Active faulting and transpression tectonics along the plate boundary in North Africa

Abstract

We present a synthesis of the active tectonics of the northern Atlas Mountains and suggest a kinematic model of transpression and block rotation that illustrates the mechanics of this section of the Africa - Eurasia plate boundary. Neotectonic structures and significant shallow seismicity (with Mw > 5.0) indicate that coeval east-west trending right-lateral faulting and NE-SW thrust-related folding, result from the oblique convergence at the plate boundary, and form a transpressional system. The strain distribution obtained from fault - fold structures and P axes of focal mechanism solutions, and the geodetic (NUVEL1 and GPS) convergence shows that shortening and convergence directions are not coaxial. The transpressional strain is partitioned along strike and the quantitative description of displacement field yields a compression to transcurrence ratio varying from 33% near Gibraltar, to 50% along the Tunisian Atlas. Shortening directions oriented NNE to NNW for the Pliocene and Quaternary, respectively, and the S shape of Quaternary anticline axes are in agreement with the 2.24° - 3.9 °/Myr modeled clockwise rotation of small tectonic blocks and paleomagnetic results. The convergence between Africa and Eurasia is absorbed along the Atlas Mountains, at the upper crustal level, by means of thrusting above decollement systems, controlled by subdued transcurrent faults. The Tell Atlas of NW Algeria that experienced numerous large earthquakes with respect to other regions is interpreted as a restraining bend that localizes strain distribution along the plate boundary.