DOCSLIB.ORG

Multiple Metamorphic Stages Within an Eclogite-Facies Terrane

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Multiple Metamorphic Stages Within an Eclogite-Facies Terrane JOURNAL OF PETROLOGY VOLUME 55 NUMBER 7 PAGES 1429^1456 2014 doi:10.1093/petrology/egu029 Multiple Metamorphic Stages within an Eclogite-facies Terrane (Sesia Zone, Western Alps) Revealed byTh^U^Pb Petrochronology D. REGIS1*, D. RUBATTO2,J.DARLING3,B.CENKI-TOK4,M.ZUCALI5 AND M. ENGI1 1INSTITUT FU« R GEOLOGIE, UNIVERSITY OF BERN, BALTZERSTRASSE 1þ3, 3012 BERN, SWITZERLAND 2RESEARCH SCHOOL OF EARTH SCIENCES, AUSTRALIAN NATIONAL UNIVERSITY, MILLS ROAD, CANBERRA, ACT 0200, AUSTRALIA 3SCHOOL OF EARTH AND ENVIRONMENTAL SCIENCES, UNIVERSITY OF PORTSMOUTH, BURNABY BUILDING, PORTSMOUTH PO1 3QL, UK 4GE¤ OSCIENCES MONTPELLIER, UNIVERSITY OF MONTPELLIER 2, UMR 5243, PLACE E. BATAILLON, 34095 MONTPELLIER, FRANCE 5DIPARTIMENTO DI SCIENZE DELLA TERRA ‘ARDITO DESIO’, UNIVERSITA' DEGLI STUDI DI MILANO, VIA MANGIAGALLI 34, I-20133 MILANO, ITALY RECEIVED MARCH 2, 2013; ACCEPTED MAY 14, 2014 Convergent plate margins typically experience a transition from sub- Bonze rocks) and the Druer slice (belonging to the Eclogitic duction to collision dynamics as massive continental blocks enter the Micaschist Complex).The new data indicate separate stages of de- subduction channel. Studies of high-pressure rocks indicate that formation at eclogite-facies conditions for each recognized independ- tectonic fragments are rapidly exhumed from eclogite facies to mid- ent kilometer-sized tectono-metamorphic slice, between 85 and 60 crustal levels, but the details of such dynamics are controversial. To Ma, with evidence of intermittent decompression (ÁP 0·5GPa) understand the dynamics of a subduction channel we report the re- within only the Fondo slice. The evolution path of the Druer slice | downloaded: 6.10.2021 sults of a petrochronological study from the central Sesia Zone, a key indicates a different P^T^time evolution with prolonged eclogite- element of the internal Western Alps.This comprises two polymeta- facies metamorphism between 85 and 75 Ma. Our approach, com- morphic basement complexes (Eclogitic Micaschist Complex and bining structural, petrological and geochronological techniques, Gneiss Minuti Complex) and a thin, dismembered cover sequence yields field-based constraints on the duration and rates of dynamics (Scalaro Unit) associated with pre-Alpine metagabbros and metase- within a subduction channel. diments (Bonze Unit). Structurally controlled samples from three of these units (Eclogitic Micaschist Complex and Scalaro^Bonze Units) yield unequivocal petrological and geochronological evidence KEY WORDS: pressure cycles; Sesia Zone; subduction channel; tectono- of two distinct high-pressure stages. Ages (U^Th^Pb) of growth metamorphic slice; yo-yo subduction zones in accessory allanite and zircon, combined with inclusion and textural relationships, can be tied to the multi-stage evolution of single samples. Two independent tectono-metamorphic ‘slices’show- INTRODUCTION ing a coherent metamorphic evolution during a given time interval Exposed high-pressure (HP) crustal rocks in Phanerozoic have been recognized: the Fondo slice (which includes Scalaro and orogens represent the result of a sequence of processes that https://doi.org/10.7892/boris.70831 *Corresponding author. Present address: Department of Environment, Earth and Ecosystems, The Open University, Walton Hall, Milton ß The Author 2014. Published by Oxford University Press. All Keynes MK7 6AA, UK. E-mail: [email protected], rights reserved. For Permissions, please e-mail: source: [email protected] [email protected] JOURNAL OF PETROLOGY VOLUME 55 NUMBER 7 JULY 2014 operate at lithosphere scale (i.e. rifting, subduction^ exhumation. Careful structural control in sampling and accretion, tectonic mixing, return flow^exhumation). The detailed