Some implications of late Cenozoic volcanism to geothermal potential in the High Lava Plains of south-central Oregon

Stratigraphic and volcanologic data obtained during reconnaissance mapping along the Brothers fault zone in the High Lava Plains of south-central Oregon (Walker and others, 1967; Greene and others, 1972) suggest a progressive decrease in age of eruption of silicic magmas from the Harney Basin westward to Newberry volcano and possibly beyond into the Cascade Range. South of the Brothers fault zone a separate, less well defined, parallel zone of silicic domes and flows extends westward from Beatys Butte; this zone also appears to show a decrease in age westward, although the time span apparently is shorter and the age decrease not as well documented. The apparent age progression and the obvious very young age and character of some of the silicic volcanic rocks at the western end of the zone is important in geologic evaluations of the geothermal potential of eastern Oregon. It suggests that igneous heat sources sufficiently young to have retained significant magmatic heat are most likely to occur at the western end of the zone. Geothermal energy associated with silicic volcanism is being explored or developed in several other parts of the world (White, 1965; Muffler and White, 1972; Grose, 1972). In areas of known geothermal potential, such as in New Zealand, Italy, Japan, and in North America at Yellowstone, the Jemez Mountains (N. Mex.), and probably the Geysers area of California, the age of silicic volcanism is very young, mostly less than a million years and commonly only a few tens or hundreds of thousands of years old, and the volume of silicic volcanism generally is large, in many places measured in terms of many cubic kilometers of magma. Because some domal masses at the west end of the Brothers fault zone are very young and some of the silicic volcanism along the zone was volumetrically large, the potential for magmatitic heat sources seems promising.

To test the concept of an age progression in and along the Brothers fault zone, as determined from field geologic evidence, and to establish the absolute ages of some of the silicic volcanic rocks in and near this zone, several potassium argon ages have been determined on crystalline phases from rhyolitic, rhyodacitic, and dacitic domes and associated flows, on rhyolitic obsidian from chilled selvages on several domes and flows, and on crystals and glass from basal vitrophyre of small and large volume ash-flow tuffs erupted from centers spatially and probably genetically related to the Brothers fault zone.