Heat generation of plutonic rocks and continental heat flow provinces

Abstract

Combined radioactivity and heat flow measurements in pluionie rocks at 38 localities in the United States define three heat flow provinces; the eastern United States, the Sierra Nevada, and a zone of high heat flow in the western United States which includes the Basin and Range province. In each of these provinces heat flow (Q) and heat production (A) are related by an equation of the form Q = a + bA. The simplest interpretation of this linear relation is that the radioactivity measured at the surface is constant from the surface to depth b but varies from place to place. Thus the fraction of heat flow from the lower crust and upper mantle, a, remains constant within a heat flow province while the variable upper crustal radioactivity generates the variable heat flow observed at the surface. In the eastern United States b = 7.5 km and a = 0.79 cal/cm2 sec, in the Sierra Nevada b = 10.1 km and a = 0.40 cal/cm2 sec, and in the Basin and Range province b = 9.4 km and a = 1.4 cal/cm2 sec. The line characteristic of the eastern United States may have broad applicability to stable portions of continents and thus be considered the reference curve for normal continental heat flow. The similarity of all the slopes indicates that most local variability of heat flow is due to sources in the uppermost 7–11 km of the earth's crust, and that the contribution from the lower crust and upper mantle is quite uniform over large regions. The intercept values can be used to infer the proportion of heat flow from the mantle and to map provinces with different mantle heat flow. These heat flow provinces correlate closely with surface geological provinces.