Fold kinematics resolved in the Sant Llorenç de Morunys growth fold, Southeastern Pyrenees, Spain

Abstract

New rock magnetic data settles a controversy over the kinematics of growth folding at Sant Llorenç de Morunys, Spain. Located in the footwall of the Vallfogona thrust fault, the fault-related growth fold is the archetype progressive unconformity, recording 113 ̊ of limb rotation. Folding and syntectonic deposition occurred synchronously with north-south shortening during the Alpine Orogeny, creating the east-west oriented growth fold. Previous studies here have reached contradictory conclusions with respect to the kinematics, including hinge behavior and limb length during folding (e.g. Riba, 1976; Ford et al., 1997; Suppe et al., 1997; Alonso et al., 2012). To evaluate these models, and determine the fold kinematics, we measured the fabric in all structural positions using anisotropy of magnetic susceptibility (AMS). Isothermal remanent magnetization, low-temperature magnetic susceptibility, and thin section microscopy of selected samples confirmed the predominance of paramagnetic minerals to the AMS. The AMS provides a useful proxy for geologic strain at very low-grade conditions, particularly in rocks rich in paramagnetic minerals. At hinge sites, AMS fabrics showing diagenetic compaction are overprinted by late LPS flattening and disjunctive cleavage. The pervasive flattening plane and related cleavage are oblique to the axial surface. Only a weak layer parallel shortening prefolding fabric is recovered from fold limbs. More notably, AMS fabrics in the hinge sites exhibit greater anisotropy than limb sites, which are largely unstrained. These data are consistent with the magnitude of finite strain determined from worm tubes exposed on bedding planes, and altogether are compatible with a fixed-hinge, limb rotation kinematic model for this fold.