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 subduction-induced 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 high-K 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 high-precision 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 north-south–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 west-southwest 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 35-k.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 central-vent-erupted 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 whole-rock geochemistry, and high-precision 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 short-lived 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
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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 1:24,000-scale 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 northwest-trending Brothers Desch Newberry Volcano HIGH Fault Zone, meets the north-trending 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 2-ka 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 high-silica 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 rear-arc 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 well-exposed 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. and ice likely covered most of the Three Sisters for all but brief periods in the
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–121°50′ –121°44′ pr50-3737-2727-2121-15<15e- 50ka ka ka A acc
mlb 6000’ mlb Little mps Brother
mmm
roc mns 012 km 44°10′ awc 10085’ 44°10′ 6000’ North contour interval 400 feet dbh dss Sister aoc awr dbh anh mrg mnh 8000’ mns
dlp msp dhr mmi dlp aef mnf 10047’ mss mhd dms awm Middle alc mnf Sister mdl
ahl mth mms 8000’ mcl msf 6000’ dlc mhl adl mcl mls asw 6000’ mmt dig mdl dsw dsw rsf mmt dcl dsn rsc dlg dcn The mmt drm Husband rdc aeg rpc mth 8000’ awf drm 0’ 600 msc rpg
alg dpg awf dnt aph South dwl aeg 800 10358’ ahc Sister mbt 0’ mtp des 44°6′ dcg aeg 44°6′ ddl dlg adh dgl –121°50′ –121°44′
Figure 2 (on this and following page). Geology of Three Sisters, Cascade Range, Oregon (USA), after Hildreth et al. (2012). (A) Geologic map of Middle Sister, including portions of South Sister, North Sister, and peripheral map units. Map unit symbols are defined in the text section “Eruptive History of Middle Sister” and in Table 2; unit symbols beginning with ‘r’ are rhyolite, ‘d’ are dacite, ‘a’ are andesite, ‘m’ are basaltic andesite, and ‘b’ are basalt. Contour interval is ~120 m (400 feet).
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Middle Pleistocene Late Pleistocene Holocene
QUATERNARY
GEOSPHERE | Volume 14 | Number 5 Calvert et al. | Eruptive History of Middle Sister 2121 Research Paper
126° 124° 122° 120° 118°
A WA 46° SH OR Columbia R. HRF Figure 3. Tectonic and volcanic features n Portland of Oregon (USA), showing selected vol- H EXPLANATION canoes (A, Mt. Adams; CL, Crater Lake; Selected large volcano DP, Diamond Peak; H, Mt. Hood; J, Mt. 45° Jefferson; M, Mt. McLoughlin; MS, Middle Area of deep graben fill Sister; N, Newberry Volcano; NS, North J SILETZIA Sister; SH, Mt. St. Helens; SS, South Sis- OceaPacific GRF TERRANE Approximate outline of ter; T, Mt. Thielson); mapped faults, with HCG SFZ major valley ticks on downthrown side of normal faults NS WILLAMETTE VALLEY (BFZ, Brothers Fault Zone; GRF, Green MS Bend Ridge Fault; HRF, Hood River Fault; SFZ, 44° Eugene SS Normal fault—Tick on Sisters Fault Zone), and grabens along B downthrown side N FZ Burns the arc axis (HCG, High Cascades Graben; DP LG LG, Lapine Graben); the direction of Cas- Direction of Cascadia cadia rotation (red arrows) (Wells et al., rotation (Wells and others, 1998) 1998); and age isochrons (Ma) of inception T Isochron, showing age (in Ma) of rhyolitic volcanism (red dashed lines) 43° (MacLeod et al., 1976; Jordan et al., 2004). CL of inception of rhyolitic volcanism (MacLeod and others, KLAMA TERRANE M 1976; Jordan and others (2004) TH Klamath OR Falls OR 42° CA NV
past 200 k.y. During the Last Glacial Maximum, ca. 20 ka, the ice sheet was con Geochemistry tinuous except for isolated nunataks above 1600 m elevation, and valley gla ciers extended down below 1200 m elevation (Scott, 1977). Lowest elevations Whole-rock compositions for 743 samples from the entire Three Sisters in the study area (Fig. 2A) are ~1700 m. Many eruptive units interacted with volcanic cluster were determined by X-ray fluorescence at the U.S. Geologi ice and display glassy quenched margins, overthickening next to valleys, and cal Survey (USGS) laboratories in Denver and the GeoAnalytical Laboratory lava flows confined to ridges between glaciated valleys (Hildreth et al., 2012). at Washington State University (Hildreth et al., 2012). Of these, 174 analyses were obtained for Middle Sister samples. Major oxide concentrations were
recalculated to sum to 99.6%. Rock names are based on SiO2 content: basalt METHODS <52 wt%, basaltic andesite 52–57 wt%, andesite 57–63 wt%, dacite 63–68 wt%, rhyodacite 68–72 wt%, and rhyolite >72 wt%. Compositions are also referred to This detailed report on Middle Sister grew out of geologic mapping of as “mafic” to represent basalt and basaltic andesite, “intermediate” for ande the Three Sisters volcanic cluster by Hildreth et al. (2012). Volcanic units were site and dacite, and “silicic” for rhyodacite and rhyolite. Rock thin sections rep mapped and sampled during ~1-month-long field seasons from 2000 to 2009. resenting all map units were scrutinized to describe and correlate map units Map units represent similar, mappable products from single vents, or occa and to choose samples appropriate for argon geochronology. sionally groups of vents, that typically erupted in a single episode (months to hundreds of years). Several map units (e.g., basaltic andesites of North Sister, mns, and Broken Top, mbt) represent similar-composition lavas that erupted Argon Geochronology over longer (tens of thousands of years) timespans. Major- and trace-element geochemistry and argon geochronology progressed simultaneously with field The USGS geochronology laboratory in Menlo Park analyzed the samples work, and those results guided subsequent mapping. Detailed map unit de using 40Ar/39Ar techniques according to procedures described by Calvert and scriptions, chemical analyses, and geochronology were made available by Lanphere (2006). Because most map units were glaciated, we were able to Hildreth et al. (2012). obtain dense, crystalline samples from lava flow and dome interiors. Detailed
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step-heating experiments on 150–220-mg groundmass separates typically andesite and dacite, and low on its northwestern flank there is a high-silica yielded plateau ages with analytical errors of ±1 ka to ±5 ka (Table 1, Fig. 4), and rhyolite coulee. This rhyolite unit, the Rhyolite of Obsidian Cliffs (roc), contains all results satisfy field constraints. Some samples contain non-atmospheric ar the highest silica content of any Quaternary rock in the Oregon Cascades.
Supplemental File 1. 40Ar/39Ar tabulated data gon contaminant; for those we interpret isochron ages as most reliable. Of the Analycal data for dated Three Sisters samples in two formats. Modern format (e.g. TS-562) for samples analyzed a er 2005 includes measured isotopes and their errors allowing re-reducon of all data. Disconnued (e.g. TS-202) format (Dalrymple, 1989) contains minimal data, including measured raos, but analycal errors were stored only on printouts. Invesgators wanng access to the printouts are welcome to contact the lead author. 40Ar/39Ar geochronology was performed using crystalline groundmass or plagioclase segregated by crushing, sieving, and magnetic techniques. Samples were carefully handpicked under a binocular microscope. 65 eruptive units represented in Figure 2A and Table 2, we dated 43 units; 22 For irradiation, 110-220 mg separates were packaged in Cu foil and placed in a cylindrical quartz vial, together with fluence monitors of known age and K-glass and fluorite to measure interfering isotopes ERUPTIVE HISTORY OF MIDDLE SISTER from K and Ca. The quartz vials were wrapped in 0.5 mm-thick Cd foil to shield samples from thermal neutrons during irradiation. The samples were irradiated for one or two hour(s) in the central thimble of the U.S. Geological Survey TRIGA reactor in Denver, Colorado (Dalrymple et al., 1981). The reactor vessel was rotated continuously during irradiation to avoid lateral neutron flux gradients. Reactor constants de 40 39 39 37 36 37 for these irradiations averaged Ar/ ArK = 0.0010±0.0004, Ar/ ArCa = 0.00071±0.00005, and Ar/ ArCa = 0.000281±0.000006. Taylor Creek sanidine (TCs) was used as a fluence monitor with an of the 32 Middle Sister eruptive units are dated (Table 2) and remaining units age of 27.87 Ma, but recalculated to 28.345 Ma (Fleck and Calvert, 2016) to match the primary intralaboratory standard, GA1550 biotite, that has an age of 98.79±0.96 Ma (McDougall and Wellman, 2011). Flu monitors and unknowns were analyzed using a continuous laser system and mass spectrometer described by Dalrymple (1989). Gas was purified continuously during extraction using two SAES ST-172 getters operated at 4A and 0A. Mass spectrometer discrimination and system blanks are important factors in the precision and accuracy of 40Ar/39Ar age determinations of Pleistocene samples because of low radiogenic yields. Discrimination is monitored by analyzing splits of atmospheric Ar from a reservoir attached to the extraction line. Typical system blanks including mass spectrometer backgrounds were 1.5x10-18 mol of m/z 36, 9x10-17 mol are well bracketed with age estimates listed in the “Accepted age” column. Eruptions at Middle Sister and in most of its immediate periphery occurred of m/z 37, 3x10-18 mol of m/z 39 and 1.5x10-16 mol of m/z 40, where m/z is mass/charge ratio. References Dalrymple, G.B., 1989, The GLM continuous laser system for 40Ar/39Ar dating; description and performance characteristics, in Shanks, W.C., III, and Criss, R.E., eds., New frontiers in stable isotopic research; laser probes, ion probes, and small-sample analysis: U.S. Geological Survey Bulletin 1890, p. 89-96. For comparison with the timing of other eruptions in the greater Three Sisters in three pulses (Fig. 2B): (1) early mafic through silicic eruptions 49–37 ka, Dalrymple, G.B., Alexander E.C., Jr., Lanphere, M.A., and Kraker, G.P., 1981, Irradiation of samples for 40Ar/39Ar dating using the Geological Survey TRIGA reactor. US Geol. Surv. Prof. Paper 1176. Fleck, R.J. and Calvert, A.T., 2016, Intercalibration of 40Ar/39Ar mineral standards with Bodie Hills sanidine, Geological Society of America Annual Meeting, paper no. 238-4. McDougall and Wellman, 2011, Calibration of GA1550 biotite standard for K/Ar and 40Ar/39Ar dating, Chemical Geology, v. 280, p. 19-25.
