Upper mantle velocity structure beneath the Arabian shield from Rayleigh surface wave tomography and its implications

We measured phase velocities at 13 periods from 20s to 143s using Rayleigh wave data recorded at recently installed, dense (135) broadband seismic stations in the Arabian shield and determined the shear-wave velocity structure. Our results clearly reveal a 300km wide upper mantle seismic low-velocity zone (LVZ) beneath the western Arabian shield at a depth of 60km and with a thickness of 130km. The LVZ has a north-south trend and follows the late-Cenozoic volcanic areas. The lithosphere beneath the western Arabian shield is remarkably thin (60-90km). The 130km thick mantle LVZ does not appear beneath the western Red Sea and the spreading axis. Thus, the Red Sea at 20 degrees-26 degrees N is an asymmetric rift, with thin lithosphere located east of the Red Sea axis, as predicted by the low-angle detachment model for rift development. Passive rifting at the Red Sea and extensional stresses in the shield are probably driven by slab pull from the Zagros subduction zone. The low shear-wave velocity (4.0-4.2km/s) and the geometry of LVZ beneath the western shield indicate northward flow of hot asthenosphere from the Afar hot spot. The upwelling of basaltic melt in fractures or zones of localized lithospheric thinning has produced extensive late Cenozoic volcanism on the western edge of the shield, and the buoyant LVZ has caused pronounced topography uplift there. Thus, the evolution of the Red Sea and the Arabian shield is driven by subduction of the Arabian plate along its northeastern boundary. Plain Language Summary A new structural model for the Red Sea and western Saudi Arabia is presented that depicts the present-day processes of seafloor spreading and continental volcanism. At 20 degrees N latitutude, we find that the main zone of mantle melting is not below the axis of the Red Sea, but beneath western Saudi Arabia, below the prominent volcanic fields there. This is an entirely new model for the Red Sea.