Cenozoic seismic stratigraphy and tectonic evolution of the Algarve margin (offshore Portugal, southwestern Iberian Peninsula)

Abstract

The Cenozoic seismic stratigraphy and tectonic evolution of the Algarve margin (southwestern border of the Iberian Peninsula) are documented from multichannel seismic reflection data, isochron and time–structure maps. Six seismic units (B to G), bounded by unconformities and probably ranging in age from Campanian to Holocene, are identified. Their ages are derived from: 1) oil exploration wells drilled in the margin; 2) geometrical relationship with the Guadalquivir Allochthonous Unit; 3) correlation with the onshore stratigraphic record of the Algarve basin; and 4) lateral correlation with unconformities dated in adjacent basins and related to tectonic events that affected Iberia during the Cenozoic. Three main tectonic domains, bounded by major fault zones that segment the Algarve margin, are characterised: a western central domain, bounded to the west by the N–S Portimão–Monchique Fault Zone and to the east by the N–S Albufeira Fault Zone; an eastern central domain, bounded to the east by the N140° São Marcos–Quarteira Fault Zone; and an eastern domain located east of this latter structure. The Cenozoic evolution of the Algarve basin results from both Hercynian basement reactivation and thin-skinned contraction above a relative thick evaporitic unit (probably Upper Triassic–Hettangian) that acted as the main décollement, controlling synchronous extensional and thrust detachments and generating both salt structures and salt-withdrawal sub-basins. A persistent halokinesis is identified, with two climax phases: during the deposition of Unit C (Lutetian to Oligocene), and at Unit E (Upper Tortonian to Messinian). The increasing flexure of the Algarve margin is mainly expressed by spatial and temporal variations of the subsidence. The overall tectonic regime is considered to be compressive, and the close juxtaposition of compressed and extended areas is related to the orientation and geometry of preexisting basement tectonic structures and to the syncline evolution of the Guadalquivir foredeep basin in response to the westward migration of the Gibraltar Arc.