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 l ayers remains unknown in large areas. The purpose of this paper is the crus tal-scale interpretation of deep seismic near-vertical reflection and refra ction/wide-angle reflection data obtained during the IAM (Iberian Atlantic Margins) project, carried out in 1993. Our results indicate that a continen tal type crust is underlying the entire Gulf of Cadiz, with progressive thi nning from east to west. The sedimentary cover shows a great thickness, rea ching 8 km in the center of the Gulf. Three main sedimentary units can be r ecognized: Jurassic-Cretaceous calcareous rocks, continuation of Algarve ou tcrops; 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 par t of the studied area, to 20 km for the westernmost part. The crustal thinn ing is more pronounced in a N-S direction than in an E-W direction. No majo r structures related with a defined Iberia-Africa plate boundary could be f ound.