Unraveling thermochronometric signatures of mineralization, deformation, and exhumation with hematite (U-Th)/He thermochronometry and thermal history modeling: an example from the Wasatch fault zone, northeastern UT, USA

Abstract:

Integrated fault rock thermochronometry, textural analysis, and thermal history modeling can fingerprint commonly synchronous thermal processes in fault zones such as mineralization, deformation, fluid circulation, and exhumation. Hematite is common in fault rocks and amenable to (U-Th)/He (HeHe) thermochrometry. We apply this integrated approach to specularite veins and hematite-coated fault surfaces (FS) and cataclasite with underlying hematite-cemented breccia hosted in the Wasatch fault zone (WFZ) to interrogate the thermal evolution of WFZ fault rocks. Hematite in veins and breccia matrix (BM) consists of undeformed specularite plates ~1-10s of μm-thick. Fault surface hematite is dominantly sub-μm cataclastic grains with sintered grains at the slip interface, suggestive of coseismic frictional heating. New and published HeHe dates from polycrystalline aliquots are ~270 to ~21 Ma (n=28), ~69 to ~18 Ma (n=7), and ~42 to ~1 Ma (n=105) from specularite, BM, and FSs, respectively. Specularite and BM dates increase with median plate width (and closure temperature) along two arrays: (1) from ~270 to ~20 Ma over ~15 to ~8 μm plate width, and (2) from ~75 to ~17 Ma from ~40 to ~10 μm plate width.

We employ forward thermal history simulations in combination with apatite and zircon (U-Th)/He and apatite fission-track data and independent geological constraints to interpret HeHe data patterns. Models vary potential hematite mineralization age and consider peak Mesozoic reheating temperatures consistent with conventional and HeHe thermochronometry. Results suggest hematite precipitation at ~1680, ~110 Ma, ~20 Ma, and ~12 Ma with superposed burial and exhumation. Modeling suggests distinct, but overlapping, HeHe date-plate width arrays form because variably aged hematite is sensitive to different portions of the ambient thermal history. Models of seismic slip (cf. McDermott et al., 2017, EPSL) reveal some FS HeHe dates reflect partial resetting by friction-generated heat. Our data and models show WFZ hematite faults developed by multiple stages of mineralization and deformation, some of which pre-date Miocene Basin and Range extension. Holistic thermal modeling approaches are useful for interpreting thermochronometric data from rocks traversing complex thermal fields.