Significance of past and recent heat-flow and radioactivity studies in the Southern Rocky Mountains region

Abstract

As reported in The Geological Society of America Bulletin of 1950, Francis Birch's innovative heat-flow research in the Colorado Front Range introduced frequently used terrain correction and thermal-conductivity measurement methods. That report also presented the first empirical evidence for a positive correlation between above-normal flux and radioactive roots in an isostatically compensated mountain area with a thick crust, an observation that strongly influenced continental heat-flow studies for the next 18 years. Birch's Front Range study clearly showed that reliable continental heat-flow research requires knowledge of bedrock radioactivity. This concept reached an unprecedented level of acceptance in 1968, when linear relations between heat flow and radiogenic heat production were discovered for three contrasting provinces in the United States. Subsequently, such lines have been central to virtually all heat-flow and radioactivity research on land. Heat-flow (Q) data for 139 boreholes provide new detail on thermal regimes in the Southern Rocky Mountains and bordering areas, as do radiogenic heat-production (A) data for 60 locales. Interpretations also emphasize reduced and residual heat-flow values; reduced values correspond to the intercepts of regional Q-A lines, whereas a residual value is the flux at a locale after the probable effect of near-surface heat production has been subtracted. The observed Q-A lines demonstrate that the Front Range and other easterly frontal ranges of the Southern Rocky Mountains in Colorado and northern New Mexico are characterized by a reduced flux (54-58 mWm-2) that is dramatically higher than that (∼27 mWm-2) in the Wyoming Basin-Southern Rocky Mountains area in southeastern Wyoming. Because the transitions between these provinces are narrow (≤50-60 km), sources in the upper crust must explain some of the contrasting reduced heat-flow values. In southeastern Wyoming, normal heat flow in Archean and early Proterozoic basement terranes probably reflects deep erosion that produced a thin (∼7 km) near-surface granitic layer that overlies a low-radioactivity lower crust. In the Colorado Front Range, Proterozoic, Mesozoic, and Cenozoic silicic rocks with relatively enriched radiogenic heat could comprise a 20- to 25-km-thick granitic layer in the upper crust that produces a large part of the above-normal reduced and residual flux. Here, partial melting of deep protolithic rocks in late Mesozoic and Cenozoic times could have produced a lower crust with low radiogenic heat production. By these views, Birch's heat flow-radioactive mountain root model of the Colorado Front Range is confirmed if a large part of the topography is isostatically compensated by low-density pre-Miocene crystal-line masses in the upper crust. Background reduced heat flow in Colorado parts of the Southern Rocky Mountains and the eastern Colorado Plateau is high (54-68 mWm-2). Zones of unusually high residual flux (88-118 mWm-2) 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 the Park Range-Mountain Parks area near the Colorado-Wyoming border. The flux in these areas implies unrealistically high equilibrium temperatures near the crustmantle boundary, and the 50- to 60-km-wide borders of the Leadville-northern Sawatch Range residual heat-flow anomaly must be caused by sources in the upper crust. There-fore, young (10- to 1-m.y. -old) intrusions in a late Cenozoic rhyolitic complex in the upper crust are preferred to explain gravity lows, late Cenozoic uplift and igneous activity, and the high residual flux in the Leadville-northern Sawatch Range area. Similar models may apply elsewhere in the northern rift zone. 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. Because very high regional flux in the northern rift zone and the San Juan Mountains areas implies above-liquidus equilibrium temperatures in the lower crust and upper mantle, serious inconsistencies arise when steady-state thermal models are examined relative to volumes of late Tertiary volcanism, uniform crustal thickness, and the absence of unusually low P-wave velocities in the mantle in the Southern Rocky Mountains.