TS-562 Andesite (adl) map area, dating of 73 of the 142 map units on the Hildreth et al. (2012) geo (2) one intermediate composition unit (alc) ca. 27 ka as South Sister was erupt Temp(°C) Age(ka)%40Ar*K/Camoles 40Ar* ∑39Ar40Ar39Ar38Ar37Ar36Ar 550-5.0±9.2-0.66 0.89 -5.85E-17 0.03 0.465193±0.0009610.269911±0.0005420.003821±0.0000440.158676±0.0017210.001629±0.000018 60020.8±4.53.751.016.34E-16 0.11 0.882676±0.0017950.702328±0.0014090.009405±0.0000700.364761±0.0029620.002978±0.000022 65020.0±2.84.861.15 1.26E-15 0.27 1.351704±0.0027331.449738±0.0029070.018678±0.0000560.662710±0.0029920.004538±0.000027 logic map are discussed. Argon geochronology in this report uses standards ing andesite/dacite lavas (Fierstein et al., 2011; Hildreth et al., 2012), and (3) the 70026.1±2.18.361.142.33E-15 0.49 1.457671±0.0029452.062497±0.0041360.026413±0.0001120.946306±0.0048800.004787±0.000028 75024.8±1.97.910.922.29E-15 0.73 1.512407±0.0030542.124285±0.0042600.028329±0.0001311.210352±0.0040100.005054±0.000026 80027.9±3.26.490.631.49E-15 0.86 1.197816±0.0024301.230990±0.0024690.016994±0.0001341.018297±0.0039050.004077±0.000026 3 8504.3±6.4 0.53 0.43 8.75E-17 0.91 0.855910±0.0017470.469196±0.0009420.007298±0.0000430.575025±0.0027870.003043±0.000021 90023.6±11.1 2.07 0.32.62E-16 0.94 0.659630±0.0013550.256128±0.0005150.004561±0.0000680.450929±0.0025290.002313±0.000020 that have been updated from those in companion papers (Fierstein et al., 2011; bulk of the volcano 25–19 ka, including the 20 ka, ~1.2 km basaltic andesite of 9757.6±11.60.460.211.12E-16 0.98 1.276558±0.0025880.339213±0.0006810.006726±0.0000740.835646±0.0041890.004535±0.000026 1050 -43.2±22.6-1.32 0.1-3.58E-16 11.414982±0.0028640.191849±0.0003860.005231±0.0000451.004774±0.0045570.005134±0.000028 1150 -49.6±77.6-2.32 0.1-4.45E-17 10.100078±0.0002430.020743±0.0000470.000440±0.0000210.111917±0.0021320.000378±0.000012 Packet IRR260-FY, Experiment #08z0232, 0.1692 g Andesite, all errors ±1 sigma Hildreth et al., 2012), so listed data in this study are 1–3 k.y. older. Analytical Middle Sister (mms) that built the upper half of the edifice. J = 0.000244621±4.81E-07
40Ar* is radiogenic argon, isotopes in volts (1.91e-14 moles/volt), corrected for blank, background, discriminaon, and decay Calculated K2O = 1.65%wt., Calculated CaO = 3.10%wt., Calculated Cl = 0.2ppm techniques, standards, and incremental-heating data are included in Supple Middle Sister is flanked by older basaltic andesite map units to the west, Total Gas Age = 20.2 ± 1.2 ka
Weighted Mean Plateau Age = 24.6 ± 1.1 ka (including ±J), 83.0% 39Ar released Weighted Mean Plateau Age = 24.6 ± 1.1 ka (without ±J) 1 MSWD = 1.24 (Good fit, MSWD < 2.77) mental File 1. north, and east (Hildreth et al., 2012). These lavas are similar to products from Steps 5 of 11 (600,650,700,750,800°C)
Isochron Age = 30.3 ± 6.035 ka (±1 sigma, including ±J) Isochron Age = 30.3 ± 4.959 ka (A Priori Errors, including ±J) monogenetic and mafic polygenetic volcanoes north and south along the High Isochron Age = 30.3 ± 13.916 ka (95% confidence, including ±J) MSWD = 1.48 (Good fit, MSWD < 3.12) 40Ar/36Ar intercept = 293.4 ± 5.3 (±1 sigma) 40Ar/36Ar intercept = 293.4 ± 4.4 (±1 sigma, A Priori) 40Ar/36Ar intercept = 293.4 ± 12.3 (95% confidence) Cascades. The basaltic andesite of The Husband (mth), the underlying basalt Steps 5 of 11 (600,650,700,750,800°C) Erupted Volumes south of The Husband (bsh), and the basaltic andesites of Little Brother (mlb) 1Supplemental Files. 1: 40Ar/39Ar tabulated data 2. Lava flow volumes were compiled using field-thickness estimates and and North Sister (mns) were not included in volume calculations for Middle Estimated eruptive volumes for all map units on the geologic map of the Three Sisters volcanic cluster graph paper overlays on the 1:24,000-scale maps for areal extent. Exposed vol Sister because they are separated from Middle Sister and are interpreted to (Hildreth et al., 2012). 3. Individual parts of Figure 6. umes were calculated and original extents were estimated for all 142 units on reflect regional volcanism. Lavas from the andesite of Obsidian Creek (aoc) Please visit https://doi.org/10.1130/GES01638.S1 or the Hildreth et al. (2012) geologic map so that we could assess the volumetric and from the basaltic andesite of North Fork (mnf) were included in volume access the full-text article on www.gsapubs.org to view the Supplemental Files. contribution of the 50–15-ka eruptive episode. Because most of the map units calculations because they are temporally and/or spatially associated with the have been eroded, there is significant uncertainty in these estimates. Volumes Middle/South Sister episode. for eruptive units represented in Figure 2A are listed in Table 2, and volumes for all units mapped by Hildreth et al. (2012) are listed in Supplemental File 2 Pre-50 ka Mafic Eruptions (footnote 1). The basaltic andesite of The Husband (mth) is a 5- by 8-km mafic volcano COMPOSITIONAL OVERVIEW (Fig. 6) with multiple vents and numerous dikes and intrusions (Sherrod et al., 2004; Taylor et al., 1987) located 4 km west of Middle Sister. Lavas range from
Eruptive units from the Three Sisters are medium-K rocks that range from 52 to 57 wt% SiO2 and are phenocryst-poor. The Husband overlies the less-
49–77 wt% SiO2; Middle Sister units span 52–77 wt% SiO2 (Fig. 5). In and silicic basalt south of The Husband (bhs), typically containing more olivine
around Middle Sister (Fig. 2A), there is not much true basalt but abundant and ranging from 51 to 53 wt% SiO2. A flank vent of The Husband with dikes basaltic andesite, andesite, dacite, and rhyolite. Similar to other eruptive units that intrude and vents that overlie other mth lavas has a 40Ar/39Ar age of 151 ± throughout the Oregon Cascades, these basalt and basaltic andesite units are 5 ka, providing a minimum age for the stratovolcano. North Sister lies less
highly variable in alkalis (K20 + Na2O = 3.8–5.7 wt%) and MgO (3.0–9.1 wt%), than 2 km north of Middle Sister and is also a glacially eroded, polygenetic and, like the nearby Crater Lake/Mazama system and other Cascade volcanoes basaltic andesite volcano (Taylor, 1987; Schmidt, 2005; Schmidt and Grunder, (Bacon et al., 1997; Bacon and Lanphere, 2006; Schmidt et al., 2008), these 2009; Hildreth et al., 2012). The basaltic andesite of North Sister (mns) includes regional lavas reflect a heterogeneous mantle source. All Three Sisters map hundreds of similar lava flows erupted between 47 ± 6 ka to at least 121 ± 6 ka units have Mg#s (100 × Mg / (Fe + Mg)) under 50, so they are not primitive. (Hildreth et al., 2012). We were not able to find fresh, crystalline textures for North Sister lavas are similar in composition to the regional basalts and ba geochronology of lavas at the deepest stratigraphic levels to assess the older saltic andesites (Fig. 5). Middle Sister, South Sister, and Broken Top contain mns dates (140–500 ka) obtained by Schmidt and Grunder (2009). The basal substantially more-evolved rocks. Broken Top analyses are limited but have tic andesite of Broken Top (mbt) is an assemblage of undifferentiated mafic a compositional range from basaltic andesite to rhyodacite. South Sister is lava flows, erupted from the 300–150-ka Broken Top stratovolcano (Hill, 1991) composed of andesite, dacite, and abundant rhyolite. Glacial cirques offer the southeast of the map area. Small-volume mafic map units are typical of central deepest exposure on South Sister and commonly expose rhyolite, so Fierstein Oregon Cascades volcanism and are exposed east and west of Middle and et al. (2011) inferred that the bulk of its unexposed core is rhyolite. Middle Sis North Sister. The basaltic andesite of Demaris Lake (mdl, 184 ± 21 ka) is a clino ter consists of abundant basaltic andesite similar to regional units as well as pyroxene–olivine–plagioclase basaltic andesite largely buried by younger lava
