ABSTRACT
The break-up of continents involves a stage of continental rifting proceeding to sea-floor spreading creating new oceanic crust. Such transitions are potentially accompanied by changes in the rugosity of the basement as sea-floor spreading processes take over from those of continental rifting, for example, changes in style and relief of normal faults or relief of volcanic features. The along-rift structure may also change, as oceanic segmentation replaces the continental rift segmentation. Whereas previous studies have investigated basement rugosity of continent-ocean transitions of mature margins, the Red Sea presents an opportunity to do so where the transition is young, only ~10 Ma in the central Red Sea studied here. We assess basement roughness (root-mean-square variation of basement relief) along profiles across and parallel to the spreading axis. The across-axis roughness is derived from the depths of basement interpreted from across-ridge seismic reflection profiles. The best estimate of mean across-ridge roughness (230 m) overlaps with, but is generally smaller than, those observed over ultra-slow 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 are computed from the free-air gravity field using densities appropriate for the oceanic crust. Roughness values are found to be within the range of values for the ultra-slow spreading Southwest Indian Ridge, which appears similarly organized into segments. These 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. Although the detailed mechanical processes are unclear, the results suggest that along-axis basement relief due to ridge segmentation can grow gradually after a continental break-up, perhaps marking a gradual establishment of the magmatic system.
