Document Type
Article
Author ORCID Identifier
Kaitlyn Crouch https://orcid.org/0000-0002-0835-2743
James P. Evans https://orcid.org/0000-0002-2181-3866
Journal/Book Title/Conference
Tektonika
Volume
1
Issue
2
Publisher
University of Aberdeen
Publication Date
9-25-2023
Journal Article Version
Version of Record
First Page
1
Creative Commons License
This work is licensed under a Creative Commons Attribution 4.0 License.
Last Page
25
Abstract
Quantifying shallow fault zone structure and characteristics is critical for accurately modeling the complex mechanical behavior of earthquakes as energy moves within faults from depth. We examine macro- to microstructures, mineralogy, and properties from drill core analyses of fault-related rocks in the steeply plunging ALT-B2 geotechnical borehole (total depth of 493 m) across the San Gabriel Fault zone, California. We use macroscopic drill core and outcrop-sample analyses, core-based damage estimates, optical microscopy, and X-ray diffraction mineralogic analyses to determine the fault zone structure, deformation mechanisms, and alteration patterns of exhumed deformed rocks formed in a section of the fault that slipped 5-12 million years ago, with evidence for some Quaternary slip. The fault consists of two principal slip zones composed of cohesive cataclasite, ultracataclasite, and intact clay-rich, highly foliated gouge within upper and lower damage zones ∼60 m and 50 m thick. The upper ∼6.5 m thick principal slip zone separates Mendenhall Gneiss and Josephine Granodiorite, and a lower 11 m thick principal slip is enclosed within the Josephine Granodiorite. Microstructures record overprinted brittle fractures, cohesive cataclasites, veins, sheared clay-rich rocks, and folded foliated and carbonate-rich horizons in the damage zones. Carbonate veins are common in the lower fault zone, and alteration and mineralization assemblages consist of clays, epidote, calcite, zeolites, and chloritic minerals. These data show that shallow portions of the fault experienced fluid-rock interactions that led to alteration, mineralization, and brittle and semi-brittle deformation that led to the formation of damage zones and narrow principal slip zones that are continuous down-dip and along strike.
Recommended Citation
Crouch, K., & Evans, J. (2023). Shallow Composition and Structure of the Upper Part of the Exhumed San Gabriel Fault, California: Implications for Fault Processes. τeκτoniκa, 1(2), 76–100. https://doi.org/10.55575/tektonika2023.1.2.30
Comments
This article is distributed under the terms of the Creative Commons Attribution 4.0 International Licence (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided appropriate credit to the original author(s) and source, as well as link to the Creative Commons license, and indication of changes that were made.
This work was funded by NSF grants EAR-920577 and 1824852, SCEC grant 18077, and USGS-NEHRP grant 1434-92-G-2184 to Evans. Grants from the Geological Society of America - Continental Scientific Drilling Division, American Association of Petroleum Geologists L. Austin Weeks grant, a Utah State University Academic Opportunity, URCO, Peak Summer Research Fellowship, and a College of Science grant supported Kaitlyn Crouch.