Tectonothermics of modern and ancient continental margins

Abstract

Characteristic patterns of heat flow are observed across present-day convergent and constructional margins. At active oceanic subduction margins heat flow varies systematically from normal oceanic values to low values, over the cold subducting slab, to high values over the zone of volcanism associated with slab melting, to normal continental values. A similar pattern is observed over active continent-continent collision zones. In both, hydrothermal circulation cells associated with the high heat-flow zone lead to ore deposition. In old continent-continent collision terrains the heat-flow pattern is broadly similar to that over active collision zones, although for quite different reasons. Low heat flow occurs over volcanic belts associated with subduction, because of their low radiogenic heat production. Higher heat flow occurs over plutons injected during subduction, because they have higher heat generation than normal crust. A zone of low heat flow may occur over middle-lower crustal terrains, now exposed by deep erosion, because of their low heat generation. The variation of heat flow can be used to indicate the direction of the ancient subduction.At active spreading ridges heat flow decreases systematically away from the axis of magma injection. The heat flow pattern across old continental rift systems is quite different, with bands of higher-than-usual heat flow delineating the edges of the rift, the result of thermal refraction at the boundary between low thermal conductivity rift-fill material and higher conductivity felsic crust. If the rift contains substantial volumes of low heat generation volcanic rocks, heat flow over the rift may be lower than that outside.Examples are taken from the Canadian Shield. Heat flow and heat-generation patterns across the boundary between the western Superior Province and Churchill Province imply subduction of Churchill crust under Superior, contrary to some previous models. Heat flow patterns across the Mid-Continent Rift in eastern Lake Superior contradict recent interpretations of seismic reflection data and suggest it is an asymmetrical rift containing a relatively small volume of volcanic rocks.KeywordsHeat FlowHigh Heat FlowCanadian ShieldSuperior ProvinceIdaho BatholithThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.