Seismic anisotropy and subduction-induced mantle fabrics beneath the Arabian and Nubian Plates adjacent to the Red Sea

Abstract

For most continental areas, the mechanisms leading to mantle fabrics responsible for the observed anisotropy remain ambiguous, partially due to the lack of sufficient spatial coverage of reliable seismological observations. Here we report the first joint analysis of shear‐wave splitting measurements obtained at stations on the Arabian and Nubian Plates adjacent to the Red Sea. More than 1100 pairs of high‐quality splitting parameters show dominantly N‐S fast orientations at all 47 stations and larger‐than‐normal splitting times beneath the Afro‐Arabian Dome (AAD). The uniformly N‐S fast orientations and large splitting times up to 1.5 s are inconsistent with significant contributions from the lithosphere, which is about 50–80 km thick beneath the AAD and even thinner beneath the Red Sea. The results can best be explained by simple shear between the lithosphere and the asthenosphere associated with northward subduction of the African/Arabian Plates over the past 150 Ma.

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

“…The fast polarization direction of shear‐wave splitting measurements (the α axis of olivine) tends to be parallel to the direction of upper mantle flow or the preferred orientation of melt shape. Shear‐wave splitting measurements show a consistent north‐south orientation of the fast direction [ Wolfe et al ., ; Hansen et al ., ; Elsheikh et al ., ; S. Chen et al ., ]. This seismic anisotropy indicates that northward flow of the asthenosphere may cause the observed seismic anisotropy in the upper mantle beneath the shield, which is in agreement with the north‐south geometry of the LVZ.…”

“…However, the geometry of the LVZ, with a nearly N‐S direction, does not parallel the Red Sea or the NE relative plate motion. Shear‐wave splitting directions [ Wolfe et al ., ; Hansen et al ., ; Elsheikh et al ., ; B. Chen et al ., ] are parallel to the trend of the LVZ. The geometry of the LVZ and the parallel shear‐wave splitting direction clearly indicate that the passive upwelling of the asthenosphere along the axis of the Red Sea rift is not the primary active process.…”

“…[] show that there are strong density variations within the lithosphere in the Arabian plate and a strong low‐density anomaly in upper mantle of the Arabian shield, which is in agreement with seismic velocity structure. The fast polarization direction of shear wave splitting for teleseismic SKS waves has a roughly north‐south orientation across the shield [ Wolfe et al ., ; Hansen et al ., ; Elsheikh et al ., ] indicating strong lattice preferred alignment due to north‐directed shearing deformation. Some studies also investigated the seismic structure beneath Cenozoic volcanic region and margin of the Arabian plate [e.g., Koulakov and Sobolev , ; Hansen et al ., ; Korostelev et al ., , ; Koulakov et al ., ] and find evidence for dike intrusions within the crust.…”

Complex seismic anisotropy beneath western Tibet and its geodynamic implications

“…Such a long term movement may produce preferred orientation of olivine in the lower lithosphere and the upper asthenosphere, leading to observable anisotropy. This mechanism has been used by Elsheikh et al (2014) to explain dominantly N-S fast orientations observed in Arabia and northern Africa.…”

Passive rifting of thick lithosphere in the southern East African Rift: Evidence from mantle transition zone discontinuity topography

“…Shear wave splitting measurements obtained adjacent to the Tanzania Craton, the Main Ethiopian Rift, and the Arabian Peninsula [ Bagley and Nyblade , ] are attributed to mantle flow modulated by northward advection of the African superplume which upwells through the mantle transition zone (MTZ) beneath the northern MRZ. However, shear wave splitting studies conducted throughout north central Africa and Saudi Arabia also demonstrate north‐south mantle flow patterns which are attributed to basal traction‐generated anisotropy from simple northward plate migration relative to the underlying asthenosphere [ Elsheikh et al , ; Lemnifi et al , ]. Similarly, shear wave splitting evidence from the Okavango rift zone (ORZ, Figure a) and southern Africa [ Silver et al , ; Yu et al , ] indicates that subcrustal mantle anisotropy can be explained by plate motion and lithospheric Archean structures and are not associated with a contemporary flow system related to the proposed African superplume.…”