R.W.H. Butler1, H.M. Griffiths1, R.A. Clark1, K. Thorp1, M. Oehlers2 & S. Spencer3
1 Dept. Earth Sci. Univ. Leeds, Leeds LS2 9JT
2 NPA Group, Crockham Park, Kent TN8 6SR.
3 Dept. Geology, American University of Beirut, Lebanon
The Dead Sea Transform system in Lebanon is represented by a distributed swathe of seismogenic faulting together with broad folds, collectively accommodating regional transpression. Geological and geomorphological data suggest that, for the past 5-6 Myr, the Roum Fault system has been an active strike-slip structure linking the transform in the Jordan valley to the Mediterranean coast in the Beirut area. Estimates for the total sinistral slip on this fault vary from at least 9 km up to 30km during this time. Nevertheless, it currently does not present a continuous surface expression that might be evident on satellite images. Here we integrate Landsat imagery with field observations to identify an array of segmented strike-slip fault strands which presumably overprint an earlier, continuous, fault zone. Additionally, we have compiled a new seismicity catalogue for the region.. Provisional estimates of b-values for the Yammouneh-Roum Faults area and the main Dead Sea Transform further south are 0.65+/-0.02 and 0.86+/-0.03 respectively, reflecting a contrast in seismotectonic style between them. Earthquake hypocentres, relocated by the Joint Hypocentre Determination method, are compared with the segmented pattern identified by structural analysis. We tentatively relate the change in transform structure, from continuous to segmented, to an evolution in plate kinematics. The current structure in south Lebanon is marked by arrays of left-stepping sinistral transcurrent faults which locally trend rightwards and are associated with local transpression. Distributed seismogenic faulting presumably reflects this combination of deformation styles.
Paper presented at Geosciences 98, Keele University, April 1998 (Active Continental Tectonics Session)