosphere. Further east, geodetic sites in the western Mediterranean basin indicate insignificant motion with respect to stable Eurasia (Nocquet and Calais, 2003). As a consequence, the 4 5 mm/yr of Nubia/Eurasia convergence must be accommodated along the Maghrebides moutain range (Nocquet and Calais, 2004), as also indicated by the spatial distribution of seismicity and the seismic moment release (Serpelloni et al., 2007). Sicily is to the first order following the motion of Nubia. Therefore, the major present day plate boundary lies along the Tyrrhenian coast of Sicily. One puzzling feature of the geodetic velocity field is the counter clockwise rotation of a relatively aseismic area embedding the Adriatic sea. This motion seems to control both the extension along the Apennines, the shortening along the Dinarides and possibily the changing regime across the central and western Alps. The location of the boundary of the southern Adriatic block with Nubia remains one of the open questions of the Mediterranean kinematics. In the eastern Mediterranean, the kinematics is dominated by a general motion towards the Hellenic trench, where the oceanic Nubia lithosphere is subducting beneath the Aegean continental domain at a rate of about 35 mm/yr. The onset of the trenchwards motion is seen as far as Romania and northwestern Bulgaria, with velocity magnitudes increasing as one gets closer to the Hellenic trench. The velocity field in Turkey shows a clockwise rotation around an Euler pole located close to the Nil delta in Egypt (e.g.McClusky et al., 2000), mostly accommodated along the North Anatolian Fault. While a rigid rotation correctly predicts the geodetic velocity field for Central Anatolia, the strain rate in western Turkey shows N S stretching increasing from the Anatolian plateau to the Aegean coast. The distributions of topography and crustal thickness in western Turkey agree with the distributions expected if the crust there had a constant thickness at 5 Ma, equal to that of the present central Anatolian plateau, and had subsequently been thinned at the present day distribution of strain rates (Aktug et al., 2009). REFERENCES B. Aktug et al., Deformation of western Turkey from a combination of permanent and campaign GPS data: limits to block like behaviour, J. Geophys. Res., 115, B10403, doi:10.1029/2009JB007040, 2009. Calais E. et al., Evidence for a post 3.16 Ma change in Nubia Eurasia North America plate motions , Earth & Planetary Science Letters, 216, 81 92, 2003. Fadil A. et al., Active tectonics of the western Mediterranean: Geodetic evidence for roll back of a delaminated subcontinental lithospheric slab beneath the Rif Mountains, Morocco, Geology, 34, 7, 2006. Nocquet, J. M., P. Willis and S. Garcia, Plate Kinematics of Nubia Somalia of using a combined DORIS and GPS solution, Journal of Geodesy, 80, 591 607, DOI : 10.1007/s00190 006 0078 0, 2006. Reilinger R., et al., GPS constraints on continental deformation in the Africa Arabia Eurasia continental collision zone and implications for the dynamics of plate interactions, J. Geophys. Res., 111, B05411, doi:10.1029/2005JB004051, 2006. Serpelloni, E. et al., Kinematics of the Western Africa Eurasia plate boundary from focal mechanisms and GPS data, Geophysical Journal International, 169, 3, 1180 1200, 2007. 66 10th Alpine workshop "CorseAlp2011" Geometry, Sequence and Amount of Thrusting in the Subalpine Molasse of Austria and Bavaria Hugo Ortner 1, Bernhard Fügenschuh 1, Michael Zerlauth 1,2 & Christoph von Hagke 3 (1) Institute of Geology and Paleontology, University of Innsbruck, Innsbruck. Austria (2) alpS GmbH, Innsbruck, Austria (3) GFZ Potsdam, Potsdam, Germany The Subalpine Molasse (SM) is the most external tectonic unit of the Alps extending from Geneva (Switzerland) to west of Salzburg (Austria). While the main part of the SM of Switzerland and Western Austria is dominated by continental, coarse grained conglomerates (Lower Freshwater Molasse, USM), a transition to more fine grained continental deposits (Lower Variegated Molasse, UBM), then brackish (“Flözmolasse”; SBM) and finally marine sediments is observed towards the east where the SM tectonic unit finally disappears. A similar eastward deepening can be observed within the detached Helvetic thrust sheets which are carbonate dominated in the west and marl dominated in the east. Moreover facies boundaries within the European continental margin strike NE SW, i.e. oblique to northern margin of the Alps. We investigate the influence of these facies transitions on the tectonic style and observed amount of shortening. Based on a newly compiled tectonic map and seismic data several cross sections were constructed to estimate the style and amount of shortening. Shortening estimates are restricted to the exposed parts of the SM, because the footwall cutoff is buried below tectonically higher units. From Western Austria to the east, shortening decreases from 32km (64%) to 11km (55%). Seismic data in westernmost Austria allow an estimation of total shortening within the SM of 40 50km as the footwall cutoff must be located below the Northern Calcareous Alps for paleogeographic reasons. Growth geometries in the Upper Marine Molasse and the Upper Freshwater Molasse point to formation and propagation of the frontal triangle structure from the Late Burdigalian onwards and outlasted deposition of the youngest preserved sediments (ca. 9Ma). Thrusts within the SM are not folded, but cut preexisting fold structures and are therefore hinterland propagating. Fault bounded young exhumation of the SM in westernmost Austria is independently confirmed by new (U Th)/He data. The general structure of the Helvetic and Flysch thrust sheets is that of an anticlinorium, with its northern limb being in contact with the SM. Fold axes in the Helvetic thrust sheets south of the SM are curved and Flysch thrust sheets appear where the strike of Helvetic fold axes diverges from the Molasse base,. The contact to the tectonically higher Helvetic and Flysch nappes is in most areas more or less bedding parallel and sub vertical to steeply north dipping. A more or less continuous sedimentary succession from the youngest sediments in the Helvetic thrust sheet to the oldest sediments of the SM exists in some areas. This suggests a bedding parallel tectonic contact, which could have been the passive roof thrust of a southern triangle zone below Molasse sediments, filled by a Helvetic and Flysch duplex buried below Molasse sediments. The southern triangle zone was then passively transported northwards on top of the basal Helvetic (Glarus) thrust. In the eastern part of the SM, this passive north dipping backthrust and fold structures are crosscut by younger out of sequence thrusts. These thrusts have systematically more offset in the east, leading to complete overthrusting of the SM. Further to the east, the SM is only known from the subsurface. There, the SM thrust sheets were emplaced in a hinterland propagating sequence and buried by Oligocene sediments, the main shortening occurring before the Lower Miocene (e.g. cross section in De Ruig and Hubbard, 2006). Major differences exist between the western and eastern SM: main thrusting in the west occurred in post Middle Miocene times while in the east such late thrusting is much less important; The thrusts within the triangle zone in the west can be seen as the most external thrusts that mark the time when the Alpine orogenic wedge stopped propagating outward, but thickened internally. In the east, the present position of the most external thrust sheets was already reached during the Oligocene, and remained stationary since then. The lack of post Middle Miocene thrusting in the east resulted in a 12° clockwise rotation. On a larger scale, Miocene shortening was probably taken up by folding, thrusting and associated exhumation in the Tauern Window. Distribution of facies in the SM and the European margin controlled the style of local deformation, but not the amount of shortening observed, as these differences can be explained by the large scale deformation pattern. REFERENCES De Ruig, M. J. & Hubbard, S. M. (2006): Seismic facies and reservoir characteristics of a deep marine channel belt in the Molasse foreland basin, Puchkirchen Formation, Austria. AAPG Bulletin, 90, 735 752, Tulsa. 67 10th Alpine workshop "CorseAlp2011" Metamorphic Zonations in the Corsican Batholith (France) and Relationships With the Post-Hercynian Events in the Mediterranean area Marie Madeleine Ottaviani Spella & Antoine Berlinghi spella@univ corse.fr University of Corsica Pasquale Paoli, Campus Grimaldi, Corte, France Secondary mineral phases (some unpublished) have been identified in the hercynian corsican batholith: chlorites (pycnochlorites, diabantites, ripidolites, brunsgivites 0.67 < Xc < 1), epidote (pistachite type 0.17 < XFe3+ < 0.41), hydrogarnet (hydrograndite 0.42 < XFe3+ < 0.67), titanite (moderately aluminous grothite 0.04 < Xal < 0.33), calcite, actinolite, pumpellyite (Fe pumpellyite), prehnite (more or less ferriferrous), stilpnomelane (ferristilpnomelane). Most of these minerals has a high iron content and a calco aluminous silicates nature. Their stability fields show a low degree of metamorphism. Chlorites, epidote, titanite and calcite secondary (only located in the Ca rich granitoïds) are widespread in the batholith. Hydrogarnet appears near some permian alkaline complexes, bounding an area from the Balagne to Bavella and approximately parallel to the boundary hercynian Corsica / alpine Corsica. Stilpnomelane, pumpellyite, actinolite and prehnite draw bands that are roughly parallel to limit hercynian Corsica / alpine Corsica. The distribution of these minerals in the corsican batholith is related either to the post hercynian period, or to the establishment of permian alkaline complexes, or the alpine tectonic phases. This shows a strong impact of the alpine Corsica on the hercynian Corsica. Investigations have been undertaken in Sardinia (in the northern half of the island) to highlight a possible continuation of low grade metamorphism and indirectly the position of the alpine range. 68 10th Alpine workshop "CorseAlp2011" Palaeogene Fold and Thrust Belt Beneath a Neogene Strike-Slip Zone in Central Hungary Márton Palotai 1 & László Csontos 2 (1) Eötvös Loránd University, Department of Geology, Budapest, Hungary (2) MOL PLC., Budapest, Hungary Lying between the enigmatic Mid Hungarian Line to the south and the well resolved Balaton Tóalmás Line (the continuation of the Periadriatic Line) to the north, the WSW ENE striking Mid Hungarian Shear Zone separates the AlCaPa and Tisza Dacia mega units ( Csontos & Vörös 2004 ). Recently shot 3D seismic data (1500 km 2) allowed for a detailed interpretation ( Palotai & Csontos 2010 ), aimed at the Palaeogene and Neogene tectonic evolution of the central part of the deformation zone. Based on detailed maps of (1) horizons for top Eocene, base Miocene and base Late Miocene, (2) thickness variations as indicators of synsedimentary deformation and (3) fault geometries from 3D seismics, the structural evolution of the studied segment of the Mid Hungarian Shear Zone can be described as follows. 1 The Mid Hungarian Shear Zone acted as a generally NW vergent fold and thrust belt in the Late Oligocene, beginning probably already in the late Early Oligocene. Observed vergencies suggest that at least in this part of the orogen the Mid Hungarian Shear Zone, intermittent between the Tisza and AlCaPa mega units, likely overrode AlCaPa, in contrary to the idea of Csontos & Nagymarosy (1998) and Schmid et al. (2008) . As an alternative, the described thrusts may form a larger backthrust zone to the main, SE verging thrust of the Mid Hungarian Line. 2 The intensity of shortening generally increased westwards, with one major internal step. The gradual change in thrusting directions to the west was likely caused by differential rotation within the shear zone between the AlCaPa and Tisza Dacia blocks, with the original shortening directions preserved in the relatively mildly deformed eastern parts. 3 Transport directions in the Early Miocene were similar to those of the Oligocene, but instead of hard linked faults, blind thrusts and related folds prevailed. 4 The Balaton Tóalmás Zone (possibly as a reactivation of a Palaeogene thrust belt) initiated most likely in the Early Miocene as a dextral strike slip zone. 5 Top NE thrusting and gentle folding in the Mid Hungarian Shear Zone partly continued in the earliest Late Miocene, and was followed by sinistral movements in the whole zone, deformation being concentrated to the Balaton Tóalmás Zone). 6 The latest observed tectonic event was the Pliocene Quaternary inversion of the Balaton Tóalmás Zone. REFERENCES Csontos L., Nagymarosy A., 1998: The Mid Hungarian line: a zone of repeated tectonic inversions. Tectonophysics 297, 51 71. Csontos, L., Vörös, A. 2004: Mesozoic plate tectonic reconstruction of the Carpathian region. Palaeogeography, Palaeoclimatology, Palaeoecology 210, 1 56. Palotai M., Csontos L. 2010: Strike slip reactivation of a Paleogene to Miocene fold and thrust belt along the central part of the Mid Hungarian Shear Zone. Geologica Carpathica 61/6, 483 493. Schmid, S. M., Bernoulli, D., Fügenschuh, B., Matenco, L., Schefer, S., Schuster, R., Tischler, M., Ustaszewski, K. 2008: The Alpine Carpathian Dinaridic orogenic system: correlation and evolution of tectonic units. Swiss J. Geosci. 101, 139 183. 69 10th Alpine workshop "CorseAlp2011" Transfer Tectonics along the Briançonnais-Dauphinois Boundary (Western Ligurian Alps) - 2 : the “Limone-Viozene Zone” Fabrizio Piana 1 , Carlo Bertok 2, Anna d’Atri 2, Luca Martire 2 & Elena Perotti 2 (1) Istituto di Geoscienze e Georisorse, CNR, Torino, Italy (2) Dipartimento di Scienze della Terra, Università di Torino, Italy The Cretaceous to Eocene diachronous closure of the Liguria Piemonte ocean and the following Oligocene onward oblique collision of the European and Adria continental plates led to the formation of a continuous orogenic chain, characterized by a northern Alpine branch with a regional vergence to the west, and a southern Apennine branch with a regional vergence to the east. This necessarily implied the presence of a regional transfer zone accomodating, for several tens of millions years, the displacement between the two opposite verging branches and controlling the evolution of the interposed “Tertiary Piemonte Basin”. A western segment of this articulated transfer zone (here labelled as “Ligurian transfer”) is assumed to have controlled the evolution of the junction zone between the western Ligurian Alps and the southern sector of the Western Alps, where its activity could have contributed to the local tight curvature of the chain, the sharp southward decrease of metamorphic grade and the uplifting of the Argentera Massif. Therefore, a regional East West sinistral strike slip zone, active since the Oligocene, was already documented since the 80’s (Stura couloir: Ricou, 1981; Lefevre, 1983), while Molli et al. (2010) claimed the existence of a wide corridor in the Western Ligurian Alps, in which several en echelon kinematic transfer zones allowed since Early Oligocene an indipendent kinematic of Ligurian Alps with respect to the Maritime Cottian Alps, with sinistral regional main transfer in the Oligocene followed by dextral movements in Late Miocene Pliocene up to now. A major structure of the “Ligurian transfer” is the Limone Viozene deformation zone (LIVZ, Piana et al., 2009), a ESE WNW transpressive shear zone that runs for several tens kilometers along the boundary between the Ligurian Brianconnais and Dauphinois Provencal domains. Its activity is documented since the first Alpine tectonic stages of the external Ligurian Alps, although stratigraphic and sedimentologic evidence points to synsedimentary tectonics along a LIVZ precursor zone since the Middle Late Jurassic. These topics will be discussed in two separate contributions: the first one regards the Stratigraphic evidence of Jurassic Eocene synsedimentary tectonics along the boundary between the Ligurian Brianconnais and Provençal Dauphinois domains, while the second contribution focuses on the LIVZ structural setting and Oligocene Miocene tectonic evolution of the Argentera Maritime Ligurian Alps junction zone. THE LIMONE VIOZENE ZONE The boundary faults of the Meso Cenozoic strike slip basins developed at the junction between the Western Ligurian Alps and the southern sector of the Western Alps were reactivated during several steps of the Alpine orogenesis. These faults originated in correspondence of Cretaceous paleo escarpments and paleofaults or within narrow belts where sharp sediment facies and thickness changes occurred. The activity of these faults started in the Oligocene, during the first tectonic stage suffered by the Brianconnais Dauphinoise fold and thrust belt and continued until at least the Early Miocene with the final emplacement of Ligurian M. Saccarello “nappe” at the top of the thrust sequence. In this kinematic frame the Limone Viozene Zone (LIVZ) developed along the external boundary of Ligurian Brianconnais as a multistage, WNW ESE, convergent wrench zone. Km scale rock slices, pertaining to Briançonnais, Provençal Dauphinois and Ligurian domains, are involved in the LIVZ that can be depicted as an asymmetric flower structure consisting of an axial zone with stretched rock slices embedded in a very steep foliation, a SW portion with thrust faults and drag folds and a NE portion dominated by mesoscale folding. Left lateral movements should have prevailed in the first stages of LIVZ activity, while dextral transfer kinematic occurred in the later stages consistently with the activity of the main Argentera boundary faults that merge with the LIVZ in its western segments. The LIVZ developed mainly in anchimetamorphic conditions, since the maximum temperature recorded is about 280° (Battaglia et al, this symposium). The long lasting occurrence of a regional strike slip zones reveals that the Western Ligurian Alps acted as a Jurassic Oligocene kinematic transfer between Iberia and Europe first, and then between the Western Alps arc and the NW termination of the early Apennines. REFERENCES Lefèvre, R. 1983. La cicatrice de Preit: une discontinuité structurale majeure au sein de la zone briançonnaise entre Acceglio et l’Argentera (Alpes Cottiennes méridionales). Compte Rendu Académie Science Paris 296, 1551 1554. Molli, G., Crispini, L., Malusà M., Mosca, P., Piana, F., Federico L. (2010) Geology of the Western Alps Northern Apennine junction area: a regional review. Journal of the Virtual Explorer, Electronic Edition, ISSN 1441 8142, volume 36, paper 10. In: (Eds.) M. Beltrando, A. Peccerillo, M. Mattei, S. Conticelli, C.Doglioni, The Geology of Italy: tectonics and life along plate margins. Piana F., Musso A., Bertok C., d’Atri A., Martire L., Perotti E., Varrone D., Martinotti G. (2009) New data on post Eocene tectonic evolution of the External Brianconnais (Western Ligurian Alps). Italian Journal Geosci., (Boll.Soc.Geol.It.), Vol. 128, No. 2, pp. 353 366. Ricou, L. E. (1981). Glissement senestre des nappes penniques le long de la bordure nord de l’Argentera: Son role dans le jeu de l’arc alpin. Compte Rendu Académie Science Paris, 292, 1305 1308. 70 10th Alpine workshop "CorseAlp2011" Structural Evolution of the Lithospheric Mantle during Continental Rifting: insights from Ligurian Ophiolitic Peridotites Giovanni B. Piccardo Dipartimento per lo Studio del Territorio e delle sue Risorse, Università di Genova, Genova, Italy Structural and petrological geochemical studies of the ophiolitic peridotites furnish significant contributions to the understanding of the geodynamic evolution of the Ligure Piemontese basin during continental extension and oceanic opening. Mesozoic continental extension in the Europe Adria realm led to thinning of the lithospheric mantle and its progressive exhumation. In fact, the original sub continental lithospheric mantle which is still preserved in ophiolites deriving from the passive margins of the basin (e.g., External Ligurides, Erro Tobbio, North Lanzo, Upper Platta) maintain structural paragenetic features indicating progressive exhumation. Their provenance from the deep lithosphere (P > 2.5 GPa) is marked by the presence of mm cm wide clusters of orthopyroxene+spinel(+clinopyroxene) formed by breakdown of precursor mantle garnet under spinel facies conditions (P < 2.5 GPa) and by the sporadic presence of garnet in pyroxenites (P > 1.5 GPa). Most of the spinel peridotites (P in the range 2.5 1.0 GPa) show km scale extensional shear zones (e.g., Vissers et al., 1991; Hoogerduijn Strating et al., 1993) with tectonite mylonite fabrics. Field and petrographic structural investigations indicate that shear zones were formed during the whole evolution of the mantle lithosphere. Some shear zones have been dated to 220 Ma (Lu Hf age) (Montanini et al., 2006) and 225 Ma (40Ar/39Ar amphibole age) (Müntener & Hermann, 2001). These ages indicate that significant lithosphere extension and mantle exhumation was already active during Upper Triassic, since the deep mantle lithosphere was already exhumed from garnet through spinel to plagioclase facies conditions by a network of extensional shear zones (Piccardo et al., 2010). Spinel facies shear zones were injected by MORB type fractional melts and, subsequently, the peridotites were percolated and impregnated by melts under plagioclase facies conditions. This early magmatism is well consistent with the reduction to about 50% of the lithosphere width and the near adiabatic upwelling and partial melting under spinel facies conditions of the DM asthenosphere during Lower Jurassic. Peridotites were impregnated and refertilised by melts percolation via porous flow when conductive heat loss prevailed on percolation heating and melt stagnated in the upper lithospheric mantle. Shear zones were infiltrated by new batches of aggregated MORB melts from the asthenosphere and transformed by reactive porous flow into dunite channels, sometimes 10 100 meters wide. These high porosity / high permeability channels are considered the fastest ways in plastic regimes to deliver oceanic MORB to shallow levels. These aggregated MORBs formed shallow gabbroic intrusions from the Lower Jurassic rifting stage [180+/ 14 Ma Erro Tobbio (Borghini et al., 2007); 179+/ 9 Ma External Ligurides (Tribuzio et al ., 2004)] to the Upper Jurassic [155 150 Ma Monte Maggiore Corsica (Rampone et al., 2009)] oceanic stage and the basaltic extrusions. REFERENCES Vissers et al., 1991, Geology, 19, 990 993; Hoogerduijn Strating et al., 1993, J. Petr., 34, 901 927.; Montanini et al., 2006, J.Petr. 47, 1943 1971; Müntener & Hermann, 2001, G.S.L., S.P. 187, 267 288; Piccardo et al., 2010, J.V.E. 36, 7; Tribuzio et al ., 2004, J. Petrol., 45, 1109 1124; Rampone et al., 2009. EPSL 15, 601 610. Borghini, G. et al, 2007. Lithos 94, 210 229. 