Chapter 23: Heat flow and thermal regimes in the continental United States

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Abstract
Heat flow and associated lithospheric thermal regimes in the continental United States are intricately related to the tectonic evolution and physical properties of the continental lithosphere. More than 2,000 published heat-flow values are now available for the United States, although the distribution of these data is very uneven, with most data sites in the western United States. The data define a single basic thermal regime in all provinces east of the Rocky Mountains and variable thermal regimes in provinces in the western United States. “Normal” heat flow east of the Rockies is interpreted to reflect a stable thermal regime with regional variations associated with redistribution of heat by ground-water convection and heterogeneity in upper crustal heat production. With the exception of Lake Superior in the Canadian Shield, there is a general trend of increasing surface heat flow with increasing crustal thickness in this stable region. Heat flow in the western United States is generally high, and the surface heat-flow pattern is not everywhere controlled by province boundaries. Calculated geotherms for much of the region suggest temperatures near the solidus close to the Moho for much of the region, with major crustal melting predicted by steady-state geotherm calculations for hot thermal subprovinces. In the Basin and Range province, extension may have been an important factor in producing or maintaining the high heat flow, but elsewhere—and perhaps in the Basin and Range also—convection of heat into the crust by mantle-derived melts not simply related to extension appears to be the primary heat source. These melts are probably related to subduction, but the exact mechanism of their origin is unclear. Low heat flow in the eastern Snake River Plain and the “Eureka Low” appears to result from downward convection by ground-water recharge. Low heat flow in the Sierra Nevada and Northwest Pacific coastal provinces appears to be related to subduction. Relatively normal heat flow in the Colorado Plateau is inconsistent with the elevation of the plateau, and an increase in heat flow with depth is predicted beneath this province. High heat flow in the California Coast Ranges is related to growth of the San Andreas fault zone and local segments of extension along its length. These thermal regimes are directly related to variations in lithospheric thickness across the United States, and regional elevation differences are associated with both crustal thickness and geotherm variations. Increased understanding of these thermal regimes and their evolution is leading to a better understanding of the tectonic evolution, physical properties, and lithospheric structure of the continental United States.

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