Extensional fault patterns of the Lepontine Dome (Central Alps, Switzerland) deduced from paleostress inversions and morphotectonics Allanic, C., Sue, C. & Burkhard, M. Institute of Geology of Neuchâtel, Group of structural geology, Emile Argand 11, 2000 Neuchâtel, Switzerland The main updoming of the Lepontine Gneiss dome NE-SW trending axis _3 (similar to those calculated started some 32-30 Ma ago with the intrusion of the all along the Simplon line). Nevertheless, it appears Bergell tonalites and granodiorites, concomitant with different sets of faults, well expressed by morpho- dextral strike-slip movements along the Tonale and logical features visible on the satellite images. Dif- Canavese Lines (Argand’s Insubric phase). Subse- ferent sets of fault can be observed with the follow- quently, the center of the main updoming has mi- ing trend: NW-SE with normal offset (the most grated slowly to the west, reaching the Simplon re- represented), N90° to N100° with normal/dextral off- gion some 20 Ma ago. The architecture of this set, and N0° to N20° with normal/sinistral offset. Oligocene-Miocene basement unit, resulting from This particular fault pattern seems concordant to ductile and semi-ductile deformations, is composed by two “sub-domes”: the Simplon dome to the west transtensional models established previously and the Ticino dome to the east, separated by the (Schreurs and Colletta, 1998; Waldron, 2005), where “Maggia steep zone”. Both of them are formed incremental strain associated with simple-shear de- mainly by stacking of different nappes consisting of formation in a strike slip zone could explain the small orthogneissic cores discontinuously mantled by a variations of fault trend. schistose paragneissic envelope and overlying The occurrences, characteristic for a specific fault Mesozoic metasediments which are tectonically in- set, of cohesive or non-cohesive cataclasites derived terposed between the older sequences and inter- from gneisses associated with pseudotachylite veins, preted as “nappe separators”. These ductile and fault planes with chlorite and quartz slickenslides, semi-ductile structures begin to be well documented and/or gouge, suggest an important brittle history (Steck and Hunziker, 1994) and imaged (Maxelon and, especially, probably a progressive evolution in and Mancktelow, 2005) contrary to the brittle post- the formation of these different sets of fault, some- nappe tectonics of the dome that still remains under- times involving reactivation of some of them. explored. These field-based studies are complemented by Thus, the network of faults and morphotectonic on-going analytical work on pseudotachylites in or- lineaments of the western parts of the Lepontine der to better constrain the timing of the deformation. dome (Central Alps) is here examined to investigate Indeed, some occurrences of true glass, resulting the late alpine kinematics from Oligocene to Quater- from melting/quenching processes, would allow tim- nary times. Calculations of the stress distributions ing constraints to be obtained. Some morphological (P-T-B axes method, numerical dynamical analysis, features suggest also a possible quaternary activity direct inversion methods, dihedra calculations) have of some WSW-ENE trending fault. yielded a stress field which may be attributed to an This late orogenic extensional tectonic evolution important phase of extension during Oligocene to of metamorphic nappe stacks appears well in line Miocene, probably following the early “core complex” with others studies on the entire alpine arc from the stage of extension leading to the development of the Bergell intrusion (Ciancaleoni, 1999) to the western Lepontine gneissic dome. Indeed, all the methods boundary (Sue et al, 2005). indicate that this crustal-scale rigid block faulting is characterised by a normal paleostress tensor with a REFERENCES Ciancaleoni L., Fügenschuh B., and Marquer D. (1999): Pa- leostress field reconstruction post-dating nappe emplace- ment in the Bergell and Insubric areas (eastern Central Alps): relationships to the late exhumation using fission- track dating, Tubinger Geowissenschaftliche Arbeiten, serie A, 52: 10-11 Maxelon, M. and Mancktelow, N. (2005): Three-dimensional geometry and tectonostratigraphy of the Pennine zone, Central Alps, Switzerland and Northern Italy, Earth Science Reviews, 71: 171-227. Schreurs, G., and B. Colletta (1998): Analogue modelling of faulting in zones of continental transpression and transten- sion, in Continental transpressional and transtensional tectonics, vol. 135, edited by R.E. Holdsworth, R.A. Strachan and J.F. Dewey, Geological Society of London Special Publications, London: 59-79. Steck, A. and Hunziker, J. (1994): The tertiary structure and thermal evolution