Implications of thrust and detachment faulting for the structural geology of the Thermo Hot Springs Known Geothermal Resource Area, Beaver County, Utah

This report presents data and conclusions concerning the role of low-angle faulting in the formation of the Thermo Hot Springs Known Geothermal Resource Area (KGRA) and the effects that such faulting may have on fluid flow and production. The conclusions are that the KGRA is formed by a low-angle normal fault that juxtaposes Mesozoic and upper Paleozoic sedimentary rock in the upper plate over metamorphic rock and granite. The Mesozoic section is in turn overlain by a sequence of Tertiary to Quaternary volcanic and sedimentary deposits. High-angle normal faults offset the sedimentary and volcanic sections, and in some, if not all, cases penetrate and offset the low-angle detachment fault. These high-angle normal faults may hydraulically compartmentalize the reservoir but also provide pathways for fluids to ascend upwards from beneath the detachment fault. The implications of the low-angle normal or detachment fault structure are significant: (1) The Thermo Hot Springs KGRA has striking similarities to the structures and stratigraphy that are exposed in the southern Mineral Mountains, which provide an excellent outcrop analog for studying the nature of the structures and fluid conduits that presumably occur at depth in the KGRA. There is good reason to suspect that the Cave Canyon detachment fault exposed in the southern Mineral Mountains is the same or similar structure as the detachment fault within the Thermo Hot Springs KGRA. The similarities include lower fault plate granitic and metamorphic rocks, hydrothermally altered cataclasite within the detachment fault zone, essentially an identical stratigraphy within the upper plate of the detachment fault, and a mosaic of north and east trending high-angle normal faults, some of which penetrate and offset the detachment fault. (2) The low-angle detachment fault model for KGRA structure has regional implications for geothermal prospecting in the Basin and Range terrain of southwestern Utah. This region is underlain by several known or suspected detachment normal faults of middle to late Tertiary age, which may act to laterally channel hot fluids at depth over large areas with little surface expression, such as springs or tufa mounds, except where the low-angle faults are breached by younger faulting.