TIMING OF VOLCANISM IN CENTRAL OREGON: DATING THE TAM MCARTHUR RIM VOLCANO

The Cascadia subduction zone on the West Coast of North America produces variable volcanism. The relatively young active volcanism in Central Oregon is of particular interest because communities are nearby that are susceptible to volcanic hazards. The Three Sisters and Tumalo Volcanic Field, among other systems, present potential hazards to Bend, Sisters and other towns in Central Oregon. The Tam McArthur Rim is a volcanic center located within the Tumalo Volcanic Field (TVF), approximately 25 km west of Bend, OR. The rim comprises of shallowly dipping bedded sequences of intermediate and silicic flows with minor tephra interbedding capped by a rhyodacite dome. Glacial activity has exposed significant portions of flow stratigraphy.

In this study we present new geochronology, petrography, mineral chemistry, and whole rock geochemistry of lava flows from the rim. Flow compositions vary significantly, ranging from andesites to thick obsidian flows. Some flows are aphyric with trachytic flow textures while others are crystal rich lacking flow textures. We provide new 40Ar/39Ar geochronology which shows lower portions of the rim are ~185 ka, suggesting volcanism is younger than previously calculated K-Ar ages. The new age data suggest the Tam McArthur edifice was constructed relatively quickly, likely within a 10 ka period. Constraining the timing of past volcanism associated with the TVF provides insights into potential future hazards, allowing for awareness and implementation of informed mitigation strategies within Central Oregon communities.