Extensional salt tectonics in the Cotiella post-rift basin (south-central Pyrenees): 3D structure and evolution

Abstract

The Cotiella Massif in the southern Pyrenees constitutes a seismic-scale exposure of an extensional basin developed during the Late Cretaceous by the gravity-driven collapse of the post-rift platform above Upper Triassic salt. The internal structure is dominated by four main middle Coniacian-early Santonian sub-basins with large rollover anticlines. They involve the carbonates of the post-rift succession which unconformably overlies the Lower Cretaceous syn-rift basins. The extensional faults are only partially inverted despite is transported tens of kilometres southwards in the Pyrenean nappes.

Data presented in this Thesis reveal that the tectono-sedimentary evolution of the basin has not only been controlled by the growth of extensional faults above salt, but also by the rising and falling of diapirs, as well as by salt migration and the development of transfer faults. At late Santonian, the extensional faults were partially inverted, salt was expulsed and salt welds were produced, leaving little trace of the original salt volume. Even though, the extensional features are outstandingly preserved. The excellent exposures offer a unique opportunity to investigate in outcrop the structure and infilling of a post-rift salt basin and gain insight into the understanding of other salt-involved passive margins.

Thus, the main purposes of the Thesis are twofold: 1) describe the structure and the facies distribution of the inverted Cotiella basin with more detail than existing works and 2) decipher the role of salt tectonics in its development. To reach these objectives, the structure has been investigated through geological mapping and structural field data acquisition, integrated with a three-dimensional (3D) 1:5.000 digital terrain model. A surface-data based 3D reconstruction; cross-sections along strike and across strike, interpretation of oblique photographs. The middle Coniacian – early Santonian succession has been sub-divided into litostratigraphic units which make the investigation of the tectono-sedimentary evolution of the Cotiella basin possible. The data presented is compared with similar structures of the world and the given interpretations are validated with sequential restorations of the salt structures. Ultimately, a structural evolutionary model of the Cotiella basin is proposed.