Flipping through field notebooks…
Abstract A sequence of mylonite-series rocks, characterized by an anastomosing pattern of variably deformed Proterozoic rock, cuts across regional foliation trends and intrusive bodies of the Gunnison Gold Belt. To date, this mylonite-series of rocks forms an unrecognized significant shear zone, the Spencer-Vulcan shear zone. The zone tracks east to west for possibly as much as 30 kilometers (km), and has an average width of 1.5 km. Proterozoic rocks involved belong to the Dubois Greenstone, Black Canyon Schist, and numerous granitoid bodies, including the Powderhorn Granite. The zone is vertical and shear sense is right lateral, with the north side east, and south side west. Shearing was an upper greenschist facies event. Shearing occurred after 1750 million years ago (Ma) but prior to 1400 Ma, possibly after 1650 Ma, and prior to 1420 Ma. There is no evidence of reactivation of the shear zone at any point after circa 1400 Ma. Deformation was probably a limited regional event that occurred between the Colorado and Berthoud orogenies.
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