40Ar/39Ar Geochronology, Paleomagnetism, and Evolution of the Boring Volcanic ﬁ Eld, Oregon and Washington, USA

40Ar/39Ar geochronology, paleomagnetism, and evolution of the Boring volcanic ﬁ eld, Oregon and Washington, USA

Robert J. Fleck1, Jonathan T. Hagstrum1, Andrew T. Calvert1, Russell C. Evarts1, and Richard M. Conrey2 1U.S. Geological Survey, 345 Middleﬁ eld Road, Menlo Park, California 94025, USA 2Peter Hooper GeoAnalytical Lab, School of the Environment, Washington State University, Pullman, Washington 99164, USA

ABSTRACT INTRODUCTION and includes details of paleomagnetic polarity and physical stratigraphy. General petrologic The 40Ar/39Ar investigations of a large suite The Boring volcanic field (BVF) in the study is routinely part of geochronology, but of fi ne-grained basaltic rocks of the Boring greater Portland and Vancouver metropolitan more detailed and systematic studies of BVF volcanic fi eld (BVF), Oregon and Washington areas of northwestern Oregon and southwest- paleomagnetism, fi eld mapping, and petrology (USA), yielded two primary results. (1) Using ern Washington (Fig. 1) consists of lava fl ows and geochemistry will be reported elsewhere. age control from paleomagnetic polarity, that erupted from dozens of monogenetic cen- A major result of this study is the development stratigraphy, and available plateau ages, ters during latest Pliocene and Pleistocene time of an approach for calculating geologically 40Ar/39Ar recoil model ages are defi ned that (Evarts et al., 2009a). Geologically youthful reasonable ages for samples whose 40Ar/39Ar provide reliable age results in the absence of volcanoes are recognizable features in the Port- analyses exhibit nuclear recoil. Such recoil an age plateau, even in cases of signifi cant Ar land area, and were referred to collectively as model ages represent 37% of the 40Ar/39Ar ages redistribution. (2) Grouping of eruptive ages Boring Lava by Treasher (1942a, 1942b) after reported here and are an integral part of the either by period of activity or by composition a group of volcanic-capped hills near the com- conclusions. defi nes a broadly northward progression of munity of Boring, Oregon, ~20 km southeast of BVF volcanism during latest Pliocene and downtown Portland. The volcanic fi eld appar- GEOLOGIC SETTING AND Pleistocene time that refl ects rates consistent ently represents a westward extension of late DISTRIBUTION with regional plate movements. Based on the Cenozoic volcanism of the Cascade volcanic frequency distribution of measured ages, arc; some vents are located as much as 90 km The BVF extends from the Cascade Range periods of greatest volcanic activity within west of the arc axis near Mount Hood (Fig. 1; westward across the southern part of the Port- the BVF occurred 2.7–2.2 Ma, 1.7–0.5 Ma, Peck et al., 1964; Hildreth, 2007). land Basin, a late Neogene to Quaternary topo- and 350–50 ka. Grouped by eruptive episode, Despite the obvious hazards and neotectonic graphic and structural depression within the geographic distributions of samples defi ne a implications of young volcanism in an urban set- Puget-Willamette forearc trough of the Cas- series of northeast-southwest–trending strips ting, surprisingly little was known about the age cadia subduction system (Evarts et al., 2009a). whose centers migrate from south-southeast and composition of the Boring volcanoes until Following Allen (1975), the eastern boundary to north-northwest at an average rate of 9.3 ± recently. Trimble (1963) mapped the distribu- of the volcanic fi eld is placed at long 122°W 1.6 mm/yr. Volcanic activity in the western tion of Boring volcanic rocks and Allen (1975) (Fig. 2), which approximates the position of the part of the BVF migrated more rapidly than inferred the locations of dozens of presumed Quaternary Cascade arc front elsewhere (Guf- that to the east, causing trends of eruptive Boring vents largely on geomorphic grounds. fanti and Weaver, 1988; Hildreth, 2007). The episodes to progress in an irregular, clock- Subsequent work in parts of the BVF (Ham- BVF extends well west of the Portland Basin,

wise sense. The K2O and CaO values of dated mond, 1980; Hammond and Korosec, 1983; occurring west of the Willamette River and the samples exhibit well-defi ned temporal trends, Madin, 1994) differentiated some Boring fl ows Portland Hills (Fig. 2). The area of the fi eld is decreasing and increasing, respectively, with based on geochemistry and limited isotopic dat- ~4000 km2; ~500 km2 of this is underlain by age of eruption. Divided into two groups by ing, and K-Ar dates for several Boring volcanic locally erupted volcanic rocks. The total erup- 3 K2O, the centers of these two distributions centers were provided in Conrey et al. (1996). tive volume for the fi eld is ~10 km , issued defi ne a northward migration rate similar to However, no systematic attempt to determine from ~80 individual volcanoes, most of which that determined from eruptive age groups. the eruptive history of the BVF had been under- erupted small volumes. The BVF is a mono- This age and compositional migration rate of taken prior to our study. genetic volcanic fi eld (Connor and Conway, Boring volcanism is similar to the clockwise In order to characterize the BVF fully and 2000) with vents that erupted composition- rotation rate of the Oregon Coast Range with assess its neotectonic and hazards signifi cance, ally similar lavas over short intervals of time. respect to North America, and might refl ect an integrated program was initiated involving BVF volcanic centers are chiefl y mafi c and, localized extension on the trailing edge of geologic mapping, petrographic and geochemi- like those in the main Cascade arc, are appar- that rotating crustal block. cal analyses, 40Ar/39Ar dating, and paleomag- ently related to subduction of the Juan de Fuca netic determinations. Our study describes the oceanic plate beneath western North America. detailed results of 40Ar/39Ar geochronology However, the tectonic position of the fi eld is

Geosphere; December 2014; v. 10; no. 6; p. 1283–1314; doi:10.1130/GES00985.1; 14 ﬁ gures; 6 tables; 3 supplemental ﬁ les. Received 20 September 2013 ♦ Revision received 23 June 2014 ♦ Accepted 11 August 2014 ♦ Published online 7 October 2014

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40Ar/39Ar GEOCHRONOLOGY

49°N The 40Ar/39Ar radiometric dating technique is BRITISH COLUMBIA a variation of the potassium-argon method that uses neutron activation of 39K to 39Ar, and analy- C sis of the resulting argon isotopic ratios to obtain

a high-precision age estimates (Merrihue and s Turner, 1966; McDougall and Harrison, 1999).

Seattle 40 39 c Details of most Ar/ Ar techniques are provided

40 39 a in Appendix 1. The use of the Ar/ Ar method

in the study of ﬁ ne-grained volcanic rocks such A d c as those of the BVF, however, has lagged behind c Mount r e e St. Helens its development with mineral separates, despite a D t i o number of very important advances in the under- e WASHINGTON n f o a OREGON standing of nuclear recoil. This study contributes

r r m y to, utilizes, and documents a model that provides Portland a reliable age results in the presence of signiﬁ cant t Mt. Hood i o

n Ar recoil effects, regardless of the presence of

Boring 40 39

an age plateau. This approach to Ar/ Ar dating

Wedge Volcanic of ﬁ ne-grained materials is discussed here as a

Field F major aspect of this study.

Ar Recoil Effects

The 40Ar/39Ar technique involves neutron irradiation of rock samples for the concurrent A measurement of 39Ar, as the surrogate for potas-

r 40 sium, and radiogenic Ar in order to deter- Juan c 42°N mine their ages (Merrihue and Turner, 1966; A NEVAD McDougall and Harrison, 1999). When nuclei de CALIFORNIA of 39K capture a neutron, they become unstable Fuca and emit a proton, transmuting to 39Ar by the nuclear reaction 39K(n,p)39Ar. Incorporation of a Plate neutron by any atomic nucleus transfers most of the energy of the neutron to the nucleus, elevat- ing it to an excited state. The excited nucleus often decays immediately by emitting a par- Pacific ticle, but may emit some or all of the energy as a photon (gamma radiation). The emission 0 200 km Plate of a particle, such as the proton emitted to pro- 124° W 120° W duce 39Ar, transfers some of the energy to the product nucleus in the form of kinetic energy, Figure 1. Cascade volcanic arc and Cascadia subduction zone, showing the location of displacing it in a direction opposite that of the the Boring volcanic fi eld and the distribution of major Cascade arc volcanoes (blue tri- emitted particle. This displacement is known angles) and volcanic vents younger than 5 Ma (orange triangles) (after Guffanti and as nuclear recoil, or the recoil effect. In their Weaver, 1988; Evarts et al., 2009a). study of lunar samples, Turner and Cadogan (1974) were the fi rst to identify loss of 39Ar by nuclear recoil as the cause of negatively inclined anomalous, being located in the forearc of the canic rocks overlie a series of mostly low-K 40Ar/39Ar age spectra (age spectra whose ages Cascadia convergent margin, well trenchward tholeiite fl ows (Evarts et al., 2009a) that were decrease progressively with increasing percent of the volcanic arc defi ned by large, long- assigned informally by Peck et al. (1964) to of 39Ar released). When the site of the 39Ar atom lived stratovolcanoes such as Mount Hood and the volcanic rocks of the High Cascade Range. involved is near the margin of the irradiated Mount Adams (Fig. 1). These largely Pliocene units evidently issued mineral grain, ejection of the proton results in The Portland Basin began to form ca. 17 Ma from vents in the Cascade arc well to the east the recoil of the 39Ar atom with suffi cient energy and gradually fi lled with sediment transported (Conrey et al., 2004). Earlier workers, includ- that it may move out of the grain in which it was from the east by the ancestral Columbia River ing Peck et al. (1964), generally grouped these produced if it recoils toward the grain margin (Evarts et al., 2009b). Signifi cant volcanism slightly older, far-traveled fl ows with the Boring (Turner and Cadogan, 1974). Similarly, neu- across the basin, however, did not commence lavas, but we exclude them because they did not tron activation of 40Ca produces the reaction until latest Pliocene time. In the southern and erupt from local vents, and may represent a dif- 40Ca(n,α)37Ar, in which the emission of an alpha eastern parts of the BVF, locally erupted vol- ferent geologic setting. particle causes recoil of the 37Ar. Argon atoms

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122°45’ 122°30’ 122°15’ 122°

East

Lewis Fork River

5 Portland

r 45°45’ Rive

205 Figure 2. Map showing vol canic rocks of the Boring vol canic Vancouver ﬁ eld (BVF; orange), volcanic Washougal rocks of the High Cascade Columbia W i 14 ll River Range that originated in the am Camas Cascade arc to the east (purple), ette and major geographic and cul- Corbett 84 tural features of the region. 26 Portland Troutdale Some major faults and the Sandy Portland Basin are also shown. 45°30’ Beaverton Green circles—locations of 40Ar/39Ar age samples; red circles—mapped volcanic vents; Boring black circles—paleomagnetic River 26 sample site. Sandy Basin River Tualatin C l Oregon a c City k a m

River

45°15’

5 0 20 km

recoiling out of a grain may be incorporated by Whereas recoil of 39Ar affects 40Ar/39Ar ages The effects of 37Ar recoil are most serious in adjacent grains in contact with the source grain directly, recoil of atoms of other Ar isotopes high-calcium and low-radiogenic materials such or be lost as Ar gas to the space between grains. may affect ages indirectly by affecting the cor- as young basalts like the Boring lavas. Argon recoiled from separated mineral grains rections made to abundances of 40Ar, 39Ar, and Huneke and Smith (1976) demonstrated that is generally lost, whereas that from multigrain 36Ar for atmospheric and irradiation-produced recoiled 39Ar is not only lost from the near-sur- aggregates, such as rock groundmass or matrix, interferences (Turner and Cadogan, 1974; face areas of grains, but the recoiled atoms may is more likely to be incorporated into an adja- Onstott et al., 1995; Jourdan et al., 2007). Recoil be embedded in adjacent grains and released cent grain. Depending on the size and shape of of 37Ar is the most signifi cant of these because according to the Ar-release characteristics of the the grains irradiated, loss of 39Ar and 37Ar may the ratio of calcium-derived 37Ar/36Ar is used to receiving grains. Their evidence of redistribu- 36 39 have a signifi cant effect on the apparent age correct for simultaneously generated ArCa. The tion of recoiled Ar nuclei confi rms the inter- determined. When 39Ar is lost from the material underrepresentation of 37Ar, because of its loss pretation made by Turner and Cadogan (1974) 36 analyzed, the potassium atoms it represents are by recoil, results in the inclusion of some ArCa for commonly observed age spectra that decline not counted in the analysis, and the potassium as atmospheric 36Ar. Atmospheric 36Ar is used progressively from older ages at low tempera- concentration of the sample is underestimated in turn to correct for atmospheric 40Ar, and its tures to minimum ages at high temperatures, proportional to the amount of loss. Because overestimation results in an overcorrection of but yield meaningful integrated ages. These 39Ar is used to determine the amount of 40K, the 40Ar and a reduced apparent age. In rocks with negatively inclined age spectra were called radioactive parent in the K-Ar technique, its limited amounts of radiogenic 40Ar this overcor- the second type of anomalous age patterns by under esti mate results in an overestimation of rection may result in removing more 40Ar than is both Huneke and Smith (1976) and Turner and the calculated 40Ar/39Ar age. measured, resulting in a so-called negative age. Cadogan (1974), the fi rst type showing only

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high-temperature decreases in age. In report- resulting 40Ar/39Ar age spectrum may be ampli- for a fi ne-grained (45–75 μm), plagioclase-rich ing the loss of both 39Ar and 37Ar by recoil from fi ed, whereas the integrated age may be unaf- (>97%) crystalline aggregate separated from sani dine and plagio clase mineral separates, fected, as noted by Turner and Cadogan (1974) a Jurassic tholeiitic basalt (81-7-2A), which Jourdan et al. (2007) noted the results of Turner and Huneke and Smith (1976). provides an excellent example of 37Ar recoil and Cadogan (1974) when observing that reim- Recoil effects of 39Ar, producing age spectra (Fig. 3). Whereas the lowest temperature frac- plantation of recoiled nuclei is greatly dimin- with progressively declining ages, were dis- tions of 81-7-2A appear to show minor recoil ished in powdered or fi nely ground samples. In cussed by Turner and Cadogan (1974) and by of 39Ar, probably related to the ≤3% cryptocrys- non vesicu lar crystalline aggregates (e.g., lunar Huneke and Smith (1976), but the recoil effects talline matrix reported in the sample (Foland basalts, Boring lavas), many or even most min- of 37Ar in fi ne-grained rocks have received less et al., 1993), the ages of the remaining incre- eral grains are bounded by other mineral grains attention, as noted by Jourdan et al. (2007). ments increase progressively with temperature or glass. In this case, rather than being lost to Jourdan et al. (2007) discussed the effects of from values 23% below the 175.3 Ma total-gas the voids between separated grains and escap- 37Ar recoil in fi ne-grained (plagioclase-rich) or integrated age to values 15% above it. Appar- ing from the sample, recoiling 39Ar and 37Ar mineral separates as a function of grain size, ently unaffected by recoil, a plateau age of are commonly redistributed to adjacent grains showing the decrease in total-gas 40Ar/39Ar and 176.8 ± 0.8 Ma is defi ned for the central 64% or glass, which may have Ar-retention charac- apparent age with increasing net loss of 37Ar and of the 39Ar released, followed by elevated ages teristics different from the grains from which resulting undercorrection for Ca-derived 36Ar. in high-temperature increments affected by the atom recoiled. In this way, the effect on the Foland et al. (1993) reported an age spectrum redistributed (migrant) 37Ar (Fig. 3). The posi-

Effects of 39Ar and 37Ar Recoil

400400

450450 t t 90-75-2 Basaltic Glass, P = 176.6 ± 0.5 Ma; TG = 176.4 Ma 350350

)

a 500500 10101010 M 550550 ( 14001400 600600 650650 e 700700

g 750750 800800 850850 900900 930930 320320 875875 14001400

A 870870 925925 360360 710710 790790 950950 975975 630630 400400 10001000 12001200 550550

470470 10501050 t t 81-7-2A Plagioclase, P = 176.8 ± 0.8 Ma; TG = 175.3 Ma

% 39Ar Released

Figure 3. The 40Ar/39Ar age spectra of two fi ne-grained Antarctic tholeiites (Foland et al., 1993) show the contrasting effects of redistribution of 39Ar and 37Ar by nuclear recoil without signifi cant net loss of either. These were selected for several reasons. The ages of these and other Antarctic tholeiites are very tightly constrained by the narrow age range (176.6 ± 1.8 Ma) established for the Ferrar continental fl ood basalt or large igneous province (Heimann et al., 1994). The samples are from the same general unit, from the same study (Foland et al., 1993), and one is dominated by 37Ar recoil effects, whereas the other is dominated by recoil of 39Ar. The samples have identical ages, similar numbers of increments, and a well-developed plateau despite clear and opposite effects of recoil. Sample 81-7-2A is a 45–75 μm plagioclase separate with a bulk K/Ca weight ratio of 0.034 (Ar age spectrum in black). Sample 90-75-2 is a 75–150 μm separate of basaltic glass with small, isolated crystals of plagioclase and pyroxene (Ar age spectrum shown in red); its bulk K/Ca ratio is ~0.184. As noted by Foland et al. (1993) and shown

in the ﬁ gure, plateau (tP) and integrated (total gas, tTG) ages are identical for the glass and nearly so for the plagioclase. Note, however, that the two age spectra are nearly mirror images of each other across the plateau (dashed line at 176.7 Ma), the glass yielding a release pattern typical of 39Ar redistribution by recoil, whereas the more Ca-rich plagioclase exhibits a release pattern more characteristic of 37Ar recoil redistribution. However, samples yield identical plateau ages in their central portions.

