STRUCTURE AND GEOCHEMICAL NATURE OF CARBONACEOUS MATTER IN FAULT ROCKS OF SAFOD PHASE 3 CORE: IMPLICATIONS FOR DEFORMATION AND FLUID-ROCK REACTIONS

The presence of carbonaceous material (CM) along or adjacent to slip surfaces in the San Andreas Fault zone sampled by Phase 3 Core at SAFOD is significant due to its unique and variable physical, chemical, electrical, and mechanical properties. The amount and distribution of this type of CM in heterogeneous and in clay-rich fault gouges is challenging to detect at the mesoscale. We probe the structure and chemistry of these materials with a variety of micro-scale and whole-rock mineralogical and geochemical methods. Scanning-electron microscopy (SEM) is used to identify textures, and quantitative elemental analysis is used to determine isotope ratios and weight proportions of C, O, and N, as well as to determine the ratio of inorganic carbon to organic carbon, and Raman Spectroscopy examines the structure and crystallinity of CM.

The spatial distribution of CM and changes observed at the micro-scale are examined as a potential indicator of mechanical pulverization or redox chemistries. This may provide insight into the deformation history observed within the actively deforming segment of the SAF sampled at SAFOD. The micro-scale properties and geochemical alteration of CM can serve as a signature for temperature-related reactions and fluid-volatile conditions and can be used to inform its potential effect on the mechanical behavior of fault zones and fluid-rock reactions within the SAF at SAFOD.

The focus of this work is on the Raman analyses performed on SAFOD core. Preliminary results from Raman analyses of previous SAFOD samples indicate that there is a correlation between the crystallinity of CM and its location relative to slip surfaces or zones of deformation with greater order being observed closer to these regions. We predict that not only will this trend hold true with the new samples and data, but that a larger trend will exist across the fault zone reflecting the same principle: more amorphous and disordered CM farther from the active fault traces, and more ordered and crystalline CM towards the core of the fault traces. The variation in Raman spectra within the SAFOD CM is not great enough to be used as a geothermometer, but systematic measurements of the degree of CM crystallinity across Phase 3 core, by proxy, may provide signatures for relative temperature variations along the sampled SAF fault at SAFOD.