Robert W.H. Butler1 (butler@earth.leeds.ac.uk)
Helena Griffiths1 (h.griffiths@earth.leeds.ac.uk)
Sara Spencer2 (spencer@layla.aub.ac.lb)
1 Dept Earth Sciences, The University of Leeds, UK
2 Dept Geology, American University of Beirut, Lebanon.
A major goal in tectonic studies is understanding how deformation is distributed in space and time within broad plate boundary zones characteristic of the continents. Integrating structural mapping with geomorphology and stratigraphy offers a useful approach by quantifying and dating vertical and horizontal movements. Dated land surfaces of known original elevation, such as coastal or lacustrine plains, are ideal for tracing vertical movements and incremental throws on faults. The map-patterns of drainage basins, corrected for stream capture, chart horizontal offsets and may be dated if linked to valley fills or known base level histories. These geomorphological tools are applied to analyse the post-Miocene evolution of the Dead Sea Transform system in Lebanon. Such studies are critical for resolving questions of plate boundary continuity through the eastern Mediterranean and for better evaluation of seismic hazards, particularly for the redevelopment of Beirut. To date, seismicity has provided only a vague picture of the distribution of active deformation because of regionally incomplete instrument cover and the low numbers of high magnitude earthquakes. The challenge is to document the relationships between vertical and horizontal movements while establishing which faults have been active over the past few Myr. Chronological constraints on geomorphic surfaces are provided by dated basalt flows and uplifted marine terraces while river valleys may be dated by linking onland drainage basins to incised canyons offshore that record end-Miocene Mediterranean lowstand. Our studies show that the regionally extensive Yammouneh fault has not been active in north Lebanon for the past 5 Ma. Plio-Quaternary wrench tectonics has focused on the Roum fault which forms part of a relay-ramp system, including the southernmost portion of the Yammouneh, linking the Dead Sea system to offshore faults. The Roum fault is marked by asymmetric flank uplift of about 250m acting on a regional uplift of western Lebanon that initiated in the Miocene. The interaction of uplift and wrenching is a product of local transpression between overlapping transform segments. Our results suggest that the coastal Lebanon has a greater risk of devastating earthquakes than the previously proposed Bekaa and northern Yammouneh areas.
Paper presented at: European Union of Geoscientists, Biannual Meeting, Strasbourg. 1997.