Kinematics and strain distribution in an orogen-scale shear zone: Insights from structural analyses and magnetic fabrics in the Gavarnie thrust, Pyrenees

Abstract

This work aims to characterize the Gavarnie thrust, one of the large-scale thrusts that define the Alpine structure of the west-central sector of the Pyrenees. A detailed comparison of structural analysis and magnetic fabrics is carried out for the Paleozoic phyllonites of the Gavarnie thrust, in order to decipher strain distribution and transport direction. The AMS at room (RT-AMS) and low (LT-AMS) temperature and the AIRM can be correlated with the structural patterns: kmin axes are mainly parallel to the pole of the S, C or C′ planes and kmax axes are parallel to the transport direction and related to ductile S-C structures. Furthermore, the Pj-T changes across the shear zone characterize strain variations: larger Pj and T are found in the basal, most deformed part of the shear zone, and lower values are found where the interaction between Alpine and Variscan-related petrofabrics is stronger. We also interpret the reactivation of Variscan inherited fabrics within the Alpine shear zone. In spite of the heterogeneous strain, markers indicate a common, top-to-the-South (N190E) Alpine transport direction, which contrasts with the strong obliquity of the genetically-related structures developed in the Southern Mesozoic sedimentary cover. In this sense, our data suggest a complete decoupling between basement and cover units during the Alpine compression.