THREE FINGERED JACK: A DISSECTED MAFIC COMPOSITE VOLCANO IN THE CENTRAL OREGON CASCADES

The central Oregon High Cascades are characterized by intra-arc extension and voluminous mafic lavas, creating an extensive mafic platform dominated by dozens of small scoria cones and ~60 km³ of lava flows. Three Fingered Jack is one of the eroded, large mafic composite volcanoes that stands out among the monogenetic small edifices of the central High Cascades, near the town of Sisters, Oregon. This ongoing study focuses on understanding magma evolution at Three Fingered Jack and documenting its eruptive history, including its activity period and volcanic stratigraphy. Crustal extension in the central High Cascades has allowed the ascent of calc-alkaline basaltic andesite and tholeiitic basalt magmas that initially accumulated at a significantly greater depth. Geochemical analysis of eruptive units from Three Fingered Jack and its surrounding will provide insight into magma evolution and recharge events that have led to subtle compositional variations between the basaltic andesites of Three Fingered Jack and the underlaying mafic platform of the central High Cascades. Two chemistries of basaltic andesites proposed in the central High Cascades are: 1) Mount Washington type basaltic andesite that has relatively higher REE concentration and LILE abundance 2) North Sister type basaltic andesite that has comparatively lower REE concentration. We suspect multiple trends of fractionation controlling magma composition in this region. As prolonged volcanic activity or more voluminous eruptions occurred to form a significantly larger edifice like Three Fingered Jack, we are particularly interested in documenting which geochemical or mineralogical characteristics differentiate basaltic andesites of Three Fingered Jack from the ones of its surrounding edifices that in turn correlate with edifice volume.

The High Cascades of Oregon have the largest concentration of mafic monogenetic volcanoes in the entire Cascade arc, some of which have erupted in the past 2,500 years. Three Fingered Jack’s dissected stratigraphy can give us the temporal activity record, which will help us better understand the eruptive patterns of the more voluminous intra-arc shield volcanoes of the central High Cascades that pose a significant risk to central Oregon communities.