HEMATITE (U-TH)/HE THERMOCHRONOMETRY REVEALS FAULT REACTIVATION AND STRAIN LOCALIZATION IN THE WASATCH FAULT ZONE, NORTHEASTERN UT, USA
Hematite He dates provide a chronologic framework for WFZ mineralization. He dates may record formation, ambient cooling, or (partial) thermal resetting from frictional heating, depending on formation depth, thermal history, and grain size/closure temperature distribution. New and previously published individual hematite aliquot He dates (±2σ) are 269.5 ± 7.6 Ma to 21.3 ± 0.6 Ma (n=28), 68.8 ± 1.9 Ma to 18.4 ± 0.5 Ma (n=7), and 42.2 ± 1.2 Ma to 1.4 ± 0.2 Ma (n=95) from veins, breccia cements, and FMs, respectively. Vein He dates are variable but increase with median plate width. This data pattern and thermal modeling of host rock apatite and zircon He data suggest vein He dates record host rock exhumation. Breccia cement and FM hematite He data patterns are inconsistent with exhumation and we interpret these results as recording mineralization and(or) subsequent thermal resetting. Hematite He data reveal mineralization at ~≥270 Ma, ~100-70 Ma, and ~17-1.5 Ma, corresponding to Ancestral Rocky Mountain orogeny, development of the Wasatch Anticlinorium, and Basin and Range extension along the Wasatch fault. These results demonstrate that meaningful date populations can be isolated from complex He data patterns. Field and hematite He data document evidence for strain localization and repeated reactivation of hematite-mineralized structures from deformation proceeding, and concomitant with, Basin and Range extension.