Analysis of terrestrial heat-flow profiles across the Rio Grande rift and southern Rocky Mountains in northern New Mexico
— Gerry W. Clarkson and Marshall Reiter

The Rio Grande rift within the southern Rocky Mountains complex of northern New Mexico is a tectonically and volcanically active region. The southern Rocky Mountains have been uplifted —1.1 km since middle Miocene time (Axelrod and Bailey, 1976), with most of the uplift occurring between 7 and 4 m.y ago (Chapin, 1979). Within the Rio Grande rift recent bimodal volcanism has been extensive; the Jemez lineament and Taos Plateau have been especially active in the last 5 m.y (Chapin, 1979; Lipman and Mehnert, 1979). On the eastern periphery of the southern Rocky Mountains in northeastern New Mexico extensive late Tertiary and Quaternary magmatic activity has occurred in the Raton and Las Vegas Basins and in the Capulin and Clayton volcanic fields (Baltz and Bachman, 1956; Gabelman, 1956; Baltz, 1965; Dane and Bachman, 1965; Johnson, 1968; Lipman and others, 1973).

The tectonic and volcanic activity in the rift is often associated with a thinned crust, upwarped mantle, and extensive subsurface magma bodies (e.g. Cordell, 1978; Baldridge, 1979; Keller and others, 1979; Lipman and Mehnert, 1979; Olsen and others, 1979; Rinehart and others, 1979). Based on heat flow, electrical conductivity, gravity, and the elevation profile across the Rio Grande rift, Cordell (1978) suggested the presence of a symmetrical anomalous crustal and upper mantle structure along the axis of the rift. Bridwell (1976) interpreted gravity anomalies in northern New Mexico along 36°N latitude in terms of an intrusion of low-velocity mantle material into an overlying thinned lithosphere across an area 400 km wide (E—W). Thermal models consistent with the gravity data also suggest that anomalous heat sources occur in the crust from 10 to 40 km depth (Bridwell, 1976). The presence of shallow magma bodies, 15-30 km deep, within the rift has been proposed on geochemical, seismic, heat-flow, and gravity data (Lipman, 1969; Sanford and others, 1973; Decker and Smithson, 1975; Cordell, 1976; Reiter and others, 1978). Cook and others (1979) suggested partial melting, or the intrusion of mantle material, at the base of the crust in the southern Rio Grande rift.

As would be expected in a region of tectonic and volcanic activity, the Rio Grande rift—southern Rocky Mountains region has high heat flow (Decker, 1969; Reiter and others, 1975; Edwards and others, 1978; Reiter and others, 1979). Edwards and others (1978) state that the heatflow pattern across the southern Rocky Mountains suggests more widely distributed and perhaps deeper heat sources than along the Rio Grande rift itself. High heat flow in the rift can be related to elevated geotherms associated with a thinned crust and to crustal magmatic activity. High heat flow in the surrounding uplifted regions may not be as obviously related to magmatic processes. Heat-flow values in the region could also be elevated by ground-water circulation (Reiter and others, 1979; Swanberg, 1979). High radiogenic-heat production can contribute to elevated heat flow, but this does not seem to be a significant anomalous heat source in the Rio Grande rift (Decker and Smithson, 1975; Edwards and others, 1978).

The purpose of this study is to consider heat-flow data across northern New Mexico, and to discuss possible trends in the data and the subsequent implications concerning heat sources in the Rio Grande rift— southern Rocky Mountains region. Heat-flow data are considered in terms of steady-state, finite-difference, Isothermal-step models similar to those models presented by Reiter and Clarkson (1983) and Clarkson (1984). Fundamental parameters in the finite-difference models used to analyze the data are the mean heat flow within regions and the variation in heat flow with distance between regions of different mean heat flows. Isothermal-step models are surely a geometrical and phenomenological simplification of actual subsurface thermal sources; however, such models do allow basic appreciation of subsurface temperature discontinuities which may generate observed heat-flow variations between regions. The time constant for isothermal sources at depths ..30 km is relatively short, and, therefore, steady-state models may be reasonable first order approximations to "relatively deep" subsurface heat sources. For the models to be considered in association with the northern Rio Grande rift and the San Juan volcanic field, heat-flow values and profile characteristics (half widths) reach about 3/4 of steady-state values 10 m.y. after step initiation, and approximate steady-state values after 20 m.y. (Clarkson and Reiter, unpubl.). As such, steady-state models may be generally consistent with the repeated volcanism in the Espaliola Basin (Manley, 1979), and may suggest heat replenishment in the San Juan volcanic field (Reiter and Clarkson, 1983).