Active tectonics and fault interactions in the Ghab Valley pull-apart basin (Dead Sea fault system) in northwestern Syria

Along the northern Dead Sea fault system (DSFS) in northwestern Syria, the left-lateral transform splays into two distinct faults bounding the 70 km long Ghab Valley. Plate tectonic models predict 4 - 7 mm/yr of slip along this part of the transform, although recent GPS results from Syria suggest slower rates of faulting. Historical records document the occurrence of large earthquakes (magnitudes greater than 7) within the vicinity of the Ghab Valley and suggest that both faults bounding the valley may be active, seismogenic structures. This study integrates new neotectonic observations with other available information on the basin structure, fault slip, and earthquake recurrence in an effort to address how slip is transferred and partitioned between the faults bounding the Ghab Valley. We also investigate how this structural framework may relate to seismogenic fault segmentation. Structural and topographic relief demonstrate that uplift is asymmetrically distributed about the Ghab Valley, with greater structural relief along the western margin of the valley. Along the both the western and eastern bounding faults, late Quaternary faulting truncates alluvial fans, Quaternary lava flows, beheaded drainages, and hanging valleys. For example, in the northeastern Ghab Valley, a displaced lava flows suggests a minimum late Quaternary slip rate of approximately 1 mm/yr. Also along the eastern fault, recent faulting, including paleoseismic indicators, is also expressed in recent sediments along the fault zone. Radiocarbon dating constrains the last coseismic event within the past 1,500 years, possibly corresponding with the historically documented earthquake of 1157. These observations provide a basis for investigating fault interactions and slip partitioning using boundary element modeling. The modeling demosntrates that transfer of fault slip produces local transtension even within an overall transpressive setting. Furthermore, the obliquity of plate motion relative to the transform influences the localization of depocenters within the pull-apart basin.