Post-Hercynian tectonic and paleogeographic evolution of SW Iberia and the Gulf of Cadiz

Abstract

[eng] This thesis addresses the tectonic and paleogeographic evolution of the SW Iberia Margin from the Mesozoic rifting stage to the post-rifting collision of Late Cretaceous through Miocene times and transpressive stages of Pliocene and Quaternary. The study is based on onshore field work, and analysis of well logs, a large set of 2D and 3D seismic reflection profiles and gravity data of the offshore.

The main contribution of this thesis is to provide an integrated model for the Mesozoic evolution of the Algarve Basin taking into account the main factors that affected the its configuration, such as the stratigraphic record, crustal structure, salt tectonics, rifting and compressional deformation.

At present, the SW Iberian margin is located along the convergent Nubia-Iberia plate boundary. In Mesozoic times, this passive margin was located to the northeast of the triple junction of the Ligurian Tethys, Central Atlantic and Northern Atlantic. Seismic interpretation and gravity modelling shows the possible presence of Ligurian Tethys oceanic crust under the Gulf of Cadiz and the existence of anomalous density bodies at lower crustal levels interpreted either as pieces of exhumed sub-continental mantle or underplated material. Tapering of Iberian crust is characterized by rapid changes in the thickness of the upper and lower crust. Gravity modelling also allows the characterization of the Cenozoic contractional overprint experienced by this Mesozoic oblique margin.

The presence of evaporites and the tectonic pulses that triggered their mobilization through diapirism in the Algarve Basin also controlled the paleogeographic configuration of the margin. From north to south, in the onshore, the evaporite unit, Hettangian in age and source for the salt tectonics, is observed to thicken basinward, in fault-controlled depocenters. Salt-related structures are only present in areas of thick initial evaporites. In the offshore, multiple salt-structures cored by the Lower Jurassic evaporites are documented by seismic reflection data and exploratory drilling. Offshore salt structures include the allochthonous Esperança salt nappe, which extends over an area roughly 40x60 km. The amount of saltrelated structures and their typology is observed to be controlled by the distribution of evaporite facies, which is in turn controlled by the structure of rift-related faulting. The study of salt tectonics of the Algarve Basin contributed on better constraining the paleogeography during the Hettangian and the units that were affected by diapirism. The comprehensive image of salt tectonics over the SW Iberian passive margin presented in this thesis covers all aspects from initial evaporite facies and thickness to the evolution of salt-related structures through Mesozoic extension and Cenozoic basin inversion.

The SW of Iberia has undergone compression during the Late Cretaceous-Cenozoic, up to present day, due to the convergence between Africa and Eurasia. Multiple contractional features and their seismic activity have been documented immediately west of the Gulf of Cadiz, in the Atlantic domain. East of the Gulf of Cadiz, the Betic-Rif orogen and its active contractional and extensional structures are well known. Significant seismic activity reflecting a dominantly compressive stress state has been observed between these two domains. The seismic activity spans the entire Gulf of Cadiz, but the features associated with this seismicity have remained elusive.

During Late Cretaceous-Cenozoic compression, basement discontinuities such as Variscan foliation and thrusts were reactivated forming south-verging monoclines. The monoclines, along with deep seawater currents, controlled the deposition of the Upper Cretaceous and Cenozoic sediments. Compression also lead to the reactivation of other basement structures (e.g., Guadalquivir Bank), which are responsible for the present-day seismicity and bathymetry in the area. The compressional phase also reactivated the salt tectonics.

In conclusion, the SW Iberian margin was firstly affected by an extensional phase during the Mesozoic that resulted into a highly-extended margin the formation of oceanic crust at its deepest part. The different extensional tectonic phases that took place in the margin controlled the thickness and depositional environments of the Mesozoic sediments. Initial rifting also contributed to the evaporite basin configuration, which, together with the basement structure, triggered diapirism in the SW Iberian margin.