Rob Butler - School of Earth Sciences, The University of Leeds, Leeds LS2 9JT UK.
Mike Oehlers - NPA Group, Crockham Park, Edenbridge, Kent TN8 6SR., UK.
Kinematic descriptions of the crustal deformation associated with the Dead Sea Transform commonly fail to satisfy the predictions of large-scale models for displacements on the plate boundary. The greatest confrontation between model and observations is presented by the Lebanese restraining bend. Yet quantifying the kinematics of crustal deformation here is critical for relating the transform to the tectonic evolution of the whole Eastern Mediterranean region. Seismicity and studies of fault zone activity based on geomorphology indicate that most of the slip on the Dead Sea Transform currently transfers from its type area northwards through southern Lebanon along the Roum Fault Zone. This structure passes offshore near Beirut indicating that the main plate boundary currently steps off the Arabian continent into the eastern Mediterranean. Fault strand architecture and off-fault strains are assessed through quantitative geomorphology (using SPOT, Landsat and DEM datasets linked to field work). The Roum Fault Zone is strongly segmented with leftward-stepping left-lateral strands that individually show oblique transpression. This implies a local slip vector towards c. N330W (Arabia relative to the SE Mediterranean). From relating this to seismicity further south along the plate boundary, a modern rotation pole for Arabia vs the eastern Mediterranean may be inferred to lie at 32±1ºE, 32±1ºN, much closer to the plate boundary than the finite pole for the transform (c. 24ºW, 34ºN). Assuming a broadly constant angular velocity for Arabia vs Africa over the period of the transform, these results imply a modern displacement on the transform that is greatly reduced (c.1mm/a) compared with its long time-averaged rate (c. 6 mm/a). These lower rates are consistent with recorded seismicity in southern Lebanon. Time variable slip vectors on the plate boundary offer a solution to the discrepancy between finite fault slip, offsets and crustal shortening within the restraining bend but require a revision of larger-scale kinematic models. Future geodetic studies offer tests of these hypotheses.
To be presented at AGU Fall Meeting, San Fransisco, 2000.