Miocene Shale Tectonics in the Alboran Sea (Western Mediterranean)

The Alboran Basin is one of the back-arc basins in the Mediterranean developed since the Miocene by the extensional collapse of a thick continental orogen: the Betic-Rif arc. Collision and basin formation occurred during the Neogene in a context of oblique convergence between the Eurasian and African plates. In the West, this basin has a major, curved depocenter (with sedimentary accumulations > 10 km) that contains a shale diapiric province with overpressured shales and associated mud volcanoes. We present a detailed reconstruction of the 3D geometry of the diapirs, analyzing in selected structural restorations the relationships between fault activity, shale diapirism, and sedimentation in the overburden. Basin formation started in the Early Miocene, and rapid initial subsidence of the basin floor was accompanied by massive sedimentation and burial of fine-grained sediments. Continuous basement subsidence during the Miocene determined gravity-driven tectonics and the down-slope migration of the shale. During this process, syn-sedimentary extension developed in basin margins and associated shale-cored thrusts occurred in the basin depocenter. Extension progressed downward and landward by means of low-angle normal faults coalescing with the basement surface, which represents a master detachment surface. We reconstruct three main pulses of thin-skinned extension that conducted the advance of allochthonous shale tongues, driven down-dip by associated thrusts and hanging wall ramp anticlines. Shale tongues advance was enhanced by antithetic high-angle normal faults that individualized non-cylindrical minibasins in the overburden.

According to these results, the Alboran Basin represents a valuable region to analyze the structural pattern associated to shale tectonics, and a key shale basin to compare the evolution of allochthonous shale tongues with those structures formed in salt basins.