GEOLOGIC MAPPING ALONG THE GUMDROP HILLS FAULT: DEXTRALLY OFFSET TUFF-FILLED PALEOVALLEYS IN THE CENTRAL WALKER LANE

The Walker Lane, a system of NW-striking dextral faults in the western Great Basin, accommodates ~25% of the dextral motion between the Pacific-North American plates. The central part of the Walker Lane (CWL) is dominated by five NW-striking dextral faults, , which strike subparallel to Pacific-North America motion. Our interpretation of 30+ year old 1:48,000 scale geologic maps across the CWL suggests that these faults dextrally offset Cenozoic tuff-filled paleovalleys. To test this interpretation and document the Miocene dextral fault slip history, we completed new geologic mapping and structural studies along an ~28 km long segment of the Gumdrop Hills fault (GHF), one of the five dextral faults in the CWL. The GHF is exposed in the Gabbs Valley Range, NV, a region dominated by exposures of Mesozoic metasedimentary and intrusive rocks and Oligocene to Miocene tuffs, lavas, and fluvial gravels. Our mapping, combined with orientation of cooling columns, flow foliation and fiamme in tuffs and lavas, and exposures of conglomerates at the base of both tuffs and lavas, suggest that the GHF cuts and offsets four nested paleovalleys incised into older Oligocene and Miocene ash-fall tuffs. These paleovalleys are in-filled with Oligocene and Miocene ash-fall tuffs and a Miocene andesitic lava with published ages that range from 24.30 ± 0.01 Ma to 22.556 ± 0.0074 Ma (Henry & Faulds, 2010, Dubyoski et al., 2016 (D2016)). Dextral offset of the paleovalleys ranges from 9.8 ± 3.8 km to 13.2 ± 1.9 km, with an average offset of 11.5 ± 2.7 km. Dividing the age of the youngest of these offset units, the ~22.6 Ma air-fall tuff (D2016), by its apparent dextral offset of 9.8 ± 3.8 km, yields a minimum Miocene dextral slip rate of 0.4 ± 0.2 mm/yr along the GHF. This slip rate is the same within error to the minimum Miocene dextral slip rate of 0.4 ± 0.1 mm/yr along the Benton Springs fault (BSF) (D2016). Our minimum long-term (10⁶ yrs) dextral slip rate along the GHF is about half the short-term (10⁴ yrs) slip rate of ~0.9 mm/yr (J. Langille et al., pers. comm., 2017); an elastic block model of GPS data does not yield decadal-scale (10¹ yrs) slip rates along the GHF, but does yield dextral slip rates of 0.47 ± 0.29 mm/yr and 0.98 ± 0.36 mm/yr along the Indian Head Hills fault to the west and BSF to the east, respectively ((Bormann, 2016).