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

 

Stefan Bucher, Stefan M. Schmid, Romain Bousquet and Bernhard Fügenschuh
Department of Earth Sciences, Basel University, Bernoullistr. 32, CH-4056 Basel, Switzerland

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.

Terra Nova, 15, 109–117, 2003

 

 

Geological map of the study area.B–B' indicates the trace of the cross-section of Fig.4(a). (b) L2 stretching lineations (L2int., area from the Front Houiller to the Valgrisenche; L2ext., area SE of the Valgrisenche) and (c) F3 fold axes (F3ext. and F3int., same separation as L2).