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TABLE 1. 40Ar/39Ar AGES FOR MAP UNITS ON AND ADJACENT TO MIDDLE SISTER, CASCADE RANGE, OREGON, USA 40Ar/39Ar weighted-mean plateau 40Ar/39Ar isochron Map Whole-rock Age Age 40Ar/39Ar total gas age 39 39 40 36 Sample Latitude Longitude unit SiO2 (ka) % Ar [steps, °C] (ka) % Ar [steps, °C] MSWD Ar/ Ari (ka) TS-562 44.1624 –121.6804 adl 61.3 24.6 ± 1.1 83 [600–800] 30.3 ± 6.183 [600–800] 1.48 293.4 ± 12.3 20.2 ± 1.2 TS-202 44.1091 –121.7686 aeg 56.5 27.5 ± 3.3 77 [650–925] 24.8 ± 4.995 [600–1100] 0.79 300.0 ± 2.3 29.0 ± 3.7 TS-577 44.1018 –121.7542 aeg 59.2 27.2 ± 1.7 73 [600–750] 29.0 ± 3.773 [600–750] 2.16 298.3 ± 3.5 28.1 ± 1.8 TS-584 44.0955 –121.7609 aeg 56.7 29.0 ± 2.1 77 [640–850] 29.5 ± 6.477 [640–850] 0.42 298.4 ± 2.7 20.1 ± 2.7 TS-44 44.1013 –121.7648 ahi 60.5 17.1 ± 2.1 78 [640–940] 23.7 ± 6.4 78 [640–940] 0.62 294.5 ± 2.3 13.3 ± 2.4 TS-283 44.1400 –121.8174 ahl 57.9 –– –––– 21.2 ± 3.4 TS-298A 44.1496 –121.8326 alc 61.9 27.7 ± 1.0 96 [600–1100] 25.9 ± 1.996 [600–1100] 1.24 300.5 ± 3.6 28.1 ± 1.1 TS-557 44.1678 –121.8842 alc 61.9 27.3 ± 1.2 74 [550–750] 31.4 ± 7.174 [550–750] 0.91 295.0 ± 12.9 22.3 ± 1.2 TS-256 44.1144 –121.8066 alg 62.2 27.6 ± 1.1 100 [550–1150]25.8 ± 2.2100 [550–1150]1.02299.6 ± 2.8 28.1 ± 1.2 TS-472 44.1547 –121.7699 anh 58.0 18.2 ± 2.5 93 [550–925] 21.4 ± 6.4 93 [550–925] 1.44 297.7 ± 4.8 15.7 ± 2.7 TS-386 44.1724 –121.8292 aoc 58.6 48.7 ± 2.2 67 [550–700] 52.9 ± 12.1 67 [550–700] 0.29 296.2 ± 7.9 42.4 ± 2.3 TS-426 44.1565 –121.8268 aoc 61.8 29.7 ± 2.3 74 [650–830] 49.7 ± 3.8 74 [650–1050] 1.14 288.0 ± 2.9 –77. 0 ± 3.9 TS-559 44.1718 –121.8997 aoc 60.1 45.4 ± 1.8 74 [600–750] 46.0 ± 12.9 74 [600–750] 0.15 298.2 ± 7.5 42.4 ± 2.0 TS-225 44.1336 –121.8021 asw 61.5 24.9 ± 4.3 58 [600–800] 37.3 ± 15.7 58 [600–800] 0.38 295.6 ± 3.5 6.7 ± 4.1 TS-372 44.1758 –121.8028 awc 62.1 22.3 ± 1.7 90 [600–1250] 25.2 ± 9.890 [600–1250] 2.20 296.9 ± 2.0 19.3 ± 1.7 TS-392 44.1695 –121.7905 awc 62.1 24.9 ± 1.5 92 [600–1100] 27.9 ± 4.592 [600–1100] 2.20 297.0 ± 1.8 21.6 ± 1.7 TS-510 44.1082 –121.7279 awf 60.0 24.1 ± 1.3 87 [600–850] 25.3 ± 4.187 [600–850] 0.30 297.7 ± 3.4 23.6 ± 1.4 TS-100 44.1649 –121.8050 dbh 64.0 18.2 ± 2.2 99 [650–1100] 20.4 ± 6.999 [650–1100] 1.50 297.5 ± 4.7 13.1 ± 3.3 TS-216 44.1573 –121.7783 dbh 64.4 27.6 ± 1.8 92 [550–925] 23.4 ± 6.9 100 [550–1200]0.32301.3 ± 3.5 29.5 ± 2.0 TS-117 44.0842 –121.8151 dcg 63.3 26.3 ± 1.2 69 [650–800] 31.2 ± 2.6 69 [650–800] 0.57 293.5 ± 1.7 11.8 ± 1.5 TS-19 44.1212 –121.7703 dcl 65.4 20.1 ± 1.6 87 [600–850] 26.4 ± 2.1 100 [550–1250]0.47293.3 ± 0.9 14.3 ± 1.7 TS-115 44.0966 –121.8310 ddl 64.3 32.9 ± 1.8 99 [600–1100] 34.7 ± 2.799 [600–1100] 1.16 297.8 ± 0.9 31.7 ± 2.1 TS-412 44.1528 –121.7835 dhr 63.6 19.4 ± 1.7 76 [630–900] 19.1 ± 3.976 [630–900] 0.75 298.7 ± 2.0 14.4 ± 1.9 TS-5 44.1285 –121.7782 dig 64.2 14.6 ± 2.7 77 [700–1000] 23.3 ± 11.2 77 [700–1000] 0.91 299.7 ± 8.4 15.0 ± 2.5 TS-581 44.0992 –121.7666 dlg 63.6 25.7 ± 1.4 70 [620–780] 26.9 ± 4.570 [620–780] 3.50 294.4 ± 3.9 19.1 ± 1.5 TS-631 44.0992 –121.7670 dlg 63.8 22.7 ± 1.6 78 [600–850] 23.6 ± 5.178 [600–850] 0.14 298.1 ± 2.3 23.3 ± 1.5 TS-103 44.1567 –121.8286 dlp 65.3 21.8 ± 1.9 81 [700–1250] 20.8 ± 7.681 [700–1250] 2.80 297.2 ± 1.8 19.3 ± 1.9 TS-511 44.1212 –121.7209 drm 64.3 29.2 ± 0.8 78 [700–1250] 29.4 ± 1.878 [700–1250] 0.89 298.2 ± 1.6 24.6 ± 0.9 TS-257 44.1258 –121.8083 dsn 64.4 26.4 ± 1.5 93 [630–1150] 19.4 ± 3.8 93 [630–1150] 0.67 302.4 ± 1.8 26.7 ± 1.6 TS-387 44.1695 –121.8153 dss 63.7 18.8 ± 1.5 94 [600–1000] 24.9 ± 3.2 100 [550–1150]1.20295.4 ± 1.4 17.3 ± 1.5 TS-223 44.1317 –121.7949 dsw 65.4 28.7 ± 2.4 77 [650–925] 17.1 ± 15.6 71 [650–850] 0.72 303.3 ± 6.2 –3.9 ± 6.0 TS-22 44.1466 –121.7370 mcl 56.5 16.4 ± 10.6 73 [640–850] 67.0 ± 15.0 100 [550–1250]8.28294.5 ± 4.7 –60.9 ± 13.0 TS-169 44.1335 –121.7535 mcl 56.4 35.7 ± 9.2 63 [650–1000] 23.7 ± 16.1 63 [650–1000] 0.36 300.7 ± 2.2 32.1 ± 8.2 TS-188 44.1283 –121.7316 mdl 53.0 209.0 ± 7.2 79 [550–850] 183.9 ± 21.1 79 [550–850] 0.86 302.7 ± 3.3 261.5 ± 8.2 TS-178 44.1483 –121.7727 mhd 54.8 37.4 ± 9.1 92 [550–925] 44.8 ± 18.3 92 [550–925] 0.52 296.8 ± 3.4 11.5 ± 11.5 TS-383 44.1704 –121.8235 mlb 53.1 48.0 ± 5.9 50 [700–750] 3.9 ± 45.050 [700–750] 0.33 305.4 ± 9.0 30.1 ± 4.7 TS-554 44.1850 –121.8601 mlb 52.6 49.2 ± 3.7 74 [550–800] 43.7 ± 17.1 74 [550–800] 0.18 299.8 ± 3.6 34.6 ± 3.9 TS-474 44.1509 –121.7816 mmi 53.3 ––18.4 ± 4.0 88 [550–925] 0.98 307.0 ± 2.5 31.5 ± 2.1 TS-429 44.1702 –121.8266 mmm 56.6 19.5 ± 7.0 50 [650–790] 19.9 ± 5.5100 [550–1075]1.40296.8 ± 1.2 9.3 ± 3.3 TS-185 44.1430 –121.7565 mms ~52.5 ––21.7 ± 19.3 79 [600–900] 1.76 302.3 ± 0.7 190.9 ± 23.2 TS-235 44.1485 –121.7843 mms 53.4 16.7 ± 11.9 83 [580–1250] 35.2 ± 30.0 83 [580–1250] 0.96 297.2 ± 1.9 –25.3 ± 12.3 TS-444 44.1481 –121.7356 mnf 54.2 48.5 ± 9.7 99 [550–1075] 93.6 ± 43.7 99 [550–1075] 1.80 294.6 ± 2.2 39.8 ± 9.5 TS-217 44.1553 –121.7749 mnh 57.1 21.3 ± 5.6 86 [550–900] 16.5 ± 17.5 86 [550–900] 0.84 299.7 ± 3.5 17.3 ± 5.9 TS-505 44.1600 –121.6950 mns 55.1 121.3 ± 5.7 76 [600–900] 138.3 ± 9.6 100 [550–1150]2.70295.9 ± 0.8 101.5 ± 5.6 NS-02-79 44.1854 –121.7609 mns 53.9 ––84.5 ± 24.9 100 [550–1150]2.99297.4 ± 5.6 72.5 ± 7.1 NS-03-160 44.1590 –121.7643 mns 54.8 71.7 ± 3.3 80 [550–800] 75.0 ± 15.6 80 [550–800] 0.81 297.1 ± 12.8 64.4 ± 3.7 NS-04-183 44.1711 –121.7741 mns ~54 55.9 ± 5.4 60 [600–750] 38.2 ± 47.8 60 [600–750] 0.23 302.8 ± 12.6 48.2 ± 6.2 TS-388 44.1748 –121.7927 mns 53.8 47.0 ± 5.7 91 [600–1250] 53.7 ± 12.2 100 [550–1250]0.77297.2 ± 1.3 40.5 ± 5.3 (continued)
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TABLE 1. 40Ar/39Ar AGES FOR MAP UNITS ON AND ADJACENT TO MIDDLE SISTER, CASCADE RANGE, OREGON, USA (continued) 40Ar/39Ar weighted-mean plateau 40Ar/39Ar isochron Map Whole-rock Age Age 40Ar/39Ar total gas age 39 39 40 36 Sample Latitude Longitude unit SiO2 (ka) % Ar [steps, °C] (ka) % Ar [steps, °C] MSWD Ar/ Ari (ka) TS-381 44.1598 –121.8099 mrg 54.6 ––19.9 ± 6.2 83 [650–1060] 1.17 292.4 ± 0.6 –78.9 ± 6.9 TS-114 44.1020 –121.8319 msc 56.8 17.9 ± 2.4 100 [550–1150] 21.8 ± 5.6 89 [550–975] 0.58 297.3 ± 1.3 17.9 ± 2.9 TS-191 44.1335 –121.7297 msf 54.0 169.1 ± 16.3 94 [600–1100] 165.4 ± 43.794 [600–1100] 0.13 298.9 ± 3.7 160.9 ± 17.0 TS-466 44.1516 –121.7268 msp 54.2 118.3 ± 2.9 96 [550–1000] 131.3 ± 5.9 96 [550–1000] 0.57 294.8 ± 1.3 112.1 ± 3.4 TS-112 44.1163 –121.8458 mth’ 52.8 151.5 ± 4.9 76 [600–850] 160.4 ± 9.8 100 [600–1150]1.44297.3 ± 1.6 149.6 ± 6.1 TS-47 44.1041 –121.7713 mtp 53.8 22.7 ± 13.1 100 [550–1150]18.4 ± 33.1 100 [550–1150]0.84298.8 ± 1.6 25.6 ± 13.4 TS-70† 44.0789 –121.7995 rmc 72.4 48.2 ± 8.3 88 [700–1150] 41.6 ± 21.4 88 [700–1150] 2.09 299.8 ± 2.5 70.8 ± 9.4 TS-141 44.0826 –121.7893 rmc 72.6 –39.7 ± 5.1 76 [690–1150] 46.0 ± 15.3 91 [580–975] 1.84 291.9 ± 2.0 –59.1 ± 5.1 TS-102 44.1641 –121.8121 roc 76.5 38.4 ± 1.8 59 [650–800] 38.2 ± 18.2 100 [500–1200]5.70298.5 ± 10.8 38.9 ± 1.6 TS-719 44.1134 –121.7359 rpc 72.5 37.5 ± 2.2 100 [550–1100]39.2 ± 5.3100 [550–1100]0.51296.3 ± 9.8 34.4 ± 3.5 TS-201 44.1095 –121.7666 rpg 68.4 33.4 ± 1.5 88 [550–800] 31.1 ± 2.6100 [550–1200]0.99300.5 ± 1.8 35.0 ± 1.7 TS-321 44.1192 –121.7891 rsc 73.3 25.8 ± 1.3 91 [580–1100] 24.1 ± 3.791 [580–1100] 0.28 300.2 ± 3.7 24.3 ± 1.3 TS-18 44.1264 –121.7544 rsf 74.0 36.4 ± 2.0 76 [700–890] 47.3 ± 6.397 [600–1020] 1.76 285.5 ± 5.3 33.8 ± 2.0 TS-156† 44.0878 –121.7735 rsw 73.3 51.4 ± 9.7 77 [850–1150] 56.2 ± 21.1 77 [850–1150] 2.21 296.7 ± 11.3 103.5 ± 9.7 †Plagioclase separate (all others are groundmass concentrates). 40 39 Notes: Samples were irradiated at the USGS reactor using 28.345 Ma Taylor Creek sanidine as the neutron-flux monitor. MSWD—mean square weighted deviation. Whole-rock SiO2: normalized wt%. Ar/ Ar age determinations by incremental-heating analysis. Weighted-mean plateau ages calculated from apparent ages on increments released within analytical error; samples without listed plateau ages yielded discordant 40Ar/39Ar age spectra, but acceptable isochron or total gas ages. Negative ages are due to large volumes of 36Ar measured on one or more steps, resulting in overcorrection of atmospheric 40Ar. Interpreted eruption ages are shown in bold.