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tively inclined age spectrum is typical of 37Ar central portions yield highly precise ages and defi ne the recoil model age for 50%. If the recoil, as 37Ar has been redistributed from sites whose integrated ages show either little or no modeled portion is increased to 0.7 or 70%, that release Ar at low temperatures, leading to net loss of either recoiling species. then 18.59% from the 700 °C step is included overcorrection of atmospheric 40Ar and young in the modeled age along with all of the 775 °C, ages in those increments. Redistribution of this Model Ages: Recoil 850 °C, and 925 °C steps plus 1.07% from the 37Ar into sites releasing Ar at high temperatures 1000 °C step (Table 1). The uncertainty in the results in undercorrection for atmospheric 40Ar As documented by Turner and Cadogan age of any fraction of an increment is taken to in those steps and apparent ages greater than the (1974), Foland et al. (1993), Heimann et al. be the same as that calculated for the whole. true age of the material. Contrary to the results (1994), and in numerous examples of Bor- The uncertainty in the model age is propagated for crushed mineral separates, the similar pla- ing volcanic rocks in our study, the effects of by standard techniques, but the mean square teau and integrated (total gas) ages for basalt recoil are most serious in the lowest and high- of weighted deviates (MSWD) is calculated 81-7-2A demonstrate that substantial recoil est temperature increments, whereas many of for the ages included in the central fraction. redistribution of 37Ar, resulting in positively these same samples exhibit age plateaus in the Where the MSWD is >1.0, errors for each age inclined age spectra, may occur without signifi - central portions of their age spectra. Because are multiplied by the square root of the MSWD, cant net loss of the recoiling species. It is impor- 40Ar/39Ar ages depend on accurate measure- increasing the uncertainty in the model age as tant to note here that this redistribution, affect- ment of undisturbed ratios of argon isotopes, the dispersion in the ages increases (Ludwig, ing gas fractions containing more than one-third not on quantitative measurement of abundances, 2003). In this study recoil model ages were of the 39Ar released, does not affect the very eliminating measurements of gas from parts of calculated for the central 50% and 70% of the tightly constrained plateau age of the remaining the grains that undergo net loss by recoil should 39Ar released by assuming that all gas released 64%. These age spectra are distinctly different also yield meaningful results, just as 40Ar/39Ar in a step was of the same age, just as plot- from positively inclined age spectra typical of age plateaus may provide reliable measures of ted in the age spectrum diagram. Model ages 40Ar loss, as discussed in detail by Turner (1968) the true ages. As shown in Figure 3, this may calculated in this way are not sensitive to the and later in the section on 40Ar/39Ar results. be accomplished by removing equal amounts direction or amount of slope on the age spec- Results for a second Antarctic tholeiite of the gas released at both the highest and low- trum, but MSWD increases dramatically as reported by Foland et al. (1993) are also shown est temperatures and avoiding the most affected slope increases or decreases from zero. Abrupt in Figure 3 to document a contrasting age spec- and discordant parts of the age spectrum. We changes in slope within the central portion trum dominated by recoil redistribution of 39Ar. defi ne here a 40Ar/39Ar recoil model age as “the have signifi cant effects and make the choice The second age spectrum is from a 75–150 μm, integrated age of contiguous temperature incre- of that portion more subjective. Selection of

high potassium (~2% K2O) basalt glass sepa- ments and fractions of increments in a centered the central fraction is based on an examination rate, sample 90-75-2 (Foland et al., 1993). The fraction of the age spectrum.” The age contribu- of the age spectrum and on the statistics of the two samples of identical age yield age spectra tion of each included increment is weighted by ages calculated, with the intent to avoid por- that are nearly mirror images of each other, the fraction of its 39Ar falling within the speci- tions involving net loss of recoil products and

although a central plateau is defi ned in each. fi ed central portion. The recoil model age, tR, is those most seriously affected by recoil effects. The age spectrum of plagioclase-rich sample calculated using the expression, We emphasize our use of the term, model, 81-7-2A is dominated by recoil of 37Ar, with with reference to this age, recognizing that the Σ low ages at low temperatures and ages above tR = (ti × fci/FC), (1) model may not always be satisfi ed. Alternative the plateau at high temperatures, defi ning a pos- approaches could involve modeling a smooth

itive sloping pattern. The high-K glassy sample where ti is the age of the ith increment, fci is the spectrum and selection of the central portion by 90-75-2 is dominated by recoil of 39Ar, with fraction of total 39Ar released in the ith incre- minimizing the error in the observed age, but ages above the plateau at low temperature and ment that falls within the speciﬁ ed central por- these approaches will be evaluated elsewhere.

below it at high temperature, deﬁ ning a nega- tion of the age spectrum, and FC is the selected 39 tive sloping pattern. In addition to statistically central fraction of Ar released. The fci value Model Ages: Excess Argon identical central plateaus, the integrated age of is a weighting factor for the portion of the age each age spectrum is indistinguishable from measured in that increment to be included in As used here, excess argon refers to 40Ar in the the plateau, demonstrating substantial recoil the integrated model age. It is calculated readily rock exceeding that generated by decay of 40K redistribution of argon without signiﬁ cant net from the cumulative fraction of 39Ar for each within components of the rock. This excludes loss (Fig. 3). From the grain sizes reported increment and the fraction of 39Ar in each of 40Ar inherited from xenocrysts or xenoliths, by Foland et al. (1993) and recoil dimensions the two tails excluded from the recoil age: where it accumulated by natural decay prior to

(Turner and Cadogan, 1974; Huneke and (1 – FC)/2. For example, if the selected central incorporation in the magma and may appear in 40 Smith, 1976; Onstott et al., 1995; Jourdan et al., fraction, FC, is 70% (or 0.7), then the fi rst 15% the age spectrum as radiogenic Ar formed in 2007), this result should not be altogether sur- (or 0.15) and the last 15% of the 39Ar released situ. Kelley (2002) provided an excellent dis- prising. Calculations similar to those of Jourdan are excluded from the model age. cussion of excess Ar and its treatment as a trace et al. (2007) show that recoil loss of 39Ar from As shown in Figure 4 and Table 1, where the element controlled by mineral-melt and mineral- equant-shaped grains should be <~0.5% for the central fraction modeled is 0.5 or 50% of the fl uid partition coeffi cients. Kelley (2002; sup- grain sizes reported for the samples in Figure 3 39Ar released, all of the 775 °C (21.62%) and ported by numerous studies cited therein), con- and barely detectable within error under most 850 °C (16.20%) steps plus 8.59% from the cluded that the dominant source of excess 40Ar circumstances. We conclude that signifi cant 700 °C increment and 3.59% from the 925 °C is from fl uid and melt inclusions incorporated in recoil redistribution of both 39Ar and 37Ar in step are within the modeled portion (totaling the rock during crystallization and/or quench- fi ne-grained crystalline and glassy aggregates 50.00%). These proportions of the measured ing. Age spectra for samples containing unsup- 40 39 40 could still result in Ar/ Ar age spectra whose ages are combined as shown above for tR to ported radiogenic Ar or excess Ar have been

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1400 07BV-G1058 Groundmass 1300°

1200 550° FC = 0.5 625° 50% Model = 853 ± 12 ka 1000

700° 800 775° 1000° 850° 925° Age (ka) 70% Model = 855 ± 17 ka 600 1075° FC = 0.7 1150°

400 Total Integrated Age = 866 ± 10 ka 200 0.0 0.2 0.4 0.6 0.8 1.0 Cumulative 39Ar Fraction

Figure 4. Age spectrum diagram for Boring lava sample 07BV-G1058 (basaltic andesite groundmass) showing the limits of 50% and 70% recoil model ages calculated as those fractions of the central portion of 39Ar released. This age spectrum is dominated by recoil of 39Ar with no plateau deﬁ ned, but a comparison of the model ages and the integrated age demonstrates their lack of sensitivity to anomalous low- or high-temperature releases, as well as to 39 37 39 bulk loss of either Ar or Ar in those increments. FC is the selected central fraction of Ar released.

TABLE 1. CENTERED RECOIL MODEL AGE CALCULATIONS FOR 07BV-G1058 GROUNDMASS Ar are calculated for a number of samples in this 39 Step ArK released f f Age mol 39Ar ci ci study, where the age defi ning the minimum of a (°C) (F = 50%) (F = 70%) (ka) (%) (cumulative %) C C well-developed saddle or U shape is considered 550 1.405E–15 3.341 3.341 0 0 1052 ± 150 625 4.178E–15 9.928 13.269 0 0 1005 ± 42 the maximum age of the sample. Where this age 700 8.547E–15 20.318 33.587 8.587 18.587 891 ± 18 is represented by more than one increment, the 775 9.095E–15 21.625 55.211 21.625 21.625 859 ± 8 model age is taken as the weighted mean. 850 6.812E–15 16.196 71.407 16.196 16.196 827 ± 10 925 5.267E–15 12.519 83.926 3.593 12.519 839 ± 18 1000 3.942E–15 9.366 93.291 0 1.074 759 ± 24 40Ar/39Ar Results 1075 1.395E–15 3.290 96.581 0 0 685 ± 55 1150 6.718E–16 1.564 98.145 0 0 674 ± 105 40 39 1300 7.876E–16 1.855 100.000 0 0 1149 ± 180 Ages determined by Ar/ Ar techniques for Weighted mean plateau age (Ma) MSWD = 2.8 846 ± 6 the Boring lavas are calculated and reported in Isochron age (Ma) MSWD = 1.7 868 ± 8 Table 21 as integrated (total gas), weighted-mean Integrated age (Ma) 866 ± 10 plateau, isochron, and modeled ages. The inte- Recoil age (Ma) (50.00%) MSWD = 3.825 853 ± 12 Recoil age (Ma) (70.00%) MSWD = 6.267 855 ± 17 grated age is calculated from the sums of all Note: Details for sample 07BV-G1058 are found with other samples in Table 2. No paleomagnetic data are radiogenic 40Ar and potassium-derived 39Ar in 39 available for this sample. fci—fraction of total Ar released in the i th increment; FC—selected central fraction of all increments of gas analyzed. A plateau age is 39Ar released (see text). MSWD—mean square of weighted deviates. defi ned as the inverse-variance weighted mean of that part of an age spectrum composed of contiguous gas fractions that together represent recognized for a long time (Dalrymple et al., under special conditions such as high pressure >50% of the total 39Ar released from the sample 1975; Lanphere and Dalrymple, 1976). Saddle- and partial resetting during metamorphism (e.g., and for which no difference in age is detected or U-shaped age spectra were fi rst recognized Foster et al., 1990; Phillips, 1991; Scaillet et al., between any two fractions at the 95% level of as typical of excess Ar (e.g., Lanphere and Dal- 1992; Scaillet, 1996). Saddle-shaped age spectra 1Table 2. To view the single-page version of rymple, 1976), but studies of excess Ar in a wide remain the most commonly observed and more Table 2, please visit the full-text article on www range of geologic environments have demon- reliably interpreted patterns, however, especially .gsapubs .org or visit http:// dx .doi .org /10 .1130 strated a complex variety of other release patterns in volcanic rocks. Model ages based on excess /GES00985 .S1.

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TABLE 2. 40Ar/39Ar AGES OF VOLCANIC ROCKS OF THE BORING VOLCANIC FIELD, OREGON AND WASHINGTON, USA 11 11 11 2,10Integrated age 3,10Plateau age Latitude Longitude Whole- Whole- Whole- Sample number 1Material rock rock rock (Ma) (Ma) 7 NAD83 NAD83 K2O CaO K/Ca Age ±1σ Age ±1σ MSWD 02BV-G10A GM 45.73237 –122.04950 0.74 8.60 0.103 1.014 0.008 1.065 0.006 1.2 02BV-G23 GM 45.65207 –122.16010 1.38 7.62 0.216 0.300 0.004 0.278 0.005 0.97 02BV-G32 GM 45.65593 –122.13570 1.17 8.41 0.166 0.235 0.006 0.260 0.006 1.11 02BV-G42B GM 45.68007 –122.13822 0.77 7.33 0.125 0.628 0.011 0.639 0.010 0.74 02BV-G90B GM 45.65147 –122.09313 0.91 8.07 0.134 0.793 0.010 none 02BV-G116A GM 45.63312 –122.10673 0.96 8.09 0.141 0.876 0.006 none 02CM-T25 WR 45.61192 –122.26407 0.85 7.29 0.139 0.698 0.008 0.693 0.009 1.01 02CM-T145 GM 45.63978 –122.40717 1.28 8.15 0.186 0.649 0.007 none 03BV-G123 GM 45.67263 –122.20835 1.64 9.11 0.214 0.087 0.005 0.128 0.004 1.8 03BV-G125 GM 45.53272 –122.08772 1.09 7.24 0.179 1.500 0.008 none 03BV-G130 GM 45.54118 –122.17337 1.22 9.28 0.157 1.972 0.013 2.308 0.014 0.91 03BV-G135 GM 45.52335 –122.18273 1.71 10.35 0.197 1.25 0.01 1.281 0.009 0.73 03BV-G146 GM 45.52977 –122.15297 0.63 9.88 0.076 2.208 0.017 none 03BV-G152 GM 45.54838 –122.01771 1.13 7.79 0.173 0.763 0.006 0.763 0.005 0.33 03BV-G154 GM 45.56338 –122.22921 1.25 7.72 0.193 0.632 0.004 0.619 0.004 0.44 03BV-G176A GM 45.50302 –122.17793 0.79 8.68 0.108 1.440 0.011 1.455 0.010 0.55 03BV-G177A GM 45.50155 –122.14167 0.81 9.09 0.106 1.318 0.006 1.381 0.008 1.9 03BV-G186 GM 45.55333 –122.12750 0.95 6.92 0.163 1.522 0.009 none 03BV-G226 GM 45.52895 –122.16420 1.23 8.26 0.177 1.232 0.008 1.253 0.006 0.70 03CM-T250 GM 45.52715 –122.33872 0.45 10.13 0.053 1.137 0.051 1.159 0.014 1.90 03CM-T253A GM 45.51773 –122.25943 1.18 9.73 0.144 2.219 0.013 none 03LM-K26A GM 45.75863 –122.00984 1.24 8.99 0.164 0.210 0.004 0.244 0.004 0.95 04BV-G194A GM 45.66937 –122.13872 0.68 8.51 0.095 0.094 0.019 0.220 0.013 0.61 04BV-G204 GM 45.63402 –122.21402 0.63 8.34 0.090 0.135 0.007 0.134 0.007 0.42 04BV-G215 GM 45.54543 –122.14182 1.25 9.16 0.163 2.263 0.016 2.270 0.006 0.60 04BV-G216 WR And 45.55387 –122.12427 0.94 7.14 0.156 1.520 0.011 none 04BV-G292B GM 45.56750 –122.14190 0.80 8.32 0.114 1.514 0.011 none 04BV-G310 GM 45.62990 –122.13513 0.85 8.27 0.122 1.162 0.010 1.140 0.010 2.1 05BV-G328C GM 45.65785 –122.20112 0.91 9.05 0.119 1.830 0.014 none 05BV-G333 GM 45.63280 –122.20578 1.22 9.86 0.147 0.834 0.014 none 05BV-G345 GM 45.67910 –122.05332 0.84 8.64 0.116 1.060 0.011 1.068 0.008 0.43 05BV-G378 GM 45.56373 –122.15192 1.02 6.83 0.177 0.981 0.008 none 05BV-G420 GM 45.66322 –122.03356 0.79 7.62 0.124 1.033 0.010 none 05BV-G450 GM 45.59511 –122.19312 1.09 7.83 0.166 0.114 0.009 0.116 0.009 0.56 06BV-G555 GM 45.57111 –122.10207 1.34 7.04 0.227 0.248 0.004 0.244 0.004 0.78 06BV-G750 GM 45.58662 –122.00273 1.03 9.80 0.126 0.915 0.007 0.928 0.005 0.66 06LM-K68 GM 45.78679 –122.03860 1.43 8.04 0.211 0.618 0.005 0.632 0.004 0.34 07BV-G810 GM 45.53520 –122.02672 1.95 9.36 0.249 1.664 0.009 1.672 0.005 1.5 07BV-G825 GM 45.56558 –122.10070 0.77 9.08 0.101 2.287 0.011 2.279 0.004 1.4 07BV-G835 GM 45.56845 –122.11512 0.66 9.81 0.080 2.339 0.015 2.311 0.009 1.3 07BV-G871 GM 45.56300 –122.07945 0.92 9.75 0.112 1.925 0.010 1.927 0.004 0.95 07BV-G881B GM 45.58653 –122.06148 1.38 9.92 0.166 1.889 0.011 1.899 0.006 1.06 07BV-G884 GM 45.58545 –122.05255 0.93 9.06 0.122 1.678 0.009 1.680 0.003 1.5 07BV-G885 GM 45.58500 –122.04987 0.76 9.07 0.100 1.074 0.007 1.062 0.006 1.3 07BV-G961A GM 45.58618 –122.01900 1.28 10.23 0.149 1.838 0.011 none 07BV-G962 GM 45.56733 –122.21037 1.00 7.57 0.157 0.123 0.004 0.126 0.003 0.70 07BV-G981 GM 45.57892 –122.09415 0.88 7.63 0.138 0.244 0.010 0.232 0.009 0.51 07BV-G997 GM 45.57132 –122.05602 0.96 7.39 0.154 0.768 0.008 0.770 0.005 1.9 07BV-G1000D GM 45.53690 –122.01217 0.75 9.63 0.093 1.691 0.009 none 07BV-G1036 GM 45.58488 –122.02623 1.33 8.13 0.195 0.962 0.006 0.945 0.005 0.67 07BV-G1058 GM 45.53733 –122.18583 1.24 8.12 0.181 0.866 0.010 none 08BV-G1084 GM 45.57850 –122.10395 0.64 8.75 0.087 0.242 0.006 0.242 0.005 0.86 08BV-G1103 GM 45.56758 –122.14378 0.94 8.96 0.125 1.499 0.010 1.503 0.008 0.67 08BV-G1116 GM 45.55428 –122.05912 0.88 7.91 0.132 1.517 0.008 none 08BV-G1177 GM 45.57112 –122.07728 0.72 8.59 0.100 1.505 0.008 none 08BV-G1185 GM 45.52447 –122.04292 1.53 8.33 0.219 1.697 0.008 none 09BV-G1252 GM 45.56486 –122.12739 1.02 7.25 0.168 0.954 0.006 none 09BV-G1318 GM 45.57072 –122.00628 0.78 8.09 0.115 0.742 0.007 0.746 0.006 0.65 09BV-G1319 GM 45.57942 –122.00028 0.71 10.02 0.084 1.901 0.010 none 99CM-T03 WR Bas 45.58628 –122.48092 1.07 7.82 0.163 0.940 0.012 none H98Y-33 WR Bas 45.80372 –122.49647 1.29 9.68 0.158 0.124 0.009 0.096 0.006 2.0 QV00-25B GM 45.65765 –122.19605 0.89 8.74 0.121 0.773 0.011 0.770 0.010 1.18 QV01-34 GM 45.55938 –122.83238 1.28 9.04 0.169 0.986 0.006 0.973 0.006 2.1 QV01-35B GM 45.62686 –122.01991 0.82 7.88 0.124 0.047 0.009 0.058 0.006 0.84 QV01-36 GM 45.63311 –122.02248 0.89 8.89 0.120 0.490 0.006 0.498 0.007 1.2 QV01-38 GM 45.62987 –122.13410 0.89 7.84 0.135 1.153 0.007 none QV01-40 GM 45.66953 –122.12520 1.64 7.56 0.259 0.250 0.004 0.280 0.005 0.84 QV01-41 GM 45.67462 –122.18495 1.32 8.33 0.189 0.068 0.006 0.109 0.007 1.1 QV01-42 GM 45.66998 –122.18655 1.72 8.67 0.236 0.068 0.009 0.135 0.005 1.9 QV01-43 GM 45.66358 –122.16030 1.27 8.21 0.185 0.281 0.006 none QV01-44 GM 45.51255 –122.78637 1.15 7.92 0.172 0.088 0.006 0.106 0.006 1.10 QV01-47 GM 45.53982 –122.37568 0.79 8.46 0.111 1.239 0.011 1.300 0.015 1.8 QV02-48B GM 45.49920 –122.56333 0.89 8.52 0.125 0.899 0.010 0.968 0.008 0.41 QV02-49 GM 45.48988 –122.51613 1.53 8.94 0.203 0.33 0.01 0.342 0.004 1.16 QV02-50 GM 45.47697 –122.50722 0.16 10.21 0.018 1.562 0.014 1.622 0.043 0.67 QV02-53 GM 45.39108 –122.49932 0.88 7.46 0.140 1.232 0.011 1.222 0.010 1.6 QV02-55C WR And 45.39770 –122.47523 0.64 7.89 0.096 2.623 0.018 none QV02-56 WR Bas 45.43448 –122.46865 0.87 7.94 0.130 0.859 0.009 none QV02-59 GM GM 45.34625 –122.53055 0.94 8.28 0.135 1.201 0.011 1.218 0.010 0.48 (continued)

Geosphere, December 2014 1289

Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/10/6/1283/3337321/1283.pdf by guest on 25 September 2021 Fleck et al.

