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Fault activity within the Lebanese transpression zone, one of the classic examples of restraining bend development on a continental transform, is here investigated using dated geomorphological features. The northern part of the Yammouneh Fault, commonly considered to be the principal active strand on this part of the Dead Sea Transform, has been inactive for the past 5 Ma. Field observations show that basalts, dated as late Pliocene in age, apparently offset by the Yammouneh fault, unconformably overlie it. The active transcurrent structure is principally, perhaps exclusively, the Roum Fault. The Lebanese transpressive zone has evolved through time, with migration of fault activity. These results confirm the overlapping transform hypothesis for the Dead Sea system, require the active triple junction between the transform and the Tethyan collision belt to lie offshore SE Cyprus, and have profound implications for assessing seismic hazard in the Levant.

Structural evolution along continental transform faults may be related to fault zone geometry and to regional variations in plate kinematics. Using a case study of the Lebanese sector of the Dead Sea Transform, the finite geometry of transpression at a restraining bend can be shown to have evolved in time. Relative structural chronologies, calibrated against dated landscape features such as lava-covered palaeosurfaces, coastal erosion surfaces and their incised drainage basins, are used to establish the timing of displacement activity on the major transcurrent faults. The early part of the region’s structural history, up to late Miocene times, was controlled by the geometry of the through-going Yammouneh Fault. Transpression on this right-trending left-lateral structure was accommodated by strike-slip and distributed crustal shortening represented by the initial uplift of Mount Lebanon. For the past 6 Ma the principal active strand of the transform has been the Roum Fault. For much of this period it is presumed to have been a through-going fault which accommodated about 30 km left-lateral displacement. During the Quaternary, the fault zone has become strongly segmented. Although the location of active transcurrent faulting has migrated during the history of the transform, the major site of crustal shortening, the Mount Lebanon-Jabel Barouk structure, has remained broadly fixed. However, the rates of amplification of this structure and the coastal flexure appear to have varied. Continuing uplift of Mount Lebanon and local Plio-Quaternary folding suggest that the offshore continuation of the Roum Fault contains a rightwards, transpressive bend. We relate this multistage history for the Lebanese sector of the transform to an evolving plate tectonic setting: the rotation pole for relative plate motion between Africa and Arabia has migrated through time and the triple junction between the transform and the Tethyan destructive plate margin to the north has moved from onshore SE Turkey to now lie in the NE Mediterranean. Our case study illustrates the transient and evolving nature of deformation in continental restraining bends.

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 - 1995: 32-35 N, 34-37 E: 1725 events: all magnitudes converted to ISN-reported M L ) 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.