The foreland deformation in the south-western Basque-Cantabrian Belt (Spain)

In the south-western portion of the Cenozoic Basque-Cantabrian Belt, structural data collected in pre-orogenic Upper Cretaceous limestones were combined with aerial images interpretation in flat-lying areas. Observations about fracture strike, type, orientation and overprinting relationships, both at the macro and mesoscales, allow to constrain the pre-thrusting and pre-folding deformational sequence that affected the area when it was forming part of the foreland basin. WNW-ESE striking extensional structures represent the oldest elements, developed during the very late stages of the Mesozoic rifting associated with the opening of the Bay of Biscay. The onset of convergence is marked by the development of extensional structures striking roughly NNW-SSE and WSW-ENE, which are associated with belt-parallel stretching in the foredeep and outer-arc extension in the peripheral bulge, respectively, and developed in a non fluid assisted environment. These elements are postdated by a pre-folding layer-parallel shortening pattern, which includes WSW-ENE striking pressure solution cleavages and reverse faults (both of them frequently reactivating inherited early-orogenic WSW-ENE striking extensional elements), and right-lateral reactivation of WNW-ESE striking pre-orogenic extensional structures. This stage was followed by a syn- to post-folding E-W oriented belt-parallel compression, WNW-ESE striking left-lateral faults being the most outstanding elements associated with this stage. The presence of such elements in Upper Cretaceous sediments, provides support to the idea that left-lateral shear zones of the area related with a Cenozoic intrabelt lateral extrusion tectonics. Our results indicate, coherently with previously published works in the Pyrenean Orogen, that most of the mesoscale deformation has been acquired before layer's tilting. (C) 2012 Elsevier B.V. All rights reserved.