Kinematics of the Western Africa-Eurasia Plate Boundary From Focal Mechanisms and GPS Data

We used earthquake and GPS data to study the present-day kinematics and tectonics of the Africa-Eurasia plate boundary in the Western Mediterranean. Crustal seismicity and focal mechanisms outline the geometry of major seismic belts and characterise their tectonics and kinematics. Continuous GPS data have been analyzed to determine Euler vectors for the Nubian and Eurasian plates and to provide the global frame for a new Mediterranean GPS velocity field, obtained by merging continuous and campaign observations. GPS velocities and displacements predicted by the Nubia-Eurasia pole provide estimates of the deformation accommodated across the tectonically active belts. Our analysis reveals a more complex fragmentation of the plate boundary than previously proposed. The roughly E-W trending mainly compressive segments (i.e., southwestern Iberia, northern Algeria and southern Tyrrhenian), where plate convergence is largely accomodated across rather localized deformation zones, and partially transferred northward to the adjacent domains (i.e., the Algero-Balearic and Tyrrhenian basins), are interrupted by regions of more distributed deformation (i.e., the Rif-Alboran-Betics, Tunisia-Libya and eastern Sicily deformation zones), or limited seismicity (i.e., the Strait of Sicily), which are characterized by less homogeneous regimes (mainly transcurrent to extensional). In correspondence of the observed breaks, tectonic structures with different orientation interfere, and we find belts with only limited deformation (i.e., the High and Mid Atlas, the Tunisian Atlas and the offshore Tunisia-Libya belt) that extends from the plate boundary into the Nubian plate, along pre-existing tectonic lineaments. Our analysis suggest that the Sicilian-Pelagian domain is moving independently from Nubia, according to the presence of a right-lateral and extensional decoupling zone corresponding to the Tunisia-Libya and Strait of Sicily deformation zone. Despite the space variability of active tectonic regimes, plate convergence still governs most of the seismotectonic and kinematic setting up to the central Aeolian region. In general, local complexities derive from pre-existing structural features, inherited from the tectonic evolution of the Mediterranean region. On the contrary, along Calabria and the Apennines the contribution of the subducted Ionian oceanic lithosphere and the occurrence of microplates (i.e., Adria) appear to substantially modify both tectonics and kinematics. Finally, GPS data across the Gibraltar Arc and the Tyrrhenian-Calabria domain support the hypothesis that slab rollback in these regions is mostly slowed down or stopped.