petrography were used to select samples for petro- dynamics of assembly of HP and UHP (ultrahigh- chronology to arrive at a coherent interpretation of the pressure) terranes is central to the tectonic study of colli- P^T^t data in the context of subduction dynamics. sion zones. Despite much interest, the mechanisms by Accessory phases proved most useful in deducing parts of which HP rocks descend to mantle depths and then return the metamorphic reaction history and to relate the specific to the surface remain controversial. stages of the evolution. Microbeam Th^U^Pb geochron- For any given HP^UHP terrane (e.g. Western Alps, ology by sensitive high-resolution ion microprobe Himalaya and Dabie-Shan), exhumation mechanisms are (SHRIMP) and laser ablation inductively coupled plasma inferred from concepts derived from geological data, mass spectrometry (LA-ICP-MS) allowed dating of these notably structural data and pressure^temperature^time stages. Single grain (or subgrain) dating turned out to be (P^T^t) paths, combined with insights derived from numer- essential to utilize the preserved fine detail of (over)- ical and analogue models. Efforts to understand the mech- growths within each accessory mineral. Where possible, anical behaviour of the lithosphere have singled out several mineral chronometers were combined to exploit their cap- mechanisms in the transition from a subduction to an ex- acity to record different stages of the metamorphic evolu- humation stage (e.g. Warren et al., 2008).These mechanisms tion. Particular attention was paid to the distribution of can be grouped into those driven by buoyancy forces (e.g. rare earth elements (REE) between major metamorphic Ernst, 2001) and those driven by a range of different tectonic minerals and the accessory phases used for geochronology processes (e.g. Platt, 1993; Kurz & Froitzheim, 2002). (zircon and allanite) to gain additional insight into the Structural and P^T^t data from several HP terranes relative timing of accessory and major mineral growth suggest two separate stages of exhumation: a first rapid (e.g. Hermann & Rubatto, 2003; Gabudianu Radulescu stage from mantle to lower crustal levels and a slower ex- et al., 2009). Two independent tectono-metamorphic ‘slices’ humation to the surface (Terry et al., 2000; Rubatto & (termed the Druer and Fondo slices, respectively) were Hermann, 2001; Carswell et al., 2003; Mukherjee et al., recognized within the Sesia Zone because they followed 2003; Parrish et al., 2006). Although various mechanisms different P^T^t paths within the subduction channel. may operate at different depths, buoyancy forces related Independent mobility of these slices has been unequivo- to the density contrast are commonly inferred to be par- cally demonstrated because they underwent different P^T ticularly significant for the first stage of exhumation, conditions at the same time. The results obtained on the whereas the second stage can be accomplished by a variety P^T^t evolution of each tectono-metamorphic slice are of syn- to post-convergent processes (e.g. Ernst, 2001). used to estimate the rates of subduction and exhumation. Moreover, some numerical models invoke extensive tec- tonic mixing during the subduction and exhumation stages (Gerya et al., 2002; Stoeckhert & Gerya, 2005; GEOLOGICAL BACKGROUND AND Roda et al., 2012), suggesting complex P^T^time paths for SAMPLING STRATEGY lithospheric fragments within a subduction channel. The Alps are a double-vergent orogen that formed as a In comparing P^T data from HP terranes with model result of the convergence between the European and predictions several questions arise, such as the following. African (Apulia) tectonic plates (e.g. Dewey et al., 1989). Under which conditions do fragments from the subducting The Sesia Zone (SZ) is the largest exposed slice of plate accrete to form an HP terrane? What temporal and Apulian continental crust in the