Three-dimensional gravity modelling of the European Mediterranean lithosphere

Abstract

SUMMARY Residual gravity anomalies characterizing the density heterogeneities in the upper mantle of the Alpine belt of Western Europe are determined. Residual anomalies were calculated on a 1 x 1 grid by subtracting the gravity effect of the density model for the Earth's crust from the observed gravity field. Our 3-D density model consists of two regional layers of varying thicknesses with a lateral variation in average density: the sedimentary cover and the crystalline crust. Offshore, the model is supplemented by a sea-water layer. This 3-D density model is based on a generalized velocity model represented by structure maps of the main seismic horizons (the 'seismic' basement and the Moho boundary) and a map of the average P-wave velocity in the consolidated crust. The density distribution within the model layers was obtained using the correlation functions between P-wave velocity and density. For sediments, sediment consolidation with depth was taken into account. The gravity effect of the model, approximated by parallelepipeds 1 x 1 in planar size, was calculated by a program designed for solving 3-D gravity problems. The program takes into account the spherical configuration of the Earth. This method also permits the estimation of the isostatic state of the crust. A mantle origin of residual gravity anomalies is confirmed by their close correlation with upper-mantle velocity heterogeneities, established by both seismic-tomography and thermal-regime data. The residual gravity field is inverted into the distribution of anomalous density within the uppermost mantle layer (between the Moho and a bottom level at 200 km depth). The most important anomalies are the high-density domains caused by the thick lithosphere of the Adriatic plate and by the lithosphere 'roots' beneath the Alps and the Calabrian Arc. Negative density anomalies over the Pannonian Basin and the Western Mediterranean basins reach -0.04 to -0.05 x lo3 kg m-3 due to thermal expansion of the asthenosphere.