Geophysical study of the crust in the central Red Sea

Abstract

The Red Sea is an important example a continental rift proceeding to an oceanic basin, but whether the crust in the central Red Sea is continental or oceanic has been controversial. Contributing to this debate, the basement geometry and roughness are assessed using seismic reflection and potential field data. An axial crustal high with a width of 70-100 km and a height of 0.8-1.6 km is found after correcting the seismically derived basement depths for evaporite and other sediment isostatic loading. Basement axial highs are commonly found at mid-ocean ridges affected by hotspots, where enhanced mantle melting results in thickened crust. Therefore, it is suggested that the central Red Sea is underlain by oceanic crust typical of a mid-ocean ridge near to a mantle hotspot, like the Reykjanes Ridge. Bouguer gravity anomalies are found strongly correlated with basement depths from seismic reflection data. The low average basement densities deduced from Bouguer-basement depth gradients imply thickened crust and/or low mantle densities beneath the ridge axis. Normal axial crust thickness predicted from fractionation-corrected sodium contents (Na8.0) implies that the earliest seafloor spreading in the central Red Sea began with thinner than average crust. To further assess the basement geometry, the inverse method of Werner deconvolution is improved and used to invert aeromagnetic anomalies for magnetic basement depths. The improved Werner deconvolution effectively maps out the axial plateau and valleys in the crustal basement. The results confirm that the basement topography in the region away from the seismic lines also has an axial plateau within ~60 km of the axis. Magnetic basement depth near the spreading axis generally co-varies with Bouguer gravity anomalies. Valleys in the derived depths coincide with fracture zones interpreted previously from shipboard gravity, aeromagnetic, bathymetric and seismic reflection data. Those valleys also correspond with areas where the evaporites have extended into the axial valley floor, as suggested by earlier researchers. Basement roughness values are computed in profiles both across and parallel to the axis. The values from axis-crossing seismic data are ~230 m, similar to those observed at other ultraslow and slow spreading ridges. The roughness values derived from axis-parallel profiles of the gravity field (200-550 m) are comparable with those of the Mid-Atlantic Ridge where it has similar along-axis segmentation. Although these basement roughness values by themselves do not exclude an extended continental crust interpretation in the central Red Sea, they are supportive of an oceanic crustal interpretation when considered along with other evidence. Finally, our new produced geological map suggests that seafloor spreading and continental rifting in the central Red Sea have been symmetric.

Date of Award	18 Sept 2019
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Neil Mitchell (Main Supervisor)