Intellectual Merit. The project provided new data sets on ages of deformation and fluid-rock alteration across a major fault (Willard thrust) in the Sevier mountain belt of Utah. Geochronologic analysis was completed for 20 samples of deformed rocks, veins, and shear zones, with ~20 ⁴⁰Ar/³⁹Ar ages of mica determined per sample from detailed laser-probe dating of different microstructures at scales down to 100 microns. Results were integrated with other structural and geochemical studies of the Willard thrust system to address the following questions. (1) What were absolute timing relations between fluid flow and alteration, internal deformation, and major thrust fault slip? (2) Were deformation and fluid flow synchronous or diachronous across the thrust fault? (3) What were strain rates and did rates vary spatially and temporally? Mica (muscovite and biotite) ages ranged mostly from 140 to 115 Ma (million years ago) for rocks above the Willard thrust (hanging wall), interpreted to record early alteration and internal deformation prior to and overlapping with onset of major thrust fault slip. Mica ages for rocks below the Willard thrust (footwall) and above the Ogden-Crawford thrust fault ranged mostly from 115 to 90 Ma, with younger ages in higher strain areas, and older ages in lower strain microstructure settings that preserve early alteration textures. Variations in laser-probe ages between microstructures within individual samples record ~10-20 m.y. duration for fluid-rock alteration and internal deformation. This duration, combined with measured strain magnitudes for different rock types in the study area, yielded natural strain rates broadly consistent with rates predicted from extrapolation of laboratory deformation studies of feldspar, quartz, and muscovite to geologic conditions. Alteration, decreasing grain size during deformation, hydrolytic weakening, and enhanced fluid pressures likely contributed to weakening of rocks and concentration of slip along major thrust faults. As the Willard thrust sheet was emplaced, it cooled, while fluid flow, alteration, and internal deformation became focused in the footwall, leading to weakening and ultimately to concentrated slip along the Ogden-Crawford thrust system. Such progression of fluid flow and weakening is important for understanding the mechanics and growth of mountain systems.

Broader Impacts. One graduate student received training in geochronologic analysis, finished a PhD, and is now working in industry, and one undergraduate student received training in electron microscopy, finished a BS, and is now working professionally. Results were presented at professional meetings and two manuscripts are in preparation for submission to scientific journals based on this project. This project promoted collaboration between Weber State University, University of Nevada Las Vegas, and the Nevada Isotope Geochronology Lab. Application of detailed laser-probe analysis for dating individual microstructures was tested and protocols refined, which can now be applied to other studies on timing of fluid flow and deformation along other fault zones.

Last Modified: 03/29/2014
Modified by: W.A. Yonkee