CRUSTAL AND VELOCITY STRUCTURE OF THE VALENCIA TROUGH (WESTERN MEDITERRANEAN) .1. A COMBINED REFRACTION WIDE-ANGLE REFLECTION AND NEAR-VERTICAL REFLECTION STUDY

In November 1988, a 2-ship seismic survey was carried out in the Valencia trough (western Mediterranean) to collect normal incidence seismic reflection profile data and six Expanding Spread Profiles along the continental shelf and trough. Velocity depth solutions derived from the analysis of the ESP show a pattern that corresponds with that of continental-type crust, except in the northeastern part of the basin where oceanic-type crust is present. Along the Iberian margin and centre of the trough, the seismic data show a well-developed Neogene and Quaternary sequence with velocities ranging from 1.6 to 4.8 km s-1, whereas along the Mallorca margin these deposits are absent. At the base of the Neogene there is a Tertiary unconformity which marks the top of the pre-rift Mesozoic carbonate platform. The carbonates, which have velocities from 5.4 to 5.9 km s-1, thin towards the axis of the trough. Along both margins the upper and middle crust is characterised by an almost transparent layer of variable thickness (6-9 km) with velocities ranging from 6.1 to 6.4 km s-1. Dipping reflections are observed, however, in the southern part of the trough. These reflections are the result of a locally thick Mesozoic sequence. A 6-km-thick highly reflective lower crust with velocities from 6.4 to 7.0 km s-1 is seen beneath the Iberian margin. Along the Mallorca margin, the lower crust is variably reflective with a velocity of about 6.6 km s-1. In the centre of the trough, the total crustal thickness is less than 14 km, which thickens asymmetrically towards the flanks reaching values of 20-21 km off-shore the Iberian Peninsula and 23 km in the Mallorca margin. The seismic Moho increases in depth from 13 to 16 km in a NE-SW direction. Relatively low, upper mantle velocities from 7.6 to 8.0 km s-1 have been found throughout the basin, indicating a slightly anomalous uppermost mantle or a crust-mantle transitional layer. In the northeast, the velocities for the sediments are similar to those described above. However, the crustal velocity solution can be characterised by three velocity-gradient layers ranging from 4.8-6.1, 6.1-6.7 to 6.7-7.2 km s-1.