253-3 Mesoproterozoic NW-SE Shortening under Greenschist-Facies to Brittle-Plastic conditions in the Skin Gulch Shear Zone and South Fork Antiform, Northern Colorado Front Range

Abstract:

Basement rocks in the northern Colorado Front Range record widespread Paleoproterozoic (~1.78–1.65 Ga) metamorphism and penetrative deformation associated with the Yavapai orogeny, but Mesoproterozoic (~1.4 Ga) deformation related to the cryptic Picuris orogeny has been increasingly recognized across the region. We present new structural and thermochronological data that provide evidence for Mesoproterozoic deformation along the NE-trending Skin Gulch shear zone (SGSZ) and South Fork antiform in Poudre Canyon northwest of Fort Collins. Amphibolite-facies mylonitic and gneissic fabrics (~1.7 Ga?) in the SGSZ are locally overprinted by mylonites and brittle-plastic reverse faults that record NW-SE shortening. These structures are characterized by quartz subgrain rotation ± bulging dynamic recrystallization, feldspar fracturing, and/or narrow (<1 mm thick) sheared chlorite+white mica zones, consistent with dominantly greenschist-facies deformation conditions. Two brittle-plastic shear zone samples have well-defined quartz crystallographic preferred orientations with c-axis opening angles corresponding to ~300–400°C dislocation creep temperatures. Mylonitization locally involves the ~1.4 Ga Silver Plume Granite and pegmatites that are likely Mesoproterozoic. A sample of Paleoproterozoic biotite paragneiss from the SGSZ yielded an apatite U-Pb date of 1317 ± 33 Ma (Tera-Wasserburg lower intercept) and a biotite ⁴⁰Ar/³⁹Ar date of 1328 ± 5 Ma. These dates collectively record the timing of cooling across greenschist-facies conditions and approximate the age of mylonitization and 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.

The South Fork antiform, ~2–3 km NW of the SGSZ, is a SE-vergent structure associated with a previously unmapped ~10 m-thick, NW-dipping reverse-sense shear zone on the overturned southeastern limb. Microstructures in the shear zone and weakly developed axial planar foliation in the antiform hinge are consistent with greenschist-facies deformation conditions. Gneissic foliation surfaces commonly preserve brittle-plastic flexural shear with lineations that parallel the antiform profile plane. As with the SGSZ, we interpret these structures to record Mesoproterozoic NW-SE shortening superposed on ~1.7 Ga amphibolite-facies fabrics. Mesoproterozoic deformation was likely more widespread across the northern Front Range than previously recognized, and future work will focus on constraining the timing of greenschist-facies and brittle-plastic deformation in the region.  

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