Abstract 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.

Abstract The origin of the subsurface fire burning since 1910 in the South Cañon Number 1 Coal Mine west of Glenwood Springs, Colorado, is unknown. Surface manifestations of the fire include gas vents (some encrusted with minerals), burnt vegetation, subsidence features, ash, sulfur, and red-oxidized shale. The minerals tschermigite, mascagnite, and gypsum formed in association with coal-fire gas exhaled from a gas vent on the western slope of South Canyon. Tschermigite and gypsum are reaction products of the gas with feldspar grains in the Williams Fork Formation. Gas collected from a vent and from soil above a burn zone in former underground workings on the eastern slope of South Canyon was found to contain numerous hydrocarbons, including n-alkanes, iso-alkanes, cyclo-alkanes, alkyl aromatics, alkenes, ketones, ethers, and a number of other volatile organic com-pounds, as well as sulfur compounds. Drill casings currently present in voids in the D coal seam on the western slope trail are useful for collecting gas samples, monitoring the temperature of subsur-face burning, and measuring the concentration of gases, such as carbon monoxide and carbon dioxide, in the field. The likely success of conventional fire-containment methodologies in South Canyon is questionable, although additional studies including drilling data may eventually suggest a useful control procedure. The 2002 “Coal Seam Fire” that burned over 12,000 acres and destroyed numer-ous buildings in and around Glenwood Springs exemplifies the potential danger an underground coal fire poses for igniting a surface fire. Coal-fire gas and the solid by-products of combustion contribute to the destruction of floral and faunal habitats and may be responsible for a variety of human diseases; hence, the study of coal gas and its by-products may prove useful in understanding environmental pollution created by coal-mine fires. Keywords: South Cañon Number 1 Coal Mine, coal-mine subsidence, coal fires, coal-fire gas, gas-vent minerals, coal-fire pollution.

Abstract The South Cañon Number 1 Coal Mine fire, in South Canyon west of Glenwood Springs, Colorado, is a subsurface fire of unknown origin, burning since 1910. Subsidence features, gas vents, ash, condensates, and red oxidized shales are surface manifestations of the fire. The likely success of conventional fire-containment methodologies in South Canyon is questionable, although drilling data may eventually suggest a useful control procedure. Drill casings in voids in the D coal seam on the western slope trail are useful for collecting gas samples, monitoring the temperature of subsurface burning, and measuring the concentration of gases such as carbon monoxide and carbon dioxide in the field. Coal fire gas and mineral condensates may contribute to the destruction of floral and faunal habitats and be responsible for a variety of human diseases; hence, the study of coal gas and its condensation products may prove useful in understanding environmental pollution created by coal mine fires. The 2002 Coal Seam Fire, which burned over 12,000 acres and destroyed numerous buildings in and around Glenwood Springs, exemplifies the potential danger an underground coal fire poses for igniting a surface fire.

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