A GLOBAL ANALYSIS OF HEAT-FLOW FROM PRECAMBRIAN TERRAINS - IMPLICATIONS FOR THE THERMAL STRUCTURE OF ARCHEAN AND PROTEROZOIC LITHOSPHERE

Previous studies of heat flow from Precambrian terrains have yielded two different relationships: a global temporal relationship between heat flow and tectonic age, and a regional spatial relationship between heat flow and the proximity of Archean cratons. We analyze heat flow from tectonically stable Precambrian terrains worldwide to address questions associated with these two heat flow patterns: (1) Is the spatial relationship a global pattern? (2) Do the two heat flow relationships have a common underpinning? In answer to the first question, our data analysis reveals a widespread spatial pattern between heat flow and the proximity of Archean cratons, which is characterized by low heat flow in Archean cratons and Proterozoic terrains adjacent to cratonic margins and higher heat flow in Proterozoic terrains that are more than a few hundred kilometers from a craton. To address the second question, we examine three previous interpretative models of Precambrian heat flow: (1) simple cooling of a thermal boundary layer, (2) thicker lithosphere in Archean terrains than in Proterozoic, and (3) greater heat production in Proterozoic crust than in Archean. Model 1 predicts essentially no change in heat flow in terrains older than approximately 1.5 Ga and therefore does not likely provide a common underpinning for the Precambrian heat flow patterns. Models 2 and 3, when combined with the special structural configuration of sutures, can independently yield both the spatial and temporal heat flow distributions and thus alone or together may be considered candidates to explain the Precambrian heat flow patterns. However, thermal models of the lithosphere in which the heat flow patterns are explained entirely by variations in crustal heat production cannot satisfy constraints on upper mantle temperatures beneath Archean cratons derived from xenolith thermobarometry. The thermobarometry constraints can be satisfied by models in which differences in lithospheric thickness are the principal factor controlling the surface heat flow distributions. If the heat flow patterns result primarily from variations in lithospheric thickness, then a different temporal heat flow relationship for Precambrian terrains can be proposed, one in which heat flow varies inversely with the age of stabilization of the lithosphere.