A GEOLOGIC MAPPING TRANSECT ACROSS THE NORTHERN WALKER LANE, NW NEVADA; DETERMINING THE LOCATION, DISPLACEMENT, AND TIMING OF MAJOR, PLATE-BOUNDARY STRIKE-SLIP FAULTS

Our STATEMAP mapping is making a transect (4.5 quads to date; 2 underway) to document the character, evolution, and earthquake hazards of the three major, NW-striking right-lateral faults in the Pyramid domain of the northern Walker Lane. These faults take up much of the Pacific-North American plate motion that is not on the San Andreas system. Major rock units include (1) Mesozoic granitoids and metavolcanics, (2) a complex sequence of 31-23 Ma ash-flow tuffs erupted from calderas in central Nevada and filling paleovalleys in Mesozoic rocks, (3) a sequence of 15-14 Ma mafic lavas that built a shield volcano, and (4) numerous alluvial fans developed along range fronts. The area consists of several structural blocks separated by the dextral and mostly older normal faults. The Virginia Mts and Pah Rah Range occupy a left step between the central and eastern dextral faults, the Warm Springs (WSF) and Pyramid Lake faults, respectively. The two ranges are N- to NW-trending, doubly plunging, extensional anticlines developed where strata roll over into oppositely dipping normal faults. Anticlines in the two ranges plunge toward one another, a curious structure that is common in the region. Dogskin Mt is a NW-plunging anticline on the N but a SW-tilted fault block with an E-dipping normal fault on the S. It is separated from the Virginia Mts to the E by the WSF, which extends ~60 km from Honey Lake, CA to near the ENE-striking, left-lateral Olinghouse fault, which marks the south end of the Pyramid domain. The WSF consists of as many as four strands over a 1.5 km width; it parallels and may have reactivated range-front normal faults (from an earlier, 3 Ma? extension episode) along much of its length. Structures along the WSF include fault-parallel, tight to locally overturned synclines in the ash-flow tuffs, linear ridges in Plio-Pleistocene sediments, and scarps and local push-ups in latest Pleistocene strata. The Honey Lake fault zone (HLF), the W-most dextral fault, bounds the SW side of Dogskin Mt. As with the WSF, the HLF consists of several strands over a 2 km width, parallels a range-front normal fault, has linear ridges in Quaternary-Tertiary strata, and locally shows dextral offset of stream courses. Preliminary estimates of total dextral displacement from offset tuff-filled paleovalleys are 8-10 km for the WSF and ?5 or ~20 km on the HLF.