A detailed study of the distribution of heat flow and radioactivity in New Hampshire (U.S.A.)

Abstract

We present a set of 39 new determinations of heat flow and radiogenic heat production for several different geological environments in the State of New Hampshire (U.S.A.). With the extensive data set now available for the state, the linear relation of heat flow and heat production appears as a very useful generalization for the study of the heat flow field of a geological province. Our measurements indicate that the vertical distribution of radiogenic heat production is similar in plutonic and metasedimentary rocks. Our data are compatible with the linear relationship established earlier by F. Birch and his co-workers in 1968. Young granites are markedly enriched in radioactive elements and those which do not outcrop are revealed by anomalies in the general relation of heat flow versus radioactivity. Heat flow is high for plutons by low elsewhere. The mean heat flow through metasedimentary formations is 1.15 μcal/cm 2 s (48 mW/m 2), a value near the mean heat flow for old cratons. The lowest heat flow measured is 0.76 μcal/cm 2 s (32 mW/m 2) for a unit poor in radioactivity. The heat flow field grades smoothly into the low heat flow regions of the Canadian Shield. The New Hampshire region is in thermal equilibrium: its heat flow is in secular equilibrium with the heat generated by crustal sources and that supplied from the mantle. In this area, the thermal perturbations due to orogenic events decrease below the detection level in less than 200–275 Ma. The thickness of the layer which is thermally affected during continent-continent collision-type orogenies cannot be greater than about 190 km.