71 10th Alpine workshop "CorseAlp2011" The Canavese Fault Jan Pleuger 1, Neil Mancktelow 1 & Richard Spikings 2 (1) Geological Institute, ETH Zürich, Switzerland (2) Department of Mineralogy, University of Geneva, Switzerland The Canavese Fault (CF), i.e. the Periadriatic Fault west of Lago Maggiore, accounts for the uplift of the relatively high grade Penninic Alps with respect to the Southern Alps and a poorly constrained amount of dextral displacement between the two blocks. In the following, we will describe several mylonite belts that occur along the CF from NE to SW. Between Lago Maggiore and Valle d’Ossola, the CF comprises two several hundred metres thick, partly overlapping greenschist facies mylonite belts. N(W) side up mylonites (Belt 1) are developed mostly within the Sesia Zone (SZ) immediately N(W) of the CF. Dextral mylonites, locally with a considerable S(E) side up component (Belt 2), are developed mostly within the Southern Alpine rocks (see also Schmid et al. 1987). The foliation of Belt 1 is the axial planar foliation S n+1 of generally NW vergent F n+1 folds which overprint the “pre Insubric” foliation S n. In moderately sheared domains, F n+1 folds are open and L n+1 is a generally subhorizontal intersection lineation of S n+1 and S n parallel to Fn+1 axes. In highly sheared domaines, F n+1 folds are tight to nearly isoclinal with axes parallel to the steeply plunging stretching lineation L n+1 . Close to the boundary between the SZ and the Southern Alps, the Belt 1 mylonites are locally refolded by F n+2 folds. F n+2 folds developed during shearing and ongoing mylonitisation of Belt 2. Belt 2 can be traced toward the W until Valle Strona where it becomes thinner (few tens of metres) and finally ends in a brittle fault. Belt 1 also becomes thinner toward the W, being restricted in Valle Mastallone to a few hundred metres of the SZ, represented there by the “Scisti di Rimella e Fobello”. Outside Belt 1, the shear sense in the “Scisti di Rimella e Fobello” is SE side up. 40 Ar/ 39 Ar data obtained from recrystallised white mica from a sample of the SE side up mylonites (UTM32 5084144/436441) yield a weighted plateau age of 37.11 ± 0.55 Ma which we interpret as the time of the mylonitisation. These mylonites are thus considerably older than shearing in Belt 1 which started at ca. 32 Ma (Mancktelow & Pleuger, this volume). 40 Ar/ 39 Ar weighted plateau ages of 34.10 ± 0.43 Ma and 33.81 ± 1.00 Ma obtained from pseudotachylite samples (5084356/437405 and 5084360/437415) collected immediately S of the protolith boundary in the Ivrea Zone (IZ) probably date brittle faulting at the onset of the N(W) side up shearing. A second belt of dextral plus SE side up mylonites (Belt 3) parallels the CF inside the IZ in Valle Sermenza and Valle Sesia. On the ridge between Valle Sesia and Valle Sessera, it approaches the protolith boundary between the Southern Alpine rocks and the SZ. In a section along the Dolca river, the same shear sense is observed in augengneiss mylonites occurring between the SZ and basic rocks of the IZ. The mylonites are crosscut by post kinematic andesitic dykes which were probably emplaced coevally with the ca. 31 Ma old Biella and Miagliano Plutons (Romer et al. 1996; Carraro & Ferrara 1968). Therefore, Belt 3 is older than Belt 2. The SZ rocks are affected by a lower greenschist facies ductile to brittle belt with a NW side up plus sinistral displacement sense (Belt 4). South of Dolca river, Belt 4 is covered by ca. 33 Ma old (Kapferer et al. 2009) andesitic volcanites. These volcanites often contain xenoliths derived from the SZ and rest on top of Sesia derived conglomerates which in turn were deposited onto the SZ basement. In most places, the succession from the SZ basement into the volcanites is not faulted. The uplift of the Biella Pluton with respect to the volcanites was largely accommodated by E side down rotation of the SZ about a subhorizontal, NNE SSW trending axis (Lanza, 1977). A largely brittle fault accommodated the relative uplift of the IZ with respect to the volcanites. In Valle Sessera, this fault is represented by kakirites and a fault breccia containing mylonite fragments. 40 Ar/ 39 Ar on white mica from such a fragment (5057936/427188) gave a weighted plateau age of 54.07 ± 0.50 Ma. South of Valle Sessera, shear zones observed along the protolith boundary between the SZ and the Southern Alps are commonly only a few metres thick and subordinate to fault breccia. Since the displacement sense of these shear zones is variable and the CF is poorly exposed, the observed structures cannot be integrated into a regional pattern. From the Serra d’Ivrea towards south, the CF splits into two branches, the Internal (ICF) and External Canavese Faults (ECF), which frame the Canavese Zone to the (S)E and (N)W, respectively. The ECF is dominated by brittle fault rocks and only rheologically weak rocks such as serpentinite and calcschist were mylonitised. From a W side up calcschist mylonite (5030790/397670), we obtained a 40 Ar/ 39 Ar white mica weighted plateau age of 29.67 ± 1.48 Ma. The displacement senses of low grade mylonites at the ICF are mostly dextral plus E side up and thus in line with a relative uplift of the IZ postdating the emplacement of the Miagliano Pluton further north. The distribution and timing of the mylonite belts observed along the CF show that coherent N(W) side up (Belt 1) and dextral (Belt 2) mylonite belts of Oligocene age are only present between Lago Maggiore and Valle Sesia. Therefore, large amounts of dextral “Insubric”, i.e. Oligocene, displacement in the order of 100 km or more (e.g. Schmid & Kissling 2000) cannot have been accommodated along the CF itself. Either there was a transfer of dextral shearing into the Penninic nappe stack (e.g. Handy et al. 2005) or a smaller amount of dextral displacement must be assumed. REFERENCES Carraro, F., Ferrara, G., 1968. Schweiz. Mineral. Petrogr. Mitt. 48, 75 80. Handy, M.R., Babist, J., Wagner, R., Rosenberg, C.L. & Konrad, M., 2005. Geol. Soc. London Spec. Publ. 243, 249 276. Kapferer, N., Mercolli, I., Berger, A., 2009. 9th Workshop on Alpine Geological Studies, Cogne/Italy, Abstract Volume. Lanza, R., 1977. Schweiz. Mineral. Petrogr. Mitt. 57, 281 290. Mancktelow, N., Pleuger, J., this volume. Romer, R.L., Schärer, U., Steck, A., 1996. Contrib. Mineral. Petrol. 123, 138 158. Schmid S. M., Kissling, E., 2000. Tectonics 19, 62 85. Schmid, S.M., Zingg, A., Handy, M., 1987. Tectonophysics 135, 47 66. 72 10th Alpine workshop "CorseAlp2011" The Giudicarie Fault System in the light of Zircon Fission Track Data Pomella Hannah 1 & Fügenschuh Bernhard 2 pomella@alps gmbh.com (1) Alps Centre for Climate Change Adaptation Technologies, Innsbruck, Austria (2) Institute of Geology and Palaeontology, University of Innsbruck, Austria In the framework of a thermochronological study, focused on the Giudicarie fault system (GFS) and related Oligocene intrusions, a contour map of the present day zircon fission track (ZFT) age distribution was constructed. The map is based on 129 ZFT data (Fügenschuh 1995, Martin et al. 1998, Stöckhert et al. 1999, Viola 2000, Most 2003, Pomella 2010) and was hand contoured. The method was necessary in order to overcome problems arising from the highly inhomogeneous spatial distribution of the data and to allow faults to be taken into account when defining the boundary of the differently aged sections. The most eye catching feature in the contour map is the corridor of young, Miocene ZFT ages from small tonalitic intrusions along the Northern Giudicarie fault. This corridor connects Early Miocene (17 23 Ma) ZFT ages of the NE Adamello with the Miocene (23 9 Ma) ZFT ages of the Meran Mauls basement and the Tauern window. To the SE the narrow corridor is bounded by Southalpine sediments characterized by only partially reset ZFT ages and towards NW by Oligocene ZFT cooling ages found in the Austroalpine units. Therefore, a tectonic model capable of explaining the presence of young tonalitic lenses, or, more generally speaking, a corridor of younger low T cooling ages between two earlier or less exhumed blocks is required. The Eo to Oligocene intrusion ages of the tonalitic lenses (Pomella et al. 2010), their granitic texture, and the lack of contact metamorphism around the bodies argue against a late and/or shallow intrusion of the tonalites in a fault zone already cooled below the zircon partial annealing zone (ZPAZ, 180 300°C; Hurford & Green). The small intrusive bodies are considered to be sheared parts of the Southalpine Adamello batholith (e.g. Martin et al. 1993; Pomella et al. 2010). Due to their position in the footwall of the sinistral transpressive, WNW dipping Northern Giudicarie fault, north directed transport would bring them to greater depths instead of exhuming them relatively to the north easternmost Adamello batholith, such as indicated by the ZFT data. This problem can be solved by a 3 phase model for the emplacement of the tonalitic lamellae. At the Eo Oligocene boundary the northeastern units of the Adamello batholith intruded along an originally straight, dextral strike slip Periadriatic line. In the Late Oligocene / Earliest Miocene the NNW ward movement of the Southalpine indenter led to a bending of the central part of the Periadriatic line and material from the northeastern part of the Adamello batholith was squeezed to the NE along the bent part of the fault. In the Early Miocene, the brittle Passeier , Northern and Southern Giudicarie faults sinistrally dissected the bent part of the Periadriatic line. Along the northern part of the bend a nearly continuous tonalitic body persisted along the Meran Mauls fault as the subsequent brittle deformation took place mostly along the Naif fault in the footwall (please see next paragraph). Along the Northern Giudicarie fault, small bodies were sheared from the former bent and already boudinaged tonalitic body, transported southwards, and exhumed by sinistral transpressive deformation. The ZFT contour map also provided new insights into the structure of the footwall of the GFS. From the three main Permian plutons present along the GFS, the northernmost (Brixen pluton) and the southernmost (Kreuzberg pluton) yielded ZFT ages of about 100 Ma, while the intermediate Ifinger pluton cooled through the zircon partial annealing zone only in the Miocene. The Ifinger granodiorite overthrusts the Southalpine basement and the Permian rocks of the Athesian Volcanic District along the NW dipping brittle Naif fault. On this thrust fault, an important age jump from Miocene to Permian ZFT ages can be observed, whereas across the branch of the GFS located in the hanging wall of the Naif fault nearly no change occurs. These data argue for an exhumation of the Ifinger pluton along the Naif fault when the Kreuzberg and the Brixen pluton had already cooled to below ca. 200°C. REFERENCES Hurford, A.J. & Green, I.R. (1983): The zeta age calibration of fission track dating. Isotope Geoscience, 1: 285 317 Martin, S.; Prosser, G. & Morten, L. (1993): Tectono magmatic evolution of sheeted plutonic bodies along the north Giudicarie line (northern Italy). International Journal of Earth Sciences, 82: 51 86 Fügenschuh B. (1995): Thermal and kinematic history of the Brenner area (Eastern Alps, Tyrol). PhD thesis, ETH Zürich Martin S., Bigazzi G., Zattin M., Viola G. and Balestrieri M.L. (1998): Neogene kinematics of the Giudicarie fault (Central Eastern Alps, Italy): new fission track data. Terra Nova 10: 217 221 Stöckhert B., Brix M.R., Kleinschrodt R., Hurford A.J. & Wirth R. (1999): Thermochronometry and microstructures of quartz a comparison with experimental flow laws and predictions on the temperature of the brittle plastic transition. Journal of Structural Geology 21: 351 369 Viola G. (2000) Kinematics and timing of the Periadriatic fault system in the Giudicarie region (central eastern Alps). PhD thesis, ETH Zürich Most P. (2003): Late Alpine cooling histories of tectonic blocks along the central part of the Transalp Traverse (Inntal Gadertal): constraints from geochronology. PhD thesis, Eberhardt Karls Universität Pomella, H. (2010): The Cenozoic evolution of the Giudicarie fault system (Eastern/Southern Alps, northern Italy). A geochronological, structural and paleomagnetic study. PhD thesis, Institute of Geology and Paleontology, University of Innsbruck Pomella, H.; Klötzli, U.; Scholger, R.; Stipp, M. & Fügenschuh, B. (2010): The Giudicarie fault system in the light of new paleomagnetic and geochronological data from boudinaged Eo /Oligocene tonalites. International Journal of Earth Sciences DOI: 10.1007/s00531 010 0612 4 73 10th Alpine workshop "CorseAlp2011" Pseudotachylytes Generated in Lower Crustal Levels: Constraints for Numerical Modelling Giacomo Prosser 1, Alfredo Caggianelli 2, Uwe Altenberger 3 & Richard Spiess 4 (1) Dipartimento di Scienze Geologiche, Università della Basilicata, Potenza, Italy (2) Dipartimento Geomineralogico, Università di Bari, Bari, Italy (3) Institut für Erd und Umweltwissenschaften, Universität Potsdam, Potsdam Golm, Germany (4) Dipartimento di Geoscienze, Università di Padova, Padova, Italy Pseudotachylyte bearing faults are commonly observed in felsic and mafic granulites located at the base of the Serre crustal section exposed in Calabria. They are exposed just above the Alpine thrust plane that brought formerly lower crustal rocks above medium to low grade metamorphic complexes. We have analysed microstructure and composition of the pseudotachylyte veins formed in lower crustal felsic granulites to obtain constraints for a numerical thermal model explaining their formation. Pseudotachylite fault veins have typically a thickness of 4 5 mm. In pull apart structures and injection veins the thickness may reach about 5 cm. In the centre of the veins the matrix is mostly composed of plagioclase and biotite microlites and it shows a glassy appearance in the chilled margins. Matrix composition, obtained by scanning microprobe analyses, ranges from basaltic trachyandesite to andesite and trachyte. The frictional melt is more basic than the protolith, since quartz often remains preserved as clasts. Interestingly, close to the vein margins tiny euhedral to subhedral garnet crystals, showing evidence of direct crystallization from the pseudotachylyte melt, are present. At the vein margin the wall rock is affected by an evident grain size reduction of quartz, related to dynamic recrystallization. In the same location, plagioclase underwent only brittle fragmentation. These observations are indicative of an initial stage of ductile to brittle deformation at temperatures comprised between 300 and 500 °C, in relation to the common range of strain rates documented for natural shear zones. Mean composition of plagioclase microlites is An 32 , whereas garnet is almandine rich (Alm 71 75 ). Considering the An content of plagioclase, a minimum estimate of peak temperature during frictional slip of about 1400 °C is obtained. An even higher temperature of 1515 °C is suggested by textural evidence indicative of incipient melting of ß quartz. Direct crystallization of garnet from the pseudotachylyte melt is instead consistent with a crustal depth of about 20 km. Therefore, all the observations coherently suggest paleoseismicity in lower crustal levels. Starting from these results, a numerical thermal model (Stella code) was built up to simulate the genesis of the pseudotachylyte fault veins, assuming an ambient temperature of 450 °C and low values for the pore fluid pressure factor (from 0 up to 0.4). The model allows to evaluate the influence of two additional effects: (1) latent heat of fusion; (2) drop of dynamical friction coefficient consequent to melting. The first effect is simulated when temperatures near the fault plane are above the melting level (1500 ± 50°C). This is modelled introducing in the melting time interval an effective heat capacity C* in place of the true heat capacity C and assuming C* = 4 x C. The second effect can be reproduced reducing the friction coefficient when conditions for melting are reached within 500 µm from the slip plane. In this case friction coefficient is reduced from 0.6 to 0.3. Model results indicate that a minimum slip on the fault plane of 4 cm in 0.6 s is required to reproduce temperature higher than 1500 °C if the pore fluid pressure factor is null and if the effects (1) and (2) are ignored. On the other hand, considering a pore fluid pressure factor of 0.4 and the influence of both additional effects (1) and (2), the minimum slip required increases to 13 cm. The model indicates that in all situations the melting temperature is exceeded after about 3 4 s at a distance of 2 mm from the fault plane. Another interesting result is the length of the melt life, ranging from 5 to 10 s. This allows to estimate the melt flow rate required to fill the space created in pull apart structures. On the basis of our results, minimum melt flow rate is in the order of 10 to 5 cm/s, in relation to dimension of typical pull apart structures. Assuming a newtonian behaviour, the estimated flow rates are compatible with viscosity of melts having compositions of the pseudotachylyte matrix. 74 10th Alpine workshop "CorseAlp2011" Melt percolation and softening of the continental lithospheric mantle: a prelude to sea-floor spreading Giorgio Ranalli Department of Earth Sciences, Carleton University, Ottawa, Canada The impregnation of the subcontinental lithospheric mantle by asthenospheric melts has important geodynamic consequences. On the basis of evidence pertaining to the Late Triassic Late Jurassic evolution of the Ligurian Tethys (Piccardo et al., 2009), a model has been proposed (Ranalli et al., 2007) suggesting that the thermomechanical effects of upward migrating melts of asthenospheric origin may be an important factor in the transition from continental extension to sea floor spreading. I revisit this model (here called Thermomechanical Erosion Model or TEM), highlighting both its potential relevance in comparison with other models, and aspects that need further study. After a brief overview of some likely lithosphere softening mechanism (heating by plume heads; strain localization and shear heating; extension embrittlement and serpentinization), an analysis of the thermomechanical effects of percolating asthenospheric melts leads to the formulation of TEM. Its basic features are: (1) The continental lithosphere is extended by far field forces until decompression melting occurs in the asthenosphere (extension factor ≥ 2). (2) Melt percolates upwards, successively by diffuse porous flow, focused flow in veins and shear zones, and channel flow in conduits and dykes. Melt velocities are of the order of 0.1 20 m a 1, resulting in times of ~ 1 100 ka to cover the free ride distance (distance travelled before melt crystallizes). (3) The effectiveness of heat transfer by melt migration is a problem left open in TEM. The roles of advection and release of latent heat cannot be quantified without a knowledge of the geometry of the melt matrix system. However, since the thermal effects of melt impregnation become more significant with an increase in melt fraction, concentration of melt migration along preferential paths such as shear zones should result in more pronounced local heating of mantle material. (4) Based on a simple parameterization in terms of the thickness of lithospheric mantle affected by the temperature increase, TEM predicts that that lithospheric softening increases nonlinearly with impregnation thickness, potentially resulting in a considerable decrease in total lithospheric strength (down to 20 40% of its initial value). The softening is achieved rapidly (≤ 2 3 Ma), and afterwards the strength is recovered asymptotically in ~ 15 20 Ma. If melt is concentrated along trans lithospheric shear zones the effectiveness of the softening is increased. “The only thing I know, is that I don’t know anything” (Socrates) “… and I’m not even sure of that” (G. Ranalli, after Michel de Montaigne) REFERENCES Piccardo G. B., Vannucci R., Guarnieri L. (2009). Lithosphere 1, 81 87. Ranalli G., Piccardo G. B., Corona Chávez P. (2007). J. Geodyn. 43, 450 464. 75 10th Alpine workshop "CorseAlp2011" Constraints on Scales and Rates of Tectonic Mobility Within Continental Collision Zones: Case Study From the Sesia Zone (Western Alps, NW-Italy) Daniele Regis 1, Martin Engi 1, Daniela Rubatto 2, James Darling 1 & Michele Zucali 3 (1) Institut für Geologie, Universität Bern, Baltzerstrasse 3, CH 3012 Bern, Switzerland (2) Research School of Earth Sciences, Australian National University, Canberra, Australia (3) Università di Milano, Dipartimento di Scienze della Terra “Ardito Desio”, Milano, Italy Convergent plate margins typically experience a transition from subduction to collision dynamics as massive continental blocks enter the subduction channel. Tectonic responses involving reverse flow are well established from studies of high pressure rocks in collisional orogens. These indicate that tectonic fragments are rapidly brought up from eclogite facies to mid crustal levels, but the details of such dynamics are controversial. Numerical models predict extensive vertical transport and mixing of tectonic blocks, but the scales and rates of such processes depend on assumed rheologies and are not well known. We report the results of a petro chronological study from the central Sesia Zone, which comprises several km size continental tectonic slices with eclogite facies imprint. Structurally controlled samples from two of these units yield unequivocal evidence of several separate HP stages: Scalaro unit : Phengite quartzites contain several successive generations of white mica and allanite, which can be correlated with structures (pervasive foliations and m scale folds) and P T data. Accessory allanite, monazite, titanite and zircon permit these stages to be dated. Mutual inclusions and overgrowth relationships allow age data on allanite and monazite to be tied to the multistage evolution of an individual sample. SHRIMP dating in this case yields consistent 208 Pb/ 232 Th ages of 75.6 ± 0.8 Ma (2σ) for allanite cores, and 69.8 ± 0.8 Ma for REE poor allanite rims. Similar age data were found for other samples from the same unit, but with a third HP stage at 65 Ma, an age well established for the main eclogite stage of the Sesia zone. Mombarone unit : Metapelites (gar ky ctd, 2 GPa, 550°C) contain allanite dated at 85.8 ± 1.0 Ma, zircon yields 74 ± 1 Ma (first rim) and ages between 70 and 60 Ma (successive rims). Given the tectonic field relations of the units, these data indicate separate stages of deformation at eclogite facies conditions for each km slice, between 86 and 65 Ma, with evidence of intermittent decompression (∆P = 0.8 GPa). The mutual position of these tectonic slices changed during this collisional interval; their juxtaposition occurred (at ca. 65 Ma?) at eclogite facies conditions, prior to the complex exhumation history of the Sesia block at amphibolite to greenschist facies conditions. Our results allow a field based calibration of the dynamics in a collisional complex, which can be compared to insights from numerical modelling. 