This study brings together a team of scientists from Georgia Institute of Technology to investigate the discrepancies between short- and long-term strain rates in the Walker Lane region of eastern California and western Nevada; an evolving segment of the Pacific-North America plate boundary. To accomplish this objective, existing geodetic infrastructure, including regional campaign and continuous GPS data, is augmented with 10 new monuments in the region. All campaign sites will be measured in three annual campaigns between 2010 and 2012. Analysis of the GPS data will yield a detailed image of the present-day strain field. Geologic mapping incorporating evaluation of aerial photography, field surveying and cosmogenic nuclide geochronology of offset alluvial fans are used to determine the dates, and hence the long-term rates, of motion across normal faults. The integration of the long-term geologic and short-term geodetic datasets will yield a comprehensive view of the distributed strain field over late Pleistocene to Recent time scales along this important plate boundary fault system. Ultimately, this will lead to improved understanding of how the structurally complex lithosphere behaves along evolving plate boundaries.
Along some plate boundary systems, including the San Andreas and North Anatolian faults, rates of tectonic deformation appear to be constant over a wide range of time scales. However, on parts of other plate boundary zones short-term rates determined from GPS measurements do not coincide with longer-term geologically inferred slip rates. For example, late Pleistocene rates of deformation across the eastern California shear zone in the Mojave Desert and in the central Walker Lane, determined from tectono-geomorphic fault studies, yield rates that are only one-half to one-third the short-term rate determined from GPS data. The mismatch between short- and long-term rates in the Walker Lane is contrary to current understanding, and has important implications for how tectonic deformation is accommodated in the lithosphere, and ultimately the relationship of this deformation to earthquake hazards. By determining rates of tectonic activity in the Walker Lave over annual to 100,000 year timescales will provide new insights into how fault systems behave and evolve over multiple earthquake cycles, and address several fundamental questions about crustal strain distribution: How constant are rates of strain accumulation and release in time and space? Are geologic slip rates averaged over multiple earthquake cycles comparable with short-term rates of deformation determined from GPS data? If discrepancies between short- and long-term rates of deformation exist, over what temporal and spatial scales does the discrepancy occur? Are these differences related to the structural complexity of a specific region or are they characteristic of entire plate boundaries?
The constancy of ground motion that drives earthquakes and crustal deformation is a fundamental issue in geodynamics and earthquake hazard assessment. Over some well established primary faults, like the San Andreas fault in California, fault motions appear nearly constant over long periods of time (> 10,000 years). However, on parts of other plate boundary zones, including the region immediately to the east of the San Andreas fault, short-term rates observed by modern geodetic methods (including GPS) do not coincide with longer-term geologic slip rates. This study aims specifically to better document and possibly rectify differences in ground motion and fault rates observed in previous studies. This is done by performing both high-density GPS campaigns of the modern plate motion, and by using cosmogenic nuclide dating of fault offsets to obtain plate motions over approximately the last 1 million years. This study focuses on the southern Walker Lane which accommodates approximately 25% of the plate motion between the North American and Pacific Plates. Results from our new GPS campaigns and observation from existing and new continuous GPS instruments across the southern Walker Lane show that current rates across the region are at least two times faster than those which can be observed through the Pleistocene using slip observed along exposed faults. While it remains possible that the tectonic driving forces are slower, through GPS, we see that the strain field is not dominated by discrete faults, but can be explained by distributed strain, presumably along buried faults. Beyond observations of deformation along the Walker Lane, comparison of our contemporary campaign GPS results with regional campaign and continuous GPS show a near complete transect of the deformational behavior of the North American/Pacific Plate transition across the region. We find that using simple, models of "locked slip" along three Walker Lane Faults, and the San Andreas Fault, we can describe approximately 90% of the modern velocity field. Beyond being an important solution for understanding the modern accommodation of strain in the region, this result also can be used to highlight the zonations of maximum seismic potential along the transect along three faults along the southern Walker Lane region. We used newly collected cosmogenic nuclide samples and observations of geologic offset along the Alluvial fans along the Lone Mountain Fault in the eastern segment of the Walker Lane, we determined new extension rates through the late Pleistocene and Holocene. Depending on interpretation of the downdip extent and dip of observed faults, we find slip rates across these faults are approximately 0.7 and 0.8 mm/yr, substantially faster than rates found for the early Pleistocene of around 0.1 to 0.4 mm/yr by members of the same team (Hoeft and Frankel, 2010). Thus, while the GPS results above show that modern increases in tectonic rates may possibly be explained by buried fault slip, or otherwise distributed strain, these results suggest that for the same fault segment, rates have changed appreciably over time. With in kind support from the National Center for Airborne Laser Mapping, we obtained a laser terrain map of the White Mountains Fault Zone along an approximately 20 km long segment between 37.57°N and 37.4°N. We used these data along with detailed on-the-ground field observations to create geologic maps of Pliocene and Quaternary alluvial fan deposits along the western side of the White Mountains. These maps were then used in conjunction with cosmogenic field samples to determine fan ages and estimate offset rates. In this work we determined slip rates of approximately 2 mm/yr across three segments of the White Mountains fault zones. In contrary to our above results along the Lone Mountain fault, we found that the White Mountains has remained remarkably constant from the mid-Pleistocene through the Holocene. These results help to reconcile a portion of the currently observed discrepancy between our and others geodetic observations of strain and other observed late Pleistocene slip rates in the Southern Walker Lane. Beyond the basic scientific advancements in this study, this project supported the development of one newly graduated PhD in the field of neotectonics, and provided substantial useful information for improved seismic hazard mapping in the California/Nevada border lands.
