Tectonic and geomorphic analysis of the Belledonne border fault and its extensions, Western Alps
Résumé
In the western Alps, the NE trending Belledonne fault system extends from the Mont Blanc massifs in the NE to the Vercors massif in the SW. It includes the Belledonne border fault, defined by an alignment of micro earthquakes (ML <= 3.5). Focal mechanisms and their respective depths indicate a crustal scale NE trending dextral strike-slip faulting. This study aims at better constraining the geometry, the fault kinematics and slip rate of the faults of the Belledonne fault system by using a multidisciplinary approach that includes tectonics, geomorphology and geophysics. New clues of potential Quaternary deformations are observed: 1 - the right-lateral offsets of morphologic markers (talwegs) along the NE trending Arcalod fault at the north-eastern termination of the Belledonne border fault; 2 - the left-lateral offset of the valley carved by the Isere glacier along the NW trending Brion fault, which is consistent with the fault kinematics deduced from the focal mechanisms. Stream network anomalies along the Belledonne border fault are related to glacial erosion processes rather than faulting. However, fault kinematics analysis along the Belledonne border fault allows us to determine a strike-slip tectonic regime characterized by horizontal ENE trending sigma 1 stress axis. It is consistent with the mean trend of the P and T axes deduced from the focal mechanisms. We display evidences that this stress state is Messinian to Quaternary in age and occurred after an Oligocene to Messinian strike-slip tectonic regime characterized by horizontal WNW trending sigma 1. Fault slip rates cannot be assessed because of the lack of morphologic features with constrained ages. However, it is likely that the presented geomorphic markers are older than Wurm. (C) 2015 Elsevier B.V. All rights reserved.