LAKE MEAD NATIONAL RECREATION AREA: STRUCTURAL CONTROL OF HOT SPRINGS IN BLACK CANYON SOUTH OF HOOVER DAM

Thermal springs in Black Canyon of the Colorado River, downstream of Hoover Dam, are important recreational, ecological, and scenic features of the Lake Mead National Recreation Area. Black Canyon exposes Miocene volcanic rocks, underlain by Proterozoic gneisses or Miocene Boulder City pluton. Spring discharge occurs along faults intruded by dacite dikes and plugs; weeping walls and seeps extend away from the faults in highly fractured rock and/or relatively porous volcanic breccias. Two major structures are important for understanding groundwater-flow paths—the Palm Tree and Salt Cedar Faults. Palm Tree Fault is a right-lateral structure that extends northwest across Black Canyon, with a diffuse right-stepping zone within the pluton where northwest-striking fault segments and associated extensional fractures are intruded by dacite dikes. Springs and seeps discharge along the fault zone, including Palm Tree and Nevada Hot Springs. Salt Cedar Fault is a north-striking structure on the west side of Black Canyon marked by a 12 kilometer-long gravity gradient. Dacite dikes, domes, and flows intrude along and overlie the fault in places. Three thermal springs discharge from the fault zone. The southern two, Salt Cedar and Boy Scout, discharge along intruded dacite. The third, Nevada Hot Spring, has several discharge points on both the Salt Cedar and Palm Tree faults where they intersect. Here, Salt Cedar Fault has right-slip striae and offsets the Palm Fault 100 meters in a right-lateral sense. The right-lateral offset likely represents reactivation of the Salt Cedar Fault, possibly including some recent activity based on earthquake data collected in the 1970s that yielded a fault plane solution indicating right-lateral slip.The filling of Lake Mead increased pore pressure below the lake and where in hydraulic connection with underlying faults and fractures, may have triggered earthquakes and opened existing structures or created new structures, thereby enhancing spring discharge near the dam. The progressive lowering in elevation of hot spring discharge at Nevada Hot Spring near Hoover Dam since this study started may be due to lowering lake levels reducing the hydrostatic pressure.