The origin and tectono-sedimentary structure of the Alboran Basin

Resum

ENGLISH: In this PhD thesis we present a geophysical study of the westernmost Mediterranean basin: the Alboran Basin. This basin is located between the Iberian Peninsula and North Africa, and it is surrounded by the Betics and Rif orogenic ranges. Along the Alboran basin, runs the plate boundary between Iberia and Africa tectonic plates. The processes that led to the basin formation in this compressive setting, and the later processes that controlled the basin deformation, remain unclear, as well as the deep structure of the basin. This study aims to: 1) characterize the crust and the basement of the basin, 2) define a seismostratigraphy coherent for the entire basin and analyse the basin evolution in the basis of the sedimentary record, 3) characterize the northeaster transition between the Alboran Basin and the Algero-Balearic Basin through the study of the Palomares Margin and 4) characterize the two most prominent tectonic structures in the area, the Yusuf Fault and the Alboran Ridge front fault. The results of this thesis have been integrated in a geodynamic model of the area, and will help to improve the regional seismic and tsunami hazard assessment model. In order to perform a regional scale study of the Alboran Basin, we have a wide Multichannel Seismic reflection (MCS) profiles grid acquired during different projects (including TOPOMED, EVENTDEEP, CAB, ESCI, CONRAD and IMPULS projects). Our results reveal that three different crusts are coexisting in the Alboran Basin: a thinned continental crust below the West Alboran and Malaga basins, a magmatic arc crust in the central and eastern parts, and the North African continental crust, below the Pytheas and Habibas basins. The basin is configured in a fore-arc basin an magmatic arc, being the back-arc basin of the system the Algero-Balearic Basin. The seismostratigraphy study supported and early Miocene initiation of the extension in the West Alboran and Malaga basins, followed by a Langhian-Serravallian extension in the North African margin. At the Tortonian, magmatic activity linked to the subduction system led to the volcanic arc formation. At the Messinian, extensional processes ended and the contractive reorganization of the basin occurred. The geomorphology study of the Palomares margin support that this contractive reorganization is not a widespread processed, as only few minor faults are reactivated. This hypothesis is confirmed by the characterizations of the Yusuf Fault and the Alboran Ridge front fault. Results in both fault systems are coherent, and point out a minimum total slip of ~20 km in a SE-NW direction since the Messinian top (5.3 Ma). Taking into account the plate convergence rates, the total convergence since the Messinian top is ~24 km, supporting that most of the strain is accommodated by these two faults. These results highlight the importance of a further seismic characterization of the area. The integration of the results presented in this thesis, together with the most recent tomographic studies (i.e., TOPOIBERIA project), bring to light the necessity of review the geodynamic models of the area. We conclude that the Alboran basin formation took place during the Miocene. The extensional processes were controlled by the geodynamics of the subduction system evolution, including westward slab roll-back and lithosphere tearing. At the ended of the Messinian, extension in the basin finished as a consequence of the ceased of the subduction. The Plio-Quaternary represented the deformational stage of the basin, led by the Iberian – African plate convergence. The distribution of the tectonic structures active in this compressive setting was defined by the inherited lithospheric structure that defined the weakness areas were these faults were developed. CASTELLANO La presente Tesis Doctoral se fundamenta en un estudio basado esencialmente en perfiles de sísmica de reflexión (mayormente adquiridos durante los proyectos TOPOMED, EVENT-DEEP y ESCI) de la Cuenca de Alborán, situada entre las cordilleras Béticas (Sur de la Penísula Ibérica) y Rif (Norte de Marruecos). El proceso de formación de la cuenca es aún discutido, al igual que los posteriores procesos de deformación. He centrado el presente estudio en: 1) caracterizar la estructura cortical de la cuenca, 2) definir su evolución, basándome en el estudio del registro sedimentario, y 3) estudiar la reorganización contractiva de la cuenca. El estudio de la reorganización contractiva lo he centrado en tres zonas: el margen de Palomares, la falla de Yusuf y el cabalgamiento frontal de la Cresta de Alborán. Los resultados revelan tres tipos de corteza que coexisten a lo largo de la cuenca: a) corteza continental adelgazada, b) corteza de arco magmático y por último, c) corteza continental del Norte de África. Las primeras evidencias de la fase extensional se localizan en la cuenca oriental de Alborán y en la cuenca de Málaga de edad Burdigaliense, seguidas por la creación de un segundo depocentro en el norte de África durante el periodo Languiense-Serravaliense. El arco magmático se formó durante el Tortoniense. Los procesos extensionales en la cuenca ocurren hasta el Mesiniense, y a partir del Mioceno tardío-Plioceno temprano, cesa la extensión y comienza la fase de deformación compresiva de la cuenca. El estudio de las estructuras activas confirma que la inversión tectónica se focaliza en unas pocas fallas que marcan los límites entre dominios corticales. El desplazamiento acumulado desde el Plioceno Inferior de dos de las principales fallas de la Cuenca de Alborán, Yusuf y el cabalgamiento frontal de la Cresta de Alborán, es cómo mínimo de 20 km. Este valor es cercano al valor total de la convergencia entre las placas de Iberia y África, y por tanto confirma que actualmente la deformación se concentra principalmente en unas pocas estructuras. Dada su importancia, es necesario realizar un estudio de la peligrosidad de estas fallas para evaluar su potencial sísmico y tsunamogénico e incluirlas en las bases de datos nacionales y europeas de riesgos geológicos.