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1 School of Earth Sciences, University of Leeds, Leeds, LS2 9JT, UK (e-mail: h.griffiths{at}earth.leeds.ac.uk)
2 Department of Geology, American University of Beirut, Beirut, Lebanon
Geological, geomorphological, and seismological data are used to postulate the existence of a lateral domain-bounding fault, the Roum fault zone in SW Lebanon. The fault zone accommodates transpression at the margins of the Lebanese restraining bend, abruptly dividing the transpressional Mount Lebanon (Jebel Barouk) uplift from the extension of the Tyre Nabatiye plateau.
Transpressional deformation at the SW margin of the restraining bend is mainly seen through large scale folding trending parallel to the restraining bend. Such folding is thought to be accommodated laterally along a 100 km length of fault zone in SW Lebanon, the Roum fault zone. Mapped lineaments and topographic expression show the fault zone to die out to the south of Beirut. Offsets of incised river valleys decrease northwards from 7.2 km to 0.2 km along the length of the fault zone over a distance of 80 km, also inferring a postulated fault tip to the south of Beirut. Strain gradients along both sides of the fault zone wall rocks shows several deformation mechanisms to be involved; pressure solution, folding, distributed shear, and normal faulting. The postulated fault tip coincides with the extent of transpression of the Mount Lebanon block.
A new seismicity catalogue (2100 BC–AD 1995: 32–35°N, 34–37°E: 1725 events: all magnitudes converted to ISN-reported ML) was compiled from published sources. Seismicity is apparently sparse around the northern Yammouneh fault but concentrates in SW Lebanon, especially in a diffuse 50–100 km wide zone around the southern Roum fault zone. Epicentral uncertainties are typically 10–25 km for modern reporting, although depths are poorly known. The seismic b-value is 0.75 ± 0.07 for the Beirut area compared to 0.88 ± 0.09 for the Dead Sea transform to the south: mapping of b-values for the SW Lebanon area suggests a gradual reduction northward along the Roum fault zone.
These observations are interpreted as the signature of a fault zone whose tip lies to the south of Beirut. The transition from transpression to extended crust, at the western edge of the Lebanese restraining bend, is accommodated along a 100 km length of fault zone. Decreasing seismic activity (over the time of the catalogue) and seismic b-values imply a differing style or mechanism of faulting in the short term along the Roum fault zone, toward Beirut.
Key Words: Dead Sea Rift • neotectonics • strain • seismiscity • transpression
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