TUFF-FILLED PALEOVALLEYS DEXTRALLY OFFSET ACROSS THE BENTON SPRINGS FAULT, CENTRAL WALKER LANE, NEVADA

The Walker Lane is a zone of dextral shear that accommodates ~25% of the Pacific-North American relative plate motion. Dextral shear across the central Walker Lane (latitude of ~38.7°) is, in part, distributed across five NW-striking dextral faults. To characterize the spatial and temporal patterns of strain release along these faults, we completed new geologic mapping, structural, kinematic, and ⁴⁰Ar/³⁹Ar geochronology studies along the 50-95 km long, NW-striking, approximately vertical Benton Spring fault, Gabbs Valley Range, Nevada, to constrain its Neogene dextral slip history. Within the field area Mesozoic bedrock is unconformably overlain by dominantly Neogene volcanic rocks and lesser sedimentary rocks. The spatial distribution of the Neogene rocks defines an inset series of five paleochannels that lie within a single major paleochannel incised into Mesozoic bedrock. Each paleochannel consists of a base of gravels or paleosols overlain by Oligocene rhyolitic tuffs and Miocene lavas ranging in composition from basaltic andesite to rhyolite, and ⁴⁰Ar/³⁹Ar ages, ranging from ~27.2 Ma to ~19.8 Ma. The volcanic units consisted of local lava flows and channelized large-volume tuffs originating upwards of 50 km from their source calderas. We used the near vertical walls of these paleovalleys as fault markers to document a mean dextral offset of 7.4 ± 1.5 km of all paleochannels across the Benton Spring fault. Dividing the age of the youngest unit, a rhyolite lava with an Ar/Ar age of~19.8 Ma, by the ~7.4 km of dextral offset yields a minimum early Miocene dextral slip rate of ~0.4 mm/yr for the Benton Spring fault. Our minimum long term dextral slip rate is ~40% of the present-day dextral slip rate of ~1.0 mm/yr based on an elastic block model of GPS results across this part of the Central Walker Lane (Borman et al., 2016). We suggest that the Benton Springs fault is one of the primary faults accommodating dextral shear across the central Walker Lane.