Strain accommodation at the lateral margin of an active transpressive zone: geological and seismological evidence from the Lebanese restraining bend

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 L ebanese restraining bend, abruptly dividing the transpressional Mount Leban on (Jebel Barouk) uplift from the extension of the Tyre Nabatiye plateau. Transpressional deformation at the SW margin of the restraining bend is mai nly seen through large scale folding trending parallel to the restraining b end. Such folding is thought to be accommodated laterally along a 100 km le ngth of fault zone in SW Lebanon, the Roum fault zone. Mapped lineaments an d topographic expression show the fault zone to die out to the south of Bei rut. 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 infe rring a postulated fault tip to the south of Beirut. Strain gradients along both sides of the fault zone wall rocks shows several deformation mechanis ms to be involved; pressure solution, folding, distributed shear, and norma l faulting. The postulated fault tip coincides with the extent of transpres sion of the Mount Lebanon block. A new seismicity catalogue (2100 BC-AD 1995: 32-35 degrees N, 34-37 degrees E: 1725 events: all magnitudes converted to ISN-reported M-L) was compiled from published sources. Seismicity is apparently sparse around the norther n Yammouneh fault but concentrates in SW Lebanon, especially in a diffuse 5 0-100 km wide zone around the southern Roum fault zone. Epicentral uncertai nties are typically 10-25 km for modern reporting, although depths are poor ly 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-val ues for the SW Lebanon area suggests a gradual reduction northward along th e Roum fault zone. These observations are interpreted as the signature of a fault zone whose t ip lies to the south of Beirut. The transition from transpression to extend ed crust, at the western edge of the Lebanese restraining bend, is accommod ated along a 100 km length of fault zone. Decreasing seismic activity lover the time of the catalogue) a nd seismic b-values imply a differing style o r mechanism of faulting in the short term along the Roum fault zone, toward Beirut.