A MEASURE OF POST-MID-MIOCENE OFFSET ON THE STATELINE FAULT, CALIFORNIA AND NEVADA

The Stateline-Amargosa Valley fault zone (SAFZ) is a NW striking zone of distributed faulting that extends along the California-Nevada state line from Ivanpah Valley northwest to as far as Bare Mountain, where some fault segments come within 20 km of the proposed nuclear waste repository site at Yucca Mountain (Schweikert and Lahren, 1997, Tectonophysics). This fault system is the easternmost zone of active dextral shear in the Eastern California Shear Zone. We present new evidence that indicates 28 ± 3 km of offset on the SAFZ since mid-Miocene time. This estimate is based on correlation of rock avalanche deposits of upper Paleozoic carbonates and intercalated rhyolitic block and ash flows at Black Butte (on the west of the SAFZ) with similar deposits associated with the Devil Peak dome, a small (~2.5 km diameter) rhyolite plug in the southern Spring Mountains ~28 km southeast of Black butte. Further evidence of correlation arises from comparison of major element analyses of obsidian and rhyolite blocks suspended in a basal surge deposit on the west flank of the Devil Peak with analyses from obsidian in a similar deposit exposed locally on the east flank of Black Butte. The small size of the Devil Peak dome and the proximal nature of both the rock avalanche and the volcanic surge deposits (obsidian blocks in the deposits at both localities exceed 50 cm in diameter) suggests that Black Butte was adjacent to Devil Peak at the time of emplacement (12.6. Ma; Armstrong, 1966, Geochimica et Cosmochimica Acta). This implies a minimum, long-term, average slip rate on the SAFZ of 2.2 ± 0.2mm/yr. This estimate is roughly twice the modern-day slip rate derived from elastic block modeling of geodetic data (Wernicke et al., 2004, JGR), and suggests either that the slip rate slowed over time or that the present-day geodetic slip rate represents a transient period of low slip. This new estimate of offset on the SAFZ indicates that this fault needs to be carefully considered in the seismic risk assessment of Yucca Mountain. In addition this offset along the SAFZ requires a re-evaluation of correlations of Mesozoic contractional structures, and suggests that the Keystone thrust and the Mesquite Pass thrust may be a contiguous structure disrupted by the SAFZ.