Thin-Skinned Extension and Shale Tectonics in a Tilted Basin Margin: The Case of the Northern Alboran Sea (Western Mediterranean)

Juan Soto, Fermin Fernández-Ibáñez, and Asrar R. Talukder

We have studied the tectonic evolution of the northern margin of the West Alboran Basin (offshore Spain), a south-dipping margin containing important sedimentary accumulations of Miocene to Recent fine-grained, marine sediments and active mud volcanoes in the seafloor.

The analysis of this sedimentary basin has been conducted through seismic interpretation of a dense grid of 2D seismic lines. This interpretation has been tied with well data, which provide lithological and bio-stratigraphic information. Our analysis of the logging data in some of the commercial wells in this margin confirms the occurrence of overpressured sediments in the lowermost levels of the basin, formed by Early Miocene, shale-rich sediments. This sequence constitutes the source rock for the widespread shale diapirs that characterized the deeper West Alboran depocentre.

We have conducted a detailed reconstruction of the 3D geometry of the shale-cored diapirs and the relationships with the overburden, analyzing the geometries and timing of the syn-sedimentary deformations. It is inferred the occurrence of a punctuated history of deformations in basin margins through the Middle to Late Miocene, conducted by syn-sedimentary normal faults that root in the basement-to-cover surface, which represents a low-angle detachment surface. We reconstruct that thin-skinned extension in basin margin promoted down-slope migration of the overpressured shales forming diapir structures that evolve basinward from shale rollers, shale anticlines, walls and allochtonous shale sheets driven by thrusts in the thicker and deeper portions of the basin. Since the Messinian times the most recent basin evolution is characterized by submarine canyons with a source area that switch during the Late Pliocene from a WE trend, with a source area in the Gibraltar Strait area, to the NW orientation, coming from the uplifting Betic mountains.

We interpret that pulses of syn-sedimentary extension in basin margin and gravity-driven shale withdraw occurred during a continuous tilting of the basin floor, which was accompanied by massive sedimentation and burial of finegrained sediments. The Alboran Basin is therefore a useful area to analyze 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.

AAPG Search and Discovery Article #90161©2013 AAPG European Regional Conference, Barcelona, Spain, 8-10 April 2013