Trevor J. Lewis

Geophysical consequences of the Cordillera–Craton thermal transition in southwestern Canada

There is a pronounced increase in heat flow and lithosphere temperatures across the transition from the stable North American Craton to the southeastern Canadian Cordillera. The heat flow increases from 40–60 mW m−2 in the Craton to 80–100 mW m−2 in the Cordillera. There are numerous reliable heat flow data in the Cordillera but measurements in the Western Canada Sedimentary Basin overlying the adjacent Craton are in petroleum exploration wells with inherent low accuracy and are affected by hydrological effects. However, the deep thermal boundary is well defined based upon contrasts in several other temperature-sensitive geophysical parameters. The boundary is located 100 km west of the mountain front in the region of the Rocky Mountain Trench, and it must occur over a distance of less than 200 km. Temperatures in the deep crust and upper mantle are first computed from the heat flow and radioactive heat generation data. These temperatures are then compared to those estimated from the temperature dependence of uppermost mantle seismic velocity, Pn, and from xenoliths in kimberlites for the Craton, and in Tertiary volcanics for the Cordillera. Pn velocities decrease from about 8.2 km s−1 in the exposed Craton to 7.8 km s−1 in the Cordillera. The temperatures just below the Moho are 400–500°C for the Craton and 900–1000°C beneath the Cordillera based upon all three constraints. Two temperature-sensitive changes across the thermal boundary are examined. There is a westward decrease in crustal thickness from 40–50 km for the Craton to 32–34 km for the Cordillera with no significant change in average elevation nor in gravity. Isostatic balance is maintained by thermal expansion and density reduction in the high-temperature Cordillera lithosphere. The average temperature to a depth of 150 km is about 400°C higher for the Cordillera compared to the Craton. There also is a pronounced westward decrease in lithosphere strength and thus deformation style. In the Craton, the crust and upper mantle are very strong to at least 100 km depth. As a result, the foreland belt deep crust and upper mantle have remained undeformed during late Mesozoic to early Tertiary tectonics. Shortening occurred primarily in overlying sedimentary thrust sheets, mediated by high pore fluid pressures. In the Cordillera hinterland, the high temperatures result in the rheological lithosphere with significant strength being limited to the upper 10–15 km of the crust. Mesozoic–Cenozoic shortening deformation and subsequent extension included this whole thin lithosphere.

Terrestrial Heat Flow in Canada

Abstract Heat flow has been measured at 214 separate sites in Canada and on the surrounding continental shelves. Most geological provinces show average heat flow that conforms approximately to world averages for areas of similar tectonic age, but the northern Prairies show an anomalously high heat flow that is probably controlled by deep water movement, and the coastal zone of southern British Columbia shows a low heat flow that is believed to be associated with recent subduction of small ocean plates.

Geothermal evidence for deforestation induced warming: Implications for the Climatic impact of land development

Analyses of temperatures from boreholes in previously forested areas in western Canada disclose sudden increases of one to two degrees in ground surface temperature at the times of deforestation at each site. This is the first clear evidence for deforestation induced warming. These findings suggest that any land development changing climatic parameters such as the amounts of water evaporated from the earths surface contributes to regional climatic change. A warming of the ground surface over a large area of central Canada, synchronous with the deforestation of southern Ontario and neighbouring regions in the nineteenth century, may be an example of climate change linked to the widespread creation of agricultural lands. Such a warming also affects the surrounding regions

The effect of deforestation on ground surface temperatures

Abstract Recorded ground surface temperatures (GSTs) over a period of a year at closely spaced sites in a temperate area (almost no snow or ground freezing) show that forested sites and one with a high water table have colder average temperatures relative to other terrains. At sites in southern British Columbia where trees have been logged and in the southern Yukon where they were burned down by a forest fire, the ground surface temperature increased at the time of deforestation. Borehole temperatures are used to show this since no GSTs were recorded. At these sites there has been no subsequent reforestation, and the ground surface temperature has remained nearly constant since deforestation. The times since deforestation range from 5 to 52 years, and the average increase in ground surface temperature is 1.8 K on northern Vancouver Island and 1.2 K in the southern Yukon. The heat required for transpiration in a forest is about 10% of the net radiative heat flux at the ground surface. If this amount of heat is surplus due to deforestation and if the earth is considered to radiate heat like a black body, then the expected increase in the GST is of the order of 1 K.

Geothermal Evidence from Canada for a Cold Period Before Recent Climatic Warming

Three deep boreholes in a small area in Quebec, each having two high-accuracy temperature logs separated by 22 years, allow reliable determination of the ground surface temperature history during the past few centuries. The temperature logs show that the recent climatic warming was preceded by a cold period near the end of the 19th century in this area. The presence of such a cold period is also suggested by borehole temperature data from other areas in Canada.