Western Alps that under- spatial scales of tectonic mixing in the subduction channel went subduction prior to the Alpine orogeny and exten- are realistic? What triggers exhumation and at what rates sively preserves eclogite-facies assemblages. It has does it occur? traditionally been divided into three subunits (from west The dynamics in a subduction channel leading to a spe- to east, Fig. 1a, Table 1): the Gneiss Minuti Complex cific HP terrane are bound to be complex. For example, (GMC), the Second Dioritic^Kinzigitic Zone (IIDK), and results presented by Rubatto et al. (2011) have already es- the Eclogitic Micaschist Complex (EMC). These tectonic tablished the existence of HP cycles for the specific case of complexes consist of a metasedimentary basement com- the Sesia Zone. To corroborate this complex evolution and posed of Variscan amphibolite-facies rocks intruded by late quantify it in detail, we adopted an integrative approach Variscan granitoids that experienced polyphase tectono- that links observations and data across a range of scales, metamorphic reworking during the Alpine orogeny (Dal from structural patterns at kilometer scale in the field Piaz et al. 1972; Compagnoni, 1977; Compagnoni et al., 1977; down to petrological and chronological data obtained at Gosso, 1977; Lardeaux & Spalla, 1991). The three units pre- micrometer scale. This field-based study of a significant serve to different degrees an early HP imprint (peak pres- portion of the Sesia Zone reveals a multi-stage evolution sures of 1·0^1·5, 1·0^1·2, and 1·6^2·4 GPa respectively) that of this HP continental terrane, from subduction to partially re-equilibrated under greenschist-facies conditions 1430 REGIS et al. SESIA ZONE EVOLUTION Fig. 1. (a) Geological map of the central Sesia Zone (SZ), Western Italian Alps. Greenschist-facies overprinted portion of the Eclogitic Micaschist Complex
Load more

Recommended publications
  • Kinematics and Extent of the Piemont-Liguria Basin
    https://doi.org/10.5194/se-2020-161 Preprint. Discussion started: 8 October 2020 c Author(s) 2020. CC BY 4.0 License. Kinematics and extent of the Piemont-Liguria Basin – implications for subduction processes in the Alps Eline Le Breton1, Sascha Brune2,3, Kamil Ustaszewski4, Sabin Zahirovic5, Maria Seton5, R. Dietmar Müller5 5 1Department of Earth Sciences, Freie Universität Berlin, Germany 2Geodynamic Modelling Section, German Research Centre for Geosciences, GFZ Potsdam, Germany 3Institute of Geosciences, University of Potsdam, Potsdam, Germany 4Institute for Geological Sciences, Friedrich-Schiller-Universität Jena, Germany 10 5EarthByte Group, School of Geosciences, The University of Sydney, NSW 2006, Australia Correspondence to: Eline Le Breton ([email protected]) Abstract. Assessing the size of a former ocean, of which only remnants are found in mountain belts, is challenging but crucial to understand subduction and exhumation processes. Here we present new constraints on the opening and width of the Piemont- Liguria (PL) Ocean, known as the Alpine Tethys together with the Valais Basin. We use a regional tectonic reconstruction of 15 the Western Mediterranean-Alpine area, implemented into a global plate motion model with lithospheric deformation, and 2D thermo-mechanical modelling of the rifting phase to test our kinematic reconstructions for geodynamic consistency. Our model fits well with independent datasets (i.e. ages of syn-rift sediments, rift-related fault activity and mafic rocks) and shows that the PL Basin opened in four stages: (1) Rifting of the proximal continental margin in Early Jurassic (200-180 Ma), (2) Hyper- extension of the distal margin in Early-Middle Jurassic (180-165 Ma), (3) Ocean-Continent Transition (OCT) formation with 20 mantle exhumation and MORB-type magmatism in Middle-Late Jurassic (165-154 Ma), (4) Break-up and “mature” oceanic spreading mostly in Late Jurassic (154-145 Ma).
