Newberry Volcano, long a target for geothermal exploration as well as an EGS demonstrator site, lies at the intersection of the northernmost Basin and Range Province, the High Lava Plains and the Cascade volcanic backarc in central Oregon. The location of the volcanic edifice and surrounding lava flows and cinder cones is structurally controlled, with the edifice intersecting the Walker Rim fault to the south-southwest, the Sisters Fault to the north-west, and the Brothers Fault to the north-east. Newberry caldera was formed at this intersection of crustal features by an explosive eruption 75,000 ybp followed by numerous rhyolitic and basaltic andesite eruptions. The Big Obsidian Flow (BOF), the most recent eruptive feature (1,300 ybp) lies inside the southern caldera rim. Pauling et al. (2023) reported at the 2023 Stanford Workshop on Geothermal Reservoir Engineering on a 2022 broadband magnetotelluric (BBMT) and gravity campaign by NREL, Enthalpion Energy LLC and personnel from Oregon State University that acquired 43 BBMT and 233 gravity stations primarily from inside the caldera, the south caldera rim and south flank of the volcano. This was carried out under the support of the DEEPEN (DErisking Exploration for multiple geothermal Plays in magmatic ENvironments) project managed by NREL. A highly conductive feature beneath the south rim and flank of the volcano that shallowed near the BOF had been previously suggested by 3-D inversion of BBMT data acquired in 2012 and 2014, as well as legacy BBMT data, but this feature was poorly constrained because of limited MT data in the south flank area. We report on a joint 3-D gravity + MT inversion of this expanded 2022 and legacy data set of 237 BBMT and 1514 gravity stations, and joint interpretation with previous seismic models. The highly conductive south rim/flank feature (heretofore referred to as SRFF) was confirmed, it is well resolved, and it is of low relative density and seismically slow. The SRFF extends from near the caldera floor at the BOF, deepening to the south beneath the south flank of the volcano to depths of at more than 4 km relative to the caldera floor, and it is not connected to the sub-caldera magma body. The trace of the SRFF lies immediately east of the Walker Rim fault, within the Basin and Range province, and it is also aligned with a system of Holocene mafic flows, cinder cones and fissure vents. Given its proximity to the Walker Rim fault and the association with faults as fluid migration pathways, the SRFF likely represents the aggregate signature of hydrothermal alteration products, hydrothermal fluids, and at depths greater than the current dataset’s resolution limit, potentially partial melt. Legacy well GEO N-1, immediately west of the SRFF shows a transition from purely conductive heat flow in the upper 1000 m, and advective/convective heat flow below presumably related to movement of hydrothermal fluids. The presence of smectite as an alteration product of basaltic ash and ash flow tuff has been associated with the top of the conductive zone 675 m below the GEO N-1 well head. In addition to the SRFF, a series of low seismic vp, low relative density, highly conductive N-S aligned finger-like features appear just within the resolution limits of the data sets ~15 km south of the caldera center, roughly parallel to the trace of the Walker Rim fault. These features bear further examination as potential extensions of the geothermal target identified beneath the volcano’s south rim and flank. For some decades Newberry volcano has been viewed strictly as an EGS development play restricted to the west flank of the volcano. Our reappraisal of legacy seismic, gravity, MT and well data, coupled with the substantial increase in MT and gravity data coverage in 2022 suggests that rather than a single EGS site, Newberry may represent a series of geothermal target fields, including a potential blind hydrothermal play as well as EGS targets ranging from conventional to superhot. Follow-on efforts to drill as close as possible to the SRFF, subject to restrictions against drilling within the boundary of the National Volcanic Monument, is a logical next step to validate this conceptual model.

Press the Back button in your browser, or search again.

Copyright 2024, Stanford Geothermal Program: Readers who download papers from this site should honor the copyright of the original authors and may not copy or distribute the work further without the permission of the original publisher.