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.
The Rachaya and Serghaya faults are the easternmost fault branches of the Dead Sea Transform Fault within the Lebanese restraining bend. They lie east of the Yammouneh fault (the main strand of the Dead Sea Transform Fault within the restraining bend), extend along the western and eastern flanks of the Anti‐Lebanon range, respectively, and show left‐lateral strike‐slip movement manifested as offset drainage. We studied both faults through combined field investigations in geomorphology and paleoseismology. Young fault scarps, mole tracks, pressure ridges and offset streams detected along the faults' traces attest to recent coseismic ruptures. Two paleoseismic investigations highlight their seismogenic potential and indicate earthquake recurrence along them: the Rachaya and Serghaya faults are active and the sources of recent historical earthquakes, the last of which might be the 30 October–25 November 1759 (Ms 6.6 and 7.4) earthquake sequence that caused severe damage in the eastern Mediterranean region. Such a possible correlation suggests that the two faults are probably structurally interconnected, as movement on one fault may stimulate movement on the other fault. In addition, both faults may define together an active seismogenic fault system that accommodates some of the regional displacement that takes place within the Lebanese restraining bend. Our results highlight that the seismogenic potential of the Rachaya and Serghaya faults must be included in any seismic hazard assessment of the region.
S U M M A R YThe left-lateral Yammouneh fault (YF) is the main active branch of the Dead Sea Transform Fault (DSTF) within the Lebanese restraining bend. Despite the overall transpressional setting, a series of sedimentary basins have developed along the trace of the YF. Consequently, palaeoseismic studies within these basins provide an opportunity to study the processes of coseismic growth of the basins, as well as elucidate earthquake behaviour of the fault, in general. Geodetic measurements of contemporary fault slip within the Lebanese restraining bend indicate that the YF accommodates most of the expected left-lateral strike-slip motion, despite the apparent lack of present-day seismicity. We studied the YF, using combined investigations of remote imagery, geomorphology and palaeoseismology. The active fault trace along a 51 km strip was delineated as relatively young surface ruptures and fault scarps that affect Holocene deposits with intermittent offset geomorphic markers. Seven closed basins that occur along-strike of the YF, were found to be related to faulting, with at least three of them displaying evident pull-apart settings. We concentrated our work on the rhombohedral Yammouneh basin, an actively evolving example of pull-apart basins, which is presently obliquely cut by the active fault, with an apparently young age of 1.4 ± 0.3 Myr. 3-D correlation and analysis of palaeoseismic investigations exposed a composite shear zone with a total subsidence that exceeds 1.6 m over the past 4000-4400 yr. Stratigraphic and geochronological constraints suggest the occurrences of at least five large faulting events during that period. By correlating the stratigraphy and ages of this trench with a previously published study located nearby, a combined palaeoseismic history for the past five events is constructed. This suggests a mean recurrence period of 1020-1175 yr for large earthquakes along this section of the YF. Our results suggest a subsidence rate due to faulting of about 0.36-0.40 mm yr −1 within the Yammouneh pull-apart basin. The geometrical relationship between the active fault trace and the older fault traces suggests that the basin may have evolved in a two stage process in which the original releasing fault bend developed a cross-cutting fault, accommodating most of the subsequent strike-slip displacement. The role of the YF in controlling basin evolution is further supported by the correspondence of the active fault with the boundary of the present depocentre. Our results highlight the interrelation between active strike-slip faulting and the coseismic incremental evolution of fault-related sedimentary basins, which may provide a conceptual model with broader implications on analogous tectonic systems worldwide.
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