The Dead Sea Transform

The Dead Sea Transform forms part of the Arabia-Nubia plate boundary between the Red Sea and the Bitlis suture zone in eastern Turkey (Fig. 8.3a). The southern part of this plate boundary provides an important example of a transtensional transform that has formed in relatively cool (45-53 mW m-2), strong continental lithosphere (Eckstein & Simmons, 1978; Galanis et al., 1986).

Since its inception in Middle Miocene times, approximately 105 km ofleft lateral strike-slip motion and ~4 km of fault-perpendicular extension has occurred within the southern part of the plate boundary (Quennell, 1958; Garfunkel, 1981). The component of extension was initiated during the Pliocene (Shamir et al., 2005). Horizontal velocities derived from GPS data (Section 5.8) suggest that relative motion between the Arabian and Nubian plates is occurring at the relatively slow rate of 4.3 mm a-1 (Mahmoud et al., 2005). Most of this motion is accommodated by faults that form a series of en echelon step-overs within a narrow, 20- to 40-km-wide transform valley (Fig. 8.3a). Rhomb-shaped grabens, elongate pullapart basins, and steep normal faults have formed where the fault segments step to the left. One of the largest of these extensional features is the Dead Sea Basin, which is ~135 km long, 10-20 km wide, and filled with at least 8.5 km of sediment (Fig. 8.3b).

Superficially the pull-apart basins and normal faults along the transtensional Dead Sea Transform resemble features that characterize narrow intracontinental rift basins (Section 7.2). Both types of basin typically are asymmetric, bounded by border faults, and display along-strike segmentations (Lazar et al., 2006). However, there are important differences between the two tectonic settings. Among the most significant of these is that, along transtensional transforms, the extension is confined mostly to the crust and displays minimal involvement of the upper mantle (Al-Zoubi & ten

Brink, 2002). Both gravity data (ten Brink et al., 1993) and wide-angle seismic reflection and refraction profiles (DESERT Group, 2004; Mechie et al., 2005) support this conclusion by indicating that the Moho is elevated only slightly (<2 km) under the Dead Sea Basin. These characteristics suggest that, although extension influences the surface morphology and shapes of extensional basins that form along transforms, it does not play a dominant role in shaping the deep structure of the fault system (Section 8.6.2) like it does in rift basins.

Seismic reflection and refraction data collected across the Arava Fault (Fig. 8.3a) reveal the deep structure of the Dead Sea Transform. Beneath the surface trace of the fault, the base of a 17- to 18-km-thick upper crust (seismic basement) is vertically offset by 3-5 km (DESERT Group, 2004; Mechie et al., 2005). The fault descends vertically into the lower crust where it broadens downward into a zone of ductile deformation (Fig. 8.11). The width of this lower crustal zone is constrained by a ~15-km-wide gap in a series of strong subhorizontal reflectors. These reflectors may represent either compositional contrasts related to lateral displacements within a narrow zone or the effects of localized horizontal flow (Al-Zoubi & ten Brink, 2002). Below the gap, the Moho displays a small amount of topography, suggesting that a narrow zone of deformation beneath the Arava Fault may extend into the mantle.

These physical characteristics provide important constraints on the dynamics of transform faults. The results from the DESERT geophysical survey (DESERT Group, 2004; Mechie et al., 2005) suggest that the ~105 km of left lateral displacement between the Arabian and Nubian plates (Fig. 8.12a) has resulted in a profile with a significantly different crustal structure east and west of the Arava Fault (Fig. 8.12b). The occurrence of extension and transtension between fault segments results in localized subsidence and crustal flexure west of the fault and a minor, similar deflection of the Moho (Fig. 8.12c). Erosion and sedimentation result in the present day structure of the plate boundary (Fig. 8.12d).

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