Carbonaceous fault-related rocks in SAFOD Phase III core: Indicators of fluid-rock interaction and structural diagenesis during slip

Krishna Borhara, Kelly K. Bradbury, & James P. Evans

Published August 14, 2017, SCEC Contribution #7633, 2017 SCEC Annual Meeting Poster #198 (PDF)

At elevated temperatures, many carbon-bearing fault zones are subjected to thermal maturation, fluid-rock interactions, and/or shear-induced phase transformations, often yielding various fault weakening agents. Black carbonaceous material documented by Bradbury et al. (2011) in SAFOD Phase III core exhibits intense comminution, shear-induced slip localization, fragmented shear zones, stylolites, calcite-cemented breccia, calcite vein fragments of varying trace element chemistry, and development of Riedel shears. Presence of calcite-cemented carbonaceous ultracataclasites in creeping segments of the Central Deforming Zone (CDZ) and Southwest Deforming Zone (SDZ) of the San Andreas Fault raises questions regarding the nature of thermochemical reactions that promote dynamic weakening and strength recovery in carbon-rich fault gouges during the seismic cycle. Preliminary analyses show that these ultracataclasites yield high total elemental carbon values between 0.4% and 1.9%, with highly negative δ13C values between -24.68‰ and -21.80‰ nearing that characteristic of organic matter. Presence of 12C-enriched material within local fractures and shear zones in the core suggests the possibility of hydrocarbon migration along the fault, consistent with previous mud-gas analyses. Microscale X-ray fluorescence (μXRF) elemental mapping of this gouge using synchrotron radiation reveals a Ti-enriched, Mn-depleted carbonaceous cataclasite cemented by Ti-depleted, Mn-enriched calcite veins with varying degrees of Ni. The nature of fluid-rock interactions in carbon-rich gouges may provide insight into how distribution and alteration of carbonaceous material in faults may affect frictional properties of the gouge. Understanding the dynamic evolution of strength and slip behavior of major tectonic faults is a fundamental problem in earthquake mechanics and seismic hazard assessment.

Key Words
carbonaceous gouge, fault weakening, SAFOD, fluid-rock interactions, phase transformations, microstructures

Citation
Borhara, K., Bradbury, K. K., & Evans, J. P. (2017, 08). Carbonaceous fault-related rocks in SAFOD Phase III core: Indicators of fluid-rock interaction and structural diagenesis during slip. Poster Presentation at 2017 SCEC Annual Meeting.

Related Projects & Working Groups
Fault and Rupture Mechanics (FARM)