Dating brittle fault slip is a research frontier essential for characterizing numerous upper crustal processes. Direct, robust, and efficiently obtained timing constraints are necessary to reconstruct structural-tectonic histories, mountain building processes, and landscape evolution, as well as to document ancient seismicity histories to assess modern seismic hazards, earthquake forecasting, and fault mechanics. However, direct dating of fault activity in the upper crust is challenging due to limited applicable radioisotopic techniques, and few robust geologic indicators of past seismic slip exist in the rock record. This project tests a new potentially transformative method to directly date fault slip by using the (U-Th)/He on iron oxide coatings on fault surfaces collected from active and ancient faults as well as explores the chemistry and physics underpinning the method. The project advances desired societal outcomes through: (1) increased public scientific literacy and public engagement with science and technology through contributions to traveling displays for K-12 students, development and presentation of elementary school educational modules, participation in university outreach activities, and development of projects and lessons for northern Utah home-schooled students; (2) development of a globally competitive STEM workforce through training of graduate and undergraduate students; (3) increased partnerships with other academic institutions and the U.S. Geological Survey.
The principal research objective of this project is to develop hematite (U-Th)/He geochronology as a method to directly date fault slip. (U-Th)/He dates from hematite-coated fault surfaces record brittle deformation events by constraining the timing of either synkinematic hematite formation or the rapid cooling from frictional heating during faulting. In some cases, these dates may track regional cooling, yielding a new tool to quantify tectonic exhumation or erosion linked to broader fault zone evolution. This method is tested on three active or ancient fault systems in the North American Cordillera: the northern Wasatch fault zone in Utah; faults in the central Colorado Front Range; and the eastern Denali fault zone in the Yukon. The project involves: (1) macroscopic characterization and structural analysis of hematite-coated faults and microscopic determination of hematite occurrence, texture, and grain size; (2) characterization of He diffusivity from 4He/3He diffusion experiments; (3) targeted (U-Th)/He dating of hematite-coated faults at each locale including multiple samples from the same fault surface; (4) development of independent constraints on fault surface thermal histories using transition metal paleothermometry from X-ray photoelectron spectroscopy and 4He/3He thermochronometry; and (5) establishment of independent constraints on fault activity timing using U-Pb dating and existing 40Ar/39Ar illite age data of related synkinematic calcite and clay-rich fault gouge, respectively, and regional cooling patterns from host-rock apatite (U-Th)/He and fission track data.