TABLE 2. 40Ar/39Ar AGES OF VOLCANIC ROCKS OF THE BORING VOLCANIC FIELD, OREGON AND WASHINGTON, USA (continued) 4,10Isochron age 5,10Modeled age 6,10Indicated age 9Magnetic Sample number (Ma) (Ma) (Ma) polarity Age ±1σ 7MSWD Intercept (2σ) 8Type Age ±1σ 7MSWD Age ±1σ 02BV-G10A 1.013 0.065 0.69 304 ± 11 RCL 50 1.060 0.008 1.521 1.065 0.006 N 02BV-G23 0.255 0.023 1.19 299.3 ± 7.7 XS 16.7 0.264 0.009 NA 0.278 0.005 N 02BV-G32 0.264 0.010 0.17 292.4 ± 1.9 RCL 50 0.258 0.007 1.46 0.260 0.006 N 02BV-G42B 0.655 0.026 0.85 293.2 ± 6.9 RCL 70 0.643 0.010 0.742 0.639 0.010 N 02BV-G90B 0.869 0.031 4.3 289.2 ± 6.7 RCL 50 0.821 0.024 7.657 0.821 0.024 R 02BV-G116A 0.887 0.010 3.2 292.5 ± 5.3 RCL 50 0.879 0.007 2.739 0.879 0.007 R 02CM-T25 0.694 0.015 1.17 295.2 ± 3.8 RCL 50 0.691 0.009 1.11 0.693 0.009 N 02CM-T145 0.639 0.193 4.9 295 ± 46 RCL 70 0.648 0.019 7.68 0.648 0.019 03BV-G123 0.122 0.011 2.2 296.8 ± 4.0 RCL 50 0.124 0.005 0.811 0.128 0.004 N 03BV-G125 1.501 0.035 3.3 285 ± 57 RCL 70 1.498 0.010 15.465 1.498 0.010 R 03BV-G130 2.302 0.016 1.04 296.7 ± 4.1 RCL 50 2.308 0.012 1.79 2.308 0.014 R 03BV-G135 1.293 0.029 2.9 293.5 ± 8.2 RCL 50 1.281 0.008 1.97 1.281 0.009 R 03BV-G146 2.282 0.041 5.9 291.5 ± 4.8 RCL 70 2.221 0.032 7.728 2.221 0.032 R 03BV-G152 0.776 0.013 1.6 291.7 ± 7.9 RCL 50 0.763 0.005 0.330 0.763 0.005 N 03BV-G154 0.632 0.004 0.18 292.8 ± 4.3 RCL 50 0.626 0.006 2.236 0.619 0.004 N 03BV-G176A 1.469 0.019 0.68 293.2 ± 6.0 RCL 70 1.469 0.015 4.741 1.455 0.010 R 03BV-G177A 1.393 0.013 2.3 291.0 ± 4.0 RCL 50 1.377 0.008 1.854 1.381 0.008 R 03BV-G186 1.531 0.024 16 292 ± 15 RCL 50 1.513 0.017 15.365 1.513 0.017 R 03BV-G226 1.259 0.008 0.36 294.3 ± 2.7 RCL 50 1.254 0.005 0.334 1.253 0.006 03CM-T250 1.165 0.041 3.2 296.5 ± 4.0 RCL 50 1.177 0.027 4.607 1.159 0.014 R 03CM-T253A 2.295 0.039 9.2 288 ± 15 RCL 50 2.284 0.016 5.108 2.284 0.016 R 03LM-K26A 0.250 0.008 1.04 294.3 ± 3.4 RCL 50 0.241 0.005 0.670 0.244 0.004 N 04BV-G194A 0.228 0.021 0.23 295.9 ± 2.2 RCL 70 0.223 0.013 0.833 0.220 0.013 N 04BV-G204 0.138 0.013 0.61 294.9 ± 3.1 RCL 50 0.136 0.007 0.333 0.134 0.007 N 04BV-G215 2.278 0.010 1.19 293.0 ± 5.2 RCL 70 2.273 0.005 0.959 2.270 0.006 R 04BV-G216 1.536 0.040 19 286 ± 23 RCL 70 1.523 0.023 26.044 1.523 0.023 R 04BV-G292B 1.506 0.011 1.8 289.9 ± 4.6 RCL 50 1.508 0.093 2.801 1.508 0.093 04BV-G310 1.173 0.018 1.4 289.4 ± 5.1 RCL 50 1.150 0.024 8.580 1.150 0.024 R 05BV-G328C 1.024 0.061 3.2 320 ± 20 XS 28.9 1.145 0.014 NA ≤1.024 0.061 N 05BV-G333 0.892 0.033 3.0 287 ± 12 RCL 70 0.882 0.027 4.819 0.882 0.027 05BV-G345 1.074 0.012 0.49 294.5 ± 3.3 RCL 50 1.067 0.009 0.570 1.068 0.008 N 05BV-G378 0.971 0.007 1.2 290.7 ± 3.7 RCL 70 0.978 0.014 8.037 0.978 0.014 R 05BV-G420 1.071 0.021 0.35 291.1 ± 4.9 RCL 50 1.055 0.022 5.022 1.055 0.022 05BV-G450 0.114 0.014 0.62 295.8 ± 3.3 RCL 70 0.112 0.009 0.734 0.116 0.009 N 06BV-G555 0.245 0.016 4.5 295.7 ± 6.4 RCL 70 0.247 0.008 4.400 0.244 0.004 N 06BV-G750 0.936 0.012 0.84 293.1 ± 5.8 RCL 70 0.928 0.004 0.864 0.928 0.005 06LM-K68 0.635 0.004 0.29 294.2 ± 2.7 RCL 70 0.633 0.005 0.100 0.632 0.004 N 07BV-G810 1.679 0.005 0.83 292.3 ± 1.9 RCL 50 1.675 0.004 0.223 1.672 0.005 R 07BV-G825 2.281 0.008 2.0 294 ± 10 RCL 50 2.280 0.005 1.866 2.279 0.004 R 07BV-G835 2.302 0.018 2.1 297 ± 10 RCL 50 2.309 0.016 3.043 2.311 0.009 R 07BV-G871 1.934 0.009 3.4 294.3 ± 2.7 RCL 70 1.927 0.003 0.946 1.927 0.004 N 07BV-G881B 1.888 0.009 1.9 299.8 ± 7.5 RCL 70 1.901 0.005 1.168 1.899 0.006 07BV-G884 1.674 0.006 1.7 298.3 ± 5.6 RCL 50 1.680 0.003 0.365 1.680 0.003 07BV-G885 1.094 0.016 0.20 289.2 ± 5.8 RCL 50 1.073 0.011 4.367 1.062 0.006 N 07BV-G961A 1.866 0.027 1.9 301 ± 13 RCL 70 1.891 0.008 1.705 1.891 0.008 07BV-G962 0.132 0.005 0.48 293.4 ± 2.6 RCL 70 0.126 0.003 0.175 0.126 0.003 07BV-G981 0.233 0.015 0.75 295.3 ± 2.6 RCL 70 0.235 0.010 0.782 0.232 0.009 07BV-G997 0.771 0.020 2.4 295.2 ± 9.7 RCL 70 0.772 0.008 2.676 0.770 0.005 N 07BV-G1000D 1.702 0.012 25 293.0 ± 6.4 RCL 70 1.691 0.008 22.310 1.691 0.008 R 07BV-G1036 0.951 0.015 5.9 296.8 ± 6.8 RCL 70 0.950 0.005 1.726 0.945 0.005 R 07BV-G1058 0.868 0.008 1.7 288.3 ± 2.7 RCL 50 0.853 0.011 3.825 0.853 0.011 08BV-G1084 0.241 0.008 0.96 295.7 ± 1.8 RCL 70 0.245 0.005 1.013 0.242 0.005 N 08BV-G1103 1.527 0.016 0.21 291.5 ± 4.8 RCL 50 1.506 0.008 0.680 1.503 0.008 R 08BV-G1116 1.540 0.006 0.95 287.8 ± 2.3 RCL 50 1.517 0.006 2.496 1.517 0.006 08BV-G1177 1.512 0.012 0.69 288.1 ± 6.3 RCL 50 1.500 0.013 6.226 1.500 0.013 08BV-G1185 1.676 0.009 1.7 293.6 ± 2.7 RCL 70 1.687 0.012 21.561 1.687 0.012 R 09BV-G1252 0.956 0.014 11.9 294.6 ± 6.7 RCL 50 0.950 0.011 10.065 0.950 0.011 R 09BV-G1318 0.744 0.020 0.66 295.6 ± 7.7 RCL 70 0.748 0.006 0.954 0.746 0.006 N 09BV-G1319 1.913 0.009 2.1 288.4 ± 3.0 RCL 50 1.898 0.015 8.878 1.898 0.015 N 99CM-T03 0.710 0.172 37 309 ± 24 XS 57.3 0.740 0.011 0.44 ≤0.74 0.011 N H98Y-33 0.101 0.061 0.009 294 ± 41 RCL 70 0.104 0.010 3.05 0.096 0.006 N QV00-25B 0.771 0.019 1.6 295.3 ± 3.1 RCL 50 0.771 0.014 1.337 0.770 0.010 N QV01-34 0.976 0.019 9.7 297.6 ± 9.8 RCL 50 0.973 0.006 2.14 0.973 0.006 R QV01-35B 0.056 0.015 1.04 296.0 ± 4.8 RCL 70 0.057 0.007 0.643 0.058 0.006 N QV01-36 0.541 0.020 1.07 292.3 ± 2.7 RCL 50 0.506 0.008 0.793 0.498 0.007 N QV01-38 1.167 0.024 3.2 289.4 ± 8.2 RCL 50 1.161 0.026 24.9 1.161 0.026 R QV01-40 0.256 0.014 7.3 301 ± 10 RCL 50 0.282 0.008 3.5 0.280 0.005 N QV01-41 0.113 0.037 4.7 293.5 ± 8.7 RCL 50 0.111 0.008 1.098 0.109 0.007 N QV01-42 0.103 0.006 3.4 299.0 ± 1.1 RCL 70 0.135 0.007 1.905 0.135 0.005 N QV01-43 0.291 0.041 4.5 295.2 ± 3.6 RCL 70 0.286 0.010 3.113 0.286 0.010 N QV01-44 0.115 0.008 1.6 293.8 ± 1.3 RCL 50 0.105 0.009 2.946 0.106 0.006 N QV01-47 1.437 0.028 0.83 288.0 ± 2.3 RCL 50 1.292 0.019 2.356 1.292 0.019 R QV02-48B 0.962 0.017 0.44 296.4 ± 4.2 RCL 50 0.964 0.007 0.832 0.968 0.008 N QV02-49 0.348 0.005 24 294.0 ± 2.0 RCL 50 0.346 0.004 0.394 0.342 0.005 N QV02-50 1.649 0.060 0.38 294.3 ± 3.0 RCL 70 1.610 0.042 0.610 1.622 0.043 R QV02-53 1.221 0.027 5.5 297.9 ± 7.2 RCL 50 1.224 0.021 8.29 1.222 0.010 R QV02-55C 2.708 0.142 12 287 ± 32 RCL 50 2.607 0.042 7.79 2.607 0.042 R QV02-56 0.819 0.012 1.8 300.0 ± 2.5 XS 44.3 0.828 0.010 0.130 ≤0.83 0.010 N QV02-59 GM 1.222 0.012 0.61 294.7 ± 2.2 RCL 70 1.221 0.008 0.137 1.218 0.010 R (continued)

1290 Geosphere, December 2014

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TABLE 2. 40Ar/39Ar AGES OF VOLCANIC ROCKS OF THE BORING VOLCANIC FIELD, OREGON AND WASHINGTON, USA (continued) 11 11 11 2,10Integrated age 3,10Plateau age Latitude Longitude Whole- Whole- Whole- Sample number 1Material rock rock rock (Ma) (Ma) 7 NAD83 NAD83 K2O CaO K/Ca Age ±1σ Age ±1σ MSWD QV02-59 WR WR Bas 45.34625 –122.53055 0.94 8.28 0.135 1.244 0.012 1.228 0.010 0.49 QV02-66B GM 45.42167 –122.55867 0.95 7.96 0.142 0.892 0.009 0.869 0.006 0.81 QV02-71 GM 45.45562 –122.56605 0.65 9.31 0.083 1.514 0.015 1.575 0.016 1.9 QV02-76 GM 45.28578 –122.45218 0.80 8.04 0.118 3.054 0.021 none QV02-80 Run 1 GM 45.26148 –122.40537 0.73 8.22 0.105 2.622 0.034 2.591 0.028 1.04 QV02-80 Run 2 GM 45.26148 –122.40537 0.73 8.22 0.105 2.578 0.014 none QV02-83 GM 45.30517 –122.56548 0.39 8.96 0.052 2.615 0.018 2.609 0.013 1.7 QV02-94 GM 45.22933 –122.52848 0.53 8.86 0.072 2.501 0.019 2.579 0.019 1.02 QV02-99 GM 45.24678 –122.49148 0.39 9.15 0.051 2.658 0.019 none QV03-106 GM 45.51892 –122.72322 1.27 8.71 0.174 0.986 0.008 1.027 0.008 1.50 QV03-111 GM 45.46742 –122.73247 1.13 9.24 0.145 1.147 0.008 1.145 0.005 0.90 QV03-129 GM 45.51887 –122.78558 0.83 8.05 0.122 1.132 0.009 1.131 0.008 0.97 QV03-134 GM 45.41325 –122.72458 1.07 9.55 0.134 1.123 0.008 1.141 0.006 1.04 QV03-140 GM 45.44150 –122.68617 1.13 8.24 0.163 1.024 0.008 1.137 0.005 1.4 QV03-142 GM 45.38978 –122.45602 1.04 7.06 0.175 0.669 0.009 none QV03-143 GM 45.41113 –122.54305 0.43 9.32 0.055 2.741 0.018 none QV03-144B GM 45.30495 –122.49213 0.89 8.08 0.131 2.586 0.015 2.567 0.014 0.13 QV03-154 GM 45.36053 –122.53242 1.08 8.26 0.155 1.076 0.008 1.204 0.005 0.31 QV03-164 GM 45.44830 –122.16795 0.76 8.62 0.105 0.727 0.020 none QV03-167 GM 45.49375 –122.13793 0.68 7.80 0.104 0.766 0.010 none QV03-205 GM 45.29807 –122.47890 0.81 8.46 0.114 2.601 0.016 none QV03-206 GM 45.28607 –122.48723 0.81 8.23 0.118 2.667 0.019 none QV03-210B GM 45.27298 –122.47173 1.02 7.83 0.155 2.403 0.018 2.514 0.014 0.47 QV04-221 GM 45.36825 –122.54288 0.91 7.15 0.151 1.277 0.009 none QV04-231C GM 45.48503 –122.48090 0.88 8.65 0.121 0.30 0.008 0.332 0.009 0.23 QV04-257 GM 45.39875 –122.48997 1.60 8.84 0.216 0.324 0.003 none QV05-293 GM 45.45318 –122.16313 0.62 8.07 0.091 1.407 0.009 none QV06-308B GM 45.43603 –122.52568 1.00 8.41 0.142 0.775 0.010 0.799 0.006 0.66 QV11-327 GM 45.83818 –121.99705 NA NA NA 0.092 0.009 0.094 0.009 0.29 QV97-02 WR Bas 45.82920 –122.52155 1.41 9.37 0.179 0.314 0.005 none QV97-03 GM GM 45.65483 –122.46772 0.73 7.78 0.112 0.658 0.009 0.583 0.007 0.91 QV97-03 WR1 WR Bas 45.65483 –122.46772 0.73 7.78 0.112 0.621 0.008 0.627 0.008 2.3 QV97-03 WR2 WR Bas 45.65483 –122.46772 0.73 7.78 0.112 0.807 0.008 none QV97-05A WR Bas 45.62813 –122.22443 0.98 8.66 0.135 1.215 0.008 1.220 0.008 1.4 QV97-08A WR Bas 45.56620 –122.21872 0.86 7.89 0.130 0.541 0.013 none QV97-12 WR And 45.63057 –122.13798 1.40 6.99 0.239 0.270 0.005 none QV97-15 WR Bas 45.63533 –122.17635 1.02 9.61 0.126 0.909 0.010 0.929 0.007 1.03 QV98-17 Run 1 WR Bas 45.62833 –122.02191 0.86 7.64 0.135 0.056 0.008 0.056 0.006 1.8 QV98-17 Run 2 WR Bas 45.62833 –122.02191 0.86 7.64 0.135 0.093 0.004 0.084 0.005 0.82 QV99-18 Run 1 WR And 45.53950 –122.56347 1.17 7.85 0.177 0.291 0.009 none QV99-18 Run 2 GM 45.53950 –122.56347 1.17 7.85 0.177 0.310 0.012 none QV99-21B GM 45.51587 –122.59705 1.27 8.21 0.184 0.198 0.005 0.206 0.005 1.4 RC02-128 GM 45.45403 –122.46305 0.79 7.86 0.119 0.524 0.020 none RC02-129 GM 45.43765 –122.39823 0.74 7.53 0.117 0.753 0.010 none RC02-131 GM 45.46300 –122.45030 0.94 7.85 0.142 0.690 0.007 none RC02-134 GM 45.41178 –122.48623 0.84 7.90 0.126 0.752 0.020 none RC02-338 GM 45.24662 –122.45303 0.80 7.96 0.119 2.642 0.016 none RC02-343 GM 45.31117 –122.47582 0.84 7.99 0.125 2.34 0.02 2.431 0.020 0.32 RC03-122 GM 45.46212 –122.12987 0.64 8.57 0.089 0.790 0.006 0.772 0.005 0.68 RC03-130 WR And 45.49482 –122.05312 1.11 6.43 0.205 0.614 0.007 0.559 0.010 0.72 RC05-30 GM 45.46178 –122.04728 0.38 9.82 0.047 2.568 0.022 none RC05-65 GM 45.50376 –122.01145 1.35 7.04 0.228 0.658 0.005 0.664 0.004 1.7 RC05-72 GM 45.54211 –122.02964 0.91 9.38 0.116 1.670 0.011 1.669 0.007 1.5 RC08-57 GM 45.52225 –122.03177 0.65 10.20 0.076 1.911 0.011 none RC09-08 GM 45.59178 –122.00638 1.20 7.29 0.196 0.983 0.006 none RC09-23 GM 45.57672 –122.03138 1.30 8.23 0.188 0.950 0.005 0.935 0.003 1.5 RC10-06A GM 45.58895 –122.00862 1.01 9.57 0.126 0.949 0.006 0.945 0.003 1.7 RC10-17 GM 45.59583 –121.99930 1.14 8.03 0.169 0.950 0.005 0.943 0.004 2.3 S91-H213 GM 45.41607 –121.98534 1.22 7.43 0.195 0.218 0.004 0.213 0.004 1.15 Tri-Met 781+84 WR Bas 45.50623 –122.75357 1.04 7.33 0.169 0.169 0.006 0.140 0.004 1.5 Tri-Met 802+53 WR Bas 45.50800 –122.74616 1.24 8.66 0.170 1.059 0.008 none Tri-Met 804+46 WR Bas 45.50823 –122.74536 0.77 8.58 0.107 1.155 0.022 1.190 0.021 1.9 Tri-Met 807+73 WR Bas 45.50852 –122.74329 0.73 9.06 0.096 3.024 0.019 1.684 0.030 0.04 Tri-Met 809+44 WR Bas 45.50877 –122.74316 0.73 8.67 0.100 1.399 0.018 0.987 0.019 0.52 Tri-Met 813+73 (1) WR Bas 45.50905 –122.74162 1.16 8.47 0.163 1.365 0.018 1.123 0.007 0.16 Tri-Met 813+73 (2) WR Bas 45.50905 –122.74162 1.16 8.47 0.163 1.205 0.010 none Tri-Met 817+31 WR Bas 45.50927 –122.74061 0.71 8.20 0.103 1.176 0.010 1.168 0.010 1.03 Z-Fisher-2 GM 45.58920 –122.47582 — — — 0.664 0.004 0.653 0.003 1.7 Note: “None” indicates that an age was not deﬁned by this reduction technique, and a dash indicates no data. 1GM indicates groundmass separates of samples of all compositions, and WR is whole rock. “Bas” and “And” indicate basaltic and andesitic rock types, respectively. 2Integrated age is the age calculated from the sum of all radiogenic 40Ar divided by the sum of all potassium-derived 39Ar in an age-spectrum (incremental-heating) experiment. “Ma” indicates age in millions of years before the present. 3An 40Ar/39Ar plateau age is the weighted mean age of contiguous steps representing at least 50% of the potassium-derived 39Ar released in an incremental-heating 36 37 experiment and for which ages are concordant at the 95% level of conﬁ dence (Fleck et al., 1977). Current reactor production ratios include: ArCa / ArCa = 2.810 ± 0.062e-4, 39 37 38 37 40 39 38 39 ArCa / ArCa = 7.10 ± 0.50e-4, ArCa / ArCa = 3.29 ± 0.75e-5, ArK/ ArK = 1.003 ± 0.379e-3, ArK/ ArK = 1.314 ± 0.001e-2. 4The isochron age is calculated by weighted-error regression of the 40Ar/36Ar and 39Ar/36Ar of contiguous gas fractions representing at least 50% of the potassium-derived 39Ar released in an incremental-heating experiment. Ar isotopic ratios are corrected for reactor-derived interfering isotopes. 5The “Modeled Age” is calculated, as discussed in the text, for either a “Recoil Model-Age,” using the indicated percentage of 39Ar released, or for the age minimum reached in a “U-shaped,” excess-Ar-model age-spectrum. 6Indicated age is the age calculated by the reduction technique considered the most reliable of the four reported (integrated, plateau, isochron, or modeled) and shown in bold type in the table. (continued) Geosphere, December 2014 1291

