GEOLOGIC, GEOPHYSICAL, AND GEOTHERMAL CHARACTERISTICS OF ST. HELENS SHEAR ZONE: RESULTS FROM WASHINGTON STATE PLAY FAIRWAY ANALYSIS

Although geothermal electricity is produced all around the Pacific Ring of Fire, the geothermal resources of the Washington Cascades remain undeveloped. Part of the difficulty of finding and developing geothermal resources in the Cascades is due to the complex pattern of crustal deformation caused by the same plate convergence that produces the magmatic arc and adds heat to the crust. Other challenges for geothermal explorers include dense vegetation, glacial veneers, and extreme precipitation and topography that leads to erosion of heat and surficial alteration.

Phase 2 of the DOE-funded Washington Play Fairway Analysis (PFA) enabled improved geologic mapping and multiple geophysical surveys of the northern St. Helens Shear Zone (SHSZ), a NNW trending fault system that connects Mt. St. Helens to geothermal leases 12-24 km to the north. New data collected included gravity, magnetic, and magnetotelluric (MT) surveys and passive seimic data from deployment of a passive 20-station seismic array which enabled improved local earthquake detection, local earthquake tomography, and ambient noise tomography. Geophysical attributes of the SHSZ that suggest it could be a conduit for upwelling hot fluid geothermal fluids are 1) low P-wave (Vp) and S-wave (Vs) velocities that infer higher temperatures and porosity, 2) high Vp/Vs anomalies that infer fluid-filled fractured rock, 3) frequent and distributed seismicity that infer critically stressed fractures and permeability, 4) a gravity low bound by high horizontal gravity gradients that infer metasediments bound by higher density Tertiary plutons - mapped as the Spirit Lake granite on the east and the Spud Mountain granite on the west, and 5) low electrical resistivity that infers geothermal brine and/or geothermal alteration.

Combined, these attributes resulted in a data-supported geothermal favorability map that can now be tested. In Phase 3, which begins October 2017, temperature-gradient holes will be drilled at two favorable locations within the SHSZ. In addition to measurement of temperature profiles, core and cuttings will be collected for microstructural and petrologic analysis. The results will be integrated into a revised favorability model that will help to refine the PFA methodology, and reduce future exploration risk.