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Isostatic gravity anomalies, Density structure of
the upper mantle, Moho, Sedimentary basins, Gravity modeling
Abstract:
The isostatic modeling represents one of the most
useful ‘‘geological’’ reduction methods of the gravity field. With
the isostatic correction, it is possible to remove a significant part of
the effect of deep density heterogeneity, which dominates in the
Bouguer gravity anomalies. Although there exist several isostatic
compensation schemes, it is usually supposed that a choice of the
model is not an important factor to first order, since the total weight
of compensating masses remains the same. We compare two
alternative models for the Arabian plate and surrounding area. The
Airy model gives very significant regional isostatic anomalies,
which cannot be explained by the upper crust structure or disturbances
of the isostatic equilibrium. Also, the predicted ‘‘isostatic’’
Moho is very different from existing seismic observations. The
second isostatic model includes the Moho, which is based on
seismic determinations. Additional compensation is provided by
density variations within the lithosphere (chiefly in the upper
mantle). According to this model, the upper mantle under the
Arabian Shield is less dense than under the Platform. In the Arabian
platform, the maximum density coincides with the Rub’ al Khali,
one of the richest oil basin in the world. This finding agrees with
previous studies, showing that such basins are often underlain by
dense mantle, possibly related to an eclogite layer that has caused
their subsidence. The mantle density variations might be also a
result of variations of the lithosphere thickness. With the combined
isostatic model, it is possible to minimize regional anomalies over
the Arabian plate. The residual local anomalies correspond well to
tectonic structure of the plate. Still very significant anomalies,
showing isostatic disturbances of the lithosphere, are associated
with the Zagros fold belt, the collision zone of the Arabian and Eurasian plates.