Structural Controls of the Neal Hot Springs Geothermal System, Eastern Oregon

Detailed geologic mapping (1:24,000 scale), structural and geochemical analyses, and integration of available geophysical and well-field data were utilized to assess the structural controls of the Neal Hot Springs geothermal field in eastern Oregon. The geothermal field lies within the intersection of two regional grabens, the middle-late Miocene, north-trending, Oregon-Idaho graben and younger late Miocene to Holocene, northwest-trending, western Snake River Plain graben. It is marked by Neal Hot Springs, which effuse from opaline sinter mounds just north of Bully Creek. Production and injection wells, with temperatures up to 142 degrees C, intersect the Neal fault zone at depths of 680-1900 m and subsidiary faults within a relay ramp or step-over within the fault zone.

The stratigraphy at Neal correlates with four regional packages. Basement rocks, discovered in one well, are granite, tentatively correlated with Jurassic Olds Ferry-Izee terrane. Nonconformably above is a thick package of middle Miocene Columbia River Basalt Group lavas, regionally known as the basalt of Malheur Gorge. Conformably above are middle to late Miocene Oregon-Idaho graben lavas, volcaniclastics, fluvial and lacustrine rocks. Overlying are the youngest rocks at Neal, which are late Miocene to Pliocene, western Snake River Plain lacustrine, fluvial, and volcaniclastic rocks.

The structural framework at Neal is characterized by northerly to northweststriking normal faults, including the geothermally related Neal fault zone. Stress inversion of kinematic data reveal an extensional stress regime, including an interpreted younger, southwest-trending (~243 degrees), least principal stress and an older, west-trending (~265 degrees) least principal stress.

The geothermal field is bounded on the east by the Neal fault, a major, westdipping, north-northwest-striking, steeply dipping normal to oblique-slip fault, along which geothermal fluids ascend, and on the west by the concealed north-northweststriking, west-dipping Sugarloaf Butte fault. The Neal fault zone can be modeled into two structural settings: an interpreted older, left-stepping, normal-slip fault zone and a younger, oblique sinistral-normal zone, suggested by the earlier west-trending and later southwest-trending extensional stress regimes. Recent sinistral-normal displacement may have generated a small pull-apart basin in the Neal area and facilitated development of the geothermal system. 'Hard-linkage' between the Neal and Sugarloaf Butte faults occurs through concealed, west-northwest-striking faults, including the Cottonwood Creek subvertical fault, along which lateral fluid-flow is likely. An inferred northplunging fault intersection at the Neal Hot Springs likely controls the location of the hot springs and sinter terraces.

Young structural features are evident at Neal. The Neal fault zone cuts Quaternary fans and late Miocene lower and upper Bully Creek Formation sedimentary rocks. In addition, the geothermal field is 4 km west of the active, north- to northweststriking, normal-slip Cottonwood Mountain fault. Furthermore, the field is within several kilometers of recently detected seismicity. This, coupled with its active hot springs (~90 degrees C), opaline sinter mounds, and geothermal fluid flow, suggest that the geothermal field lies within an active (Quaternary), southward-terminating, left-stepping fault zone, which locally acts as a pull-apart basin with sinistral- and normal-slip components.