Play Fairway Analysis of the Central Cascades Arc-Backarc Regime, Oregon: Preliminary Indications

摘要

We are assessing the geothermal potential including possible blind systems of the Central Cascades arc-backarc regime of central Oregon through a Play Fairway Analysis (PFA) of existing geoscientific data. A PFA working model is adopted where MT low resistivity upwellings suggesting geothermal fluids may coincide with dilatent geological structural settings and observed thermal fluids with deep high-temperature contributions. A challenge in the Central Cascades region is to make useful Play assessments in the face of sparse data coverage. Magnetotelluric (MT) data from the relatively dense EMSLAB transect combined with regional Earthscope stations have undergone 3D inversion using a new edge finite element formulation. Inversion shows that low resistivity upwellings are associated with known geothermal areas Breitenbush and Kahneeta Hot Springs in the Mount Jefferson area, as well as others with no surface manifestations. At Earthscope sampling scales, several low-resistivity lineaments in the deep crust project from the east to the Cascades, most prominently perhaps beneath Three Sisters. Structural geology analysis facilitated by growing LiDAR coverage is revealing numerous new faults confirming that seemingly regional NW-SE fault trends intersect N-S, Cascades graben-related faults in areas of known hot springs including Breitenbush. Major element geochemical modeling of high-chloride thermal springs west of Three Sisters using Geo-T analytical software implies subsurface equilibration temperatures in the 130-140°C range. Subsurface temperatures and deep source contributions will be refined using ToughReact reactive transport analysis including isotopic data. To date, results appear consistent with our PFA working model described above.