FEATURES OF DEEP CRUSTAL STRUCTURE AND THE ONSHORE-OFFSHORE TRANSITION AT THE IBERIAN FLANK OF THE VALENCIA TROUGH (WESTERN MEDITERRANEAN)

First results are presented of a recent onshore seismic survey complementary to the Valsis-2 Cruise, which consisted of ESP, COP and CDP marine seismic profiles across the Valencia Trough (Western Mediterranean). The marine energy source used was an airgun array of 5800 cubic inch recorded at 2 land stations on the western flank of the Valencia Trough, at distances between 10-120 km. The experiment has resulted in an extended sampling of the deep crustal structure of the eastern Mediterranean flank of the Iberian peninsula, as well as the offshore-onshore transition. Three transverse NW-SE profiles have been interpreted. Local thinning of the sedimentary cover has been determined towards the centre of the basin which, together with the shallow high velocities observed on the southern profile, could be related to volcanic episodes. A seismic continental basement has been found at depths between 3 and 5 km. A thin lower crust (3-5 km) with velocities around 6.8 km/s has been identified in the northern part of the basin. Alternative crustal models considered for the 3 profiles have been tested, not only from arrival times but also from relative amplitude distributions. A first-order Moho discontinuity fits the data best. The well-defined Moho boundary results in energetic P(M)P reflections, and a clear updoming is observed towards the interior of the basin, from depths about 20-21 km inshore of Barcelona to 15-17 km depths 60 km offshore. An anomalous upper mantle with low P(n) velocities of about 7.7 km/s is confirmed in most of the sampled areas.