The transition from thick-skinned to thin-skinned tectonics in the Basque-Cantabrian Pyrenees: the Burgalesa Platform and surroundings

Interpretation of seismic data in the margins of the Burgalesa Platform in the Basque-Cantabrian Pyrenees has allowed proposition of a new structural model that combines different modes of deformation during oblique tectonic inversion, conditioned by the distribution of Triassic salts. Deformation was decoupled by the presence of the salt horizon between basement-involved thrusts inverting formerly Triassic and Late Jurassic–Early Cretaceous extensional faults and a detached thrust system involving the Upper Triassic to Neogene sedimentary package. Structural units experiencing different styles of deformation are not only stacked vertically above and below the salt, but most importantly, they change from one to the other along-strike across the transversal edges of the Triassic salts. The Burgalesa Platform detached thrust system was confined between the basement-involved structures of the Cantabrian Mountains westward and the NW tip of the Iberian basement-involved structures (San Pedro) southward. This together with the obliquity between the Pyrenean shortening direction and the strike of the previous extensional faults, mostly during the late stages of deformation, determined the strike-slip reactivation of the basement-involved inverted faults and the lateral extrusion of the Burgalesa Platform detached Mesozoic successions above the salt towards the SE to form a prominent thrust salient oblique to the main Pyrenean trend. The proposed model combines thick-skinned with thin-skinned structural styles during oblique tectonic inversion and is consistent with the surface data, including the fracture system, the available subsurface data and the mechanical stratigraphy.