The ocean-continent boundary in the Gulf of Lion from analysis of expanding spread profiles and gravity modelling

Abstract

A R Y Two-ship multichannel seismic profiles, deep penetration (ECORS) and conventional seismic lines (LIGO surveys) are used to study the crustal structure of the Gulf of Lion (Western Mediterranean). 11 full ESPs (Expanded Spread Profiles) with total shot-receiver ranges up to 60 km were shot in 1981 perpendicular to the margin of the Gulf of Lion and in 1988 a deep MCS seismic profile (ECORS-CROP program) was performed parallel to the ESPs. These ESPs were analysed by matching traveltime and amplitude variations in both the x -f and z-p domains. The resulting P-wave velocity/depth model has the following features, (a) beneath the continental slope of the Provenqal margin a rapid rise of the Moho from 20 to 14 km and the existence of an anomalous 7.2-7.4 km s-' velocity layer, (b) from the base of the slope to the extensive salt-domes domain a 5-6 km thin crust which does not appear typically oceanic in nature, (c) quite typical oceanic crust up to the Sardinian margin. Gravity modelling is consistent with the seismic results. The OCB (ocean-continent boundary) could be placed north of that postulated by previous authors, where the data indicate a remarkably narrow transition between continental and 'oceanic' crust, or south where a typical oceanic crust, which correlates well with the domain of the salt domes and of large magnetic anomalies, has been determined.