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American University of Beirut, Beirut, Lebanon
The Levant Fracture System is a transform plate boundary that connects the extensional Red Sea spreading centre, in the south, with the collisional Tauride region, in the north, across the Levant. The debate regarding its origin, evolution and development is reviewed, with proposed models ranging from a primordial geosuture, along which only vertical movements have occurred, to a Neogene transform, along which the Arabian Plate has moved 107 km left-laterally northwards in two stages, 62 km in the Miocene and 45 km in the Pliocene-Recent. Overwhelming field evidence supports this degree of motion south of Lebanon but in Lebanon, where the system branches into a braided fault system generally trending north-northeast, evidence for such motion is lacking. Proponents of the transform model have explained away this lack of evidence by invoking division of the total horizontal motion to the south amongst the main fault strands across Lebanon and ascribing the uplift/ depression of the Lebano-Palmyride structural units to transpressional-transtensional deformation by this horizontal motion. The inversion of the Lebano-Palmyride belts had, in fact, been largely achieved by the Late Oligocene as a consequence of oblique collision between the Eurasian-Arabian Plates during closure of the Neo-Tethys. The Neogene initiation of the Levant Fracture System, resulting from the opening of the Gulf of Aden-Red Sea, could only, therefore, have modified existing structures, not been responsible for them. Pliocene bypassing of the principal Lebanese Yammouneh Fault requires that active motion in the south be transmitted through other faults. The currently active north-south Roum Fault has been proposed as the principal candidate and its trace northwards appears identifiable on satellite images. Field checks suggest it has broken into short left-stepping segments and it probably cuts the coast obliquely just south of Beirut. Current research to establish the geometry and magnitude of horizontal motion on this fault is discussed.
Deceased, reprint requests should be addressed to the editors.
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