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 E-W-trending, right-lateral faulting and NE-SW, thrust-related folding result from oblique convergence at the plate boundary, which forms a transpressional system. The strain distribution obtained from fault–fold structures and P axes of focal mechanism solutions, and the geodetic (NUVEL-1 and GPS) convergence show that the shortening and convergence directions are not coaxial. The transpressional strain is partitioned along the strike and the quantitative description of the displacement field yields a compression-to-transcurrence ratio varying from 33% near Gibraltar, to 50% along the Tunisian Atlas. Shortening directions oriented NNE and NNW for the Pliocene and Quaternary, respectively, and the S shape of the Quaternary anticline axes, are in agreement with the 2.24˚/Myr to 3.9˚/Myr modeled clockwise rotation of the small tectonic blocks and with the paleomagnetic data. The convergence between Africa and Eurasia is absorbed along the Atlas Mountains at the upper crustal level, by means of thrusting above decollement systems, which are controlled by subdued transcurrent faults. The Tell Atlas of northwest Algeria, which has experienced numerous large earthquakes with respect to the other regions, is interpreted as a restraining bend that localizes the strain distribution along the plate boundary.
Highlights
Along the Gibraltar–Sicily section, the Atlas Mountains are among the most active zones in the Mediterranean region, due to the Africa–Eurasia plate convergence (Figure 1)
Large and moderate-sized shallow seismic events have been recorded over the last few decades along this plate boundary, with the largest being the October 10, 1980, El Asnam Ms 7.3 earthquake that was associated with a NWdipping, emerging thrust fault [Philip and Meghraoui 1983, Yielding et al 1989]
The thrust-and-fold belts of North Africa appear, as a zone of diffuse deformation, with no clear interplate limit striking across the Alboran Sea and the Rif and Tell Atlas Mountains [Grimison and Chen 1986, Serpelloni et al 2007]
Summary
Along the Gibraltar–Sicily section, the Atlas Mountains are among the most active zones in the Mediterranean region, due to the Africa–Eurasia plate convergence (Figure 1). The plate boundary is often indicated as an E-W straight line parallel to the Mediterranean coast, or drawn along the foreland–hinterland limit of the Tertiary fold-and-thrust belt, or inferred from hypothetical tectonic blocks This is probably due to the low level of seismicity associated with the relatively low convergence rate (3 to 6 mm/yr) [McKenzie 1972, Buforn et al 2004, Nocquet and Calais 2004], and to the complex tectonic pattern of the Tertiary contractional orogen [Frizon de Lamotte et al 2000, Faccenna et al 2004]. On the basis of some mechanical properties of shear zones, we propose a kinematic model of transpression and block rotation that accounts for the main tectonic processes along the plate boundary in North Africa
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