Dynamics of the Central Mediterranean Region

The Aegean is one of the fastest deforming regions on the planet. The northward motion of the Arabian plate, the westward drift of Turkey, right-lateral shear in Northern Aegean Sea, and complicated deformation patterns in Greece have long been of geophysical interest. In this work we seek to quantify the relative driving forces for active deformation within the Aegean. Specifically, we seek to quantify the role of body forces associated density variations within the lithosphere as well as the role of plate interaction in driving the active deformation there. Our numerical grid covers the entire Aegean region and the Turkish mainland. We first interpolate GPS data with continuous bi-cubic spline functions to define a kinematic solution. The density of grid areas used in the kinematic and dynamic modeling, together with the GPS data, are sufficient to delineate both rapidly deforming and rigid areas. Moreover, the model strain rate tensor field, defined by the fit to the GPS data, is in good agreement with the styles of active deformation inferred from earthquake moment tensor solutions. Our dynamic solutions involve a direct estimate of vertically averaged deviatoric stresses, using a thin-sheet parameterization. The deviatoric stress solution satisfies a set of balance equations for a non-accelerating continuum, where the governing force comes from the lateral variations in the graviatational potential energy, calculated assuming Airy-type isostatic compensation of topography. A second solution to the linear equations, without the body force term, is sought in a formal inversion to define a complete stress boundary solution. The deviatoric stress field associated with the boundary condition solution is added to the deviatoric stress field associated with the gravitational potential energy distribution to define a total solution. In the formal inversion the directions and relative magnitudes of principal axes of the total deviatoric stresses are constrained to be a best-fit with directions and relative magnitudes of principal strain rates, inferred from the interpolation of GPS observations. We obtained quite small vertically averaged deviatoric stress values for Western Turkey of 50-60 bars, which is roughly half of that calculated for the Western United States. The right lateral shear stresses are diffuse across a wide NE-SW trending band connecting the Marmara Sea region to Corinth Strait, where the GPS data show the largest strain rates (around 4.5 x 10-14s-1 within our grid cells). A remarkable feature is that gravitational potential energy distributions alone are sufficient to nearly completely define the right-lateral shear stress pattern in Marmara region. Elsewhere, gravitational potential energy differences and plate interaction act approximately equally to define the deviatoric stress field that drives deformation in the region. Using the vertically averaged deviatoric stress magnitudes from the dynamic solution, and the rates of strain from the kinematic solution, we solve for the vertically averaged effective viscosity field in the Aegean region. The Black Sea shows up as a region of high effective viscosity. Surprisingly the Anatolian block is defined by low deviatoric stress magnitudes, rather than by abnormally high effective strength.