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Suggested Citation:"HEAT FLOW." National Research Council. 1987. Scientific Value of Coring the Proposed Southern Appalachian Research Drill Hole. Washington, DC: The National Academies Press. doi: 10.17226/18690.
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Page 32
Suggested Citation:"HEAT FLOW." National Research Council. 1987. Scientific Value of Coring the Proposed Southern Appalachian Research Drill Hole. Washington, DC: The National Academies Press. doi: 10.17226/18690.
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Page 33
Suggested Citation:"HEAT FLOW." National Research Council. 1987. Scientific Value of Coring the Proposed Southern Appalachian Research Drill Hole. Washington, DC: The National Academies Press. doi: 10.17226/18690.
×
Page 34
Suggested Citation:"HEAT FLOW." National Research Council. 1987. Scientific Value of Coring the Proposed Southern Appalachian Research Drill Hole. Washington, DC: The National Academies Press. doi: 10.17226/18690.
×
Page 35

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8 Heat Flow Historically, heat flow on continents has been used as a con- straint on models for chemical and physical processes occurring within the earth. Systematic differences in heat flow between physiographic or tectonic units are now apparent. These differ- ences are an important factor in formulating models of the tectonic and thermal history of continents. The definition of equilibrium (steady-state) heat flow is: q = K(dT/dz) where q is heat flow, K is rock thermal conductivity, and dT/dz is the geothermal gradient. In the absence of heat sources, determi- nations of heat flow at various depths in the southern Appalachian hole should be the same, irrespective of the thermal conductivity of the interval over which the product K(d T/dz) is determined. A corollary to this is that several determinations of heat flow should be made in the hole at depths where the average rock thermal conductivity is different to confirm the equality, and therefore the reliability, of a heat flow determination. THERMAL CONDUCTIVITY Methods of measurement of thermal conductivity have been 32

33 summarized by Roy et al. (1981). Reliable determinations of ther- mal conductivity of rocks from depths of several km or greater must be made on representative core samples. Conductivities measured on contiguous samples over a volume of approximately 10 cm3 can differ by 50 percent even though the precision of each deter- mination is 1 percent. An average value for many samples must therefore be taken over the interval for which the average ther- mal conductivity is desired. Conductivity measurements should be made on at least one sample per meter of core for a typical 30 m interval used for a heat flow determination. Several methods are used to estimate thermal conductivity for a vertical interval given many determinations on small samples (Roy et al., 1981). All emphasize the heterogeneity of rocks, and the desirability for con- tinuous core over the interval where heat flow is to be determined. The most accurate determinations are made on approximately 1 cm thick disks cut from cores between 2 and 4 cm in diameter (Table 2). Conductivity of isotropic rocks can be calculated from measurements on cuttings. Conductivity anisotropy can be signif- icant in the deformed metamorphic rocks that will be penetrated in the southern Appalachian drill hole. Meaningful determination of vertical conductivity will require core samples. Oriented core is unnecessary because only the inclination of planar structures or fabrics, not their azimuth, is needed. GEOTHERMAL GRADIENTS Temperature logs are readily obtained to a precision of ±0.01°C. For a reliable heat flow determination, the geothermal gradient (dT/dz) must be measured over the same interval as that for which a representative thermal conductivity has been de- termined. This interval must be one over which the geothermal gradient is not disturbed by ground-water circulation. Intervals of about 30 m should suffice for accurate gradient determinations, although shorter intervals probably could be tolerated. Measure- ments must be repeated after drilling has ceased, commonly several times, until drilling-related thermal disturbances have relaxed. Although it may not be possible to determine a steady-state gradient over a small interval while drilling is in progress, it is pos- sible during drilling to identify the presence of cracks that transmit water and therefore might preclude a reliable heat flow determi- nation after steady-state thermal conditions have been reached.

34 Cracks that transmit water have characteristic thermal signatures (Drury and Jessup, 1982). Precision temperature logs should therefore be obtained at every opportunity during the drilling program to characterize, where possible, the hydraulic transmis- sivity of rock units penetrated by the deep hole. These data may be used to help decide on the amount of core to be taken over a deeper part of the same lithostratigraphic interval for thermal conductivity measurements. HEAT GENERATION Variations in heat flow in the eastern United States are caused primarily by differences in crustal concentrations of the radioac- tive, heat-producing isotopes of U (contributes 40 to 45 percent), Th (40 to 45 percent), and K (10 to 20 percent). A U atom gives off about 4 times the heat of a Th atom, but because Th/U ratios in rocks are commonly around 4, Th is about as important as U for heat production. Minimum sizes of representative samples are determined by the same criteria as for other geochemical analyses. Sampling intervals depend on the degree of heterogeneity of the geologic unit of interest. Meaningful data for radioactive element distribution in layered metamorphic rocks, like those we expect to encounter in the southern Appalachian hole, will require careful examination and extensive sampling of essentially continuous core. Because the bulk of the U and Th in rocks commonly resides in rare grains of accessory minerals, along grain boundaries, or in veins or segregations, the distribution or redistribution of these el- ements in rocks cannot be understood without thin sections made from core. SIGNIFICANCE OF HEAT FLOW DATA The highest heat flow values in the eastern United States are associated with syn- and post-metamorphic granites, and most heat flow determinations have been made in granitic rocks (Sass et a/., 1981). Birch et al. (1968) determined heat flow in New England and the Adirondacks and found that heat flow (?) was related to surface heat generation (A) by q=q*+DA

35 where q* is a constant ("reduced heat flow") and D is a constant with units of depth. Continental heat flow provinces are denned on the basis of q*, assumed to be the flux from the lower crust and upper mantle. The simplest models that result in a linear relation between q and A are: (1) the concentration of heat-producing elements is constant from the surface to a depth D\ and (2) the distribution of heat-producing elements decreases exponentially downward from the surface to a depth of approximately 3D, where D is a logarithmic decrement describing the rate of change of the concentration of heat-producing elements. These models place severe constraints on crustal structure wherever a linear relation- ship is observed. Lachenbruch (1968) pointed out that Model 2 will account for effects of differential erosion above heat flow sites, whereas Model 1 will not. The Piedmont heat flow and heat generation values, deter- mined in post- and late synmetamorphic (254-330 Ma) granite plutons and metagranites, lie in a belt approximately parallel to major structural trends in the Appalachians. These data can be modeled with q* = 30 mW/M2 and A = 8 km. Costain and Glover (1980) have proposed that the occurrence of a master sole thrust that truncates granites in the allochthonous plate may be respon- sible for the linear relation between heat flow and heat generation from many of the plutons of the eastern Piedmont. In this inter- pretation, £> is a measure of the depth to the master thrust. The interpretation of D as a logarithmic decrement, however, would require an uninterrupted exponential decrease of U and Th to a depth of approximately 3D or about 20 km. Redistribution of mobile heat-producing elements during metamorphism might ac- count for an exponential distribution. The proposed deep hole would make possible a unique test of these competing models for radioelement distribution. An understanding of the linear relation between heat flow and heat production has important tectonic implications not only for the eastern United States, but for any tectonic province where it is observed. The proposed deep hole, together with supplementary shallow heat flow sites, offers a unique opportunity to resolve the significance of the linear relation between heat flow and heat production in this region and perhaps provide constraints that are applicable globally. Because of the known low heat production of carbonates, quartzites, and Grenville basement below the master decollement, it should be possible to determine q* directly from measurements in the 10-km hole.

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