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TABLE 2. 40Ar/39Ar AGES OF VOLCANIC ROCKS OF THE BORING VOLCANIC FIELD, OREGON AND WASHINGTON, USA (continued) 4,10Isochron age 5,10Modeled age 6,10Indicated age 9Magnetic Sample number (Ma) (Ma) (Ma) polarity Age ±1σ 7MSWD Intercept (2σ) 8Type Age ±1σ 7MSWD Age ±1σ QV02-59 WR 1.224 0.017 0.056 296.2 ± 5.8 RCL 70 1.228 0.009 0.851 1.228 0.010 R QV02-66B 0.868 0.006 1.5 295.8 ± 1.2 RCL 70 0.876 0.009 3.993 0.869 0.006 R QV02-71 1.617 0.037 2.1 292.2 ± 3.4 RCL 50 1.591 0.015 0.623 1.575 0.016 R QV02-76 1.329 0.152 10.9 481 ± 54 XS 41.0 2.718 0.013 0.320 ≤2.72 0.013 R QV02-80 Run 1 2.615 0.046 1.6 292 ± 10 RCL 50 2.587 0.031 0.795 2.591 0.028 R QV02-80 Run 2 2.638 0.022 1.3 283 ± 6 RCL 50 2.583 0.028 10.885 2.583 0.028 R QV02-83 2.607 0.061 1.9 296.1 ± 9.4 RCL 70 2.613 0.016 1.997 2.609 0.013 R QV02-94 2.609 0.046 0.62 291.1 ± 8.2 RCL 50 2.573 0.015 1.011 2.579 0.019 R QV02-99 2.799 0.071 4.4 287.5 ± 8.7 RCL 70 2.666 0.036 7.474 2.666 0.036 R QV03-106 1.027 0.011 2.0 295.5 ± 3.2 RCL 50 1.027 0.006 1.124 1.027 0.008 N QV03-111 1.148 0.006 1.12 294.8 ± 1.7 RCL 70 1.147 0.005 0.796 1.145 0.005 R QV03-129 1.161 0.023 6.5 293.1 ± 4.8 RCL 70 1.142 0.014 4.581 1.142 0.014 R QV03-134 1.148 0.020 0.24 294.6 ± 2.3 RCL 70 1.142 0.006 0.269 1.141 0.006 R QV03-140 1.148 0.008 1.9 292.9 ± 3.9 RCL 50 1.139 0.007 1.928 1.137 0.005 R QV03-142 0.712 0.044 3.9 289.0 ± 9.1 RCL 70 0.673 0.021 8.003 0.673 0.021 N QV03-143 1.157 0.040 1.12 384.0 ± 6.4 RCL 50 1.966 0.054 17.283 1.157 0.040 R QV03-144B 2.538 0.022 0.18 302.3 ± 8.5 RCL 50 2.576 0.009 1.997 2.567 0.014 R QV03-154 1.208 0.012 0.42 294.7 ± 3.4 RCL 50 1.204 0.006 0.110 1.204 0.005 R QV03-164 0.406 0.710 177 303 ± 50 RCL 33 1.135 0.096 9.657 1.135 0.096 R QV03-167 0.849 0.030 4.4 290.4 ± 4.3 RCL 70 0.784 0.022 6.668 0.784 0.022 T QV03-205 2.597 0.061 5.2 288±15 RCL 50 2.595 0.038 21.407 2.595 0.038 QV03-206 2.597 0.061 5.2 295.2 ± 4.8 RCL 50 2.601 0.057 20.457 2.601 0.057 N QV03-210B 2.535 0.043 7.6 289.7 ± 6.0 RCL 50 2.506 0.022 4.464 2.514 0.014 R QV04-221 1.268 0.051 21 296 ± 17 RCL 70 1.278 0.024 18.967 1.278 0.024 R QV04-231C 0.349 0.008 0.64 293.5 ± 1.5 RCL 50 0.332 0.009 0.454 0.332 0.009 QV04-257 0.348 0.006 3.6 286 ± 8 RCL 50 0.337 0.003 2.423 0.337 0.003 N QV05-293 1.415 0.011 1.6 291.4 ± 2.5 RCL 70 1.413 0.021 23.85 1.413 0.021 R QV06-308B 0.806 0.007 1.5 293.9 ± 1.3 RCL 70 0.800 0.006 0.157 0.799 0.006 QV11-327 0.095 0.021 0.24 295.6 ± 7.7 RCL 70 0.097 0.009 0.203 0.094 0.009 N QV97-02 0.112 0.023 3.6 299 ± 13 RCL 70 0.129 0.008 5.016 0.129 0.008 N QV97-03 GM 0.587 0.019 1.18 295 ± 4 RCL 70 0.586 0.008 0.337 0.583 0.007 N QV97-03 WR1 0.638 0.033 4.50 293.5 ± 6.8 RCL 50 0.617 0.018 6.255 0.627 0.008 N QV97-03 WR2 0.535 0.027 0.48 319.3 ± 8.2 XS 37.0 0.658 0.009 0.140 ≤0.658 0.009 N QV97-05A 1.224 0.026 1.8 294 ± 18 RCL 50 1.220 0.007 1.459 1.220 0.008 R QV97-08A 0.564 0.028 11.7 295.1 ± 4.3 RCL 50 0.543 0.013 3.693 0.543 0.013 N QV97-12 0.286 0.042 2.8 293 ± 13 RCL 50 0.270 0.011 3.085 0.270 0.011 N QV97-15 0.938 0.020 1.5 293 ± 12 RCL 50 0.930 0.008 1.617 0.929 0.007 R QV98-17 Run 1 0.055 0.021 2.1 295.7 ± 7.2 RCL 70 0.056 0.010 2.160 0.056 0.006 N QV98-17 Run 2 0.079 0.030 0.66 298 ± 19 RCL 50 0.086 0.005 0.484 0.084 0.005 N QV99-18 Run 1 0.338 0.048 13 293.3 ± 4.8 XS 19.3 0.133 0.025 0.96 ≤0.133 0.025 N QV99-18 Run 2 0.30 0.05 8.8 295.4 ± 4.3 XS 9.7 0.132 0.057 NA ≤0.132 0.057 N QV99-21B 0.215 0.015 1.9 294.1 ± 4.9 RCL 50 0.208 0.009 2.436 0.206 0.005 RC02-128 0.659 0.081 4.5 291.2 ± 6.9 RCL 50 0.671 0.052 0.643 0.671 0.052 N RC02-129 Invalid due to recoil RCL 50 0.893 0.025 8.124 0.893 0.025 N RC02-131 0.676 0.023 6.0 294.1 ± 4.2 RCL 50 0.670 0.017 10.05 0.670 0.017 N RC02-134 0.822 0.051 3.5 289 ± 5 RCL 70 0.770 0.050 7.093 0.770 0.050 N RC02-338 2.607 0.071 4.9 291 ± 12 RCL 50 2.624 0.042 20.51 2.624 0.042 R RC02-343 2.444 0.061 0.74 294.4 ± 7.3 RCL 50 2.434 0.018 0.352 2.431 0.020 R RC03-122 0.772 0.018 1.02 295.6 ± 7.5 RCL 30 0.775 0.020 18.64 0.772 0.005 T RC03-130 0.594 0.059 0.80 292.3 ± 5.0 RCL 50 0.607 0.028 8.432 0.607 0.028 N RC05-30 2.597 0.071 9.3 294.1 ± 9.6 RCL 50 2.559 0.047 8.076 2.559 0.047 R RC05-65 0.681 0.007 1.6 288.7 ± 4.0 RCL 70 0.657 0.008 5.709 0.657 0.008 N RC05-72 1.695 0.012 3.9 293.0 ± 4.4 RCL 50 1.683 0.013 5.649 1.683 0.013 R RC08-57 none RCL 50 1.952 0.012 2.406 1.952 0.012 N RC09-08 0.983 0.033 5.0 288 ± 24 RCL 50 0.984 0.021 41.07 0.984 0.021 RC09-23 0.948 0.008 1.08 290 ± 5.0 RCL 50 0.941 0.004 2.064 0.941 0.004 RC10-06A 0.931 0.015 1.8 301 ± 13 RCL 70 0.945 0.006 2.791 0.945 0.003 RC10-17 0.972 0.018 6.7 290.5 ± 8.8 RCL 50 0.951 0.008 5.430 0.951 0.008 S91-H213 0.215 0.015 1.5 295 ± 3 RCL 50 0.214 0.004 0.970 0.213 0.004 N Tri-Met 781+84 0.122 0.008 1.04 299.1± 2.8 RCL 50 0.136 0.005 1.40 0.122 0.008 N Tri-Met 802+53 1.042 0.031 43 295.6 ± 4.6 RCL 50 1.047 0.019 4.614 1.047 0.019 N Tri-Met 804+46 1.237 0.071 3.4 294.9 ± 5.7 RCL 50 1.195 0.047 4.287 1.190 0.021 R Tri-Met 807+73 1.491 0.081 42 322 ± 13 XS 33.7 1.686 0.030 0.039 ≤1.69 0.030 R Tri-Met 809+44 0.969 0.035 8.1 296.0 ± 1.8 XS 71.8 0.983 0.018 0.666 0.987 0.019 N Tri-Met 813+73 (1) 1.045 0.101 81 320 ± 47 XS 43.1 1.123 0.007 0.212 ≤1.045 0.101 N Tri-Met 813+73 (2) 1.063 0.031 5.9 316 ± 11 XS 16.8 1.075 0.020 NA ≤1.075 0.020 N Tri-Met 817+31 1.186 0.015 0.61 293.9 ± 2.4 RCL 50 1.174 0.009 0.864 1.168 0.010 R Z-Fisher-2 0.656 0.007 1.1 293.5 ± 4.1 RCL 50 0.650 0.004 2.625 0.653 0.003 N 7MSWD represents mean square of weighted deviates, a measure of goodness of fit, comparing the observed scatter to that expected from calculated analytical errors (McIntyre et al., 1966). 8Type of Model Age: RCL 50 = Recoil model age of central portion of 39Ar released, percentage shown; XS = Excess 40Ar age minimum of U-shaped age spectrum, percentage of 39Ar represented shown. 9Magnetic polarity of the samples: N = Normal, R = Reversed, or T = Transitional. A blank in this column indicates that the magnetic polarity of the sampled unit was not determined. 10All ages are calculated to a monitor age equivalent to an age of 28.02 Ma for Fish Canyon Tuff sanidine. 11 K2O and CaO concentrations were determined using X-Ray Fluorescence in the Peter Hooper GeoAnalytical Laboratory at Washington State University. Intensities were measured with a Thermo-ARL Advant’XP+ spectrometer on low-dilution glass pellets made using methods described in Johnson et al. (1999). The spectrometer was calibrated with 105 certifi ed reference materials; matrix corrections to concentrations for all major and minor elements were calculated iteratively using infl uence coeffi cients

derived from the NBSGSC database. Two sigma uncertainties in K2O and CaO values estimated from hundreds of replicate experiments are 0.015 and 0.043 wt %, respectively. Molar K/Ca values were calculated from the whole-rock K2O and CaO values.

1292 Geosphere, December 2014

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conﬁ dence (Fleck et al., 1977). An isochron σ age is calculated using the algorithm of York ±1 40 36 39 36

(1968) for Ar/ Ar versus Ar/ Ar correlation (Ma) with conventions adopted by Ludwig (2003). Indicated age Modeled ages, discussed herein, represent the 6 age calculated for a speciﬁ ed fraction of an age = 1.003 ± 0.379e-3,

spectrum, based on a speciﬁ c model for the ori- K 3.043 3.126 0.054 3.79 3.228 0.052 6.87 3.228 0.025 0.231 3.052 0.027 7.761 2.942 0.077 1.892 3.548 0.030 79 13.99 0.21 24.35 14.17 0.15 53.47 13.42 0.16 32.43 13.70 0.02 15.42 3.109 0.042 Ar MSWD Age 114.6 13.86 0.02 7 gin of that pattern. Columns in Table 2 labeled 39 / K σ

“Indicated age” report the age determined to be Ar 40 ±1

the best age estimate for the sample from evalu- Ar represented shown. ation of the different analytical approaches and 39 (Ma) from consideration of relevant geologic, paleo- type in the table. 3.126 0.053 3.228 0.051 3.109 0.042 3.228 0.025 3.052 0.027 3.548 0.030 2.942 0.077 Modeled age Ar released in an incremental-heating 5 13.99 0.21 13.42 0.16 14.17 0.15 13.6913.65 0.14 0.10 bold magnetic, petrologic, and chemical results. 39 Tables of incremental heating results of individ- = 3.29 ± 0.75e-5, Type Age

ual samples summarized in Table 2 are available Ca 8

2 Ar

in the Supplemental Table . 37 / ) Our results identify two groups of dated Ca σ Ar

olivine-phyric ﬂ ows that were previously con- 38 sidered part of the BVF, but represent older volcanic activity. The earliest group consists of lavas that yield ages of ca. 13.6 Ma and belong Intercept (2

to inadequately studied Miocene sedimentary from calculated analytical errors (McIntyre et al., 1966). 40 39 = 7.10 ± 0.50e-4, MSWD and volcanic units (Table 3). The Ar/ Ar (Ma) 7 Ca , most not derived locally. Ar results for these Miocene rocks are reported Isochron age 4 σ 37 Ar released, or for the age minimum reached in a “U-shaped,” excess-Ar-model / lateau, isochron, or modeled) and shown in Ar released in an incremental-heating experiment and for which ages are concordant at here because of past confusion concerning their 39 Ca 39

relationship with the BVF, but the geology of Ar 39 Ar age minimum of U-shaped spectrum, percentage

these older units is not part of this paper. Ar in an age-spectrum (incremental-heating) experiment. 40 The second group of (largely) pre-Boring 39 volcanic rocks consists of more voluminous, far- traveled, predominantly low-potassium tholeiite 1.00 3.195 0.132 0.73 293.6 ± 4.3 70 RCL 3.109 0.075 0.588 3.058 0.074 1.8 3.142 0.029 2.4 279 ± 13 50 RCL 3.086 0.023 2.77 3.088 0.009 1.9 1.942 0.020 1.8 298.2 ± 6.5 70 RCL 1.965 0.015 0.751 1.964 0.010

ﬂ ows that were sourced in the High Cascades MSWD Age ±1 7

to the east and followed drainages, including = 2.810 ± 0.062e-4, Ca σ

the ancestral Columbia River, into the Portland Ar ±1 (Ma) 37 Basin (Fig. 2). These rocks, which yield ages / Ca Plateau age 3

between ca. 3.6 Ma and 2.9 Ma (and one age of Ar 36 Ar of contiguous gas fractions representing at least 50% the potassium-derived Age 36 3.088 0.009 1.964 0.010 1.96 Ma) (Table 3), are excluded from the BVF 3.058 0.074 Ar/

because their vents are outside the deﬁ ned area. 39

These are not to be confused with locally erupted σ ±1

low-potassium Boring lavas that are part of the Ar and 36

ﬁ eld and for which results are shown in Table 2. Ar released, percentage shown; XS = Excess (Ma) Ar/ 39 Ar divided by the sum of all potassium-derived 40 Results for early low-potassium tholeiite lavas 40 Integrated age are included in Tables 3 and 43 despite their erup- 2 tion from distant vents because they occur within the area of the BVF, bear on the local stratigraphy, and their ages and paleomagnetic directions were measured as part of this study. The earliest eruptions of the Boring lavas are Ar AGES OF SAMPLES PREDATING OR ORIGINATING OUTSIDE THE BORING VOLCANIC FIELD, OREGON AND WASHINGTON, USA AND WASHINGTON, THE BORING VOLCANIC FIELD, OREGON OUTSIDE OR ORIGINATING AGES OF SAMPLES PREDATING Ar

typiﬁ ed by sample QV02-83 (Fig. 5), basalt from 39 Ar/

southeast of Oregon City and west of Estacada, 40 NAD83 NAD83 Age that yields a plateau-type age spectrum and an age Latitude Longitude

2Supplemental Table. Boring Lava 40Ar/39Ar Incre- TABLE 3. TABLE mental-Heating Data: Ages and J based on Taylor Creek Rhyolite Sanidine = 27.87 Ma. If you are dence (Fleck et al., 1977). Current reactor production ratios include: viewing the PDF of this paper or reading it ofﬂ ine, Geologic unit 1 please visit http://dx .doi .org /10 .1130 /GES00985 .S2 or the full-text article on www .gsapubs .org to view

the Supplemental Table. Ar plateau age is the weighted mean of contiguous steps representing at least 50% potassium-derived 39

