21 Advancements in the Proterozoic tectonics of western North America

This session highlights recent developments in understanding of the Proterozoic tectonic record of western Laurentia, particularly its relation to critical mineral resources. Insights derived from all disciplines and approaches are welcome.

Tuesday, 19 May 2026
9:00 AM - 12:00 PM
Alvarado F

Mesoproterozoic (~1.33 Ga) Greenschist-Facies Mylonites and Brittle-Plastic Faults Record NW-SE Shortening in the Skin Gulch Shear Zone, Northern Colorado Front Range

Paper number: 21-3
Program: Technical Sessions
Time: 9:45 AM – 10:05 AM

Authors

Abstract

Most metamorphism and penetrative deformation of basement rocks in the northern Colorado Front Range is interpreted to record late Paleoproterozoic (ca. 1.78–1.65 Ga) convergent tectonism; however, Mesoproterozoic (ca. 1.4 Ga) deformation associated with the cryptic Picuris orogeny has been increasingly recognized across the Yavapai Province. We present new macro- and microstructural data and high-temperature thermochronological data that provide evidence for Mesoproterozoic deformation along the NE-striking Skin Gulch shear zone in Poudre Canyon, northern Colorado. Amphibolite-facies (ca. 1.7 Ga?) mylonitic and gneissic fabrics in the Skin Gulch shear zone are locally overprinted by moderately to steeply SE-dipping mylonites and ultramylonites that predominantly record SE-side-up reverse-sense shear. Chlorite-coated brittle-plastic faults with reverse slip commonly parallel these mylonitic fabrics or cut across gneissic fabrics and consistently record NW-SE shortening. The mylonites and brittle-plastic faults are characterized by quartz subgrain rotation recrystallization ± bulging recrystallization, brittle feldspar deformation, and/or narrow (<1 mm thick) foliated chlorite+white mica zones with shear bands, consistent with greenschist-facies conditions. Electron backscatter diffraction (EBSD) analysis of two brittle-plastic shear zone samples yielded well-defined quartz crystallographic preferred orientations with c-axis opening angles corresponding to 370±50°C to 335±50°C dislocation creep temperatures. A sample of biotite gneiss from this area yielded an apatite U-Pb date of 1317 ± 33 Ma (Tera-Wasserburg lower intercept age) and a biotite 40Ar/39Ar date of 1328 ± 5 Ma (weighted mean of flat part of age spectra). These dates collectively record the timing of cooling across greenschist-facies conditions, and the 40Ar/39Ar biotite date likely records the timing of cooling below ~380–300°C (closure temperature) and thus approximates the age of brittle-plastic strain. The ~1.33–1.32 Ga timing of this NW-SE contraction is notably younger than Mesoproterozoic magmatism and deformation elsewhere in the Rocky Mountain region and suggests that the Picuris orogeny may have at least locally been a more protracted event than previously thought. Our reconnaissance work on adjacent structures such as the South Fork antiform suggests that Mesoproterozoic (?) greenschist-facies to brittle-plastic NW-SE shortening is not restricted to map-scale shear zones.

Geological Society of America Abstracts with Programs. Vol. 58, No. 4

doi: 10.1130/abs/2026RM-410641

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