The northern extension of the Dead Sea transform fault in southern Lebanon
bifurcates into several faults that cross Lebanon from south to north. The main
strand, the Yammouneh fault, marks the boundary between the Levantine (eastern
Mediterranean) and Arabian plates and separates the western mountain range (Mount
Lebanon) from the eastern mountain range (Anti-Lebanon). Bouguer gravity contours
in Lebanon approximately follow topographic contours; i.e., positive Bouguer
anomalies are associated with the Mount Lebanon and Anti-Lebanon ranges. This
suggests that the region is not in simple isostatic compensation. Gravity observations
based on 2.5-dimensional modeling and other available geological and geophysical
information have produced the following interpretations. (1) The crust of Lebanon
thins from 35 km beneath the Anti-Lebanon range, near the Syrian border, to
27 km beneath the Lebanese coast. No crustal roots exist beneath the Lebanese
ranges. (2) The depth to basement is 3.5-6 km below sea level under the ranges
and is 8-10 km beneath the Bekaa depression. (3) The Yammouneh fault bifurcates
northward into two branches; one passes beneath the Yammouneh Lake through the
eastern part of Mount Lebanon and another bisects the northern part of the Bekaa
Valley (i.e., Mid-Bekaa fault). The Lebanese mountain ranges and the Bekaa depression
were formed as a result of transtension and later transpression associated with
the relative motion of a few crustal blocks in response to the northward movement
of the Arabian plate relative to the Levantine plate.
This paper was published in Geology by the Geological Society of America (GSA), and GSA retains the copyright.
Geological Society of America, P.O. Box 9140, Boulder, CO 80301-9140
Figure 3. Crustal models based on modeling Bouguer gravity anomalies along profiles A-A', B-B', and C-C'. Exaggerated topography and observed (dotted line) and calculated (solid line) Bouguer gravity values are also shown. Density values are in g/cm^3. See Figures 1 and 2 for locations of profiles