AGE, SETTING, AND REACTIVATION OF THE GRIZZLY CREEK SHEAR ZONE: TECTONIC IMPLICATIONS OF A NEWLY RECOGNIZED BRITTLE/PLASTIC THRUST IN CENTRAL COLORADO

The Proterozoic Grizzly Creek shear zone (GCSZ, Allen et al., this volume) in the White River uplift of west-central Colorado records a long history of overprinting brittle and plastic deformation suggesting that the GCSZ corresponds to a previously unrecognized crustal-scale structure. The GCSZ (orientation ~260 40NE cross-cuts shallowly N-dipping augen gneisses in the footwall produced by the deformation of the margin of an undated megacrystic granite. The GCSZ comprises protomylonite, mylonite, pseudotachylyte (pst), and mylonitized pseudotachylyte (mst). Mutually overprinting mylonite, pst, and mst indicate broadly synchronous brittle and plastic deformation under mid-crustal conditions similar to ca. 1.4 Ga shear zones of the Colorado mineral belt shear zone system (CMB). Mylonite and pst in the GCSZ must be Precambrian because it is truncated by the unconformity beneath the Cambrian Sawatch formation. We tentatively correlate the GCSZ with ~1.4 Ga shear zones of the CMB based on the similarity of deformation conditions and regional 40Ar/39Ar data that show that most exposed basement rocks in central Colorado cooled below the threshold for quartz ductility (~280°C) by about 1.35 Ga. Mylonite in the GCSZ records S-vergent reverse/thrust motion, consistent with regional strain involving partitioned N-S shortening, E-W extension, and complex transpression along NE-trending subvertical shear zones of the CMB. The GCSZ coincides with the southern Repeated reactivation of the GCSZ probably influenced Phanerozoic tectonics in the region. The GCSZ is overprinted by the Laramide Grizzly Creek fault (>200 m reverse displacement), lies near the margin of the Eagle River evaporite basin, and roughly coincides with the a major left-step in the Grand Hogback that demarcates the western limit of basement-cored Laramide uplifts in Colorado. We suggest that the GCSZ represents a long-lived lithospheric weakness initiated during ~1.4 Ga intracontinental tectonism.