Late Paleozoic polyphased tectonics in the SW Belledonne massif (external crystalline massifs, French Alps)

Abstract

In the southwestern area of the Belledonne massif (external crystalline massifs, French western Alps), the finite strain pattern reflects the geometry and kinematics of Late Devonian-Late Carboniferous deformational events. Previous works suggested that the nappe stacking occurred during Late Devono-Dinantian times. New observations allow us to constrain the subsequent tectono-metamorphic evolution of the various stacked units. Westward thrusting is responsible for the early dismembering of the Rioupéroux-Livet plutono-volcanic units. Micaschist slices have also been inserted within the Rioupéroux-Livet units during this Dinantian tectonics, and have recorded metamorphic conditions of about 7–8 kbar, 600–650 °C. The local NW direction of shortening is similar to the overall shortening direction recorded at this time in the Hercynian belt of western Europe. During the Late Visean, the Early Paleozoic Chamrousse ophiolite was thrust towards the northeast on top of the Rioupéroux-Livet units. The inverted geometry of the Chamrousse ophiolite may be interpreted as a consequence of this tectonics. Northeastward thrusting, also observed in the northeastern part of the French Massif Central is interpreted as the result of lateral escape in a global convergent context. Finally, the Late Carboniferous period is characterized by two extensional events: one leading to southwest dipping low-angle extensional shear zones, during the Westphalian, and a second one during the Stephanian. This latter is characterized by ductile/brittle normal faults, with a northwest-southeast direction of extension and related to the gravitational collapse of the whole Hercynian belt. © 2002 Éditions scientifiques et médicales Elsevier SAS. All rights reserved.