Ephemeral thermal and solute plumes formed by upwelling groundwaters near salt domes

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Abstract
Geochemical observations made by previous workers indicate that rapid upward advection of hot water has occurred along the flanks of some salt domes in the Northern Gulf of Mexico Basin and in the Danish Central Trough. The observations are based on pore fluid chemistry and on high temperatures of formation of metal sulfides and other diagenetic minerals near the crests of salt domes. Some of the diagenetic minerals formed at 40°C to 80°C above the ambient temperature of the host rocks. Numerical simulations of coupled heat and dissolved salt transport show that thermal perturbations in excess of 60°C can be repeatedly produced near the crest of a salt dome by focused dewatering of a thick strongly overpressured sedimentary interval around the dome. In our simulations, dewatering was triggered and stopped by cyclically changing the permeability at the salt‐sediment interface. The thermal plumes dissipated within 100 kyr after dewatering ceased. Stratigraphically shallow brine plumes were also formed by dewatering and these gravitationally unstable plumes dissipated more slowly by sinking within 1 m.y. Simulations showed that free thermohaline convection can also generate thermal perturbations near the crest of a dome. In this mode of upwelling, thermal perturbations reached 30°C when the vertical permeability of the hydrostatically pressured interval was 10−14 m2 (10 mdarcy). Because such permeabilities seem large for the hydrostatically pressured interval as a whole in the Northern Gulf of Mexico Basin, the formation of high‐temperature metal sulfide deposits near the crests of salt domes seems more likely to be due to dewatering of the overpressured interval with very strong focusing of expelled waters along the flanks of salt domes.