BOUGUER GRAVITY AND CRUSTAL STRUCTURE OF THE DEAD-SEA TRANSFORM-FAULTAND ADJACENT MOUNTAIN BELTS IN LEBANON

The northern extension of the Dead Sea transform fault in southern Leb anon bifurcates into several faults that cross Lebanon from south to n orth. The main strand, the Yammouneh fault, marks the boundary between the Levantine (eastern Mediterranean) and Arabian plates and separate s the western mountain range (Mount Lebanon) from the eastern mountain range (Anti-Lebanon). Bouguer gravity contours in Lebanon approximate ly follow topographic contours; i.e., positive Bouguer anomalies are a ssociated with the Mount Lebanon and Anti-Lebanon ranges. This suggest s that the region is not in simple isostatic compensation. Gravity obs ervations based on 2.5-dimensional modeling and other available geolog ical and geophysical information have produced the following interpret ations. (1) The crust of Lebanon thins from approximately 35 km beneat h the Anti-Lebanon range, near the Syrian border, to approximately 27 km beneath the Lebanese coast. No crustal roots exist beneath the Leba nese ranges. (2) The depth to basement is approximately 3.5-6 km below sea level under the ranges and is approximately 8-10 km beneath the B ekaa depression. (3) The Yammouneh fault bifurcates northward into two branches; one passes beneath the Yammouneh Lake through the eastern p art of Mount Lebanon and another bisects the northern part of the Beka a Valley (i.e., Mid-Bekaa fault). The Lebanese mountain ranges and the Bekaa depression were formed as a result of transtension and later tr anspression associated with the relative motion of a few crustal block s in response to the northward movement of the Arabian plate relative to the Levantine plate.