Salt Flows in the Central Red Sea

Abstract

The central Red Sea is a nascent oceanic basin. Miocene evaporites, kilometers in thickness, were deposited during its continental rifting phase and early seafloor spreading. With further seafloor spreading, increasing dissolution due to increasing hydrothermal circulation as well as normal fault movements removed lateral constraint of the evaporites at the walls of the axial rift valley. Because halite is a ductile material that forms a large part of the evaporite sequence, the evaporites started to move downslope, passively carrying their hemipelagic sediment cover. Today, flowlike features comprising Miocene evaporites are situated on the top of younger magnetic seafloor spreading anomalies. Six salt flows, most showing rounded fronts in plan view, with heights of several hundred meters and widths between 3 and 10 km, are identified by high-resolution bathymetry and DSDP core material around Thetis Deep and Atlantis II Deep, and between Atlantis II Deep and Port Sudan Deep. The relief of the underlying volcanic basement likely controls the positions of individual salt flow lobes. On the flow surfaces, along- slope and downslope ridge and trough morphologies parallel to the local seafloor gradient have developed, presumably due to extension of the hemiplegic sediment cover or strike-slip movement within the evaporites. A few places of irregular seafloor topography are observed close to the flow fronts, interpreted to be the result of dissolution of Miocene evaporites, which contributes to the formation of brines in several of the deeps. Based on the vertical relief of the flow lobes, deformation is taking place in the upper part of the evaporite sequence. Considering a salt flow at Atlantis II Deep in more detail, strain rates due to dislocation creep and pressure solution creep were estimated to be 10−14 1/s and 10−10 1/s, respectively, using given assumptions of grain size and deforming layer thickness. The latter strain rate, comparable to strain rates observed for onshore salt flows in Iran, results in flow speeds of several mm/year for the offshore salt flows in certain locations. Thus, salt flow movements can potentially keep up with Arabia–Nubia tectonic half-spreading rates reported for large parts of the Red Sea. P.