76 10th Alpine workshop "CorseAlp2011" Plio-Pleistocene Changes in Drainage Evolution and Implications for the Denudation History of the Central European Alps Wolfgang Reiter, Simon Elfert & Cornelia Spiegel wreiter@uni bremen.de Geodynamics of Polar Regions, University of Bremen, Germany We are investigating the relation between denudation history, drainage pattern and climate evolution. In our study we use apatite fission track and apatite (U Th Sm)/He data of detrital sediments from the circum Alpine area for obtaining denudation history of the Central European Alps and changes in the associated drainage system. For this purpose we analyse samples from the Sundgau gravel (France), the Upper Rhine Graben (boreholes of Parkinsel and Viernheim, Germany), and the Swiss foreland (Höhere Deckenschotter, Irchel). Our analyses of the Sundgau gravel revealed the Aar massif and the Lepontine Dome as the most likely source areas during the Late Pliocene. The influence of the Lepontine area seems to have decreased during this period reflecting a northward shift of the main drainage divide. During the lowermost Pleistocene, the Upper Rhine Graben received alpine detritus for the first time, with the Aar massif as one of the possible main source areas. Surprisingly, the Alpine signal of the Rhine sediments indicate constant denudation rates during various glacials and interglacials. Whereas the sediments of the Swiss foreland over the same period predominantly originated in the Eastern Alps, and thus belonged to a different Alpine area than to the Aar massif. Lag times are defined as the difference between thermochronological age and the age of deposition, providing a measure for palaeodenudation rates and also for changes of denudation rates through time. For most of our samples we could distinguish between two age populations, with the Aar massif as one of the main source areas. The lag time plot of our samples reveal an interesting pattern: i) The youngest population shows constant lag time of ~6 Ma between 4 and 2 Ma decreasing to ~4 Ma after 2 Ma. This change indicates an increase of the palaeodenudation rate of the Aar massif from ~0.6 to 0.8 km year 1. ii) The lag time of the oldest population decreased continuously between 4 and 1.2 Ma reaching a value of ~4 Ma, and iii) at around 1 Ma the lag time is rising to ~14 Ma. Compared to our results, the constant lag time of the younger population agrees with the lag time reported from the Western Alps [Glotzbach et al., in press], whereas after 2 Ma, the Central Alps seem to pass through a period of a more rapid exhumation than the Western Alps. The accelerated exhumation between 2 and 1.2 Ma is most likely related to valley erosion and relief formation due to glacial processes. The strong lag time increase at around 1 Ma may reflect reworking and transport of older detritus which has been stored temporarily in the depocenters within the Central Alps, such as mountain lakes and valleys. This is consistent with a reported overdeepening of glacial basins/valleys in the Aare catchment before 0.26 Ma [Anselmetti et al., 2010]. REFERENCES Anselmetti, F.S., Drescher Schneider, R., Furrer, H., Graf, H.R., Lowick, S.E., Preusser, F., Riedi, M.A. (2010). A ~180,000 years sedimentation history of a perialpine overdeepened glacial trough (Wehntal, N Switzerland). Swiss J Geosci. DOI 10.1007/s00015 010 0041 1. Glotzbach, C., Bernet, M., van der Beek, P. (in press). Detrital thermochronology records changing source areas and steady exhumation in the Western European Alps. Geology. 77 10th Alpine workshop "CorseAlp2011" Petrographic and Fission Track Analyses on Modern Rriver Sands from the Western Alps: Insights on Short-term Erosion Distribution Alberto Resentini, Marco G. Malusà & Eduardo Garzanti Laboratory for Provenance Studies, University of Milano Bicocca Detecting sources of detritus is a fundamental step to investigate erosion patterns and feedback interactions between tectonic and surface processes (Beaumont et al., 2001; Koppes and Montgomery, 2009). In order to evaluate the short term (10 3 10 5 a) erosion distribution in the Western Alps, we collected modern river sands from the Dora Baltea and Arc Rivers. Samples were quantitatively analyzed in terms of bulk composition and apatite fission track ages. The relative contributions from diverse geological units were assessed by applying linear mixing models to sediment composition data sets (Weltje, 1997; Vezzoli et al., 2004). Fission track analysis on detrital apatites were used to quantify apatite contribution from source areas with different geochronological signature. Combining relative apatite supply data, geochemical analyses and apatite fertility estimates in source rocks, the apatite budget was converted into a relative sediment budget, and mean erosion rates were calculated by using the available information on annual sediment fluxes. Results indicate that on short term timescales erosion is focused in the External Massifs, characterized by four to five times higher sediment yields than the axial belt. Such short term erosion pattern, consistent with westward migration of transpressional tectonic activity through time (Malusà et al., 2009), cannot be explained by climatic forcing and appears to be unrelated to differences in slope angle or other morphological features (Resentini and Malusà, submitted). Active tectonic uplift, rather than exogenous processes, is thus held to be the principal driving force of erosion at both long and short timescales. REFERENCES Beaumont, C., Jamieson, R.A., Nguyen, M.H., and Lee, B., 2001. Himalayan tectonics explained by extrusion of a low viscosity crustal channel coupled to focused surface denudation. Nature, v. 414, p. 738 742. Koppes, M., Montgomery, D.R., 2009. The relative efficacy of fluvial and glacial erosion over modern to orogenic timescales. Nature Geoscience, 2, 644 647. Malusà, M.G., Polino, R., and Zattin, M., 2009. Strain partitioning in the axial NW Alps since the Oligocene. Tectonics, v. 28, p. 1 26 Resentini, A., and Malusà, M.G., submitted. Sediment budgets by detrital apatite fission tracks (Rivers Dora Baltea and Arc, Western Alps). Geol. Soc. Am. Special Paper. Vezzoli, G., Garzanti, E., and Monguzzi, S., 2004. Erosion in the Western Alps (Dora Baltea basin). 1. Quantifying sediment provenance. Sedimentary Geology, 171, 227 246. Weltje, G.J., 1997. End member modeling of compositional data: Numerical statistical algorithms for solving the explicit mixing problem. Math. Geol., 29, 503 549. 78 10th Alpine workshop "CorseAlp2011" Miocene Exhumation of the Southern Alps: New Constraints from Low-Temperature Thermochronology Rebecca L. Reverman, M. Giuditta Fellin, Frederic Herman & Sean D. Willett ETH Zurich, Switzerland During the Miocene, the European Southern Alps went through a complete tectonic cycle from beginning of deformation at about 18 Ma, to maximum expansion and cessation of thrust propagation (Pieri and Groppi, 1981; Schmid et al., 1996; Schumacher et al., 1996). At the end of the Miocene, the cessation of outward expansion occurred at the same time with the onset of strong climatic oscillations related to the Messinian salinity crisis in the Mediterranean and to the Plio Pleistocene cooling in the Northern hemisphere. Given the high sensitivity of tectonics to erosion, a relationship between tectonics and climatic changes has been postulated in the Alps at the end of the Miocene (Willett et al., 2006). The interplay of tectonic and climatic factors on the evolution of the alpine orogen are potentially best recorded in the Southern Alps, which were tectonically active during the Miocene and were affected by the Messinian fluvial erosion and Plio Pleistocene glacial reshaping. The aim of this study is to provide new constraints on the Miocene erosional and exhumational processes in the Southern Alps based on low temperature thermochronologic data. The Adamello Complex represents the ideal study area for this aim because (i) it has a considerable relief (> 2 km), (ii) its lithological composition (granitic rocks) is potentially rich in inclusion free euhedral apatite grains,(iii) it is the largest glaciated area in the Southern Alps, (iv) major rivers dissecting the complex flow into inferred Messinian valleys and (v) finally because it is close to one of the main Miocene fault systems (the Giudicarie belt, Castellarin et al 2006),. Low temperature thermochronometers, such as, apatite (U Th Sm)/He dating (AHe) and apatite fission track dating (AFT), constrain near surface (<5 km) exhumation rates that can be used to characterize climate or tectonic forcing. We present AHe and AFT ages for samples collected in the two largest valleys of the Adamello Complex, along with samples from the Mt.Sabion line and Val Trompia Thrust. All ages span the Miocene and display a normal age elevation relationship, where age increases with elevation. However, AHe ages from the Adamello Complex reveal a possible base of an exhumed AHe partial retention zone located at a modern elevation of ~2200m, which suggests a maximum of 3 km of exhumation has occurred since ~8 Ma and inferring a minimum of 4 km of exhumation between 29 Ma and 8 Ma, as constrained by pluton emplacement age and depth. The fast cooling of the low elevation samples as recorded by their AHe ages indicate that at least 1.5km of rock was exhumed rapidly at 8 Ma. The magnitude and timing of this event constrains a period of transpressional activity along the South Giudicarie line, further supported by AHe and AFT ages from a sample located between the S. Giudicarie line and Mt. Sabion line. Older AHe ages determined for the Val Trompia thrust area indicate that it was inactive during this event, but may have played a role in earlier exhumation. The final 1.5 km of exhumation since 6 Ma may be due to erosion driven by external climatic conditions. In order to test possible exhumation and landscape development scenarios, our data was modelled using Pecube (Braun, 2003), a 3 D heat conduction model including topographic relief and erosion. REFERENCES Braun, J. 2003: Pecube: a new finite element code to solve the 3D heat transport equation including the effects of time varying, finite amplitude surface topography. Computers & Geosciences 29, 787 794. Castellarin, A., G. B. Vai, et al. (2006). "The Alpine evolution of the Southern Alps around the Giudicarie faults: A late Cretaceous to Early Eocene transfer zone." Tectonophysics 414: 203 223. Pieri, M. and G. Groppi (1981). Subsurface geological structure of the Po Plain, Publication 414 del Progetto Finalizzato Geodinamica, CNR. Schmid, S. M., O. A. Pfiffner, et al. (1996). "Geophysical geological transect and tectonic evolution of the Swiss Italian Alps." Tectonics 15(5): 1036 1064. Schumacher, M. E., G. Schönborn, et al. (1996). Rifting and collision in the Southern Alps. Deep structure of the Swiss Alps: Results of the National Research Program 20 (NRP 20). O. A. Pfiffner, P. Lehner, P. Heitzmann, S. Müller and A. Steck, Birkhäuser: 186 204. Willett, S. D., F. Schlunegger, et al. (2006). "Messinian climate change and erosional destruction of the central European Alps." Geology 34: 613 616 79 10th Alpine workshop "CorseAlp2011" On the Relationship Between Shortening, Exhumation and Orogen Width During Collision in the Central and Eastern Alps Claudio Rosenberg 1, Romain Bousquet 2 & Sebastian Garcia 1 [email protected] berlin.de (1) Freie Universität Berlin, Germany (2) Université de Rennes I, France We investigate the relationship between the width of the Alpine orogen, the amount of syncollisional finite shortening and syncollisional exhumation, comparing six cross sections through the Alpine orogen, from the Simplon Fault in the west, to the Styrian basin in the east. We do not consider the western Alps s.s., because their convergence direction, their timing of exhumation, and the amount of shortening differ markedly from those in the Central and Eastern Alps. The width of the Alpine pro wedge in map view changes along strike, both in the Central and in the Eastern Alps. In the Central Alps, the width progressively increases from a minimum in the area of the Simplon Fault, to a maximum in the area of the Engadine Window. In the Eastern Alps, the width progressively increases from a minimum in the area of the Brenner Fault, to a maximum at the easternmost limit of the chain, at the western border of the Styrian Basin. These changes of width correlate with changes in the amount of syncollisional shortening and exhumation as shown by a series of cross sections through the Alpine orogen. In order to estimate the latter quantity, we define a reference marker, which is the top of the Penninic or the base of the Austroalpine, i.e., the base of the „rigid lid“. From the cross sections we calculate the surface of penninic nappes below this marker and above the sea level, and consider this area as proportional to the amount of exhumation. Where the pro wedge is shorter the orogen is more intensely folded, shortened, and exhumed. The thickness of the sediments in the northern Alpine foreland basin (including the eroded portions) increases in front of the areas that are more intensely shortened, possibly pointing to greater amounts of flexure, consistent with greater orogen thickness. Geobarometers for the Tertiary Alpine metamorphism also point to higher pressures in the areas of larger orogen shortening and smaller orogen width. All the above observations indicate that the orogenic wedge can be shortened to very different amounts without a readjustment of its width by the nucleation of new foreland thrusts, suggesting that the orogen was not in a critical state. Averaged over the entire Miocene the amount of eroded material reconstructed from the cross sections points to erosion and exhumation rates varying by a factor 5 between the western (Simplon area: > 1mm/a) and the eastern (Engadine section: < 0,2 mm/a) margins of the Central Alps. A similar difference exists between the western and the eastern margins of the Eastern Alps. The positive correlation between highest amounts of exhumation, largest amplitude of crustal scale folding and highest rates of erosion within small (250 km long) segments of the Alpine chain precludes differential climatic effects to explain differential exhumation and varying orogen widths. Greater shortening is the primary cause of greater exhumation and erosion and smaller orogen width. 80 10th Alpine workshop "CorseAlp2011" Fe-Mg metasomatism in the Mont Blanc Massif Magali Rossi 1, Yann Rolland 2 & Catherine Chauvel 3 magali.rossi@univ savoie.fr (1) Edytem, UMR 5204, Technolac, Le Bourget du Lac, France (2) Géoazur, UMR7526, Parc Valrose, Nice, France (3) ISTerre, UMR 5275, Grenoble, France The Mont Blanc massif (External French Alps) consists in a Hercynian granite that intruded a polymetamorphic basement. As the granite mineralogy and chemical composition is rather homogeneous at the massif’s scale, it is a good area to study fluid rock interactions in relation with the Alpine deformation. Several alpine structures have been described in the Mont Blanc granite : (i) a network of ductile shear zones dated at 16 Ma (Rolland et al., 2008); (ii) alpine veins bearing quartz chlorite ± calcite ± adularia, dated at 14 Ma (unpublished data) and (iii) a metasomatic zone dated at 29 Ma (Cenki Tok et al. 2011). The metasomatic zone crops out only along Glacier de Leschaux, where a digitated metasomatic front is clearly visible, but similar altered samples were found further south in the inner part of the massif, during digging of the Mont Blanc tunnel. Metasomatized rocks are characterized by the intense precipitation, under stress, of chlorite and phlogopitic biotite that replace magmatic quartz and feldspars. The compositions of the metasomatic biotites and chlorites differ from those in the granitic protolith and in other shear zones. Metasomatic phyllosilicates are indeed more silica rich and present a much higher X Mg than the granitic minerals: in average, X Mg = 0.65 0.70 in the metasomatic zone whereas X Mg = 0.30 0.45 in the granite. Comparison of whole rock chemical composition of the metasomatized rocks with the granitic protolith allows mass balance calculation related to fluid rock interaction. The metasomatized rocks evidence huge mass transfer: they are depleted in SiO 2 (strong depletion), Al 2O3 and alkali elements, and are strongly enriched in MgO and Fe 2O3 (up to +3932 % and +407 % respectively). Mass loss due to strong silica depletion is partially or totally compensated by gains of MgO and Fe 2O3. This metasomatism thus results from intensive interaction between the Mont Blanc granite and a fluid extremely rich in MgO and Fe 2O3 (perfect correlation between MgO and Fe 2O3). In order to investigate the origin of the metasomatic fluid, Pb and Nd isotopic compositions were analyzed in 10 samples covering a wide range of alteration degrees. The Nd isotopic compositions are remarkably constant with an average ƐNd value of 0.5 suggesting that no significant difference exists between the sources of REE in the various samples. In contrast, the Pb isotopic compositions define a remarkable isochron at ~295 Ma. These results suggest that most of the U/Pb fractionation occurred during a tardi Hercynian deformation event, shortly after granite emplacement, and that only relatively minor changes (<30%) were introduced during the Alpine deformation. Fluid circulation during Alpine deformation may thus have occurred through reactivation of (tardi )hercynian structures and fluid pathways. REFERENCES Cenki Tok et al., 2011. Allanite Behavior in Upper Mid Crustal Shear Zones: Can we Date Deformation Directly? Example from the Mont Blanc Massif and Implications on Its Alpine Evolution. Corsalp meeting. Rolland et al., 2008. 40 Ar 39 Ar dating of synkinematic white mica: insights from fluid rock reaction in low grade shear zones (Mont Blanc Massif) and constraints on timing of deformation in the NW external Alps. Wibberley, C.A.J., Kurtz, W., Imber, J., Holdsworth, R.E. & Collettini, C. (eds). The Internal Structure of Fault Zones: Implications for Mechanical and Fluid Flow Properties. 299, 1 23. 81 10th Alpine workshop "CorseAlp2011" Stress Distribution at the Northwestern and Central Betic Cordillera-Alboran Sea: the Transition From Subduction to Continental Collision Ana Ruiz Constán 1, Jesús Galindo Zaldívar 2, 3 , Antonio Pedrera 4 & Carlos Marin Lechado 4 [email protected] montp2.fr (1) Géosciences Montpellier, CNRS Université Montpellier 2, Montpellier, France (2) Departamento de Geodinámica, Universidad de Granada, Granada, Spain (3) Instituto Andaluz de Ciencias de la Tierra, CSIC Universidad de Granada, Granada, Spain (4) Instituto Geológico y Minero de España, Madrid, Spain Seismic tomography models performed in the Mediterranean region had shed light on the deep structure of most of the belts that surround the Mediterranean Sea (Apennines, Dinarides, Maghrebides) confirming the presence of subduction slabs. In the western Mediterranean, subduction has also been invoked as a development mechanism of the Betic Rif Cordillera although its nature oceanic or continental , geometry, polarity, recent evolution and relationships with other mechanisms are not as well established as in other Alpine Mediterranean chains. Seismic activity in the Gibraltar Arc (Fig. 1) is characterized by shallow (0 40 km) and intermediate (40 110 km) earthquakes separated by a seismic gap of a zone of very deep earthquakes (620 660 km). The present day stress distribution along the slab is determined from the analysis of earthquake focal mechanisms. Most of the analyzed seismicity is concentrated in the Iberian continental lithosphere. Shallow seismicity (<40 km) is mostly located at the mountain front, its focal mechanism inversion mainly revealing NW SE horizontal compression, parallel to the Africa Eurasia convergence. At intermediate depths (50 80 km), the bending of the slab produces down dip compression and along strike horizontal extension in its inner part, whereas NE SW extension prevails in the external part. The presence of T axes highly dipping to the southeast at around 90 110 km depth coincides with the transition between oceanic and continental lithosphere. This down dip extension could be explained as the pull associated to the negative buoyancy of the oceanic lithosphere slab sinking into the mantle. Very deep events clearly show down dip compression and suggest the continuity of the oceanic lithosphere down to the 660 km discontinuity, where slabs experience resistance to further descent. Figure 1. A) General location of the Betic Cordillera in the framework of the Mediterranean Alpine chains; B) Distribution of small to moderate seismicity (Mb>3.5) occurring from 1986 to March 2010 (USGS/NEIC database, http://earthquake.usgs.gov/); GA, Gibraltar Arc; CA, Calabrian Arc. In this context, during the development of the Gibraltar Arc, the westward emplacement of the Alboran Domain occurred above an eastward subducted Euroasiatic oceanic lithosphere during the Early Middle Miocene, interacting with N S to NW SE Europe Africa plate convergence. Since Late Miocene, the subduction stopped due to the consumption of most of the oceanic lithosphere, and the NW SE convergence between the Eurasian and African major plates became the main geodynamic mechanism. Therefore, present day subduction could be considered practically deceased (greatly reduced intermediate and deep seismogenic areas) due to the unlikelihood of subducting continental lithosphere. The Iberian continental crust is forced to sink into the mantle by plate convergence at shallow levels, linked in depth to the pull of the oceanic lithosphere slab that is restricted to the western and central Betic Cordillera and Alboran Sea. The transition zone between the continental and oceanic lithospheric slab would be located around 90 110 km, in view of available seismic tomography images. The Gibraltar Arc is now in the transition from a subduction to a continental collisional setting. 82 10th Alpine workshop "CorseAlp2011" Lu-Hf Garnet Ages from the Southeastern Part of the Pohorje Mountains, Eastern Alps, Slovenia Sascha Sandmann 1, Daniel Herwartz 1, Frederik Kirst 1, Nikolaus Froitzheim 1, Thorsten Nagel 1 & Carsten Münker 2 s.sandmann@uni bonn.de (1) Steinmann Institut, Universität Bonn, Bonn, Germany (2) Institut für Geologie und Mineralogie, Universität Köln, Köln, Germany The Pohorje Mountains form the southeastern part of the Eastern Alps. The basement of the Pohorje Mountains mainly consists of high grade metamorphic gneisses and micaschists with bodies and lenses of metabasics and an up to 8 km wide body of ultramafic rocks (Kirst et al., 2010), the Slovenska Bistrica Ultramafic Complex (SBUC). The grade of Cretaceous metamorphism in the Eastern Alps increases to the southeast (Hoinkes et al., 1999), with maximum pressures and temperatures reaching up to 4 GPa and 900 °C in our study area, the southeastern Prohorje Mountains (Janák et al., 2006). The goal of this study is the comparison of the retrograde metamorphic history of samples from inside the SBUC with samples from outside the SBUC. Therfore two garnet peridotite samples and one eclogite sample from the SBUC and one eclogite sample from about two km north of the SBUC were dated by Lu Hf geochronology. All 11 garnet separates from all four samples give an isochron age of 96.8 ± 1.6 Ma (MSWD of 3.9), identical with an omphacite garnet isochrone of 96.68 ± 0.87 Ma (MSWD of 0.022) obtained from eclogite sample S34. These ages are higher than previously published Sm Nd garnet and U Pb zircon ages of ca. 91 Ma for eclogites from the southeastern Pohorje Mountains interpreted to reflect pressure peak conditions (Miller et al., 2005). Interestingly these ages concur with our Lu Hf whole rock garnet isochrones, which tend to give younger ages (88 Ma to 94 Ma), because our whole rock samples frequently plot above the isochron defined by the garnet separates. This points towards disequilibrium between the individual mineral phases and/or partial resetting of the Lu Hf isotope system. Clearly, 900 °C is higher than the assumed closure Temperatures of the U Pb, Sm Nd and Lu Hf isotope systems. Moreover , mobility of divalent cations is evident from the lack of major element growth zonation (Fe, Mg, Ca, and Mn) in single garnet grains of all our samples. Hence at least partial isotopic resetting can be expected. However, the Lu Hf system is assumed to be more robust against reequilibration than Sm Nd (c.f. Scherer et al., 2000) and our 11 point garnet isochron may still represent a garnet growth age, while the younger Lu Hf garnet whole rock ages probably reflect partial sample reequilibration. The Sm Nd garnet ages (Miller et al., 2005) may then either reflect a cooling age, or the discrepancy of ca. 6 Ma between Sm Nd and Lu Hf simply reflects prolonged timescales of garnet growth, where Lu Hf is strongly biased towards garnet cores (Lapen et al., 2003). In summary, the four samples of this study evidently share the same retrograde metamorphic history, since they form a well defined 11 point garnet isochron. If this isochron truly dates garnet growth, garnet growth in all samples is cogenetic. REFERENCES Hoinkes, G., Koller, F., Rantisch, G., Dachs, E., Höck, V., Neubauer, F., Schuster, R. (1999): Alpine metamorphism of the Eastern Alps. Schweizerische Mineralogische und Petrographische Mitteilungen, 79, 155 181. Janák, M., Froitzheim, N., Vrabec, M., Krogh Ravna, E.J., De Hoog, J.C.M. (2006): Ultrahigh pressure metamorphism and exhumation of garnet peridotite in Pohorje, Eastern Alps. Journal of metamorphic Geology, 24, 19 31. Kirst, F., Sandmann, S., Nagel, T., Froitzheim, N., Janák, M. (2010): Tectonic evolution of the southeastern part of the Pohorje Mountains (Eastern Alps, Slovenia). Geologica Carpathica, 61, 6, 451 461. Miller, C., Mundil, R., Thöni, M., Konzett, J. (2005): Refining the timing of eclogite metamorphism: a geochemical, petrological, Sm Nd and U Pb case study from the Pohorje Mountains, Slovenia (Eastern Alps). Contributions to Mineralogy and Petrology, 150, 70 84. Lapen, T.J., Johnson, C.M., Baumgartner, L.P., Mahlen, N.J., Beard, B.L., Amato, J.M. (2003): Burial rates during prograde metamorphism of an ultra high pressure terrane: an example from Lagi di Cignana, western Alps, Italy. Earth an Planetary Science Letters, 215, 57 72. Scherer, E.E., Cameron, K.L., Blichert Toft, J. (2000): Lu Hf garnet geochronology: Closure temperature relative to the Sm Nd system and the effect of trace mineral inclusions. Geochemica et Cosmochemica Acta, 64 (19), 3413 3432. 