Project Abstract
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We have developed a new fault surface paleothermometer that exploits the temperature- sensitive oxidation state changes in transition metals that are a result of shear heating during seismic slip (Evans et al., 2014). We use transition metal surface chemistry to document iron and Mn oxidation state changes and determine the peak temperatures attained during slip on faults coated by Fe-oxides. We have also developed methods to directly date the slip on faults using U- Th/He geochronology. With these methods, we are dating several faults of the San Andreas Fault, exhumed in the Mecca Hills, and determining the likely peak temperatures of faults of the San Andreas fault system. We have examined fault-related textures with optical and scanning electron microscopy from the Painted Canyon, Platform, Eagle Canyon, and Hidden Spring fault, all part of the San Andreas fault system in the Mecca Hills.
We have used, for the first time, XANES analyses of natural fault surfaces fron the San Andreas system to examine the microchemical and microtextural nature of seismic slip surfaces and estimate frictional ΔT of ~450°C.
We use the results of the microstructural study and geochemistry to select aliquots to test the U-Th/he thermochronology method on the Painted Canyon and Platform Faults, and show that they were heated 400,000 – 650,000 years ago. The thermochronology method appears to be robust, and we are currently separating seven other sample sites to examine the evolution of the Mecca Hills, and to further estimate fault heating temperatures. |
