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Peace and Science in the Middle East

Interpreting the data:
The results of a seismic refraction experiment.

      We used the recorded seismograms from this experiment to construct a model of the seismic velocity structure of the earth's crust in the Dead Sea region. The seismic velocities often correspond to different rock types. Using forward and inverse ray tracing modeling techniques, we made a 2-dimensional model slice of the earth's crust along an East-West line, which crosses the Dead Sea near Sedom. A priori data, such as the existing surface topography and geology, and the geology deduced from drill holes, are used to constrain the velocity model and interpret the model in terms of subsurface geology.

      Several significant conclusions can be ascertained from the 2-dimensional model of the West-East line. (Numbered boxes in figure (b) at right correspond to numbers in text.) Box (1) in the adjacent figure shows the approximate depth to the bottom of the Dead Sea Basin. Low P-wave velocities in the top two model layers within the basin are interpreted to represent Miocene and younger basin fill of mostly continental and lacustrine sediments. The thickness of these layers (6.2 km) is similar to that encountered in Sedom Deep-1 borehole nearby (6.45 km). The third and deeper model layer under the basin is considerably slower (5-5.9 km/s) than in the surrounding regions and is interpreted to be the pre-basin sedimentary rocks, which were down-dropped by the opening of the pull-apart basin. This layer is ~4 km thick, slightly thicker than the inferred 3 km thick pre-basin, but similar in thickness to the nearly uneroded sedimentary section west of the rift valley.

      As shown in box (2) in the figure, the velocity contours of the highlands surrounding the Dead Sea Basin indicate that uplift is constrained to only the uppermost 10-15 km of the crust. Previous models of the shoulders of the basin assumed uplift involving the whole crust. If the uplift arises from bending of an elastic layer, this layer must be thin (~7 km) and confined to the upper and middle crust.

      Box (3) in the figure above, shows that the seismic velocity of the crust to a depth of 18 km under the basin is lower than in the surrounding areas. The seismic velocity under the Dead Sea, below a depth of 18 km, is similar to that in the surrounding areas.

      Box (4) in the figure above, shows that the seismic crust-mantle boundary (known as Moho) below the basin is not associated with a major depth discontinuity. However, small steps with various shapes or perturbation to the shape of the Moho (dotted lines in the figure) with an amplitude < 2.5 km, may exist and would not be detected by the seismic waves. A perturbation with this amplitude is too small to compensate isostatically for the low density of sediments in the basin, or to explain the observed large negative gravity anomaly over the Dead Sea Basin.

      The lack of isostatic compensation and the large depth of the basin evidenced in the seismic data indicates that the previous assumptions about the strength of the crust in this region do not apply. The measured surface heat flow from the Dead Sea Basin and its surrounding areas is too low (40-50 mW/m2) for the traditional explanation of ductile flow of the lower crust. We propose an alternative explanation for the mechanics of the opening of the Dead Sea Basin. The lower lithostatic pressure (rock overburden) at mid-crust levels under the basin relative to the surrounding crust and the fracturing of rock under the basin due to earthquakes at that depth (13-18 km) promote fluid flow from the surrounding crust into the middle crust below the basin. The presence of fluids within the fractured crust help create weak shear zones along which the opening is concentrated.

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