of the central Alps - compressional and extensional structures in an orogenic belt. -Tectonophysics, 238: 229-254. Sue C., Delacou B., Champagnac J. D., Allanic C., Burkhard M., Tricart P. (submitted). Extensional neotectonics in the western Alps. Special volume: Continental extension. In: Frontiers in Earth Sciences, Ed.: Geologische Vereinigung and Societe Geologique de France, Springer Verlag. Waldron, J. (2005): Extensional fault arrays in strike-slip and transtension. J. Struct. Geol., 27:23-34. Figure1. Examples of late brittle extensional structures. a) Post mylonitic fault near Val Calnegia. Mylonitic foliation, dipping at ca. 20° to the NW is cut by a later set of conjugate brittle normal faults, representing a graben-like-structure, in bi- otite/muscovite gneiss of the Antigorio unit. b) Close up of outcrop shown in a). Feldspar clasts as well as asymmetrically sheared quartz veins indicate a normal “top to the NE” shear sense. c) Close up of outcrop shown in b). A steep normal fault plane mineralized with quartz and chlorite, of which striations indicate a normal sense of shear. HP-rocks in the Alps: Review of isotopic ages based on petrology and their geodynamic consequences Berger, A. & Engi, M. lnstitute of Geological Sciences, University of Bern, Switzerland [email protected] To understand the spatial and temporal evolution of western Alps is directly linked to the location the Se- collisional orogens, such as the Alps, age data on sia unit. This unit is seen as a continental fragment the timing of subduction/extrusion of individual units inside the Piemont-Liguria ocean. If so, the age of may set important markers. Regrettably, the inter- HP-metamorphism in the Sesia unit sets a lower limit pretation of age data is rarely unambiguous, and this for the subduction age of this paleogeographic unit. limits their value despite their high precision. On Provided the Sesia unit was subducted in sequence compiling isotopic ages and petrological information with subsequent parts of the preserved Piemont- for the main oceanic and mélange units in the West- Liguria ocean, the western Alps underwent subduc- ern and Central Alps, this difficulty translates to sub- tion from late Cretaceous (Sesia unit) to Eocene stantial uncertainty in the geodynamic scenarios. (Zermatt unit). This implies a time interval for sub- Individual isotopic mineral ages from high pressure duction along this plate boundary is ~25 Ma, which (HP) metamorphic samples are interpreted as either translates to an average subduction rate of 1.6 cm/a (1) early metamorphic growth on a prograde for the size (400 km) of this paleogeographic unit. path, This rate is consistent with published plate-motion (2) strong metamorphic/deformational overprint, data for the Alps at this time. (3) diffusional resetting of the chronometer, or The same paleogeographic unit (i.e. Piemont- (4) uncertain due to currently limited under- Liguria-ocean) is represented by the Platta-Forno- standing of the mineral chronometer . Malenco units in Eastern Switzerland and Italy. These units are incorporated in the Apulia realm in Where data on the PT-evolution are lacking, individ- the Cretacous (110-90 Ma; Villa et al. 2000), no HP- ual age data do not easily translate to constraints for metamorphism has been documented from these the geodynamic setting in a given geological period. units. The tectonics related to the Platta unit is a It is commonly assumed that (2) reflects conditions nappe-like incorporation during west directed thrust- near “peak metamorphism” (Tmax?). However, unless ing. The timing and the tectonic processes are dif- based on robust thermobarometry, this interpretation ferent in the Platta and the Zermatt units. This indi- is generally at odds with current understanding of cates that some pieces of the Piemont-Liguria ocean petrologic evolution. For some published data sets of were obducted early in their history during west- HP-ages in the Alps, there is insufficient petrological vergent thrusting, whereas the main portion was evidence to discern between (1) and (2), and (4). For subsequently subducted, during the lower Tertiary. most of the relevant data sets (3) is of minor con- Isotope age data from the Central Alps indicate cern. high-pressure relics in a mélange zone, with HP- Available age data do provide some markers on phases forming in different location within an ex- the subduction history. We connect some Zir U/Pb tended time interval of 65-37 Ma (Brouwer et al. and Lu/Hf data in eclogites of the Sesia zone with 2005, Gebauer 1999). The timing is thus roughly the timing of HP-equilibration at 65 Ma (Duchêne et consistent with age data of HP-events in the western al. 1997, Rubatto et al., 1999). For the Zermatt unit, Alps. However, it