83 10th Alpine workshop "CorseAlp2011" Lower crust generation in the Jurassic Ligurian-Piedmontese Basin (Pineto gabbroic complex, Corsica) Alessio Sanfilippo 1 & Riccardo Tribuzio 1,2 (1) Dipartimento di Scienze della Terra, Università di Pavia, Pavia, Italy (2) C.N.R. Istituto di Geoscienze e Georisorse, U.O. di Pavia, Pavia, Italy The Pineto gabbroic complex from Northern Corsica consists of a layered gabbroic section locally intruded by basalt dykes with N MORB geochemical signature [1,2]. Despite the local occurrence of orogenic brittle deformation structures, the modal/grain size layering in the gabbroic rocks allowed us to reconstruct the original architecture of the Pineto gabbroic complex. We thus evaluated the original thickness of the gabbroic section, which reaches ~2000 m. The lowermost sector of the Pineto gabbroic complex is mainly made up of troctolites, locally including up to 50 m thick olivine gabbros layers. Olivine gabbros are predominant in the central sector of the sequence, where they are associated with minor clinopyroxene rich gabbros and troctolites. The uppermost sector of the sequence consists mostly of clinopyroxene rich gabbros. In addition, it contains two troctolite lenses (up to 100 m thick) that are associated with olivine rich troctolites. The magmatic layering of the clinopyroxene rich gabbros is locally crosscut by oxide gabbros showing diffuse contacts with respect to the host rocks. Several bodies of serpentinised spinel plagioclase mantle peridotites (up to 100 m thick) were also found within the gabbroic section. As to the gabbroic rocks, micro structures and the positive correlations between An in plagioclase and Fo in olivine (and Mg# in clinopyroxene) are reconciled with a process of fractional crystallisation of plagioclase + olivine → clinopyroxene by MORB type melts. We evaluated the cooling rate of the gabbroic complex using Ca in Olivine geospeedometer [3]. Our results document fairly constant and relatively high cooling rates, similar to gabbroic sections from (ultra )slow spreading ridges [4]. Taken as a whole, the large scale structure of the Pineto gabbroic complex bear many structural and compositional similarities with the two deepest sections documented at (ultra )slow spreading ridges (i.e. Atlantic Bank at South East Indian Ridge and Atlantis Massif at Mid Atlantic Ridge). Finally, we propose a model for the growth of the Pineto gabbroic complex as accretion of different melt lenses crystallizing in situ. REFERENCES Beccaluva, L., Ohnenstetter, D., Ohnenstetter, M., Venturelli G. (1977); Contrib. Mineral. Petrol., 64, 11 31; [2] Saccani, E., Principi, G., Garfagnoli, F., Menna, F. (2008); Ofioliti 33:187 207; [3] Coogan, L.A., Hain, A., Stahl, S., Chakraborty, S., 2005; Geochim. Cosmochim. Acta 69, 3683 3694; [4] Coogan L.A., Jenkin G.R.T, Wilson R.N. (2007); Journal of Petrology. 84 10th Alpine workshop "CorseAlp2011" Melt transport and deformation history in a “non-volcanic” ophiolitic section (Northern Apennine, Italy): implications for crustal accretion at slow spreading settings Alessio Sanfilippo 1 & Riccardo Tribuzio 1,2 (1) Dipartimento di Scienze della Terra, Università di Pavia, Pavia, Italy (2) C.N.R. Istituto di Geoscienze e Georisorse, U.O. di Pavia, Pavia, Italy Field observations, petrological and geochemical data are used to constrain a conceptual model for the formation and evolution of the Scogna Rocchetta Vara gabbro peridotite section from Internal Ligurian ophiolites (Italy). The studied section is attributed to an intra oceanic domain of the Jurassic Ligurian Piedmontese basin and is characterised by the lack of basalt flow layer, similar to non volcanic segments from (ultra )slow spreading ridges. The data presented allowed us to reconstruct the exhumation of a moderately depleted, spinel facies mantle section. This exhumation was associated with repeated events of reaction with migrating melts and was followed by the intrusion of MORB type gabbroic rocks and the exposure of the gabbro peridotite association at the seafloor. The proposed model shows a “hot” lithospheric evolution in which melt transport occurred in form of grain scale porous flow. We recognised a series of melt/peridotite interaction events, either diffuse or channeled, which modified the composition of the precursor mantle. In particular, localized infiltrations of MORB type melts gave rise to formation of spinel websterite layers. Websterite layers were transposed by a deformation event that developed a tectonitic fabric in the host peridotites. The spinel facies deformation event most likely occurred under lower lithospheric temperature conditions. In the plagioclase stability field, the peridotites were subjected to impregnation by depleted melts and formation of replacive dunitic conduits. The “hot” lithospheric evolution was followed by an evolution characterized by melt transport through fractures, which started with melt crystallization into troctolite to olivine gabbro dykes. Both mantle structures and gabbroic dykes are locally crosscut by gabbroic sills. As the mantle section cooled significantly, therefore, the dip of the melt migration structures evolved from sub vertical to sub horizontal. The growth of a gabbroic pluton (up to ~400 m thick), which contains mantle and olivine rich troctolitic bodies, is attributed to accretion of gabbroic sills. The tectono magmatic history recorded by the gabbroic pluton after its solidification is characterised by ductile shearing developed from near solidus to amphibolite facies conditions. The following brittle evolution is associated with injections of dykes with N MORB type geochemical signature and finally led to exposure of the gabbro peridotite association at the seafloor. 85 10th Alpine workshop "CorseAlp2011" Exhumation-Related Structures at the Eastern Margin of the Tauern Window (Eastern Alps, Austria) Andreas Scharf 1, Silvia Favaro 1, Mark R. Handy 1 & Stefan M. Schmid 2 [email protected] berlin.de (1) Freie Universität Berlin, Germany (2) ETH Zürich, Switzerland Cross sections across and correlations of tectonic units within the Tauern Window (Figs. 1 3) yield insight into its deformation history (Schmid et al., this vol.). This, together with structural analysis, reveals early subduction (D1) and thrusting accompanied by isoclinal folding (D2) of slices of oceanic lithosphere (Glockner Nappe, GN = Valais branch of Alpine Tethys) and continental crust (Rote Wand Seidlwinkl Nappe = RS, Venediger Nappe Complex = VNC) derived from the distal European margin (Favaro et al., this vol.). N directed thrusts (D3) led to the accretion of Europe derived basement in a duplex (“Venediger Duplex”). Here we focus on exhumation (D4) during which these duplexes underwent further N S shortening, contemporaneous with orogen parallel normal faulting and strike slip ductile shearing under retrograde amphibolite to subgreenschist facies conditions. The Katschberg Shear Zone (KSZ, Figs. 1 3) is the key element of this D4 system of kinematically linked shearing and folding. In map view this 3 5 km thick belt of mylonites is associated with a penetrative (S4) foliation that swings around a series of simultaneously formed antiforms (Hochalm, Sonnblick) and a synform (Mallnitz). The eastern segment of the KSZ is a low angle (25 30°, Fig. 2), SE dipping, top E to SE ductile normal fault capped by a 10 100 m wide zone of cataclasites known as the Katschberg Normal Fault (KNF). At the northern and southern terminations of the KNF, the ductile KSZ curves into an orogen parallel orientation and runs within calc schist (Bündnerschiefer) of the GN. It lacks a cataclastic overprint and is characterized by moderately to steeply dipping mylonitic foliation (S4) with subhorizontal stretching lineations Ls4 (Fig. 1). The northern, E W trending branch exhibits dextral sense of shear, whereas the southern branch is sinistral and strikes NW SE, i.e. subparallel to the brittle D5 Mölltal Fault with predominantly dextral shear sense indicators that overprint D4 mylonite. The kinematic continuity of top E to SE directed normal faulting along the KNF with ductile dextral and sinistral strike slip motion at its ends, combined with the coincidence of the KNF with the greatest amount of tectonic omission around the Hochalm Dome all indicate that N S shortening, strike slip shearing and orogen parallel normal faulting were broadly coeval. The kinematics of the KSZ system indicates that units in the Tauern Window underwent exhumation by a combination of extensional unroofing and erosional denudation. The cooling history inferred from the literature constrains the KSZ to have formed between 28 and 17 Ma. Exhumation initiated with orogen parallel extension, with initially rather flat lying branches at the N and S ends of the KNF. As the amount of N S shortening increased, these branches acquired progressively steeper orientations and finally accommodated predominantly strike slip motion. The steepened parts of the KSZ represent stretching faults, such that the offset along them decreases from E to W. This coincides with an overall decrease in the intensity of D4 N S shortening and E W extension towards the central part of the Tauern Window. Dextral motion along the brittle Mölltal Fault occurred after the KSZ was deactivated at about 17 Ma. In summary, the KSZ at the eastern end of the Tauern Window may pre date a substantial part of the N S to NE SW directed indentation of the Adriatic microplate that was accommodated by continued and more intense upright folding in the W part of the Tauern Window, as well as by strike slip motions along the Periadriatic Line and the SEMP line (Schneider et al., this vol.). Fig.1 Fig.2 Fig.1: Simplified tectonic map (after Schmid et al., this vol.) Fig.3 showing traces of profiles in Fig.2 and Fig.3. Units: VNC- Venediger Nappe Complex, HA-Hochalm Nappe, RS-Rote Wand-Seidwinkl Nappe, GN-Glockner Nappe, MA- Matrei Unit, AA-Austroalpine Nappes. Equal area plots: Circles-poles to S4, Crosses - Ls4 86 10th Alpine workshop "CorseAlp2011" The Fate of a Composite Permian Intrusion During Alpine Collision: the Rofna Porphyry Complex of the Suretta Nappe (E Switzerland) Thomas Scheiber, O.A. Pfiffner & G. Schreurs Institute of Geological Sciences, University of Berne, Switzerland Stacking of crystalline basement nappes is observed in the core of collisional orogens. Using a field based approach we try to get a better understanding of strain accumulation during basal accretion processes in the Alps. In eastern Switzerland, the Suretta nappe comprises a stack of lithospheric slices derived from the Briançon domain and assembled in a south dipping subduction zone during the Cenozoic orogenic cycle. Eocene top north directed stacking (Ferrera phase after Milnes & Schmutz, 1978; Schmid et al, 1997) predates a phase of backfolding (Niemet Beverin phase). The present day axial plunge of about 30° towards the ENE exposes the basal thrust of the Suretta nappe over tens of kilometres and provides nearly continuous outcrop from bottom to top of the nappe. The most conspicuous features in map and cross sectional view are sediment intercalations, reaching fairly deep into the crystalline basement. These intercalations of Triassic sediments in the upper parts of the nappe appear to be isoclinal folds at first sight, but closer inspection reveals a component of thrusting as well. Emphasis was laid on the frontal part of the nappe composed essentially of late to post Variscan intrusive rocks of the Rofna Porphyry complex. Porphyritic rock types are in intrusive contact with rocks having the same mineralogical composition but containing much bigger feldspar clasts. This augengneiss facies is in many cases directly underlying autochthonous Triassic sediments and was interpreted to be of Ordovician age by various authors. Preliminary results of LA ICP MS measurements on zircons, however, indicate late Variscan intrusive ages for the augengneiss as well. Detailed structural mapping combined with strain analyses using the center to center (Fry) method based on the distribution of K feldspar clasts (Genier and Epard, 2007) yield different strain patterns of the magmatic rocks : Mylonites were not only detected at the base of the Suretta nappe, but also at the base of one internal thrust slice overlying highly deformed autochthonous Triassic sediments (“nappe separators”). These mylonites bear NNW SSE trending streching lineations that are related to Ferrera phase nappe stacking. Despite the fact that shear sense indicators are generally rare, a top to the NNW directed transport is assumed. In the lower and interior parts of the nappe, weakly to undeformed boudins are generally surrounded by L tectonites indicating WSW ENE stretching; foliated equivalents reveal variable strain intensities. The upper part of the nappe which was strongly affected by retro shearing and back folding generally shows higher strains. In this part of the nappe, the Ferrera phase strain ellipsoid was overprinted by strong top to the SSE directed shearing. This resulted in an oblate finite strain ellipsoid. In the interior and lower parts of the nappe, contrariwise, the Ferrera phase strain ellipsoid was affected by minor horizontal shortening, which lead to a predominantly WSW ENE constrictional finite strain state. In order to unravel fold thrust relationships in the Suretta nappe, a stepwise retro deformation was performed combining field observations and data gathered from strain analyses. REFERENCES Schmid, S. M.; Pfiffner, O. A.; Schreurs, G. 1997: Rifting and collision in the Penninic Zone of eastern Switzerland. In. Pfiffner, O.A., Lehner, P., Heitzmann, P., Mueller, St. & Steck, A. (Eds) Deep Structure of the Swiss Alps: Results of NRP 20, Birkhäuser Verlag , p 160 185. Milnes, A.G. & Schmutz, H. U. 1978: Structure and history of the Suretta nappe (Pennine zone, Central Alps) a field study. Eclogae geologicae Helvetiae, 71/1, 19 33. Genier, F.; Epard, J.L. 2007: The Fry method applied to an augen orthogneiss: Problems and results. Journal of Structural Geology, 29, 209 224. 87 10th Alpine workshop "CorseAlp2011" Kinematic and Metamorphic Evolution of the Lukmanier Pass Region (Switzerland): Extension Related History? Filippo L. Schenker 1, Eric Reusser, Mark James Caddick, Claudio Madonna, Daniele Grassi & Jean Pierre Burg (1) ETH Zurich, Department of Earth Sciences, Zurich, Switzerland Detailed mapping, structural and petrological studies of the Lukmanier Pass area, in the Swiss Alps, document the kinematic and metamorphic evolution of a key segment of the northern margin of the Lepontine Dome. The mapped area covers three tectonic units, from north to south: (1) the Gotthard Massif, (2) its parautochthonous Mesozoic cover and (3) the Lucomagno Massif. The intermediate pressure metamorphism increases from greenschist facies in the north to amphibolite facies in the south. The main foliation generally strikes E W and dips steeply to the north. The contact between the Gotthard Massif and the parautochthonous Mesozoic series is characterized by top to the S sense of shear, which indicates back thrusting in the context of the Alps. The contact between the parautochthonous series and the Lucomagno Massif is characterized by top to the N shear sense, thus defining ductile normal faulting. Structural and metamorphic relationships suggest that a single protracted deformation event formed these distinct and opposite shear senses (although geochronological data are required to confirm the ages of the shear). Elongated minerals of the peak temperature metamorphic assemblages (i.e. staurolite, kyanite and amphibole) locally define the kinematic lineation and attest to north dipping ductile normal faulting. This juxtaposed the “cold” Gotthard massif in the hanging wall with the “hot” Lucomagno massif in the footwall, sandwiching the Mesozoic units during peak metamorphic conditions. The peak condition was calculated from a garnet kyanite staurolite biotite metapelite near the contact with the Lucomagno massif. Detailed petrological work combined with thermodynamic modeling show a well constrained clockwise P T path with an early temperature estimate of ca. 550 °C obtained from inclusions in prograde garnet and followed by a HT peak at ca. 650 °C and 8 kbar. The duration of these metamorphic phases will be discussed by means of 1D modeling of major element intra crystalline diffusion in garnet. At a regional scale, the contact between the Gotthard massif in the north and the Penninic units in the south is characterized by top to the NW thrusting with a dextral strike slip component. This dextral component created a pull apart extension responsible for top to the N normal faulting in the Lucomagno region. The apparent back thrusting of the Gotthard massif is probably related to a local mechanical instability during the top to the N normal faulting. 88 10th Alpine workshop "CorseAlp2011" New Tectonic map of the Alpine collision zone between Eastern Alps and Western Turkey Stefan M. Schmid 1,2 , Daniel Bernoulli 2, Bernhard Fügenschuh 3, Liviu Matenco 4, Roland Oberhänsli 5, Senecio Schefer 2 & Kamil Ustaszewski 6 (1) ETH Zürich, Switzerland (2) Univerität Basel, Switzerland (3) Universität Innsbruck, Austria (4) Vrije Universiteit Amsterdam, Holland (5) Universität Potsdam, Germany (6) GeoForschungsZentrum Potsdam, Germany We present a map that extends a previously published tectonic overview (Schmid et al. 2008) into Greece and Western Turkey. The Dinarides, linked to the Alps Western Carpathians along the present day Mid Hungarian fault zone (a former transform fault), represent an orogen of opposite subduction polarity with respect to the Alps. Dinarides and Hellenides alike consist of far travelled nappes detached from the Adriatic continental margin along Paleozoic or younger decollement horizons during Cretaceous and Cenozoic orogeny. The more internal nappes (i.e. Drina Ivanjica, Pelagonian) are composite nappes in which the allochthonous Adriatic margin sequences passively carry previously (during the latest Jurassic) obducted ophiolites (Western Vardar Ophiolitic Unit). Hence, such obducted ophiolitic units (for example, the Mirdita ophiolites) have to be rooted in one single oceanic Neotethys suture, the Sava Izmir Ankara suture. Therefore such ophiolitic “massifs” found SW or S of the Sava Izmir Ankara suture zone do not mark oceanic sutures nor do the Drina Ivanjica and Pelagonian “massifs” represent independent continental fragments (terranes). The same logic applies to Western Turkey with the difference that the ophiolites that overly Tavsanli zone, Ören Afyon unit (Pourteau et al. 2010) and the Lycian composite nappe were obducted in Late Cretaceous rather than Late Jurassic times. The Sava Zone defined in the northern part of the Dinarides comprises the following rock associations (Ustaszewski et al 2010) that define a suture zone (Sava suture) located between a non metamorphic upper plate (Tisza Mega Unit) and a lower plate that consists of a nappe stack derived from the Adriatic margin: (1) Intra oceanic magmatic associations covered by pelagic sediments, (2) Maastrichtian age siliciclastic sediments, deposited along the Adria Europe plate boundary and incorporated into an accretionary wedge and (3) Metasediments indicating temperatures of 350 to 630 °C and pressures up to 5 7 kbar reached at around 65 Ma. Across the Aegean Sea the Sava suture follows the southern rim of the Circum Rhodope Belt, finally ending up in the Izmir Ankara zone. This zone separates the Anatolides Taurides as parts of the Adria microplate from the Pontides located north of the Neotethys suture, made up of highly sheared flysch units of latest Cretaceous to earliest Paleogene age in western Turkey. The Budva Cukali Krasta Pindos paleogeographic zone represented an intra platform pelagic seaway during the Triassic (Bernoulli 2001) that persisted into Eocene times. In the southern part of the external Dinarides and in the western Hellenides the incompetent pelagic strata and overlying Cenozoic flysch became the site of a decollement zone also for the far travelled higher nappes such as the High Karst Parnassos nappes and the Korabi Pelagonian composite nappe that carries previously stacked nappes on its back. This thrusting was active in Late Eocene times. Since the amount of Cenozoic age shortening substantially increases eastward, the Budva Cukali Krasta Pindos nappe laterally develops into an intracontinental suture zone marked by the Cycladic Blueschist Zone of the eastern Hellenides that can be traced into Western Turkey (Dilek nappe) were this blueschist belt overlies and surrounds the Menderes tectonic window. Conversely, the amount of Cenozoic age displacements accommodated within the Budva Cukali Krasta Pindos tectonic unit dramatically decreases towards the northwest, i.e. within the northern external Dinarides where the Budva zone wedges out. The reasons for this are that (1) much of the Europe Adria convergence is here taken up by the Alps that exhibit an opposite subduction polarity and (2) that the amount of Africa Europe convergence decreases westward. Important lateral changes are also observed when comparing the present day lithospheric configuration of the Dinarides with that of the Hellenides. While the Adriatic lithospheric slab can only be traced down to a depth to some 200km and its deeper parts broke off (Spakman &Wortel 2004), an over 2100 km long slab is still preserved below the Aegean part of the Hellenides (Hafkenscheid et al. 2006), indicating long lasting subduction of one single lithospheric slab that initiated during the onset of closure of Neotethys (Jurassic?) and accommodated some 1500km of plate convergence after Neotethys closed, i.e. after the Sava Izmir Ankara suture zone formed some 65Ma ago according to the crustal scale retrodeformation provided by Jolivet & Brun (2010). REFERENCES Bernoulli, D., 2001. Mesozoic Tertiary carbonate platforms, slopes and basins of the external