Miocene Shale Tectonics in the Northern Alboran Sea (Western Mediterranean)

Abstract

The Alboran Basin is a back-arc basin in the Mediterranean developed during the Miocene by the extensional collapse of the thick continental orogen known as the Betic-Rif arc. Collision and basin formation occurred in the Neogene as a result of oblique convergence of the Eurasian and African plates. A major, curved depocenter (with sedimentary accumulations 10 km [6 mi]) is located to the west of this basin, containing a diapiric province with overpressured shales and mud volcanoes. This study presents a detailed reconstruction of the three-dimensional geometry of the diapirs and associated minibasins in the northern margin of this major depocenter (offshore Spain). Basin formation began in the early Miocene when rapid initial subsidence of the basin floor was accompanied by massive sedimentation and burial of fine-grained sediments. Gravity-driven tectonics and continuous basement subsidence during the Miocene led to downslope migration of mobile shales, whereas the basin margins were affected by synsedimentary extension, and associated shale-cored thrusts occurred in the basin depocenter. Extension occurred by means of low-angle normal faults coalescing with the basement surface, which represents a master detachment surface. Thin-skinned extension during the Miocene was accompanied by punctuated diapir ascent and the advance of shale sheets. Downslope shale advance was enhanced by counterregional high-angle normal faults isolating noncylindrical minibasins in the overburden. The Alboran Basin therefore is a useful area for analysis of the structural pattern associated with shale tectonic processes and a key basin for comparing the geometries and evolution of shale with structures formed in salt basins.