Penetrative strain of the Aragüés fault-propagating folds in the Southern Pyrenees revealed by magnetic fabric analysis

Abstract

The Aragüés thrust system, in the Southern Pyrenees, is a superb and well-exposed sequence of thrust imbricates to study the deformation and development of fault-propagating folds. The mechanisms of folding and thrusting, including the importance of material contrast during the development of the thrust sequence, are well-studied in this area. However, insights into the distribution and magnitude of penetrative strain remain unknown. Therefore, analysis of anisotropy of magnetic susceptibility (AMS) is used to reveal penetrative deformation within the sedimentary rocks of the Aragüés thrust system. Rock samples from 66 locations are collected across the Aragüés thrust imbricates with a focus on the layered limestone unit. A total of 489 cylinders were retrieved from the oriented hand samples and their AMS signal was measured. Thermomagnetic and magnetic remanence measurements show that the magnetic susceptibility of the samples is dominated by paramagnetic minerals and the magnetic lineation (axis of maximum susceptibility) is mainly parallel to subparallel to bedding. However, the more strongly magnetic samples (kmean: 3-5 x 10-4SI) show a magnetic lineation parallel to the general NNE-SSW shortening direction at the back- and forelimbs of fault-propagation folds. In contrast, the samples with a lower magnetic susceptibility (kmean: 1-3 x 10-4 SI), which are also closer to fold hinges, reveal a magnetic lineation perpendicular to the main shortening direction. We interpret the differences in fabric alignment and magnetic susceptibility to mineral composition and structural evolution. For example, the magnetic lineation parallel to the shortening direction is a consequence of flexural slip and flow along bedding surfaces or incompetent beds of the layered limestone units in the fold limbs. Additionally, a relationship between the magnetic lineation and the development of cleavage and stylolites within the limbs can be identified, but is not consistent. The magnetic lineation at the fold hinge is perpendicular to the main shortening direction and needs further investigation for strain accommodation prior to folding, an effect of buckling and/or development as intersection lineation. These first interpretations of the magnetic fabric provide additional insights into the deformation of the Aragüés thrust system and the development of fault-propagating folds. Moreover, the AMS data adds new information on penetrative strain in this region of the Southern Pyrenees and reveals a general tectonic overprint on grain-scale within sedimentary rocks during deformation.