Buttressing and reverse reactivation of a normal fault in the Jurassic rocks of the Asturian Basin, NW Iberian Peninsula

A detailed structural analysis was carried out on the Jurassic rocks cropping out along the cliffs of La Conejera Inlet (Asturias, Spain). It includes a geological map and a distortion-free cross-section constructed via photogrammetric methods. La Conejera Inlet is located within the Asturian Basin, a Permian-Mesozoic extensional basin partially formed during the opening of the Bay of Biscay. It suffered selective basin inversion during a Cenozoic contraction responsible for the Pyrenees and its western prolongation along the north margin of the Iberian Peninsula. The study of the structures (folds, faults, joints and veins) of the hangingwall of two normal faults with opposite dip senses reveals that it underwent a later compressional stage in which one fault block acted as a buttress. The contractional deformation in the hangingwall, interpreted as a deformed rollover anticline with an associated antithetic fault, diminishes on moving away from one of the main faults. The positive inversion tectonics produced not only a buttressing effect, but it also involved a certain amount of reverse reactivation of one of the main faults that still preserves a normal displacement. The original normal motion would have taken place during the Middle?-Late Jurassic, related to an embryonic stage of the opening of the Bay of Biscay. The later contractional stage would have been caused by the Cenozoic Alpine shortening. The good outcrop quality allows a relative chronology for the observed structures to be established. Employing all the available information we tried to reconstruct the structure at depth and predict the detachment depth, and to estimate the amounts of extension (the present-day value and that before the compression) and compression.