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

Title of Publication: 
Linking hematite (U-Th)/He dating with the microtextural record of seismicity in the Wasatch fault damage zone, Utah, USA
Author: 
Ault, Alexis K., Reiners, Peter W., Evans, James P. and Thomson, Stuart N.
Publication Info: 
Geology, doi: 10.1130/G36897.1 v. 43 no. 9 p. 771-774
Abstract: 

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.