3 = 1.314 ± 0.001e-2. Table 4. To view the single-page version of K Ar/ Ar 40

Table 4, please visit the full-text article on www 39 / K

.gsapubs .org or visit http:// dx .doi .org /10 .1130 indicates pre-Boring, low-potassium tholeiitic lava from Cascade arc Miocene units include volcanic and sedimentary strata. LKT Integrated age is the calculated from sum of all radiogenic An The isochron age is calculated by weighted-error regression of the Age” is calculated, as discussed in the text, for either a “Recoil Model-Age,” using indicated percentage of The “Modeled Indicated age is the calculated by reduction technique considered most reliable of four reported (integrated, p Deviates, a measure of goodness ﬁt, comparing the observed scatter to that expected MSWD represents Mean-Square of Weighted 50 = Recoil model age of central portion Age: RCL of Model Type 1 2 3 4 5 6 7 8 Ar experiment. Ar isotopic ratios are corrected for reactor-derived interfering isotopes. experiment. the 95% level of conﬁ age-spectrum. Sample number QV01-46B unit Miocene 45.38468 –122.20472 13.47 0.08 none 13.40 0.09 8.3 278 ± 18 50 RCL 38 QV01-46 Miocene unit 45.38468 –122.20472 13.92 0.07 none 13.97 0.14 3.5 255 ± 20 70 RCL QV03-135 Miocene unit 45.39712 –122.70057 13.70 0.02 none 13.61 0.08 3.1 258 ± 18 50 RCL QV03-104A Miocene unit 45.25225 –122.30177 13.77 0.09 none 14.23 0.18 18 ± 20 280 70 RCL RC02-179 Miocene unit 45.25690 –122.28197 13.87 0.11 none 13.86 0.02 121 261 ± 40 50 RCL 03BV-G133B LKT 45.54078 –122.17037 3.014 0.085 none 3.165 0.091 10.1 290.1 ± 9.4 50 RCL 05BV-G462 LKT 45.50017 –122.08620 3.272 0.021 none 3.267 0.051 5.3 292.7 ± 5.5 50 RCL ALDCK-1 Pre-Boring 45.31750 –122.05792 3.050 0.018 CLRV-1QV04-256 LKT LKT 45.17662 45.41087 –122.14892 –122.21388 2.004 3.117 0.015 0.021 none 3.195 0.051 3.7 ± 12 282 70 RCL BLRN-1 LKT 45.44015 –122.22379 3.155 0.094 none 3.269 0.034 4.3 286 ± 10 70 RCL BVCK-1 LKT 45.54498 –122.16970 3.151 0.073 RC02-187 LKT 45.19698 –122.33115 2.993 0.024 none 3.053 0.061 7.8 291.3 ± 7.9 50 RCL RC02-203 LKT 45.22295 –122.36257 2.62 0.03 none 3.073 0.091 3.3 289 ± 10 90 RCL /GES00985 .S3. WSP LKT 45.68199 –121.63699 3.523 0.024 none 3.580 0.061 7.5 294.2 ± 8.1 70 RCL

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1.0 1.5 1250 0.3 02CM-T145 1145 06BV-G750 QV01-44 1400 0.8 550

600 950 Plateau Age = 0.106 ± 0.006 Ma 700 1085 1.0 700 750 800 850 0.2 1025 650 1200 0.6 775 910 MSWD = 1.10 970 900 625 850 81% gas released 1000 775 0.4 1000 1350 850 925 0.5 1075 0.1 700 Recoil Age = 0.648 ± 0.019 Ma Plateau Age = 0.928 ± 0.005 Ma Apparent Age (Ma) 0.2 MSWD = 0.66 1100 550 MSWD = 7.68 92% gas released 70% gas released 1175 625 550 0.0 0.0 0.0 2.0 1.5 1.5 1400 03BV-G186 QV01-34 QV02-56 1400 1200 550 1.8 625 925 1.0 700 775 850 1075 1.0 1250 1.6 1000 550 950 1100 625 700 625 1125 850 925 1000 626 775 700 750 800 1.4 875 1025 0.5 1200 0.5 Age Minimum = 0.828 ± 0.010 Ma Apparent Age (Ma) Plateau Age = 0.973 ± 0.006 Ma 1100 MSWD = 0.13 1.2 Recoil Age = 1.513 ± 0.017 MSWD = 2.1 MSWD = 15.37 63% gas released 44% gas released 50% gas released 550 1.0 0.0 0.0 1.5 0.3 3.0 03CM-T250 QV01-35B QV02-83 1250 2.8 925 1000 850 925 1100 775 1075 Plateau Age = 0.058 ± 0.006 Ma 1.0 700 0.2 MSWD = 0.84 1150 960 2.6 850 1400 MSWD = 95% gas released 700 625 1250 625 775 1000 Plateau Age = 1.159 ± 0.014 Ma 2.4 0.5 0.1 625 1025 MSWD = 1.90 775 840 900 550 Plateau Age = 2.609 ± 0.013 Ma

Apparent Age (Ma) 57% gas released 700 2.2 MSWD = 1.70 95% gas released 1100 550 550 1200 0.0 0.0 2.0 2.0 0.5 3.0 04BV-G292B QV01-40 QV02-94 1150

1.8 0.4 2.8 550 665 725 950 1075 625 1300 900 1.6 0.3 700 800 850 2.6 625 1000 850 700 775 601 775 925 1075 1000 850 925 600 1100 1250 1.4 1000 0.2 2.4 1150 Plateau Age = 2.579 ± 0.019 Ma Recoil Age = 1.508 ± 0.093 Ma Plateau Age = 0.280 ± 0.005 Ma 1400 1.2 0.1 MSWD = 0.84 2.2 MSWD = 1.02 MSWD = 2.80 88% gas released 50% gas released 53% gas released 550 550 1.0 0.0 2.0 1.5 0.3 1.5 1400 1100 1400 05BV-G333 QV01-42 1025 QV03-129

625 700 1200 1250 775 1.0 0.2 850 925 875 775 1000 1050 700 725 975 850 825 925 1.0 625 775 650 Recoil Age = 1.142 ± 0.014 Ma 925 550 0.5 0.1 MSWD = 4.58 1100

Apparent Age (Ma)Apparent Age (Ma) Apparent 1100 Recoil Age = 0.882 ± 0.027 Ma Plateau Age = 0.135 ± 0.005 70% gas released MSWD = 4.82 MSWD = 1.90 550 MSWD = 70% gas released 600 69% gas released 0.0 0.0 550 1200 0.5 0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0 Cumulative 39Ar released Cumulative 39Ar released Cumulative 39Ar released

Figure 5 (on this and following page). The 40Ar/39Ar age spectra of selected samples of the Boring volcanic ﬁ eld lavas. Those portions of age spectra used in weighted mean plateau ages, recoil model ages, or age minima are shown by lines with arrows. The placement of the arrows and the percentages cited indicate the portion of the total 39Ar released that is included in that age calculation. The mean square of weighted deviates (MSWD; see Table 2 footnote) increases dramatically with the slope of points in inclined model ages. In these cases it provides little measure of precision or the validity of the model used, but may be useful in determining the appropriate central portion.

of 2.609 ± 0.013 Ma (Table 2). Small amounts of Na-rich groundmass plagioclase) to those releas- 17 , QV01-35B) (Fig. 5). The Beacon Rock sam- gas, released in the earliest and latest increments, ing at high temperature (e.g., Ca-rich pyroxene). ples were collected at two localities and analyzed probably represent recoil redistribution of small The youngest samples analyzed in this study are three times. The ﬁ rst was analyzed twice as a amounts of 37Ar during neutron irradiation from from the Beacon Rock volcanic neck along the whole-rock sample and yielded widely disparate sites releasing argon at lower temperature (e.g., Columbia River east of Multnomah Falls (QV98- ages of 55.8 ± 6.1 ka and 84.2 ± 5.1 ka. A second

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3.0 1.2 QV03-144B RC02-129 1275 2.8 1400 1150 700 775 1150 700 0.8 625 775 850 2.6 850 925 1000 1075 925 1000

1075 2.4 Plateau Age = 2.567 ± 0.014 Ma Recoil Age = 0.893 ± 0.025 Ma 0.4 MSWD = 0.13 MSWD = 8.12 1350 50% gas released Apparent Age (Ma) 2.2 55% gas released 625

550 550 2.0 0.0 2.0 1.5 QV03-164 RC02-134

700 1150 775 1.0 1.0 650 850 700 750 800 925 1075 600 1100 850 900 950 1000 550 1000 0.0 550 Recoil Age = 1.135 ± 0.096 Ma 0.5 MSWD = 9.66 Recoil Age = 0.770 ± 0.050 Ma Apparent Age (Ma) 33% gas released MSWD = 7.09 1050 70% gas released

-1.0 0.0 1.0 30

550 625 QV03-167 Tri-Met 807+73 Run 1 700 775 0.8 850 1400 925 20 450 1000 500 Age Minimum = 1.686 ± 0.030 Ma 1200 0.6 MSWD = 0.039 34% gas released 10 Recoil Age = 0.784 ± 0.022 Ma 1130 0.4 MSWD = 6.67 520 1250 570601635 680 730 780 830 880 930 980 1080 70% gas released 1030 1100 0 Apparent Age (Ma) 0.2

0.0 -10 0.3 15 QV98-17 Run 1 Tri-Met 809+44

0.2 Plateau Age = 0.056 ± 0.006 Ma 1200 MSWD = 1.80 10 100% gas released 480 Plateau Age = 0.987 ± 0.019 Ma 1150 1060 1160 1100 0.1 MSWD = 0.52 1275 60% gas released 900 1050 675 825 5 Apparent Age (Ma) 1350 750 0.0 975 1400 1005 982 1025 570 635 675 720 760 800 840 885 930960 600 530 600 980 985 1020 -0.1 0 975 0.3 3.0475 1400 QV98-17 Run 2 500 TriMet 813+73 1200

Plateau Age = 0.084 ± 0.005 Ma 2.5 600 Age Minimum = 1.123 ± 0.007 Ma 0.2 MSWD = 0.82 550 75% gas released MSWD = 0.21 36% gas released 1200 2.0 550 675 750 1100

975 600 0.1 825 900 1050 Apparent Age (Ma) 1.5 625 1025 690 930 730 810 890 975 655 770 850 0.0 1100 1.0 0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0

Cumulative 39Ar released Cumulative 39Ar released

Figure 5 (continued).

sample was collected and analyzed as a ground- complex patterns that may not be successfully ate. As should be expected, recoil model ages are mass separate and yielded an age of 58.4 ± 6.4 ka interpreted. The age patterns for samples 06BV- virtually identical to the plateaus. Age spectra that is in agreement with the younger of the two G750, QV01-34, and QV03-129 (Fig. 5) exhibit of samples such as 03BV-G186, 04BV-G292B, whole-rock analyses. This age is accepted as the age plateaus in their central portions with ages QV03-167, and RC02-134 (Fig. 5), however, most reliable estimate of the age of the unit. above the plateau in low-temperature steps and show negatively inclined patterns typical of severe The remainder of this section relates to rep- below it in high-temperature fractions in a classic 39Ar recoil redistribution with no age plateaus resentative age spectra of samples characteristic 39Ar recoil pattern (see Fig. 3). The well-defi ned defi ned, but the ages are also well constrained. of specifi c patterns, ranging from plateaus, to plateaus show no evidence of age discordance, The ages of the fi rst two are well controlled as positively or negatively inclined spectra, to more however, and recoil effects are minor to moder- part of a group of four samples located ~5 km

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northwest of Larch Mountain that have the same Tri-Met 813+73 (Robertson light rail tunnel netic mineralogy in many of these rocks is most reversed magnetic polarity and nearly the same west of Portland, i.e., “Tri-Met tunnel”; Fig. 5; likely some form of titanomagnetite. Where AF stratigraphic position. Recoil model ages and Walsh et al., 2011). Where the magnitude of demagnetization is incomplete and a signifi cant integrated ages of all four are within analytical the saddle is great, however, the probability amount of remanent intensity remains (Figs. error at 1.5 Ma (Table 2). The last two samples of excess Ar is much more likely. The pres- 6C, 6E), titanohematite is also likely present. represent lavas erupted very close to the Brunhes- ence in whole-rock samples of large amounts The characteristic magnetizations are inferred Matuyama reversal at 0.781 Ma (Ogg, 2012). of coarse-grained olivine, a common source to be primary thermal remanent magnetizations Normal polarity sample RC02-134 was erupted of excess Ar, undoubtedly contributed to this (TRM) acquired during initial cooling of the at 0.770 ± 0.050 Ma, shortly after the reversal, effect in those samples. rock unit, and this inference is corroborated whereas sample QV03-167, with an age of Sample-related complexities also contribute by the excellent correspondence of observed 0.784 ± 0.022 Ma, is from a unit with a transi- to a few age spectra that defy interpretation, paleomagnetic polarities with the geomag- tional magnetic orientation (discussed herein as probably due to recoil with substantial net loss netic polarity time scale (Table 4; Ogg, 2012). perhaps the best constrained example of agree- of the recoiling nuclides. Sample 05BV-G333 Due to the absence of measurable posteruptive ment between recoil model ages and magnetic (Fig. 5; Table 2) has no plateau, a disparate inte- deformation in the region, no structural cor- polarity). grated age, and no paleomagnetic sampling for rections were applied to the in situ site-mean Recoil redistribution of 37Ar is also apparent control. Low ages in both early and late incre- directions. in samples of the BVF. Age spectra for samples ments suggest net recoil loss. Sample QV03- Site-mean directions for the BVF (Table 4) 02CM-T145, 03CM-T250, and QV01-42 defi ne 164 shows large amounts of both 39Ar and 37Ar are shown in Figure 7. For the most part, dis- positively inclined patterns typical of 37Ar recoil, recoil producing an enormous range in ages, persal of these directions about the long-term with younger ages in the low-temperature steps between 0.13 and 1.4 Ma (Fig. 5; Table 2). The dipole direction is associated with paleosecular and older ages at high temperature (Fig. 5). recoil model age of 1.13 ± 0.10 Ma is consistent variation of the geomagnetic fi eld resolvable As noted here, these cannot be confused with with its reversed polarity, but little confi dence by the paleomagnetic method over centuries to results of 40Ar loss, which cannot produce ages can be attached. Sample RC02-129 also shows millennia. Such changes in direction, along with less than zero, and whose positively inclined low ages in both early and late increments and geologic mapping and geochemical studies, age spectra either fl atten (small to moderate large amounts of recoil redistribution (Fig. 5; have been used to correlate elements within the loss) or steepen (large to extreme loss) mono- Table 2). Its isochron age is invalid due to recoil Boring units, but this analysis will be the sub- tonically with increasing temperature (Turner, and its recoil model age and integrated age are ject of a separate report. Here, a unit’s geomag- 1968). In two of these three examples the recoil discordant. The integrated age for this sample netic polarity (normal, reversed, transitional, of 37Ar out of sites where it would be released is more consistent with its normal polarity, but or excursional) and associated 40Ar/39Ar ages, at low temperature results in total 40Ar/36Ar together with the other two samples discussed particularly those near polarity boundaries, are below that of atmosphere and negative apparent here provides evidence that recoil may result in compared with an established polarity time ages. In each of these three samples signifi cant age spectra that are not interpretable. scale (Ogg, 2012) to identify any inconsisten- amounts of 37Ar are redistributed to sites where cies between the paleomagnetic and geochrono- it is released at high temperatures, resulting in PALEOMAGNETISM OF THE BVF logic methods. One likely excursional direction undercorrection for atmospheric 40Ar and ages (Table 4; Fig. 7) was found in a hand sample well above the true age of the sample. Age Techniques used in paleomagnetic sampling from a fl ow dated as 1.500 ± 0.013 Ma, within spectra such as those of samples QV01-40 and and measurement of the Boring volcanic rocks the Matuyama polarity chron. QV01-44 (Fig. 5) yield central plateaus, but are provided in Appendix 2. Although paleomagnetic directions can be exhibit negative ages in both lowest and highest affected signifi cantly by ancient (and uncon- temperature intervals. These indicate substantial Paleomagnetism Results strained) magnetic anomalies and terrain effects bulk loss of 37Ar, rather than redistribution, even that deformed the local geomagnetic fi eld at in high-temperature sites where total 40Ar/36Ar is The natural remanent magnetization (NRM) the time of a rock unit’s cooling and magnetic also below that of atmosphere. These cannot be intensities for samples of the Boring lavas and acquisition (Baag et al., 1995), the paleomag- accounted for by redistributed 39Ar because no intrusions, measured prior to any demagnetiza- netic polarity is far more robust and within a amount of 39Ar can result in statistically nega- tion treatments, are ~1–0.1 A/m. Both of the geochronologic framework is considered the tive ages. Negative 40Ar/39Ar ages at high tem- alternating fi eld (AF) demagnetization proce- most reliable indicator of the rock unit’s rel- peratures indicate that improper corrections of dures were successful in defi ning stable char- ative age. atmospheric 40Ar are being made as a result of acteristic directions of magnetization for the bulk loss of 37Ar. rock samples (Fig. 6). Pilot and later stepwise DISCUSSION Complex age spectra such as those of sam- demagnetizations indicate that the specimens ples QV02-56, QV02-94, and QV03-144B contain primarily univectorial magnetizations Constraints on the Reliability of (Fig. 5) defi ne patterns with modest saddle or that form linear components on orthogonal vec- Measured Ages U shapes characteristic of excess Ar. Where tor plots, which decay toward the origin with amounts of possible excess are modest, recoil increasing demagnetization. Signifi cant sec- Despite complexities introduced by sample- of both 39Ar and 37Ar, or bulk loss of 39Ar, pos- ondary magnetic overprints were rare (Fig. 6G), related issues such as fi ne grain size and pheno- sibly with some high-temperature release of but those found might be due to local reheating cryst- and/or fl uid-inclusion–borne excess Ar, redistributed 37Ar, might produce these pat- from subsequent dike emplacement. Success 40Ar/39Ar geochronology of the Boring lavas is terns. True excess Ar patterns, however, are also of AF demagnetization treatments in reduc- remarkably successful. The 40Ar/39Ar age spec- identifi ed in Boring lava samples such as tunnel ing NRM intensities to <10% of their initial tra, isochron, recoil or excess Ar model ages, samples Tri-Met 807+73, Tri-Met 809+44, and values by 80–100 mT indicates that the mag- and integrated (total gas) ages were calculated

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TABLE 4. PALEOMAGNETIC MEASUREMENTS OF VOLCANIC ROCKS WITHIN THE CONFINES OF THE BORING VOLCANIC FIELD, OREGON AND WASHINGTON, USA

1 Paleomagnetic samples 3 3 Unit age 2 Inc Dec 4 Unit name or volcanic center Latitude Longitude Ar sample N/No (ka) ID (°) (°)