    [Show full text]
  • Alps the Role of the Periadriatic Line in the Tectonic Evolution of The
    Geological Society, London, Special Publications The role of the Periadriatic Line in the tectonic evolution of the Alps S. M. Schmid, H. R. Aebli, F. Heller and A. Zingg Geological Society, London, Special Publications 1989; v. 45; p. 153-171 doi:10.1144/GSL.SP.1989.045.01.08 Email alerting click here to receive free email alerts when new articles cite this service article Permission click here to seek permission to re-use all or part of this article request Subscribe click here to subscribe to Geological Society, London, Special Publications or the Lyell Collection Notes Downloaded by on 30 May 2007 © 1989 Geological Society of London The role of the Periadriatic Line in the tectonic evolution of the Alps S. M. Schmid, H. R. Aebli, F. Heller & A. Zingg SUMMARY: The Periadriatic Line and related lineaments formed as a result of post- collisional deformations which severely modified the Alpine chain. This post-late Oligocene deformation is the result of dextral transpression between the Adriatic sub-plate and the European foreland. Indentation of the western edge of the southern Alps caused uplift, related to backthrusting and associated deformations of the Lepontine region combined with E-directed escape of the central Alps. In the eastern Alps the response to dextral transpression is mainly by lateral escape along conjugate strike slip zones with minor or no vertical movements. Older deformations along this essentially late Alpine lineament can still be inferred locally and include: extension and transfer faulting in the late Palaeozoic to early Mesozoic, Cretaceous deformations, and Tertiary phases of compression (Eocene) and possibly extension (Oligocene).
    [Show full text]
  • Geologic Map of the Southern Ivrea-Verbano Zone, Northwestern Italy
    •usGsscience for a changing world Geologic Map of the Southern Ivrea-Verbano Zone, Northwestern Italy By James E. Quick,1 Silvano Sinigoi,2 Arthur W. Snoke,3 Thomas J. Kalakay,3 Adriano Mayer,2 and Gabriella Peressini2·4 Pamphlet to accompany Geologic Investigations Series Map I- 2776 1U. S. Geological Survey, Reston, VA 20192- 0002. 2Uni versita di Trieste, via Weiss 8, 341 27 Trieste, ltalia. 3Uni versity of Wyoming, Larami e, WY 8207 1- 3006. 4Max-Planck-lnstitut ft.ir Chemi e, J.J. Becherweg 27, 55 128 Mainz, Germany. 2003 U.S. Department of the Interi or U.S. Geological Survey COVER: View of the Ponte della Gula, an ancient bridge spanning the Torrente Mastellone approximately 2 kilometers north of the village of Varallo. Diorite of Valsesia crops out beneath the bridge. Photograph by ADstudia, Silvana Ferraris, photographer, Pizza Calderini, 3-13019 Varallo Sesia ([email protected]) INTRODUCTION REGIONAL SETTING The intrusion of mantle-derived magma into the deep conti­ The Ivrea-Verbano Zone (fig. 1) is a tectonically bounded sliver nental crust, a process commonly referred to as magmatic of plutonic and high-temperature, high-pressure metamorphic underplating, is thought to be important in shaping crustal com­ rocks in the southern Alps of northwestern Italy (Mehnert, position and structure. However, most evidence for this process 1975; Fountain, 1976). To the northwest, it is faulted against is indirect. High P-wave velocities and seismic-reflection profiles the basement of the Austro-Alpine Domain by the lnsubric Line, reveal that much of the deep continental crust is dense and a major suture zone that separates the European and Apulian strongly layered, consistent with the presence of layered mafic plates (Schmid and others, 1987; Nicolas and others, 1990).
