Author: Kimberly Diem Chi Blisniuk
Publisher:
Published: 2011
Total Pages:
ISBN-13: 9781267238320
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Understanding the mechanics of plate boundaries and associated seismic hazards requires evaluation of how slip is distributed over Quaternary timescales along individual faults, across fault zones, and entire fault systems. Documenting such slip distributions over multiple time scales is challenging due to the difficultly of obtaining reliable ages for offset landforms. For instance, exposure ages from cosmogenic isotopes can be significantly affected by surface processes, and U-series dating of pedogenic carbonate provides only minimum ages because carbonate accumulation occurs after deposition. Fortunately, the controlling factors for the resulting age and age uncertainties of each method are relatively independent from each other, so a combination of cosmogenic isotope and U-series dating may significantly improve the reliability of landform dating and yield more reliable slip rate estimates. To understand how deformation is shared across the Pacific and North American plate boundary, a comprehensive slip rate history is presented at multiple locations and time-intervals from 6 sites across the paired Clark and Coyote Creek faults of the southern San Jacinto fault zone. Offsets are constrained from field mapping and high-resolution LiDAR topography data, and displaced alluvial fans were dated with U-series on pedogenic carbonate clast-rinds and/or in situ cosmogenic 10Be. In general, these results show that, in an arid setting where post-depositional processes are limited and multiple dating techniques can be applied, self-consistent landform ages may be obtained to yield reliable slip rate estimates. The results from this study show that (1) the rate of deformation across the southern San Jacinto fault zone has remained uniform and constant in time and space over at least the past 40 kyr, and probably since its early Quaternary initiation, (2) pronounced slip rate gradients exist along the length of both the Clark and Coyote Creek faults, declining from the northwest to the southeast, and (3) summed slip rates of ~13 to 17 mm/yr across the bedrock portion of southern San Jacinto fault zone suggests that since at least the latest Pleistocene, deformation across the Pacific-North America plate boundary has been partitioned fairly evenly between the southern San Andreas fault zone and San Jacinto fault zone.