Climatic changes in central and eastern Canada inferred from deep borehole temperature data

Abstract We analyzed data from 23 boreholes at 19 sites in central and eastern Canada, for the purpose of estimating ground surface temperature (GST) histories. These boreholes were logged down to at least 550 m depth with thermistor probes. Thermal conductivity measurements had been previously made at small depth intervals for the entire depth ranges of most of the boreholes. The temperature profiles of these boreholes do not indicate water disturbance. We estimated terrain effects for each borehole using a time dependent solid-angle method. The thermal perturbations caused by lakes or deforestation near the borehole sites are insignificant in most cases. However, four of the holes were found to be severely influenced by terrain effects. GSTs estimated from the borehole data less influenced by the terraineffects form two groups. The first group, which are generally from data of better quality, show a cold period near the end of the last century before the recent warming trend; the second show it 80–100 years earlier. We consider the former typical of the climate of the Boreal climatic region of Canada. The difference between the two groups may reflect the spacial variability of the climate. Four GST estimates do not belong to either type, and the reasons are discussed.

Water movement within lac du bonnet batholith as revealed by detailed thermal studies of three closely-spaced boreholes

Abstract Successive temperature logs have been obtained over a period of two years in three closely-spaced boreholes in the Lac du Bonnet batholith of the Superior Province of the Canadian Shield. Two of the boreholes, of depth 450 m and 830 m, intersect a dipping fracture zone at 435–450 m. In both holes water is flowing from near the surface to the fracture zone at approximately 1.5–1.9·10−5 m3 s−1, the flow being inferred from analysis of the temperature logs. Below 25 m, temperatures in these two holes are 0.22–0.28 K lower than those in the third, 145 m, hole. The temperature data have been combined with over 200 thermal conductivity measurements on core samples to produce heat flow values. In the deepest hole heat flow above the fracture zone is 16% higher than that below the zone. This indicates that water is flowing up the fracture zone. The flow rate is approximately 0.3 g s−1 m−1, and the flow has existed for thousands of years. Observation of thermal effects of water flow in massive, relatively unfractured plutons in a region having little topographic relief causes one to be concerned about the reliability of heat flow values measured in similar environments. The regional heat flow is taken to be 50 mW m−2 after correction for glaciation effects. The average value of 24 determinations of radioactive heat generation in granitic core samples is 5.23 ± 1.11 μW m−3, which is more than three times higher than expected for such a heat flow in the Superior Province. This implies that the layer of high radioactive heat generation is thin, being not more than 4 km and probably about 1.3 km thick.

Differences in recent ground surface warming in eastern and western Canada: Evidence from borehole temperatures

Because of the perturbation caused by a recent climatic warming, most borehole temperature-depth profiles in Canada show reduced or negative gradients at shallow depths. However, the onset time and magnitude of the warming are different in eastern and western (British Columbia and southern Yukon) Canada. We determined the average subsurface temperature perturbations associated with the recent warming from borehole data for 34 and 51, respectively, well distributed sites in these two regions of Canada. If the ground surface temperature is assumed to have increased linearly since and onset time, the results indicate that the ground surface has warmed by 1.5 K since the mid-19th century in eastern Canada but only by 0.8 K since the late 19th century in British Columbia and southern Yukon.

The thermal nature of the Canadian Appalachian crust

Abstract Heat flow values for 17 new sites in the Canadian Appalachians of Nova Scotia, Prince Edward Island, New Brunswick and Quebec are reported. They consist of ten high-quality measurements obtained from a combination of accurate temperature gradient and thermal conductivity measurements, and seven values obtained from sites at which accurate temperature data were obtained but at which lack of core material meant that conductivity had to be estimated from lithological information. The mean and standard deviation of heat flow from the new sites is 58 ± 11 mW/m 2 , not significantly different from the mean and standard deviation obtained by incorporating seventeen previously published values, 57 ± 11 mW/m 2 . Heat flow varies across this part of the Canadian Appalachians, being lowest in the central part, which is underlain by a major Carboniferous basin, and highest near the coast. There is no obvious relationship between heat flow and age of orogenic imprint. In southern New Brunswick there is an area of high heat flow, greater than 70 mW/m 2 , that appears to be related to the presence of radiogenic Devonian granitic batholiths, both exposed and buried. Using a previously defined heat flow-heat generation relationship for the Appalachians of Canada, the radiogenic layer is estimated to be approximately 1.4 to 3.3 km thick in the St. George batholith of New Brunswick, and surficial in the Wedgeport pluton of Nova Scotia. Gravity interpretation suggests that the maximum thickness of the St. George batholith is 7.5 km. Very low values of heat flow are reported for the Magdalen Basin. Up to 10 km of crust may have been eroded from the basement prior to the formation of the southern part of the basin, removing much of the radiogenic source of heat.

Correlation between the depth to the lower-crustal high conductive layer and heat flow in the Canadian Cordillera

Abstract In much of the southern Canadian Cordillera, the top of the lower-crustal electrical conductor coexists with the 450°C isotherm. A statistical correlation between the depth to the top of the lower-crustal electrical conductor, heat flow and 450°C isotherm are shown, based on MT and heat flow results in southern British Columbia, Canada. The depth to the lower-crustal conductor and the depth to the 450°C isotherm vary from 20–35 km in western B.C. (Insular Belt, Coast Plutonic Complex, Intermontane Belt) to less than 10 km in the eastern part of the Canadian Cordillera (Omineca Belt). The largest depths are in the Insular Belt (35 km). The coexistence of the top of the lower-crustal electrical conductor, top of the zone of lower-crustal seismic reflectors, and 450°C isotherm supports the hypothesis that the 450°C isotherm defines the top of the zone over which interconnected fluid can exist in the lower crust.