Apennines and Sicily. In: Anatomy of a Orogen: the Apennines and adjacent Mediterranean basins (Eds G.B. Vai and P. Martini), pp. 307 325. Kluwer Academic Publishers, Dordrecht. Hafkenscheid, E., Wortel, M.J.R., Spakman, W., 2006. Subduction history of the Tethyan region derived from seismic tomography and tectonic reconstructions. Journal of Geophysical Research 111: doi:10.1029/2005/B003791, B08401. Jolivet, L. & Brun, J.P., 2010. Cenozoic geodynamic evolution of the Aegean. Int J Earth Sci 99: 109 138. Pourteau, A., Candan, O., Oberhansli, R., 2010. High pressure metasediments in central Turkey: Constraints on the Neotethyan closure history. Tectonics 29, TC5004. Schmid, S.M., Bernoulli, D., Fügenschuh, B., Matenco, L., Schefer, S., Schuster, R., Tischler, M. & Ustaszewski, K., 2008. The Alpine Carpathian Dinaridic orogenic system: correlation and evolution of tectonic units. Swiss Journal of Geosciences, 101(1): 139 183. Spakman, W., Wortel, M.J.R., 2004. A tomographic view on Western Mediterranean geodynamics. Chapter 2 in: W. Cavazza, F.M. Roure, G.M. Stampfli, P.A. Ziegler (Editors), The Transmed Atlas The Mediterranean region from crust to mantle. Springer, Berlin, New York, pp. 31 52. Ustaszewski K., Kounov A., Schmid, S.M., Schaltegger, U., Krenn, E., Frank, W. & Fügenschuh, B., 2010. Evolution of the Adria Europe plate boundary in the northern Dinarides: From continent continent collision to back arc extension. Tectonics 29, TC6017, 1 34, doi:10.1029/2010TC0026 89 10th Alpine workshop "CorseAlp2011" New Tectonic map and Cross-Sections of the Tauern Window (Eastern Alps, Austria) Stefan M. Schmid 1,2 , Andreas Scharf 2, Mark R. Handy 2, Claudio Rosenberg 2, Silvia Favaro 2 & Audrey Bertrand 2 (1) ETH Zürich, Switzerland (2) Freie Universität Berlin, Germany A new tectonic map of the Tauern Window was compiled for three reasons: (1) we felt that the traditional and local nomenclature using terms such as “Zentralgneise”, “Altes Dach”, “untere Schieferhülle”, “obere Schieferhülle” and “Nordrahmenzone” is nowadays outdated. It needs to be replaced by more adequate terminology in order to reflect the progress made in geology since the last 100 years and to better understand orogenic processes, (2) we needed a unified basis for understanding the complex 3 dimensional architecture of the Tauern Window and surrounding Austroalpine units in order to construct a series of profiles, and, (3) we think that map and profiles are useful for a better understanding of the kinematics and dynamics of deformation in a first class natural laboratory. The tectonic map of the Tauern Window provided by Kurz et al (1998), introducing the concept of nappes rather than “Schieferhüllen”, was influental for attempting a new map compilation and provided a solid basis. The number of published articles and maps is by far too long to be mentioned and here we can only point out the most important ones, namely the Salzburg map and explanatory notes (Pestal et al. 2005, 2009), together with all the detailed maps available from the Geologische Bundesanstalt (http://www.geologie.ac.at/). The major tectonic units are, from top to bottom (1) the Austroalpine Nappe System that was subdivided according to the scheme proposed by Schmid et al. (2004); (2) the Matrei Zone, remnants of the Alpine Tethys (Piemont Liguria Ocean) accreted to the Austroalpine already in Cretaceous times; (3) the Glockner Nappe comprising units deposited on oceanic lithosphere of the Valais branch of Alpine Tethys, not accreted to the upper plate before Eocene times; (4) units derived from the European distal margin that entered the Alpine subduction zone early on and are currently found above the roof thrust of a crustal scale duplex, the Venediger Nappe Complex (VNC) that formed later (Seidlwinkl Rote Wand Nappe, Eclogite Zone and Wolfendorn Nappe) and finally (5) the VNC crustal scale duplex itself, subdivided into three major thrust sheets, from top to bottom, the Zillertal Riffl Sonnblick Storz Nappe, the Tux Granatspitz Hochalm Nappe, and the Ahorn Göss Nappes forming horse type imbricates above a sole thrust that remained buried at depth. Analysis of the 3D geometry, together with literature data, reveals the following sequence of kinematic steps during the Cenozoic, i.e. after pre Cenozoic suturing between Austroalpine Nappes and Matrei Nappe. D1: early subduction: Eclogite facies metamorphism in parts of the Glockner Nappe, within the Eclogite Zone and parts of the Seidlwinkl Rote Wand Nappe (Kurz et al. 2008) documents subduction of an ocean continent transition zone below Austroalpine and Matrei Nappes during the onset of collision, as the Alpine Tethys closes at around 42 Ma (Ratschbacher et al.2004). D2: Isoclinal folding of the contact between Glockner Nappe and Seidlwinkl Rote Wand Nappe. This event is only locally preserved in the central part of the Tauern Window that escaped significant overprint during the final stages of exhumation. It is linked to the gradual exhumation of high p rocks arriving at a depth where amphibolite grade conditions prevail at around 32Ma (Kurz et al. 2008) and linked with a first Cenozoic event of nappe emplacement. D2 structures are restricted to units located above the roof thrust of the VNC duplex. D3: formation of the Venediger Nappe Complex: During the mature stages of collision massive volumes of Europe derived upper crust entered the subduction zone and, due to buoyancy forces, were accreted to the upper plate before reaching the critical depth for high p metamorphism. Massive accretion of upper continental crust provides the heat source for Barrow type metamorphic overprint. D4: predominantly ductile components of final exhumation: The activity of the Katschberg and Brenner kinematic systems starting at around 25 Ma ago leads to contemporaneous orogen perpendicular compression, orogen parallel extension and strike slip motions that shape the final geometry of the Tauern Window. They involve both tectonic and erosional unroofing and are driven by the indentation of the Adriatic plate (Rosenberg et al. 2007) as well as by a component of orogen parallel extension due to roll back in the Carpathians. D5: brittle components of final exhumation: These heavily affect the western Tauern window where they lead to a through going brittle component of normal faulting, also across parts of a pre existing Brenner Line, while the dextral Mölltal Fault develops as a Riedel shear of the dextral Periadriatic Line; brittle faulting within and outside the window is strike slip dominated. REFERENCES Kurz, W., Neubauer, F., Genser, J. & Dachs, E. 1998: Alpine geodynamic evolution of passive and active continental margin sequences in the Tauern Window (eastern Alps, Austria, Italy): a review. Geologische Rundschau 87, 225 242. Kurz, W., Handler, R. & Bertoldi, C. 2008: Tracing the exhumation of the Eclogite Zone (Tauern Window, Eastern Alps) by 40Ar/39Ar dating of white mica in eclogites. Swiss Journal of Geosciences 101, S191 S206. Pestal, G., Hejl, E. and others 2005: Geologische Karte von Salzburg 1:200.000, Geologische Bundesanstalt Wien. Pestal, G., Hejl, E., Braunstingl, R. & Schuster, R. 2009: Erläuterungen Geologische Karte von Salzburg 1 : 200 000. Land Salzburg & Geologische Bundesanstalt, 1 162. Ratschbacher, L., Dingeldey, Ch., Miller Ch., Hacker, B.R. and McWilliams, M.O. 2004: Formation, subduction, and exhumation of Penninic oceanic crust in the Eastern Alps: time constraints from 40Ar/39Ar geochronology. Tectonophysics 394, 155 170. Rosenberg, C.L., Brun, J.P., Cagnard, F. & Gapais, D. 2007: Oblique indentation in the Eastern Alps: Insights from laboratory experiments. Tectonics 26, TC2003. doi:10.1029/2006TC001960. Schmid, S.M., Fügenschuh, B., Kissling, E. and Schuster, R., 2004. Tectonic map and overall architecture of the Alpine orogen. Eclogae geologicae Helvetiae 97: 93 117. 90 10th Alpine workshop "CorseAlp2011" An Element Mobility and Geochronological Investigation of a Mid-crustal Alpine Shear Zone - La Valetta Shear Zone (Argentera-Mercantour) Julie Schneider 1, Clément Ganino 1, Laure Fernandez 2, Yann Rolland 1, Delphine Bosch 2 & Guillaume Sanchez 1 (1) GeoAzur, Université de Nice Sophia Antipolis, CNRS, Nice, France (2) Géosciences Montpellier, Université Montpellier II, CNRS, Montpellier, France La Valetta shear zone, which is located in the Argentera Mercantour massif (SW Alps, France) consists of a 500m wide N140°E dextral transpressive Alpine shear zone. This mylonitic shear zone is characterized by a deformation gradient from almost undeformed Variscan migmatitic gneiss protolith to ultramylonite. It allows investigations of 1) element mobility in the course of fluid assisted deformation and metamorphism in the middle crust, and 2) the behavior of radiochronometers during these processes. A 200m wide profile transverse to the shear zone was sampled in order to perform petrological, geochemical and geochronological analyses. Mineralogical reactions have been established for the various stage of mylonitisation and show enhancement of element mobility toward the most deformed rocks. It mainly consists of plagioclase and biotite breakdown into phengite, chlorite and Ti bearing minerals. These reactions indicate greenschist facies conditions (375+/ 30°C; 6+/ 1kbar) in water saturated system. Mineralogical reactions operate at 500µm scale textural domains in protomylonites, whereas the scale of reactions is about 5mm in ultramylonite. Enhancement of element mobility is further supported by whole rock major and trace elements analyses along the shear zone profile. Composition profiles show almost linear increase or decrease of elements in the last 20m of the shear zone. In particular, Ti, Fe, Ca and LREE are strongly enriched and Si depleted. Moreover, Ar/Ar dating has been performed on single muscovite grains from the same samples. The muscovites are porphyroclasts inherited from the Variscan protolith and show only slight resetting (total ages range between 254 and 325 Ma). However, the age profile obtained along the shear zone is perfectly correlated with the composition profile, especially in the last 20m (see figure below). The whole of these results allows us to discuss element mobility in the light of mineralogical reactions, fluid/rock ratio and fluid saturation and their relationship with deformation. Moreover, the consequences on the Ar/Ar system and implications on dating techniques will be discussed. 91 10th Alpine workshop "CorseAlp2011" Erosive Denudation vs. Extensional Unroofing; A Discussion on Exhumation of the Tauern Window Susanne Schneider 1, Audrey S. Bertrand 1, Silvia Favaro 1, Sebastian Garcia 1, Konrad Hammerschmidt 1, Mark R. Handy 1, Claudio L. Rosenberg 1, Andreas Scharf 1 & Stefan M. Schmid 2 (1) Freie Universität Berlin, Institute of Geological Sciences, Germany (2) Eidgenössische Technische Hochschule Zürich, Institute of Geophysics, Switzerland In this combined study we present new structural and geochronological results of the Tauern Window (Eastern Alps) from sub millimetre scale to map view and compile them with literature data. We focus on the kinematics related to exhumation in Oligocene and Miocene time. Exhumation is higher and younger in the western than in the eastern Tauern Window. To understand the results in a dynamic sense, we discuss the influences of the driving forces South Alpine indentation and subduction rollback within the Carpathians acting during Oligocene/Miocene time. On the base of a new tectonic map and sections we divide the structural evolution of the Tauern Window into five deformation stages (Schmid et al. this vol.). Deformational stage (D4) is ductile and mainly responsible for the present day appearance of the tectonic window. The Tauern Window is a structural and metamorphic dome, to its western and eastern ends it is bordered by two km scale normal fault systems (NFSs), the Brenner NFS in the west and the Katschberg NFS in the east. These NFSs consist of mylonitic shear zones (SZs), which are capped by co genetic brittle normal faults (NFs) during cooling (D5). To its northern and southern margins it is bordered by a series of ductile shear zones and brittle strike slip faults. During D4 pre existing structures (D1 D3) were folded in upright fashion and form an orogen parallel dome. Due to different style in deformation (D4) the Tauern Window is organised in eastern, central and western sub dome. The central sub dome preserves isoclinally folded nappe contacts (D2), which are less affected by D3 and D4 overprinting. The more open folds of the eastern sub dome are slightly elongated and have broad hinges. They strike SE NW and are separated by a tight syncline (Favaro et al., this vol.). Main foliation within the eastern sub dome is a composite foliation (S1/S2) imbricated during D3 and folded during D4. The eastern end of the sub dome is surrounded by the ductile Katschberg NFS, whose northern and southern ends bend into steep dextral and sinistral shear zones, respectively, which remain within the window. Main foliation along the Katschberg NFS is a mylonitic S4 foliation formed during doming and coeval extensional unroofing (Scharf et al., this vol.). In contrast in the west, upright, ENE WSW trending folds (D4) are tight with km scale amplitudes and associated with m to km scale, predominantly sinistral shear zones in their steep limbs and tight synclines. Main foliation within the western sub dome is a sub vertical axial plane foliation (S4) with a sub horizontal stretching lineation. Along the bordering and intervening sinistral shear zones S4 foliation is mylonitic. To the western margin foliations (S4) become flat and form the Brenner SZ, a mylonite belt of ca. 5 km thickness with normal sense of shear. The spatial change over from steep into flat S4 foliations occurs continuously along the Olperer SZ within the window and along the Jaufen SZ outside, both showing combined sinistral and normal sense of shear. Towards the NE steep S4 foliations acquire NE trends in the vicinity of Mittersill and merge into the brittle sinistral SEMP line at its western end. This structural difference between the eastern and western sub domes coincides largely with a distinctive cooling history defined by Rb/Sr cooling ages of micas, Ar/Ar cooling ages of micas, apatite fission track ages (AFT) and zircon fission track ages (ZFT) but crosscut folded nappe contacts (D1 D3). In the eastern sub dome isochronal cooling ages form a broad, triangular pattern, which runs parallel to the Katschberg NFS. More or less the entire sub dome cooled from above 550° C below 230° C between 27 17 Ma. Because of consistency between structural and thermal patterns within the eastern sub dome, ductile activity along the Katschberg NFS is constrained by cooling ages. Extensional unroofing (D4) terminated earlier in the east at ~ 17 Ma. In contrast the cooling pattern in the west is narrow, concentric and ENE WSW elliptical. It has steep gradients in age values to its northern, southern and western margins and ZFT ages as young as ~11 Ma in the centre and the Brenner NFS. The western sub dome cooled from above 600° C below 230° C between 20 11 Ma. Distribution of cooling ages displays two D4 structural features, the Brenner NFS on the one hand and upright tight folds on the other. These cooling ages overlap with formation ages of syn kinematically grown phengites and K feldspars, which range between 33 7 Ma. Post kinematic blasts post date mineral formation during sinistral shear at 15, 12 and 9 Ma. Within the eastern sub dome the cooling pattern is dominated by extensional unroofing along the Katschberg NFS, whereas for the western sub dome the cooling pattern is influenced by both, extensional unroofing along the Brenner NFS and upright, tight D4 folding followed by erosive denudation. The influence of the fold geometry on the cooling pattern nearly reaches the Brenner NFS, therefore we interpret the western sub dome to be dominated by doming and consequently erosive denudation. Published, late stage (D4) uplift rates are about 3.6 mm/yr for the western and 4.5 mm/yr for the eastern sub dome. During cooling the uplift rate decreased markedly to 0.4 mm/yr for the entire Tauern window, deduced from calculated age elevation relation of ZFT data. Post D4 uplift rates within the eastern sub dome decreased further to 0.1 mm/yr, whereas uplift rate within the western sub dome keep constant 0.4 mm/yr, deduced from age elevation relation of AFT data. Brittle deformation (D5) is mostly taken up by the Brenner NF at the western sub dome and the fault zones bordering the South Alpine indenter (Meran Mauls and Pustertal line). The interior of the Tauern Window is mainly affected by strike slip faulting passing into extensional faults close to the Brenner and the Katschberg NFSs. The direction of the minimal stress (σ3) is ESE WNW to ENE WSW for the strike slip regimes inside the Tauern Window, whereas along the Brenner and Katschberg NFs σ3 direction is oriented ESE WNW and SE NW, respectively. The lack of evidence for a stress field compatible with upright folding suggests that the rocks affected in the brittle domain by the related inverse regime have been eroded since (Bertrand et al., this vol.). Active tectonics was deduced from combining GPS data and seismicity over the entire Eastern Alps. Present motion of the entire Tauern Window is slightly northward. East of the window lateral extrusion of the Gurktal block and the Styrian basin towards the northeast still occurs (Garcia, this vol.), with respect to stable Eurasia. Ongoing N S convergence is mostly taken up within the Southern Alps while eastward extrusion occurs north of the SEMP line and east of the Katschberg NFS. We interpret the central and eastern sub dome to be decoupled at ~17 Ma from the western sub dome and preserving an earlier structural stage, which has been overprinted in the western sub dome. Continued N S shortening was then taken up within the western sub dome by brittle normal faulting along the Brenner NF, upright folding and sinistral strike slip motion, which was transferred through the western sub dome into the brittle SEMP line. We favour subduction rollback of the Carpathians and related E W extension at ~25 Ma as the main reason for decoupling the central and eastern sub dome from D4 doming induced by South Alpine indentation. 92 10th Alpine workshop "CorseAlp2011" The Eastern and Western Alps boundary in Vorarlberg (western Austria) Marcel Schulz 1, 2 , Hannah Pomella 1, Michael Zerlauth 1, 2 , Hugo Ortner 2 & Bernhard Fügenschuh 2 schulz@alps gmbh.com (1) AlpS Centre for climate change adaption technologies, Innsbruck, Austria (2) Institute of Geology und Palaeontology, University of Innsbruck, Austria In the context of a deep geothermal potential study for Vorarlberg several crustal scale cross sections were constructed between the Rhine valley in the west and the Kleinwalser valley in the east. The construction of these cross sections is based on surface geology, drillings, as well as on reinterpreted seismic lines and incorporates published sections. The general geological architecture of the examined area can be described as a typical foreland fold and thrust belt, comprising the tectonic units of the Subalpine Molasse, (Ultra )Helvetic, Penninic, and Austroalpine nappes. Along the south dipping listric Alpine basal thrust these units overthrust the autochthonous Molasse. The Subalpine Molasse itself is multiply stacked, forming a large triangle zone which can be identified in seismic lines (AICHHOLZER et al. 2010 ). Other well defined seismic features are the top of the European basement and its autochthonous Mesozoic cover, both dipping southwards from about 3500m BSL near Bregenz to approx. 6500m BSL in Liechtenstein. Furthermore a discontinuous double reflector, interpreted as the base of the Helvetic nappe complex, could be identified. Because of the quality of the seismic lines, shot between the 1970s and 1990s, the complicated internal structure of the Helvetic nappe stack could not be determined. The assumed hinterland dipping duplex structure of the Helvetic nappes results from surface outcrop and borehole data. However, at least two Helvetic nappes are necessary to fill the space between their basal reflector and Earth’s surface. The deeper nappe, most often labeled “Hohenemser nappe” (WYSSLING 1985), was probably overthrusted by the superficially exposed “Säntis nappe”. Thereby, the Cretaceous cover units of the “Hohenemser nappe” were possibly detached from the more competent Jurassic units, pushed northwards, and folded, before finally also being overthrusted. In the area of Bludenz, the Helvetic basal reflector can be recognized at a depth of about 5000m BSL. Its top is interpreted to lie at approx. 2000m BSL, overlain by the Vorarlberger Flysch. This Helvetic nappe stack can possibly be interpreted as the southward extension of the Säntis nappe. Alternatively, a third Helvetic nappe, overthrusted by both aforementioned nappes, is conceivable. The southward extension of the entire Helvetic nappe stack remains, due to a lack of seismic data, undisclosed to this point. Based upon our construction, the shortening within the Helvetic nappes amounts to approx. 50%, as determined this far for the Cretaceous Kieselkalk and the Jurassic Quinten Limestone . This value is of the same order of magnitude as compared to cross sections from eastern Switzerland (SCHMID et al. 1997, TRÜMPY 1969). REFERENCES Aichholzer, S; Ortner, H. (2010): Das Molasse Profil im Bregenzer Wald und Westallgäu: Hinterland wandernde Überschiebungssequenz bei der Schuppenstapelung. In: Journal of Alpine Geology Gesellschaft der Geologie und Bergbaustudenten in Österreich 52, 80 81. Schmid, S.M., Pfiffner, O.A. and Schreurs, G. (1997): Rifting and collision in the Penninic zone of eastern Switzerland. In: Pfiffner O.A. et al. (eds.), Deep Structure of the Alps, Results from NFP 20, 160 185. Trümpy, R. (1969): Die helvetischen Decken der Ostschweiz: Versuch einer palinspastischen Korrelation und Ansätze zu einer kinematischen Analyse. Eclogae geol. Helv. , 62/1, 105 138. Wyssling, G. (1985): Palinspastische Abwicklung der helvetischen Decken von Vorarlberg und Allgäu. Jb. Geol. B. A., 127/4, 701 706. 