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The Grizzly Creek shear zone is a newly discovered Proterozoic structure coincident with the southern margin of the Laramide White River uplift in west-central Colorado. The shear zone includes a 10-15 m basal mylonite oriented 255/44°NW that truncates foliation and magmatic fabrics in the footwall. The mylonite is overlain by >400 m of moderately north-dipping tectonites dominated by protomylonitic to mylonitic gneiss intercalated with mainly concordant pseudotachylyte veins and cmscale ultramylonite bands. At the upper boundary of the shear zone, the high-strain tectonic fabrics grade into relatively undeformed rocks of the hanging wall. The shear zone separates supracrustal gneisses and coarse-to-megacrystic granitoids in the footwall (south block) from fine-grained, foliated granite and supracrustal gneisses in the hanging wall (north block). Mutually overprinting brittle and plastic fault rocks, including cataclasites, pseudotachylyte fault veins, mylonitized pseudotachylyte, and ultramylonite bands, record cyclic deformation by both seismogenic rupture and plastic creep within the shear zone. Mineral lineations on mylonitic foliation surfaces plunge N-NE, and a variety of microscopic and mesoscopic shear-sense indicators indicate top-to-south reverse displacement.

The Grizzly Creek shear zone is truncated by the Cambrian-Precambrian nonconformity, which is underlain by a 1-2-m-thick paleoregolith containing altered pseudotachylyte. We interpret the shear zone to record cyclic seismogenic faulting and aseismic plastic flow at the brittle-plastic transition during Proterozoic ~N-S shortening. The age, kinematics, and tectonic style of the Grizzly Creek shear zone are consistent with the 1.4 Ga Colorado mineral belt shear zone system; a NE-trending intracontinental deformation zone that formed under a regional strain field involving N-S shortening, E-W extension, and complex transpression on subvertical shear zones 55 km to the southeast. The Grizzly Creek shear zone was brittley reactivated as a Laramide, south-vergent reverse fault with a displacement of > 200 m. We suggest that the Proterozoic Grizzly Creek shear zone represents a persistent zone of weakness in the lithosphere that significantly impacted the Cenozoic structural evolution and present geomorphology of the region.

Keywords: pseudotachylyte, ultramylonite, brittle-plastic deformation, tectonic inheritance, White River uplift, Glenwood Canyon.

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