Constraints on the post ∼25-ka slip rate of the Yammoûneh fault (Lebanon) using in situ cosmogenic 36Cl dating of offset limestone-clast fans

The most active seismogenic structure along the eastern shore of the Mediterranean is the N–S-trending left-lateral Levant Fault System (LFS), the plate boundary between Arabia and Africa. In Lebanon, it forms a 160-km-long restraining bend responsible for the uplift of Mount Lebanon. The resulting transpression is partitioned between the offshore Tripoli–Roûm thrust and the Yammoûneh strike–slip fault. There are few quantitative constraints on the Quaternary slip rate along the LFS. Here we present a direct estimate of the ∼25-ka mean slip rate on the Yammoûneh fault. Mapped offsets of alluvial fans at two sites ∼50 km apart on the eastern flank of Mount Lebanon range between 24±2 and 80±8 m. About 30 limestone cobbles sampled on these fans yield in situ cosmogenic 36Cl exposure ages mostly between 6 and 27 ka. A statistical assessment of offsets versus ages provides bounds on the Late Pleistocene–Holocene slip rate on the fault: 3.8–6.4 mm/yr. These results are consistent with long-term geological inferences, confirming that the Yammoûneh fault is the main strike–slip branch of the LFS in Lebanon. They illustrate both the potential and the difficulties of using in situ cosmogenic 36Cl dating of limestone-clast fan deposits for deciphering tectonic and geomorphic processes in the Mediterranean.