93 10th Alpine workshop "CorseAlp2011" Relation of the Tethys ocean to the Cretaceous (Eoalpine) subduction in the Alps: data, framework and ideas Ralf Schuster 1, Friedrich Koller 2, Hans Jürgen Gawlick 3, Wolfgang Frank 4 & Richard Lein 5 (1) Geologische Bundesanstalt, Wien, Austria (2) University of Vienna, Department of Lithosperic Research, Wien, Austria (3) University of Leoben, Dept. of Applied Geosciences and Geophysics, Leoben, Austria (4) CEAL, Geological Inst. of Slovak Academy of Science, Bratislava, Slovak Republic (5) University of Vienna, Department of Geodynamic and Sedimentology, Wien, Austria The relation of the Tethys (Meliata Vardar Hallstatt) ocean to the Cretaceous (Eoalpine) subduction in the Alps is an enigmatic problem. Several models with oceanic and intra continental subduction zones were proposed in the past, but none of these models explain all geological observations. Remnants of the Triassic to Cretaceous, westernmost part of the Tethys ocean are very rare in the Alps and Western Carpathians (Meliata slices), but form large nappes in the Dinarides (Vardar Zone, Sava Zone). The continental units below these oceanic units formed segments on a curved, east to south facing continental margin of the Adriatic plate towards the Tethyan ocean in Triassic time (Haas et al. 1995). During complex tectonic evolutions the segments of this margin were deformed in different ways since the Middle Jurassic. Recently these segments represent the nappes of the external Dinarides, the Austroalpine and the Inner Western Carpathians. In the individual segments some stages of the tectonic evolution are well preserved, others are only badly preserved or completely missing. However, taking together the information from different areas the history of the margin can be reconstructed. The aim of this contribution was the investigation of basal conglomerates (Santonian) of the Gosau Group from Pfenningbach (Lower Austria) in the eastern part of the Austroalpine nappes. There, a very specific pebbles association including amphibolites, basic volcanics and radiolarites as exotic material occurs (Gruber et al. 1992). Most of the volcanics and amphibolites show mostly OIB affinity, representative of within plate magmatism typical for oceanic seamounts. The amphibolites exhibit a metamorphic fabric and a mineral assemblage including garnet and clinopyroxene, indicating metamorphic temperatures of more than 600 °C. Based on Sm Nd and Ar Ar ages the metamorphic imprint occurred in Jurassic time (Schuster et al. 2007). In general these amphibolites show many similarities to amphibolites of the metamorphic sole below ophiolites in the Dinarides . Associated radiolarites of greyish to blackish colour yield biostratigraphic ages of several time intervals between Upper Anisian to Middle Jurassic. Upper Anisian Lower Ladinian (Fassan) radiolarites are frequent in the sections from the Tethys distal shelf, whereas Langobardian Lower Carnian (Cordevol) radiolarites are expected to come from an oceanic area, because of the high carbonate production rate on the shelf at these times (Suzuki et al. 2007). Characterizing of the pebbles and their surroundings we suggest that they derived from obducted ophiolite nappes and/or melange of the Tethys ocean. These nappes continue into the Dinarides and were obducted onto the continental margin in the Early Callovian. As they are missing on top of the Austroalpine nappes today, they have been completely eroded and their detritus is now only present within Late Jurassic and Cretaceous sediments (Faupl & Wagreich 2000). Accepting this interpretation we have to conclude that the history of the Adriatic continental margin in the recent Dinarides and Eastern Alps was very similar until the Late Jurassic: In the early Middle Jurassic (~170 Ma) an east to south dipping intraoceanic subduction developed in the Tethys ocean. There Triassic oceanic lithosphere was subducted below a Jurassic oceanic crust. Since the late Middle Jurassic (~165 Ma) ophiolite nappes were obducted onto the continental margin and further on also continental material was entering the subduction zone. Shortening and tectonic activity continued throughout the Late Jurassic (Missoni & Gawlick 2010). However, in this time interval a decoupling of the evolution in the recent Dinarides with respect to the Alps and Western Carpathians occurred: While the Dinarides still evolved at the margin of the Adriatic plate towards the Tethys ocean, similar units of the Eastern Alps were transferred from a marginal to a more internal position inside the Adriatic plate. This has to be concluded, because during the onset (at ~135 Ma) of the south(east) dipping Cretaceous (Eoalpine) subduction these parts were overthrusted by the Adriatic plate without remnants of the Tethys ocean in between. Due to the missing oceanic suture Janak et al. (2004) and Stüwe & Schuster (2008) argued for the onset of the Cretaceous subduction in an intracontinental setting, decoupled from subduction processes in the Tethys ocean. The re arrangement of the Adriatic margin was explained by transpressive, Late Jurassic, sinistral strike slip tectonics between the northern part of the Adriatic plate (recent Austroalpine and Inner Western Carpathian units) and its southern main part. During these movements the former south facing continental margin of the northern part came in contact with the internal continental parts of the southern part of the Adriatic plate. This interpretation bears two challenging points: 1) The south dipping Cretaceous (Eoalpine) slab contains no oceanic lithosphere and in succession buoyancy effects are missing. 2) The position of the sinistral strike slip faults have to run exactly along the east west oriented continental margin of the Austroalpine unit towards the Tethys ocean segregating oceanic from continental lithosphere. These two points can be avoided by the following setting and interpretation: a) Complete subduction of the oceanic lithosphere in an embayment can cause extension along strike in the also curved slab. b) Tearing of the slab between the north south striking segment of the recent Dinarides and the east west striking segment of the recent Eastern Alps and Western Carpathians. c) Formation of strike slip faults in east west direction in the continuation of the teat fault and along the south dipping slab of the northern plate (recent Alps and Western Carpathians). d) Shifting of the slab into the continental domain along the strike slip fault zone. In the following the oceanic (Tethys) slab can contribute a part of the buoyancy for the onset of the Cretaceous (Eoalpine) subduction. This interpretation is in agreement with the missing oceanic suture and it may also explain the increasing pressures in the Cretaceous eclogites from the Texel Complex in the west towards the Saualpe Koralpe Complex in the east (Hoinkes et al. 1999, Hoinkes et al, 2010). 94 10th Alpine workshop "CorseAlp2011" New insights into the seismotectonics of the Gulf of Cadiz area based on instrumental seismicity and seismic reflection profiles reinterpretation: preliminary results Sónia Silva 1, Henrique Duarte 1, Pedro Terrinha 1, Luis Matias 2 & NEAREST Working group 3 (1) Unidade de Geologia Marinha, Laboratório Nacional de Energia e Geologia, Portugal (2) Instituto Dom Luis, Lisboa, Portugal (3) http://nearest.bo.ismar.cnr.it/Partners The Gulf of Cadiz offshore SW Iberia is an area where episodic destructive seismic and tsunamigenic events have occurred, such as the 1st November 1755 Lisbon earthquake. The association of active faults to this kind of high magnitude event has been intensively studied specially due to the contribution of several international team projects, working in this area for more than two decades. However, the meaning of the persistent small to intermediate magnitude seismicity recognized in this region is still misunderstood. This problem is mostly related to the lack of an accurate hypocenter location of these events resulting from an asymmetrical geographical distribution of the permanent seismic network. To address this problem the EC project NEAREST conducted a passive seismic experiment in the Gulf of Cadiz where 24 BB seismometers (plus the GEOSTAR) were deployed from September 2007 to July 2008. The examination of the continuous data stream allowed the detection of a large number of local events that were not detected by the land networks. The analysis of the complete data set reveals 3 main clusters of earthquakes: 1) in the Gorringe Bank; 2) in the SW limit of the Horseshoe Fault and 3) in the São Vicente Canyon. Most of the small magnitude earthquakes are located in the mantle (at depths between 30 and 60 km), like the hypocenters of 3 larger instrumental earthquakes in the area: i) the 28th February 1969 (Mw~8.0); ii) the 12th February 2007 (Mw=6.0) and iii) the 17th December 2009 (Mw=5.5). However, focal mechanisms show a mixed pattern, mostly strike slip and reverse dip slip with a very few normal fault mechanisms, showing that processes related with this seismicity must be complex, probably involving the interaction between different active geological structures and reflecting particular local rheological contrasts. A preliminary reevaluation of the relation between this intermediate depth seismicity and the active faults in the Gorringe Bank cluster is presented by integration of this new instrumental seismicity data with reprocessed depth migrated seismic reflection profiles AR92 3 and AR92 8 (fig.1). Fig.1- A-Gulf of Cadiz area location (adapted from Jimenez-Munt et al., 2010); B- Simplified tectonic map with hypocenters distribution derived from NEAREST Project experiment and the location of the seismic reflection profiles AR92-3 and AR92-8(GB- Gorringe Bank, SVC- São Vicente Canyon, GF-Gorringe Fault, HF-Horseshoe Fault, MF- Marquês de Pombal Fault, SWIM- SWIM Fault) REFERENCES Jiménez Munt, I., Fernàndez, M. , Vergés, J. , Afonso, J. C. , Garcia Castellanos D.and Fullea, J. (2010) Lithospheric structure of the Gorringe Bank: Insights into its origin and tectonic evolution, TECTONICS, VOL. 29, TC5019, doi:10.1029/2009TC002458, 95 10th Alpine workshop "CorseAlp2011" Melt Present Deformation of the Mafic Lower Crust Exposed in Calabria Richard Spiess 1, Alfredo Caggianelli 2, Giacomo Prosser 3 & Antonio Lucchiari 1 (1) Dipartimento di Geoscienze, Università di Padova, Padova, Italy (2) Dipartimento di Scienze Geologiche, Università della Basilicata, Potenza, Italy (3) Dipartimento Geomineralogico, Università di Bari, Bari, Italy In the Serre Massif of Calabria an about 8 km thick sequence of granulite facies rocks constitutes the lower crust of the former Variscan orogen. The base of this lower crustal section is made up of layered garnet bearing metagabbros which are interspersed with lenses of metaperidotites and layers of metapyroxenites and metahornblendites (Moresi et al., 1978; Schenk, 1984). Single zircon conventional and spot U Pb isotopic analyses place the age of the gabbroic protoliths into the Precambrian (Schenk, 1984, 1990; Micheletti et al. 2008). The main metamorphic overprint under granulite facies conditions of the gabbros has occurred during the Variscan orogeny. Zircon dating constrained the metamorphic peak close to 300 Ma (Schenk, 1984; Caggianelli et al., 2000; Micheletti et al., 2008) in coincidence with the emplacement of huge masses of granitoid magmas in the middle crust. Reaction microstructures have induced Schenk (1984) to infer that the lower crust of the Variscan orogen in Calabria was subjected to isothermal decompression immediately after the thermal maximum. He has deduced that the lower crust became decompressed from about 750 to 550 MPa at 800° C and then affected by slow isobaric cooling down to 200° C over the next 250 million years. Acquafredda et al. (2008) have shown that decompression occurred during two different stages and involved a much thicker continental crust than previously assumed. Internally consistent thermodynamic modelling of the pre decompression mineral assemblage results in 900° C and 1.1 GPa for the metamorphic peak conditions. We can now show that at the thermal peak conditions the metagabbro at the base of the Calabria crust was affected by incipient partial melting. In amphibole bearing metagabbros reaction seams of quartz+phlogopite around Opx suggest that a dehydration reaction of the type phlogopite (Phl) + quartz (Qtz) = orthopyroxene (Opx) + melt has been crossed. Peterson & Newton (1989) place this reaction, which is not pressure sensitive, close to 850° C, but fluorine contents within phlogopite will shift it to higher temperatures (Peterson et al., 1991). Partial melting was accompanied by deformation of the metagabbros, leading to the localisation of cm thin shear zones wherein the partial melt was drained. Shear zone development occurred in a general deformation regime, with partitioning of the shortening strain next to the shear zones resulting in intense progressive crenulation of the earlier Variscan foliation. Ti rich pargasitic amphibole layers were involved in this crenulation deformation. Application of the semiquantitative “Ti in amphibole” geothermometer of Ernst and Liu (1998) to the recrystallised pargasites results in a deformation temperature of 870° C, whereas application of amphibole plagioclase thermometry (Holland & Blundy, 1994) results in temperatures of 818 to 857° in the pressure range 500 to 1000 MPa. Estimating the pressure conditions using the semiquantitative Al in amphibole geobarometer of Ernst and Liu (1998) results in pressures of at least 500 MPa. However, application of the crystal structure modelling geobarometer of Nimis & Ulmer (1998) to clinopyroxene in equilibrium with the plagioclase rich melt of the shear zones shows that pressures during partial melting have been close to 1 GPa. Partial melting of the lower crust must have significantly affected the tectonics of the entire crust of Calabria during the Variscan orogeny. Indeed melting must result in weakening of the lower crust and may have triggered the onset of late orogenic extension. REFERENCES Acquafredda, P., Fornelli, A., Piccarreta, G., Pascazio, A., 2008. Multi stage dehydration decompression in the metagabbros from the lower crustal rocks of the Serre (southern Calabria, Italy). Geological Magazine 145, 397 411. Caggianelli, A., Prosser, G., Del Moro, A., 2000. Cooling and exhumation history of deep seated and shallow level, late Hercynian granitoids from Calabria. Geological Journal 35, 33 42. Ernst W. G. & Liu J. (1998). Experimental phase equilibrium study of Al and Ti contents of calcic amphibole in MORB A semiquantitative thermobarometer. American Mineralogist, 83, 952 969. Holland T., Blundy J. (1994) Non ideal interactions in calcic amphiboles and their bearing on amphibole plagioclase thermometry. Contributions Mineralogy Petrology, 116, 433 447. Micheletti, F., Fornelli, A., Piccarreta, G., Barbey, P., Tiepolo, M., 2008. The basement of Calabria (southern Italy) within the context of the Southern European Variscides: LA ICPMS and SIMS U Pb zircon study. Lithos 104, 1 11. Moresi, M., Paglionico, A., Piccarreta, G., Rottura, A., 1978. The deep crust in Calabria (Polia Copanello Unit): a comparison with the Ivrea Verbano zone. Mem. Sci. Geol. XXXIII, 233 242. Nimis, P. & Ulmer, P. (1998) Clinopyroxene geobarometry of magmatic rocks. Part 1. An expanded structural geobarometer for anhydrous and hydrous basic and ultrabasic systems. Contributions to Mineralogy and Petrology, 133, 122 135. Peterson, J.W., and Newton, R.C. (1989a) Reversed experiments on biotite quartz feldspar melting in the system KMASH: Implications for crustal anatexis. Journal of Geology, 97, 465 485. Peterson J. W., Chacko T. & Kuehner S. M. (1991). The effects of fluorine on the vapor absent melting of phlogopite * quartz: Implications for deep crustal processes. American Mineralogist, 76, 470 476. Schenk, V., 1984. Petrology of felsic granulites, metapelites, metabasics, ultramafics, and metacarbonates from Southern Calabria (Italy): prograde metamorphism, uplift and coolinh of a former lower crust. Journal of Petrology 25, 255 298. Schenk, V., 1989. P T t path of the lower crust in the Hercynian fold belt of Southern Calabria. In Evolution of metamorphic belts ( eds. J.S. Daly, R.A. Cliff and B.W.D. Yardley). Geological Society of London, Special publication 43,337 342. 96 10th Alpine workshop "CorseAlp2011" Reconstruction of the pre-Glacial Alpine Topography to Address the Glacially-Induced Modifications Pietro Sternai, Frédéric Herman, Sean D. Willett, Jean Daniel Champagnac & Bernhard Salcher Geologisches Institut, Eidgenössiche Technische Hochshule (ETH), Zürich, Switzerland We use numerical modeling to investigate the effects of glaciation on the pre glacial topography of the Alps. Amongst the models that have been proposed, the stream power law has been profitably used for modeling the dynamics of fluvial bedrock channel incision (e.g. Whipple and Tucker, 1999): ∂z m n = U − KA S (1) ∂t where ∂z/ ∂t (m*a 1) is the time rate of change of channel elevation, U (m*a 1) is rock uplift rate, A(m) is upstream drainage area, S is local channel gradient, K is a dimensionless coefficient of erosion and m and n are positive constants related to basin hydrology and erosion process. Under steady state conditions ( ∂z/ ∂t = 0), equation (1) can be solved to yield an expression for equilibrium channel gradient: 1 m n − U n S= A (2) K where the ratios U / K and m/ n are generally referred to as the steepness and concavity index, respectively. Particular focus is put in the spatial variability of the steepness index over the Alpine mountain belt. Assuming a constant concavity index, the pre glacial topography of the Alps is obtained through an inversion technique that resolves the local slopes (as described in equation (2)) by minimizing the misfit between the elevations of the actual and modeled channel heads. Subsequently, a glacial erosion model (ICE CASCADE, Braun et al., 1998) is applied to obtain information about the glacially induced modification of the modeled fluvial topography. We make model based predictions of sediment production, mass redistribution and morphologic evolution. We present the possible evolution of the pattern of glacial erosion through time, when topography is subject to different tectonic and climatic conditions. Our results highlight the major morphologic diversities between the pre glacial and the actual Alpine topographies such as smaller relief and maximum elevation in turn providing insights into the major modifications that glacial action has induced on the morphology of the Alps. REFERENCES Braun, J., D. Zwartz, and J. Tomkin (1998), A new surface process model combining glacial and fluvial erosion, Ann. Glaciol., 28 (1), 282 290. Whipple, K. X., and G. E. Tucker (1999), Dynamics of the stream power river incision model: implication for height limits of mountain ranges, landscape response timescale, and research needs, J. Geophys. Res., 104, (B8), doi:10.1029/1999JB900,120. 97 10th Alpine workshop "CorseAlp2011" Kinematic Evolution of the Corsica Triple junction Eugenio Turco, Antonio Schettino, Pietro Paolo Pierantoni, & Chiara Macchiavelli Università di Camerino The tectonic evolution of Alpine Corsica and surrounding areas is described through a series of 2 D and 3 D plate reconstructions from the onset of the Alpine convergence in the early Cretaceous (~120 Ma) to the early Miocene (~19 Ma). The southward subduction of Eurasian lithosphere beneath Adria and Iberia, and the subduction of Ligurian lithosphere beneath the eastern Iberian margin, led to the formation of a trench trench trench triple junction close to the northern tip of Corsica, whose kinematic evolution determined the structural complexity of eastern Corsica and the Ligurian accretionary prism. During a first phase, from the early Aptian (~120 Ma) to the late Maastrichtian (~68 Ma), the triple junction migrated eastward along the Alpine subduction zone, determining the juxtaposition of the Alpine and Ligurian accretionary prisms. A second phase, which started during the Eocene (~55 Ma) and terminated at the beginning of the Oligocene (~33 Ma), was characterized by the northeastern migration of the triple junction along the Alpine trench and by the overthrusting of the Alpine accretionary prism onto the eastern Iberian margin. During this time interval the Alpine and Ligurian subduction zones were linked by a left lateral transform fault. Finally, from the early Oligocene (~33 Ma) to the early Miocene (~19 Ma), during the rotation of the Corsica and Sardinia blocks, slip partitioning along the southern part of the Alpine accretionary wedge determined the formation of a small sliver (Alpine Corsica) which migrated northwestward with respect to Corsica along a left lateral transform fault. 98 10th Alpine workshop "CorseAlp2011" Neotectonics of the Dinarides - Pannonian Basin Transition Area and possible sources of the M L 6.5 Banja Luka Earthquake of 1969 Kamil Ustaszewski 1, Marijan Herak 2, Bruno Tomljenović 3, Davorka Herak 2 & Srebrenka Matej 4 kamilu@gfz potsdam.de (1) German Research Center for Geosciences GFZ Potsdam, Potsdam, Germany (2) Department of Geophysics, University of Zagreb, Zagreb, Croatia (3) Faculty of Mining, Geology and Petroleum Engineering, Zagreb, Croatia (4) INA Industrija nafte d.d., Zagreb, Croatia The fold and thrust belt of the Dinarides on the western Balkan Peninsula hosts some of the strongest instrumentally recorded earthquakes in Europe. Seismicity distribution (e.g., D’Agostino et al., 2008) and GPS derived shortening rates between Italy and the Pannonian Basin amount to c. 3 5 mm/a (e.g., Bennett et al., 2008) and suggest that the Adria Europe convergence is largely taken up within the external Dinarides near the Adriatic Sea coast. However, instrumentally strong earthquakes were also recorded within the internal Dinarides, as evidenced by the ML 6.5 Banja Luka earthquake (northern Bosnia and Hercegovina) of 27 October 1969. This suggests that the convergence zone is diffusely distributed across the entire Dinarides up to the Pannonian Basin. Our study first provides a neotectonic framework for this transitional area. We then review possible source mechanisms for the still very poorly understood 1969 earthquake sequence. Subduction of Meliata Vardar oceanic lithosphere and subsequent collision of the European derived Tisza Dacia and Adriatic plates in the late Cretaceous early Paleogene led to the development of the SW vergent Dinarides fold and thrust belt. Severe extension affected the northern Dinarides in conjunction with the formation of the Pannonian Basin since the latest Paleogene (24 Ma) and throughout much of the Miocene. This extension reactivated former thrust faults related to foregoing collision as low angle extensional detachments and led to the formation of core complexes, exposing greenschist to amphibolite grade rocks. This is documented on several inselbergs along the southern margin of the Pannonian Basin, i.e. along the southern margin of the Sava depression between Bosnia and Hercegovina and Croatia. The Sava depression formed as a supradetachment basin on top of the north dipping detachment. The initially coherent detachment was disrupted by both N and S dipping high angle normal faults since about Ottnangian times (c. 18 Ma). We presume that the Miocene extensional fault system extended at least to mid crustal depths based on the fact that amphibolite grade rocks are exposed in the footwall of the detachment. After the Pontian (c. 5 Ma), numerous high angle normal faults were compressively or transpressively inverted, rejuvenating an ‘inselberg’ morphology in the southwestern Pannonian Basin that had initiated during Miocene extension. The available focal mechanisms for the main 1969 Banja Luka earthquake and its largest foreshock on 26 October 1969 (M L 6.0) indicate reverse faulting along c. E W trending nodal planes and c. N S trending pressure axes based on p wave first motion polarities. We analyzed the aftershock distribution in order to better understand which fault ruptured during the main event, restricting the analysis to events with M ≥ 2 and an epicenter location uncertainty < 10 km. The events during the first 36 hours following the largest foreshock cluster in an ENE trending area, 40 km long and 10 km wide. This cluster parallels the trend of the frontal thrust of the East Bosnian Durmitor thrust sheet cropping out a few km to the south. We hence suspect that this thrust could have been compressionally or transpressionally reactivated during the recorded seismic events. Alternatively, the event could have nucleated along one of the Neogene detachment faults itself. We next analysed the seismicity in the larger epicentral area during the entire time span covered by the earthquake catalogue. This analysis was restricted to M L ≥ 3.0 earthquakes, for which the determination of hypocenter depth has smaller uncertainties. The average depth distribution for the considered events tends to parallel the ENE to NE oriented thrusts in the area. The shallower events tend to occur close to the surface trace of the thrusts and deepen towards NE. This is taken as circumstantial evidence for ongoing seismic activity along the East Bosnian Durmitor thrust in the internal Dinarides. REFERENCES Bennett, R. A., S. Hreinsdóttir, G. Buble, T. Bašić, Ž. Bačić, M. Marjanović, G. Casale, A. Gendaszek, and D. Cowan (2008), Eocene to present subduction of southern Adria mantle lithosphere beneath the Dinarides, Geology, 36(1), 3 6. D'Agostino, N., Avallone, A., Cheloni, D., D'Anastasio, E., Mantenuto, S. and Selvaggi, G., 2008. Active tectonics of the Adriatic region from GPS and earthquake slip vectors. J. Geophys. Res., 113. 