Deep crustal configuration of the Valencia trough and its Iberian and Balearic borders from extensive refraction and wide-angle reflection seismic profiling

Abstract

New constraints on the crustal structure of the Valencia trough and its Iberian and Balearic borders have been obtained from a seismic experiment carried out in autumn 1989. A total of 290 sea shots were fired along three refraction/wide-angle reflection profiles recorded by 10 Ocean Bottom Seismometers (OBSs) in the Valencia trough and 110 land stations distributed on both flanks. The huge amount of recorded data in each profile has been integrated and jointly interpreted to obtain a single 2-D velocity-depth model along each line. The geometry and deep crustal structure of the basin are fairly well defined in terms of laterally inhomogeneous layered media. The sedimentary cover shows a layer of consolidated sediments, overlain by Neogene and Quaternary sediments, with a velocity of 5.1 km/s interpreted as Mesozoic carbonates at the northwestern side of the Valencia trough. A fairly homogeneous crystalline basement is found on all profiles at a mean depth of 4 km below sea-level, with a velocity of 6.0–6.1 km/s. The observed critical distances and arrival times of the Moho-reflected waves are firm evidence supporting a thinning of the crust towards the centre of the basin. The Moho rises up to 13 km in the axis of the trough, in the NW-SE and E-W profiles, and up to 15 km in the NE-SW profile. Moreover, the analysis of the well defined P MP phase shows differences between the deep structure of the flanks and the trough, respectively. A sharp velocity contrast at the Moho from a fairly uniform lower crust of 6.4 km/s velocity is proposed in both flanks, in contrast to a high velocity gradient within the lower crust interpreted near the axis of the trough. An anomalous upper-mantle velocity of 7.8 km/s fits the data throughout the Valencia trough and Mallorca, with slightly higher values of 7.9 km/s required towards the continent. These velocity values do not favour an interpretation in terms of underplating. The geometry of the Moho discontinuity shows a gradual deepening towards the Iberian Peninsula, in contrast to a more abrupt slope towards Mallorca. Depth values of 30–32 km, typical of a continental crust, are inferred about 40 km inland on the Iberian Peninsula, in agreement with the crustal thicknesses already reported for the Iberian range and the Ebro basin. The present crustal thinning resulting from rift episodes in the Valencia trough and the Iberian border is thus confirmed and constrained from our seismic results of the deep crust and upper mantle.