SCEC Award Number 17207 View PDF
Proposal Category Individual Proposal (Data Gathering and Products)
Proposal Title Co-seismic Weakening Mechanisms within the San Andreas Fault System, Mecca Hills and SAFOD, California: The roles of Carbon and Focused Alteration on the Strength of Faults
Investigator(s)
Name Organization
Kelly Bradbury Utah State University
Other Participants Krishna Borhara, PhD student
James P. Evans
1 Undergraduate Student
SCEC Priorities 3d, 3f, 3b SCEC Groups Geology, SAFS, FARM
Report Due Date 06/15/2018 Date Report Submitted 11/14/2018
Project Abstract
 Previous examination of the lithology and microstructures in the San Andreas Fault Observatory at Depth (SAFOD) Phase 3 core revealed distinct intervals of sheared black carbonaceous rocks throughout the ~ 40 m length of core. Our key project objectives included determining the source of the carbonaceous matter, documenting the physio-chemical changes within the carbonaceous matter across the sampled core and actively creeping fault traces, determining the potential effects of temperature, composition and degree of shear localization on dynamic fault weakening of carbon-rich fault zones, and identifying any evidence of thermal decomposition of the carbonaceous material. To address these questions we utilized an integrated micro-structural and micro-geochemical approach to identify the zones enriched in carbonaceous matter and any potential evidence for weakening or alteration in these carbon-rich zones in SAFOD Phase 3 Core. Methods include optical and high-resolution Scanning Electron Microscopy (SEM), Synchrotron Radiation, Total Organic Carbon (TOC) Analysis, X-Ray Diffraction Analysis, and Raman Microspectroscopy. Total Organic Carbon values ranged from 0.5 to 4.0 wt%, with the highest concentrations located within the actively creeping shear zones. Shifts within the Raman spectra of the carbonaceous matter within the black fault rock southwest of the SDZ indicate increasing crystallinity and localized increases in temperature within micro- to nano-scale slip surfaces. Synchrotron radiation and µXRF mapping highlight the distribution of Fe and Ca in association with deformation or fluid-related textures within the fault rocks. This approach is useful for studying fluid-rock interactions and resolving the evolution of physio-chemical changes at the micro- to nano-scale.
Intellectual Merit This interdisciplinary approach spans a wide range of scales and methods used in earthquake mechanics to understand the evolution of strength and slip behavior of major tectonic faults for seismic hazard assessment. Faults are dynamic structures capable of accommodating large crustal stresses and frequent fluid influxes that incorporate compositionally diverse materials. Mechanical and chemical processes in fault zones often increase the reactivity of constituent minerals and produce complex microstructural changes. As faults experience frequent changes in environmental conditions, evidence of earlier physiochemical processes may over-printed by late-stage processes, and only be preserved as microscopic remnants. Geochemically significant elements may be present in dilute concentrations not detectable by conventional analytical methods. This approach is particularly useful for studying fluid-rock interactions and resolving the evolution of major and trace element chemistry with microstructural changes at the micro- to nano-scale.
Broader Impacts Project funding supported a summer stipend for Krishna Borhara (USU PhD student) and provided travel funding to attend and present her work at both the 2017 SCEC Annual Meeting and the 2018 Gordon Research Conference. Funds have also provided 2 annual SEM memberships and direct training and experience in SEM imaging and analysis at the USU Microscopy Core Facility for the PhD student and an undergraduate student. The PhD student gained new expertise in preparing and analyzing rock samples through the geochemical work at USU’s Stable Isotope Laboratory and in examining rock samples as a visiting scientist at the Stanford Synchrotron Radiation Lightsource (SLAC) lab. Project funding also enabled the PI to foster a new research direction in microscopy of fault rocks by conducting analyses at the Raman Micro-spectroscopy Lab in Geological Sciences at the University of Colorado-Boulder. This work is currently in preparation for publication by the PI, and will be co-authored by the PhD student and the 2 undergraduate students.
Exemplary Figure Figure 3. Example results from Raman Spectra on a Sample from the Black Fault Rock at G-2-5-3194.8 m MD (See Fig. 1). The red line represents measurements across a bounding slip surface in the black fault rock that is < 50 μm thick (see red region in image). The blue line represents measurements within a zone of ultracataclasite that is ~ 2 mm thick. D represents the defect-activated peaks and G is the graphite activated peak (Buseck and Beyssac, 2014). The Raman signatures between the two regions examined show an increase to a more ordered form of carbonaceous matter within the bounding slip surface relative to the cataclasite zone, indicating an increase in temperature localized to an extremely thin zone. Based on the shape profiles shifts of the characteristic peaks in the first-order region, the change in temperature is on the order of 50 – 100 °C with peak temperatures up to 300° or greenschist facies. Data reduction plots created by Dr. Eric Ellison at Raman Microspectroscopy Lab at UC-Boulder, CO.
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