Abstract: Analysis of the Isostatic Equilibrium in the Gulf of Lion (France): Implications for the Geodynamic and Thermal Evolution

J. M. Gaulier, N. Chamot-Rooke, F. Jestin

We compute the tectonic subsidence within the Liguro-Provencal basin and associated continental margins using 3D elastic flexural modelling and display results on a detailed map centered on the Gulf of Lion margin sensu stricto and a larger one covering the entire basin as well as the margins of Corsica and Sardinia. We first combine seismic refraction and reflection data to a 3D gravity model to obtain basement and Moho depth. Tectonic subsidence is then computed assuming both local and regional isostasy. We compared observed relationships between the present-day tectonic subsidence and crustal thickness to predicted ones based on both homogeneous and inhomogeneous stretching models. For stretching factors smaller than 2 (upper margin), both types of models can ccount for the observed tectonic subsidence/crustal thickness relationship. For stretching factors greater than 2 (lower margin), the subsidence is anomalously high and departs significantly from the predicted ones. The entire Liguro-Provencal basin is also deeper than the expected depth for an oceanic basin of the same age. Previous studies attribute the abnormal subsidence of the Gulf of Lion to a recent regional compression, still active and well expressed in the Ligurian area. However, similar anomalously large subsidence has been widely documented in some of the marginal basins of the Western Pacific. In these, as well as in the Gulf of Lion, the free-air gravity is close to zero whereas a significant mass deficit would be expected if the anomalous subsidence was driven by lateral f rces. An alternative explanation would be a low potential temperature of the asthenosphere as expected in areas where subduction of cold material has occurred for some time. A low potential asthenospheric temperature would imply a small amount of melting during oceanic accretion, compatible with the extremely thin oceanic crust in the Gulf of Lion.

AAPG Search and Discovery Article #90956©1995 AAPG International Convention and Exposition Meeting, Nice, France