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.