Seismic crustal structure in the Gulf of Cadiz (SW Iberian Peninsula)

The sedimentary structure in the Gulf of Cadiz has been extensively studied by oil exploration companies. However, up to now little is known about its deep crustal structure. Moreover, the total thickness of the sedimentary layers remains unknown in large areas. The purpose of this paper is the crustal-scale interpretation of deep seismic near-vertical reflection and refraction/wide-angle reflection data obtained during the IAM (Iberian Atlantic Margins) project, carried out in 1993. Our results indicate that a continental type crust is underlying the entire Gulf of Cadiz, with progressive thinning from east to west. The sedimentary cover shows a great thickness, reaching 8 km in the center of the Gulf. Three main sedimentary units can be recognized: Jurassic-Cretaceous calcareous rocks, continuation of Algarve outcrops; the Allochthonous Units of Guadalquivir/Gulf of Cadiz, the offshore continuation of the inland Carmona nappe; and sub-horizontal post-Miocene marine sediments. The crystalline crust is divided into three main layers: the upper crust is characterized by P-wave velocity values of 5.7–6.1 km/s; the middle crust shows values of 6.3–6.4 km/s; the lower crust has a mean vertical velocity gradient of 0.02 km/s/km, with velocities between 6.9 to 7.1 km/s. The total crustal thickness varies from 27 km for the eastern part of the studied area, to 20 km for the westernmost part. The crustal thinning is more pronounced in a N-S direction than in an E-W direction. No major structures related with a defined Iberia-Africa plate boundary could be found.