STRUCTURAL CONTROLS AND PRELIMINARY GEOLOGIC MAP OF THE NEAL HOT SPRINGS GEOTHERMAL AREA, MALHEUR COUNTY, OREGON

Amagmatic, geothermal systems are commonly controlled by certain structural relationships that foster deep fluid flow. Studying these relationships in producing geothermal fields yields important insights. Knowing this, detailed geologic mapping (1:24,000 scale), structural analyses, and integration of geophysical and well-field data were utilized to assess the structural controls of the Neal Hot Springs geothermal field in eastern OR. The geothermal field lies within the intersection of two regional grabens, the middle-late Miocene, N-trending, OR-ID graben and younger late Miocene to Holocene, NW-trending, western Snake River Plain graben.

The local structural framework at Neal is characterized by northerly to NW-striking normal faults, including the geothermally related Neal fault zone (NFZ). The geothermal field is bounded on the east by the Neal fault, a major, W-dipping, N- to NNW-striking, steeply dipping normal to oblique-slip fault, along which geothermal fluids ascend, and on the west by the concealed N- to NNW-striking, W-dipping Sugarloaf butte fault. Stress inversion of kinematic data reveal an extensional stress regime, including an interpreted younger, SW-trending (~243°), and older, W-trending (~265°), least principal stresses. The NFZ 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 W-trending and later SW-trending extensional stress regimes. Recent sinistral-normal displacement generated a small pull-apart along the NFZ. ‘Hard-linkage’ between the Neal and Sugarloaf Butte faults likely occurs through concealed, WNW-striking faults. An inferred N-plunging fault intersection controls the location of the hot springs and sinter terraces.

Young faulting is evident at Neal with Quaternary fans cut by the NFZ. In addition, the geothermal field is 4 km west of the active, N- to NW-striking, normal-slip Cottonwood Mountain fault and its recently recorded seismicity. This, coupled with its active hot springs (~90°C) and opaline sinter mounds, suggests that the geothermal field lies within an active (Quaternary), southward-terminating, left-stepping fault zone, which locally acts as a pull-apart with sinistral- and normal-slip components.