flows and glacial deposits. The basaltic andesite of South Fork Whychus Creek from the unit overlap at 48 ± 6 ka and 49 ± 4 ka, within analytical error of two (msf) is a crystal-poor lava flow and tuff erupted 169 ± 16 ka. Bothmdl and msf out of three age determinations by Schmidt and Grunder (2009). may have erupted from beneath the later site of Middle Sister. The basaltic The basaltic andesite of North Fork Whychus Creek (mnf) is a set of thin andesite of Soap Creek (msp, 131 ± 6 ka) is a small scoria cone exposed east of lava flows exposed through glacial till along and north of the North Fork of Middle Sister that produced two phenocryst-poor lava flows exposed 2–4 km Whychus Creek. It contains 5–10% plagioclase, 1–3% olivine, and sparse clino downstream. The basaltic andesite of Linton Spring (mls, undated), west of pyroxene. The mafic flows are the earliest identified lavas erupted from Middle Middle Sister, is also likely to fall within this age range. It underlies Middle Sister, dated at 49 ± 10 ka. Sister lavas but is not well bracketed by stratigraphy and is undated due to The andesite of Obsidian Creek (aoc) is an extensive apron of crystal-poor
glassy textures. This cliff-forming crystal-poor unit has a cube-jointed texture andesite lava flows (58–63 wt% SiO2) that overlie Little Brother (mlb) lava flows that suggests ice-contact emplacement, and it is likely to have erupted during (Figs. 6 and 7). The unit, aoc, appears to have vented from Middle Sister and Marine Isotope Stage (MIS) 6 glaciation, ca. 190–130 ka. Lavas interpreted to is as thick as 120 m at medial exposures and far thicker at its terminus 7 km have erupted from Middle Sister prior to 50 ka total ~0.4 km3 (Table 2). from Middle Sister. Rare phenocrysts include commonly rounded 0.5–1.5-mm plagioclase and rarer <0.5-mm olivine, which is typically resorbed and has re Mafic and Silicic Units in Immediate Periphery (50–37 ka) action rims. Geochemically, with mixing trends between basaltic andesite and rhyolite (such as the younger roc, described below), samples from aoc are In the 50–37 ka time interval, multiple eruptive units vented from and ad unique in the map area (Fig. 8). Major and trace elements (Hildreth et al., 2012) jacent to Middle Sister, particularly in its northwest sector (Figs. 6 and 7). The from aoc are distinct from all other andesite units in the Three Sisters cluster, basaltic andesite of Little Brother (mlb) is a ~2-km diameter, mafic (52–53 wt% and the inferred silicic component is the oldest rhyolite at Middle Sister. Two 40 39 SiO2) edifice at the northwest toe of North Sister. Lavas from the vent extend Ar/ Ar plateau ages from the unit are 49 ± 2 ka and 45 ± 2 ka, and an isochron over 6 km to the west and northwest. The edifice consists of hundreds of thin age from a third sample yielded 50 ± 4 ka. These eruptions are coeval with lava flows containing 1–5% plagioclase, 3–5% olivine, and abundant olivine– early cone-building rhyolite eruptions at South Sister (Fierstein et al., 2011). plagioclase and all-plagioclase crystal clots. Several comagmatic dikes and a The roc coulee emerges from beneath younger Middle Sister lavas ~3 km small plug cut the edifice. A lack of unconformities between lava flows sug northwest of the summit and extends another 3 km northwest (Figs. 2A, 6, 40 39 gests Little Brother was a short-lived eruption, and two Ar/ Ar plateau ages and 7). The aphyric, high-silica rhyolite (76.4–76.7 wt% SiO2) is the most silicic
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TS-412 (dhr) Dacite of Hayden-Renfrew col The basaltic andesite of Hayden Glacier–Diller Glacier cleaver (mhd) is a 10 stack of several mafic lava flows and intercalated scoria exposed in the east face of Middle Sister (Fig. 9). Phenocryst abundance is variable: 7–15% plagio 5 clase and 1–3% olivine. A flow on the east end of the cleaver yielded a 37 ± 9 ka 0 40Ar/39Ar plateau age. The basaltic andesite of the headwaters of Linton Creek –5 2 (mhl) is a small window of glacially scoured lava flows that are chemically %Radiogenic 1.5 identical to mhd and inferred to be coeval. The underlying dacite of Linton 1 Creek (dlc) apparently also erupted in this interval. Unit mhd overlies ~45 m
K/Ca 0.5 of stratified proximal andesite ejecta, the andesite tephra fall at the base of 100 0 the east face of Middle Sister (aef), comprising two sequences separated by Total gas age = 14.4 ± 1.9 ka an angular unconformity and consisting of 2–15-cm clasts, coarse ash, and small (<5 mm) lapilli. The unconformity likely represents a small change in WMPA = 19.4 ± 1.7 ka vent location or eruptive style rather than significant time. Sampled clasts are 75.6% 39Ar released 50 MSWD = 0.54 compositionally indistinguishable (62.6–63.1 wt% SiO2) and lithologically simi 1150 Figure 4. Age data for dacite of Hayden– lar to andesites erupted subsequently at South Sister. Although undated and
Renfrew col, Cascade Range, Oregon Age (ka) difficult to access, its preservation suggests aef erupted shortly before mhd (USA). Representative 40Ar/39Ar age spec- 630 670 700 730 820 and is considered to represent the earliest erupted silicic andesite in the Three trum and isochron from a dacite lava flow 770 900 0 590 3 (sample TS-412) from the Hayden–Renfrew Sisters cluster. During 50–37 ka, ~2.1 km of lava and tephra erupted from Mid
col; this flow (dhr) overlies and provides Apparent 550 dle Sister (Table 2). an age bracket for the voluminous basaltic andesite of Middle Sister, Cascade Range, 1000 Oregon (USA). The 40Ar/39Ar age experi South Sister Eruptive Pulse (37–27 ka) –50 ment yielded a plateau age and con- 0.00.2 0.40.6 0.81.0 cordant isochron. WMPA—weighted Volcanism shifted southward for ~10 k.y. and built South Sister (Fig. 6). mean plateau age; MSWD—mean square Cumulative 39Ar released weighted deviation. Eruptive units, summarized here, are described in detail by Fierstein et al. 320 (2011). The rhyolite of South Fork Whychus Creek (rsf) erupted from a flank (±1 ) vent ~2 km northeast of South Sister and ~3 km southeast of Middle Sister. 630 315 The glacially scoured coulee is ~1 km wide, 150–300 m thick, and composed Age = 19.1 ± 3.9 ka (630-900°C steps) MSWD = 0.75 of three compositionally identical flow units (73.8–74.4 wt% SiO2). This rhyolite 40 36 Ar/ Ari = 298.8 ± 4.5 (±2 ) 40 39 310 770 yielded a Ar/ Ar plateau age of 36 ± 2 ka.
Ar Cone-building eruptions at South Sister were principally rhyolite prior to 820 700
36 670 305 36 ka, followed by voluminous andesite and dacite 33–27 ka (Fierstein et al.,
Ar / 900 730 2011); Middle Sister did not erupt again until the end of this interval. Rhyo 40 lite and dacite erupted from South Sister summit 33–31 ka, then andesite and 300 Air 590 dacite erupted from South Sister’s Hodge Crest flank vent 29–27 ka. One of 1150 550 these South Sister andesite map units, the andesite west of Lost Creek Glacier 295 1000 (alg), forms an apron of uniform phenocryst-rich lava that is remarkably sim 0100 200 300 ilar to the Middle Sister-sourced andesite of Linton Creek (alc), a 120–450 m thick stack of lava flows. The alc unit locally has jointing patterns indicating 39Ar/36Ar ice-marginal emplacement and appears to have flowed along the northeast margin of a large Linton Creek valley glacier. Dates of the two map units are Quaternary lava in the Oregon Cascades. The coulee is glacially scoured and is indistinguishable; alg yielded a 27.6 ± 1.1 ka plateau age, and two samples from 1 km wide and 120 m thick near Obsidian Falls and 135 m thick at its distal flow alc yielded 27.7 ± 1.0 ka and 27.3 ± 1.2 ka plateau ages. While tempting to lump front. A syneruptive lapilli-fall deposit containing pumice, obsidian, and felsite alc and alg into a single eruptive unit, topographic constraints require that alc is preserved on the west ridge of Little Brother. Dense felsite from roc yielded was erupted from a vent on the Middle Sister edifice. These coeval South and a 40Ar/39Ar plateau age of 38 ± 2 ka. Additionally, a small circular rhyolite lava Middle Sisters eruptions signal a shift in eruption locus to Middle Sister. The 3 dome (r58, 74 wt% SiO2, 41 ± 1 ka) lies ~9 km northwest of Middle Sister (well volume of alc, the only Middle Sister lava erupted in this interval, is ~0.74 km beyond area mapped in Fig. 2A) (Hildreth et al., 2012). (Table 2).
