Rocky Mountain Section - 72nd Annual Meeting - 2020

Paper No. 19-4
Presentation Time: 2:30 PM


MCDERMOTT, Robert G. and AULT, Alexis K., Department of Geosciences, Utah State University, 4505 Old Main Hill, Logan, UT 84322

Extensional fault systems exhibit complex relations between new fault growth and reactivation of pre-existing fabrics and structures. The Wasatch fault zone (WFZ), UT, demarcates the eastern limit of Basin and Range extension, but represents only the most recent component of the Wasatch Front’s ~1.7 Byr deformation history. We combine field observations with hematite (U-Th)/He (He) thermochronometry of fault and vein systems in the Brigham City segment of the WFZ to reveal spatiotemporal patterns of fault reactivation. Three types of hematite occur in the WFZ: undeformed specularite veins, high-gloss fault mirrors (FMs), and hematite-cemented breccia reactivated as FMs. Field and microtextural observations provide evidence for multiple episodes of mineralization within individual vein sets. FM surfaces are co-located with veins and show multiple striae, but normal-sense slip is most recent.

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.