Salt Tectonics of the Offshore Tarfaya Basin (NW Africa): A Regional to Local Approach

Abstract

[eng] Although there is certain consensus on the Middle Jurassic to Present evolution of the Central Atlantic, the Early Jurassic rift to drift transition stage is still under discussion. Through the interpretation of unpublished, recently acquired deep seismic reflection data, integrated with legacy 2D surveys from offshore Morocco, this thesis (Chapter 2) presents new evidences supporting a revised passive margin evolutionary model with focus on the rift to drift transition phase. Assuming evaporite deposition was synchronous along the margin, a northward propagation of rifting and breakup is interpreted, with salt deposited during the late crustal thinning stage in the Tarfaya and Agadir basins (Southern segment), during crustal thinning in the Agadir, Essaouira and Safi basins (Central segment), and during continental stretching in the Mazagan Plateau (Northern segment). Furthermore, structural inheritance from Paleozoic tectonic boundaries, such as the South Atlas Fault Zone, were reactivated as transfer zones during rifting separating segments with different crustal deformation styles and extensional rates. Finally, from Late Cretaceous times, the inherited Paleozoic and Mesozoic structural fabric controlled the localization of thick and thin-skinned tectonic inversion caused by the compressional events related to the convergence between the African and Eurasian plates. Chapter 3 from this thesis presents the key events that conditioned the evolution of the offshore Tarfaya Basin (SW Morocco). From Late Triassic to Early Jurassic times, salt was deposited with a basinward thickening wedge-shaped geometry on a narrow trough developed over thinned continental crust. During the Jurassic, salt withdrawal was probably triggered by local depocenters related to the progradational loading on the demised carbonate slope on a proximal setting in the central and northern areas of the Tarfaya Basin. Moreover, thermal subsidence might have led to the regional tilting of the salt basin probably playing an important role as triggering mechanism in the distal basin. During the Early Cretaceous, the progradation of the Tan-Tan Delta promoted a continued basinward expulsion of salt and the proximal extrusion of salt sheets. Finally, from Late Cretaceous to the Present-day, shortening related to the convergence between Africa and Eurasia resulted in thick-skin inversion and the rejuvenation of precursor salt structures. Finally, Chapter 4 of this thesis focuses on the effect of salt tectonics on seabed topography and how it controlled deep-water sedimentation in the Tarfaya Basin. The study area is subdivided in three structural domains: I (Proximal), II (Intermediate) and III (Distal), with distinctive characteristics. Domain I is characterized by secondary welded, buried salt sheets

with counterregionally dipping feeders. Domain II is distinguished by contractionally rejuvenated diapirs and salt sheets and Domain III by passively growing diapirs actively deforming the modern seabed. During the Paleogene, the active growth of structures located in Domain I imposed a high paleotopographic relief blocking turbiditic flows sourced from the shelf. The Early Oligocene is characterized by continued active diapirism and a low sediment flux. This triggered the widespread deposition of mass transport deposits and the diversion of turbiditic channel- lobe systems. In contrast, the Late Oligocene is marked by the exhumation of the Atlas, Anti Atlas and Reguibat Shield systems which provided large amounts of sediments that bypassed the shelf and were deposited forming large turbiditic fans. The Miocene is characterized by sediment bypass from the shelf to deep-offshore settings and a gradual burial of the proximal salt structures that were ultimately secondary welded. The identified depositional systems evidence a progressive amalgamation of the different minibasins. The Plio-Quaternary evolution of the study area is marked by a second major orogenic pulse in the Atlas System resulting in an increase of onshore exhumation rates and sediment flux from the High Atlas, Anti Atlas and Reguibat Shield.

[spa] Esta tesis presenta un estudio integrado multiescala de la tectónica salina del margen Atlántico de África Noroccidental. El capítulo 2 presenta nuevas evidencias que soportan una revisión del modelo evolutivo de la transición rift – drift en el margen pasivo de Marruecos. A través de la interpretación de secciones sísmicas 2D, la caracterización de los estilos estructurales observados en la sal autóctona (Triásico Superior – Jurásico Inferior), evidencian la propagación del rifting en sentido sur-norte. En la Cuenca de Tarfaya y Agadir Sur (segmento sur), la sal fue depositada en la etapa de adelgazamiento cortical tardío, en las cuencas de Agadir Norte, Essaouira y Safi (segmento central) durante la etapa de adelgazamiento cortical, y en el Plateau de Mazagan, la sal fue depositada en la etapa de extensión cortical. El capítulo 3 de la presente tesis se focaliza en el estudio de la tectónica salina de la Cuenca de Tarfaya. La sal fue depositada en un estrecho depocentro con una geometría en forma de cuña con máximos espesores en el sector distal. Durante el Jurásico, la tectónica salina en sectores proximales fue probablemente iniciada a partir de depocentros locales relacionados a cargas diferenciales provocadas por la progradación de la plataforma. En sectores distales, la subsidencia térmica relacionada a la etapa de post-rift puede haber jugado un rol importante en la iniciación de depocentros locales. Durante el Cretácico Inferior, la progradación del Delta de Tan-Tan contribuyó a la expulsión de sal hacia sectores distales y la extrusión de salt sheets. Finalmente, a partir del Cretácico Superior, la convergencia entre las placas africana y euroasiática provocó el comienzo de una etapa donde los esfuerzos compresivos causaron la inversión tectónica de estructuras y el rejuvenecimiento del diapirismo. Finalmente, el capítulo 4 de la presente tesis, presenta un estudio sobre la interrelación entre tectónica salina y sistemas deposicionales marinos profundos durante el Cenozoico. Partiendo del modelo presentado en el capítulo3, las estructuras salinas ejercen una marcada influencia en la paleotopografía marina bloqueando o desviando corrientes gravitacionales. Asimismo, en períodos donde la relación entre crecimiento de las estructuras salinas / sedimentación proveniente del continente es alta, se observan sistemas asociados a mass transport deposits.