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TABLE 2. SELECTED ERUPTIVE UNITS, THREE SISTERS, CASCADE RANGE, OREGON, USA Accepted Exposed Maximum estimated Cumulative exposed Cumulative maximum 40 39 SiO2 Ar/ Ar age age volume volume volume, Middle Sister volume, Middle Sister Eruptive unit nameMap unit (wt%) (ka) (ka) (km3) (km3) (km3) (km3) Andesite of Collier Cone acc* 57.5 –1.5 0.0240.024 –– Rhyolite of Devils Chain rdc* 72.6 –2.0 0.3200.350 –– Dacite of Irving Glacier dig 64.2 14.6 ± 2.714.60.197 0.2164.804 7.001 Dacite domes southwest of Middle Sister dsw 65.4 17.1 ± 15.3 17.1 0.0450.046 4.6086.785 Basaltic andesite of Middle Sister summit mss 56.1 18.0 0.0000.000 4.5636.739 Dacite of Black Hump dbh 64.0 18.2 ± 2.2 18.7 0.3250.488 4.5636.739 23.4 ± 6.9 Dacite of Middle Sister summit dms 65.6 –19.00.001 0.0014.238 6.251 Dacite of Hayden–Renfrew col dhr 63.6 19.4 ± 1.719.40.001 0.0034.237 6.250 Basaltic andesite intrusion of Middle Sister mmi 53.3 18.1 ± 3.920.50.002 0.0034.237 6.248 Basaltic andesite of Middle Sister mms 53.4 16.7 ± 11.7 20.5 1.0901.190 4.2356.245 21.4 ± 19.0 Basaltic andesite tuff of Middle Sister mmt 53.4 –20.50.004 0.0393.145 5.055 Basaltic andesite of Montague Memorial mmm 56.6 19.5 ± 6.921.00.008 0.0233.141 5.016 Andesite agglutinate north of Hayden Glacier anh* 58.0 21.4 ± 6.321.40.003 0.006– – Dacite north of Separation Creek dsn 64.4 19.4 ± 3.721.50.015 0.0183.134 4.994 Basaltic andesite north of Hayden Glacier mnh 57.1 21.3 ± 5.521.50.001 0.0013.119 4.976 Dacite of Lane Plateau dlp 65.3 21.8 ± 1.921.80.185 0.2403.118 4.974 Basaltic andesite of Separation Creek msc* 56.8 21.8 ± 5.521.80.180 0.230– – Basaltic andesite of Renfrew Glacier mrg 54.6 19.9 ± 6.122.00.011 0.1162.933 4.735 Andesite of headwaters of Linton Creek ahl 57.7 21.2 ± 3.322.50.014 0.1042.922 4.619 Basaltic andesite of Teardrop Pool mtp* 53.8 22.7 ± 12.9 22.7 0.0060.008 –– Dacite north of Teardrop Pool dnt* 63.0 –22.70.001 0.001– – Dacite ejecta of South Sister summit des* 63.5 –22.70.077 0.117– – Dacite of Lewis Glacier headwall dlg* 63.7 22.7 ± 1.6 22.7 0.0440.054 –– 25.7 ± 1.4 Andesite west of Middle Sisterawm 60.0 –23.50.006 0.0162.908 4.515 Andesite west of Renfrew Glacier awr 61.3 –23.50.006 0.0262.902 4.499 Basaltic andesite of Camp Lake mcl 56.5 16.4 ± 10.4 23.5 0.3600.440 2.8964.473 23.7 ± 15.8 Andesite west of Collier Glacier awc 62.1 22.3 ± 1.7 23.8 0.0250.100 2.5364.033 24.9 ± 1.5 Andesite of West Forkawf* 60.0 24.1 ± 1.324.10.300 0.375– – Andesite of Demaris Lake adl 61.9 24.6 ± 1.124.60.420 0.6202.511 3.933 Andesite of southwest slope of Middle Sister asw 61.5 24.9 ± 4.224.90.001 0.0062.091 3.313 Dacite of Sister Spring dss 63.7 24.9 ± 3.124.90.024 0.0362.091 3.307 Basaltic andesite south of Scott Pass mps* 56.8 –25.40.180 0.280– – Rhyolite of Separation Creek rsc* 73.3 25.8 ± 1.325.80.188 0.225– – Dacite of Chambers Lake dcl* 65.4 26.4 ± 2.126.40.056 0.081– – Andesite of Eugene Glacier aeg* 58.0 27.2 ± 1.7 27.5 0.7000.910 –– 27.5 ± 3.2 29.0 ± 2.1 Dacite west of Lost Creek Glacier dwl* 63.9 –27.60.045 0.065– – Andesite of Linton Creek alc 62.0 27.7 ± 1.0 27.6 0.5400.740 2.0673.271 27.3 ± 1.2 Andesite west of Lost Creek Glacier alg* 62.5 27.6 ± 1.127.60.300 0.405– – Andesite dome of Lewis Glacier headwall adh* 62.6 –27.60.015 0.045– – Dacite north of Carver Lake dcn* 67.1 –28.00.034 0.044– – Dacite of Green Lakes dgl* 63.9 –28.50.380 0.510– – (continued)
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TABLE 2. SELECTED ERUPTIVE UNITS, THREE SISTERS, CASCADE RANGE, OREGON, USA (continued) Accepted Exposed Maximum estimated Cumulative exposed Cumulative maximum 40 39 SiO2 Ar/ Ar age age volume volume volume, Middle Sister volume, Middle Sister Eruptive unit name Map unit (wt%) (ka) (ka) (km3) (km3) (km3) (km3) Dacite of Red Meadowdrm* 64.3 29.2 ± 0.829.20.720 0.900– – Andesite fall of Hodge Crest ahc* 60.4 –30.50.050 0.075– – Dacite southwest of Clark Glacier dcg* 63.3 31.2 ± 2.631.21.310 1.640– – Rhyolite of Park Creek rpc* 72.0 31.9 ± 8.731.90.019 0.029– – Dacite of Dew Lake ddl* 64.3 32.9 ± 1.832.90.790 0.950– – Dacite pumice fall of Prouty Glacier headwall dpg* 64.6 –32.90.005 0.008– – Rhyodacite of Prouty Glacier rpg* 68.4 33.4 ± 1.533.40.007 0.007– – Andesite of Prouty Glacier headwall aph* 61.6 –34.60.004 0.046– – Rhyolite of South Forkrsf* 74.0 36.4 ± 2.036.40.675 0.900– – Basaltic andesite of headwaters of Linton Creek mhl 55.1 –37.00.001 0.0121.527 2.531 Dacite of Linton Creek dlc 66.8 –37.00.002 0.0231.525 2.519 Basaltic andesite of Hayden–Diller cleavermhd 54.8 37.4 ± 8.937.40.008 0.0831.523 2.496 Andesite fallout at base of east face of Middle Sister aef 62.7 –37.60.001 0.0041.516 2.414 Rhyolite of Obsidian Cliffroc 76.5 38.4 ± 1.838.40.360 0.4801.515 2.410 Andesite of Obsidian Creek aoc 58.5-63.0 48.7 ± 2.2 47.1 1.1301.530 1.1551.930 49.7± 3.7 45.4 ± 1.8 Basaltic andesite of North Forkmnf 54.2 48.5 ± 9.548.50.001 0.1010.025 0.400 Basaltic andesite of Little Brother mlb* 52.7 48.0 ± 5.9 48.9 0.4200.620 –– 49.2 ± 3.7 Basaltic andesite of North Sister mns* 54.0 47 ± 6 to 121 ± 6 >478.000 16.000 –– Basaltic andesite of Soap Creek–N. Fork Whychus Creek confluence msp* 54.2 131.3 ± 5.8 131.30.006 0.018– – Basaltic andesite of Linton Spring mls 55.1 150.00.004 0.0390.024 0.299 Basaltic andesite of The Husband mth* 52.8 151.5 ± 4.9 151.55.000 8.000– – Basaltic andesite of South Forkmsf* 54.0 169.1 ± 16.3169.1 0.0040.044 –– Basaltic andesite north and south of Demaris Lake mdl 53.0 183.9 ± 21.1183.9 0.0200.260 0.0200.260 Basaltic andesite of Broken Topmbt* 55.5 –>2005.000 7.000– – Notes: Eruptive units (Fig. 2) (Hildreth et al., 2012) are listed in eruptive order (youngest to oldest). Accepted ages are based on integrating field relations and radiometric ages. Eruptive volumes are estimated for all units, but only those interpreted to have erupted from, or adjacent to, Middle Sister are included in the cumulative volumes. *Not included in Middle Sister volume calculation.
