Linking hematite (U-Th)/He dating with the microtextural record of seismicity in the Wasatch fault damage zone, Utah, USA


Ault, Alexis K.
Reiners, Peter W.
Evans, James P. and Thomson
Stuart N.

Techniques directly dating fault slip are few, limiting the ability to interpret the rock record of seismicity. Hematite is commonly found in fault zones, amenable to (U-Th)/He dating, and slip surface hematite may be reset by shear heating events and/or recrystallization. Glossy hematite-coated fault surfaces in the Wasatch fault footwall damage zone, Utah (USA), exhibit evidence of hematite cataclasis and preserve Pliocene hematite (U-Th)/He dates. Apatite (U-Th)/He and fission track data from the host gneiss indicate footwall unroofing through ∼2 km by ca. 4.5 Ma. Internally reproducible but disparate hematite (U-Th)/He dates 4.5 Ma and younger from isolated locations on a single fault surface do not reflect ambient cooling. We hypothesize that these dates, and associated iridescence and annealed crystal texture, document rapid cooling from friction-generated heat during small seismic slip events between ca. 4.5 and 2.5 Ma. Thus, hematite (U-Th)/He dating offers the potential to decipher thermal anomalies in the rock record associated with slip on 105–106 yr time scales.

Full article

A: Faults and thermochronology of north-central Wasatch Mountains, Utah (USA). Dashed blue lines are boundaries between Salt Lake City, Weber, and Brigham City fault segments. Apatite He (AHe; Armstrong et al., 2003, 2004) and apatite fission-track (AFT) data (Naeser et al., 1983) are classified by dates with red circles and green squares, respectively. The yellow rectangle is the location of B. B: Study area with hematite He sample locations (yellow squares) and results (std. dev.—standard deviation); apatite He-AFT host-rock samples from this study (circles with blue outlines) and Armstrong et al. (2004) are classified by apatite He date according to legend in A. C: Photograph of iridescent hematite-coated faults.

Publication Listing

Geology, doi: 10.1130/G36897.1 v. 43 no. 9 p. 771-774