Oceanic basement roughness in the central Red Sea

The Red Sea is a rare example of a continental rift proceeding to an oceanic basin. In the central Red Sea, the transition to oceanic spreading appears to have occurred at ~10 Ma, but unusually low frequencies of magnetic anomalies suggest that spreading may have occurred in spreading centres that were buried beneath the evaporites before ~5 Ma. We continue our investigation of this area by assessing the basement roughness (the root-mean-square variation of basement relief) along profiles across and parallel to the spreading axis. The across-axis roughness can be compared with that of typical abyssal hill topography formed by faulting and volcanism, whereas the axis-parallel roughness can be compared with variations due to the ridge segmentation. We estimated roughness values from depths of basement interpreted from across- ridge seismic reflection profiles. The best estimate of mean across-ridge roughness of 230 m overlaps with, but is generally smaller than, those observed over ultraslow spreading ridges, consistent with a ridge affected by a hotspot (here the Afar), which typically leads to smaller fault relief. Basement roughness values along ridge-parallel profiles were computed from the free-air gravity field using densities appropriate for oceanic crust and a modified Bouguer slab formula, since suitable ridge-parallel seismic profiles are not available. Errors arising from the slab approximation were investigated using forward 2D modelling and found to be ~30%. Correcting for this bias leaves roughness values within the range of values for the ultra-slow spreading Southwest Indian Ridge, which appears organised into segments due to magmatism concentrated at segment centres. Such an organisation has been suggested for the central Red Sea previously based on the segmented structure of gravity anomalies. The axis-parallel roughness values reach minima roughly mid-way between the coast and the axial trough, where the suggested transition from stretched continental to predominantly oceanic crust occurs. The basement relief due to ridge segmentation therefore appears to have grown gradually since continental breakup.