99 10th Alpine workshop "CorseAlp2011" Tethys-Atlantic Interaction Along the Boundaries of the Iberian Plate: Pyrenees and Betics Mountain Belts Jaume Vergés & Manel Fernàndez Group of Dynamics of the Lithosphere (GDL), Institute of Earth Sciences “Jaume Almera”, CSIC, Barcelona, Spain Detailed movements of small tectonic plates between larger plates have been difficult to assess due to their facility to move independently of those limiting plates and because the frequent vertical axis rotations they experienced during their evolution. The Iberian plate, located at the western boundary of the Tethys Ocean, is one of these conflicting plates. As has been pointed out in several published works the combined study of large scale plate motions together with field based results along plate boundaries must constrain as much as possible their evolution. The feedback between these two different scales of work should provide a better understanding of the evolution of the Iberian plate although there are still large conflicting areas and time periods that need to be understood. In this paper we try to discuss field observations from the Pyrenees and Betics within the frame of published large scale Iberian plate reconstructions. Starting in the Early Middle Jurassic times the central Atlantic connected to the Alpine Tethys separating Iberia from the north of Africa. The Iberia Africa boundary resulted in a segmented margin characterized by a thinned crust with strong and multiple evidences of upper mantle exhumation and formation of oceanic crust (e.g., Schettino and Turco 2010). The northern propagation of the Northern Atlantic in Early Cretaceous times produced the abandonment of the Alpine Tethys but the opening of the Bay of Biscay along the already rifted Pyrenean corridor with the concomitant counterclockwise rotation of Iberia. Upper mantle rocks were exhumed along the western side of the Pyrenees in the eastern oceanic crust pinpoint (e.g., Jammes et al., 2010). Further northern propagation of the North Atlantic Ocean also produced the end of the Pyrenean transtension close to the Early to Late Cretaceous boundary. The Iberian plate was thus an independent plate when Africa started northwards convergence against Eurasia during the youngest part of the Late Cretaceous (Santonian to Maastrichtian). During this northwards convergence the two Iberian plate boundaries closed producing the Pyrenees and the Betics (Betic Rif system). The Pyrenees show double vergence and about 150 km of shortening and a partial north dipping subduction of Iberia beneath Europe that is clearly imaged in the ECORS crustal seismic profile (e.g., Choukroune et al., 1989). The foreland developed as under filled and marine from Late Cretaceous to the latest middle Eocene time at about 36 Ma and then overfilled and continental from deposition of the Cardona evaporitic level to the end of shortening during the Oligocene (e.g., Vergés et al., 2002). The end of deformation determined by magnetostratigraphy on growth strata attached to the front of the Pyrenean fold and thrust belt occurred during upper Oligocene times (~24.7 Ma) although major basement fast uplift, determined from fission track cooling ages in central Pyrenees, ended at about 30 Ma followed by a much slower rate up to about 25 Ma (e.g., Metcalf et al. 2009). This apparent straightforward model for the Pyrenees is not applicable to the Betics where both the geodynamic evolution and field observations have been used to provide multiple and many times conflicting interpretations. The arcuate shape of the Betic Rif system, the single NNW directed compressive external belt along the central External Betics and foreland basin, the HP/LT pile of thin upper crustal units sandwiched between less or non metamorphic units in the discontinuous Internal Betics, the extensional set of faults cutting through the Internal Betics and generating the Alboran domain after 27 Ma together with the extensive volcanism fit with a geodynamic scenario triggered by the subduction of this segment of the Alpine Tethys ocean beneath Africa. The NE SW lithospheric thickening beneath the western Betics, Rif and Atlantic Moroccan Margin, reaching a maximum thickness of 230 km, is related to the superposition of two different mechanisms: the NW SE Africa Eurasia convergence acting since Late Cretaceous; and the early Miocene to late Tortonian mechanism responsible for the formation of the Alboran Basin (subduction, slab roll back and mantle delamination) (Fullea et al. 2010). REFERENCES Choukroune, P. and ECORS Team. 1989. The ECORS Pyrenean deep seismic profile reflection data and the overall structure of an orogenic belt. Tectonics 8, 23 39. Fullea, J., M. Fernàndez, J.C. Afonso, J. Vergés, and H. Zeyen. 2010. The structure and evolution of the lithosphere asthenosphere boundary beneath the Atlantic Mediterranean Transition Region. Lithos 120, 74 95. Metcalf, J. R., P. G. Fitzgerald, S. L. Baldwin, and J. A. Muñoz. 2009. Thermochronology of a convergent orogen: Constraints on the timing of thrust faulting and subsequent exhumation of the Maladeta Pluton in the Central Pyrenean Axial Zone. Earth and Planetary Science Letters 287, 488 503. Schettino, A., and E. Turco. 2010. Tectonic history of the western Tethys since the Late Triassic. Geological Society of America Bulletin , 123, 89 105. Suzon, J., L. Lavier, and G. Manatschal. 2010. Extreme crustal thinning in the Bay of Biscay and the Western Pyrenees: From observations to modeling. Geochemistry Geophysics Geosystems 11, Vergés, J., M. Fernàndez, and A. Martínez. 2002. The Pyrenean orogen: pre , syn , and post collisional evolution. Journal of the Virtual Explorer 8: 55 58. 100 10th Alpine workshop "CorseAlp2011" Along-strike variability in structural style, exhumation history and overall nappe architecture of distal Adria-derived units: northern Calabria vs. Alpi Apuane Stefano Vitale 1, Alessandro Iannace 1, Stefano Mazzoli 1, Giancarlo Molli 2, Steven Reddy 3 & Fred Jourdan 3 (1) University of Naples Federico II, Italy (2) University of Pisa, Italy (3) Curtin University of Technology, Perth, Australia The aim of this contribution is that of comparing the structural styles, exhumation histories and overall nappe architecture of metasedimentary units derived from two transects originally located hundreds of kilometers away along the strike of the former distal Adria continental margin: the Alpi Apuane (Tuscany) in the north, and the Lungro Verbicaro Unit (N Calabria) in the south. The correlation between these two areas in terms of their main units, litostratigraphy, deformation and tectonic history was first introduced by Cortese (1895) and later supported by Haccard et al. (1972), Elter and Scandone (1980), and Dewey et al (1989). More recently it has been re proposed and reinterpreted by Iannace et al., (2005), Perrone et al (2006), and Iannace et al. (2007). Here we further extend this correlation by combining the results of research we have been carrying out on in the last decades (Iannace et al., 2007; Molli, 2008, and references therein) in order to better understand: (i) the evolution of the distal Adria continental margin and its orogenic history, and (ii) the role of paleotectonic heritages and the structural controls exerted by the subduction retreating setting. While a reappraisal of the stratigraphy involving a correlation between Alpi Apuane and northern Calabria units (Lungro Verbicaro) in terms of paleotectonic setting has already been addressed (Iannace et al., 2005; Perrone et al., 2006), we point out here the major differences that exist with respect to: (i) styles of deformation during underplating: distributed with associated development of kilometer scale fold nappes in the Alpi Apuane vs. strongly localized within high strain domains at the meters to hundreds of meters scales in the Lungro Verbicaro Unit; (ii) timing of underplating, becoming younger from north (27 20 Ma in the Alpi Apuane; Kligfield et al., 1981) to south (18 11 Ma in N Calabria; Iannace et al., 2007, and new Ar Ar data from this study); (iii) peak P T conditions of the metamorphic signature: HP greenschist facies in the Alpi Apuane (Mg clorithoid, kyanite, phengite; Molli et al., 2000) vs. Fe Mg carpholite and blueschist assemblages in N Calabrian metasedimentary units (Rossetti et al., 2004; Iannace et et al., 2007); and (iv) timing and rates of exhumation: vertical average exhumation rate of c.1 km/Ma in the Alpi Apuane and more than 2 km/Ma in N Calabria. Moreover, using the complete stratigraphic and geochronological data sets including low T thermochronometry (Fellin et al., 2007; Iannace et al., 2007) two different exhumation paths can be traced; for instance, as far as final exhumation (from c. 4 Km to the surface) is concerned, this is younger (Pliocene), slower (0.8 km/Ma) and mainly erosion driven in the Apuane, whereas it is faster (1.6 km/Ma), older (Tortonian) and tectonically driven in N Calabria. The aforementioned major features in the structural setting and tectonic evolution of Alpi Apuane and N Calabria are related to differences in the geodynamic framework, and particularly to the activity of the continental subduction retreating Apennine system in the two areas as well as to the different paleotectonic heritages of the two former transects of the distal Adria continental margin. 101 10th Alpine workshop "CorseAlp2011" Inherited Ocean-Continent Transition zones in deeply subducted terranes: Insights from Alpine Corsica Alberto Vitale Brovarone 1,2, Marco Beltrando 1, Jacques Malavieille 2, Francesco Giuntoli 1, Ermete Tondella 1, Chiara Groppo 1, Olivier Beyssac 3 & Roberto Compagnoni 1 (1) Dept. of Mineralogical and Petrological Sciences, University of Torino, Torino, Italy (2) Laboratoire Géosciences Montpellier, University of Montpellier 2, Montpellier, France. (3) Institut de Minéralogie et de Physique des Milieux Condensés (IMPMC), UMR CNRS IPGP Universités Paris 6 et 7, Campus Jussieu, Paris France In the Schistes Lustrés of Alpine Corsica (France) serpentinized mantle rocks are associated with continental basement and meta volcanic/ sedimentary cover rocks. The continental basement slivers associated with meta ophiolites have been previously interpreted (i) as basement sheets that have been tectonically displaced during the Alpine orogeny, (ii) as reworked, i.e. detrital, sequences that have been deposited within the ocean, resulting from erosion of the nearby continental crust or iii) as continental basement slivers and related sedimentary cover represent relics of continental crust thinned during rifting. Despite that, relationships among lithologies are reworked by the pervasive Alpine deformation, leading authors to incline to orogenic juxtapositions. The relationships among ultramafics, continental basement rocks and metasediments are especially well exposed in the Monte San Petrone unit, where Alpine metamorphism reached lawsonite eclogite facies conditions. The contact between serpentinites and slivers of continental basement, relatively flat lying over several kilometers, is characterized by evidence of cataclastic deformation pre dating Alpine High Pressure ductile fabrics. The serpentinite/continental basement pair is stratigraphically overlain by metasediments with a typical Jurassic Cretaceous supra ophiolitic lithostratigraphy, with metaradiolarites passing upward to marbles and calcschists. Noticeably, no evidence of cataclastic deformation is found in metasediments, indicating that motion along the brittle tectonic contact predated the deposition of the Mesozoic cover. These observations indicate that the lithostratigraphy of the Monte San Petrone unit was established during a pre Alpine polyphase evolution, which culminated in extensive brittle deformation along a flat lying detachment fault prior to the deposition of Jurassic and Cretaceous sediments. We stress that the inferred Mesozoic extensional tectonics originated during the opening of the Western Tethys. This Mesozoic architecture is comparable with those described in modern and fossil Ocean Continent Transition (OCT) zones (e.g. Manatscal et al., 2006). These Tethyan architecture was preserved despite Alpine deformation and metamorphism, when the different lithologies (i.e. meta ophiolites, continent derived rocks and metasediments) underwent a common metamorphic evolution, culminating at T = 490 550°C and P = 2.2 2.6 GPa). Similar tectono stratigraphic associations are observed in other high pressure terranes of Alpine Corsica and in the Western Alps (e.g. Beltrando et al., 2010), suggesting that inherited OCT type domains may be common in Alpine type orogens. REFERENCES Beltrando, M., Rubatto, D. & Manatschal, G., 2010. From passive margins to orogens: The link between ocean continent transition zones and (ultra)high pressure metamorphism. Geology, 38(6), 559 562. Manatschal G., Engström A., Desmurs, L., Schaltegger, U., Cosca, M., Müntener, O., Bernoulli, D., 2006. What is the tectono metamorphic evolution of continental break up: the example of the Tasna Ocean Continent transition. Journal of Structural Geology 28, 1849 1869. 102 10th Alpine workshop "CorseAlp2011" What happens during subduction? A perspective from the architecture of Alpine Corsica Alberto Vitale Brovarone 1,2 , Olivier Beyssac 2, Jacques Malavieille 3, Giancarlo Molli 4, Marco Beltrando 1 & Roberto Compagnoni 1 (1)Dept. of Mineralogical and Petrological Sciences, University of Torino, Torino, Italy (2) Institut de Minéralogie et de Physique des Milieux Condensés (IMPMC), UMR CNRS IPGP Universités Paris 6 et 7, Campus Boucicaut, Paris, France (3) Laboratoire Géosciences Montpellier, CNRS University Montpellier 2, Montpellier Cedex 5, France (4) Dept. of Earth Sciences, University of Pisa, Pisa, Italy Alpine Corsica consists of a complex stack of variably metamorphosed units of continental and ocean derived material, and offers a complete section through the Alpine orogenic wedge over a 40 km long section. Because of the strong Alpine deformation, only locally preserved primary stratigraphic successions point to precise paleogeographic domains. Consequently, large regions of the belt have been interpreted either as complex tectonic mixing formed during the Alpine subduction, or as strongly reworked coherent portions of continental and/or oceanic lithosphere. The lack of detailed PT estimates over wide regions results in several uncertainties in identifying the boundaries separating units that experienced different tectono metamorphic evolutions and, consequently, in the definition of an exhaustive architecture of the belt. Several samples of metasediments have been investigated by means of Raman Spectroscopy of Carbonaceous Material (RSCM) thermometry in order to provide a complete metamorphic dataset over all the units of Alpine Corsica. Metamorphic conditions range from the very low grade, up to the lawsonite eclogite facies, and a good correspondence between the metamorphic conditions obtained on metasediments and the previous estimates on other lithologies (such as metamafics or continental basement rocks) is found. Our dataset shows the occurrence of large, homogeneous metamorphic domains, boundaries of which correspond to sharp and systematic metamorphic gaps. Nine main tectono metamorphic units are defined. These units are laterally continuous and occupy a specific position within the belt. We stress that only a very limited number of major orogenic tectonic contacts occurs, thus indicating a very limited effect of tectonic mixing in the subduction channel. Our results point to the preservation of large and continuous portions of Tethyan lithosphere, whose locally preserved stratigraphic features allow their paleogeographic origin to be assigned. Similar conclusions have been recently proposed for the high pressure units of the Western Alps and of Alpine Corsica (Angiboust et al., 2009;Beltrando et al., 2010; Vitale Brovarone et al., in press; Angiboust et al., unpublished data), indicating that the preservation of large pre orogenic domains represent a first order feature of Alpine belts. Results provided in this study suggest a link between pre orogenic arrangements and the present day structure of Alpine Corsica, providing important constrains for deciphering the evolution of the Tethys Alps system. REFERENCES Angiboust, S., Agard, P., Jolivet L., Beyssac, O., 2009, The Zermatt Saas ophiolite: the largest (60 km wide) and deepest (c. 70 80 km) continuous slice of oceanic lithosphere detached from a subduction zone?, Terra Nova , 21 : 171 180. Beltrando, M., Rubatto, D. & Manatschal, G., 2010. From passive margins to orogens: The link between ocean continent transition zones and (ultra) high pressure metamorphism. Geology, 38(6), 559 562. Vitale Brovarone, A., Beltrando, M., Malavieille, J., Giuntoli, F., Tondella, E., Groppo, C., Beyssac, O. & Compagnoni, R., in press. Inherited Ocean Continent Transition zones in deeply subducted terranes: Insights from Alpine Corsica. Lithos. 103 10th Alpine workshop "CorseAlp2011" Role of Seafloor Processes During Subduction: heterogeneous densification of subducting oceanic crust and Coexistence of Lawsonite-Eclogite and Blueschist From Alpine Corsica Alberto Vitale Brovarone 1,3 , Chiara Groppo, György Hetényi 2, Roberto Compagnoni 1 & Jacques Malavieille 3 (1) DSMP, University of Torino, Torino, Italy (2) Dept. of Earth Sciences, ETH Zurich, 8092, Zurich, Switzerland (3) Laboratoire Géosciences Montpellier, CNRS University of Montpellier 2, Montpellier Cedex 5, France In Alpine Corsica (France), deeply subducted metabasalts are well preserved as lawsonite bearing eclogite (Law Ecl), occurrence of which is restricted to about 10 localities worldwide. The Corsican Law Ecl, consisting of omphacite + lawsonite + garnet + phengite + titanite, occur as both single undeformed metabasaltic pillows surrounded by lawsonite blueschist (Law Bs), and carbonate bearing eclogitic veins. Law Bs are found as variably deformed metabasaltic pillows locally crosscut by eclogitic veins and consist of glaucophane + actinolite + lawsonite + garnet + phengite + titanite. Despite these two rock types (i.e. Law Ecl and Law Bs) are classically considered as characteristic of two distinct metamorphic conditions, field evidence and microstructures reveal that both assemblages are stable at the same peak metamorphism conditions. Isochemical phase diagrams (pseudosections) calculated for representative Law Ecl and Law Bs samples indicate that both lithologies equilibrated at the same conditions of about 520 ± 20 °C and 2.3 ± 0.1 GPa. These two rock types are characterized by MORB chemical compositions, but minor differences are observed between the two rocks, being the Law Bs slightly depleted in CaO with respect to the Law Ecl. These chemical variations depend on different degrees of hydrothermal alteration affecting basaltic rocks at seafloor. Consequently, different inherited microchemical domains result at HP in Law Ecl assemblages, in primary, poorly altered basalts, and Law Bs assemblages in altered, Ca depleted basalts. The coexistence at the same peak metamorphic conditions of Law Ecl and Law Bs implies that different portions of deeply subducted oceanic crust may be characterized by significantly different mineralogical associations and, as a consequence, by different physical properties. In particular, thermodynamic modelling of phase equilibria indicates that the progressive dehydration and eclogitization reactions of different portions of subducting oceanic crust significantly depend on the inherited bulk rock chemical composition. This feature results in an earlier densification of primary basaltic composition with respect to the altered basalts. These observations indicate that primary oceanic structures and microchemical domains can be preserved at high pressure conditions and play a major role during subduction influencing rock densification and eclogite formation in subducting slabs. 104 10th Alpine workshop "CorseAlp2011" How much material was eroded in the Northern Alpine Foreland Basin and why and when did it happen? Christoph von Hagke 1, Onno Oncken 1, Daniel F. Stöckli 2 & Hugo Ortner 3 vonhagke@gfz potsdam.de (1) Helmholtz Centrum Potsdam GFZ, Telegrafenberg, Potsdam, Germany (2) University of Kansas, Lawrence KS, USA (3) Institute of Geology and Palaeontology, University of Innsbruck, Austria Increased sedimentation and erosion rates since the late Miocene, mainly around 5Ma, have been argued for based on data from different study areas all over the world and the Alps in particular. This increase in erosion has mainly been attributed to climatic forcing and possible tectonic response to climate changes. Also, it was debated whether this increase is not merely an artefact of observational biases in the sedimentary record (Willenbring and von Blanckenburg, 2010). We chose the Northern Alpine Foreland Basin (NAFB) as a suitable area to investigate reported increased erosion rates. Being among the best studied foreland basins worldwide with unparalleled data availability, the NAFB provides almost unique prerequisites for understanding the uplift and erosion history of foreland basins. On the base of new apatite fission track and (U Th Sm)/He data in conjunction with previously published data, our aim is to shed light on the late phase of evolution of the Molasse basin and to discuss different competing hypotheses for explaining the observed thermochronology age pattern. We investigated three profiles, all of them crossing the triangle zone between the thrusted Subalpine Molasse and the flat lying Plateau Molasse (von Hagke et al., 2009). The two westernmost horizontal transects in our study area, Entlebuch and Rigi, are located south of the Jura fold and thrust belt, whereas the profile in the Bregenzerach is lying east of the Jura Mountains and thus crosses the present day Alpine front. Based on field observations, it was argued for out of sequence thrusting for the eastern part of the Molasse basin (Ortner et al., 2008). In the west, it remains an open question whether the present day Alpine front is located in a distal position, i.e. north of the Jura fold and thrust belt, which implies northward propagation of the orogenic wedge, or if the current deformation front is located south of the Swiss Molasse basin, implying a back stepping of the orogenic front during Plio Pleistocene times. The common view is that thrusting proximal to the Alps ceased at c.12Ma, contemporaneous with the onset of folding in the Jura Mountains in the north. 