Hematite Fault Rock Thermochronometry and Textures Inform Fault Zone Processes

Document Type

Article

Journal/Book Title/Conference

Journal of Structural Geology

Volume

133

Publisher

Elsevier Ltd

Publication Date

2-2-2020

Award Number

NSF, Division of Earth Sciences (EAR) 1654628

Funder

NSF, Division of Earth Sciences (EAR)

Abstract

Minerals on fault surfaces record fluid-rock interaction, strain, and/or heat during fault slip. Fault-hosted hematite is amenable to (U–Th)/He (He) thermochronometry. Hematite He dates reflect fault-related processes or exhumation, depending on hematite growth above or below the grain-size controlled closure temperature and processes that promote open system behavior. Deconvolving these phenomena requires multi-scale hematite textural characterization and constraints on the ambient thermal history from conventional thermochronometry. Two examples of this approach include investigations of hematite-coated fault surfaces from exhumed damage zones in the seismically active Wasatch fault zone (WFZ), UT, and Painted Canyon fault (PCF) paralleling the southern San Andreas fault, Mecca Hills, CA. WFZ fault mirrors preserve sub-μm-scale polygonal grain textures and hematite He data patterns that, together with host-rock apatite He data, are consistent with localized friction-generated heat, hematite He loss, and hematite recrystallization during damage zone seismicity <4.5 Ma. In contrast, PCF hematite slip surfaces comprise foliated, nm-thick, high-aspect ratio plates, analogous to scaly clay fabrics, and hematite He dates record syn-kinematic mineralization episodes ~0.7–0.4 Ma via creep processes. These case studies highlight the power of integrated hematite textural observations and targeted fault rock thermochronometry to inform the timing and conditions of seismic to aseismic slip processes.

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