NEOGENE EXTENSION OF SOUTHERN YUCCA MOUNTAIN AND NORTHERN AMARGOSA DESERT, NEVADA, BASED ON NEW SUBSURFACE DATA

Yucca Mountain is a faulted volcanic terrane within the Crater Flat extensional basin, separated from the Amargosa Desert to the south by buried structures interpreted to be strike-slip faults on the basis of geophysical and borehole data. Aeromagnetic data show two east-trending en-echelon lineaments south (Highway 95 fault) and southeast (Little Skull Mountain fault) of Yucca Mountain that separate the complex magnetic pattern typical of the faulted volcanic strata at Yucca Mountain from a more uniform magnetic pattern to the south. The gravity data show steep, north-dipping gradients correlative with the aeromagnetic lineaments and a deep northeast-striking gravity low between them. Data from boreholes on opposite sides of the geophysical lineaments show a sharp change in lithology, from thick Miocene volcanic rocks and thin interbedded volcaniclastic rocks in Crater Flat basin, to poorly consolidated gravels dominated by Paleozoic carbonates and quartzites, thick interbedded volcaniclastic deposits, and thin Miocene tuffs in the Amargosa Desert. Paleozoic rocks were penetrated in boreholes at relatively shallow depths south of the Highway 95 fault, and they crop out just south of the Little Skull Mountain fault. Thick, coarse alluvium interbedded with tuff representing an early-eruption facies occur within the gravity low between the two lineaments, indicating development of an elongate transtensional basin. We propose that the Highway 95 fault and the Little Skull Mountain fault are sinistral wrench faults, connected by a northeast-striking transtensional basin. The wrench faults appear to be en echelon with the western end of the Las Vegas Valley shear zone and probably were active as the Crater Flat basin developed during Neogene extension and volcanism.