Research Paper GEOSPHERE Eruptive history of Middle Sister, Oregon Cascades, USA—Product of GEOSPHERE; v. 14, no. 5 a late Pleistocene eruptive episode Andrew T. Calvert, Judy Fierstein, and Wes Hildreth https://doi.org/10.1130/GES01638.1 Volcano Science Center, U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025, USA 12 figures; 2 tables; 1 set of supplemental files ABSTRACT ternary vents (Hildreth, 2007). Study of the entire history of various volcanic CORRESPONDENCE: acalvert@ usgs .gov segments informs the underlying subduction­induced magmatic and tectonic New mapping, geochemistry, and argon geochronology illuminate a brief, conditions found in convergent margins. The Three Sisters arc segment is an CITATION: Calvert, A.T., Fierstein, J., and Hildreth, remarkably silicic episode set in a mafic segment of the Cascade arc. Middle attractive target for study because of the wide range of chemical compositions W., Eruptive history of Middle Sister, Oregon Cas‑ cades, USA—Product of a late Pleistocene eruptive Sister was constructed during a 35-k.y. episode in the late Pleistocene from of eruptive products, access to internal portions of the stratovolcanoes due to episode: Geosphere, v. 14, no. 5, p. 2118–2139, mafic, intermediate, and silicic eruptions adjacent to the primarily rhyolitic glaciation, and the relatively high­K compositions of volcanic materials allow­ https:// doi .org/ 10 .1130 /GES01638.1. South Sister. Eruptions in the Three Sisters volcanic cluster prior to 50 ka were ing high­precision geochronology. exclusively mafic (<57 wt% SiO2), and several basaltic andesite lava flows can Most of the volcanoes in the Three Sisters arc segment are mafic (<57 wt% Science Editor: Shanaka de Silva be traced to Middle Sister or a predecessor volcano (prior to 150 ka). Lava SiO2), and many are aligned in north­south–trending arrays. At least four of flows erupted 50–37 ka at Middle Sister and on its periphery were chemically these volcanoes (North, Middle, and South Sisters; Broken Top) have a long Received 20 November 2017 Revision received 6 April 2018 diverse, with abundant basaltic andesite, a high-silica rhyolite flow, and an eruptive history (>10 k.y.), and others (e.g., The Husband [Fig. 2A], The Wife Accepted 11 June 2018 andesite produced from mixing of a rhyolite and mafic magma. Abundant [6 km southeast of South Sister], Sphinx Butte [8 km west­southwest of South Published online 10 August 2018 rhyolite and rhyodacite erupted in this interval also at South Sister. Eruptive Sister]) may also have a protracted history, whereas most peripheral vents ap­ activity paused at Middle Sister 37–27 ka but continued at South Sister with pear to be products of single, brief eruptions. Middle Sister (3062 m altitude), large volumes of dacite and andesite lavas. Middle Sister erupted mafic, inter- the smallest and youngest stratovolcano in the Three Sisters volcanic cluster, mediate, and silicic lava flows 27–15 ka and then ceased to erupt. Calculated was constructed during a 35­k.y. episode in the late Pleistocene and is broadly eruptive rates for the entire Three Sisters volcanic cluster quadrupled from contemporaneous with South Sister. ~0.2 to ~0.8 km3/k.y. between 50 and 15 ka, largely owing to the eruptions The Three Sisters stratovolcanoes are of similar elevation and appear­ focused at Middle and South Sisters, and the cluster has now returned to its ance but are substantially different in composition. North Sister is exclusively modest eruptive output, mainly away from the stratovolcanoes. Time–volume mafic with central­vent­erupted lavas ranging from ~120 ka to ~50 ka. South results for the volcanic cluster are compared to studies of other well-mapped, and Middle Sisters are relatively young (<50 ka) and compositionally anom­ well-dated stratovolcanoes. Nearly all centers record similar eruptive-volume alous for this mafic portion of the Cascade arc. South Sister is composed behavior with long histories of relatively constant output punctuated by short largely of rhyolite and dacite erupted over the past 50 k.y., with andesite episodes of voluminous eruptions. In addition to the Three Sisters, two of eruptions during brief intervals (Fierstein et al., 2011). Middle Sister is a ba­ these centers (Mt. Mazama, Crater Lake, Oregon, and Puyehue/Cordon Caule saltic andesite to dacite stratocone (Figs. 2A and 2B) built in narrow inter­ OLD G in the southern Andes) record significant compositional changes associated vals during 50–37 ka and 27–15 ka on a glacially eroded highland of basaltic with the voluminous eruptive episodes. andesite. Rhyolite and rhyodacite erupted adjacent to Middle Sister but not from the central vent. This report focuses on Middle Sister, particularly on its profound late Pleistocene eruptive episode, and complements Fierstein et al. OPEN ACCESS INTRODUCTION (2011), which focuses on South Sister. The resulting combination of detailed field mapping, comprehensive whole­rock