Chron Event NAD83 NAD83 T3374 45.62640 –122.01970 — 72.0 27.3 9/9 Basaltic andesite of Beacon Rock 58 ± 6* — — — QV98-17 Run 1 — — — — — — QV98-17 Run 2 — — — — — — QV01-35B — — — Basalt of Trout Creek Hill 94 ± 9 BL8097 45.75478 –122.83220 QV11-327 53.6 314.4 8/8 T2287 45.51255 –122.78637 QV01-44 48.9 357.7 8/8 Basalt of Barnes Road 106 ± 6 4T064 45.49375 –122.77808 — 38.8 333.3 7/8 T8065 45.82920 –122.52155 QV97-02 70.3 0.4 8/8 Basalt of Battle Ground 108 ± 16 ———H98Y-33——— Basaltic andesite of Canyon Creek 116 ± 9 T5049 45.59511 –122.19312 05BV-G450 44.4 2.5 8/8 Tri-Met 781+84 Polarity information from Walsh et al. (2011) Basaltic andesite of Elk Point 122 ± 8 T2319 45.50620 –122.75328 — 45.5 354.5 8/8 T3004 45.66917 –122.19527 — 32.1 1.4 8/8 T3012 45.66800 –122.19437 — 47.1 29.2 8/8 Basalt of Texas Creek 127 ± 6 — — — QV01-41 — — — — — — QV01-42 — — — ———03BV-G123——— T0260 45.53950 –122.56347 QV99-18 Run 1 59.1 1.8 7/8 Basaltic andesite of Rocky Butte ≤133 T0260 45.53950 –122.56347 QV99-18 Run 2 — — — ———04BV-G204——— Basaltic andesite of Vogel Creek 134 ± 7 T0276 45.63568 –122.21387 — 48.0 5.1 9/9 T6169 45.41607 –121.98534 S91-H213 66.1 354.9 8/8 Basaltic andesite of Aschoff Buttes 213 ± 4 T6161 45.39957 –122.02237 — 60.1 349.5 8/8 Basaltic andesite of Elevator Shaft 242 ± 5 BL1011 45.57850 –122.10395 08BV-G1084 41.7 330.5 1/1 ———06BV-G555——— Andesite of Multnomah Basin 244 ± 4 BL8017 45.56775 –122.09228 — 45.4 355.3 8/8 T7074 45.75863 –122.00984 03LM-K26A 75.7 358.6 8/8 Basaltic andesite of North Fork Rock 244 ± 4 Creek T7066 45.75869 –122.01049 — 76.8 350.8 8/8 T7058 45.76809 –122.02254 — 74.9 18.6 8/8 T3020 45.66953 –122.12520 QV01-40 61.5 290.9 8/8 Basaltic andesite of Hard Scramble 268 ± 12 T3052 45.65593 –122.13570 02BV-G32 80.6 355.8 8/8 Creek T3044 45.66109 –122.12719 — 71.5 303.6 8/8

Brunhes T5033 45.66937 –122.13872 04BV-G194A 77.9 308.8 7/8 Andesite of McCloskey Creek 270 ± 11 T0285 45.63057 –122.13798 QV97-12 67.9 307.4 7/8 T3028 45.66358 –122.16030 QV01-43 84.6 102.8 8/8 4T056 45.65207 –122.16010 02BV-G23 79.0 310.7 8/8 Basaltic andesite of Boyles Creek 280 ± 4 T5017 45.64583 –122.14993 — 80.7 308.0 7/8 T5025 45.66498 –122.14882 — 77.6 306.8 8/8 T5041 45.64948 –122.14365 — 76.1 307.5 8/8 ———QV04-257——— Basalt of Carver 337 ± 3 BL8073 45.39722 –122.49577 — 60.4 5.1 7/8 Basalt of Powell Butte 342 ± 5 T2271 45.48988 –122.51613 QV02-49 50.4 10.0 8/8 — — — QV01-36 — — — Basalt of Little Beacon Rock 498 ± 7 T3383 45.63361 –122.02183 — 54.7 353.6 8/8 T6319 45.64670 –122.03120 — 64.9 12.6 8/8 Andesite of Latourelle Prairie 607 ± 28 T5294 45.49482 –122.05312 RC03-130 56.1 335.5 8/8 T8089 45.56620 –122.21872 QV97-08A 40.2 10.3 9/9 Basaltic andesite of Mount Pleasant 612 ± 21* — — — 03BV-154 — — — T8073 45.65483 –122.46772 QV97-03 58.4 349.1 8/8 Basaltic andesite of Green Mountain 627 ± 8 — — — QV97-03 — — — Basaltic andesite of Mowich Butte 632 ± 4 BL1004 45.77708 –122.04250 06LM-K68 40.1 335.7 1/1 Basaltic andesite of Dougan Creek 639 ± 10 4T025 45.68007 –122.13822 02BV-G42B 72.8 9.0 7/7 Basaltic andesite east of North Fork BL8129 45.50347 –122.02320 — 53.0 335.9 8/8 657 ± 8 Bull Run River T5302 45.50376 –122.01145 RC05-65 65.5 352.2 8/8 T3080 45.46300 –122.45030 RC02-131 41.9 14.1 4/8 Basaltic andesite of Rodlun Road 670 ± 17 A3001 45.46900 –122.41900 — 39.7 6.8 12/12 Basalt of Borges Road 671 ± 52 T3072 45.45403 –122.46305 RC02-128 74.0 13.1 8/8 Basaltic andesite of Hardscrabble 673 ± 21 A3013 45.38978 –122.45602 QV03-142 39.3 12.6 12/12 quarry — — — 02CM-T25 — — — Andesite of Mount Norway 693 ± 9 T6231 45.60612 –122.25223 — 38.1 2.6 8/8 Basaltic andesite of Prune Hill ≤740 T0268 45.58628 –122.48092 99CM-T03 49.2 22.8 7/8 (continued)

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Brunhes 6.9872 470 2.8 56.1 204.2 N 220 ± 13 Andesite of McCloskey Creek 6.9764 254 3.8 55.4 176.2 N 270 ± 11 7.9811 370 2.9 42.4 252.0 N 286 ± 10 7.9396 116 5.2 56.3 208.2 N 278 ± 5 Basaltic andesite of Boyles Creek 6.9127 69 7.3 54.5 212.9 N — 7.9728 258 3.5 55.4 203.3 N — 7.9672 214 3.8 56.1 198.8 N — — — — — — N 337 ± 3 Basalt of Carver 6.9802 303 3.5 84.5 13.2 N — Basalt of Powell Butte 7.9731 260 3.4 73.7 25.5 N 342 ± 5 — — — — — N 498 ± 7 Basalt of Little Beacon Rock 7.9061 75 6.5 78.5 85.4 N — 7.9653 202 3.9 81.2 315.5 N — Andesite of Latourelle Prairie 7.9415 120 5.1 69.6 130.9 N 607 ± 28 8.9142 93 5.4 65.9 34.1 N 543 ± 13 Basaltic andesite of Mount Pleasant — — — — — N 619 ± 4 7.9815 377 2.9 79.6 112.0 N 627 ± 8 Basaltic andesite of Green Mountain —————N≤658 ± 9 Basaltic andesite of Mowich Butte 1.0000 na na 59.8 106.8 N 632 ± 4 Basaltic andesite of Dougan Creek 6.9293 85 6.6 76.4 258.3 N 639 ± 10 Basaltic andesite east of North Fork 7.9866 522 2.4 68.0 123.5 N — Bull Run River 7.9796 343 3.0 84.2 173.0 N 657 ± 8 3.9895 286 5.5 65.8 24.7 N 670 ± 17 Basaltic andesite of Rodlun Road 11.9877 897 1.5 66.4 41.7 N — Basalt of Borges Road 7.9917 847 1.9 73.4 260.6 N 671 ± 52 Basaltic andesite of Hardscrabble 11.9756 451 2.1 64.7 29.4 N 673 ± 21 quarry —————N693 ± 9 Andesite of Mount Norway 7.9193 87 6.0 65.9 51.8 N — Basaltic andesite of Prune Hill 6.9699 199 4.3 66.4 0.6 N ≤740 ± 11 (continued)

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TABLE 4. PALEOMAGNETIC MEASUREMENTS OF VOLCANIC ROCKS WITHIN THE CONFINES OF THE BORING VOLCANIC FIELD, OREGON AND WASHINGTON, USA (continued)

1 Paleomagnetic samples 3 3 Unit age 2 Inc Dec 4 Unit name or volcanic center Latitude Longitude Ar sample N/No (ka) ID (°) (°)

Chron Event NAD83 NAD83 BL8113 45.54838 –122.01772 03BV-G152 64.4 3.6 8/8 Basaltic andesite of Palmer Peak 761 ± 7 — — — 07BV-G997 — — — — — — 09BV-G1318 — — — T5278 45.55767 –122.02328 — 70.3 15.3 8/8 Basaltic andesite of Damascus 770 ± 50 A3025 45.41178 –122.48623 RC02-134 39.8 8.3 12/12 Basalt of Bobs Mountain 770 ± 10 T6119 45.65765 –122.19605 QV00-25B 63.9 340.0 8/8 Brunhes Basaltic andesite of Winston Road ≤828 A3061 45.43448 –122.46865 QV02-56 78.6 4.1 11/12 T5153 45.49375 –122.13793 QV03-167 55.0 195.5 8/8 Basaltic andesite of Lookout Point 773 ± 5 BL9001 45.46212 –122.12987 RC03-122 62.3 179.5 9/9

Basaltic andesite of Mount Talbert 869 ± 6 4T104 45.42167 –122.55867 QV02-66B –51.0 189.4 8/8 ———02BV-G90B——— Basaltic andesite of Duncan Creek 874 ± 16 4T040 45.65117 –122.09190 — –66.4 174.9 8/8 — — — 02BV-G116A — — — Basalt of Tote Road 929 ± 7 4T032 45.63533 –122.17635 QV97-15 –37.5 167.3 8/8 Basaltic andesite of Yeon Mountain 945 ± 5 BL9026 45.58488 –122.02623 07BV-G1036 –64.2 169.2 8/8 BL8041 45.56373 –122.15192 05BV-G378 –54.5 153.3 8/8 Basaltic andesite of Devils Rest 961 ± 14 — — — 09BV-G1252 — — — Basalt of Kaiser Road 973 ± 6 T2295 45.55938 –122.83238 QV01-34 –71.7 273.7 7/8

Basaltic andesite of Kelly Butte 968 ± 8 4T256 45.49920 –122.56333 QV02-48B 65.8 306.9 8/8 T6225 45.65785 –122.20112 05BV-G328C 54.9 0.7 4/8 Basaltic andesite of Wildboy Creek ≤1024 T5073 45.65765 –122.19838 — 43.4 11.0 6/8 Basaltic andesite of Burnside Road 1027 ± 8 4T088 45.51892 –122.72322 QV03-106 68.0 320.3 6/8 Basalt of Sunset Hill 1047 ± 19 — — — Tri-Met 802+53 Polarity information from Walsh et al. (2011) Basalt of Upper Horsetail Creek 1062 ± 6 BL1007 45.58500 –122.04987 07BV-G885 40.5 79.7 1/1 Jaramillo Basaltic andesite of Three Corner 1065 ± 6 4T048 45.73237 –122.04950 02BV-G10A 46.8 19.9 7/8 Rock Basaltic andesite of Woodward — — — 05BV-G345 — — — 1068 ± 8 Creek T6217 45.68325 –122.05423 — 48.5 28.1 8/8

— — — QV03-140 — — — 4T192 45.46742 –122.73247 QV03-111 –64.4 164.7 8/8 Basalt of Mount Sylvania 1141 ± 3 4T001 45.41325 –122.72458 QV03-134 –82.6 149.5 8/8 4T017 45.44783 –122.72892 — –65.5 159.4 7/8 4T096 45.43825 –122.69133 — –58.9 152.2 8/8 — — — QV03-129 — — — Matuyama Basaltic andesite of Bonney Slope 1142 ± 14 4T080 45.52653 –122.79958 — –45.7 189.3 8/8 4T072 45.49788 –122.75588 — –44.7 191.3 7/8 T3036 45.62987 –122.13410 QV01-38 –55.3 178.5 8/8 Basaltic andesite of Fletcher Flat 1155 ± 18 — — — 04BV-G310 — — — Basalt of Chamberlain Hill 1159 ± 14 T5105 45.52715 –122.33872 03CM-T250 –57.9 182.7 8/8 Basalt of Cornell Mountain 1190 ± 21 — — — Tri-Met 804+46 Polarity information from Walsh et al. (2011) T2231 45.34625 –122.53055 QV02-59 –56.2 168.7 7/8 Basaltic andesite of Hunsinger 1207 ± 6 T3088 45.36695 –122.52338 — –75.9 171.8 8/8 4T128 45.33962 –122.53640 QV03-154 –45.7 163.4 4/8 T8081 45.62813 –122.22443 QV97-05A –58.5 180.9 8/8 Basaltic andesite of Bear Prairie 1220 ± 8 T0293 45.61653 –122.30115 — –67.7 195.3 7/8 Basaltic andesite of Outlook 1222 ± 10 A3049 45.39108 –122.49932 QV02-53 –56.1 161.6 12/12 Basaltic andesite of Holcomb Creek 1278 ± 24 4T200 45.36825 –122.54288 QV04-221 –60.4 154.2 8/8 Basalt of Pepper Mountain 1281 ± 9 4T152 45.52335 –122.18273 03BV-G135 –60.8 201.7 6/7 Basaltic andesite of Broughton Bluff 1292 ± 19 T2279 45.53982 –122.37568 QV01-47 –54.6 193.0 8/8 Basalt of S Fork Gordon Creek 1381 ± 8 T5145 45.50155 –122.14167 03BV-G177A –52.9 186.3 7/8 T5184 45.44830 –122.16795 — –65.7 193.0 8/8 Basaltic andesite of Trout Creek 1413 ± 21 — — — QV05-293 — — — T5169 45.47690 –122.26923 — –62.5 166.0 6/8 T3280 45.48848 –122.31010 — –59.4 189.9 8/8 Basaltic andesite of Gordon Creek 1455 ± 10 T5129 45.50302 –122.17793 03BV-G176A –57.1 175.6 8/8 Basaltic andesite of Franklin Ridge 1500 ± 13 BL1012 45.57112 –122.07728 08BV-G1177 19.8 179.5 1/1 Basaltic andesite east of Angels — — — 08BV-G1103 — — — 1503 ± 8 Rest BL8049 45.56732 –122.14252 — –47.8 184.7 8/8 (continued)

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TABLE 4. PALEOMAGNETIC MEASUREMENTS OF VOLCANIC ROCKS WITHIN THE CONFINES OF THE BORING VOLCANIC FIELD, OREGON AND WASHINGTON, USA (continued) 5VGP 7Age Unit name or volcanic center 4R 4k 4α 6Polarity 8Expected polarity 95 Latitude Longitude (ka) Chron Event 7.9888 628 2.2 87.4 311.7 N 763 ± 5 Basaltic andesite of Palmer Peak — — — — — N 770 ± 5 — — — — — N 746 ± 6 7.9692 227 3.7 76.8 280.5 N — Basaltic andesite of Damascus 11.9810 578 1.8 66.2 38.3 N 770 ± 50 Basalt of Bobs Mountain 7.9598 174 4.2 76.1 154.7 N 770 ± 10 Brunhes Basaltic andesite of Winston Road 10.9705 339 2.5 67.3 241.5 N ≤828 ± 10 7.9524 147 4.6 7.8 45.2 T 784 ± 22 Basaltic andesite of Lookout Point 8.9272 110 4.9 0.9 58.2 T 772 ± 5 781 Basaltic andesite of Mount Talbert 7.9952 1451 1.5 74.4 26.2 R 869 ± 6 — — — — — R 821 ± 24 Basaltic andesite of Duncan Creek 7.9801 352 3.0 85.3 192.7 R — —————R879 ± 7 Basalt of Tote Road 7.9653 202 3.9 63.2 85.0 R 929 ± 7 R Basaltic andesite of Yeon Mountain 7.9623 186 4.1 82.5 154.8 R 945 ± 5 7.7729 31 11.8 67.2 129.7 R 978 ± 14 Basaltic andesite of Devils Rest — — — — — R 950 ± 11 Basalt of Kaiser Road 6.9922 769 2.2 34.8 260.7 R 973 ± 6 988 Basaltic andesite of Kelly Butte 7.9882 593 2.3 54.3 171.2 N 968 ± 8 3.8920 28 17.7 79.8 54.6 N ≤1024 ± 61 Basaltic andesite of Wildboy Creek 5.9770 218 4.6 67.8 30.6 N — Basaltic andesite of Burnside Road 5.9834 300 3.9 63.4 173.9 N 1027 ± 8 Basalt of Sunset Hill Polarity information from Walsh et al. (2011) N 1047 ± 19 Basalt of Upper Horsetail Creek 1.0000 na na 23.3 318.1 N 1062 ± 6 Jaramillo Basaltic andesite of Three Corner 6.7788 27 11.8 66.3 9.6 N 1065 ± 6 Rock Basaltic andesite of Woodward — — — — — N 1068 ± 8 Creek 7.9785 326 3.1 62.7 354.6 N — 1072 — — — — — R 1137 ± 5 7.9729 258 3.5 79.3 156.8 R 1145 ± 5 Basalt of Mount Sylvania 7.9607 178 4.2 57.3 223.7 R 1141 ± 6 6.9811 318 3.4 75.7 163.3 R — 7.9561 159 4.4 68.8 141.4 R — — — — — — R 1142 ± 14 Matuyama Basaltic andesite of Bonney Slope 7.9910 776 2.0 70.1 32.1 R — 6.9561 137 5.2 68.8 28.2 R — 7.9603 176 4.2 80.1 65.0 R 1161 ± 26 Basaltic andesite of Fletcher Flat — — — — — R 1150 ± 24 Basalt of Chamberlain Hill 7.9812 373 2.9 82.7 40.8 R 1159 ± 14 Basalt of Cornell Mountain Polarity information from Walsh et al. (2011) R 1190 ± 21 6.9734 226 4.0 77.9 73.8 R 1218 ± 10 Basaltic andesite of Hunsinger 7.8127 37 9.2 71.5 225.8 R — 3.9937 477 4.2 67.5 99.2 R 1204 ± 5 7.9891 643 2.2 83.6 51.5 R 1220 ± 8 R Basaltic andesite of Bear Prairie 6.9853 408 3.0 78.6 295.7 R — Basaltic andesite of Outlook 11.9883 940 1.4 73.7 121.7 R 1222 ± 10 Basaltic andesite of Holcomb Creek 7.9351 108 5.4 70.9 144.5 R 1278 ± 24 Basalt of Pepper Mountain 5.9637 138 5.7 73.9 333.4 R 1281 ± 9 Basaltic andesite of Broughton Bluff 7.9847 459 2.6 75.6 115.1 R 1292 ± 19 Basalt of S Fork Gordon Creek 6.9898 588 2.5 77.0 33.7 R 1381 ± 8 7.9300 100 5.6 80.8 307.9 R — Basaltic andesite of Trout Creek — — — — — R 1413 ± 21 5.9836 306 3.8 79.9 144.5 R — 7.9888 623 2.2 81.0 0.3 R — Basaltic andesite of Gordon Creek 7.9892 648 2.2 81.5 82.3 R 1455 ± 10 Basaltic andesite of Franklin Ridge 1.0000 na na 34.2 58.5 E 1500 ± 13 Basaltic andesite east of Angels — — — — — R 1503 ± 8 Rest 7.9771 306 3.2 72.9 43.7 R — (continued)

1300 Geosphere, December 2014

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TABLE 4. PALEOMAGNETIC MEASUREMENTS OF VOLCANIC ROCKS WITHIN THE CONFINES OF THE BORING VOLCANIC FIELD, OREGON AND WASHINGTON, USA (continued)

1 Paleomagnetic samples 3 3 Unit age 2 Inc Dec 4 Unit name or volcanic center Latitude Longitude Ar sample N/No (ka) ID (°) (°)