Middle Sister Eruptive Pulse (27–15 ka) Several dacite and andesite units on the western flank of Middle Sis ter appear to have erupted from Middle Sister during this time period, but The 12-k.y. Middle Sister eruptive pulse has been divided into two time their vents are obscured. The dacite of Sister Spring (dss) is a 60–80 m thick intervals (27–21 ka and 21–15 ka) in Figure 6. In the saddle between South and lava flow (Fig. 7), dated at 25 ± 3 ka and overlying roc. Andesite lava flows Middle Sisters, two ca. 26-ka units (rsc and dcl) mark the shift of volcanism in the west and northwest sectors (Fig. 7) are similar in chemical composi toward Middle Sister (Fig. 6). The rhyolite of Separation Creek (rsc) is a 1-km- tion, variable in phenocryst assemblage, and yield ages or brackets between diameter dome equidistant (2.5 km) from South and Middle Sisters summits 25 and 22 ka. Several have evidence of interaction with ice and water during (Fig. 10) that yielded a 40Ar/39Ar plateau age of 25 ± 1 ka. The dome appears emplacement. These include the (1) andesite of the southwest slope of Mid contemporaneous with the neighboring dacite of Chambers Lakes (dcl), which dle Sister (asw), a small window of crystal-poor andesite surrounded by the is composed of orange and black agglutinated tephra fallout that vented from younger mms; (2) andesite west of Collier Glacier (awc), a fan of numerous the Chambers Lakes depression (Fig. 10). The dacite, which is widely exposed shingled lava flows, mainly 5–25 m thick but with a 70 m thick flow apparently in the saddle, is crudely stratified and eroded into ledges with variable dips as ponded against an expanded Collier Glacier; (3) andesite west of Renfrew Gla much as 25°, suggesting buried preexisting topography. The main texture is cier (awr), a fan of thin (2–5 m) and thick (15–40 m) moderately porphyritic loose or tack-welded pumice and scoria grading to vitrophyre. On the ridge lava flows; (4) >70 m thick andesite of headwaters of Linton Creek (ahl); and crest between the Chambers Lakes, the deposit was locally thick enough to (5) andesite west of Middle Sister (awm), a fan of lavas that are 2–50 m thick spread southeastward as rheomorphic flows, one of which is dated at 26 ± 2 ka. and that are lithologically similar to awr but are chemically distinct with higher
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4.0
3.5
3.0
Rhyolite 2.5
Figure 5. SiO2 versus K2O variation dia- gram for the Three Sisters volcanic cluster Andesite 2.0 (Hildreth et al., 2012), Cascade Range, Ore Rhyodacite gon (USA). Map units that vented within ~3 km of the central vent of the North, Middle, or South Sister are included with O (wt. %)
2 that volcano; more distal vents are consid-
K Dacite 1.5 Basaltic ered peripheral. Andesite
High K
1.0 Medium KBasalt
Broken Top
0.5 South Sister and vicinity K Middle Sister and vicinity Medium K North Sister and vicinity Low Peripheral vents 0.0 45 50 55 60 65 70 75 80 SiO2 (wt. %)
Fe, Mg, and Ca and lower K. The andesite of Demaris Lake (adl), of similar age the southernmost units of the Matthieu Lakes fissure, 2−10 km of North Sister, and composition, is exposed 4 km east of Middle Sister. Its source is uncer described by Schmidt and Grunder (2009). tain, but small windows of this lava project toward Middle Sister. Also in this The summit sequence at South Sister was coeval with these cone-building interval, basaltic andesite lavas encircle Middle Sister at low elevations: The Middle Sister lavas between 23 ka and 22 ka. The thick dacite of Lewis Glacier voluminous and well-exposed basaltic andesite of Camp Lake (mcl) vented headwall (dlg) immediately preceded the dacite ejecta of South Sister summit low on the southeastern slope of Middle Sister and overlies adl; the basaltic (des), the dacite north of Teardrop Pool (dnt) and, the youngest South Sister andesite south of Scott Pass (mps) erupted north of North Sister; the basaltic edifice unit, the andesite of Teardrop Pool (mtp) (Fierstein et al., 2011). andesite north of Hayden Glacier (mnh) and the andesite agglutinate north Two dacite units, the dacite of Lane Plateau (dlp) dated at 22 ± 2 ka and the of Hayden Glacier (anh) erupted between Middle Sister and North Sister; the less-precisely dated dacite north of Separation Creek (dsn) at 19 ± 4 ka, were basaltic andesite of Renfrew Glacier (mrg) and basaltic andesite of Montague emplaced on the western and southwestern flanks of Middle Sister shortly be Memorial (mmm) likely erupted on the northwestern flank of Middle Sister, but fore the cone-building eruption described below. Map unit dlp (Fig. 7) intruded their vents are obscured by younger eruptive units; and the basaltic andesite of into and then apparently flowed over a glacier to feed the Lane Plateau coulee. Separation Creek (msc) vented low on the southwestern flank of Middle Sister Intricately jointed columns are abundant in the vent area and there is a con and drapes flows from The Husband and South Sister. Units mps and anh are spicuous gap between the vent and the bulk of the lava flow. Thedlp coulee
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–121°50′ –121°44′ 5320–7–1–>< 5015372721 ka 10 rsc acc 9 dcl mlb mps 8 mlb desrpg 7 rpc 6 dlg mmm rsrdfc 5 dlp dcndbh
roc MgO mns 4 dnt rodcgdhrc awc 3 44°10′ 44°10′ dsdlddlcn 2 mbtdig North dsdgls dbh 1 aefmdldms dss Sister 0 adldpg mlaocdssw aoc anh ahldrm awr dbh mlmnsmmbi mrg mnh 9 asdwlw mns mnfmsmmfm 8 dlp awmsadhcp msp mhdmms dhr mmi awaegf aef O 7 mhlmth dlp mnf 2 mmt Middle mss mhd awahcm Sister dms 6 mss awm awalcr alc mnf O + K mdl 2 5 mclalg Na mrgaph ahl 4 mth mms mcl msc msf dlc mhl adl mcl 3 mtp mls asw 45 50 55 60 65 70 75 80 mps SiO mmt dig mdl 2 dsw dsw rsf mmt dcl lakes dsn rsc < 15 ka dlg ice dcn mmt drm alluvium 21–15 ka rdc aeg glacial deposits rpc 27–21 ka mth above mms awf drm msc rpg mms 37–27 ka
alg dpg awf beneath mms dnt aph South 50–37 ka dwl aeg mbt mtp ahc Sister des 44°6′ dcg aeg 44°6′ > 50 ka ddl dlg adh dgl –121°50′ –121°44′ units bracketing mms 012 km
contour interval 400 feet
Figure 6 is interactive when viewed with Adobe Figure 6. Geologic units and chemistry over different time intervals, Three Sisters volcanic cluster, Cascade Range, Oregon (USA). Mapped geologic units for given time intervals are shown with Acrobat. View age-separated maps and plots by corresponding chemical variation diagrams for SiO2 versus total alkalis (Na2O + K2O) and MgO (in wt%). Chemical data from Middle Sister are plotted as symbols colored to correspond to map units; hovering mouse over time-interval buttons. Indi- for comparison, data from all Three Sisters cluster analyses are shown as gray symbols in each map view. (Figure is dynamic when viewed in Adobe Acrobat or Adobe Acrobat Pro; view age-sepa- vidual parts of the figure are also accessible in rated maps and plots by hovering mouse over time-interval buttons). Individual parts of the figure are also accessible in Supplemental File 3 [text footnote 1]). Map unit symbols are defined in the Supplemental File 3 (text footnote1). text section “Eruptive History of Middle Sister” and in Table 2.
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sits atop a ridge suggesting it was bounded by 100–300 m thick ice during emplacement. The voluminous mms inundates older eruptive units in the south and west sectors (Figs. 7 and 10) of the Middle Sister volcano and is well exposed (Figs. 6 and 9) on its steep eastern face, where it makes up the upper half (~225 m) of the exposed cone. It consists of thin flows of vesicular, porphyritic mafic lava flows with intercalated blocky rubble. The lava flows contain 25–35%
plagioclase and 5–10% olivine; most contain 52.2–52.9 wt% SiO2, but several
late flows and dikes contain 53.3–54.1 wt% SiO2 (Fig. 6). One of these dikes fed the basaltic andesite intrusion of Middle Sister (mmi), which cut through early-erupted mms flows and is well exposed high above the Hayden Glacier (Fig. 9). Discontinuous patches of mafic tuff exposed low on the southwestern flank of Middle Sister are mapped as the basaltic andesite tuff of Middle Sister (mmt) and are interpreted as surges and/or lahars, coeval with mms, perhaps initiated by water produced by lava–ice interaction. Minor undated remnants of mafic lava flows (basaltic andesite of Middle Sister summit, mss) and silicic lava flows (dacite of Middle Sister summit, dms) are exposed at the summit of Middle Sister (Fig. 9). Dating the mms eruptive episode was difficult be cause of the glassy, vesicular textures. 40Ar/39Ar dating of two lavas and a dike yielded large errors: 17 ± 12 ka and 21 ± 19 ka for the flows and 18 ± 4 ka for Figure 7. Photograph looking southeast the intrusive unit (mmi), but precise dating of bracketing lava flows (Fig. 6) toward Middle Sister, Cascade Range, Oregon (USA), showing generalized constrain the episode to ca. 20.5 ka. Fourteen units that directly underlie mms eruptive units drawn as an overlay were dated, the youngest two being dlp (22 ± 2 ka) and dsn (19 ± 4 ka) (Fig. 7). on oblique aerial photograph (taken Four units directly overlying mms were dated, the oldest two being dhr, the by John Scurlock) and labeled with preferred ages. Map unit symbols are dacite of Hayden Glacier–Renfrew Glacier col (19 ± 2 ka), and dbh, the dacite defined in the text section “Eruptive Mt of Black Hump (18 ± 2 ka). The dacite domes southwest of Middle Sister (dsw) Broken Bachelor Top History of Middle Sister” and in Table 2. South and the dacite of Irving Glacier (dig) erupted at 17 ± 15 ka and 15 ± 3 ka, re Sister Middle spectively (Fig. 10). Chill joints at the terminus of dig indicate ice was at least Sister mms rsc 200 m thick in the saddle between South and Middle Sisters at 15 ka. During 20 ka 26 ka 3 3 dbh dsw 27–15 ka, ~3.7 km of lava and pyroclastic material erupted, of which ~1.2 km 19 ka dig 17 ka dsn 15 ka 22 ka is the widespread mms (Table 2). asw 25 ka awm 24 ka ahl awr awm 23 ka 23 ka Regional Volcanism and Holocene South Sister Rhyolite Flows (<15 ka) awc North dlp alc 22 ka Sister 28 ka dbh Following the unusually productive episode of Middle and South Sisters
mrg construction between 50 ka and 15 ka, volcanism in the Three Sisters cluster awc 22 ka 24 ka waned (Fig. 6), and, except for two late Holocene rhyolite flank eruptions south dss 25 ka and northeast of South Sister, was no longer focused at the central vent vol dlp canoes. Two eruptive units located mostly north (acc) and mostly south (rdc) aoc 47 ka mlb of Figure 2A were dated at <15 ka. The rhyolite of Devils Chain (rdc) erupted at 49 ka mmm ~2 ka to the northeast, east and southeast of South Sister (Fig. 2); the related, 20 ka but slightly older, rhyolite of Rock Mesa (rrm) coulee (Scott, 1987; Stelten and roc 38 ka Cooper, 2012) erupted south of South Sister (beyond the limits of Fig. 2A). The andesite of Collier Cone (acc) is a map unit of the mafic periphery that erupted 1.5 ka and is exposed several kilometers northwest of the North Sister summit (Fig. 2A, upper left). Nothing erupted from Middle Sister in this <15-ka interval (Table 2).
