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1 Department of Geology, Portland State University, P.O. Box 751, Portland, Oregon 97207, USA
2 U.S. Geological Survey, West 904 Riverside, Room 202, Spokane, Washington 99210, USA
3 Oregon Department of Geology and Mineral Industries, 1831 First Street, Baker City, Oregon 97814, USA
4 Department of Earth Sciences, Open University, Milton Keynes, UK
The Oregon-Idaho graben is a newly identified north-south–trending synvolcanic graben in southeastern Oregon and southwestern Idaho within the middle Miocene backarc rift system that extends 1100 km from southern Nevada to southeastern Washington. The graben formed along the western margin of the North American craton shortly after the largest volumes of tholeiitic flood basalt erupted (Columbia River Basalt Group, Steens-Pueblo Basalt, basalt of Malheur Gorge, basalt and latite unit of Ekren et al., 1981). Rhyolite flows and ash-flow tuffs (16.1–14.0 Ma) erupted from northeastern Oregon (Dooley Volcanics) to northern Nevada (McDermitt volcanic field) shortly after the flood basalt was emplaced. Subsidence of the Oregon-Idaho graben (15.5–15.3 Ma) coincides with eruption of rhyolite flows and caldera-related ash-flow tuffs from vents along the margins and within the graben. Mafic and silicic intragraben volcanism accompanied sedimentation from about 15.3 to 10.5 Ma. Sedimentary and volcanic rocks from extrabasinal sources, especially southwestern Idaho, were introduced periodically.
After initial subsidence, the evolution of the Oregon-Idaho graben is divided into three stages. Stage 1 (15.3–14.3 Ma) followed intragraben caldera collapse and was marked by deposition of fluvial and lacustrine sediment across the graben. Stage 2 (14.3–12.6 Ma) movement on intragraben fault zones divided the graben into distinct subbasins and marked the onset of calc-alkalic volcanism. Fine-grained tuffaceous sediment derived from glassy rhyolite and pyroclastic deposits and basalt tuff cones interbedded with rhyolite ash and lapilli-fall deposits and locally erupted basalt hydrovolcanic deposits predominated during synvolcanic subsidence. Synsedimentary hot-spring alteration and precious-metals mineralization of graben fill were controlled by the same intragraben fault zones that served as magmatic conduits. During stage 3 (12.6–10.5 Ma) the subbasins were filled, and graben-wide fluviatile and lacustrine sedimentation resumed. At about the same time, renewed rhyolitic volcanism occurred on both flanks, and tholeiitic volcanism resumed within the Oregon-Idaho graben. Subsidence in the Oregon-Idaho graben ceased as west-northwest–striking faults related to the formation of the western Snake River plain became active.
Key Words: extension tectonics • fault zone • geologic history • graben • Malheur County Oregon • Miocene • volcaniclastic rocks
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