DATING BRITTLE DEFORMATION WITH HEMATITE (U-TH)/HE CHRONOMETRY: AN EXAMPLE FROM THE WASATCH FAULT ZONE, UTAH
Hematite is preserved on a suite of highly polished, iridescent, minor fault surfaces in a 4 km long, 300 m wide zone in Paleoproterozoic Farmington Canyon Complex gneiss in the Brigham City segment of the Wasatch fault. Petrographic, SEM, and XRD observations indicate that these surfaces comprise 0.2-1 micron thick, 2-10 micron long crystalline hematite plates, consistent with formation at temperatures in excess of 100°C. Plates are not comminuted or pervasively fractured and show increasing parallel alignment with proximity to the fault plane. SEM imaging of the slip surfaces show multiple directions and likely generations of striae.
We acquired hematite (U-Th)/He data from 26 aliquots from highly polished slip surfaces on four fault planes. Dates from three faults overlap within error and are 4.9 ± 0.8, 4.9 ± 0.3 and 5.7 ± 0.7 Ma (mean and standard deviation). A fourth fault yields dates of 12.3 ± 4.2 Ma. Apatite He dates from this same locality and elevation range from 3.7 - 6.9 Ma, overlapping with the hematite He results from three samples. Based on a compilation of available hematite He diffusion kinetics, closure temperatures are a function of plate thickness and are ~110-140°C in these aliquots. Differences between apatite and hematite He dates are too small to be explained by tectonic or erosional exhumation. Hematite He dates instead require rapid cooling from hydrothermal fluid flow along these structures associated with fault activity and/or shear heating creating the iridescent, highly polished surfaces. Older, scattered dates on the fourth fault plane may reflect earlier hematite formation (~16-18 Ma) and incomplete resetting or localized U-Th loss in hematite on the fault. These results imply that the last significant seismogenic event involving hot fluids on these footwall structures occurred ~5 Myr ago.