Crustal and Upper Mantle Structures Beneath the Arabian Shield and Red Sea

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 M_w > 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, we obtain a north-south oriented resultant that matches our splitting observations and supports models of the active rifting processes. This explains why the north-south orientation of the fast polarization direction is so pervasive across the vast Arabian Plate. Seafloor spreading in the Red Sea is non-uniform, ranging from nearly 0.8 cm/a in the north to about 2 cm/a in the south. The Moho and LAB are shallowest near the Red Sea and become deeper towards the Arabian interior. Near the coast, the Moho is at a depth of about 22–25 km. Crustal thickening continues until an average Moho depth of about 35–40 km is reached beneath the interior Arabian Shield. The LAB near the coast is at a depth of about 55 km; however, it also deepens beneath the Shield to attain a maximum depth of 100–110 km. At the Shield-Platform boundary, a step is observed in the lithospheric thickness where the LAB depth increases to about 160 km. This study supports multi plume model, which states that there are two separated plumes beneath the Arabian Shield, and that the lower velocity zones (higher temperature zones) are related to volcanic activities and topographic characteristics on the surface of the Arabian Shield. In addition, our results suggest a two-stage rifting history, where extension and erosion by flow in the underlying asthenosphere are responsible for variations in LAB depth. LAB topography guides asthenospheric flow beneath western Arabia and the Red Sea, demonstrating the important role lithospheric variations play in the thermal modification of tectonic environments.