Normal fault thermal regimes: conductive and hydrothermal heat transfer surrounding the Wasatch fault, Utah

Abstract

The thermal regime across an active normal fault is affected by tectonic processes of exhumation and erosion of the footwall and burial and sedimentation on the hanging wall. An enhanced thermal regime in the footwall is juxtaposed against a thermally depressed regime in the hanging wall causing significant two-dimensional (2D) heat flow. These thermal processes have been simulated with 2D numerical models and applied to the Wasatch fault of central Utah. Simulations included variable fault angles of 90°, 60°, and 45° and a vertical displacement profile accounting for hanging wall and footwall tilt. After 20 m.y. of fault movement, 60° fault, isotherms are displaced ∼1 km on the footwall and hanging wall. Furthermore, model surface heat flow is enhanced by 25% above the footwall and depressed by 15% above the hanging wall 10 km from the fault trace; the heat flow transition has a half width of 10 km. Present-day surface heat flow was compiled for 23 sites along the Wasatch Front. Heat flow has a mean value of 92 mW/m 2 (standard deviation 25 mW/m 2) but, unlike model predictions, does not show a discernible variation in heat flow across the fault. Part of the discrepancy between predicted and observed conductive heat flow may be caused by groundwater which recharges in the uplifted footwall, is heated in the subsurface, and discharges as thermal water along the range bounding fault or into the hanging wall valley. Flow rates and water temperatures from 29 tectonic hot springs along the Wasatch Front indicate a minimum hydrothermal thermal power loss of 90 MW or 0.24 MW per kilometer of strike along the fault. This thermal power is equivalent to a heat flow of 21 mW/m 2 captured uniformly between the range crest and range front, and discharged in hot springs.