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Berkeley, United States

Gold P.O.,University of Texas at Austin | Behr W.M.,University of Texas at Austin | Rood D.,Imperial College London | Sharp W.D.,Berkeley Geochronology Center Berkeley | And 2 more authors.
Journal of Geophysical Research B: Solid Earth | Year: 2015

Northwest directed slip from the southern San Andreas Fault is transferred to the Mission Creek, Banning, and Garnet Hill fault strands in the northwestern Coachella Valley. How slip is partitioned between these three faults is critical to southern California seismic hazard estimates but is poorly understood. In this paper, we report the first slip rate measured for the Banning fault strand. We constrain the depositional age of an alluvial fan offset 25±5m from its source by the Banning strand to between 5.1±0.4ka (95% confidence interval (CI)) and 6.4+3.7/-2.1ka (95% CI) using U-series dating of pedogenic carbonate clast coatings and 10Be cosmogenic nuclide exposure dating of surface clasts. We calculate a Holocene geologic slip rate for the Banning strand of 3.9+2.3/-1.6mm/yr (median, 95% CI) to 4.9+1.0/-0.9mm/yr (median, 95% CI). This rate represents only 25-35% of the total slip accommodated by this section of the southern San Andreas Fault, suggesting a model in which slip is less concentrated on the Banning strand than previously thought. In rejecting the possibility that the Banning strand is the dominant structure, our results highlight an even greater need for slip rate and paleoseismic measurements along faults in the northwestern Coachella Valley in order to test the validity of current earthquake hazard models. In addition, our comparison of ages measured with U-series and 10Be exposure dating demonstrates the importance of using multiple geochronometers when estimating the depositional age of alluvial landforms. © 2015. American Geophysical Union. All Rights Reserved.

Bibby T.,University of North Dakota | Putkonen J.,University of North Dakota | Morgan D.,Vanderbilt University | Balco G.,Berkeley Geochronology Center Berkeley | Shuster D.L.,University of California at Berkeley
Geophysical Research Letters | Year: 2016

Cosmogenic nuclide measurements associated with buried glacier ice in Ong Valley, in the Transantarctic Mountains, suggest the preservation of ancient ice. There are three glacial tills on the valley floor which have formed from the concentration of regolith contained within sublimating glacier ice. Two tills are less than 1m thick and underlain by ice. Measurements of cosmogenic 10Be, 26Al, and 21Ne show that (i) the youngest buried ice unit and corresponding till are at least 11-13ka, (ii) another ice unit and corresponding intermediate-age till are at least 1.1Ma old under any circumstances and most likely older than 1.78Ma, and (iii) the oldest till is at least 1.57Ma and most likely greater than 2.63Ma. These observations highlight the longevity of ice under thin debris layers and the potential to sample ancient ice for paleoclimate/paleoatmosphere information close to the present land surface. ©2016. American Geophysical Union. All Rights Reserved.

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