Numerical modelling of the Aegean-Anatolian region: Geodynamical constraints from observed rheological heterogeneities

High strain rates and intense seismic activity characterize both the boundaries and the interior of the Aegean-Anatolian plate: the availability of geodetic and geophysical data makes this region ideal to make detailed models of continental deformation. Although the deformation occuring in the Aegean-Anatolian plate may be regarded as the primary effect of the Arabian indenter push, it has already been demonstrated that this mechanism cannot account for the observed extrusion/rotation of the whole plate. We investigate the present-day steady-state anelastic deformation of the Aegean-Anatolian plate by a thin plate thermomechanical finite element (FE) model that accounts for realistic rheological mechanisms and lateral variations of lithospheric properties. Studying the region with uniform models, where average values for thermal and geometric parameters are chosen, we find that two tectonic features, in addition to the Arabian plate push, are critical to reproduce a velocity field that gives a reasonable fit to the observations. The first is the E-W constraint of NW continental Greece, related to the collision between the Aegean-Anatolian plate and the Apulia-Adriatic platform, required in the model to attain the SW orientation of the velocity field along the Hellenic Arc. The second is the trench suction force (TSF) due to subduction of the African lithosphere, which is needed to fit the observed mean extrusion velocity of 30 mm yr-1 along the Hellenic Arc. Uniform models are useful to study the sensitivity to the interplay of rheological/thermal parameters in a simplified framework but, in all cases, predict a strong deformation localized along the Hellenic Arc, whereas geodetic and seismological data show that the highest strain rates are located in western Anatolia. Furthermore, uniform models are non-unique in the sense that since we model a vertically averaged thin plate, different thermal and rheological parameters can be combined to yield the same lithospheric strength. We account for internal sources of deformation with heterogeneous models, where the available constraints on lateral variations of crustal thickness and surface heat flow have been included. The heterogeneous distribution of lithospheric strength contributes to ameliorate the fit to geodetic and stress data, since the predicted velocity field is characterized by an acceleration from E to W, with a sharp increase in the proximity of the western margin of the Anatolian peninsula, where the highest rates of intraplate deformation are observed. In our model this partitioning of the deformation is due to the different rheology of the Aegean Sea, which, being slightly deformable, transmits the TSF to the western margin of Anatolia. Our results are consistent with the interpretation of the Aegean-Anatolian system as a single, rheologically heterogeneous plate.