Coseismic growth of sedimentary basins along the Yammouneh strike-slip fault (Lebanon)

Abstract

SUMMARY The 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.