HEAT FLOW AND GEOTHERMAL RESOURCES OF THE ALASKAN INTERIOR

WILLIAMS, Colin F.¹, GALANIS, S. Peter¹, GRUBB, Frederick V.², and SASS, John H.³, (1) US Geological Survey, Mail Stop 977, 345 Middlefield Road, Menlo Park, CA 94025, [email protected], (2) US Geol Survey, Sacramento, CA 95819, (3) US Geological Survey, Flagstaff, AZ 86001

As part of a project to compile a new national geothermal resource assessment, we have collected and analyzed new and previously unpublished temperature and thermal conductivity measurements to determine conductive heat flow at 11 sites in the Alaskan interior from the Brooks Range in the north to Cook Inlet in the south. The heat flow values range from 42 to 130 mW m^-2 with a mean of 88 mW m^-2. Heat flow at four of the sites exceeds 100 mW m^-2. The high heat flow values at two of these four sites most likely reflect the effects of magmatic heating from the nearby Wrangell Mountains and Prindle volcanic areas. Thermal conditions at the other two sites may be due to the combined effects of erosion and rapid near-surface ground-water flow. Although the limited spatial coverage of measurements leaves significant uncertainties in some areas, such as southwestern Alaska, we can draw some preliminary conclusions about the thermal regime of central Alaska and the degree to which crustal heat flow influences the formation of hydrothermal systems.

Geothermal manifestations in central Alaska are dominated by the belt of thermal springs that extends along an east-west trend from the Canadian border to the Seward peninsula. Heat flow measurements at five sites cover the portion of this trend from Fort Yukon in the east to the base of the Seward peninsula in the west and average 77±7 mW m^-2. Limited temperature gradient data from the Seward peninsula itself are consistent with a higher background heat flow. The 77 mW m^-2 average is elevated relative to average continental heat flow, and simple thermal models indicate that it is favorable for the formation of low to moderate temperature (up to 150 ^oC) hydrothermal systems from the deep circulation of water along fault zones or within permeable strata. Geothermometer estimates of temperatures for geothermal reservoirs located in the same area are consistent with this result, and such systems are capable of supporting local geothermal direct use and electric power generation from binary power plants. Large, high temperature hydrothermal systems are unlikely to be found outside of areas of Quaternary magmatic activity, but comparisons with other thermally active regions indicate that the identified thermal springs and manifestations in central Alaska are only a fraction of the number likely to be found with more extensive exploration.