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Heat flow in the eastern ranges of the Southern Rocky Mountains in Colorado substantially exceeds that in the Wyoming Basin-Southern Rocky Mountains area in southeastern Wyoming. The transitions between these areas are narrow (≤ 60 km wide), estimated near-surface crustal radiogenic heat productions are different, and there is no evidence for young magmatism in the easternmost mountains in northern Colorado and southern Wyoming. Therefore, radioactive heat contrasts in the upper crust are used to explain significant amounts of the heat-flow differences in these regions. In southern Wyoming, normal heat flow in Archean and Proterozoic terranes probably reflects a deeply eroded, thin (7–15 km) granitic layer that overlies low-radioactivity units in a 37–40 km thick crust. In the Colorado mountains to the south, silicic metamorphic and igneous rocks with relatively enriched radiogenic 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 reduced heat flow and isostatic equilibrium. Areas of unusually high heat flow occur in the Rio Grande rift zone in the environs of the Colorado Mineral Belt in the Leadville-northern Sawatch Range region, eastern parts 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 near the crust-mantle boundary, and the narrow borders (50–60 km wide) of the Leadville-northern Sawatch Range heat-flow anomaly must be caused 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 uplift and extension of the northern Rio Grande rift-Southern Rocky Mountains system in Colorado. |
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