3D Geometries of Shale Diapirs in the Alboran Sea (Western Mediterranean)

Behind the orogenic belt formed by the Betic and Rif mountains in the westernmost Mediterranean, the Alboran Basin appears as an arcuate basin formed during the Neogene by severe late-orogenic extension in the context of Africa-Eurasia plate convergence. The western portion of this basin is characterized by a tight sedimentary depocenter that mimics the curvature of the Betic-Rif orogenic arc. In the depocenter axis (>10 km depth) the lower-most sedimentary layers (Early Miocene in age) corresponds to overpressured shales, that developed pervasive diapiric structures. Using different commercial multichannel seismics, structural restorations, fault- and diapir-activity graphs we have reconstructed in 3D the geometry of the shale diapirs and of the overburden in the associated minibasins, evaluating the possible link between marginal thin-skinned extension and basinal shale diapirism.

Basin floor dips centripetally towards the main basin depocenter. Accompanying basin subsidence since the Early Miocene, gravity-driven tectonics promoted thin-skinned extension in the basin margins and associated downslope shale tectonics. In this direction shale migrated, and similarly to salt structures, it is developed progressively pillows, shale roller and welts, diapir walls, and finally allochthonous shale sheets. Thin-skinned extension seems to promote subsequent shale migration and major pulses of allochthonous emplacement by toe thrusts. Diapir crests show non-cylindrical geometries, with culminations and double axis immersions associated to lateral terminations of updip syn-sedimentary faults. Extension direction and associated thrusting are parallel to basement dip, whereas shale moved also along perpendicular paths, probably induced by oblique fault structures in the basement. The resulting structural pattern provides therefore valuable information to decipher subsurface fluid migration patterns in shale basins like the Neogene Alboran Basin.