Lithospheric inheritance controls on early sea-floor spreading: new insights from magmato-structural patterns along the Red Sea axis

Lithospheric inheritance is known to strongly influence the spatial and temporal patterns of continental deformation in all geodynamic contexts, emphasizing the role of rheological feedbacks between time-spaced geodynamic events. In principle, the transition from continental rifting to sea-floor spreading at diverging plate boundaries marks a threshold beyond which these long-term feedbacks no longer apply. This is because sea-floor spreading is accompanied by the creation of new lithosphere from melting and cooling of the underlying and uprising mantle, which should make lithospheric inheritance negligible at oceanic plate boundaries. However, whether and how lithospheric inheritance continues to affect oceanic plate boundary processes after the continental rifting to sea-floor spreading transition is reached has so far not been explored in detail.

As a young oceanic rift that broke up the Arabia-Nubia Shield and its mosaic of Proterozoic accreted blocks, the Red Sea (RS) represents an ideal case to study these specific lithospheric inheritance effects. We performed a quantitative morpho-structural analysis designed to track along-axis variations of the magmato-structural architecture of the RS plate boundary and to explore its relationships with the inherited structures of the rifted continental plates. Specifically, faults and sea-floor morphology have been mapped over the post-5.3Ma extent of oceanic crust from Global Multiresolution Synthesis (including multibeam surveys) bathymetry. The structural and magmatic patterns have then been extracted by quantifying four metrics: the axial depth, the slope of the central-trough flanks, the proportion of exposed volcanic sea-floor, and the distribution of normal-fault offsets.

This analysis reveals that anomalously deep segments bounded by steeper-than-average flanks bound the central RS in the North and South. Furthermore, it shows that this specific axial topography occurs where the structural pattern locally switches from regularly-spaced and moderate-displacement (~400m) normal faults to one dominant large-displacement (~1200m) fault as well as coinciding with a lower proportion of volcanic sea-floor (15-20% versus 70% on average along the rest of the axis). This distinct magmato-structural signature is commonly interpreted to reflect a decreased fraction of plate separation accommodated magmatically along slow and ultra-slow spreading ridges, in agreement with tectono-magmatic interaction models: individual faults that form near the axis remain active longer and accumulate more displacement when this fraction decreases. On the other hand, a decreased magma input would result in a thinner crust, and thus isostatically account for the anomalous depth of these segments.

The location of these two magma-starved segments appears unrelated to variations in spreading rate or to the segmentation of the RS axis, but stands in the prolongation of two major Proterozoic suture zones within the Arabia-Nubia Shield. On the Arabian side, both of these two inherited structures coincide with a rise of the lithosphere-asthenosphere boundary (LAB) as mapped from S-to-P receiver functions. We therefore propose that on-axis magma starving results from local outward spreading of the upper-mantle upwelling, in turn driven by its off-axis channeling along the LAB topographic highs. Thereby, heat and eventually melts would be transferred from beneath the axis to beneath the onshore suture zones, possibly fueling the Plio-Pleistocene volcanic activity observed there.