North-south variations in structure, topography, and melting regime along the ultra-slow spreading Red Sea Ridge

Abstract

The Red Sea rift is a nascent ultra-slow spreading ridge superimposed on the Afar plume. Based on high-resolution seismic data, the southernmost (south of the Danakil rift at 17.05°N), southern (17.05-19.75°N), and central (19.75-23.8°N) segments display seafloor spreading that is anomalously magma-rich compared to other ultra-slow spreading centers. In contrast, the northern segment (23.8-28°N) exhibits magma-poor extension along large-offset east- and west-dipping detachments. Sediment-corrected basement depths along the northern Red Sea reveal an axial valley as deep as the Gakkel Ridge (4200-5100 m). South of 19.75°N, plume-supported axial shoaling matches that of adjacent parts of Arabia, Africa, and the Gulf of Aden. Geochemically, the southernmost Red Sea is the locus of plume-ridge interaction. Here, E-MORBs are enriched in alkali, incompatible, and light rare-earth elements.

High mantle potential temperatures (T[subscript p]; 1326±5°C), melting pressures (12±0 kbars) and temperatures (1306±6°C), and fractionation pressures (5.3±1.6 kbars) calculated using the reverse fractional crystallization model of Brown (2019) suggest thickened oceanic crust created by high-degree partial melting of a plume-like source. North of the Danakil rift, T[subscript p] (1307± 11°C) spans a narrow range and is within the range of ambient mantle. The southern Red Sea contains N- to E-MORB depleted in alkali, incompatible, and light rare-earth elements indicating limited mixing with Afar plume material, while the central segment is host to the most depleted magmas along the ridge (La/Sm[subscript N] < 0.8). Within the southern and central regions, fractionation pressures (2.0±1.2 and 4.8±2.1 kbars, respectively) indicate lithosphere (5-15 km) thinner than that of normal ultra-slow spreading ridges (15-35 km).

In the northern Red Sea, high Na₈ and deep pressures of melting (10.4±1.4 kbars) suggest thickened lithosphere, undulations in which induce melt focusing into volcanic deeps. Based on these results, we propose that the Red Sea south of at least 26.5°N is an oceanic spreading center. We find that anomalously magma-rich spreading in the central and southern segments cannot be related to the Afar plume. Instead, the Danakil rift diverts plume-related mantle flow northeast beneath Arabia. Thus, the southern and central Red Sea must be characterized by vigorous mantle upwelling that causes heightened melt production and lithospheric thinning.