Miocene to Present kinematics of the NW-Alps: evidences from remote sensing, structural analysis, seismotectonics and thermochronology

At the end of their Cretaceous-Tertiary subduction-collision history, the Alps underwent a complex brittle reworking. An integrated study has been undertaken in order to explain the Miocene to Present kinematic evolution of the north-western Alps. Data from remote sensing, structural geology, seismotectonics and thermochronology have been analysed and compared. Remote sensing and structural analysis evidence that the Pennine-Graian nappe stack constitutes a continuous block, characterised by high-strain border-zones and by a quite homogenous internal deformation. The border-zones of this block are: (i) the normal SW-dipping Simplon fault; (ii) the broad dextral strike-slip system, constituted by the Phone and Chamonix lines and by the Penninic and Brianconnais reactivated frontal thrusts, and, to the SE of the block, (iii) the recently discovered sinistral strike-slip Ospizio Sottile fault. A complex network of faults and fractures dissects the Pennine-Graian block, the most important being the E-W striking Aosta-Ranzola fault system. Even if these faults do not generally show large displacements in the Miocene, they constitute an important strain marker, since they have a regular frequency and indicate a general NE-SW extension of the block. Seismicity is mainly concentrated along the strike-slip border-zones and focal plane solutions generally agree with surface data (within discussed errors). Thermochronology indicates that, from the Late Miocene onwards, no major changes took place in the exhumation pattern, and hence in the overall kinematics. This evidence supports the general agreement between present-day deformation (seismotectonic data) and kinematics in the recent-past (structural geology data). Different tectonic reconstructions are checked with the discussed data set. In the authors' opinion, the SW-ward orogen-parallel escape model better explain the presented body of structural, seismotectonic and thermochronology evidences. (C) 2000 Elsevier Science Ltd. All rights reserved.