    [Show full text]
  • Kinematics and Extent of the Piemont–Liguria Basin – Implications for Subduction Processes in the Alps
    Solid Earth, 12, 885–913, 2021 https://doi.org/10.5194/se-12-885-2021 © Author(s) 2021. This work is distributed under the Creative Commons Attribution 4.0 License. Kinematics and extent of the Piemont–Liguria Basin – implications for subduction processes in the Alps Eline Le Breton1, Sascha Brune2,3, Kamil Ustaszewski4, Sabin Zahirovic5, Maria Seton5, and R. Dietmar Müller5 1Department of Earth Sciences, Freie Universität Berlin, Berlin, Germany 2Geodynamic Modelling Section, German Research Centre for Geosciences, GFZ Potsdam, Potsdam, Germany 3Institute of Geosciences, University of Potsdam, Potsdam, Germany 4Institute for Geological Sciences, Friedrich-Schiller-Universität Jena, Jena, Germany 5EarthByte Group, School of Geosciences, The University of Sydney, Sydney, NSW 2006, Australia Correspondence: Eline Le Breton ([email protected]) Received: 18 September 2020 – Discussion started: 8 October 2020 Revised: 3 March 2021 – Accepted: 5 March 2021 – Published: 21 April 2021 Abstract. Assessing the size of a former ocean of which only 84–35 Ma, 260 km between 35–0 Ma), which greatly exceeds remnants are found in mountain belts is challenging but cru- the width of the ocean. We suggest that at least 63 % of the cial to understanding subduction and exhumation processes. subducted and accreted material was highly thinned conti- Here we present new constraints on the opening and width nental lithosphere and most of the Alpine Tethys units ex- of the Piemont–Liguria (PL) Ocean, known as the Alpine humed today derived from OCT zones. Our work highlights Tethys together with the Valais Basin. We use a regional tec- the significant proportion of distal rifted continental margins tonic reconstruction of the Western Mediterranean–Alpine involved in subduction and exhumation processes and pro- area, implemented into a global plate motion model with vides quantitative estimates for future geodynamic modeling lithospheric deformation, and 2D thermo-mechanical mod- and a better understanding of the Alpine Orogeny.
    [Show full text]
  • Facies and Late Triassic Fossils in the Roisan Zone, Austroalpine Dent Blanche and Mt Mary-Cervino Nappe System, NW Alps
    Swiss J Geosci (2016) 109:69–81 DOI 10.1007/s00015-016-0207-6 Facies and Late Triassic fossils in the Roisan zone, Austroalpine Dent Blanche and Mt Mary-Cervino nappe system, NW Alps 1 1 2 3 Gloria Ciarapica • Leonsevero Passeri • Franco Bonetto • Giorgio Vittorio Dal Piaz Received: 11 August 2015 / Accepted: 7 January 2016 / Published online: 12 February 2016 Ó Swiss Geological Society 2016 Abstract The Roisan zone is a metamorphic cover unit Roisan zone could have been deposited in a more distal exposed along the ductile shear zone between the Dent area. Blanche s.s. and Mont Mary-Cervino Upper Austroalpine outliers, Aosta Valley, north-western Italian Alps. It is Keywords Austroalpine covers Á Sedimentology Á characterized by the occurrence of dolostones, pure mar- Stratigraphy Á Foraminifers Á Dasycladales bles, marbles with quartz, calcirudites and ophiolite-free calcschists. Locally, dolostones preserve alternances of thick massive beds and thinner levels of planar stromato- 1 Introduction lites and other sedimentary structures and textures typical of a carbonate platform. In Mt Grand Pays they contain The Roisan zone is the most extended sedimentary cover unit Dasycladales and foraminifers referable to the Norian. Pure associated to the Austroalpine Dent Blanche and Sesia– marbles and marbles with quartz grains are tentatively Lanzo nappe system, the upper part of the subduction-related referred to the end of Triassic–Early Jurassic, thin-bedded collisional wedge in the northwestern Alps. As shown in marbles and calcirudites to the Early and Middle Jurassic, Fig. 1, classic Argand’s (1908, 1909) Dent Blanche fold- calcschists from Middle Jurassic to Late Cretaceous.
    [Show full text]
  • GEOLOGICAL EXCURSION the WESTERN ALPS Field Guidebook
    Orléans University-Institute of Geology and Geophysics Cooperation program GEOLOGICAL EXCURSION IN THE WESTERN ALPS June 22 -July 2, 2018 Field guidebook excursion leaders: M. Faure & Y. Chen Monviso from Agnel Pass Orléans University-Peking University Cooperation program 1 A GEOLOGICAL EXCURSION IN THE WESTERN ALPS Field guide book 2018 M. Faure, Y. Chen PART I: GEOLOGICAL OUTLINE OF THE FRENCH-ITALIAN ALPS INTRODUCTION 1. The Alpine system in Europe. The European continent was progressively edificated by several orogenic events since the Archean (Fig. 1). Paleoproterozoic belts are restricted to Scandinavia. A Neoproterozoic orogen, called the Cadomian Belt, from the name of the Caen city in Normandy, and formed around 600 Ma, is observed in the northern part of the Massif Armoricain and also in Great Britain, in Spain, and East Europe. During the Paleozoic, three collisional belts are recognized, namely i) in western Scandinavia, Scotland, Ireland, Wales and Britain, the Caledonian Belt results of the collision between North America (or Laurentia) and Scandinavia (or Baltica) that gave rise to the Laurussia continent in Silurian; ii) the Variscan (or Hercynian) Belt that develops in Middle Europe from SW Iberia to Poland, results of the collision between Laurussia and Gondwana in Devonian and Carboniferous; iii) the Urals formed by the collision between Laurussia and Siberia in Carboniferous. As the result of the Paleozoic orogenies, in Permian, Europe and Africa belonged to the Pangea megacontinent. Fig. 1: Tectonic map of Europe During the Cenozoic, several orogenic belts are recognized in southern Europe (Fig. 1). The Pyrénées are due to the Eocene closure of a continental rift opened in Mesozoic between France and Iberia (= Spain and Portugal).
