Constraints on the post-25-ka slip rate of the Yammouneh 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-Roum thrust and the Yammo (u) over cap neh strike-slip fault. There are few quantitative constraints on the Quaternary slip rate along the LFS. Here we present a direct estimate of the similar to25-ka mean slip rate on the Yammo (u) over cap neh fault. Mapped offsets of alluvial fans at two sites similar to50 km apart on the eastern flank of Mount Lebanon range between 24+/-2 and 80 8 in. About 30 limestone cobbles sampled on these fans yield in situ cosmogenic Cl-36 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, continuing that the Yammo (u) over cap neh 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 Cl-36 dating of Iimestone-clast fan deposits for deciphering tectonic and geomorphic processes in the Mediterranean. (C) 2004 Published by Elsevier B.V.