Rob Butler1, Helena Griffiths1 and Sara Spencer2
1Department of Earth Sciences, The University of Leeds, Leeds LS2
2Department of Geology, American University of Beirut, Beirut, Lebanon
The deformation of continental lithosphere is complex with plate boundaries represented by broad zones of deformation rather than by single discrete faults. The bulk tectonic evolution of these zones is poorly understood, commonly because of difficulties of integrating displacements across major discontinuities, later deformation obliterating earlier structures and the difficulty in resolving large-scale structure through the fog of small-scale complexity. Studies of active plate boundaries where the broad kinematics are well-established and examples where bulk displacements are relatively low offer the best opportunities for obtaining better kinematic descriptions of continental deformation. This contribution is concerned with such an example and aims to evaluate plate boundary evolution at a continental transform.
The Dead Sea fault zone of the Levant has a bulk sinistral offset of about 110km during the Cenozoic. For much of its length the fault zone is narrow and weakly transtensional. In Lebanon the fault splays into several strands to form a gross restraining bend. Significant crustal shortening has generated 3000m topography along the Mount Lebanon range. This range contains the regionally continuous Yammouneh fault which runs north into Syria. The map pattern of the Homs basalt shows a sinistral offset at the north of the range. Our fieldwork shows this to be a topographic artefact - the Homs basalt stratigraphically seals the Yammouneh fault and later drainage basins are not offset. New Ar ages on the basalt (5.02 ±0.2 Ma) show that the north of the Lebanon restraining bend has been largely inactive in Plio-Quaternary times. Active uplift continues throughout the Mount Lebanon range suggestive of distributed crustal thickening. However, at the southern end of the restraining bend, strike-slip introduced along the Dead Sea fault zone transfers via an array of relay ramps that link the southern end of the Yammouneh fault across to the Roum fault. The Roum fault is marked by rhomboid basins, intense shoulder uplift and offset drainage basins indicative of sinistral strike-slip. However, the coastal plateau to the west is cut by NE-SW normal faults. Timing and kinematics of these faults is established from uplifted marine terraces and drainage basins. The coastal plateau shows NW-SE crustal extension, presumably relaying sinistral displacements from the Dead Sea fault zone to the Levant coast. However, this distributed releasing bend formed prior to the offset of geomorphic surfaces by the Roum fault. Thus, for the past c. 5Ma, fault strands in south Lebanon have begun to develop into a distributed releasing bend while crustal shortening continues by distributed strain as a restraining bend.
The locus of deformation in the upper crust of the Levant has evolved as have the patterns of partitioning between localised faulting and more distributed strain. This may reflect a combination of work hardening in response to changes in the orientation of fault strands, variations in mechanical coupling at depth and variations in plate kinematics. The study illustrates the utility of systematic 'geomorphological stratigraphy' on understanding fault zone evolution.
Paper presented at: Annual Meeting of the Tectonic Studies Group, December 1996. University of Birmingham.