Present-day dynamics of the Aegean region: A model analysis of the horizontal pattern of stress and deformation

In order to gain quantitative insight into the forces that control the present-day stress field and pattern of horizontal motions in the Aegean region, we adopt a forward model approach. Mechanisms that have been proposed to explain the present-day tectonics are represented, to first approximation, in terms of the expected force distributions. On the basis of a thin elastic shell representation of the Aegean lithosphere we then calculate the stress field and displacements associated with these force sets and compare the results with observations of the state of stress based on geological studies of fault kinematics, strain as expressed by earthquake focal mechanisms, and horizontal motions measured by means of satellite geodesy. The two most commonly cited mechanisms suggested to control the Aegean tectonics, (1) the push exerted by the westward moving Anatolian block and (2) forces associated with the Hellenic subduction zone, are first analyzed separately. It is found that the westward Anatolian push alone does not explain the observed prevalence of tensional stress. This is also expressed in a mismatch between the displacements computed to result from the westward push and the observed horizontal velocities. Regarding the forces on the Aegean overriding margin, we find that a model with outward pulling forces of uniform magnitude, acting normal to the arc from the SW Peloponnese to Rhodes, yields a stress field that matches the observed pattern of tension to a large extent. This distribution of forces is consistent with the notion of gravitational spreading of the Aegean lithosphere. The pattern of seismic strain of the overriding margin may evidence the occurrence of a small additional resistive force. Although the prevalence of tension thus appears due mainly to the subduction-related forces, other aspects of the stress field and also the pattern of horizontal motions indicate that these forces act in combination with westward push. Our model results allow us to address the relation between the forces experienced by the upper plate and the kinematics of subduction, not just in terms of a vertical cross section but also by taking into account the along-arc dimension.