Late‐stage deformation in a collisional orogen (Western Alps): nappe refolding, back‐thrusting or normal faulting?

Abstract

ABSTRACT Nappe refolding, back‐thrusting and normal faulting frequently cause severe late‐stage overprinting of the architecture of an orogen. A combined investigation of nappe stack polarity, kinematics of shearing and metamorphic gradients in the Western Alps develops criteria for distinguishing between these three modes of late‐stage deformation. This distinction is a prerequisite for any retro‐deformation necessary for understanding the main tectonic and metamorphic evolution of collisional orogens. In the case of the Western Alps overprint was by mega‐scale nappe refolding in the Oligocene. This implies exhumation of the HP‐rocks prior to postnappe folding, i.e. during nappe stacking and by foreland‐directed ascent within a subduction channel.