geochemistry, and high­precision Most Cascade volcanic segments host composite andesite–dacite cones argon geochronology done on most eruptive units allows spatial, chemical, (Mt. Adams, Mt. Hood, Mt. Rainier, Mt. Mazama, Mt. Shasta) with eruptive his­ and temporal analysis of the volcanic cluster. The relatively short­lived epi­ tories spanning 200–700 k.y. The Three Sisters volcanic cluster of the Cascades sode that produced both Middle and South Sisters yielded a broad chemical (Fig. 1) near Bend, Oregon, is young and compositionally diverse in compar­ range of eruptive units. Detailed investigation of this episode provides in­ This paper is published under the terms of the ison, containing three late Pleistocene stratovolcanoes made of basalt, ande­ sight into the kind of episodic behavior that is common at most arc strato­ CC‑BY‑NC license. site, dacite, and rhyolite, nestled in a volcanic segment with at least 466 Qua­ volcanoes. © 2018 The Authors GEOSPHERE | Volume 14 | Number 5 Calvert et al. | Eruptive History of Middle Sister Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/14/5/2118/4347032/2118.pdf 2118 by guest on 26 September 2021 Research Paper 124° 122° 120° PREVIOUS WORK Mt. Mt. Adams Early work by Hodge (1925) and Williams (1944) described the physiog­ St. Helens 46° raphy and composition of the Three Sisters volcanic cluster. Prior mapping at various scales (Williams, 1957; Taylor, 1978, 1987; Taylor et al., 1987; Sher­ N rod et al., 2004) conveys some field relations and compositional ranges, but Hood River WASHINGTO lack of geochronology resulted in substantial overestimate of eruptive ages. OREGON Columbia R. Middle Sister, in particular, has a glacially ravaged east face, despite being T Portland S slightly younger than the more intact South Sister. The North Sister edifice N E Mt. R was mapped by Schmidt and Grunder (2009) as part of a petrologic study. C Hood A 1:24,000­scale geologic map (Hildreth et al., 2012), incorporating geochem­ E E istry and geochronology, established the framework for a detailed paper on G G N South Sister (Fierstein et al., 2011) and for the present study of Middle Sister. 45° Salem A R Petrologic investigations of mafic components of the cluster include Hughes AN R E and Taylor (1986), Hughes (1990), and Schmidt and Grunder (2011), and silicic PACIFIC OCEA Mt. D John Day R. petrogenesis was studied by Hill (1991) and Stelten and Cooper (2012). A Jefferson Corvallis C S A C GEOLOGIC SETTING T Sisters S THREE The Cascade Arc in Central Oregon is exceptionally broad with abundant, Eugene Bend A SISTERS distributed mafic centers and north–south alignments of vents (Hildreth, 44° Willa tes R. O me u 2007). The Three Sisters lie at a complicated section of the arc where the north tte R. C boundary of the Basin and Range province, the northwest­trending Brothers Desch Newberry Volcano HIGH Fault Zone, meets the north­trending High Cascades Graben (Sherrod et al., Diamond LAV A 2004), a weakly extensional basin stretching north to Mt. Jefferson (Fig. 3). Peak PLAINS The Three Sisters volcanic cluster as mapped by Hildreth et al. (2012) con­ Cappy tains three stratovolcanoes, several eroded mafic shields, and numerous flank Mountain Mt. vents. Lines of mafic volcanoes are common away from the stratovolcanoes, Thielsen and even the 2­ka Devils Chain (northern terminus is rdc on Fig. 2A, southern terminus 8 km south of South Sister) on the flank of South Sister forms a north– 43° Crater Lake south array (Hildreth et al., 2012). The alignment of the Three Sisters and the simultaneous construction of Middle and South Sister may also reflect these favored vent alignments, which are parallel to the High Cascades Graben. Older, eroded map units are chiefly basaltic andesite and basalt; younger Mt. BASIN AND units range from basalt to high­silica rhyolite. The abundance of true rhyolite McLoughlin RANGE PR (72–77.5 wt% SiO ) in this arc segment is unusual for the Cascade arc axis (Hill, KLAMATH MOUNTAINMedford OVINCE 2 1991; Hildreth, 2007), though somewhat older rhyodacite is prevalent to the east, where Hill and Taylor (1990) described widespread silicic pyroclastic de­ S Klamath Falls posits erupted over the past 1 m.y., and rhyolite/rhyodacite is abundant at the 42° OREGON CALIFORNIA great rear­arc Newberry Volcano to the southeast as well as farther east across 0 100 KM the High Lava Plains (Figs. 1 and 3). 0 50 MILES The Three Sisters each are strongly glacially eroded, especially North Sis­ Mt. Shasta Medicine Lake ter with deep cirques in all sectors and Middle Sister with its well­exposed eastern flank. Glaciers are small now, but during the Pleistocene there is evi­ Figure 1. Map of western Oregon, southern Washington, and northern California (USA), showing High Cascade stratovolcanoes dence for a broad mountain ice sheet covering the Cascade crest (Scott, 1977), (red triangles), population centers (black squares), and selected geomorphic features.