Synsedimentary Tectonic Strain and Fold Kinematics Recorded by Anisotropy of Magnetic Susceptibility in Pyrenean Flysch

Depositional, compaction, and tectonic rock fabrics are elucidated by magnetic fabrics in growth strata at Pico del Aguila, a transverse fold in the Spanish Pyrenees. The anisotropy of anhysteretic remanence (AAR) and magnetic susceptibility (AMS, measured at 77K and 295K) suggests that the magnetic fabric is predominately carried by platy paramagnetic minerals. The burial compaction fabric is superposed by tectonic strain, which can be used to infer fold kinematics. The phyllosilicate fabric is deformed by layer parallel shortening oriented E-W creating a magnetic lineation on both limbs and bounding synclines parallel to the N-S fold axis and in bedding. This fabric is probably due to an intersection lineation created by the platy paramagnetic grains. In contrast, the magnetic lineation is fold axis perpendicular at sites near the anticlinal hinge resulting from either tangential longitudinal strain and/or flattening related to south-directed thrusting. Minimum principal axes are nearly perpendicular to bedding across the fold suggesting that the paramagnetic grains have not been additionally deflected by bedding- parallel shear strain. The magnitude of the lineation, a proxy for strain magnitude, is nearly consistent across the fold and throughout the growth strata. Equivalent strain within the synclinal hinges and limbs suggests migrating synclines while strain orientation and fanning growth layers records a pinned anticlinal hinge. AMS is a sensitive marker for depositional, compaction, and tectonic fabrics in marine clastic rocks, even in the near surface environment (less than 100˚C and 1km depth).