HEAT FLOW AND ISOSTASY REVISITED FOR THE RIO GRANDE RIFT AND SOUTHERN ROCKY MOUNTAINS IN COLORADO

New geothermal gradient and heat flow maps are in preparation for Colorado as a contribution to the National Geothermal Database System (NGDS). In a reexamination of existing data and integration with new data an attempt is being made to separate the heat flow into three components: 1) shallow geothermal anomalies associated with hydrothermal fluid movements; 2) upper crustal radiogenic heat production; and 3) tectonic and regional variations in heat flow. The purpose of this separation is to calculate crustal and upper mantle geotherms associated with the Rio Grande rift, southern Rocky Mountains, and adjacent tectonic provinces. The results are consistent with the conclusions of Decker et al. (1988; GSA Bull, 100, p. 1851-1885): they indicate that steady-state geotherms calculated for the higher heat-flow areas of the southern Rocky Mountains and Rio Grande rift indicate super-solidus temperatures in the lowermost crust unless radiogenic heat production values, that are not indicated by rocks that outcrop at the surface, are assumed in the crust. The alternative solution is that the geotherms are transient, perturbed either by tectonic disturbances, and/or by very young (< 2 Ma) upper crustal intrusions. The crustal thickness is not more than a few kilometers different between the southern Rocky Mountains and the Great Plains but the average surface elevation increases by more than two kilometers going west from the plains to the mountains. Both provinces are in regional isostatic equilibrium and the difference in elevation is not supported by the observed change in crustal thickness. Part of the high mountain elevation is isostatically supported by low density, thick granites in the upper crust of the mountains. Other support is provided by thin lithosphere beneath the mountains resulting in lower uppermost mantle densities relative to the adjacent plains uppermost mantle. This conclusion is supported by the contrast in heat flow between the southern Rocky Mountains (high heat flow - thin lithosphere) and the Great Plains (lower heat flow - thicker lithosphere).