Strain patterns within part of the Willard thrust sheet, Idaho-Utah-Wyoming thrust belt

The Willard thrust sheet, located in northern Utah, USA, contains a 10-15-km-thick sequence of strata that were internally deformed and translated about 50 km ESE. Upper levels of the sheet are relatively little deformed, lower levels have minor folds and cleavage that increase in intensity downward and rotate into parallelism with the thrust, and the basal fault zone contains mylonite and ultracataclasite. Finite strain displays systematic changes with structural position and lithology. Basal levels have strain ratios (R≈RXZ) of 3-5, and angles (θ) between stretch directions and the thrust of <10° in the profile plane. Diamictite, slate, and greywacke at lower levels have R of 1.5-3 and θ of 10-35°, with overall correlations between increasing R, decreasing θ, and greater mica content. Overlying micaceous quartzite has relatively low strain, with R of 1.3-2 and θ>50°. Principal stretching (X) directions trend perpendicular to oblique to minor fold axes, which are locally oblique to the overall transport direction, reflecting partial decoupling of internal deformation from thrust slip. Principal axial ratios have RYZ>RXY, but microfabrics indicate only limited extension in the intermediate stretching direction (Y), corresponding to volume losses of about 10-40%. Strain is factored into components of thrust-perpendicular thinning and thrust-parallel shear that increase downward and in mica-rich layers, minor thrust-parallel shortening to extension, minor longitudinal extension, and minor longitudinal (wrench) shear. These patterns reflect litho-mechanical layering of the thrust sheet, which includes a very weak basal fault zone that formed at elevated fluid pressures, a weak mica-rich lower level with widespread crystal plastic and mass transfer deformation, and a stronger upper part with limited internal deformation.