HYDROTHERMAL HEAT DISCHARGE IN THE CASCADE RANGE, NORTHWESTERN UNITED STATES

Hydrothermal heat discharge in the Cascade Range includes the heat discharged by thermal springs, by “slightly thermal” springs that are only a few degrees warmer than ambient temperature, and by fumaroles. Thermal-spring heat discharge is calculated on the basis of chloride-flux measurements and geothermometer temperatures and totals ~230 MW in the U.S. part of the Cascade Range, excluding the transient post-1980 discharge at Mount St. Helens (currently ~80 MW). Heat discharge from “slightly thermal” springs is based on the degree of geothermal warming (after correction for gravitational potential energy effects) and totals ~650 MW. Fumarolic heat discharge is calculated by a variety of indirect and direct methods and totals ~150 MW, excluding the transient mid-1970s discharge at Mount Baker (~80 MW) and transient post-1980 discharge at Mount St. Helens (currently >230 MW). Other than the pronounced transients at Mount St. Helens and Mount Baker, hydrothermal heat discharge in the Cascade Range appears to be fairly steady over a ~30-year period of measurement. Of the total of ~1,000 MW of “steady” hydrothermal heat discharge identified in the U.S. part of the Cascade Range, less than 50 MW occurs north of latitude 45^o15’ N (~0.1 MW per km arc length from 45^o15’ -49^oN). Much greater rates of hydrothermal heat discharge south of 45^o15’N (~1.7 MW per km arc length from 40-45^o15’N) may reflect the influence of Basin and Range-style extensional tectonics (faulting) that impinges on the Cascades as far north as Mount Jefferson but is not evident farther north.