Using salt diapirs and minibasins to constrain interpretations of crustal rifting and inversion in the Basque Pyrenees, Spain

Published interpretations across the Basque Pyrenees vary significantly in their depictions of rifting and subsequent inversion. Major points of disagreement relate to: (i) the asymmetry of the margin, i.e., whether the major extensional and contractional detachment dipped toward the north or south; and (ii) the degree of decoupling between supra- and subsalt deformation and thus the amount of thin-skinned translation of the cover relative to basement. Here we use outcrop and subsurface data to analyze the salt structures along a regional transect in order to resolve this ongoing debate. Several aspects of the salt-related geometries are diagnostic of thin-skinned deformation. First, Villasana de Mena diapir has significantly thicker synrift strata on its basinward (northern) flank, contains stringers of Paleozoic rocks, and was growing passively during crustal extension. Its origin was consequently related to an underlying basement fault, yet it is situated today above an unfaulted detachment, suggesting that the diapir was translated above the salt during rifting and/or shortening. Second, Salinas de Rosio diapir is located at the southern termination of landward-shifting synrift depocenters and developed as a postrift to synorogenic salt pillow and then diapir. Thus, thin-skinned translation over a fault-related ramp in the base salt created synrift ramp-syncline basins, with the basement fault subsequently localizing salt inflation due to differential loading and then contractional buttressing. Third, Poza de la Sal diapir is at the basinward end of another set of synrift ramp-syncline basins and along a thin-skinned fold and thrust structure that was active during the synrift. Fourth, the large-scale geometry from the Bilbao Anticlinorium to the south documents contractional translation above a continuous salt detachment with a ramp-flat geometry. In summary, the salt and suprasalt geometries in a large part of the Basque Pyrenees demonstrate two phases of thin-skinned translation above a north-dipping salt detachment: (i) decoupled, basinward (northward) translation during rifting; and (ii) thin-skinned, southward-directed thrusting during inversion. The geometries are incompatible with thick-skinned inversion on a major south-dipping crustal detachment and smaller, basement-rooted faults that cut through the salt and its overburden.