Cretaceous-Eocene compression in the central Southern Alps (N Italy) inferred from Ar/Ar dating of pseudotachylytes along regional thrust faults |
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| Authors: | Stefano Zanchetta Paolo D’Adda Andrea Zanchi Valentina Barberini Igor M Villa |
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| Institution: | 1. Dipartimento di Scienze Geologiche e Geotecnologie, Università degli Studi di Milano Bicocca, P.za della Scienza 4, Milano 20126 Italy;2. Institut für Geologie, Universität Bern, 3012 Bern, Switzerland |
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| Abstract: | The integration of structural analyses with 40Ar/39Ar dating of fault-related pseudotachylytes provides time constraints for the reconstruction of the Alpine evolution of the central portion of the South Alpine orogenic wedge. In the northern sector of the belt a Variscan basement is stacked southward on the Permian to Mesozoic cover along regional faults (Orobic and Porcile thrusts). Fault zones, slightly postdating a first folding event of Alpine age, experienced a complex evolution through the ductile and brittle deformation regime, showing greenschist facies mylonites overprinted by a penetrative cataclastic deformation. Generation of fault-related pseudotachylyte veins marks the onset of brittle conditions, lasting up to the youngest episodes of fault activity. 40Ar/39Ar dating of the pseudotachylyte matrix of 9 samples give two separated age clusters: Late Cretaceous (80–68 Ma) and latest Palaeocene to Middle Eocene (55–43 Ma). These new data provide evidence that the pre-Adamello evolution of the central Southern Alps was characterised by the superposition of different tectonic events accompanying the exhumation of the deepest part of the belt through the brittle–ductile transition. The oldest pseudotachylyte ages demonstrate that south-verging regional thrusting in the central Southern Alps was already active during the Late Cretaceous, concurrently with the development of a synorogenic foredeep basin where the Upper Cretaceous Lombardian Flysch was deposited. |
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| Keywords: | Pseudotachylyte 40Ar/39Ar dating Orobic thrust Porcile thrust Compressive deformation |
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