DEEP-STRUCTURE OF MEDICINE-LAKE VOLCANO, CALIFORNIA

Medicine Lake volcano (MLV) in northeastern California is the largest- volume volcano in the Cascade Range. The upper-crustal structure of th is Quaternary shield volcano is well known from previous geological an d geophysical investigations. In 1981, the U.S. Geological Survey cond ucted a teleseismic tomography experiment on MLV to explore its deeper structure. The images we present, calculated using a modern form of t he AGH-inversion method, reveal that there is presently no hint of a l arge (>100 km(3)), hot magma reservoir in the crust. The compressional -wave velocity perturbations show that directly beneath MLV's caldera there is a zone of increased seismic velocity. The perturbation amplit ude is +10% in the upper crust, +5% in the lower crust, and +3% in the lithospheric mantle. This positive seismic velocity anomaly presumabl y is caused by mostly subsolidus gabbroic intrusive rocks in the crust . Heat and melt removal are suggested as the cause in the upper mantle beneath MLV, inferred from petro-physical modeling. The increased sei smic velocity appears to be nearly continuous to 120 km depth and is a hint that the original melts come at least partly from the lower lith ospheric mantle. Our second major finding is that the upper mantle sou theast of MLV is characterized by relatively slow seismic velocities ( -1%) compared to the northwest side. This anomaly is interpreted to re sult from the elevated temperatures under the northwest Basin and Rang e Province.