Introduction
Since 1979 the Geothermal Research Program of the U.S.
Geological Survey has carried out a multidisciplinary research
effort in the Cascade Range. The goal of this research is to
understand the geology, tectonics, and hydrology of the
Cascades in order to characterize and quantify geothermal
resource potential. A major goal of the program is compilation
of a comprehensive geologic map of the entire Cascade Range
that incorporates modern field studies and that has a unified
and internally consistent explanation.
This map is one of a series presently being compiled that
shows Cascade Range geology by fitting published and unpublished
mapping into a province-wide scheme of lithostratigraphic
units; map sheets of the Cascade Range in California
and in Oregon complete the series. The complete series forms a
guide to exploration and evaluation of the geothermal resources
of the Cascade Range and will be useful for studies of volcanic
hazards, volcanology, and tectonics.
For geothermal reasons, the maps emphasize Quaternary
volcanic rocks. Large igneous-related geothermal systems that
have high temperatures are associated with Quaternary volcanic
fields, and geothermal potential declines rapidly as age increases
(Smith and Shaw, 1975). Most high-grade recoverable geothermal
energy is likely to be associated with silicic volcanism
less than 1 Ma. Lower grade (= lower temperature) geothermal
resources may be associated with somewhat older rocks;
however, volcanic rocks older than about 2 Ma are unlikely
geothermal targets (Smith and Shaw, 1975).
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First posted October 16, 2014
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