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

Abstract

SUMMARY The purpose of this paper is to explain the kinematics of the inverted extensional Organy` basin(CentralSouthernPyrenees)duringitsextensionalevolution.Thestudyoftheanisotropy 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 thestructuralevolutionofthebasin,whichsupportanewtectonicmodelfortheAptian–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 kmin axes perpendicular to bedding and kmax 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` a basin was acquired during or shortly after deposition of the Aptian-Lower Albian marls without any subsequent overprint. The direction of the magnetic lineation obtained from the AMS at room and low temperatures, agrees with the orientation of the stress axes obtained from the analysis of brittle mesostructures (mainly faults and calcite-filled tension gashes) indicating its tectonic origin. We interpret the orientation of the magnetic ellipsoids related to the transtensional movements linked to the formation of the Organy` a basin during the Aptian–Albian interval, which are consistent with the eastward displacement and rotation of Iberia with respect to Europe. Analysis oftensiongashesalsoindicatesamainNW–SEextensiondirection,whereaspalaeostressanalysis of striated faults shows a more N–S trending σ3. The transtensional model proposed from magnetic fabric and brittle mesostructures for the Organy` a basin during the Aptian–Albian is consistent with the creation of a strongly subsiding area south of the Axial Zone during the Cretaceous and implies that rotation and separation of Iberia respect to Europe took place simultaneously.