BRECCIAS IN STRONGLY EXTENDED ROCKS ABOVE THE FLUORSPAR-BULLFROG HILLS DETACHMENT: IMPLICATIONS FOR GROUNDWATER FLOW NEAR BEATTY, NEVADA

Bodies of sedimentary, volcanic, and tectonic breccia are common in the hanging wall of the Bullfrog Hills detachment (BHD) north of Rhyolite, Nevada. The BHD extends eastward into Fluorspar Canyon where the detachment dips 40 degrees north and separates Late-Proterozoic and Paleozoic strata from Tertiary volcanic and sedimentary rocks comprising the hanging wall. Farther east, the Fluorspar Canyon Fault links with the moderately dipping Tate’s Wash Fault. After removing tilt, these structures form a listric normal fault that cuts through the Paleozoic section and onto crystalline basement. Paleozoic units above the detachment have been carried tens of km westward. Extension was accompanied by episodic eruptions, some of which may have vented along faults. The Rainbow Mountain half graben formed after eruption of the Ammonia Tanks tuff (11.4 Ma). Stratigraphic units, such as the Buttonhook Wash tuff, and underlying tuff are restricted to structural compartments bounded by faults. The basin, bounded by normal and strike-slip faults, was enlarged during westward movement accommodated by progressive development of NE-striking left-lateral faults on the southern flank and an E-striking transverse fault to the north. The principal basin fill comprises tens of m of basal basalt (10.3 Ma), overlain by up to 120m of cobbly volcanogenic sedimentary debris and slide blocks. Overlying rhyolite flows and bedded falls of ash with entrained bodies of volcanic and sedimentary breccia, especially near the base, comprises Rainbow Mountain Group (10.3Ma) a few hundred m thick. We speculate that many faults recorded by hanging wall units do not cross the detachment. Groundwater moving southwest from Pahute Mesa in the Nevada Test Site, along the NE-striking Thirsty Canyon lineament, may pond against the east side of the N-trending Hogback fault as suggested by the presence of numerous springs. Units mapped as breccia in the study area generally are not directly fault related and do not underlie the detachment. Water that emerges near Beatty, Nevada continues southward via the Amargosa River.