STRUCTURE AND EVOLUTION OF THE WARM SPRINGS VALLEY FAULT (WSF), NORTHERN WALKER LANE, NEVADA: POST-3-MA INITIATION(?)

The WSF is the middle of three northwest-striking, right-lateral faults of the Pyramid domain of the northern Walker Lane. The WSF is a continuous fault zone that extends ~60 km from Honey Lake, CA to near the east-northeast-striking, left-lateral Olinghouse fault, which marks the southern end of the Pyramid domain. Along the central part, the WSF is superposed on structures from an earlier (3 Ma?) episode of extension. Dogskin Mt on the west is a SW-tilted block with a major E-dipping normal fault; northwards, displacement on this Dogskin fault dies out, and the northern end of the range is a north-plunging anticline. The Virginia Mts on the east are tilted southwest into the Dogskin fault. The WSF consists of as many as four strands over a 1.5 km width and parallels but is ~1.5 km east of the Dogskin fault. At the north end of Dogskin Mt, Oligocene (30-23 Ma) ash-flow tuffs between strands are folded into open to tight synclines with locally overturned limbs. Southward for ~10 km, a series of linear ridges along the WSF are cored mostly by uplifted and probably folded late Tertiary sediments and Pleistocene fan deposits. Sediments in the southernmost ridge contain a 3.57±0.17-Ma tephra that is tilted 38°, and probably correlative deposits are locally vertical. The paucity of coarse clasts in the late Tertiary sediments suggests that relief in early Pliocene time was much less than today. Distal fan deposits that are also deformed along the WSF are not measurably displaced from their undeformed proximal fans. This limits total Pleistocene displacement along some individual strands to much less than 1 km. Displacement locally blocked drainages to create closed basins that were filled with Holocene lacustrine deposits. Potential piercing points for total displacement include distribution of tuff-filled paleovalleys and of megabreccia formed by collapse off 23.5-Ma rhyolite intrusive domes. Megabreccia that crops out on both sides of the WSF allows as little as 2 km or a probable maximum of 12 km total displacement. One initial interpretation is that WSF displacement is relatively small and began after a 3 Ma episode of E-W extension that is widely recognized in the region. Also, the WSF is locally reactivating normal faults of that 3-Ma episode.