Hydrothermal systems of the Cascade Range, north-central Oregon

Quaternary volcanoes of the Cascade Range form a 1,200- kilometer-long arc that extends from southern British Columbia to northern California. The section of the Cascade Range volcanic arc in central Oregon is characterized by relatively high Quaternary volcanic extrusion rates and hot-spring discharge rates. Stableisotope data and measurements of hot-spring heat discharge indicate that gravity-driven thermal fluid circulation transports about 1 MW (megawatt) of heat per kilometer of arc length from the Quaternary arc into Western Cascade rocks older than about 7 Ma (millions of years before present). Inferred flow-path lengths for the Na-Ca-Cl thermal waters of the Western Cascades are 10 to 40 kilometers (km), and an average topographic gradient as large as 0.1 separates the inferred recharge areas from the hot-spring groups. Thermal-fluid residence times are probably 10² to 10⁴ years: sulfate-water isotopic equilibrium indicates residence times of more than 10² years, and our interpretation of stable-isotope data implies residence times of less than 10⁴ years.