From extension to inversion of lateral back-arc basins in the Western Mediterranean (Alboran Sea)

Workshop. Alboran Domain and Gibraltar Arc: Geological Research and Natural Hazards - El dominio de Alborán y el Arco de Gibraltar: Investigación geológica y riesgos naturales - Le Domaine Alboran et l'Arch de Gibraltar: Recherche géologique et risques naturels, 16-18 octubre 2019, Granada..-- 2 pages

In converging plate boundaries, upper-plate extension can occur in a limited amount of space and time according to plate kinematics and to lower plate behaviour. It is considered to be the result of slab roll-back and can be associated to a segmentation of the downgoing slab with the development of sub-vertical lithospheric-scale tear faults on its edges. The development of the slab tear triggers formation of STEP faults (Subduction Tear Edge Propagator). These STEP faults, that bound laterally the back-arc basins, accommodate the progressive tear of the slab and its lateral motion. The transition from back arc extension to inversion leads to a complex structural setting. The thinned continental crust of the Alboran Sea and its overlying sedimentary cover are deformed since the Miocene by both convergence between Eurasia and Africa and deep processes related to the Tethyan slab retreat. Part of the deformation is recorded in the southern Alboran Sea through the Xauen and Tofiño banks (named the South Alboran Ridge or SAR) where oceanographic surveys acquired swath bathymetry and seismic reflection. Major questions concerning the setting up of these structures and the present-day deformation remain still open: (i) what was the initial geometry of the southern margin of the west Alboran Basin? (ii) How and where is accommodated the shortening related to the Africa-Eurasia convergence? (iii) What is the role of weak Miocene undercompacted shales and muds on the structural style of the Banks (Comas, Platt, et al., 1999)?. Four main stages of deformation have been recognized in the SAR region. Early-Miocene extension occurs with syn-tectonic wedges related to normal faulting, as already observed in the North Alboran Ridge. The SAR region is interpreted as the continuity of the very thick Miocene West Alboran depocenter related to the west Alboran mud province. Linked to the Gibraltar slab retreat, we suggest that the southern part of the WAB depocenter was deformed by the activity of a STEP fault, which is subsequently inverted. We then interpret the SAR zone to have formed by contractional overprinting of a shear zone. The present-day SAR region is dominated by E-W trending folds and thrusts with evidence of compressional deformation active from late Miocene to presentday. An acceleration in the uplift and compressional activity is evidenced during the top Miocene. The structural style of the fold and thrust suggests a weak decollement layer corresponding to the overpressure shale, but can also reflect the existence of an intracrustal fault. The Tortonian inversion and associated gravity-driven movement, superimposed with the Africa indentation, induced the present-day style of deformation of the SAR. Our study allows to understand which tectonic and sedimentary processes are responsible of the final structure observed through the South Alboran basin. We show that weak layers and inherited crustal structures play a major role on the style and mode of deformation of the back-arc basin. In particular, the major tectonic imprint of the STEP faults, accommodating slab-roll back, as well as the sedimentary imprint of the undercompacted shale acting as a decollement layer, are highlighted. Understanding deformation processes around the tip of back-arc basins will provide key information on the forces and dynamics controlling the inversion of STEP faults on the edge of subduction areas. Wide-angle seismic studies are necessary to elucidate the presence and geometry of the deep crustal structure as the STEP fault and intra-crustal thrusts