Heat Flow in the United States and the Thermal Regime of the Crust

Abstract

A contour map of heat flow based on 625 observations now available in the conterminous United States shows new detail. Sub-provinces of exceptionally high heat flow (>2.5 HFU (1 HFU = 10−6 cal/cm2 s)) in the western states are beginning to emerge as regional features, but their boundaries are still largely unknown. The ‘Battle Mountain High', previously described in north central Nevada, probably extends northeastward to Utah and Idaho and westward almost to California. With the eastern Snake River Plain, a region that probably has large convective loss, it could form a zone of exceptionally high heat loss that extends almost continuously for 1000 km from the vicinity of Steamboat Springs near Reno, Nevada, to Yellowstone Park in Wyoming and possibly northward into the Idaho batholith. A sinuous high heat flow subprovince of comparable length is emerging in the Rio Grande Trough in New Mexico and southern Colorado. The linear relation between surface heat flow and radioactive heat production, so successful in the Sierra Nevada and eastern United States provinces, does not apply in the Basin and Range province. There the variations in heat flow caused by hydro-thermal and magmatic convection are probably greater by a factor of 3 or 4 than those caused by crustal radioactivity, and heat flux into the lower crust is not uniform; it is probably controlled by the mass flux of intruding magma. Regional variations in this mass flux, probably associated with crustal spreading, can account for the high heat flow subprovinces, and more local anomalies and silicic volcanic centers as well. Although convective processes cause a large dispersion of heat flow in the Basin and Range province, modal values of reduced heat flow can be used to construct generalized crustal temperature profiles for comparison with profiles for more stable areas and with melting relations for crustal rocks. Theoretical profiles are consistent with the widespread magmatic manifestations observed in the Basin and Range province. Laterally extensive silicic partial melts are possible at midcrustal levels in ‘hot' subprovinces like the Battle Mountain High. Effects of hydrologic convection (whether driven by thermal density differences or regional piezometric conditions) are important to an understanding of regional heat flow, especially in tectonically active areas. The ‘Eureka Low', a conspicuous subprovince (∼3×104 km2) with anomalously low heat flow in southern Nevada, is probably caused by interbasin flow in deep aquifers fed by downward percolation of a small fraction of the annual precipitation. Heat flow observations in such areas provide useful information on regional hydrologic patterns.