Recent heat-flow studies in the Western United States, especially the Cordillera, are discussed and summarized and a new heat-flow map is presented. The major features of the map have already been described: high heat flow in the Northern Rocky Mountains, Columbia Plateau, High Cascades, and Basin and Range provinces (the Cordilleran thermal anomaly zone); high heat flow along the San Andreas-Gulf of California transform system; high heat flow in the Southern Rocky Mountains; moderate heat flow in part of the Colorado Plateau; and low heat flow along the Sierra Nevada and the coastal provinces of Oregon and Washington. In addition, much detail is apparent in the Cordilleran thermal anomaly zone. Very high heat flow (greater than 2.5 HFU) is found in the Cascades, and part of the Brothers fault zone in Oregon, part of the Snake River Plain in Idaho, Yellowstone in Wyoming, the Battle Mountain “high” in Nevada, the Geysers area and the Imperial Valley in California, and the Rio Grande rift in New Mexico. Areas of low heat flow are associated with part of the Columbia Basin in Washington, the eastern part of the Snake River Plain in Idaho, and the Eureka “low” in Nevada. The heat-flow map is very complicated because it includes the effects of crust and mantle radioactivity and magmatic heat sources, regional hydrology, and thermal refraction due to structurally related thermal conductivity contrasts.
In the active tectonic areas there are energy losses associated with volcanism, intrusion, and hydrothermal convection. Such losses may not be measured in a typical heat-flow survey. These losses are evaluated and shown to be a significant part of the total heat flow in many areas. Heat flow is compared to the geographical distribution of volcanism, plutonism, hydrothermal activity, and average topography. The regions of high heat flow correlate well with the areas of Cenozoic volcanism, plutonism, and thermal spring activity. The lack of a one-to-one correlation of areas of active plutonism to regional concentrations of thermal springs is shown. The relationship of topography to heat flow is complicated, and it appears that in general the average composition of the crust changes during a major continental thermal event so that the relationships between topography and heat flow may change during the evolution of the thermal event.
Detailed heat-flow interpretation relies on the relationship between heat flow (Q) and radioactive heat generation (A). The Basin and Range plot of Q versus A applies only to areas where the most recent volcanic event is older than 17 m.y. In areas of younger thermal events, the reduced heat flow (that is, the heat flow measured at the surface less heat production from crustal radioactive sources) is generally higher than 1.4 HFU, and hydrothermal convection and volcanism are major mechanisms involved in the total energy transfer. Furthermore, transitions between thermal provinces are narrow (generally less than 20 km); therefore, the sources directly responsible for the surface-measured variations in heat flow must be in the crust. Thermal boundaries also usually appear in areas of contemporary seismicity.
As a synthesis of the discussion, a map of energy release (as opposed to heat flow measured at the surface) and a simple model of a Cordilleran thermal event are presented. The generalized map of energy release shows total thermal energy transfer from the mantle, including nonconductive energy losses. This map shows highest heat flow along the eastern and western borders of the Cordilleran thermal anomaly zone and a smoother variation of heat flow within the zone than does the heat-flow map. During a continental thermal event typical of the Cordilleran thermal anomaly zone, which may be infinitely more varied in intensity and duration than an oceanic spreading event, the conductive heat flow will be only a part of the total energy loss, and the dominant heat-transfer mechanisms change with time over the life of the event. For the areas where a thermal event is young, volcanism and plutonism may be the prime energy-loss mechanisms, but in spite of the high overall energy loss, large areas of young volcanism may have very low conductive heat flow because of the dominant effect of hydrothermal convection as a mechanism for plutonic energy loss. As the thermal event decays, regional hydrothermal convection and heat conduction become the dominant heat-transfer mechanisms. For areas where thermal events are older than 17 m.y., heat conduction is the dominant heat-transfer mechanism although hydrothermal convection may be locally significant.