Hydrothermal systems of the Cascade Range, north-central Oregon

Quaternary volcanoes of the Cascade Range form a 1,200kilometer-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 10 2 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. A large area of near-zero near-surface conductive heat flow occurs in the younger volcanic rocks of the central Oregon Cascades, due to downward and lateral flow of cold ground water. A heat-budget analysis shows that heat advected from areas where rocks younger than about 7 Ma are exposed could account for the anomalously high advective and conductive heat discharge measured in older rocks at lower elevations. Magmatic intrusion at rates ranging from 9 to 33 km3 per kilometer arc length per million years could account for the total heat-flow anomaly. Two alternative models for the high heat flow observed in older rocks on the flanks of the Cascade Range involve (1) an extensive midcrustal heat source or (2) a narrower deep heat source that is confined to the Quaternary arc and is flanked by a relatively shallow conductive heat-flow anomaly caused by regional ground-water flow. This lateral-flow model implies a more limited geothermal resource base, but a better-defined exploration target. Analysis of available regional gravity, magnetic, and electrical geophysical data does not clearly favor either of the two models. We simulated ground-water flow and heat transport through two cross sections west of the Cascade Range crest: one in the Breitenbush area, where there is no major arc-parallel normal faulting, and one in the McKenzie River drainage, where major graben-bounding faults exist. Measured temperature profiles, hot-spring discharge rates, and geochemical inferences constrain the results. In the simulations, the alternative conceptual models for the deep thermal structure were represented as wide or localized deep heat sources. We found that either model can satisfy the observations. Thermal observations in the Breitenbush area seem to require significant advective heat transfer, whereas the sparser observations in the McKenzie River area can be satisfied with either advectionor conduction-dominated simulations. The numerical simulations provide some estimates of regional-scale permeabilities: simulated bulk permeabilities of about 10~14 m2 (meter squared) in the youngest (0-2.3 Ma) rocks and 10~17 m2 in the oldest (18-25 Ma) rocks allow the thermal observations to be matched. In general, permeability decreases downsection, but for rocks of any age, permeability at very shallow (less than about 50 m) depths may be much higher than the bulk permeability values required by the thermal observations: this is indicated by high recharge rates in 0to 7-Ma rocks (greater than 1 meter per year) and well-test data from domestic wells in rocks older than 7 Ma (which indicate permeability values of 10~14 to 10~12 m2). The actual thermal structure is probably more complex than either of the models considered here. Deep drilling in areas of high heat flow in the older rocks would be the most definitive test of the models. Comparison of natural heat discharge from the central Oregon Cascade Range with that from the relatively well-explored Taupo volcanic zone suggests that published resource estimates for the Cascades are optimistic. INTRODUCTION The Cascade Range is a 1,200-km-long volcanic arc that extends from southern British Columbia to northern California. High-temperature igneousrelated geothermal resources are assumed to exist in the Cascade Range (for example, Brook and others, 1979), but their magnitude and extent are poorly known. Several lines of evidence suggest relatively high geothermal potential in the central Oregon Cascade Range, a part of the arc characterized by relatively high rates of Quaternary volcanic extrusion (Sherrod and Smith, 1990), hot-spring discharge (Mariner and others, 1990), and conductive heat flow (Blackwell and others, 1982a, 1990a; Blackwell and Steele, 1987; Blackwell and Baker, Ll L2 GEOHYDROLOGY OF GEOTHERMAL SYSTEMS 1988b). The central Oregon Cascade Range also includes several silicic volcanic systems that are probably young enough and large enough to retain substantial amounts of heat (R.L. Smith and Shaw, 1975, 1979). Extrusion rates and hot-spring discharge rates decrease north and south of the area, and conductive heat flow decreases to the north and possibly to the south. The Cascade Range in Oregon is customarily divided into two physiographic subprovinces, the relatively uneroded High Cascades and the deeply dissected Western Cascades (Callaghan and Buddington, 1938). That distinction is useful here because of the fundamental control that topography exerts on regional hydrology. The High Cascades subprovince forms the crest of the range and is built mainly of permeable upper Pliocene and Quaternary volcanic rocks that create a broad ridge receiving heavy snowfall. The High Cascades are a regional ground-water recharge area; approximately half of the incident precipitation infiltrates and recharges ground-water systems. In contrast, the Western Cascades is a deeply incised terrane underlain by less permeable Oligocene to lower Pliocene volcanic and volcaniclastic strata. The boundary between the two subprovinces is controlled partly by volcanic onlap and partly by major normal faults. Topographically driven ground-water flow from the High Cascades feeds springs to the west and east. Most hot springs in the study area discharge at nearly the same elevation in deep valleys of the Western Cascades, as much as 20 km west of the High Cascades (fig. 1). One set of hot springs discharges east of the Cascade Range in a valley on the Deschutes-Umatilla Plateau. No hot springs are found in the High Cascades between latitudes 44° and 45°15' N. Two end-member models describe the deep thermal structure of the Oregon Cascade Range: one model invokes an extensive, uniform midcrustal heat source, the other a relatively narrow, spatially variable heat source. The relative contribution of regional versus localized heat sources is important to understanding the accessible geothermal resource base of the Cascade Range, and the heat-budget analysis and numerical simulations discussed in this report demonstrate implications of each model. This report is a chapter in the U.S. Geological Survey Professional Paper series "Geohydrology of Geothermal Systems. " Previous chapters in this series describe hydrothermal systems at Long Valley, California; Hot Springs, Arkansas; Yellowstone National Park; Klamath Falls and Warner Valley, Oregon; and Ennis, Montana. The series also includes surveys of hydrothermal systems over broader areas (for example, the State of Utah and the Appalachians) and several related topical studies.