| Field Trip Description: This trip will examine Proterozoic mid-crustal deformation and Phanerozoic brittle reactivation in the newly recognized Grizzly Creek shear zone, central Colorado. Our recent mapping defines the Grizzly Creek shear zone (GCSZ) as a 0.6-km-thick, brittle–plastic deformation system that dips moderately to the north along the southern margin of the White River uplift. Deep canyon incision provides spectacular cross-strike exposures of Proterozoic basement beneath Paleozoic cover strata. In Proterozoic basement, the shear zone consists of a 20-m-thick basal mylonite overlain by highly strained gneisses and megacrystic granitoids, both of which host hundreds of pseudotachylyte fault veins and thin mylonite zones. Locally, part of the shear zone is defined by a strongly foliated, fine-grained granitoid cut by pseudotachylyte and ultramylonite. The presence of mylonitized pseudotachylyte in part of the shear zone suggests seismogenic faulting and plastic flow occurred cyclically at mid-crustal depths. Kinematic indicators are consistent with top-to-south, Proterozoic compression. The GCSZ is cut by the Phanerozoic Grizzly Creek fault; an east-west striking, north-dipping reverse fault that coincides with the hinge of a monocline that bounds the southern margin of the Laramide White River uplift. We will discuss applications of our work to understanding earthquake rupture dynamics, Proterozoic tectonics, and the Precambrian ancestry of Phanerozoic structures and fluvial incision in the southern Rockies. The trip will involve a four-mile round trip trail hike and a steep, off-trail traverse on both days. |