Anisotropy of magnetic susceptibility (AMS) records synsedimentary deformation kinematics at Pico del Aguila anticline, Pyrenees, Spain

Abstract

Pico del Aguila anticline is a transverse décollement fold located at the Pyrenean thrust front. The anticline is a synsedimentary structure buried during growth by delta front mudstones and sands of the Eocene Arguis and Belsué-Atares formations. Both the anisotropy of magnetic susceptibility measured at 77 K and 294 K and the anisotropy of anhysteretic remanence show that susceptibility is dominated by paramagnetic clay minerals and can be used as a proxy for depositional and tectonic fabric orientations. In general, the maximum and intermediate principal susceptibilities (k1 and k2) of the AMS lie in bedding and the minimum principal susceptibility (k3) is oriented nearly normal to bedding. Layer-parallel shortening (LPS) produced a c. north–south-trending magnetic intersection lineation in bedding on anticline limbs and in the adjacent Belsué and Arguis synclines by deforming the depositional and diagenetic compaction fabric. The degree of magnetic anisotropy is higher along axial surfaces than on limbs. At the anticline hinge, oblate magnetic ellipsoids with an east–west-aligned lineation and a bedding-parallel magnetic foliation demonstrate the overprinting of the LPS magnetic fabric during the emplacement of the underlying thrust sheet. AMS data record fold kinematics characterized by constant-length limb rotation about pinned hinges and are compatible with kinematics recorded by growth strata geometries. This study emphasizes that AMS is a very sensitive measure of depositional, compaction and tectonic fabrics in marine clastic rocks in the diagenetic realm.