Etude thermomécanique de la zone de transition mer-continent de la marge algérienne : implication géodynamique

摘要

Understanding how subduction initiates at a passive margin is a major geodynamic question, which remains debatted because of the forces necessary to overcome bending and frictional resistance of the lithosphere and initiate this subduction.Along the southern shore of the Western Mediterranean Sea, the Algerian margin undergoes ~NS compression due to the African-Eurasian convergence at a rate of less than ~ 1 cm / year. This setting causes tectonic inversion of this North African passive margin and possibly incipient subduction. Indeed, recent geophysical marine data acquired in the Algerian Basin (MARADJA, 2003, 2005MARge Active d’el Djazaïr) and SPIRAL 2009 (Deep Seismic and Regional Investigations in Algeria campaigns) showed evidence of recent compressive deformation.We used SPIRAL wide-angle seismic profiles to determine the state of isostatic equilibrium at the vicinity of the continent-ocean boundary. This allowed us to image a too deep Moho in the oceanic part, and a too shallow one in the continental domain, with a boundary between both domains located at the margin toe. These results indicate that the Algerian margin display the same isostatic anomalies as an active margin.The flexural bending of two lithospheric plates that can be modelled by a finite element procedure may simulate this isostatic disiquilibrium. This modeling shows larger vertical deflection in the central part of the study area (6-7 km) compared to the earsternmost and westernmost profiles (3 km).Aiming at understanding the mechanisms that led to this localized deformation at the margin toe and to the possible initiation of a subduction zone, we used a thermomechanical modeling method where we changed the rheological parameters and rheology of a weakness zone located at the ocean-continent boundary. It appears that the presence of a thermal anomaly at the ocean-continent transition, the latter being a thinned continental crust, is paramount to the location of deformation at the margin toe in the Algerian basin.