A transtensional basin model for the Organyà basin (central southern Pyrenees) based on magnetic fabric and brittle structures

Abstract

The purpose of this paper is to explain the kinematics of the inverted extensional Organy basin (Central Southern Pyrenees) during its extensional evolution. The study of the anisotropy of magnetic susceptibility (AMS) and the study of brittle mesostructures are the instruments used to decipher the extensional kinematics. Separation of magnetic subfabrics (analyses of AMS at low temperature and anisotropy of the anhysteretic remanence -AARM) has been also done to properly interpret the obtained magnetic fabric data. The combination of all magnetic fabric and brittle mesostructures analyses has provided new information to clarify the structural evolution of the basin, which support a new tectonic model for the Aptian-Albian basinal stage. 20 sites for the magnetic fabric analysis were sampled in Aptian-Lower Albian marls. The AMS ellipsoids obtained at room temperature show their k min axes perpendicular to bedding and k max axes oriented NW-SE in 70 per cent of the samples, N-S in 15 per cent and NE-SW in 15 per cent. Analyses of AMS at low temperature show a better clustering of the axes of the magnetic ellipsoid pointing out that phyllosilicates constitute the carriers of the AMS, whereas the analyses of the AARM confirm the scattering of the ferromagnetic grains. These results suggest that the obtained AMS in the Organy basin was acquired during or shortly after deposition of the Aptian-Lower Albian marls without any subsequent overprint.