THERMAL REGIMES OF THE SOUTHERN ROCKY-MOUNTAINS AND WYOMING BASIN IN COLORADO AND WYOMING IN THE UNITED-STATES

Heat flow in the eastern ranges of the Southern Rocky Mountains in Col orado substantially exceeds that in the Wyoming Basin-Southern Rocky M ountains area in southeastern Wyoming. The transitions between these a reas are narrow (less than or equal to 60 km wide), estimated near-sur face crustal radiogenic heat productions are different, and there is n o evidence for young magmatism in the easternmost mountains in norther n Colorado and southern Wyoming. Therefore, radioactive heat contrasts in the upper crust are used to explain significant amounts of the hea t-flow differences in these regions. In southern Wyoming, normal heat flow in Archean and Proterozoic terranes probably reflects a deeply er oded, thin (7-15 km) granitic layer that overlies low-radioactivity un its in a 37-40 km thick crust. In the Colorado mountains to the south, silicic metamorphic and igneous rocks with relatively enriched radiog enic heat could comprise a 20-30 km thick granitic layer in the upper parts of the 50-52 km thick crust, and explain much of the high reduce d heat flow and isostatic equilibrium. Areas of unusually high heat fl ow occur in the Rio Grande rift zone in the environs of the Colorado M ineral Belt in the Leadville-northern Sawatch Range region, eastern pa rts of the San Juan Mountains in southern Colorado, and in Park Range- Middle Mountain Park areas near the Colorado-Wyoming border. The flux in these areas implies unrealistically high equilibrium temperatures n ear the crust-mantle boundary, and the narrow borders (50-60 km wide) of the Leadville-northern Sawatch Range heat-flow anomaly must be caus ed by sources in the upper crust. Hence, young (10-1 Ma) intrusions in a late Tertiary rhyolitic complex in the upper crust are preferred to explain gravity lows, late Cenozoic uplift and igneous activity, and the excess heat flux in the Leadville-northern Sawatch Range area. If this interpretation is correct, magmatic thickening of the crust, not extensional-subsidence mechanisms, probably explains late Cenozoic upl ift and extension of the northern Rio Grande rift-Southern Rocky Mount ains system in Colorado.