SCALING ANALYSIS AND NUMERICAL MODELING OF HYDROTHERMAL CIRCULATION IN COOLING AND EXHUMING HOT CONTINENTAL ARC CRUST

The formation of continental crust in magmatic arcs involves rapid cooling of hot magmas (~800-1000 C) to relatively cold crust (~400-500 C) within 1-10 Myrs. Such rapid cooling must have occurred in part by rapid syn-magmatic exhumation. However, hydrothermal circulation may further enhance cooling altering the thermal and rheological states in a continental arc. In this study, we develop a 1-D thermal evolution model that invokes exhumation, conductive cooling and convective cooling by hydrothermal circulation in a crust whose permeability depends on both temperature and pressure. We used a self-adaptable equivalent thermal conductivity for the convective layer as an approximation of hydrothermal cooling. We have derived a semi-analytical solution of the steady state thermal profile and developed a numerical model to solve the transient problem. Our study shows that hydrothermal circulation could penetrate up to 8-9 km depth within 0.1-1 Myr time scale. The hydrothermal convective layer effectively acts as a highly conductive lid whose equivalent thermal conductivity is one order of magnitude higher than normal conductivities. Hydrothermal circulation results in a higher surface heat flux and a shallower thermal gradient (dT/dz) in the convective layer. At low Peclet number (Pe#≤4), hydrothermal cooling promotes the deepening of brittle-ductile transition (BDT), stabilizing the BDT at a depth deeper than the pure exhumation case. When Pe#>4, hydrothermal circulation more quickly responds to exhumation, prevents the overheating in the upper crust caused by advection, and stabilizes upper crustal temperature at a relatively lower temperature for a longer period (~10 Myrs). Hydrothermal cooling thus helps to maintain a stable BDT depth and promote brittle deformation to be localized in the upper crust. We conclude that hydrothermal cooling could be a potential driver for cooling and deformation in continental arcs. Such hydrothermal cooling could occur in some areas in the Late Cretaceous Sierra Nevada Arc, where fast exhumation occurred during and after the magmatic flare-up.