Seismic imaging of deep low‐velocity zone beneath the Dead Sea basin and transform fault: Implications for strain localization and crustal rigidity

Abstract

New seismic observations from the Dead Sea basin (DSB), a large pull‐apart basin along the Dead Sea transform (DST) plate boundary, show a low velocity zone extending to a depth of 18 km under the basin. The lower crust and Moho are not perturbed. These observations are incompatible with the current view of mid‐crustal strength at low temperatures and with support of the basin's negative load by a rigid elastic plate. Strain softening in the middle crust is invoked to explain the isostatic compensation and the rapid subsidence of the basin during the Pleistocene. Whether the deformation is influenced by the presence of fluids and by a long history of seismic activity on the DST, and what the exact softening mechanism is, remain open questions. The uplift surrounding the DST also appears to be an upper crustal phenomenon but its relationship to a mid‐crustal strength minimum is less clear. The shear deformation associated with the transform plate boundary motion appears, on the other hand, to cut throughout the entire crust.

Rising of the lowest place on Earth due to Dead Sea water‐level drop: Evidence from SAR interferometry and GPS

“…If this anomaly were to be compensated according to the Airy isostasy model, upper mantle should have reached to a depth of 15 km [ten Brink et al, 2006]. Seismic studies, however, place the Moho at a much greater depth of about 30 km [Ginzburg et al, 1979;ten Brink et al, 2006].…”

“…In Figure 14 we show profiles of Young modulus as a function of depth inferred from P wave velocities [ten Brink et al, 2006] and densities [Götze et al, 2007] using a Poisson's ratio of 0.31 (profiles locations are labeled as 2 and 3 on Figure 1a of ten Brink et al [2006]). Note that the inferred Young modulus within the Dead Sea basin (solid dark curve in Figure 14) is by more than a factor of two lower than that of the best fitting model for depths lower than about 3 km, and that the depth-averaged Young modulus is similar to that of the best fit model down to about 15 km.…”

“…[32] It is somewhat surprising that a simple homogeneous elastic model provides a good fit to the data on both sides of the Dead Sea, since upper-crust seismic velocities inferred from seismic tomography indicate larger values east of the Dead Sea than west of it [ten Brink et al, 2006], and since the Dead Sea transform juxtaposes Precambrian rocks in the east next to Paleozoic to Cenozoic sediments on the west [Freund et al, 1970]. In Figure 14 we show profiles of Young modulus as a function of depth inferred from P wave velocities [ten Brink et al, 2006] and densities [Götze et al, 2007] using a Poisson's ratio of 0.31 (profiles locations are labeled as 2 and 3 on Figure 1a of ten Brink et al [2006]).…”

“…Thus, isostatic compensation is not a viable mechanism for the Dead Sea basin. Instead, the upper crust may be supported by either lithospheric rigidity or an alternative mechanism [ten Brink et al, 2006].…”

Lithospheric composition and thermal structure of the Arabian Shield in Jordan

“…All three scenarios of higher (55, 60, and 65 ) predict the generation of micro-seismicity in the transition zone between the upper and lower crust, at minimum depths between 22 and 16 km, respectively. These depths are slightly lower than the focal depths for the Dead Sea basin calculated by Ten Brink et al (2006), implying earthquake clustering in the upper crust (between 8-10 and 12-16 km). The shallower observed focal depths relative to what our geotherms imply may be related to the previously discussed slight uprise of the brittle/ductile transition zone, triggered by recent geodynamic processes.…”