Lithospheric gravitational instability beneath the Southeast Carpathians

The Southeast corner of the Carpathians, known as the Vrancea region, is characterised by a cluster of strong seismicity to depths of about 200 km. The peculiar features of this seismicity make it a region of high geophysical interest. In this study we calculate the seismic strain-rate tensors for the period 1967-2007, and describe the variation of strain-rate with depth. The observed results are compared with strain-rates predicted by numerical experiments. We explore a new dynamical model for this region based on the idea of viscous flow of the lithospheric mantle permitting the development of local continental mantle downwelling beneath Vrancea, due to a Rayleigh-Taylor instability that has developed since the cessation of subduction at 11 Ma. The model simulations use a Lagrangean frame 3D finite-element algorithm solving the equations of conservation of mass and momentum for a spatially varying viscous creeping flow. The finite deformation calculations of the gravitational instability of the continental lithosphere demonstrate that the Rayleigh-Taylor mechanism can explain the present distribution of deformation within the downwelling lithosphere, both in terms of stress localisation and amplitude of strain-rates. The spatial extent of the high stress zone that corresponds to the seismically active zone is realistically represented when we assume that viscosity decreases by at least an order of magnitude across the lithosphere. The mantle downwelling is balanced by lithospheric thinning in an adjacent area which would correspond to the Transylvanian Basin. Crustal thickening is predicted above the downwelling structure and thinning beneath the basin.