COORDINATED FAULT SLIP-RATE VARIATIONS OVER MULTIPLE TIME SCALES ACROSS THE SIERRA NEVADA FRONTAL FAULT ZONE

Our research is designed to examine variations in fault slip rates through time and space. Our strategy is to focus on a region where slip rates are sufficiently slow that we can expect to assess the presence or absence of rate differences at 10 ky to 100 ky time scales. Our field area comprises three sites along the eastern Sierra Nevada frontal fault system; each of which contains an abundance of geomorphic markers, such as moraines and terraces, which are displaced by a suite of range-front faults. Our methods include geomorphic mapping, surveying, and cosmogenic ¹⁰Be surface exposure dating. Not only will the data collected at these sites enable us to define mean deformation rates, but also by utilizing markers of different ages, they will define changes in rates through time and interactions among multiple faults.

Our data from three study areas (Sonora Pass, Bridgeport Basin, and Mono Basin) define along-strike variations in deformation rates over several temporal intervals. Surface exposure dating results demonstrate that we can define tightly clustered ages of boulders on individual surfaces; that the age uncertainties can be somewhat less than 10% (setting a lower bound on the rate differences we should be able to define reliably); and that we can readily date features ranging from both the last and the penultimate glaciations, thereby spanning >100 ky. In each of three areas where multiple markers dating back 10s to 100s of thousands of years are present, we can quantify changes (if any) of slip rates across the intervals between moraine or terrace formation and the present. Furthermore, we can evaluate the synchronous or asynchronous slip history of each fault segment, and utilize a compilation of all rates to assess the regional slip distribution and its changes through time. Additionally, in each area where faults cut multiple dated units, we can assess whether intervals of slower slip on one fault are synchronously balanced by more rapid slip on another fault. In this way, we can examine spatial gradients in slip magnitude and rates along single faults, and compare them with gradients on nearby faults. Lastly, our late Quaternary data will also be compared with modern geodetic rates in this same region, thereby providing tests of key assumptions about both the temporal constancy and spatial variations of slip rates.