Seismic crustal structure in the southwest of the Iberian Peninsula and the Gulf of Cadiz

Abstract

The crust under the southwestern Iberian Peninsula and the Gulf of Cadiz has been sampled by 1200 km of deep seismic refraction=wide-angle reflection profiles, together with many seismic reflection lines and bore-holes. Wide-angle seismic data were collected during the last three decades. Commercial multichannel data provide a detailed image of the uppermost crust, improving the confidence about the models of the deeper structures. P-wave velocities within the thick column of sediments in the Gulf of Cadiz range from 2.0 to 3.8 km=s, while the Algarve and the Sines areas have higher velocities of 4.3 to 4.8 km=s. The top of the Palaeozoic basement rises to the northwest, outcropping in the South Portuguese zone of the Iberian Massif, and is characterized by P-wave velocities of 5.7–5.9 km=s. High velocities of 6.4 km=s have been found at shallow depths of 7 to 10 km in the South Portuguese zone, that could be related to the mafic and ultramafic rocks in the Beja-Acebuches zone. Lower crustal velocities are in the range of 6.7–6.9 km=s. The crustal thickness shows important lateral changes from 29 km beneath the Guadalquivir Basin=Iberian Massif contact to 35 km in the southeastern part of the South Portuguese zone. From the interpretation of these seismic data, a geodynamic model of the evolution of the crust in Southwestern Iberia and the Gulf of Cadiz is proposed. The Guadalquivir Basin, and its continuation at sea, the Gulf of Cadiz, is a flexure area of the crust that could be related to the overloading due to the overthrusting of the Alboran Domain over the Iberian plate.