1D forward and inverse modelling of the apatite fission track and (U Th)/He data with HeFTy reveals that the observed strong cooling signal since the Middle Miocene cannot be explained by constant erosion, but requires increased uplift rates in distinct time windows. We are able to quantify the maximum amount of section removed by the cooling events. These values go in line with estimations, which can be drawn from topographic or vitrinite reflectance data. Our data shows a pronounced cooling (i.e. erosion) signal at c.10Ma. In addition, we can show that a 5Ma event is not mandatory to explain the observed thermochronology age distribution. We conclude that thrusting in the Subalpine Molasse did not cease at 12Ma. REFERENCES Ortner, H., Thöny, W., Aichholzer, S., Zerlauth, M., Pilser, R., and Tomek, C., 2008, Geometry and sequence of thrusting in the Alpine Foreland of western Austria and Bavaria, EGU General Assembly 2010, Volume 11: Vienna, Austria, Geophysical Research Abstracts, p. EGU2009 12318 von Hagke, C., Cederbom, C.E., Oncken, O., Schlunegger, F., Ortner, H., and Lindow, J., 2009, Coupling between Climate & Tectonics? Insights from Low Temperature Thermochronology and Structural Geology applied to the Pro Wedge of the European Alps, 9th Workshop on Alpine Geological Studies: Cogne, Italy Willenbring, J.K., and von Blanckenburg, F., 2010, Long term stability of global erosion rates and weathering during late Cenozoic cooling: Nature, v. 465, p. 211 214 105 10th Alpine workshop "CorseAlp2011" Geochronological Constraints on the pre-Alpine Evolution of the II DK (‘Seconda Zona Diorito Kinzigitica’, SW Sesia Zone, Italy) Brigitte von Niederhaeusern 1, Bénédicte Cenki Tok 1,2 , James Darling 1 & Martin Engi 1 (1) Institut für Geologie, Universität Bern, Bern, Switzerland (2) Géosciences Montpellier, Université de Montpellier 2, Montpellier, France The ‘Seconda Zona Diorito Kinzigitica’ (II DK) of the Sesia Zone represents Apulian lower crust, subducted during the Alpine convergence. Although this unit largely preserves assemblages of a pre Alpine HT metamorphism, blueschist facies assemblages linked to the HP/LT Alpine metamorphism developed within shear zones and mylonitic contacts to the adjacent units. In the SW Sesia zone, the II DK occurs as lenses along the boundary of Gneiss Minuti (GM) to the Eclogitic Micaschist Complex (EMC). Our study focussed on samples from shear zones with clear evidence of the Alpine HP metamorphism. To date specific metamorphic stages of the Alpine evolution, detailed microstructural and petrologic analysis of select samples was followed by in-situ U Th Pb dating of zircon, monazite, and allanite. Surprisingly, the obtained ages do not reflect the Alpine HP stage, but preserve evidence from the pre Alpine HT evolution of the studied samples. Data from two shear zones are presented here: (1) Vasario (Valle di Ribordone): Within the II DK unit, a m scale shear zone, which is composed of basic and pelitic domains was studied. Blueschist facies assemblages are best developed in high strain domains of the shear zone, where glaucophane + phengite ± clinozoisite define the mylonitic foliation. In intermediate strain domains of pelitic composition, large rounded zircon grains show detrital (igneous) cores and metamorphic rims, with textures characteristic of growth during HT metamorphism (patchy sector zoning). Further evidence for growth of these zircon domains under HT metamorphic conditions includes Ti in zircon thermometry (800 850°C) and trace element patterns indicating presence of plagioclase and garnet during zircon crystallisation. LA ICP MS U Pb dating of these rims yield uniform ages of 270 ± 3 Ma. In the same lithological domain, monazite is preserved as inclusions in garnet and as relic cores in pseudomorphs of allanite + apatite replacing the monazite. Chemical microprobe dating of these monazite grains reveals two growth episodes, with cores crystallized at around 330 ± 14 Ma and rims at around 275 ± 6 Ma. (2) Ingria (Val Soana): A shear zone developed along the northern contact of II DK to GM in the SW Sesia zone. The less deformed domains of the shear zone preserve blueschist facies assemblages (glaucophane + phengite). Strongly deformed domains near the contact show a greenschist facies (epidote + albite + chlorite) overprint. Metapelitic domains contain rounded zircon crystals, again with textures characteristic of growth at HT conditions. Dating of these grains will be performed soon. A studied mylonite sample (quartz + white mica + garnet) contains igneous zircon without distinct metamorphic recrystallization. LA ICP MS U Pb dating of these grains reveals two main age populations of ca. 296 ± 7 and 275 ± 6 Ma, as well as older cores of ca. 557 and 426 Ma. In a blueschist facies pelitic domain, preliminary LA ICP MS U Pb data of homogeneous, igneous allanite clasts indicate Permian crystallization ages. Although the studied rocks show a clear structural and metamorphic Alpine overprint, the mineral chronometers largely reflect the pre Alpine history. So far, only few radiometric ages exist for the II DK. The published chronological constraints largely relied on the petrological similarity of the II DK rocks with those from the Ivrea zone and its structural position. While Permian magmatism is well established in the Sesia zone with mafic intrusives (mostly gabbros) and more evolved types (e.g. Mucrone granodiorite), the HT metamorphism was ascribed either to the Variscan orogenic cycle (Desmons et al.1999) or to a Permian extensional period (e.g. Marotta and Spalla 2007). The latter is now widely recognized in the Eastern Alps (Schuster and Stüwe 2008). While the new data from this study show some indication of a pre Permian metamorphic and igneous evolution, they mainly reflect a marked HT imprint on the II DK metapelites at Permian times. This imprint includes HT metamorphism at 800 850°C in the stability field of garnet and plagioclase as well as intrusive rocks. REFERENCES Desmons J., Compagnoni R., Cortesogno L., 1999. Pre Alpine metamorphism of the internal zones of the Western Alps. Schweizerische mineralogische und petrographische Mitteilungen, 79: 23 39. Marotta A.M. & Spalla M.I., 2007. Permian Triassic high thermal regime in the Alps: Result of late Variscan collapse or continental rifting? Validation by numerical modelling. Tectonics 26: TC 4016. Schuster R. & Stüwe K., 2008. Permian metamorphic event in the Alps. Geology, 36(8): 603 606. 106 10th Alpine workshop "CorseAlp2011" 3D Modeling of the Western Alpine Arc Naomi Vouillamoz 1, Christian Sue 2 & Jean Daniel Champagnac 3 christian.sue@univ fcomte.fr (1) Fribourg University, Switzerland (2) Besançon University, France (3) ETH Zurich, Switzerland We built a 3D structural model of the western alpine arc, a highly non cylindrical mountain belt, using the 3D geomodeler of the French BRGM (Calcagno et al., 2008). The model allows to handle the large scale 3D structure of 15 major crustal units modelized in the belt (from the lower crust to the sedimentary cover nappes), and 2 main discontinuities (the Insubric line and the Crustal Penninic Front), and to better understand their structural relationships. The study area comprises the western alpine arc, from the Jura belt to the Northwest, up to the Bregell granite and the Lepontine Dome to the East, and is limited to the South by the Ligurian basin. It is limited vertically by the topography at the top and the moho interface at the bottom. We discarded the structural relationships between the Alps sensus stricto and the surrounding geodynamic systems such as the Rhine graben or the connexion with the Apennines. The 3D model is based on the global integration of various data such as the DEM of the Alps (Gtopo30), the moho isobaths (Waldhauser et al., 1998), the simplified geological and tectonic maps of the belt, the crustal cross sections ECORS CROP and NFP 20 (Schmid & Kissling 2000), and complementary cross sections specifically built to precise local complexities. The database has first been integrated in a GIS project to prepare their implementation in the geomodeler , by homogenizing the different spatial referencing systems. The global model is finally interpolated from all these data, using the potential field method, and represented by a Voronoï diagram (Courrioux et al., 2001). Evolutions of this model could be the basis for more precise regional models, and/or for integration of structural and tectonic data such as earthquakes (Sue et al., 2010), faulting, nappes’ geometry (Maxelon & Mancktelow, 2004)... It could also be a starting point for a large scale dynamic modeling of the alpine arc, including its recent evolution (Sue et al., 2007), toward a volume balanced retro deformation of the belt. REFERENCES Calcagno et al., 2008, Phys. Earth Planet. Int., 171, 147 157 Courrioux et al., 2001, Tectonophysics, 331, 1 2, 181 196 Maxelon & Mancktelow, 2005, Earth Science Reviews, 71, 171 227. Schmid & Kissling, 2000, Tectonics, 16, 62 85 Sue et al., 2007, Int. J. Earth Sci., 96 6, 1101 1129 Sue et al., 2010, BSGF, 181 6, 583 590 107 10th Alpine workshop "CorseAlp2011" Refined Exhumation History of the Ivrea-Verbano Zone, Southern Alps Reinhard Wolff, István Dunkl, Gesine Kiesselbach, Siegfried Siegesmund & Hilmar von Eynatten Geoscience Center, Georg August University of Göttingen, Germany The Ivrea Verbano Zone is interpreted as exposing a tilted section of the lower continental crust, which makes it to a key region to examine a profile through the crust to the upper mantle (Mehnert 1975; Fountain 1976, 1981). It was rotated into an upright position along the Insubric mylonite belt (Schmid et al. 1987). To refine the exhumation history of this area we performed K/Ar mica, zircon fission track and (U Th)/He thermochronology in the Ivrea Verbano Zone, the Strona Ceneri Zone, the Sesia Lanzo Zone and the adjacent Lower Penninic nappe. We also analyzed the pink Baveno granite. In the Strona Ceneri Zone large granite plutons and mafic intermediate stocks and dykes were emplaced in post Variscan time. The Baveno granite belongs to this NE SW elongated batholith, composed of the plutons Valsessera Biellese, Alzo Roccapietra, Quarna, Mottarone Baveno, and Montorfano. These plutons are also called Graniti dei Laghi (Boriani, Giobbi 2004). The granites are metaluminous and have the geochemical character of a medium K, calc alkaline series (Pinarelli et al. 2002). The shallow intrusion depth of less than ~4 km is based on miarolithic cavities in the Baveno granite (Boriani et al. 1990b). In addition Boriani (1990a) suggested that the Strona Ceneri Zone was, at least in part, exposed in the Permian, and the Baveno granite had already cooled down in the lower Permian. The contact of the miarolithic and the underlying non miarolithic granites indicate, that the Baveno granite has been rotated 15 ° 20 ° eastwards after its emplacement (Boriani et al. 1990a). The Permian emplacement of the Baveno granite is confined by geochronology data by Jäger and Faul (1960) with biotite K/Ar ages of 268 Ma and 269 Ma and Rb/Sr ages of 290 Ma for biotite and 291 Ma for potassic feldspar. Siegesmund et al. (2008) dated the emplacement age of the granite to 291 Ma and 284 Ma with biotite K/Ar. Our new zircon (U Th)/He data of the Baveno granite indicates that after the Permian emplacement the granitic body cooled below the closure temperature of (U Th)/He (~185°C) in Triassic times at around 220 Ma. Probably the early Mesozoic rifting generated an elevated heat flow which was responsible for the Triassic reset. Along the NW margin of the IVZ the Tertiary cooling and tilting history was refined by the new (U Th)/He ages into the early Miocene. REFERENCES Boriani, A.; Burlini, L.; Sacchi, R. (1990a) Tectonophysics 182, 91 102. Boriani, A.; Origoni, E.G; Borghi, A.; Caironi, V. (1990b) Tectonophysics 182, 103 118. Boriani, A.; Giobbi, E. (2004 ) Periodico di Mineralogia 73, 5 22. Fountain, D. M. (1976) Tectonophysics 33, 145 165. Fountain, D. M. (1981) Earth and Planetary Science Letters 56, 263 277. Jäger, E.; Faul, H. (1960) Schweizer. Min. Pet. Mitt. 40, 10 12. Mehnert, K. (1975) Neues Jahrbuch Mineralogie Abhandlungen 125, 156 199. Pinarelli, L.; Moro, A.D.; Boriani, A.; Caironi, V. (2002) Eur. J. Min. 14 (2), 403 415. Schmid, S. M.; Zingg, A.; Handy, M. (1987) Tectonophysics 135 (1 3), 47 66. Siegesmund, S.; Layer, P.; Dunkl, I.; Vollbrecht, A.; Steenken, A.; Wemmer, K.; Ahrendt, H. (2008) Geological Society, London, Special Publications 298, 45 68. 108 10th Alpine workshop "CorseAlp2011" Is Lateral Extrusion in the Eastern Alps Really Lateral? Andreas Wölfler, Walter Kurz, Harald Fritz & Kurt Stüwe andreas.woelfler@uni graz.at & walter.kurz@uni graz.at Karl Franzens University of Graz, Institute of Earth Sciences, Graz, Austria This study comprises a reassessment of the classical model of lateral extrusion in the Eastern Alps by using recently published geochronological data, sedimentary ages from intramontane basins, the ages and distribution of magmatic rocks, and information from seismic profiles. Extrusion related faulting continuously propagated from the western towards the central eastern part of the Eastern Alps during Oligocene to Mid Miocene times. This is confined by oblique convergence between the Adriatic and European plates. During Mid Miocene times extrusion became not only lateral in terms of parallel to the trend of the Eastern Alps, but was characterized by a displacement vector at a high angle to the strike of the orogen. This resulted in the exhumation of the Schladming and Pohorje Blocks that were exhumed within extensional bridges at the northern and southern terminations of the Pöls Lavanttal fault system, respectively. From Mid Miocene (~15 Ma) to recent times extrusion was controlled by overall extension between the Dinaric and Carpathian subduction zones. The influence of north directed compression triggered by the northward moving Adriatic plate diminished and the influence of the Pannonian plate, extending between the slowly westward retreating Dinaric and quickly retreating Carpathian subduction zones, increased. This gave rise to Miocene volcanism that is exclusively found east of the Dinaric trench. We therefore consider that lateral extrusion in the Eastern Alps can be subdivided into distinct tectonic phases, with less pronounced eastward extension related displacement between Late Oligocene and Mid Miocene times. As soon as the Eastern Alps passed the Dinaric trench the entire domain became highly extensive. 109 10th Alpine workshop "CorseAlp2011" HP-UHP metamorphic belts in the Eastern Tethyan Orogenic System in China Jingsui Yang 1，Zhiqin Xu 1 & Paul T. Robinson 2 (1) Key Laboratory for Continental Dynamics, MLR, Institute of Geology, Chinese Academy of Geological Science, Beijing, China (2) Department of Earth Sciences, Dalhousie University, Halifax, Canada Many HP UHP metamorphic belts with eclogite facies rocks occur in the Eastern Tethyan orogenic system in China and adjacent regions. Based on their age of formation/exhumation and tectonic settings, the HP UHP metamorphic belts in China can be divided into 3 broad groups. Group I includes three Proto Tethyan (Early Paleozoic) HP UHP belts, specifically the North Qaidam South Altun UHP belt, the North Qilian North Altun HP belt and the East Qinling UHP belt. Group II includes three Paleo Tethyan HP UHP belts, namely the Dabie Sulu HP UHP belt, the Shuanghu HP belt in the Qiangtang terrane and the Sumdo (U) HP belt of the Lhasa terrane in the central Tibet Plateau. Two Neo Tethyan HP UHP metamorphic belts in the eastern and western Himalayan Syntaxes comprise Group III. Those HP UHP metamorphic belts formed by deep subduction of oceanic lithosphere occur in the eastern part of the Proto Paleo Tethyan system between microcontinental blocks. In contrast, the HP UHP metamorphic belts formed by deep subduction of continental crust are located mainly along suture zones marking continent continent collision. The Dabie Sulu HP UHP metamorphic rocks, which represent root materials of the Paleo Tethyan Dabie Sulu orogenic belt, are superimposed onto the Yangtze block as domed nappes. These were produced by scissors style collision between the North China and Yangtze blocks during the Triassic, and exhumed along multi layer channels. The HP UHP metamorphic belts in the Western Himalayan Syntaxis represent a transition from deep subduction of oceanic lithosphere to deep subduction of continental crust in the Neotethyan tectonic domain. The formation of these HP UHP metamorphic rocks was related to subduction of two tongue shaped lithospheric slabs during collision between corners of the wedge shaped Indian plate and the Asian plate. Rapid exhumation of these rocks was associated with strike slip faults on both sides of the HP UHP metamorphic belts. 110 10th Alpine workshop "CorseAlp2011" Structure and Evolution of the early Paleozoic orogenic belt on the northeast Margin of Tarim Basin Cai Zhihui, Xu Zhiqin, Tang Zhemin & He Bizhu Institute of Geology, Chinese Academy of Geological Sciences, Beijing, China The northeastern margin of Tarim basin is located along the southern margin of the Central Asia Orogenic belt, has undergone an accretional orogenic process, connects the East Tianshan Orogen and Tarim Basin, it is an important area that geologists study accretion type orogens and basin mountain coupling relations. With field investigation and lab study, the author analyzed the orogenic process of orogenic belt. Identify pre Sinian metamorphic and magmatic events and discover a large ductile detachment shear zone in the basement of the orogenic belt at the northeast Margin of Tarim Basin. According to isotope dating results, the Kuruktag area has late archean early proterozoic (2565 2296 Ma) or older basement and experienced major metamorphic and magmatic events of proterozoic period at 1880~1817Ma and 1000~811Ma. The existences of massive mylonites with gentle dip angle, shear folds, stretching lineations and asymmetric porphyroclasts indicate that there is a large ductile detachment with detachment direction NE SW which formed at 383 Ma (Ar Ar dating result in this paper). Identify Caledonian orogenic event on the orogenic belt in the northeastern margin of Tarim, the northeastern margin of Tarim Basin. The evidences are: (1) An early Paleozoic suture zone (Yushugou Tonghuashan suture zone) exists in the south Tianshan; (2) The Cambrian and Ordovician rocks in Kuruktag (south part of the orogeny) have the feature of passive continental margin abyssal and slope facies. (3) Early Paleozoic abyssal sedimentary accretionary wedge, the product of paleo South Tianshan Ocean subduction, outcrops in Kumish (in the south Tianshan). (4) Early Paleozoic magmatic arc type granites outcrop widely in Central Tianshan. (5) Fold thrust belt thrusting southward formed at early Paleozoic in Kuruktag. (6) Silurian Devonian conglomerates develop widely in orogenic belt and basin, northeastern margin of Tarim Basin. (7) Carboniferous rocks and Ordovician Silurian rocks contacted with an angular unconformity have different structural features. Deformation in Carboniferous rocks occur broad and gentle upright folds and Ordovician Silurian upright compact folds with flow cleavage. (8) Dextral strike slip fault system in Central Tianshan and Kuruktag, the product of intra continental deformation after early Paleozoic close of the south Tishan Ocean, formed before 340 383Ma (Ar Ar dating results) Redefine four large dextral strike slip shear zones in orogenic belt of the northeastern margin of Tarim Basin and identify their formation duration time. The north Central Tianshan shear zone, the south Central Tianshan shear zone, Xinger shear zone and Xingdi shear zone are identified as ductile brittle dextral shear zones. These dextral strike slip shear zones formed before 340 383 Ma (Ar Ar dating results), and large scale Intra continental ductile dextral strike slipping event occurred at 241~290 Ma (Ar Ar dating results). Propose a structural evolution pattern of the orogenic belt on the northeastern margin of Tarim. (1) At Sinian the South Tianshan Ocean opened. (2) Paleo South Tianshan ocean shrinked at Sinian early Paleozoic period and the Tarim block subducted underneath the Jungar block resulting accretionary arc formed at Central Tianshan. (3) At Caledonian Tarim block and Central Tianshan accretionary magma arc collided which lead to formation of a series of asymmetric similar folds and flow cleavages. Molasses basins formed at the same time, the majority part of which was still under the sea level. (4) After the end of arc continental collision, intra continental translational thrusting occurred at orogenic belt, and large ductile detachment belt formed between cap and basement. (5) Carboniferous igneous rocks, indicating formation setting of extensional cracking at back arc, develop widely in northeastern margin of Tarim due to subduction southward of paleo Asia Ocean at north. (6) Early Permian massive continental red molasses along orogenic belt are the signal that molasse basins began to change from marine to continental. (7) At Permian early Triassic, large dextral ductile brittle strike slip happened in northeastern margin of Tarim. (8) The east part of orogenic belt in northeastern margin of Tarim show deformation characteristics of sinistral strike slip. (9) During Mesozoic period large uplift erosion occurred commonly in the orogenic belt of northeastern margin of Tarim. (10) At Cenozoic, the northeastern margin of Tarim experienced intra continental compressinal orogenic process again due to the compression of India plate towards Tibetan plateau. 111 10th Alpine workshop "CorseAlp2011" Late Palaeozoic Deformation of Post-Variscan Lower Crust:Shear Zone Widening due to Strain Localization During Retrograde Shearing Ivan Zibra Geological Survey of Western Australia, East Perth, Western Australia The Santa Lucia Shear Zone (SLSZ, Corsica) is a granulite facies Permian shear zone that developed after the emplacement of a deep seated gabbroic intrusion. New structural data shows that the SLSZ results from the juxtaposition of three spatially distinct mylonite belts, which are the product of the interaction between magmatism, metamorphism and shearing over a temperature range from ~800 to~400°C (Zibra et al., 2010). During the earlier high grade deformation stage, which was accompanied by decompression from ~7 to ~5 kb at ~800°C, the SLSZ has accommodated high finite strain on a shear zone ~1 km wide. Strain became increasingly localized as temperature decreased, but rather than reactivating pre existing shear zones as commonly expected, younger mylonites expanded into previously unsheared rock, extending the total width of the shear zone. The zonation of different fabrics across the SLSZ suggests that pre existing compositional and grain size heterogeneities in the starting material played a key role in governing superposed generations of shear zones. Our data suggest that Permian granulites were incorporated into the upper crust prior to Mesozoic. Moreover, the occurrence of MORB type dolerite dykes and ~ 180Ma thermal pulses recorded by granulite facies metasediments (Rossi et al., 2005), indicate that during Jurassic times the S. Lucia basement was located within the thinned continental margin prior to oceanization. Boudinaged pegmatoid Opx-bearing tonalite, previously injected into the host granulite-facies gabbro. Boudin neck is locally filled by undeformed leucotonalite (white arrow), interpreted to have been a melt. Note the severe stretching recorded by Opx porphyroclasts adjacent to the vein boundaries (black arrows; base of photo: ~45 cm) REFERENCES Rossi P, Cocherie A, Fanning CM, Deloule E (2006). Variscan to Eo Alpine events recorded in European lower crust zircons sampled from the French Massif Central and Corsica, France. Lithos 87:235 260. doi:10.1016/j.lithos.2005.06.009. Zibra I, Kruhl JH, Braga R (2010). Late Palaeozoic deformation of post Variscan lower crust: shear zone widening due to strain localization during retrograde shearing. Int J Earth Sci (Geol Rundsch) DOI 10.1007/s00531 009 0441 5 112