Paleoseismology from Paleoshorelines: Combining Lidar Data and Geochronology to Resolve Displacement of Pleistocene Pluvial Shorelines along Normal Faults in the Northwestern Basin and Range

Paleoshorelines of pluvial lakes in the Basin and Range are commonly used to determine lake highstands and to assess past climate in a now-arid region. However, because paleoshorelines record a paleohorizontal datum, these features can provide insight into tectonic processes at a variety of scales as well. Deviations of a paleoshorelines from a modern horizontal plane may be caused by isostatic rebound of the crust resulting from lake removal or by offset along faults since a given lake stillstand. While isostatic rebound has a significant effect over large lake basins such as Bonneville and Lahontan, the effect is negligible in smaller lakes that fill a single valley, often within terminal basins. As a result, variability in shoreline elevations can be attributed primarily to offset along normal faults. Pluvial lakes occupied valleys in the Basin and Range between >1 Ma to <12 ka, a time range for which fault slip rates have proven difficult to determine using traditional paleoseismic and geologic records. Trenches provide paleoseismic records across a single fault, but a single valley may contain several seismically active faults, and the time resolution is typically limited by the maximum age of radiocarbon dating and the earthquake recurrence interval. Geologic records typically provide average slip rates over time rather than timing of individual seismic events. In the northwestern Basin and Range, where the strain rate is low and earthquake recurrence intervals are long, it is particularly critical to extend the traditional paleoseismic record as far back as possible. By precisely dating paleoshorelines and using high-resolution topographic data to correlate them across active faults, it is possible to greatly enhance the record of slip rate along normal fault systems. Newly acquired airborne lidar data in Surprise Valley, a small terminal basin in northeastern California, reveals a mappable series of shorelines that occur throughout the valley on both sides of the seismically active Surprise Valley fault (SVF). The lidar data, collected by the National Center for Airborne Laser Mapping through a NASA-funded proposal, elucidate relationships between geomorphological features such as landslides, fault scarps, and paleoshorelines. We combine these data with our preliminary ²³⁰Th-U ages on two shoreline tufa deposits of 13.9 ± 1.2 ka to 31.2 ± 3.9 ka and 180 to 50m above modern playa. Our preliminary analysis of these combined data suggest up to 20 m of elevation difference between the 13.9 ± 1.2 ka paleoshoreline over approximately 60 km of distance along the valley. This change in elevation may be due to differential slip along the SVF, with a greater amount in the north than in the south, or it may be due to motion along smaller faults parallel to the SVF. Additional dating of paleoshoreline tufa deposits over a broader spatial area will help distinguish between these possibilities and their implications for seismic activity. Our analysis of pluvial lake shorelines will extend the paleoseismic record in this valley both spatially, beyond the main Surprise Valley fault, and temporally, beyond the currently known 18 ka record.