Date of Award:

8-2022

Document Type:

Thesis

Degree Name:

Master of Science (MS)

Department:

Geosciences

Committee Chair(s)

Alexis K. Ault

Committee

Alexis K. Ault

Committee

Kelly K. Bradbury

Committee

James P. Evans

Abstract

Fault slip relieves stress in the shallow crust by slipping suddenly during earthquakes, but some faults also slip slowly in between earthquakes. Exhumed faults, brought up to the Earth’s surface from depth, preserve a record of fault processes and slip rates informed by fault rock structures, textures, and chemistry. Hematite, a common iron oxide mineral that precipitates on fault surfaces, exhibits crystal textures that potentially indicate past slip rate. Hematite can be dated using the radioisotopic system of (U-Th)/He thermochronometry, which constrains the time when He is trapped within a crystal, a process that is a function of temperature. Exhumed faults that are parallel and connect to the San Andreas Fault (southernmost California) at depth cut crystalline rock and contain networks of small, hematite-coated faults. Here, hematite displays crystal morphology and structures that indicate hematite formed episodically a formation, repeatedly slipped at slow rates. Hematite (U-Th)/He dates show hematite formed at shallow depths ( < 1.5km) within the past million years. These data imply the shallow portion of the San Andreas fault and nearby faults with hematite-bearing rock deformed at slow rates in the past. We compare these observations to fault surfaces that were generated in a lab using shear experiments at slow rates and characterize rock friction properties influencing fault behavior. Experiments show hematite is weak and strengthens with increasing velocity, which suggests hematite can accommodate slow slip. (U-Th)/He data from lab faults indicate He was released from hematite at cooler temperatures than those that typically cause He loss, solely because grain size reduction during faulting creates new particle surfaces where He was previously trapped. This implies natural, tiny hematite particles produced during faulting may have younger (UTh)/He dates than expected. Natural and experimental observations reveal hematite fault networks can host transient slow slip.

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