Stratigraphic and structural evolution of the middle Miocene synvolcanic Oregon-Idaho graben

The Oregon-Idaho graben is a newly identified north-south-trending synvolca nic graben in southeastern Oregon and southwestern Idaho within the middle Miocene backarc rift system that extends 1100 km from southern Nevada to so utheastern Washington. The graben formed along the western margin of the No rth American craton shortly after the largest volumes of tholeiitic flood b asalt erupted (Columbia River Basalt Group, Steens-Pueblo Basalt. basalt of Malheur Gorge, basalt and latite unit of Ekren et al,, 1981), Rhyolite flo ws and ash-flow tuffs (16.1-14.0 Ma) erupted from northeastern Oregon (Dool ey Volcanics) to northern Nevada (McDermitt volcanic field) shortly after t he flood basalt was emplaced. Subsidence of the Oregon-Idaho graben (15.5-1 5.3 Ma) coincides with eruption of rhyolite flows and caldera-related ash-f low tuffs from vents along the margins and within the graben. Mafic and sil icic intragraben volcanism accompanied sedimentation from about 15.3 to 10. 5 hla, Sedimentary and volcanic rocks from extrabasinal sources, especially southwestern Idaho, were introduced periodically. After initial subsidence, the evolution of the Oregon-Idaho graben is divid ed into three stages. Stage 1 (15.3-14.3 R-la) followed intragraben caldera collapse and was marked by deposition of fluvial and lacustrine sediment a cross the graben, Stage 2 (14.3-12.6 Ma) movement on intragraben fault zone s divided the graben into distinct subbasins and marked the onset of calc-a lkalic volcanism, Fine-grained tuffaceous sediment derived from glassy rhyo lite and pyroclastic deposits and basalt tuff cones interbedded with rhyoli te ash and lapilli-fall deposits and locally erupted basalt hydrovolcanic d eposits predominated during synvolcanic subsidence. Synsedimentary hot-spri ng alteration and precious-metals mineralization of graben fill were contro lled 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 fl uviatile and lacustrine sedimentation resumed. lit about the same time, ren ewed rhyolitic volcanism occurred on both flanks, and tholeiitic volcanism resumed within the Oregon-Idaho graben, Subsidence in the Oregon-Idaho grab en ceased as west-northwest-striking faults related to the formation of the western Snake River plain became active.