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

Abstract

We measured phase velocities at 13 periods from 20 s to 143 s 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 300 km wide upper mantle seismic low‐velocity zone (LVZ) beneath the western Arabian shield at a depth of 60 km and with a thickness of 130 km. The LVZ has a north‐south trend and follows the late‐Cenozoic volcanic areas. The lithosphere beneath the western Arabian shield is remarkably thin (60–90 km). The 130 km thick mantle LVZ does not appear beneath the western Red Sea and the spreading axis. Thus, the Red Sea at 20°–26°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 (4.0–4.2 km/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.

Crustal Magmatism and Anisotropy Beneath the Arabian Shield—A Cautionary Tale

“…Several lines of evidence (seismic, petrologic, and geochemical) support the hypothesis of a localized channel of north directed asthenospheric flow beneath the MMN line within a corrugation in the overriding lithosphere (Camp & Roobol, ; Hansen et al, ; Yao et al, ). Such flow may reasonably couple into the ductile lower crust through traction along the base of the thin rigid lithosphere.…”

“…This hypothesis is consistent with seismic splitting data over the entire Arabian shield which observe a north‐south oriented fast direction originating in the asthenosphere but poorly constrained in depth (Hansen et al, ). Seismic tomography models further image a low‐velocity zone (LVZ) as shallow as 60‐ to 90‐km depth, suggestive of shallow asthenosphere beneath thinned lithosphere of the western Arabian Shield (Chang et al, ; Yao et al, ). Low heat flow, deep mantle earthquakes, and crustal xenoliths further suggest the lithosphere is not in thermal equilibrium, all consistent with lithospheric thinning occurring around 12 Ma (Blanchette et al, ; McGuire & Bohannon, ) near the end of Afro‐Arabian doming and the onset of Harrat volcanism.…”

Crustal and uppermost mantle shear wave velocity structure beneath the Middle East from surface wave tomography

“…This possible northward flow of the upwelled material affects not only the upper mantle but also the crustal structure beneath the Red Sea and Arabian Shield. One important question that seismic tomography studies have attempted to address is the mechanism of emplacement of Cenozoic volcanism in the Arabian Shield (Hansen et al, 2007(Hansen et al, , 2008Koulakov et al, 2016;Yao et al, 2017;Tang et al, 2016Tang et al, , 2018Tang et al, , 2019. Tang et al, (2019) and Koulakov et al, (2015) argue that the crustal low-velocity zones imaged beneath the western Arabian Shield may reflect weakened zones in the crust caused by magma ascent rather than significant partial melt or steady-state crustal reservoir.…”

“…Two end-member mechanisms are proposed: passive and active rifting. In passive rifting model, spreading is driven by far-field forces such as slab pull from Zagros subduction (Koulakov et al, 2016;Yao et al, 2017). In active rifting model, spreading is driven by local mantle upwelling (e.g.…”

Shear Velocity Structure Beneath Saudi Arabia From the Joint Inversion of P and S Wave Receiver Functions, and Rayleigh Wave Group Velocity Dispersion Data

“…While the entire Gulf of Aden displays a well‐developed mid‐ocean ridge with seafloor spreading (Cochran, ), the evolution and current state of rifting of the Red Sea is still controversial. A few studies (e.g., Ebinger & Sleep, ; Hansen et al, ) suggest that the Red Sea has evolved into active rifting, whereas other studies (e.g., Koulakov et al, ; McGuire & Bohannon, ; Yao et al, ) argue that the Red Sea is still at a stage of passive rifting.…”