    [Show full text]
  • Miocene Basement Exhumation in the Central Alps Recorded by Detrital
    https://doi.org/10.5194/se-2019-98 Preprint. Discussion started: 14 June 2019 c Author(s) 2019. CC BY 4.0 License. 1 Miocene basement exhumation in the Central Alps recorded 2 by detrital garnet geochemistry in foreland basin deposits 3 Laura Stutenbecker1*, Peter M.E. Tollan2, Andrea Madella3, Pierre Lanari2 4 5 1Institute of Applied Geosciences, Technische Universität Darmstadt, Schnittspahnstr. 9, 6 64287 Darmstadt, Germany 7 2Institute of Geological Sciences, University of Bern, Baltzerstrasse 1+3, 3012 Bern, 8 Switzerland 9 3Department of Geosciences, University of Tuebingen, Wilhelmstr. 56, 72074 Tübingen, 10 Germany 11 *corresponding author: [email protected] 12 Abstract 13 The Neogene evolution of the European Alps was characterized by the exhumation of crystalline 14 basement, the so-called external crystalline massifs. Their exhumation presumably controlled the 15 evolution of relief, distribution of drainage networks and generation of sediment in the Central Alps. 16 However, due to the absence of suitable proxies, the timing of their surficial exposure, and thus the 17 initiation of sediment supply from these areas, are poorly constrained. 18 The northern alpine foreland basin preserves the Oligocene to Miocene sedimentary record of tectonic 19 and climatic adjustments in the hinterland. This contribution analyses the provenance of 25 to 14 My- 20 old alluvial fan deposits by means of detrital garnet chemistry. Unusually grossular- and spessartine- 21 rich garnets are found to be unique proxies for identifying detritus from the external crystalline 22 massifs. In the foreland basin, these garnets are abundant in 14 My-old deposits, thus providing a 23 minimum age for the surficial exposure of the crystalline basement.
    [Show full text]
  • Handy Etal 10 ESR.Pdf
    This article appeared in a journal published by Elsevier. The attached copy is furnished to the author for internal non-commercial research and education use, including for instruction at the authors institution and sharing with colleagues. Other uses, including reproduction and distribution, or selling or licensing copies, or posting to personal, institutional or third party websites are prohibited. In most cases authors are permitted to post their version of the article (e.g. in Word or Tex form) to their personal website or institutional repository. Authors requiring further information regarding Elsevier’s archiving and manuscript policies are encouraged to visit: http://www.elsevier.com/copyright Author's personal copy Earth-Science Reviews 102 (2010) 121–158 Contents lists available at ScienceDirect Earth-Science Reviews journal homepage: www.elsevier.com/locate/earscirev Reconciling plate-tectonic reconstructions of Alpine Tethys with the geological–geophysical record of spreading and subduction in the Alps Mark R. Handy a,⁎, Stefan M. Schmid a,c,1, Romain Bousquet b,2, Eduard Kissling c,3, Daniel Bernoulli d,4 a Institut für Geologische Wissenschaften, Freie Universität Berlin, Malteserstrasse 74-100, D-12249 Berlin, Germany b Institut für Geowissenschaften, Universität Potsdam, Postfach 601553, D-14415 Potsdam, Germany c Institut für Geophysik, ETH-Zentrum, Sonneggstrasse 5, CH-8092 Zurich, Switzerland d Geologisch-Paläontologisches Institut, Universität Basel, Bernoullistrasse 32, CH-4056 Basel, Switzerland article info abstract Article history: A new reconstruction of Alpine Tethys combines plate-kinematic modelling with a wealth of geological data Received 23 November 2008 and seismic tomography to shed light on its evolution, from sea-floor spreading through subduction to Accepted 1 June 2010 collision in the Alps.
