Graduate Thesis Or Dissertation

A Burst of Silicic Volcanism in the Central Oregon Cascades: Insights into the Eruption History of the Tumalo Volcanic Center

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  • The Tumalo volcanic center (TVC) is a major but relatively understudied ~700 km2 geologic area in the Cascade arc, located west of the city of Bend, OR, and east of South Sister Volcano. The TVC is composed of intermediate to silicic pyroclastic flows, tephra, and domes and lava flows, totaling at least 30-50 km3 of erupted material. Located within a complex tectonic region, the TVC resides at the intersection of the High Cascades subduction zone and the intraplate age-progressive High Lava Plains volcanism trend. In addition, it is flanked by several normal fault systems, including the Brother’s Fault Zone, the Sisters Fault Zone, and the High Cascades Graben. This study aims to determine whether TVC magmatism is attributed to subduction zone or intraplate volcanic processes by 1) using petrography and geochemistry to determine magma storage conditions (“hot-dry” vs. “cold-wet”) and 2) using 40Ar/39Ar age information to thoroughly evaluate the timing of TVC eruptions relative to regional tectonics. Petrographic analysis reveals that most TVC units are crystal-poor (<10 vol %) with an anhydrous mineral assemblage (plagioclase + clinopyroxene + orthopyroxene + oxides). Additionally, TVC units exhibit high FeO*, FeO*/MgO, and Zr/Sr, undepleted middle REE trends, and high temperature (~800 to > 1000°C) thermometry (plagioclase-liquid, clinopyroxene, two-pyroxene, and Fe-Ti oxide); all of which indicate “hot” storage conditions. Fe-Ti fO2 data indicate slightly more reducing conditions (between NNO-1 and NNO+1) than average Cascades data, including rhyolites from nearby South Sister volcano, which lies on the Cascade arc front. Oxygen isotope values (δ18Oplagioclase=4.82 to 7.18 ‰) are consistent with the assimilation of high and low δ18O crust. 40Ar-39Ar ages indicate volcanism occurred between ~180 to 668 ka, with a major magmatic episode at ~400-440 ka that includes a large ignimbrite, tephra fallout, and several domes and lava flows along a NW-SE trend that is parallel to local normal faults. Overall, chemical, geochronological, and volume results indicate that the TVC may represent the “missing link” between the hot-dry-reduced, tholeiitic High Lava Plains (HLP) volcanic activity that occurs within the back arc, including the “hot-damp” activity at Newberry Volcano (Mandler et al., 2014; Parker, 2021), and the increase in “hot-damp” silicic volcanic activity that occurs at Three Sisters Volcanic Center after 50 ka (Fierstein et al., 2011; Parker et al., 2023).
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  • NSF Grant No. 1764360
  • George and Danielle Sharp Fellowship
  • Central Oregon Geological Society Research Grant
  • NSF-INTERN Fellowship



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