Digital Appendix of Masters Thesis "Shallow Composition and Structure of the San Gabriel Fault, California in Drill Core and Geophysical Logs: Implications for Fault Slip and Energetics"


Characterizing the behavior and structure of shallow faults is critical to evaluating earthquake hazards and modeling seismic energy dissipation. Macro- to microanalysis of geotechnical drill core acquired across the San Gabriel fault (SGF), California provides insight into the composition, properties, and structure of shallow faults and the upper transition from seismic slip to aseismic creep within the fault zone. The SGF accommodated right-lateral slip 12 to 5 million years ago and has since been exhumed in the San Gabriel Mountains. Borehole ALT-B2 plunges 68° to the south and forms a shallow angle with the 80° north- dipping SGF. The borehole provides a near continuous sampling suite of ~500 m of protolith and fault-related rocks that formed at depths of ~2-2.5 km, within the upper seismic-aseismic transition zone. In this study, we present integrated mesoscopic drill core data and observations, microstructural and geochemical data, and synchrotron-based XRF elemental maps from rocks from borehole ALT-B2. We supplement results from the drill core with some field samples that encompass the surface traces of the SGF in Little and Big Tujunga Canyons, ~8-16 km southeast of the borehole site. Results from these analyses provide a suite of physical and chemical properties of moderately to highly indurated fault-related rock from the shallow SGF. We document overprinting and cross-cutting relationships of brittle and plastic deformation and evidence for hydrothermal alteration. These results indicate that the SGF accommodated both seismic slip and aseismic creep mechanisms in the shallow crust and that fluid-rock interactions vary spatially in the fault zone and occurred throughout multiple cycles of slip accommodation. We suggest that the upper transition from seismic-dominated slip to aseismic-dominated creep is influenced by fluid-rock interactions and the associated formation of and changes in clay minerals. The implications of this study may be used as an analog for deciphering the nearby active San Andreas fault system and other transform faults. Synchrotron X-ray fluorescence (XRF) mapping is a novel technique in shallow fault studies that allows us to examine the spatial distribution of elemental changes within samples at the micron- to grain-scale and how these elemental signatures are related to the different rock types/fabrics in the deformed rocks. Our XRF mapping results show concentrations of Fe, Mn, Ni, Cr, V, Ti, As, and Zn in altered and damaged fault-related rock, including shear zones, slip surfaces, breccia matrices, veins, fractures, and injections into host rock and existing fabrics. These data reveal variations in the nature, expression, and degree of fluid-rock interactions at the microscale and the segregation and mixing of elements within deformation fabrics in the shallow fault.

Author ORCID Identifier

James P. Evans

Document Type




File Format

.xslx, .docx, .jpg, .pdf, .txt, .doc, .hpf, .xrdml, .bak, . rpm, .hdf5, .dat

Viewing Instructions

Sam's microprobe Analysis toolKit (SMAK) or similar software is needed to process synchrotron XRF map data. See README for further information.

Publication Date



Geological Society of America

Utah State University

American Association of Petroleum Geologists

NSF, Division of Earth Sciences (EAR)

Southern California Earthquake Center


Utah State University

Award Number

NSF, Division of Earth Sciences (EAR) 1824852; Southern California Earthquake Center 18077



Referenced by

Crouch, Kaitlyn A., "Shallow Composition and Structure of the San Gabriel Fault, California in Drill Core and Geophysical Logs: Implications for Fault Slip and Energetics" (2022). All Graduate Theses and Dissertations. 8451.

Start Date


End Date



San Gabriel Mountains



Code Lists

TS: Thin Section, XRF: X-ray fluorescence, XRD: X-ray diffraction, PSS: Principal slip surface, m: meter, ft: feet, DZ: Damage zone, PSZ: Principal slip zone, LT: Little Tujunga, BT: Big Tujunga, Pm: Mendenhall Gneiss, Kj: Josephine Granodiorite, ppm: parts per million


Earth Sciences


Creative Commons License
This work is licensed under a Creative Commons Attribution 4.0 License.


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