Fourteen new heat flow and radiogenic heat production measurements have been obtained in the Paleo-Proterozoic Trans-Hudson Orogen of the Canadian Shield. This orogen, which consists of several distinctive belts, corresponds to a pulse of crustal growth through island arc magmatism between 1.9 and 1.8 Ga. The data now available include 17 previously published measurements. Heat flow variations that are related to the history of magmatism and internal differentiation of the belts provide constraints on the crustal assemblages in the different belts of the orogen. The average and standard deviation of heat flow values for the entire orogen, 42¿11 mW m-2, are identical to those of the older Superior Province and of the younger Grenville Province. For the orogen as a whole, heat flow is weakly correlated to the heat production of surface rocks. High heat flow values are found in the Thompson belt, consisting of metasedimentary rocks deposited on the ancient continental margin of the Superior craton. There the accumulation of sediments derived from older and differentiated continental upper crust has resulted in significant concentrations of radioelements in large volumes of rocks. The heat flow is low in the belts that expose juvenile Proterozoic crust consisting mostly of arc-related volcanic rocks. In the Flin Flon-Snow Lake Belt, the average heat flow is the same as the average of the orogen. The low heat production and the lack of correlation between heat flow and heat production suggest that the supracrustal volcanics exposed at the surface are thin and rest on a basement richer in radioelements. In the Lynn Lake belt, the heat flow is significantly lower than the average for the orogen although the surface heat production is not low. The heat flow data require a thin (<10 km) surface layer overlying the mid and lower crust depleted in radioelements. Around the town of Lynn Lake, heat flow is consistently low over a distance of ≈40 km. The coincidence between this cold spot and anomalously thick crust suggests that deep crustal roots may be preserved because of the stronger rheology implied by the low temperatures. The evolution of the Trans-Hudson Orogen exemplifies the interplay between the processes generating rocks of evolved composition, which require crustal thickening, and those forming normal continental crust with average thickness, which require crustal flow and soft crustal rheology. ¿ 1999 American Geophysical Union