ALBERT

Description: Recent models support the view that the Pyrenees were formed after the inversion of a previously highly extended continental crust which included exhumed upper mantle rocks. Mantle rocks remain near to the surface after compression and mountain building, covered by the latest Cretaceous to Paleogene sequences. 3D lithospheric-scale gravity inversion demands the presence of a high-density mantle body placed within the crust in order to justify the observed anomalies. Exhumed mantle, having ~50 kilometers of maximum width, continuously extends beneath the Basque-Cantabrian Basin and along the northern side of the Pyrenees. The association of this body with rift, post-rift and inversion structural geometries are tested in a balanced cross-section across the Basque-Cantabrian Basin that incorporates a major south-dipping ramp-flat-ramp extensional detachment active between Valanginian and early Cenomanian times. Results indicate that horizontal extension progressed circa 48 km at variable strain rates which increased from 1 to ~4 mm/yr in middle Albian times. Low-strength Triassic Keuper evaporites and mudstones above the basement favor the decoupling of the cover with formation of minibasins, expulsion roll-overs and diapirs. The inversion of the extensional system is accommodated by doubly-verging basement thrusts due to the reactivation of the former basin bounding faults in Eocene-Oligocene times. Total shortening is estimated in ~34 km and produced the partial subduction of the continental lithosphere beneath the two sides of the exhumed mantle. Obtained results help to pinpoint the original architecture of the North-Iberian Margin and the evolution of the hyperextended aborted intracontinental basins.