SCEC Award Number 19166 View PDF
Proposal Category Collaborative Proposal (Data Gathering and Products)
Proposal Title Collaborative Research: Insights into the episodicity of fault-rock development from Pb-isotope studies of San Andreas fault gouge
Name Organization
Randolph Williams University of Wisconsin, Madison Christie Rowe McGill University (Canada)
Other Participants An undergraduate research assistant (TBD) would be hired in the Spring of 2019.
SCEC Priorities 3f, 3d, 3g SCEC Groups FARM, SAFS, Geology
Report Due Date 03/15/2020 Date Report Submitted 04/01/2020
Project Abstract
The main goal of this project is to quantify the fluid budgets, time scales, and mechanisms governing the growth of frictionally weak clay minerals in San Andreas fault (SAF) gouge. To accomplish this, we are examining pristine fault rocks acquired from recent cores taken across the Mojave segment of the SAF near Elizabeth Lake, California. Our previous work on these core samples used a combination of field, microstructural, and geochemical analysis to define the combined mechanical and geochemical pathways of fault-rock development (SCEC #18189; PI’s Savage, Williams, Rowe). The combination of those results with recent triaxial deformation experiments show that the formation of even modest amounts of mixed-layer illite-smectite during fluid-rock interactions results in profound decreases in the frictional strength of fault rocks. To better constrain the processes leading to illite-smectite formation, our current work is employing a novel application of lead (Pb) isotope analysis. By comparing the Pb isotope composition of illite-smectite extracted from fault gouge with the parent minerals from which it formed in the protolith, we are determining if clay-forming fluids were dominated by local Pb sources (implying low fluid-rock ratios) or external sources, possibly during co-seismic pulses of fluid flow (implying higher fluid-rock ratios). The latter may imply clay growth that is genetically related to individual fault-slip events, and punctuated changes in fault-rock friction through time.
Intellectual Merit The stability and strength of upper crustal faults are controlled in part by the physical properties of fault rocks. Within this, the incorporation of frictionally weak phyllosilicates is a well known control on mechanical strength. Our previous work on an exceptional suite of fault-rock samples collected from core across the Mojave segment of the San Andreas fault (SAF) documented the combined mechanical and geochemical pathways of fault-rock evolution as they pertain to the incorporation and/or formation of phyllosilicates. Of these, the geochemical aspects are relatively poorly understood by the fault mechanics community. For example, virtually nothing is known about the volumes or sources of fluids during clay-mineral authigenesis. The timescales of fluid-rock interaction contributing to clay-forming reactions are similarly unknown. Our current work represents a fundamentally novel attempt to apply Pb isotopic analyses to solve these problems by providing direct constraints on fluid-rock ratios during fault-rock development, likely permitting inferences on the relative time scales (i.e. continuous versus episodic) of clay mineral formation. These results will inform our understanding of the processes contributing to fault-rock weakening in the SAF system and beyond.
Broader Impacts Our project has significant societal implications for the Los Angeles area. The original coring for this work was commissioned by the LA Department of Water and Power to assess seismic hazards associated with the Elizabeth Lake tunnel, part of the LA aqueduct system that crosses the San Andreas fault near Lake Hughes, CA. This tunnel is responsible for delivering a substantial percentage of potable water to the LA metropolitan area. A recent LiDAR survey through the tunnel revealed no visible deformation in the vicinity of the known fault trace, suggesting that the San Andreas has been locked in this area (at least at this depth) since 1911. Our work on the Elizabeth Lake core samples is helping to define the width of the deforming zone around the fault, in addition to the fundamental mechanical properties of the fault rocks. We remain in close contact with representatives of the LADWP with respect to project results. In addition, this grant supported an undergraduate student researcher (Annabelle Sobotik) at the University of Wisconsin-Madison. Sobotik gained significant experience in a variety of protocols related to sample preparation and analysis as part of her work. She has subsequently begun a senior research project with PI Williams examining fault rock development in a different part of the San Andreas system (unrelated to this proposal).
Exemplary Figure Figure 2. Friction coefficients determined for fault rocks through triaxial deformation experiments. Illite/smectite(I/S)-rich gouges are profoundly weak when compared to chlorite-rich gouges and high-strain fault-damaged rocks (FDR). Protolith granodiorites and low-strain FDR samples are considerably stronger than all other samples. Our research is focused on determining the origin and timescales of formation of I/S, which appears to exert a significant control on fault-rock frictional strength even at low abundances.