Relationship between AMS and folding in an area of superimposed folding (Cotiella-Bóixols nappe, Southern Pyrenees)

Abstract

We present the results of anisotropy of magnetic susceptibility (AMS) analysis carried out in weakly deformed, Upper Cretaceous marine marls from the Southern Pyrenees. The data refer to 23 sites distributed in the fine-grained intervals of the Vallcarga and Barbaruens formations, extending along the hangingwall and footwall of the Cotiella-Bóixols thrust. The observed magnetic fabrics are typical of early stages of deformation, and show oblate, triaxial and prolate magnetic ellipsoids. In most of the sites, a magnetic foliation parallel to bedding can be observed. A well-defined magnetic lineation, which is correlated with the main tectonic features of the studied region, can be recognised. We interpret the observed magnetic fabric as related to a tectonic overprint on an original, compaction, sedimentary fabric. The tectonic AMS fabric is related to the superimposition of two sequential, and non-coaxial fold-and-thrust events: N–S to NNW–SSE folding, related to oblique ramps of Late Cretaceous thrusts, and WNW–ESE folding, related to the main, Eocene Pyrenean thrusting stage.