Pliocene-Quaternary deformational structures in the eastern Algarve continental shelf, Gulf of Cadiz

Abstract

Evidences of tectonic deformation from the Pliocene to the Quaternary are evaluated on the northern Gulf Cadiz continental margin along the eastern Algarve continental shelf between the Guadiana River and Faro city, in a regional compressive context close to the Eurasia-Africa plate boundary. We studied an extensive set of 2D highresolution seismic reflection profiles framed within previously published seismic-stratigraphic models. The seismic stratigraphic interpretation and regional correlation allow us to identify a major Middle Pleistocene (0.9–026 Ma) angular unconformity along the shelf, plus deformational structures affecting widespread depositional hiatuses in the Pliocene-Quaternary sedimentary units of the study area. Several deformation features indicate neotectonic activity: ENE-WSW to NE-SW thrusts moving pre-Middle Pleistocene shelf deposits and N-S to NW-SE folds and NNE-SSW to NNW-SSE high-angle normal faults affecting Middle to Upper Pleistocene shelf deposits. These structures are interpreted under the light of two major deformation phases that took place in the Algarve continental shelf. A first phase of Pre-Middle Pleistocene thrusting agrees with a dominant transpressive regime along the southwestern Iberian margin since the Tortonian, which is also recorded in the deep-water contouritic record of the southern Iberian margin, where compressional events caused intensification of the Mediterranean Outflow Water. A Middle to Upper Pleistocene tectonic phase is characterized by less intense deformation of the sedimentary cover involving individual structures (i.e., folds and faults), some of which remained active at least up to the Last Glacial Maximum. This more recent tectonic deformation is compatible with slowly uplifting trend recognized in the Portuguese mainland, and coetaneous diapiric reactivation that locally deformed Late Quaternary sediments along the southwestern Iberian margin. The study lends additional evidence of regional neotectonic activity producing Pliocene to Pleistocene deformation, and having ultimate implications for seismic hazard assessment, in offshore settings close to coastal population centers along the southern Portuguese coast.