THE FLUORSPAR CANYON FAULT: A TILTED MIOCENE DETACHMENT

The well mapped (Cornwall and Kleinhampl, 1961; Monsen et al, 1992) Fluorspar Canyon fault (FCF) strikes easterly along the northern margin of Bare Mountain between Beatty, Nevada and Crater Flat. The fault separates strongly deformed and commonly brecciated Cambrian strata from Miocene volcanic units broken by normal faults. Collapse of the southern wall of an open pit mine reveals acres of exposure of the FCF surface that dips 40o north. Hundreds of meters southward, the FCF links to the Tates-Wash Fault (TWF) with apparent left-lateral displacement. Restoration of the dip recorded by the FCF reveals that the two faults compose a normal fault that cuts downward about 5.5 km through Oligocene (?) conglomerate and underlying mid-Paleozoic strata before gently cutting further downward across the Cambrian strata. Remnants of the capping conglomerate crudely delineate the trace of the tip line of the normal fault. The hanging wall of the listric fault comprising mainly the mid-Paleozoic strata does not crop out in the Beatty-Bullfrog region (Maldonado, 1990) and was probably displaced westward as much as 35 km. Tuffs and sediments that began to accumulate in the half graben about 15 Ma ago were tilted about 12 Ma ago and overlain by carbonate-clast conglomerate in the persisting half-graben. An additional pulse of extension is recorded by the Ammonia Tanks Tuff (11.0 Ma) (Fridrich et al. 2007). Westward, the FCF is mapped as gently cutting down section into the Wood Canyon Formation that forms the sub-horizontal footwall in the vicinity of the Bullfrog mine near Rhyolite. Northwest of Beatty and west of the Amargosa River, exposures of Zabriskie Quartzite in the Bullfrog Hills suggest that the sub-horizontal footwall extends to the north. The discontinuity that bounds the western margin of the tilted FCF segment may be coincident with the north-striking Beatty Fault close to the Amargosa River and nearby springs.