F. Rolandone

Surface heat flow, crustal temperatures and mantle heat flow in the Proterozoic Trans-Hudson Orogen, Canadian Shield

The mean and standard deviation of heat flow values are 42 ± 9 mW m � 2 .I n this province, distinctive geological domains are associated with specific heat flow distributions. The heat flow pattern follows the surface geology with a central area of low values over an ancient back arc basin (Kisseynew) and an ancient island arc (Lynn Lake Belt) made of depleted juvenile rocks. Higher heat flow values found in peripheral belts are associated with recycled Archean crust. Within the Canadian Shield, there is no significant variation in heat flow as a function of age between provinces spanning about 2 Gyr. There is no geographic trend in heat flow across the Canadian Shield from the THO to the Labrador Sea. Low heat flow areas where the crustal structure is well-known are used to determine an upper bound of 16 mW m � 2 for the mantle heat flow. Present and paleogeotherms are calculated for a high heat flow area in the Thompson metasedimentary belt. The condition that melting temperatures were not reached in Proterozoic times yields a lower bound of 11–12 mW m � 2 for the mantle heat flow. INDEX TERMS: 8130 Tectonophysics: Evolution of the Earth: Heat generation and transport; 8120 Tectonophysics: Dynamics of lithosphere and mantle—general; 1020 Geochemistry: Composition of the crust; 8015 Structural Geology: Local crustal structure; KEYWORDS: heat flow measurements, crustal thermal structure, mantle heat flow

Low mantle heat flow at the edge of the North American Continent, Voisey Bay, Labrador

Heat flow measurements in 4 deep drillholes near Voisey Bay, Labrador, have yielded the lowest value ever reported in the Canadian Shield, 22 mW m−2. This very reliable estimate is also one of the lowest continental heat flow values world wide. It requires the crust to be very poor in radioelements in this part of the Archean Nain Province. It also strongly supports the view that mantle heat flow is low (<15 mW m−2) throughout the Canadian Shield, with no trend of increasing mantle heat flow near the edges of the continent. It also raises questions about the controlling mechanism for rifting and the opening of the Labrador Sea at 100 Ma.

Heat flow variations in a deep borehole near Sept-Iles, Québec, Canada : Paleoclimatic interpretation and implications for regional heat flow estimates

A deep (> 2000m) borehole in the Sept-Iles intrusion, on the north shore of the Saint Lawrence River, in Quebec, Canada, was repeatedly logged for temperature. Systematic variations of the temperature gradient with depth are not correlated with the thermal conductivity. We interpreted the temperature profile as follows: (1) During the last glacial maximum, the temperature at the base of the ice sheet was cold (≈−5°C); (2) When the region was below sea level, between 10 and 5ky B.P., the ground surface temperature was warm (≈ 6°C); (3) The average ground surface temperature dropped to ≈ 2°C at 5ky B.P. when the region rebounded above sea level; (4) The long time averaged ground surface temperature before the last glacial maximum was ≈0 - 1°C; (5) The reference heat flow (36 – 37mW m−2) is 4–5mW m−2 higher than estimated from the upper 1000m of the heat flow profile. This interpretation can not be extrapolated to the entire region covered by the Laurentide ice sheet. Except for extremely deep (> 1500m) boreholes, the small uncertainty (< 15%) affecting heat flow estimates can not be eliminated.

Heat flow in the Nipigon arm of the Keweenawan rift, northwestern Ontario, Canada

[1] In the Archean Superior Province, the Nipigon Embayment, in the area of Lake Nipigon north of Lake Superior, is covered by MidProterozoic sediments intruded by Keweenawan diabase sills. It has been interpreted as a failed arm of the ca. 1100 Ma Keweenawan rift. Six new heat flow values in this area show that the region of low heat flow associated with the Keweenawan rift in Lake Superior extends northwards along the western margin of the Nipigon Embayment. The average heat flow in the Nipigon area (39 ± 5 mWm � 2 ) is only slightly lower than

Simultaneous inversion of gravity and heat flow data: constraints on thermal regime, rheology and evolution of the Canadian Shield crust☆

Abstract Heat flow and gravity are sensitive to crustal composition and thickness. In this paper, we show that heat flow and gravity data, combined with constraints from seismic refraction, can be inverted to obtain the gross composition of the crust. The inversion method is used to determine the crustal structure in different parts of the Canadian Shield. The long wavelength variations in heat flow imply a difference in thermal regime between and within provinces and subprovinces of the Canadian Shield. The crustal heat production determined by inversion is used to construct temperature and rheological profiles for the lithosphere in different parts of the Canadian Shield. Regions enriched in radioelements (the western part of the Abitibi subprovince and the Thompson Belt) had a weak rheology after they stabilized and could not withstand stresses for geologically long times. In contrast, subprovinces that are depleted in radioelements (the eastern part of the Abitibi and the Lynn Lake Belt) had a stronger rheology and might have preserved crustal roots after their formation.