Origin of the hot springs along the Arabian Escarpment in Saudi Arabia and Yemen

Study region The foothills of the Asir-Yemen mountains bordering the eastern margin of the Red Sea in Saudi Arabia and Yemen. Study focus The hot springs in the Al Lith and Jizan region were previously considered as potential geothermal resources due to presumed magmatic or fault-controlled origins. Our purpose is to understand their origin, and we present evidence that they represent discharge points of artesian meteoric groundwater originating from the 2–3 km-high Asir-Yemen Plateau. New hydrological insights for the region The hot springs are not related spatially to rift bounding faults or to post-rift magmatism. The hot springs emerge from the fractured basement in the floors of major incised valleys at the base of the foothills. Their low elevations of ~160 m downstream to 288 m upstream corresponds to the top of the underlying groundwater aquifer. The light H-O isotopic composition of the spring waters indicates a meteoric source from high elevations in the mountains, and tritium content (in 1982) indicates < 25 years residence time in the basement. We conclude that meteoric rainfall which infiltrated over the Asir-Yemen mountains are heated to ~100 °C by the continental geothermal gradient as they descend 2.5 vertical kilometers through the fractured Precambrian basement, before discharging along the base of the foothills. A coupled groundwater-flow and heat-transport numerical simulation independently supports this interpretation by reproducing the observed discharge elevations and spring temperatures under non-magmatic boundary conditions. The weight of the evidence, coupled with low discharge rates, indicates that these hot springs are unsuitable for conventional geothermal energy applications.