Constraining kinematic and temporal evolution of a normal-sense shear zone: Insights into the Simplon Shear Zone (Western Alps)

The exhumation of the Lepontine Dome in the Central Alps was mainly driven by extensional shear zones at its borders. The Simplon Shear Zone (SSZ), formed as a consequence of east-west lateral extrusion perpendicular to north-south convergence between Adria and Europa plates, has been the leading structure in the exhumation of the western sector of the Lepontine Dome where the deepest rocks of Central Alps are nowadays exposed. We present here a multidisciplinary study of the SSZ combining fieldwork, microstructural analyses, vorticity estimates, quartz c-axis fabric analysis, quartz paleopiezometry and 40Ar/39Ar geochronology. The SSZ evolved from epidote-amphibolite to greenschist facies and then brittle conditions during shearing. A decrease of simple shear component from 88% to 37% towards the top of the shear zone is observed, with mylonites displaying ages within the 12-8 Ma time interval. Differential stress (59–78 MPa) and strain rate (10−11-10−12 s−1) estimates are in agreement with values obtained for crustal-scale low-angle normal faults developed at medium to shallow crustal levels. Our multiscale and multidisciplinary approach points out that the SSZ experienced a complex evolution, with shear strain heterogeneously distributed across the shear zone in the frame of a decrease of the simple shear component and increase of the differential flow stress toward the top of the shear zone.