Earthquake recurrence and modes of deformation in the central and northern Walker Lane: Observations from paleoseismic trenches across the Carson and Sierra Nevada Range fronts

Observations acquired through the mapping of Quaternary deposits, the measurement of fault scarps and the excavation of two trenches along the Sierra Nevada Mountain Range provide information bearing on the rate and style of deformation in the central and northern Walker Lane. Active uplift along the eastern flank of the Sierra Nevada Mountain Range is manifest through triangular facets, oversteepened range fronts and scarps in young alluvium at fan heads. Structural, stratigraphic and pedogenic relations exposed in a trench in Antelope Valley (∼39.59°N latitude) record a three-event earthquake history, including two Holocene surface ruptures and third event of an older but unknown age. Radiocarbon dating of bulk samples shows the most recent and penultimate events occurred ∼1350 cal ybp and ∼6250 cal ybp, respectively. These dates imply a late Holocene average recurrence interval of ∼5000 years. Division of the 3.6 m most recent offset by the average recurrence interval yields a late Holocene slip rate of mm/yr. A second trench was excavated across the Carson Range front fault at the latitude of Reno, Nevada (∼39.4°N), along a nearly continuous, ∼4 km long range front scarp. The exposure revealed a sharp, planar, low-angle failure surface dipping 33° and coincident with a bedding plane in the Hunter Creek formation. Slip along the failure surface juxtaposes a massive alluvial package against the Hunter Creek sandstone. The low-angle failure plane invokes two interpretations of the driving force behind the displacement: (1) mass wasting in a landslide; and (2) slip along a seismogenic low-angle normal fault. A landslide origin for displacement is unfavorable based on the length and continuity of the scarp on the surface and the absence of landslide debris downslope. The preferred low-angle normal fault interpretation is supported by documented surface rupture along the fault ∼2 km south of the site and the observation of an exceptionally high density zone of short discontinuous faults in the pediment outboard the range front that may be a manifestation of the low-angle structure at depth. Previous paleoseismic studies show the low-angle slip proposed here is not representative of the entire Carson Range, and that is likely restricted to a few kilometers. Lastly, structural. stratigraphic and pedogenic relations in the trench are insufficient to elucidate an earthquake event history.