Date of Award

5-2024

Degree Type

Report

Degree Name

Master of Science (MS)

Department

Geosciences

Committee Chair(s)

Kelly Keighley Bradbury

Committee

Kelly Keighley Bradbury

Committee

Elizabeth Petrie

Committee

James P. Evans

Abstract

Differing crystallinities of carbonaceous material are common in fault rocks across a range of geologic settings and spatial scales and may provide constraints on strain rate, the nature of fault slip, fluid-rock interactions, and temperature variations over the earthquake cycle. The Pioneer fault at Little Fall Creek in south-central Idaho provides an excellent opportunity to study nanostructure changes of carbonaceous matter as a function of fault deformation. At this location, the Pioneer Fault exhibits a well-defined principal slip zone (PSZ) composed of multi-layered white siliceous mineralization and black carbon to graphite with a continuously exposed adjacent damage zone that includes an array of carbonaceous- to graphite-bearing deformation structures. The Pioneer Fault is a post-Mississippian fold-and-thrust belt structure with geologic evidence for low-angle Eocene normal slip reactivation. Carbonaceous-bearing fault gouge is generally weak, and the combination of silica and carbon coated slip-surfaces may demonstrate evidence for multiple episodes of fluid-rock interaction and/or variations in strain rate during fault slip within the Pioneer Fault zone. We examined the mineralogical, geochemical, and textural relationships of a select suite of carbonaceous- bearing fault rock structures that extend from the principal slip zone (PSZ), across the adjacent footwall damage zone of the Pioneer Fault, and up-section to where the hanging wall meets the footwall with exposed protolith. We begin the process of correlating the nanostructure of carbonaceous matter determined from Raman spectroscopy analyses to the meso- to micro-scale compositional and textural observations. This work contributes to understanding the potential spectrum of CM/graphite-bearing fault rocks formed by varying deformation processes and/or fluid rock interactions during fault slip.

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