Er. Decker, THERMAL REGIMES OF THE SOUTHERN ROCKY-MOUNTAINS AND WYOMING BASIN IN COLORADO AND WYOMING IN THE UNITED-STATES, Tectonophysics, 244(1-3), 1995, pp. 85-106
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.