DO MOUNTAINS SERVE AS RECHARGE ZONES FOR THE EARTH'S UPPER CRUST?

Multiple studies have invoked topographically driven continental-scale ground-water flow in sedimentary rocks recharged in mountainous areas. In general, these studies neglect flow in underlying crystalline rocks due to presumed low basement permeabilities. However, stable isotope data from relict hydrothermal systems indicate that meteoric water commonly circulates to depths up to 10km in crystalline basement. A careful review of published permeability measurements and estimates, including recent pumping test results from deep drill holes, suggests that typical hydraulic conductivity (K) values for the crystalline basement are actually 10^-9 to 10^-7 m/s to a depth of about 6km. These K values are high enough to potentially allow significant heat, mass, and fluid advection on a geologic time scale, raising a fundamental question: Can mountain recharge drive continental-scale ground-water flow in crystalline basement rocks? To explore this question, a simple 2-D numerical model of heat and fluid transport was constructed that includes the upper 10km of the crust from the Rocky Mountain Front Range in Colorado to northwestern Missouri 1000km to the east. Reasonable boundary conditions and heat flow parameter values were assumed. The geologic framework and K of sedimentary rocks overlying the basement were taken from existing studies. Preliminary model runs suggest that temperature data from deep tunnels within the Front Range provide an upper limit on K on the order of 10^-9 m/s for the crystalline mountain mass. Although too low to allow significant advective heat flow, this K is sufficient to allow substantial fluid flow on a geologic time scale within the upper 1-2km of the basement below the Denver Basin sedimentary units from the Front Range well into Kansas. Modeled fluid ages in this basement high-flow zone are <10Ma, much younger than Proterozoic metamorphic fluids or Cretaceous connate waters that presumably would be displaced by this topographically driven flow. The modeling exercise thus suggests that mountains may serve as recharge zones for broad regions of the upper continental crust, and that fluid in the crystalline basement in these regions may be more dynamic than previously thought.