Abstract

Along with magma supply rates, glacial ice and local tectonics influence the eruptive style and preservation of arc volcanoes. We present the results of geologic mapping and 20 new 40Ar/39Ar dates for the mafic composite North Sister Volcano, the adjoining Little Brother shield volcano, and Matthieu Lakes Fissure in the central Oregon Cascade Arc, along with major-and trace-element analyses. North Sister is the oldest and most mafic of the Three Sisters Volcanoes and has produced dike-fed, low-K basaltic andesitic (52.5%–55% SiO2) agglutinated lava, scoria, and tuffs over its < ca. 400 k.y. history. Severe erosion of the North Sister edifice has revealed the largest radial dike swarm in the Cascade Arc as well as abundant subglacially erupted, palagonitic tuffs and breccias and dramatic unconformities. Slight, but systematic changes in composition coincide with changes in eruptive style, indicating that North Sister was constructed in four central volcano eruptive stages. Stages are bounded by unconformities and include (1) the Lower Shield Stage (ca. 400 ka), (2) the Glacial Stage (99–182 ka), (3) the Upper Shield Stage (ca. 80 ka), and (4) the Stratocone Stage (55–70 ka). We estimate that ~90% of the total 40 km3 volume of North Sister was produced during the first two stages. The >11-km-long, north-trending Matthieu Lakes Fissure (75–11 ka) transects North Sister and erupted in three magmatic pulses, yielding a series of thick lavas, scoria cones, and subglacially erupted flow-dominated tuyas.

Glacial ice has been present during much of the eruptive history of North Sister. Palagonitic tuffs, tuyas, and ice-contact lava flows as well as major unconformities between eruptive stages correlate with the δ18O of seawater global climate record at ca. 250 ka, 180–130 ka, and 80–18 ka. Time-integrated eruption rates at North Sister appear to have slowed from 0.18 to 0.12 km3 per k.y. between < ca. 400–300 ka and 100 ka to 0.08 km3 per k.y. between ca. 100 ka and 50 ka. The waning of volcanism at North Sister coincides with a decrease in Ni and MgO concentrations, indicating that olivine fractionation dominated the later stages. Also, toward the end of volcanism at North Sister, dikes changed from a radial pattern to a N-S pattern, parallel to faults and vent alignments associated with E-W extension of the High Cascades graben. The Matthieu Lakes Fissure represents the final overprinting of the edifice and distribution of magma supply away from the North Sister center.

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