Mechanisms of margin inversion in the external Western Alps: Implications for crustal rheology

Abstract

The kinematics of the collisional crustal wedge in the external Western Alps is discussed in the light of new cross sections of the whole external zone at the latitude of the Belledonne–Grandes Rousses–Oisans massifs (External Crystalline Massifs, ECM), as well as a detailed analysis of the deformation of their basement and cover. The cross sections were built from new field data and published geological maps, and were restored through time (Oligo-Miocene times) to unravel the successive stages of the Dauphinois margin contractional deformation. During Oligocene times, the Variscan basement was deformed essentially by greenschist facies Alpine distributed shear zones with no significant reactivation of the inherited Jurassic normal faults or the Variscan foliation. However, the inherited syn-rift basins localized the deformation characterized by a thick-skinned style and a cover disharmonically folded over basement shear zones, with no major décollement in between (shortening of 11.5km, 20%). Those early deformations progressively localized on the frontal crustal ramp and caused shortening in the folded belt (16.5km of shortening, 23%). A similar sequence of deformation has been inferred in other Alpine ECMs. This strongly suggests that the crust of Dauphinois proximal passive margin was thin enough, and its buoyancy low enough (as a result of the Liassic rifting), to experience significant tectonic burial during the Tertiary collision. As a consequence of a weakening by both the presence of inherited basins (and their weak syn-rift sedimentary rocks) and the P–T conditions (greenschist facies), the crust was shortened and the structural style was thick-skinned.