The Thermal Structure of the Continental Crust

Abstract

In the United States, the surface heat flow can be separated into two main components. The first component is due to radioactive heat sources in the upper crust, and the second component is due to sources in the lower crust and upper mantle. The heat flow from the lower crust and upper mantle is constant over large regions, called heat flow provinces, and the transitions between provinces are very narrow (less than 100 km). High values of mantle heat flow (>1.4 ical/cm 2 sec) occur in the Basin and Range, Columbia Plateaus, Northern Rocky Mountains, Southern Rocky Mountains physiographic provinces, and in the Franciscan rocks east of the San Andreas fault zone. Normal or near-normal mantle heat flow (0.8) is found in th e United States east of the Rocky Mountains, in the Colorado Plateaus, the Southern California batholith, and in the Puget Sound Region. Subnormal mantle heat flow (0.4, the lowest known anywhere) occurs in the Sierra Nevada Mountains. The variations in ma ntle heat flow are attributed to the thermal effects of sea -floor spreading during the Cenozoic. Measurements on other continents suggest heat flow provinces there with mantle heat flow values similar to those found in the United States. Temperatures are calculated for the three heat flow provinces under a variety of assumptions. The mantle heat flow is the main determinant of crustal temperatures, and thus vertical variations in electrical conductivity due directly or indirectly to temperature effects must vary in depth between the heat flow provinces. The region in the United States with the lowest crustal temperatures in crystalline terrain is the western foothills of the Sierra Nevada; hence this area should be the most favorable for feasibility studies of crustal transmission of electromagnetic waves.