Mechanics of low-angle normal faulting: An example from Roosevelt Hot Springs geothermal area, Utah

Abstract

Roosevelt Hot Springs geothermal area is located in the Mineral Mountains of southern Utah. The geothermal reservoir is formed by systems of faults and joints in Cenozoic plutonic and Precambrian metamorphic rocks. Low-angle denudation faults, dipping between 5° and 35° to the west, form an important component of the reservoir's structure. These faults developed simultaneously with steeply dipping faults that dissect the low-angle fault plates and merge into denudation faults at depth. Gently westward dipping joints provided planes of weakness along which the denudation faults nucleated. The average coefficient of sliding friction along the faults was less than 0.5 and probably ranged between 0.15 and 0.4. The maximum depth for formation of the denudation faults was estimated as 5 km. Hydrothermally altered cataclasite preserved in the fault zones indicates that faulting occurred under brittle conditions in the presence of chemically reactive fluids. The hydrothermal alteration may have significantly reduced friction in the fault zones. Hydrothermal alteration along fractures in the present geothermal reservoir is similar to that observed in the exhumed denudation faults, indicating that the frictional resistance along faults and joints in the reservoir could be significantly lower than along similar structures in unaltered granitic rock. Studies of the structural stability of the reservoir as a consequence of fluid withdrawal and reinjection should consider possible mechanical effects of this hydrothermal alteration.