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Evaluation of the Past Seismic Activity at Yucca Mountain, Nevada, Based on the Rate of Landscape Evolution Determined with Cosmogenic Cl-36 and Ne-21 in Feldspar Mineral Separates

Rood, D.H., Lawrence Livermore National Laboratory, Center for Accelerator Mass Spectrometry, Livermore, CA 94550, United States, rood5@llnl.gov; Finkel, R.C., Earth and Planetary Science Department, University of California, Berkeley, CA 94720, United States, rfinkel@berkeley.edu; Balco, G., Berkeley Geochronology Center, Berkeley, CA 94709, United States, balcs@bgc.org.

In order to determine characteristics of past seismic activity at Yucca Mountain, Nevada, we use cosmogenic Cl-36 and Ne-21 analyses on landscape features that can be interpreted as being indicative of seismic activity. This procedure requires that we know (1) the ground motion necessary to destroy or create the feature, and (2) the age of the feature of interest. Our study addresses the preservation history of geomorphic features including precariously balanced rocks, precipitous cliffs, and joint openings that provide a potential tool for constraining unexceeded ground motion. Cosmogenic dating of hillslope boulder deposits can be used to discriminate potential rockfall induced by extreme ground motion from deposits generated by climatic processes and to constraint the rates of geomorphic processes leading to their formation or destruction.Lacking quartz in the volcanic rocks that underlie Yucca Mountain, we have successfully analyzed Cl-36 and Ne-21 in alkali feldspar mineral separates from the Tiva Canyon and Topopah Springs Tuffs. These mineral separates have low stable chloride concentrations and sufficient target element abundances (e.g. potassium for Cl-36 and sodium for Ne-21), to make them ideally suited for determining spallation-produced cosmogenic nuclides. Our preliminary results from Yucca Mountain indicate low rock erosion rates (a few m/My) and long exposure times (some 100’s ky) for the geologic features that we have studied. These results indicate that evidence for extreme ground motion deposits may be preserved on the Yucca Mountain landscape for >0.5 My. Using our newly developed methodology, additional samples from a full suite of geologic features with a range of fragilities will be analyzed in order to refine the paleoseismic history of Yucca Mountain.

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Last Modified: 2011 Aug 10

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