Crustal Conditions Favoring Convective Downward Migration of Fractures in Deep Hydrothermal Systems

Cooling magma plutons and intrusions are the heat sources for hydrothermal systems in volcanic settings. To explain system longevity and observed heat transfer at rates higher than those explained by pure conduction, the concept of fluid convection in fractures that deepen because of thermal rock contraction has been proposed as a heat-source mechanism. While recent numerical studies have supported this half a century old hypothesis, understanding of the various regimes where convective downward migration of fractures can be an effective mechanism for heat transfer is lacking. Using a numerical model for fluid flow and fracture propagation in thermo-poroelastic media, we investigate scenarios for which convective downward migration of fractures may occur. Our results support convective downward migration of fractures as a possible mechanism for development of hydrothermal systems, both for settings within active zones of volcanism and spreading and, under favorable conditions, in older crust away from such zones. Geothermal energy is transferred through and stored in the rock and fluids of the Earth's crust. If temperature increases sufficiently with depth and the crust provides sufficient pathways for water to flow through, colder water sinks and percolates downward, gets heated at depth and then rises due to its lower density at higher temperature. This creates a hydrothermal circulation system that transports heat from the deep crust to shallower depths from where it can be produced. Wells drilled into these systems produce hot water and/or steam for direct heat utilization or electricity production. To understand the renewability of hydrothermal systems, we need to understand how heat is transferred deep in the crust. A hypothesis has been proposed, suggesting that fractures, propagating downwards because of contraction of the water-cooled surrounding rock, are central to maintaining the heat transfer from the deep crust. Our computer simulations corroborate this hypothesis. Based on settings found in Iceland, we show how fluid flow and propagation of fractures can be important for development of hydrothermal systems both in active regions of volcanism and, under favorable conditions, also in older crust away from such regions. The latter results are important for the identification of hidden geothermal systems. Numerical modeling supports convective downward migration of fractures as a source mechanism for hydrothermal systemsFluid flow, fracture opening and propagation in a thermo-poroelastic rock mass are simulated in different geological settings in the crustCrustal stresses are key to understanding whether a hydrothermal system can evolve in regions away from active zones of volcanism