restraining bend of the Dead Sea Fault (Lebanon and SW Syria) Strain partitioning of active transpression within the Lebanese Geological Society, London, Special Publications

Recent neotectonic, palaeoseismic and GPS results along the central Dead Sea fault system elucidate the spatial distribution of crustal deformation within a large (c.180-km-long) restraining bend along this major continental transform. Within the ‘Lebanese’ restraining bend, the Dead Sea fault system splays into several key branches, and we suggest herein that active deformation is partitioned between NNE–SSW strike-slip faults and WNW–ESE crustal shortening. When plate motion is resolved into strike-slip parallel to the two prominent NNE– SSW strike-slip faults (the Yammouneh and Serghaya faults) and orthogonal motion, their slip rates are sufficient to account for all expected strike-slip motion. Shortening of the Mount Lebanon Range is inferred from the geometry and kinematics of the Roum Fault, as well as preliminary quantification of coastal uplift. The results do not account for all expected crustal shortening, suggesting that some contraction is probably accommodated in the Anti-Lebanon Range. It also seems unlikely that the present kinematic configuration characterizes the entire Cenozoic history of the restraining bend. Present-day strain partitioning contrasts with published observations on finite deformation in Lebanon, demonstrating distributed shear and vertical-axis block rotations. Furthermore, the present-day proportions of strike-slip displacement and crustal shortening are inconsistent with the total strike-slip offset and the lack of a significantly thickened crust. This suggests that the present rate of crustal shortening has not persisted for the longer life of the transform. Hence, we suggest that the Lebanese restraining bend evolved in a polyphase manner, involving an earlier episode of wrench-faulting and block rotation, followed by a later period of strain partitioning. Restraining bends, by definition, involve plate motions that are oblique to the associated transcurrent faults. An important question pertains to how oblique convergence is accommodated by crustal deformation. In an active tectonic setting, understanding the spatial distribution of crustal strain is important for assessing the earthquake hazard, as well as for understanding the tectonic evolution of the restraining bend. The Dead Sea fault system (DSFS) is a prominent continental transform in the eastern Mediterranean region (Fig. 1). Along this transform system, a large (c.180-km-long) restraining bend along the Dead Sea fault system encompasses present-day Lebanon and SW Syria. Recent palaeoseismic (Gomez et al. 2001, 2003; Daeron et al. 2004, 2005; Nemer & Meghraoui 2006) and geodetic (e.g. Wdowinski et al. 2004; Reilinger et al. 2006) studies have shown that the strike-slip faults are unambiguously active within the Lebanese restraining bend, refuting suggestions to the contrary (e.g. Butler et al. 1997, 1998). The ‘Lebanese’ restraining bend of the DSFS also can serve as an analogue for other large restraining bends along continental transform systems, including the Big Bend of the San Andreas Fault (e.g. Chaimov et al. 1990). In contrast to the Big Bend, the Lebanese restraining bend appears to be structurally less complicated, with fewer fault branches. Hence, it may be easier to assess the distribution and controls of crustal deformation, owing to the relative simplicity of the structure. This paper synthesizes new results and recently published data to assemble a kinematic model for present-day tectonism in the Lebanese restraining From: CUNNINGHAM, W. D. & MANN, P. (eds) Tectonics of Strike-Slip Restraining and Releasing Bends. Geological Society, London, Special Publications, 290, 285–303. DOI: 10.1144/290.10 0305-8719/07/$15.00 # The Geological Society of London 2007. Fig. 1. Regional tectonic map of the Dead Sea fault system. Directions (white arrows) and rates (mm/a) of motion of the Arabian plate relative to the Sinai plate are based on the plate model of Reilinger et al. (2006). Abbreviations: JAA, Jebel Abdel Aziz; EFS, Euphrates fault system; NAF, North Anatolian fault; EAF, East Anatolian fault. Inset depicts the plate-tectonic setting of the Arabian–Eurasian collision. F. GOMEZ ET AL. 286