UV LASERPROBE 40AR/39AR DATING OF EARLY INTERNAL STRAIN AND LATE FLUID-INDUCED STRAIN LOCALIZATION ASSOCIATED WITH THE WILLARD THRUST, UTAH

Determining the absolute ages of deformation fabrics is critical to resolve progressive deformation histories in orogenic belts and to address orogenic-scale kinematic and mechanical evolutions. In situ isotopic analysis is particularly advantageous because the relationships between the deformation fabric, analyzed mineral, and isotopic age can be established. Greenschist-facies rocks provide opportunities to directly date deformation by ⁴⁰Ar/³⁹Ar as muscovite ages may reflect deformation ages when isotopic closure temperatures are higher than crystallization or growth temperatures. We have applied in situ ⁴⁰Ar/³⁹Ar UV laserprobe to date deformation fabrics related to early and late stages in the history of the Willard thrust, the westernmost major thrust in the Wyoming salient of the Sevier fold-thrust belt. Two structural settings were studied: (1) the basal thrust sheet at Fremont Island (FI) where Neoproterozoic diamictite and greywacke exhibit a NW-dipping cleavage relative to bedding and record variable thrust-parallel ESE shear and thrust-perpendicular flattening strains; and (2) a stratigraphically-localized shear zone recording westward increasing top-east shear in Neoproterozoic diamictite, which lies depositionally on Paleoproterozoic basement in the footwall at Antelope Island (AI). Analyzed textural sites of syntectonic mica from 13 rock chips from AI and 6 rock chips from FI include microshears, strain shadows, veins, and mica beards within diamictite matrix and clasts. UV laserprobe ages of phengitic muscovite from AI dominantly range from 115 to 90 Ma, with the youngest ages from high-strain zones, similar to the age range of heating steps for 4 furnace step-heated samples. Syntectonic phengitic muscovite at AI was produced by fluid-induced alteration of K-feldspar and plagioclase. Estimated deformation temperatures of 300-350°C from microstructures, consistent with local preservation of Proterozoic argon ages in coarse muscovite, support interpretation of 115 to 90 Ma ages as growth ages, recording progressive reaction-softening and strain localization. Laserprobe ages of phengitic muscovite and biotite from FI yield dominant age populations of 130-145 Ma, interpreted as metamorphism related to ingress of fluids from the tectonically-thickened hinterland.