    [Show full text]
  • Italian Alps S
    TECTONICS, VOL. 15, NO. 5, PAGES 1036-1064, OCTOBER 1996 Geophysical-geological transect and tectonic evolution of the Swiss-Italian Alps S. M. Schmid,10. A. Pfiffner,2 N. Froitzheim,1 G. Sch6nborn,3 and E. Kissling4 Abstract. A complete Alpine cross section inte- relatively strong lower crust and detachmentbetween grates numerous seismicreflection and refraction pro- upper and lower crust. files, across and along strike, with published and new field data. The deepest parts of the profile are con- strained by geophysicaldata only, while structural fea- Introduction tures at intermediate levelsare largely depicted accord- ing to the resultsof three-dimensionalmodels making Plate 1 integrates geophysicaland geologicaldata useof seismicand field geologicaldata. The geometryof into one singlecross section acrossthe easternCentral the highest structural levels is constrainedby classical Alps from the Molasse foredeep to the South Alpine along-strikeprojections of field data parallel to the pro- thrust belt. The N-S section follows grid line 755 of nounced easterly axial dip of all tectonic units. Because the Swisstopographic map (exceptfor a southernmost the transect is placed closeto the westernerosional mar- part near Milano, see Figure i and inset of Plate 1). gin of the Austroalpinenappes of the Eastern Alps, it As drawn, there is a marked difference between the tec- contains all the major tectonic units of the Alps. A tonic style of the shallower levels and that of the lower model for the tectonic evolution along the transect is crustal levels. This difference in style is only partly real. proposedin the form of scaledand area-balancedprofile The wedgingof the lower crust stronglycontrasts with sketches.Shortening within the Austroalpinenappes is the piling up and refolding of thin flakesof mostly up- testimonyof a separateCretaceous-age orogenic event.
    [Show full text]
  • The Tectonometamorphic Evolution of the Sesia–Dent Blanche Nappes (Internal Western Alps): Review and Synthesis
    Swiss J Geosci DOI 10.1007/s00015-014-0172-x The tectonometamorphic evolution of the Sesia–Dent Blanche nappes (internal Western Alps): review and synthesis Paola Manzotti • Michel Balle`vre • Michele Zucali • Martin Robyr • Martin Engi Received: 29 May 2014 / Accepted: 6 October 2014 Ó Swiss Geological Society 2014 Abstract This study reviews and synthesizes the present zone, indicating that the Sesia–Dent Blanche nappes rep- knowledge on the Sesia–Dent Blanche nappes, the highest resent a stack of several individual nappes. During the tectonic elements in the Western Alps (Switzerland and subsequent subduction of the Piemonte–Liguria Ocean Italy), which comprise pieces of pre-Alpine basement and large-scale folding of the nappe stack (including the Roi- Mesozoic cover. All of the available data are integrated in a san-Cignana Shear Zone) took place under greenschist crustal-scale kinematic model with the aim to reconstruct facies conditions, which indicates partial exhumation of the the Alpine tectono-metamorphic evolution of the Sesia– Dent Blanche Tectonic System. The entrance of the Bri- Dent Blanche nappes. Although major uncertainties remain anc¸onnais micro-continent within the subduction zone led in the pre-Alpine geometry, the basement and cover to a drastic change in the deformation pattern of the Alpine sequences of the Sesia–Dent Blanche nappes are seen as belt, with rapid exhumation of the eclogite-facies ophiolite- part of a thinned continental crust derived from the Adriatic bearing units and thrust propagation towards the foreland. margin. The earliest stages of the Alpine evolution are Slab breakoff probably was responsible for allowing partial interpreted as recording late Cretaceous subduction of the melting in the mantle and Oligocene intrusions into the Adria-derived Sesia–Dent Blanche nappes below the most internal parts of the Sesia–Dent Blanche nappes.
