Geophysical modelling detects an intrusive magmatic body in the lower crust atop an underplated Moho at the Red Sea rifted margin, Central Saudi Arabia

Abstract

Nearly ∼43% of the Red Sea margin in central Saudi Arabia is occupied by a Bouguer gravity ‘high’ extending over ∼700+ km that covers the Jeddah-Al Lith Terrane, acronymed JALA (after Jeddah-Al-Lith Anomaly). The anomaly is asymmetric along the shoreline and it is mostly uniform in terms of amplitude and wavelength. It is also comprised of a shorter wavelength coastal gravity high (CGH), which in turn camouflages a ∼160 km + wide regional low Bouguer anomaly (B.A.). The JALA is interpreted by undertaking integrated crustal modelling for six selective gravity traverses (having variable lengths from 140 to 260 km) across the region. The modelling is constrained by crustal rheological parameters derived from the Receiver Function Analysis (REF) results. The 2D distribution of the rheological parameters and their regression relations are used to infer the probable rock composition at crustal depth (up to 42.5 km). Three primary results obtained from the integrated crustal modelling are: (i) a shorter wavelength coastal gravity high (CGH), (ii) this corresponds to a lens-shaped intrusion of higher density (3.20 g/cm3) ponded in the lower crust (average 18.5 km depth) which produces the CGH, and (ii) the longer wavelength gravity low is attributable to a laterally extensive hot and lower-density layer at the Moho-margin. The inferred magmatic intrusion at the shoreline formed over the past millions of years and is essentially inactive today, while the origin of the anomalous layer is ascribed to magmatic underplating at the rifted margin where the Afar fossil plume presumably remained active.