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2.0
1.5 2
O 1.0 Ti
0.5 all
bbr
10 bcc
9 bes bsc 8 Figure 8. Variation diagrams of SiO plot- bsq 2 7 ted against major oxides TiO2, MgO, and btk Na2O (in wt%) for selected mafic units, 6 andesite unit aoc, and rhyolite unit roc of mbs the Three Sisters volcanic cluster, Cascade 5 Range, Oregon (USA). Complete Three mhr MgO Sisters chemical data are plotted in gray. 4 mms Unlike other andesites (gray) in the map area, linear array of aoc suggests mixing 3 mnf of mafic and rhyolitic material. Array of other andesites suggests control by frac- 2 mnh tional crystallization. Map unit symbols are defined in the text section “Eruptive mtb 1 History of Middle Sister” and in Table 2. myl 7 aoc
roc 6
5 O 2 Na 4
3
2 45 50 55 60 65 70 75 80 SiO2
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dmdms ~~1919 ka mss ~~1818 kkaa
mmmss mmi 20.520.200 55k kkaa 220.2020.555k kkaa
dbdbhh 191 kaa dhdhrr mhmhdd 19.5 kkaa aeeff 377 kak aefaeaeeff aefefe HHaydenayden Glacier dbdbhh >37 ka
anh 2121.21.55k5 kkaa
Diller Glacier Dillerr GGlacierr tilll
Figure 9. Photograph looking west toward Middle Sister, Cascade Range, Oregon (USA). Upper half of the volcano is composed of thin, rubbly basaltic andesite flows of the mms eruption. Units mhd, aef, and anh underlie mms. Units dbh, dhr, dms, and mss overlie mms. Age of mms and mmi based on ages of overlying and underlying lava flows. Map unit symbols are defined in the text section “Eruptive History of Middle Sister” and in Table 2.
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Middle Sister North Sister
Figure 10. Photograph looking north to- ward Middle Sister and North Sister (gray) from the northern flank of South Sister, Cascade Range, Oregon (USA). Silicic units rsc, dlp, dsn, and dcl and andesites asw and mcl underlie the climactic mms eruption. Dacite units dsw, dms, and dig dms overlie mss. Both rsc and dig have textures ~19 ka suggesting ice-marginal emplacement. Map unit symbols are defined in the text dlp section “Eruptive History of Middle Sister” 22 ka mms 20.5 ka and in Table 2. asw 25 ka mcl 23.5 ka dig dsn dsw 22 ka 17 ka 15 ka rsc dcl 26 ka 26 ka
till
ERUPTIVE VOLUMES AND RATES of the volcanic field and its rate of eruption both changed dramatically. In the interval 50–15 ka, Middle Sister grew to ~7 km3 and South Sister grew from ~5 Volcanic rocks around the Three Sisters are as old as 600 ka (Hildreth et al., to ~20 km3. Both composite volcanoes erupted significant amounts of ande 2012). Eruptive volumes were estimated for each map unit (Supplemental site, dacite, and rhyolite in this episode. Maximum estimated eruptive rates File 2 [footnote 1]) using Hildreth et al.’s (2012) geologic map, though estimates for the entire cluster in the 50–15-ka interval increased to ~0.8 km3/k.y., four for units older than 150 ka are considered minima because of erosion and times the 200–50-ka rate. Following the 15-ka cessation of volcanism at Middle burial by younger units. The eruptive rate from 600 ka to 50 ka appears to have Sister, no central vent eruptions have occurred in the cluster, and the regional been consistent at ~0.1–0.2 km3/k.y., and from 200 ka to 50 ka, at ~0.2 km3/k.y. eruptive rate appears to have returned to its modest long-term average of ~0.2 (Fig. 11). Middle Sister is a small stratovolcano and appears to have erupted km3/k.y. Approximately half of the total volume of the Three Sisters cluster only a negligible pre-50 ka volume. Starting at 50 ka, the eruptive composition erupted since 200 ka was produced in the 35-k.y. episode (50–15 ka).
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200 150 100 50 0
more ice A 6 2 5
4 O 3 18 4 5 1 less ice 80 3
B r Basalt 3 /ky 70 Basaltic Andesite
) Andesite 3 60 Three Sisters Dacite ~0.8 km Volcanic Cluster Figure 11. Eruptive volumes, rates, and Rhyolite composition data for 200-ka and younger 50 volcanic units of the Three Sisters volcanic 3 /kyr cluster, Cascade Range, Oregon (USA), and relation to Marine Isotope Stages (MIS).
olume (km 40 ~0.2 km (A) Oceanic oxygen isotope curve (MIS 1–6). (B) Exposed (solid lines) and estimated (dotted lines) cumulative eruptive volumes 30 for Middle Sister (green lines), South Sister (red lines), and the entire cluster (blue lines) are plotted by composition against time. 20 (C) 40Ar/39Ar ages and 1σ errors for Three South Sister Sisters dated samples, plotted by SiO2 con- (Fierstein et tent (Hildreth et al., 2012).
Cumulative V al., 2011) 10 Middle Sister
0 80 C 2 70
60 %wt. SiO 50 200 150 100 50 0 Age (ka)
RELATIONSHIP OF VOLCANISM TO GLACIATION volcanic cluster do reflect the gradual cooling from MIS 3 to MIS 2 globally. The coincidence of this 35-k.y. magmatic episode with this cooling trend The 50–15-ka eruptive episode coincided with cold temperatures and thick before and during the Last Glacial Maximum glaciation is intriguing. Many ice, best recorded by oxygen isotope variation (Fig. 11) in marine sediments studies (Miller and Smith, 1987; Singer et al., 1997, 2008; Bacon and Lanphere, (Lisiecki and Raymo, 2005). Timing of ice thickness in the Oregon Cascades 2006; Huybers and Langmuir, 2009; Watt et al., 2013; Rawson et al., 2015) sug may not be exactly synchronous with trends in marine oxygen isotopes, but gest increased eruptive rates associated with deglaciation at arc volcanoes. abundant ice-contact features in lavas erupted 50–15 ka in the Three Sisters This study reveals a well-mapped and dated example that shows the opposite.
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Peak eruptive behavior was coincident with the buildup to peak glaciation of neither a magmatic system undergoing fractional crystallization producing the Last Glacial Maximum (MIS stage 2) (Lisiecki and Raymo, 2005) and ended variable-composition lavas nor enough magmatic input to cause melting of immediately prior to deglaciation (Fig. 11). wall rocks or antecedent mafic intrusions. The eruption of rhyolite starting ~50 ka and the apparent mixing of mafic material with rhyolite in aoc heralds a significant increase in intrusive inputs. Previous study of rhyolite at South DISCUSSION Sister (Hill, 1991) attributes rhyolite to partial melting of amphibolitic crust; however, the subsequently abundant dacite and andesite erupted at Middle Middle Sister is a relatively young stratovolcano that grew during an anom and South Sister imply development of a fractionating magmatic system that alous episode in the Three Sisters volcanic cluster’s history. From 600 to 50 ka waxed during 50–30 ka, culminated during 30–20 ka, then waned by 15 ka. The the Three Sisters segment of the Cascade volcanic arc was characterized by 50–15 ka episode that built Middle and South Sisters has no precedent on the eruptions of basalt and basaltic andesite, both from discrete vents and from arc axis here and appears to have ended at 15 ka. Eruption of rhyolite ~2 ka composite cones (Hildreth et al., 2012). Peripheral units that include basaltic south and east of South Sister may have introduced a new eruptive episode andesite of The Wife, basalt of Sphinx Butte, basaltic andesite of the Husband, or it may have been a cryptic rejuvenation of residual rhyolite magma beneath and basaltic andesite of North Sister typify the cluster prior to 50 ka, and rhyo South Sister. Notably, zircons separated from rrm and rdc yielded ages as old dacite (rhyodacite southwest of Golden Lake and rhyodacite of Upper Chush as 80 ka, although most yielded ages between 20 ka and 50 ka, when South Falls) and dacite (dacite of Todd Lake) associated with either Broken Top or Sister was most active (Stelten and Cooper, 2012), so at least some of the rhyo with rhyodacite lavas near Tam McArthur Rim (Hildreth et al., 2012; Hill, 1991) lite appears to be related to the 50–15-ka eruptive episode. erupted east of the Three Sisters cluster. During this interval, several volumi Previous workers have pointed out that the silicic volcanism at the Three nous silicic eruptions inundated lowlands to the east and southeast from vents Sisters is at the western end of a 10-m.y. rhyolite sweep (Fig. 3) across the High in the Tumalo volcanic field, 20 km east of Middle Sister (Sherrod et al., 2004). Lava Plains of Oregon (MacLeod et al., 1976; Jordan et al., 2004; Fierstein et al., Our map focuses on South, Middle, and North Sisters, so volumes tabulated 2011). Arguments over the cause of this sweep range from westward spreading here do not include appreciable amounts of these voluminous silicic deposits of the eastward-propagating Yellowstone hotspot (Draper, 1991; Jordan et al., or Broken Top lavas. Using estimated volumes from 200 to 50 ka yields ~0.2 2004), to crustal-block rotation (Carlson and Hart, 1987), to asthenospheric km3/k.y. productivity rates across the Three Sisters map extent. The eruptive flow related to the rollback of the downgoing Cascadia slab (Ford et al., 2013), rate appears to have been somewhat lower prior to 200 ka; however, erosion to lithospheric extension at the edge of the Basin and Range province (Fitton and inundation by younger eruptive units renders volume estimates uncertain. et al., 1991). This study does not resolve this controversy, but it does document Beginning at 50 ka, the eruptive character at Middle Sister and in the clus the late Pleistocene introduction of rhyolite to this formerly mafic portion of ter as a whole changed from mafic to bimodal mafic/felsic, and the eruption the Cascade Arc—arguably the leading edge of the propagating anomaly. rate approximately quadrupled (Figs. 6 and 11). No rhyolite has been identified along the High Cascades crest older than 50 ka. Mafic eruptions, the high-silica rhyolite roc, and the chemically mixed aoc, interpreted as a mixture of basaltic COMPARISON WITH OTHER ARC VOLCANOES andesite and rhyolite, erupted adjacent to and from Middle Sister coincident with inferred silicic eruptions at South Sister (Fierstein et al., 2011). In the time Comprehensive eruptive histories for active volcanoes are time-consuming interval 37–27 ka, volcanism waned near Middle Sister and flared up at South endeavors and have been completed at relatively few volcanoes, mainly in the Sister. Abundant andesite and dacite lava flows (Fierstein et al., 2011) covered Cascades (Hildreth and Lanphere, 1994; Hildreth et al., 2003a; Bacon and Lan South Sister, and several rhyolite flows erupted on its flanks. Copious amounts phere, 2006), Chilean Andes (Singer et al., 1997, 2008; Hora et al., 2007), Mexico of phenocryst-rich andesite erupted at 27 ka at South Sister and one of these (Frey et al., 2004), and Alaska (Hildreth et al., 2003b; Jicha and Singer, 2006). eruptive packages, alg, is identical in composition and time to andesite alc Despite being compositionally diverse and active over varied timescales, all erupted from Middle Sister. Andesite/dacite composition lavas erupted at Mid the studied volcanoes display some degree of episodic behavior, typically with dle Sister 26–23 ka. The cone-building mms eruption that built the upper half tens of thousands of years of modest output punctuated by brief episodes of Middle Sister is bracketed between 21 and 19 ka. Several small dacite erup (1–20 k.y.) of high output. A subset of these detailed time–volume studies are tions (dig, dsw, dhr, dsn, dms, and dbh) at Middle Sister overlie mms and are compiled in Figure 12. waning eruptions of the 50–15-ka episode. Eruptive rates calculated for the Three Sisters cluster average 0.2 km3/k.y. The temporary quadrupling of the eruption rate (Fig. 11) and introduction before and after the 50–15-ka uptick (to 0.8 km3/k.y.) in volcanism, yielding of andesite/dacite compositions are a profound departure from the productive, a long-term average of ~0.3 km3/k.y. since 200 ka and likely similar back to but consistently mafic, earlier eruptive history of the Three Sisters volcanic >500 ka, though units older than 250 ka are poorly preserved because of exten cluster. The mafic character of the field prior to 50 ka implies that there was sive erosion and exposures are limited because of deposition of younger erup
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400
350
300 Mt. Adams
Cumulative V Figure 12. Time–volume plot for Three Sisters, Cascade Range, Oregon (USA), 250 compared to other mapped and dated arc volcanoes (Hildreth and Lanphere, 1994; Singer et al., 1997; Hildreth et al., 2003a; Frey et al., 2004; Bacon and Lanphere, 2006; Jicha and Singer, 2006; and Hora
200 olume (km et al., 2007). Volumes plotted are maxi- mum estimates of erupted volumes from Mt. Baker each study. Mt. Baker volumes include Kulshan caldera deposits (Hildreth et al., 2003a), and Crater Lake/Mt. Mazama vol- 150 umes include regional volcanism exposed 3 in the study area (Bacon and Lanphere, ) 2006), and Volcán Ceboruco includes the adjacent San Pedro and Amado Nervo volcanic deposits (Frey et al., 2004). Is.— Three Sisters 100 Island; V.—Volcán.
