Crustal and Upper Mantle Structures Beneath the Arabian Shield and Red Sea
Amri, Abdullah Al . 2017
Abstract
The Arabian Shield and Red Sea region is considered one of only a few places in the world
undergoing active continental rifting and formation of new oceanic lithosphere. We
determined the seismic velocity structure of the crust and upper mantle beneath this region
using broadband seismic waveform data. We estimated teleseismic receiver functions from
high-quality waveform data. The raw data for RF analysis consist of 3-component broadband
velocity seismograms for earthquakes with magnitudes Mw > 5.8 and epicentral distances
between 30° and 90°. We performed several state-of-the-art seismic analyses of the KACST
and SGS data. Teleseismic P- and S-wave travel time tomography provides an image of upper
mantle compressional and shear velocities related to thermal variations. We present a
multi-step procedure for jointly fitting surface-wave group-velocity dispersion curves (from 7
to 100 s for Rayleigh and 20 to 70 s for Love waves) and teleseismic receiver functions for
lithospheric velocity structure. The method relies on an initial grid search for a simple crustal
structure, followed by a formal iterative inversion, an additional grid search for shear wave
velocity in the mantle and finally forward modeling of transverse isotropy to resolve
surface-wave dispersion discrepancy. Inversions of receiver functions have poor sensitivity to
absolute velocities. To overcome this shortcoming we have applied the method of Julia et al.
(Geophys J Int 143:99–112, 2000), which combines surface-wave group velocities with
receiver functions in formal inversions for crustal and uppermost mantle velocities. The
resulting velocity models provide new constraints on crustal and upper mantle structure in the
Arabian Peninsula. While crustal thickness and average crustal velocities are consistent with
many previous studies, the results for detailed mantle structure are completely new. Finally,
teleseismic shear-wave splitting was measured to estimate upper mantle anisotropy. These
analyses indicate that stations near the Gulf of Aqabah display fast orientations that are aligned
parallel to the Dead Sea Transform Fault, most likely related to the strike-slip motion between
Africa and Arabia. The remaining stations across Saudi Arabia yield statistically the same
result, showing a consistent pattern of north-south oriented fast directions with delay times
averaging about 1.4 s. The uniform anisotropic signature across Saudi Arabia is best
explained by a combination of plate and density driven flow in the asthenosphere. By
combining the northeast oriented flow associated with absolute plate motion with the
northwest oriented flow associated with the channelized Afar plume along the Red Sea.
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