Are There Significant Hydrothermal Resources in the U.s. Part of the Cascade Range?

The Cascade Range is a geothermal dichotomy. On the one hand, it is an active volcanic arc above a subducting plate and is demonstrably an area of high heat flow. On the other hand, the distribution of hydrothermal manifestations compared to other volcanic arcs is sparse, and the hydrothermal outflow calculated from stream chemistry is low. Several large estimates of undiscovered geothermal resources in the U.S. part of the Cascade Range prepared in the 1970s and early 1980s were based fundamentally on two models of the upper crust. One model assumed that large, partly molten, intrusive bodies exist in the upper 10 km beneath major volcanic centers and serve as the thermal engines driving overlying hydrothermal systems. The other model interpreted the coincident heat-flow and gravity gradients west of the Cascade crest in central Oregon to indicate a partly molten heat source at 10*2 km depth extending -30 km west from the axis of the range. Investigations of the past ten years have called both models into question. Large long-lived high-temperature hydrothermal systems at depths < 3 km in the U.S. part of the Cascade Range appear to be restricted to silicic domefields at the Lassen volcanic center, Medicine Lake volcano, Newberry volcano, and possibly the Three Sisters. Federal land-use restrictions further reduce this list to Medicine Lake and Newberry. Dominantly andesitic stratocones appear to support only small transitory hydrothermal systems related to small intrusive bodies along the volcanic conduits. The only young caldera, at Crater Lake, supports only lowto intermediate-temperature hydrothermal systems. Most of the Cascade Range comprises basaltic andesites and has little likelihood for highlevel silicic intrusions and virtually no potential for resultant large high-temperature hydrothermal systems. Undiscovered hydrothermal resources of the Cascade Range of the United States are substantially lower than previous estimates. The range does have potential for intermediatetemperature hot dry rock and localized lowto intermediate-temperature hydrothermal systems. INTRODUCTION This paper presents preliminary conclusions from a multi-year effort of the U.S. Geological Survey to evaluate the geothermal potential of the Cascade Range. A detailed assessment will be published as a Bulletin of the U.S. Geological Survey (Muffler and Guffanti, in preparation). Here we draw heavily on a manuscript (Guffanti and Muffler, 1995) submitted to the May 1995 World Geothermal Congress in Florence, Italy, as well as several recent publications of our USGS colleagues, particularly Ingebritsen etal. (1989, 1992, 1994), Mariner e t a l . (1990), and Blakely (1994). TECTONIC AND VOLCANIC SETTING The Cascade volcanic arc lies above the Cascadia subduction zone along which the small Gorda, Juan de Fuca, and Explorer plates are being subducted eastward under the large North American plate. The Cascade volcanic arc consists of two physiographic and geologic provinces (Peck et al., 1964; Duncan and Kulm, 1989): the 42-10 Ma Western Cascades, and the 10-0 Ma High Cascades. Just east of the High Cascades are two large, essentially bimodal, basalt-rhyolite volcanoes: Newberry volcano in Oregon, and Medicine Lake volcano in northern California. Major composite volcanoes that have erupted andesites, dacites, and even rhyolites occur at intervals of =lo0 km along the Cascade Range (Figure 1). In addition, there are several thousand smaller, discrete volcanoes that over the past five million years have erupted only once, or at most a few times, producing primarily basaltic andesite (see Guffanti and Weaver, 1988). Sherrod and Smith (1990) estimated the following extrusion rates since 2 Ma in km' per km of arc length per million years: north of Mount Rainier: 0.21 southern Washington and northern Oregon: 1.6 central Oregon: 3-6 northern California: 3.2.