STRUCTURAL CONTROLS ON THERMAL SPRINGS IN BLACK CANYON, ARIZONA AND NEVADA

Thermal springs in Black Canyon of the Colorado River below Hoover Dam are important recreational and scenic features of the Lake Mead National Recreation Area. The U.S. Geological Survey is completing a study of the geohydrologic framework of the springs for the National Park Service.

Springs discharge along faults intruded by dacite dikes and plugs; weeping walls extend away from the faults where they intersect highly fractured rock and/or relatively porous volcanic breccias. Two major structures were identified as key flow pathways-- Palm Canyon and Salt Cedar faults. Palm fault is a right-lateral fault zone that extends NW across Black Canyon, cutting older Patsy Mine volcanic rocks east of the river. West of the river, the fault forms a diffuse right-stepping zone within the Boulder City pluton in which NW-striking fault segments and associated extensional fractures are intruded by dacite dikes related to younger Mt. Davis volcanic rocks. Palm fault zone marks a fundamental change in chemical and isotopic compositions of thermal water to the north and south. Justet and others (2010) indicate the thermal waters are a mixture of shallow groundwater from Lake Mead and deeper thermal groundwater derived from crystalline basement. This mixing signature is traceable from the dam to the fault but not detected southward. Palm fault therefore forms a significant barrier to the southerly mixing of lake water.

Salt Cedar fault is a northerly-striking, west-side-down normal fault zone on the west side of Black Canyon, marked by a 12 km long strong gravity gradient (Langenheim and others, 2010). Mt. Davis dacite dikes, domes and flows are locally intruded along and overlie its trace. Three thermal springs discharge from the fault zone: the southern two, Salt Cedar and Boy Scout, discharge along dacite intruded into the fault. To the north, Nevada Hot Spring is located where Salt Cedar and Palm faults intersect, discharging from orifices on both of the fault planes. Here, Salt Cedar fault has strike-slip striae and offsets the Palm fault 100 m in a right-lateral sense. Right-lateral offset is interpreted as young reactivation of the Salt Cedar fault, based on fault plane solutions from nearby 1970s’ earthquakes indicating right-lateral slip on a northerly striking fault plane, and on young unstable rockfall deposits along the fault zone.