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Structural Framework beneath the Alboran Sea: Inputs to Understand the Tectonic Evolution of the Gibraltar Arc System

Menchu Comas and Ana Crespo-Blanc

During the last decades, numerous works have evidenced that the pervasive Neogene collision between the Eurasian and Africa plates resulted in the complex crustal structure of the Gibraltar Arc System (GAS), which includes the Betic-Rifean orogenic belt, the adjacent external accretionary prism, and the Alboran and Algerian back-arc basins that straddle the actual plate boundary. In this context, the Alboran Sea is believed a key region in deciphering the Neogene tectonic evolution of the GAS, and with this assumption this contribution focuses in getting new clues on the crustal structure of the Alboran Sea basin and at their transition to the neighboring Algeria basin. Based on reflection seismic profiles and mapping offshore, we document the different structural domains existing at present, and three main phases on the tectonic deformation happened during the Neogene. The earliest phase of deformation, well depicted in the West Alboran Basin, started in the late-Early Miocene (about 18 Ma) and denotes a NE-SW directed extension responsible for the initial basin-opening and substantial sedimentary infill. It resulted in two major half-graben affected by differential tectonic subsidence and connected by transfer faults. This deformational event constrains the timing and amount of the coeval crustal thinning. The tectonic regime then changed in the late Tortonian (about 9 Ma) to a compressional one with mainly N-S directed compression causing inversion of the extensional grabens, discrete folding, and strike–slip faulting on former NE-SW-trending transfer-faults. This compression leads to the noticeable shale-tectonics and shale-diapirism in the West Alboran Basin. The third phase of deformation, caused by a regime of NNW-SSE compression and consequent perpendicular extension, prevails from the late Messinian onwards (5-0 Ma) and activates the contractive reorganization of the basin at a crustal scale by major wrench faulting and SSE-dipping thrusting, which involved significant bending of former structures and segmentation of the basin margins. These recent crustal thrusts and wrench fault-zones illustrate the location and types of the existing crustal boundaries, control the deformation partitioning and delimit the actual structural domains. The spatial and temporary evolution of the tectonic deformation documented beneath the Alboran Sea by geological and geophysical observables provides, among others, new insights into the influences of major transfer fault-zones in the geodynamic history of the GAS.

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