Kinematics of the Iberia-Maghreb plate contact from seismic moment tensors and GPS observations

The Iberian Peninsula and the Maghreb experience moderate earthquake activity and oblique, similar to NW-SE convergence between Africa and Eurasia at a rate of similar to 5 mm/yr. Coeval extension in the Alboran Basin and a N35 degrees E trending band of active, left-lateral shear deformation in the Alboran-Betic region are not straightforward to understand in the context of regional shortening, and evidence complexity of deformation at the plate contact. We estimate 86 seismic moment tensors (M-W 3.3 to 6.9) from time domain inversion of near-regional waveforms in an intermediate period band. Those and previous moment tensors are used to describe regional faulting style and calculate average stress tensors. The solutions associated to the Trans-Alboran shear zone show predominantly strike-slip faulting, and indicate a clockwise rotation of the largest principal stress orientation compared to the regional convergence direction (a, at N350 degrees E). At the N-Algerian and SW-Iberian margins, reverse faulting solutions dominate, corresponding to N350 degrees E and N310 degrees E compression, respectively. Over most of the Betic range and intraplate Iberia, we observe predominately normal faulting, and WSW-ENE extension (sigma(3) at N240 degrees E). From GPS observations we estimate that more than 3 mm/yr of African (Nubian)-Eurasian plate convergence are currently accommodated at the N-Algerian margin, similar to 2 mm/yr in the Moroccan Atlas, and similar to 2 mm/yr at the SW-Iberian margin. 2 mm/yr is a reasonable estimate for convergence within the Alboran region, while Alboran extension can be quantified as similar to 2.5 mm/yr along the stretching direction (N240 degrees E). Superposition of both motions explains the observed left-lateral transtensional regime in the Trans-Alboran shear zone. Two potential driving mechanisms of differential motion of the Alboran-Betic-Gibraltar domain may coexist in the region: a secondary stress source other than plate convergence, related to regional-scale dynamic processes in the upper mantle of the Alboran region, as well as drag from the continental-scale motion of the Nubian plate along the southern limit of the region. In the Atlantic Ocean, the similar to 3.5 mm/yr, westward motion of the Gibraltar Arc relative to intraplate Iberia can be accommodated at the transpressive SW-Iberian margin, while available GPS observations do not support an active subduction process in this area. (c) 2006 Elsevier B.V. All rights reserved.