Early development of a Mesozoic basin edge controlled by the reactivation of Variscan structures: the Western Asturian basin, NW Spain

Abstract Detailed structural and geological mapping, along with the construction of deep cross-sections, focused on the border of a Mesozoic extensional basin (the Western Asturian Basin), has enabled the reconstruction of a complex evolution of the initial sedimentary infill. This evolution was driven by the reactivation of Variscan Paleozoic faults. The comprehensive surface and subsurface characterization of the basin was made possible through fieldwork and borehole data. Using these methods, five fault types have been identified: Variscan thrusts, Late Variscan right-lateral faults, Late Variscan thrusts, Mesozoic (Alpine) normal faults, and Cenozoic (Alpine) reverse and strike-slip faults. The Mesozoic faults commonly reactivated the Paleozoic faults during four extensional phases: Middle–Late Triassic, Early Jurassic, Late Jurassic, and Early Cretaceous. As a result of this reactivation, an asymmetric graben with complex geometry developed, controlled by the inherited Variscan and Late Variscan faults. Furthermore, this area has been demonstrated, for the first time, to represent the western boundary of a large Middle–Late Triassic rift basin, a boundary where extensional activity continued during the Early Jurassic, unlike in the inner Asturias Basin, where the extension almost stopped. The studied basin margin was partially inverted during the Eocene–Oligocene Alpine compression, leading to the reactivation of Paleozoic and Mesozoic faults with favorable orientations as reverse faults, and the development of associated open folds. Buttressing is also common, due to the geometry of most of the pre-existing faults.