High velocity lower crust with anomalous rheological parameters under the Red Sea Passive Margin, SW Saudi Arabia – insight into the evolution of the Hijaz-Asir Escarpment Zone

Abstract

SUMMARY The Red Sea Passive Margin (RSPM) in SW Saudi Arabia is characterized by two morphological provinces; the Red Sea Coastal Plains (RSCP) and the Hijaz-Asir Escarpment Zone (HAEZ) at the Arabian Shield Edge (ASE). Of these, the HAEZ is conspicuous by its seaward-facing heights of > 3 km. Based on the available receiver function analysis (REF) results from 25 seismic stations, we first present the Moho relief map underlying the RSPM to demonstrate that: (i) the transitional crust under the coastal region is 20–25 km thick which steadily increases landward to attain a thickness of ∼40 km below ASE, across an intervening region of depressed Moho with depths of 45–52 km. The latter exhibits significant regional extent and displays a close correlation with the topography of the HAEZ, suggesting a tectonic control on its formation. Eight rheological parameters for the upper as well as the lower crust are computed from the REF data, namely; Vp, Vp/Vs, density (ρ), Lame's first constant (λ), Lame's second constant (μ), Poisson's ratio (σ), bulk modulus (K) and acoustic impedance (AI). Their 2-D distribution for the lower crust unravels, for the first time, a high velocity lower crust (HVLC) underneath the HAEZ. The rheological character of the HVLC is further elucidated by investigating the relations between six rheological parameters: Vp–Vp/Vs, Vp/Vs–lower crustal thickness (hLC), Vs/ρ–hLC, λ–ρ, σ–ρ and λ–μ. The results indicate that lithology of the lower crust varies from diorite–felsic gneiss under RSCP to gabbro–diabase and mafic gneiss under HAEZ. We conjecture that the HVLC is an outcome of the exchange of materials from the subcrust at the hinterland of the RSPM directly underlying the HAEZ. Deep seismic control can only resolve the finer details of such crustal transformation.