Fabric Analysis of Survivor Clasts in the Southwest Deforming Zone of the San Andreas Fault at Three Kilometers Depth

The research goal is to constrain the kinematics and deformation mechanisms of the Southwest Deforming Zone (SDZ) of the San Andreas Fault through fabric analysis of survivor clasts recovered during Phase 3 drilling at the San Andreas Fault Observatory at Depth (SAFOD). Evidence of pressure-and chemical-dependent processes have been reported for the actively creeping SDZ. Fault rock particle size distributions, fit to a power law probability distribution, are used to discriminate deformation mechanisms. Previous studies have not demonstrated whether the power law is the most appropriate probability distribution, and whether the scaling parameter can discriminate between different deformation processes. I propose to characterize particle size-and shape-distributions of survivor clasts in the SDZ through 3D analysis of X-Ray Computed Tomography (XCT) images over a range of scales. Following a published statistical method to estimate the best-fit power-law scaling parameter, I will assess the goodness-of-fit of different probability distribution functions to observations. The scaling parameter will be compared to published model predictions to infer the dominant deformation mechanism. Preliminary analysis suggests the observed size distributions are not well-fit by the power law distribution, unlike previous work. A poor fit may reflect bias in the measurement of small particles resulting from the XCT imaging resolution limits. Accordingly, I will conduct additional imaging to extend characterization of smaller particle sizes. Also, I will particle shape analysis as an independent means to infer kinematics and deformation mechanisms. These results will lead to better particle distribution analysis methods and improve understanding of the SDZ creep mechanisms.