GRIZZLY CREEK SHEAR ZONE: A NEWLY RECOGNIZED FRICTIONAL–PLASTIC DEFORMATION SYSTEM IN THE WHITE RIVER UPLIFT, COLORADO

We report the discovery of a diverse suite of tectonites that include mutually overprinting brittle and plastic fault rocks within Paleoproterozoic basement along the southern margin of the Laramide White River uplift in west-central Colorado. The deformation system, herein referred to as the Grizzly Creek shear zone (GCSZ), includes mylonite, ultramylonite, mm- to cm-scale pseudotachylyte veins (pst), mylonitized pseudotachylyte (mpt), cataclasite, and younger brittle faults hosted by gneisses and two sheared granitoids. The GCSZ strikes W to SW and dips north (30–60º) above subvertically foliated biotite-muscovite gneisses and a porphyritic granodiorite. In steep canyon exposures above Grizzly Creek, the GCSZ consists of a 10–15-m-thick mylonite, overlain by >300 m of gneisses cut by pst fault veins (n >75). Cumulative vein thickness measured across the shear zone indicates that >0.25% of the host rock was frictionally melted during multiple episodes of pst generation. The upper 120 m of the pst zone changes character 4 km westward along strike, where a strongly foliated granitoid that hosts pst, mpt, and thin mylonites is in pst-bearing fault contact with underlying gneisses. Mineral lineations on mylonitic foliation planes plunge N-NE, and diverse kinematic indicators, including plastically deformed pst injection veins, indicate top-to-S displacement. The GCSZ is truncated by the Cambrian-Precambrian nonconformity, where pst and mpt is included in a weathered paleoregolith below the Sawatch Formation, documenting a Proterozoic origin for the shear zone.

We interpret the GCSZ to record cyclic seismogenic faulting and plastic flow at the brittle-plastic transition during ~N-S compression. The system may be temporally correlative with Mesoproterozoic intracontinental transpression along the NE-striking Colorado mineral belt shear zone system, which incorporates a similar suite of tectonites 55 km to the SE. The GCSZ was brittly reactivated in the Phanerozoic and is cut by a S-vergent reverse fault with ~200 m of stratigraphic offset. On a regional scale, the GCSZ is along strike of a prominent E-W bend in the Grand Hogback monocline to the west and is coincident with the Laramide flexural hinge of the White River monocline, suggesting a Precambrian ancestry for these structures.