Page 238 - Global Tectonics

224 CHAPTER 8

8.3 THE DEEP Brink, 2002). Both gravity data (ten Brink et al., 1993)

and wide-angle seismic reflection and refraction pro-

STRUCTURE OF files (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
CONTINENTAL Basin. These characteristics suggest that, although
extension influences the surface morphology and

TRANSFORMS shapes of extensional basins that form along trans-
forms, it does not play a dominant role in shaping the
deep structure of the fault system (Section 8.6.2) like
8.3.1 The Dead Sea Transform it does in rift basins.

Seismic reflection and refraction data collected
across the Arava Fault (Fig. 8.3a) reveal the deep struc-
The Dead Sea Transform forms part of the Arabia– ture of the Dead Sea Transform. Beneath the surface
Nubia plate boundary between the Red Sea and the trace of the fault, the base of a 17- to 18-km-thick upper
Bitlis suture zone in eastern Turkey (Fig. 8.3a). The crust (seismic basement) is vertically offset by 3–5 km
southern part of this plate boundary provides an impor- (DESERT Group, 2004; Mechie et al., 2005). The fault
tant example of a transtensional transform that has descends vertically into the lower crust where it broad-
−2
formed in relatively cool (45–53 mW m ), strong conti- ens downward into a zone of ductile deformation (Fig.
nental lithosphere (Eckstein & Simmons, 1978; Galanis 8.11). The width of this lower crustal zone is con-
et al., 1986). strained by a ∼15-km-wide gap in a series of strong
Since its inception in Middle Miocene times, approxi- subhorizontal reflectors. These reflectors may repre-

mately 105 km of left lateral strike-slip motion and ∼4 km sent either compositional contrasts related to lateral
of fault-perpendicular extension has occurred within the displacements within a narrow zone or the effects of
southern part of the plate boundary (Quennell, 1958; localized horizontal flow (Al-Zoubi & ten Brink, 2002).

Garfunkel, 1981). The component of extension was initi- Below the gap, the Moho displays a small amount of
ated during the Pliocene (Shamir et al., 2005). Horizontal topography, suggesting that a narrow zone of deforma-
velocities derived from GPS data (Section 5.8) suggest tion beneath the Arava Fault may extend into the
that relative motion between the Arabian and Nubian mantle.
−1
plates is occurring at the relatively slow rate of 4.3 mm a These physical characteristics provide important
(Mahmoud et al., 2005). Most of this motion is accom- constraints on the dynamics of transform faults. The
modated by faults that form a series of en echelon results from the DESERT geophysical survey (DESERT
step-overs within a narrow, 20- to 40-km-wide transform Group, 2004; Mechie et al., 2005) suggest that the ∼105 km
valley (Fig. 8.3a). Rhomb-shaped grabens, elongate pull- of left lateral displacement between the Arabian and

apart basins, and steep normal faults have formed where Nubian plates (Fig. 8.12a) has resulted in a profile with a
the fault segments step to the left. One of the largest of significantly different crustal structure east and west of

these extensional features is the Dead Sea Basin, which the Arava Fault (Fig. 8.12b). The occurrence of exten-
is ∼135 km long, 10–20 km wide, and fi lled with at least sion and transtension between fault segments results in
8.5 km of sediment (Fig. 8.3b). localized subsidence and crustal flexure west of the fault

Superficially the pull-apart basins and normal faults and a minor, similar deflection of the Moho (Fig. 8.12c).

along the transtensional Dead Sea Transform resemble Erosion and sedimentation result in the present day
features that characterize narrow intracontinental rift structure of the plate boundary (Fig. 8.12d).
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, 8.3.2 The San Andreas Fault
there are important differences between the two tec-
tonic settings. Among the most significant of these is

that, along transtensional transforms, the extension is The San Andreas Fault formed in Oligocene times

confined mostly to the crust and displays minimal (Atwater, 1970, 1989) when the Pacifi c–Farallon spread-
involvement of the upper mantle (Al-Zoubi & ten ing ridge collided with the western margin of North