Salt Formation, Accumulation, and Expulsion Processes During Ocean Rifting—New Insight Gained from the Red Sea

Abstract

Recent observations of thick carpets of mobile salt slurries on the Red Sea floor (Salt Flows) and huge accumulations of salts in the sub-surface (‘Salt Walls’ and ‘Salt Ridges’), associated with topographical lows (Deeps), suggest that the Red Sea currently produces new volumes of brines and solid salts underground. The salt producing zone is focused around the central rifting axis and represents about 15% of the entire Red Sea area. The brines and solid salts are formed by boiling and supercritical phase separation in forced convection cells (hydrothermal circulation), located above shallow-seated magmatic intrusions along the spreading axis. The descending water of the convection cells attains increasing pressure and temperature, resulting in supercritical water conditions, giving rise to phase separation. Salts are therefore deposited underground and accumulate in the heavily fractured country rocks in the rift zone. Dense brines also migrate further down and concentrate beyond saturation. Conversely, the ascending limbs of the hydrothermal cells consist of low salinity vapor which condenses upon cooling, hence dissolving previously deposited salts. The different solubilities of sea salts lead to a refining of the salt types. When reaching the seafloor, the newly formed brines are cooled further, eventually becoming oversaturated in salts, which results in precipitation onto the seafloor. The dense brine layers also protect seafloor salts from re-dissolution by normal seawater. In the continued process, brines will migrate through salt deposits (as they build up) eventually giving rise to salt glaciers, salt walls, salt pinnacles, and ‘diapirs’ (injectites). Ores, hydrocarbons and clays are often associated with salts, being part of such a robust, self-organizing and self-sustaining hydrothermal system. Although this model arose from geological and geophysical observations performed in the Red Sea area, it may also be applicable to present and past rift zones worldwide, especially those with low spreading rates, in their early rifting stage. In addition to solar evaporation of seawater, our model also considers the significance of temporal variations in mass- and heat-flow and its impact on hydrothermal flow that governs underground salt formation, accumulation and mobility.