Kinematics of ductile shearing from outcrop to crustal scale in the Monte Rosa Nappe, Western Alps

Abstract

Significant ductile deformation has produced widespread mylonitic structures in the basement orthogneisses and the Mesozoic cover quartzites and marbles of the Monte Rosa nappe, in the inner part of the Western Alps. We summarize here the results of a detailed microstructural study at several scales of observation in the Northern part of the nappe and discuss the kinematic significance and compatibility of such results. Most of the observed deformations can be accounted for by progressive WNW‐vergent shearing during and after the late Eocene, within a ductile shear zone of crustal scale. The displacement of the hanging wall may have been in excess of several tens of kilometers. However, some ESE‐vergent shear criteria cannot be reconciled with this general picture and suggest that local backthrusting occurred later near the toe of the Monte Rosa nappe.