Chron Event NAD83 NAD83 T5270 45.53283 –122.15100 03BV-G125 –53.6 185.7 8/8 Andesite of Larch Mountain 1504 ± 8 ———03BV-G186——— T5121 45.55387 –122.12427 04BV-G216 –47.1 190.3 7/8 4T159 45.53732 –122.11537 — –49.3 181.7 7/8 T3256 45.41113 –122.54305 QV03-143 –53.2 184.1 8/8 T3060 45.45562 –122.56605 QV02-71 –49.9 199.9 7/8 T3068 45.45562 –122.56605 — –58.4 194.8 4/4 Basalt of Mount Scott 1574 ± 14 4T240 45.44025 –122.56797 — –54.2 177.4 8/8 4T248 45.46888 –122.55993 — –43.2 165.9 7/8 T2255 45.41275 –122.52422 — –54.7 205.7 8/8 4T112 45.44317 –122.54905 — –57.8 170.8 8/8 T2263 45.47697 –122.50722 QV02-50 –71.8 173.6 8/8 Basalt of Sycamore 1622 ± 43 T3264 45.42742 –122.49277 — –57.3 192.6 8/8 Basaltic andesite east of Larch BL8105 45.53522 –122.02672 07BV-G810 –54.1 207.7 8/8 1674 ± 5 Mountain BL9018 45.52447 –122.04292 08BV-G1185 –55.8 195.4 8/8 Basaltic andesite of Bell Creek 1683 ± 13 T6279 45.54211 –122.02963 RC05-72 –63.7 196.5 8/8 Basaltic Andesite of North Fork Bull 1691 ± 8 BL9010 45.53690 –122.01217 07BV-G1000D –57.8 211.8 8/8 Run River

BL1005 45.58653 –122.06148 07BVG881B 76.6 330.4 1/1 Basalt of Tumalt Creek 1896 ± 5 — — — 07BVG961A — — — Basalt of Horsetail Creek 1898 ± 15 BL1008 45.57942 –122.00028 09BV-G1319 75.2 324.7 1/1 ———07BV-G871———

Olduvai Basaltic andesite of Oneonta Creek 1927 ± 4 BL8001 45.56325 –122.09387 — 53.5 0.6 5/8 Basalt of Deer Creek 1952 ± 12 BL1010 45.52225 –122.03177 RC08-57 51.8 12.4 1/1

———CLRV-1——— Low-potassium tholeiite 1964 ± 10 T5208 45.17912 –122.15208 –73.0 166.8 5/8 4T167 45.52977 –122.15297 03BV-G146 –61.6 172.3 7/8 Basalt of North Fork Gordon Creek 2221 ± 32 T5137 45.50142 –122.13893 — –58.6 174.0 8/8 4T136 45.54118 –122.17337 03BV-G130 –61.4 181.4 8/8 Basalt of Bridal Veil Creek 2277 ± 9 — — — 03CM-T253A** — — — Matuyama T7146 45.54543 –122.14182 04BV-G215 –55.2 189.9 7/8 4T272 45.51882 –122.25687 — –64.0 174.6 8/8 Basaltic andesite of Multnomah BL8033 45.56562 –122.09945 07BV-G825 –62.7 177.7 8/8 2284 ± 12 Basin Road ———07BV-G835——— 4T232 45.31117 –122.47582 RC02-343 –62.8 182.4 7/8 Basaltic andesite of Fischers Mill 2431 ± 20 T6009 45.32217 –122.46547 — –57.4 184.7 8/8 T3161 45.33983 –122.43963 — –38.2 198.5 7/8 — — — QV03-210B — — — Basaltic andesite of Root Creek 2514 ± 14 T2223 45.27245 –122.50463 — –53.0 184.4 8/8 Low-potassium tholeiite 2559 ± 47 T6185 45.46178 –122.04728 RC05-30 –67.9 179.0 8/8 — — — QV03-144B — — — Basaltic andesite of Four Corners 2567 ± 14 4T224 45.30445 –122.49235 — –51.0 171.3 4/8 T3169 45.32098 –122.48060 — –56.6 195.5 8/8 T3112 45.23045 –122.58430 — –58.8 193.7 7/8 Basaltic andesite of Fallsview 2579 ± 19 T5081 45.25188 –122.52190 — –61.3 163.9 8/8 T3120 45.22933 –122.52848 QV02-94 –55.0 192.7 8/9 T3145 45.26148 –122.40537 QV02-80 –60.4 203.0 8/8 Basaltic andesite of Highland Butte 2596 ± 23 T3129 45.24662 –122.45303 RC02-338 –53.7 198.6 8/8 — — — QV03-206 — — — Basaltic andesite of Beaver Creek 2601 ± 57 T3153 45.28578 –122.45218 QV02-76 –55.7 206.5 6/8 T5089 45.28292 –122.49823 –41.6 181.6 7/8 4T120 45.39770 –122.47523 QV02–55C –56.2 178.7 8/8 Andesite of Tong Road 2607 ± 42 T2247 45.39678 –122.47898 –59.0 177.3 7/8 — — — QV02-99 — — — — — — QV02-83 — — — T3096 45.28787 –122.54222 — –52.1 184.5 8/8 Basalt of Canemah 2616 ± 18 T3137 45.26212 –122.47612 — –52.8 190.5 8/8 4T208 45.17958 –122.50173 — –31.0 162.8 8/8 T2239 45.34883 –122.60997 — –51.5 192.1 8/8 T3104 45.24237 –122.58547 — –52.7 189.9 8/8

(continued)

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Olduvai Basaltic andesite of Oneonta Creek 4.9944 708 2.9 78.5 55.4 N — Basalt of Deer Creek 1.0000 na na 73.8 17.5 N 1952 ± 12 1945 — — — — — R 1964 ± 10 Low-potassium tholeiite 4.9676 123 6.9 74.4 211.6 R — 6.9618 157 4.8 83.8 123.8 R 2221 ± 32 Basalt of North Fork Gordon Creek 7.9938 1129 1.7 82.4 95.6 R — 7.9495 139 4.7 86.4 41.3 R 2308 ± 14 Basalt of Bridal Veil Creek — — — — — R 2284 ± 16 Matuyama 6.9839 374 3.1 77.7 17.1 R 2270 ± 6 7.9317 102 5.5 86.2 153.0 R — Basaltic andesite of Multnomah 7.9680 219 3.8 87.8 106.5 R 2279 ± 4 Basin Road — — — — — R 2311 ± 9 6.9571 140 5.1 87.9 1.1 R 2431 ± 20 Basaltic andesite of Fischers Mill 7.9470 132 4.8 81.9 30.3 R — 6.9563 137 5.2 61.7 19.0 R — — — — — — R 2514 ± 14 Basaltic andesite of Root Creek 7.9613 181 4.1 77.8 140.1 R — Low-potassium tholeiite 7.9803 355 2.9 84.5 231.4 R 2559 ± 47 — — — — — R 2567 ± 14 Basaltic andesite of Four Corners 3.9755 122 8.3 74.8 87.0 R — R 7.9615 182 4.1 75.8 357.0 R — 6.9872 468 2.8 78.4 352.1 R — Basaltic andesite of Fallsview 7.9705 237 3.6 78.1 139.7 R — 7.9392 115 5.2 76.3 8.3 R 2579 ± 19 7.9771 305 3.2 72.9 332.5 R 2583 ± 28 Basaltic andesite of Highland Butte 7.9368 111 5.3 72.0 359.0 R 2624 ± 42 — — — — — R 2601 ± 57 Basaltic andesite of Beaver Creek 5.9667 150 5.5 68.2 342.6 R ≤2718 ± 13 6.9682 188 4.4 68.6 53.6 R — 7.9753 283 3.3 81.3 64.7 R 2607 ± 42 Andesite of Tong Road 6.9649 171 4.6 84.0 102.4 R — — — — — — R 2666 ± 36 — — — — — R 2609 ± 13 7.9621 185 4.1 77.0 40.4 R — Basalt of Canemah 7.9730 259 3.5 75.6 19.6 R — — [–32,–3] [6.9,17.3] 58.1 89.9 R — 7.9899 693 2.1 73.8 162.0 R — 7.9581 167 4.3 75.8 21.7 R — 2581 (continued)

1302 Geosphere, December 2014

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TABLE 4. PALEOMAGNETIC MEASUREMENTS OF VOLCANIC ROCKS WITHIN THE CONFINES OF THE BORING VOLCANIC FIELD, OREGON AND WASHINGTON, USA (continued)

1 Paleomagnetic samples 3 3 Unit age 2 Inc Dec 4 Unit name or volcanic center Latitude Longitude Ar sample N/No (ka) ID (°) (°)

Chron Event NAD83 NAD83 No representative units

T3185 45.22295 –122.36257 RC02-203 –56.9 223.7 8/8 Low-potassium tholeiite 3055 ± 29 4T216 45.19698 –122.33115 RC02-187 –61.5 198.2 8/8 T5192 45.41087 –122.21388 QV04-256 –81.2 182.6 8/8 Kaena Basalt of McIntyre Ridge 3088 ± 9 T6017 45.31750 –122.05792 ALDCK-1 –60.4 195.6 8/8 Gauss

4T175 45.54498 –122.16970 BVCK-1 54.5 10.9 5/9 Low-potassium tholeiite 3105 ± 41 T5113 45.54078 –122.17037 03BV-G133B 54.3 2.2 7/8 Low-potassium tholeiite 3228 ± 27 T3217 45.44015 –122.22379 BLRN-1 57.0 29.6 8/8 Note: Red text indicates magnetically reversed samples, and green text indicates transitional magnetizations. Dash indicates no data. 1Unit age is the indicated age of single samples or the weighted mean age of multiple samples, except where a discordant value is rejected. Signiﬁcant discordance is marked with an asterisk (*). 2Sample Locations for Ar samples in Table 2. Where age and paleomagnetic data are shown on the same line, sample locations are the same except where samples marked with asterisks (**). 3Inclination (Inc) and declination (Dec) of site mean paleomagnetic direction in degrees. (continued)

for nearly 150 samples (Table 2). Plateaus were magnetic results provide the strongest indepen- consistent with an age of ca. 28.2 Ma for Fish defi ned by data for 92 of these, representing dent constraints on the measured ages through Canyon sanidine (Kuiper et al., 2008; Rivera 62% of those studied. Recoil model ages in all comparison to the time scale of geomagnetic et al., 2011) were used, these ages would be fur- 92 samples are concordant with these plateau reversals (Ogg, 2012; shown graphically in ther increased, suggesting that the Ogg (2012) ages (i.e., agree within the quoted 2σ uncertain- Fig. 8 and in the fi nal column of Table 4). Dated value of 2.581 Ma may underesti mate the best ties). Although plateaus are not always precisely samples near the Brunhes, Matuyama, and age for that boundary. The third age inconsis- central, they commonly contain most of the Gauss boundaries agree within uncertainties tent with its polarity is the 0.968 ± 0.008 Ma age same steps included in the recoil model age, so (Table 4; Fig. 6); 6 samples are within 20 k.y. measured for the normally magnetized basaltic this is not unexpected. The 40Ar/39Ar isochron of the Brunhes-Matuyama boundary, 2 of which andesite of Kelly Butte. This age is ~20 k.y. ages were calculated for all but two samples, have transitional magnetic directions. Of the 8 younger than the age given by Ogg (2012) for and all except one of these are also concordant samples within 20 k.y. of the Matuyama-Gauss the end of the Jaramillo event, slightly greater with their plateaus for similar reasons (Table 2). boundary, 6 also have consistent polarity. The than the 2σ uncertainty. Because inconsistencies It is important that, where no 40Ar/39Ar plateau agreement is also refl ected in samples erupted in the data set are so limited, the most probable was defi ned, recoil model ages and isochron during brief periods of the appropriate polarity explanation is that propagated analytical errors ages that exclude strongly divergent steps were in the Jaramillo, Olduvai, and Kaena geomag- inadequately represent the total uncertainties also concordant for all samples except those few netic polarity events. Two reversed samples plot in those samples. We conclude that 40Ar/39Ar with extreme recoil effects or excess Ar. This within 15 k.y. after the Jaramillo normal polarity ages reported here, including recoil model ages indicates that recoil ages are a good proxy for event and 11 samples of reversed polarity closely calculated as discussed herein, agree well with plateaus, and that both the recoil model age and bracket the brief 1.185–1.173 Ma Cobb Moun- paleomagnetic constraints and ages on the same the isochron age may represent the age of the tain normal event, which was not represented in units with no apparent age bias. sample in the absence of an age plateau where our sampling. The Olduvai event is represented they agree within a 2σ level of uncertainty. The by the basaltic andesite of Oneonta Creek Age and Geographic Trends in the BVF value of recoil model ages in rocks like those (Tables 2 and 4), and the Kaena event is well rep- of the BVF, however, is demonstrated by the resented by low-potassium tholeiites assigned Figure 8 presents a relative probability plot Lookout Point basaltic andesite, which has by Peck et al. (1964) to the volcanic rocks of the of the 40Ar/39Ar indicated-age results from transitional magnetic polarity (Table 4). The High Cascade Range (Tables 3 and 4). Of the Table 2. Measured ages included are shown in isochron age of sample QV03-167 from this dated samples for which paleomagnetic results the upper diagram with their uncertainties. Vol- unit is 0.849 ± 0.030 Ma and not within 2σ of are available (Table 4), only three yield ages canic rocks with eruptive vents outside the BVF a known magnetic reversal. The corresponding that are inconsistent with their observed paleo- and Miocene units (Table 3) are not included in recoil model age, however, is 0.784 ± 0.022 Ma magnetic polarity beyond their 2σ uncertainties the fi gure or in the following discussion. Dated (Fig. 5) and compares to an age of 0.781 Ma for calculated following the approach of Ludwig samples, covering ~80% of the identifi ed vol- the Brunhes-Matuyama boundary (Ogg, 2012). (2003). The ages of two of these three samples, canic centers in the BVF, document persistent A second sample of the unit was less affected from the basalt of Canemah, are statistically but intermittent small-volume volcanic activity by recoil, yielding plateau, isochron, and recoil within the uppermost part of the Gauss, but have in the Portland Basin during the past 2.7 m.y. model ages that are consistent with the bound- reversed polarity (Table 4). The measured ages The fi gure includes more than one sample from ary age (Table 4). of four other reversed polarity samples of the a center in several cases, but the broad cover- Although geologic mapping, petrology, earliest lavas of the BVF are also older than the age of eruptive centers lends credence to the and geochemistry provide constraints on ages 2.581 Ma age accepted as the Matuyama-Gauss representative nature of the data set. Based on through correlation of units, stratigraphic posi- boundary, although these others are not statisti- our sampling, peak activity within the BVF tion, and structural level, as shown above, paleo- cally different at an uncertainty of 2σ. If J values occurred between ca. 1.3 and 0.5 Ma, most of

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TABLE 4. PALEOMAGNETIC MEASUREMENTS OF VOLCANIC ROCKS WITHIN THE CONFINES OF THE BORING VOLCANIC FIELD, OREGON AND WASHINGTON, USA (continued) 5VGP 7Age Unit name or volcanic center 4R 4k 4α 6Polarity 8Expected polarity 95 Latitude Longitude (ka) Chron Event No representative units 3032 7.9751 281 3.3 56.7 325.3 R 2942 ± 77 Low-potassium tholeiite 7.9877 569 2.3 76.7 332.2 R 3052 ± 27 R 7.9941 1184 1.6 62.6 239.5 R 3109 ± 49 Kaena Basalt of McIntyre Ridge 7.9768 302 3.2 78.0 341.7 R 3088 ± 9 Gauss 3116 4.9648 114 7.2 76.6 15.9 N 3058 ± 74 Low-potassium tholeiite 6.9777 269 3.7 79.1 48.2 N 3126 ± 50 Low-potassium tholeiite 7.9706 238 3.6 66.7 337.0 N 3228 ± 27 4 α Number of samples analyzed (N) over number of samples collected (N0). R is the vector sum of N unit vectors, k is the concentration parameter of Fisher (1953), and 95 is the radius of 95% conﬁdence (na = not available). 5VGP is virtual geomagnetic pole calculated from the site-mean direction. 6Magnetic polarity of the samples: N = Normal, R = Reversed, T = Transitional, E = Excursional. 7Ages from Table 2 (in ka, with 1-sigma uncertainties). 8Solid where Normal, vertically lined where Transitional, and R where Reversed. Ages shown for paleomagnetic boundaries are from Ogg (2012). Unrepresented polarity events only shown where necessary for clarity.

which was erupted from monogenetic centers Evarts et al. (2009a) summarized the distri- was reported (Fleck et al., 2002; Evarts et al., (Treasher, 1942a, 1942b; Allen, 1975). Breaks bution and age of the volcanic centers within 2009a). To evaluate spatial, temporal, and com- of 100 k.y. or more in the record of dated vol- the BVF, based on many of the results cited positional trends more thoroughly, we elected to canic activity occurred between 2.4 and 2.3 Ma, herein. The beginning of BVF volcanism in the subdivide the data set into age groups of suffi - 2.2 and 2.0 Ma, 1.9 and 1.7 Ma, and 500 and southern part of the fi eld with a northward and cient size to provide some measure of statistical 350 ka (Fig. 8). westward migration of activity within the fi eld relevance, but adequately spaced in time to eval-

A T3374 Brunhes B T5153 transitional C 4T104 Matuyama D 4T256 Jaramillo

Figure 6. Orthogonal vector W, UP W, UP W, UP W, UP plots of alternating ﬁ eld (AF) S N NRM demagnetization results for S N 1E-3 A/m Boring lava samples from different polarity intervals (see 1E-2 A/m S N Table 4). Solid circles indicate

projection onto the horizontal 1E-2 A/m plane and open circles indicate

projection onto the vertical 1E-3 A/m plane. NRM is natural remanent magnetization end point S N for each diagram. Demagneti- NRM zation steps are 2.5, 5, 10, 15, NRM NRM E, DOWN 20, 30, 40, 50, 60, 70, and 80 mT. E, DOWN E, DOWN E, DOWN Diagrams primarily show the decay of univectorial magnetizations, which are inferred to be E 4T080 Matuyama F BL8001 Olduvai G 4T216 Gauss

primary thermoremanent mag- W, UP W, UP W, UP netizations acquired during ini- NRM tial cooling. Samples from site S N BL8001 (F) had both normal NRM S N and transitional directions, and 1E-3 A/m a transitional one is shown here. (transitional) In G, the primary reversed magnetization acquired dur- 1E-1 A/m ing the Kaena event has been 1E-2 A/m overprinted by a normal component, possibly during a ther- S N mal event related to subsequent NRM dike emplacement. E, DOWN E, DOWN E, DOWN