    [Show full text]
  • Polyphase Greenschist-Facies Reactivation of the Dent Blanche Basal Thrust (Western Alps) During Progressive Alpine Orogeny
    Swiss J Geosci (2017) 110:503–521 DOI 10.1007/s00015-017-0264-5 Polyphase greenschist-facies reactivation of the Dent Blanche Basal Thrust (Western Alps) during progressive Alpine orogeny Frederik Kirst1 Received: 24 May 2016 / Accepted: 13 February 2017 / Published online: 28 February 2017 Ó Swiss Geological Society 2017 Abstract This study assesses the significance, geometry, shearing, partly conjugate top-NW/top-SE shearing, and and kinematics of greenschist-facies deformation along the resulting orogen-perpendicular crustal extension. Subse- Dent Blanche Basal Thrust (DBBT), a major tectonic quently, the DBBT was affected by a phase of orogen- contact in the Internal Western Alps of Switzerland and perpendicular shortening (D4) and formation of folds and Italy. The DBBT separates continental units of the Dent crenulations at ca. 34–33 Ma due to increasing compres- Blanche nappe, the structurally highest unit in the Western sional tectonics. Finally, a phase of semi-ductile to brittle Alps, from underlying Piemont-Ligurian ophiolites. normal-sense top-NW and conjugate shearing (D5) from Mylonites and deformation structures along the contact ca. 32 Ma onwards particularly affected the southeastern provide a record of its retrograde greenschist-facies evo- segment and indicates exhumation of the DBBT through lution after earlier high-pressure metamorphism. A first the ductile–brittle transition. This was followed by brittle phase of foreland-directed, reverse-sense, top-(N)W NW–SE extensional deformation. This study suggests that shearing (D1) occurred between ca. 43 and 39 Ma, related the DBBT experienced a polyphase deformation and to exhumation of the Dent Blanche nappe from high- reactivation history under decreasing greenschist-facies pressure conditions.
    [Show full text]
  • The Underground of the Western Alps
    Mémoires de Géologie (Lausanne) No. 15, 1993 - I - The Underground of the Western Alps by Robin Marchant IMPRIMERIE CHABLOZ S.A., TOLOCHENAZ ISSN 1015-3578 - II - This eloctronic version is identical to the printed edition, except for some figures which are in color here but in black and white in the printed edition. - I - Université de Lausanne Institut de Géologie Faculté des Sciences et de Paléontologie The Underground of the Western Alps Thèse de doctorat présentée à la Faculté des Sciences de l’Université de Lausanne par Robin Marchant géologue diplômé Jury de thèse: Prof. Gérard Stampfli (Directeur) Prof. Raymond Olivier (Lausanne) Dr. Riccardo Polino (Torino) Prof. Albrecht Steck (Lausanne) Prof. François Thouvenot (Grenoble) Mémoires de Géologie (Lausanne) No. 15, 1993 - II - Cover image (after a photo from the book "CERVIN, une montagne de PUB"; by courtesy of Yvan Hostet- tler): The Matterhorn, the most outstanding mountain of the Western Alps, has puzzled many generations of geologists, of which Emile Argand (see drawing above) was one of the most eminent. This peak is made of gneisses (the series of Arolla and Valpelline) originating in the Austroalpine terrain; they overlie the Piemont accretionary prism (the “Schistes Lustrés” of the Tsaté nappe) and remnants of the Piemont oceanic crust (the ophiolites of Zermatt-Saas). A few km down in the Matter valley, a large backfold affects the Siviez-Mischabel nappe (corresponding to Argand’s “Série de Casanna”), which originates in the Briançonnais exotic terrain. In order to puzzle out the deep structures in this region, the PNR/NFP-20 research programme shot two deep seismic lines at the foot of the Matterhorn: the W3 line runs up the Matter valley, passing through the village of Zermatt, up to Zmutt, where it is relayed by a short perpendicular profile, the W4 line.
    [Show full text]