Crater Lake 50
V. Ceboruco . Seguam Is V. Parinacota 0 600 500 400 300 200 100 0 Age (thousands of years)
tive units. Compilations of various well-dated centers (Bacon and Lanphere, Puyehue/Cordon Caule complex produced basaltic andesite early in its history 2006; Jicha and Singer, 2006) identify long-term average eruption rates that and rhyodacite and basalt in the past 40 k.y. (Singer et al., 2008). The Three vary from ~0.1 km3/k.y. for Tartara–San Pedro complex (Singer et al., 1997) and Sisters transition from monotonous basaltic andesite to rhyolite is similar to the Mt. Baker volcanic field (Hildreth et al., 2003a) to 0.8 km3/k.y. for Mt. Katmai the Puyehue/Cordon Caule complex; however, with no prior rhyolite emplaced (Hildreth et al., 2003b). Short-term eruptive rates for typical stratovolcanoes along the arc front, the 50–15-ka eruptive episode at the Three Sisters pro commonly are 0.5–1 km3/k.y. but can range as high as 32 km3/k.y., such as duced high-silica rhyolite and then extruded andesite/dacite at four times the at Klyuchevskoy (Fedotov et al., 1987), and even higher for other, caldera- earlier eruptive rate. forming eruptions. Several of these studies identify compositional variation All the compared eruptive histories document that arc magmatism is fo with time. Mt. Mazama/Crater Lake was an andesite/dacite stratovolcano until cused at individual stratovolcanoes for hundreds of thousands of years, but ~27 ka, when rhyodacite began to dominate (Bacon and Lanphere, 2006). The eruptive flux can vary over short periods by an order of magnitude or more.
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The form of several of these detailed eruptive histories are remarkably similar: Draper, D.S., 1991, Late Cenozoic bimodal magmatism in the northern Basin and Range Province Seguam Island, the Volcán Ceboruco, and the Three Sisters produced ~80 km3 of southeastern Oregon: Journal of Volcanology and Geothermal Research, v. 47, p. 299–328, https://doi.org/10.1016/0377-0273(91)90006-L. over 400 k.y. at rates that accelerated in the late Pleistocene (<150 ka). The se Fedotov, S.A., Khrenov, A.P., and Jarinov, N.A., 1987, Klyuchevskoy volcano, its activity and evo lection of these stratovolcanoes may be biased toward modest-sized edifices lution, 1932–1986: Vulkanologiya i Seysmologiya, v. 4, p. 3–16. that are clearly constructional and active today yet are small enough to present Fierstein, J., Hildreth, W., and Calvert, A.T., 2011, Eruptive history of South Sister, Oregon Cas cades: Journal of Volcanology and Geothermal Research, v. 207, p. 145–179, https://doi .org tractable field projects. Other, larger centers, with eruptive-history studies in /10.1016/j.jvolgeores.2011.06.003. progress (e.g., Mt. Shasta, Mt. Veniaminof, and Newberry Volcano [Calvert, Fitton, J.G., James, D., and Leeman, W.P., 1991, Basic magmatism associated with late Cenozoic 2015]), display episodic volcanism at times different from those in Figure 12. extension in the western United States: Compositional variations in space and time: Journal of Geophysical Research. Solid Earth, v. 96, no. B8, p. 13,693–13,711, https://doi .org/10.1029 Erosion and inundation by younger eruptive units clearly impacts exposure, so /91JB00372. sampling and volume estimation is challenging, particularly for deposits older Ford, M.R., Grunder, A.L., and Duncan, R.A., 2013, Bimodal volcanism of the High Lava Plains than the end of the penultimate glaciation (MIS 6, ~135 ka). and northwestern Basin and Range of Oregon: Distribution and tectonic implications of age-progressive rhyolites: Geochemistry Geophysics Geosystems, v. 14, no. 8, p. 2836–2857, https://doi.org/10.1002/ggge.20175. Frey, H.M., Lange, R.A., Hall, C.M., and Delgado-Granados, H., 2004, Magma eruption rates con CONCLUSIONS strained by 40Ar/39Ar chronology and GIS for the Ceboruco–San Pedro volcanic field, western Mexico: Geological Society of America Bulletin, v. 116, p. 259–276, https://doi.org/10.1130 /B25321.1. Middle Sister is the product of a profound 50–15-ka eruptive episode that Hildreth, W., 2007, Quaternary magmatism in the Cascades: Geologic perspectives: U.S. Geologi also built South Sister and abundant lava flows in their periphery. At 7 km3, cal Survey Professional Paper 1744, 125 p., http://pubs.usgs.gov/pp/pp1744/. Middle Sister’s eruptive volume is modest, but its diverse chemistry and its Hildreth, W., and Lanphere, M.A., 1994, Potassium–argon geochronology of a basalt–andesite– dacite arc system: The Mount Adams volcanic field, Cascade Range of southern Washing sudden onset and abrupt end are intriguing. Built on an older basalt and basal ton: Geological Society of America Bulletin, v. 106, p. 1413–1429, https://doi .org/10.1130/0016 tic andesite platform, this episode produced volcanism ranging from basaltic -7606(1994)106<1413:PAGOAB>2.3.CO;2. andesite to high-silica rhyolite at four times the previous or current volumetric Hildreth, W., Fierstein, J., and Lanphere, M.A., 2003a, Eruptive history and geochronology of eruption rates in the Three Sisters cluster. In studies of other arc volcanoes, the Mount Baker volcanic field, Washington: Geological Society of America Bulletin, v. 115, p. 729–764, https://doi.org/10.1130/0016-7606(2003)115<0729:EHAGOT>2.0.CO;2. Hildreth and Lanphere (1994), Bacon and Lanphere (2006), and others have Hildreth, W., Lanphere, M.A., and Fierstein, J., 2003b, Geochronology and eruptive history of discussed intense eruptive periods succeeded by long periods of quiescence. the Katmai volcanic cluster, Alaska Peninsula: Earth and Planetary Science Letters, v. 214, The Sisters are notable because detailed mapping and high-resolution geo p. 93–114, https://doi.org/10.1016/S0012-821X(03)00321-2. Hildreth, W., Fierstein, J., and Calvert, A.T., 2012, Geologic map of Three Sisters volcanic cluster, chronology show that two adjacent stratovolcanoes were concurrently active Cascade Range, Oregon: U.S. Geological Survey Scientific Investigations Map 3186, scale over the same short, but measurable, interval. The episode epitomizes and 1:24,000, 2 sheets, pamphlet 107 p., http://pubs.usgs.gov/sim/3186/. informs similar episodic behavior at many other stratovolcanoes. Hill, B.E., 1991, Petrogenesis of compositionally distinct silicic volcanoes in the Three Sisters region of the Oregon Cascade Range: Effects of crustal extension on the development of continental arc silicic magmatism [Ph.D. thesis]: Corvallis, Oregon State University, 247 p. Hill, B.E., and Taylor, E.M., 1990, Oregon central High Cascade pyroclastic units in the vicinity of ACKNOWLEDGMENTS Bend, Oregon: Oregon Geology, v. 52, no. 6, p. 125–140. We thank Ed Taylor for sharing mapping and insight into the volcanic field. Andy Ouimette care Hodge, E.T., 1925, Mount Multnomah, ancient ancestor of the Three Sisters: University of Ore fully prepared geochronology and geochemistry samples, Tim Debey handled irradiations, and gon Publications, v. 2, no. 10, 160 p. James Saburomaru oversaw argon analysis. Willie Scott, Brian Jicha, Mariek Schmidt, and Sci Hora, J.M., Singer, B.S., and Wörner, G., 2007, Volcano evolution and eruptive flux on the thick ence Editor Shan de Silva provided helpful reviews. Any use of trade, firm, or product names is for crust of the Andean Central Volcanic Zone: 40Ar/39Ar constraints from Volcán Parinacota, Chile: descriptive purposes only and does not imply endorsement by the U.S. Government. Geological Society of America Bulletin, v. 119, p. 343–362, https://doi .org /10 .1130 /B25954 .1 . Hughes, S.S., 1990, Mafic magmatism and associated tectonism of the central High Cascade Range, Oregon: Journal of Geophysical Research, v. 95, p. 19,623–19,638, https://doi .org/10 REFERENCES CITED .1029/JB095iB12p19623. 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