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uate spatial change. Using observed breaks in N the age data as indicative of true breaks in eruptive activity seemed the most logical approach, but using all the breaks noted here resulted in groups of 5 and 6 ages as well as groups of 31 and 91 ages. To provide a more representative distribution of ages a more arbitrary subdivi- sion was necessary, utilizing shorter breaks in dated activity and combining early periods of infrequent activity. The following age groups Boring were selected, and are shown in Figure 8: Volcanic Field (1) 2.7–2.1 Ma, (2) 2.1–1.3 Ma, (3) 1.3 Ma to 900 ka, (4) 900–450 ka, (5) 450–200 ka, and 270 90 (6) 200–0 ka. Although admittedly subjective, Polarity even more arbitrary divisions of the data set normal gave similar results, including ones based on transitional/excursional dividing the data into 6 groups of equal numbers reversed of ages and into 2 equal groups based on composition (discussed herein). When locations of dated BVF samples are plotted by age groups defi ned by pulses of activity, these distributions confi rm a northward migration of activity and suggest a somewhat more systematic evolution of the BVF (Fig. 9). Evarts et al. (2009a) noted that the eruptive vents for most lava fl ows of the BVF can be identifi ed, 180 but are not known for all of the fl ows sampled. This introduces a small uncertainty that could Figure 7. Equal-area plot of site-mean paleomagnetic directions for lava fl ows be reduced if the locations of all vents were of the Boring volcanic fi eld (data from Table 4). Filled symbols indicate positive known or if the ages of each recognizable cone inclinations, and open symbols indicate negative inclinations. had been determined. This may be practical in the future with more detailed work, but in this study the dated locations are considered to ade- trend of activity in the BVF is represented by a tances of each center from some reference must quately represent the sources of small-volume mean trend of N70°E for the six fi elds. The age- be determined. Calculating the perpendicular BVF volcanism. The distributions outlined by group trends show an irregular clockwise rota- distances for each of the age-group centroids the samples defi ne a suite of elongate fi elds that tion with time that appears to be the result of a from an average trend line (N70°E) through the documents a northward migration of vol canism greater northward migration of volcanic activity group 6 center provides snapshots of the aver- in the BVF. Fields for the oldest (2.7–2.1 Ma) on the western side of the BVF than to the east. age location of volcanism in the BVF during and the youngest (200–0 ka) age groups are Correlation coeffi cients (R) for the regressions each pulse of activity (Fig. 11). These distances shown by shaded patterns in Figure 9, empha- ranging from 0.37 to 0.89 (Table 5) are suffi - and the average age of each group (Table 6) are sizing the almost complete absence of overlap ciently well defi ned to establish the generally regressed to calculate an average rate of north- in their areas. The 2.1–1.3 Ma group also has northeast-southwest trends of the age groups northwestward migration of volcanism in the little overlap with the youngest volcanism of and the change in trend after the earliest activity BVF between 2.7 Ma and the present (Fig. 12). the BVF, but the distribution of volcanism dur- of age group 1. An average migration rate of 9.3 ± 1.6 m/k.y. ing this period is in a more central area of the The geographic centers of the age groups (Fig. (mm/yr) (1σ) is calculated in the N20°W direc- BVF, not extending south of the town of Boring 10) migrate north-northwestward, roughly per- tion, normal to the average N70°E trend of the (Fig. 9). It is noteworthy that the fi elds of all four pendicular to the N70°E average trend of the age age groups. It is important that the Gales Creek– intermediate aged groups (spanning the 2.1 Ma groups. To evaluate the effect of the group selec- Mount Angel, Canby-Molalla, Portland Hills– to 200 ka period) defi ne a northeast-southwest tion on this migration, 6 different age groups Clackamas River, and Sandy River faults also trend between those of the youngest and old- were selected by dividing the data set into equal have northwestward trends, as shown by the est age groups with signifi cant overlap between groups of 24 samples, except for groups 4 and dextral displacement of aeromagnetic anoma- them. Locations of points within the fi elds are 5, which were given 25 samples. As expected, lies (Blakely et al., 2000). dispersed, but geographic trends within these group centers are different, but the latitude of The northeastward trend of age groups of the distributions are apparent. To defi ne these each group center is north of the next older cen- BVF is broadly parallel to the northeast trend trends, linear trend lines, numbered from oldest ter, confi rming the northward migration defi ned of mapped normal faults within the Portland to youngest, were determined by least-squares by the original grouping. This pattern is also con- region (e.g., Wells et al., 1995; Blakely et al., regression of the Universal Transverse Merca- fi rmed by compositional grouping of the data, as 2000), and to the most probable trend and slip

tor (UTM) coordinates for the samples of each seen in the following discussion. mechanism that produced the Mw 5.2 November age group (Table 5; Fig. 10). The trends range To address the magnitude of the north-north- 1962 Portland-Vancouver earthquake (Yelin and from N46°E to N80°E, and the time-averaged westward migration of the age groups, the dis- Patton, 1991). Local extension normal to these

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Figure 8. Relative probabil- Boring Lava Geochronology ity plot of eruptive ages of the Boring lavas, created using 6 5 4 3 2 1 the 40Ar/39Ar indicated ages and uncertainties shown in Table 2. The upper portion of the diagram shows the ages in

order of ascending age with Age Order error bars indicating their 1σ uncertainties. The lower portion of the diagram plots relative probabilities of lavas of a Polarity given age against age, provid- ing a plot of the eruptive activity of the Boring volcanic ﬁ eld (BVF) based on the ages available. Earliest volcanic activity in the ﬁ eld began ca. 2.7 Ma with lulls in activity from ca. 2.2 Ma to 2.0 Ma, from 1.85 Ma to 1.70 Ma, from 1.38 Ma to 1.30 Ma, and from 480 ka to

350 ka. These periods of inactiv- Relative Probability ity divide the Boring lavas into eruptive suites by age and facil- itate evaluation of spatial and compositional trends within the ﬁ eld. The two periods of greatest activity, 1.30 Ma to 500 ka 0.0 0.5 1.0 1.5 2.0 2.5 and 350 ka to present, are sub- Age (Ma) divided at shorter lulls centered ca. 900 ka and 200 ka, respectively, to divide the BVF into six eruptive groups. These eruptive episodes are (1) 2.7–2.1 Ma, (2) 2.1–1.3 Ma, (3) 1.3 Ma to 900 ka, (4) 900–450 ka, (5) 450–200 ka, and (6) 200–0 ka. A bar summarizing a recent version of Earth’s magnetic polarity (Ogg, 2012) is shown between the upper and lower parts of the diagram.

trends is consistent with interpretations of active et al., 1990a, 1990b), eruption of small-volume Evolution of Potassium and Calcium tectonism in this region during late Pliocene and lavas in the extensional regimes between rotat- in the BVF Quaternary time (Yelin and Patton, 1991; Wells ing crustal blocks may provide an explanation et al., 1998; Blakely et al., 2000), and with the for the anomalous forearc location of the BVF. Measurements of K and Ca are byproducts anomalous location of Boring volcanism in the The pattern of migration of volcanism within the of 40Ar/39Ar analyses, but because most analy- Cascadia forearc. Beeson et al. (1985, 1989) BVF suggests that the locus of greatest exten- ses used groundmass separates, bulk-rock

concluded that the Portland Basin is a pull-apart sion or pulling apart in the Portland Basin may measurements of K2O and CaO are reported

basin, formed east of the Portland Hills fault. also have shifted to the north-northwest during in Table 2. Variations in bulk-rock K2O and Whereas the western margin of the basin fi ts the past 2.7 m.y. The similarity of the 9.3 ± 1.6 CaO for the suite of Boring lavas show an this model with the northwest-trending, dextral mm/yr migration rate determined for the Boring impressive correlation with age with an evolu- Portland Hills–Clackamas River fault (Yelin lavas to rates of northwestward motion of crustal tion toward more alkaline magmas (Fig. 13). and Patton, 1991; Blakely et al., 1995, 2000), blocks in the Cascadia forearc (England and Because BVF activity migrates in a northward a basin-bounding fault on the eastern margin Wells, 1991; Wells et al., 1998; McCaffrey et al., direction, a geographic control of the chemi- is poorly defi ned by geologic mapping and the 2007), and to extension in the northern Basin cal evolution should be considered. Evarts Sandy River fault zone is not at the basin margin. and Range province (Magill et al., 1982; Wells et al. (2009a) noted that no low-K tholeiites Dextral shearing may be more distributed along et al., 1998), suggests that within-block defor- were erupted in the BVF after ca. 1.6 Ma, the eastern side of the basin and extend still far- mation may be comparable in magnitude to, or but the trends defi ned in Figure 13 show a ther east, as a substantial portion of Boring vents even accommodate, the majority of differential more coherent pattern of chemical evolution.

extend east of the basin. North-northwestward motion between blocks. In the case of the BVF, K2O and K/Ca values increase progressively migration of age groups is less apparent and the frequency of volcanism could thus reﬂ ect from the oldest (2.6–2.7 Ma) Boring volcanic extension may be slower in the eastern part of periods of locally greater and lesser extension, or rocks to the youngest without abrupt increases

the BVF. Because the Portland Basin is not an block rotation, with the 1.7–0.5 Ma period being or decreases (Fig. 13). K2O increases from area of high heat ﬂ ow typical of volcanic ﬁ elds the most rapid and the 2.2–1.7 Ma period repre- an average of ~0.7% in 2.6 Ma samples to supplied by large mantle heat sources (Blackwell senting an interval of reduced extension. ~1.1% in the youngest groups. CaO decreases

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122° 30’ 122° 00’

20200–0–0 kaka Lewis FFork East River ColumbiaC

a 0 k 45° 45’ - 45 45° 45’ 00 9009 - 450 rkka

West River

Riverver VANCOUVERV R Washougalshou

WWillamette Camas Riverier Columbia TROUTDALE a PORTLANDORTLT COCORBETT a

BEAVERTON 0 45° 30’ River 45° 30’ 1.1 0 .3–3 2002 ka – – 0.90 Ma 0–0

2.2.1–1–1.31.3 MaMa 5 .9 M 454 BORING Little SandyS a River Tua latinn River TUALATIN SANDY er Sandy Riv a y OREGONO ON M .1 CITY –2.12 Ma River .7–7 Clack 2.2 ackamasack Estacada Willamette c ma 45° 15’ 45° 15’

River er

02040 km

Molalla

122° 30’ 122° 00’

Figure 9. Distribution of Boring lava samples by eruptive group, showing the fi elds enclosing all dated samples for each period of activity. Fields are patterned for the oldest (2.7–2.1 Ma; purple) and youngest (200–0 ka; red) age groups to emphasize the south-southeast to north- northwest migration of volcanic activity in the Boring volcanic fi eld (BVF), with minimal overlap of those groups. Note that samples of intermediate age fi ll the central area of the volcanic fi eld, overlapping the fi elds of the oldest and youngest samples, and that south to north migration of activity is more rapid in the western part of the BVF than in the eastern part.

TABLE 5. TRENDS AND DISTRIBUTIONS OF AGE GROUPINGS OF THE BORING VOLCANIC FIELD, OREGON AND WASHINGTON, USA Correlation Center of distribution Age Calculated trend Age range coefﬁ cient group of group Latitude Longitude (R) NAD83 NAD83 1 2.70–2.10 Ma N46.16°E 0.895 45.37206 –122.35879 2 2.10–1.30 Ma N79.58°E 0.614 45.53758 –122.14864 3 1.30–0.90 Ma N73.91°E 0.627 45.53101 –122.37410 4 900–450 ka N73.61°E 0.392 45.55621 –122.26494 5 450–200 ka N67.50°E 0.553 45.58240 –122.21601 6 200–0 ka N78.19°E 0.366 45.64192 –122.30910 Mean N69.82°E SEM 5.04° (7.22%) Note: SEM represents the standard error of mean.

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122° 30’ 122° 00’

Lewis FFork East River ColumbiaC

45° 45’ 45° 45’

Fork

West iver R ka River 6 450450 ka ver 90900–0– VANCOUVERV R Washougalshou a 0.90.9 MMa 1.1.3–3– WWillamette Camas a ka 5 Riverier .3 M 00–0–0 ka 2.2.1–1–1.31 Ma 202 Columbia4 TROUTDALE 3 a PORTLANDORTLT COCORBETT

BEAVERTON 2 45° 30’ River 45° 30’

ka Little 00 BORING SandyS Tualat–2002 ka River 50–0 in 454 River TUALATIN SANDY Sandy River y OREGONO ON 1 CITY River Clack amette ackamasack Estacada Will c ma 45° 15’ 45° 15’ a Rive M e 1 r 2.12. Ma – 7–7 2.2. 02040 km Molalla

122° 30’ 122° 00’

Figure 10. Geographic trends of age-grouped Boring volcanic fi eld (BVF) lava samples are defi ned by linear regression of Universal Trans- verse Mercator (UTM) sample locations. The center of each group’s distribution (larger symbols, numbered from oldest to youngest) is calculated as the centroid of the UTM coordinates of its samples. The trend lines of the age groups progress irregularly but clearly from northeast-southwest for the oldest to east-northeast–west-southwest for the youngest, documenting the more rapid south to north migration of volcanic activity in the western part of the BVF. Age-group centers document a progressive northward migration of the volcanic fi eld, which slows between 2.1 Ma and 450 ka, the period of greatest activity. The average trend of the six groups is N70°E, with an average center for the BVF located ~1 km northeast of Corbett, Oregon, near that of group 4. Fields for the oldest and youngest groups are patterned for reference to Figure 9.

progressively from an average of ~9% in the Additional chemical and isotopic studies are above and one below the median K2O value 2.6 Ma rocks to ~8% in the most recent group. needed to deﬁ ne and distinguish between causes (0.9355). As expected from age and chemical

Because of the inverse variation in K2O and for the correlation of age and composition of the trends, samples with K2O values higher than the

CaO, the average K/Ca in BVF lavas increases lavas, such as time-dependent changes in depth median (mean 1.213% K2O) are younger (mean by ~70%, from ~0.10 to ~0.17 during this of melting and melt fraction, crustal involve- of 0.848 Ma versus 1.342 Ma) than those with

period. Variations among rocks of the same ment, or water content. To aid in evaluating values below the median (mean 0.752% K2O). age are substantial as is apparent from the the possible effect of geographic control on the The locations of samples in each group are plot- dispersion in K/Ca, but regressions of the data chemical trends, the sample suite was divided ted in Figure 14 and show similar areal distribu- deﬁ ne clear trends in the plots (Fig. 13). equally into two compositional groups, one tions, although the northernmost samples have

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122° 30’ 122° 00’

45° 45’ 45° 45’ Average Trend

ka 6 450450 ka 90900–0– a 0.90.9 MMa 1.1.3–3– ka 5 1.31.3 MMaa 20200–0–0 ka 4 2.2.1–1– 3

45° 30’ 2 45° 30’

0 ka –20020 ka 45450–0 Perpendicular Distances 1

45° 15’ 45° 15’ a M 1 2.12. Ma 02040 km – 7–7 2.2.

122° 30’ 122° 00’

Figure 11. Migration of the age groups relative to the (N70°E) average trend is shown by the perpendicular distances of their centers from a trend line drawn through the center for the 200–0 ka age group (group 6). These distances provide a measure of the distance that the locus of volcanic activity of the Boring volcanic fi eld has migrated through time. The center of the oldest age group (fi eld in purple) is almost 27 km south-southeast of the average trend through the center of the youngest age group (fi eld in red).

TABLE 6. MIGRATION OF AGE-GROUP TRENDS IN THE BORING VOLCANIC FIELD, OREGON AND WASHINGTON, USA Mean age Error Distance of centroid Assumed Age Age of group in age from mean trend location error group range (ka) (ka) (m) (m) 1 2.70–2.20 Ma 2495.9 35.0 26918 2000 2 2.00–1.30 Ma 1637.9 36.25 15124 2000 3 1.30–0.90 Ma 1093.8 17.51 9891 2000 4 900–450 ka 715.0 17.87 10127 2000 5 450–200 ka 265.7 11.33 8668 2000 6 200–0 ka 109.9 6.47 0 2000 Slope Regression Regression parameters (m/k.y) Simple X-Y 9.3 ± 1.6 R2 = 0.895; R = 0.946 Equal error-weighted 9.3 ± 1.6 MSWD = 2.6 Note: MSWD is the mean square of weighted deviates; m/k.y. = mm/yr.

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30000 above-median K2O values and the most southern Gp1 have below-median values. The geographic center for the above-median K O group is 5.3 km 25000 2 Age-Group due north of the lower group, indicating a 10.7 Migration Rate mm/yr migration rate based on the mean ages of the K O groups. Considering the grouping and 20000 2 averaging of lavas from multiple periods of volcanic activity, this rate compares favorably with the 9.3 ± 1.6 mm/yr average calculated above 15000 Gp2 from activity-based groupings (Fig. 11). It is signifi cant that this grouping of samples by con- Gp4 Distance (m) centration is completely independent of the cri- 10000 Gp5 Gp3 teria used to select the age groupings shown in Migration Rate = 9.3±1.6 m/k.y. Figure 8, yet defi nes a similar migration of volcanism within the BVF. 5000 R2 = 0.895 R = 0.946 Tectonic Implications of Migration Gp6 of BVF Volcanism 0 0 500 1000 1500 2000 2500 3000 McCaffrey et al. (2007) noted that the Juan Age (ka) de Fuca plate currently subducts obliquely Figure 12. A measure of the average north-northwestward migration rate of the Boring northeastward beneath North America at a rate volcanic fi eld is obtained by linear regression of the average ages of the age groups and their of 30–45 mm/yr with most of the relative plate calculated distances from the most recent center. Distances between centers were calculated motion taken up by subduction, but that the over- normal to the average trend of the age groups through the group 6 center, as shown in riding continental plate is broken into a series of Figure 11. Ages are the average ages of each group. Simple X-Y (shown here) and error- crustal blocks rotating clockwise about nearby weighted regressions of these data (see Table 6) defi ne the average migration rate for the poles in the backarc. The resulting northward center of Boring volcanic activity with time. The mean rate of migration normal to the aver- motion of the Coast Range, at 7.1 ± 0.5 mm/yr age trend of the fi eld is 9.3 ± 1.6 m/k.y. (1σ) with a high degree of correlation (R = 0.946). (McCaffrey et al., 2007), is similar to the volcanic migration rate discussed here for the BVF. Paleomagnetic and global positioning system Chemical Evolution of Boring Lavas (GPS) measurements also indicate a westward 2.5 increase in the northward motion of the Coast K2O = 1.1455 – 0.1515 × Age Ranges that results in their clockwise rotation 2.0 relative to North America (McCaffrey et al., 2007). The greater north-northwestward migra- 1.5

O tion of volcanic activity determined for the west-

Figure 13. Linear regres- 2 ern side of the BVF may refl ect this westward sion of K O and CaO K 1.0 2 increase in dextral shear during the past 2.7 m.y. with time documents a 0.5 Both volcanic activity and magmatic com- quite regular chemical positional changes of the Boring lavas migrate evolution of the Boring northward within the BVF and both distribu- volcanic fi eld. Because of CaO = 7.9732 + 0.3934 × Age tions are broad, spanning much of the volcanic the inverse relationship 12.0 fi eld. Because the distribution of volcanic activ- between these compo- ity within each of the age-grouped intervals nents, the average molar 8.0 (Fig. 9) covers as much as half of the areal extent

K/Ca increases by ~70% CaO of the BVF, crustal extension (Yelin and Patton, in the 2.6 m.y. period. 1991; Wells et al., 1998; Blakely et al., 2000) is Data used in the figure 4.0 presumed to have had at least a similar breadth. are presented with other The increasing alkalinity of the Boring lavas sample data in Table 2. 0.3 K/Ca = 0.1705 – 0.02746 × Age and simultaneous northward migration of activ- (Analytical methods used ity permit both temporal and geographic inter- in their determination pretations of this evolution. Temporal changes are noted in Table 2 foot- 0.2 that involve increasing depth of melting and/or note.) decreasing melt fraction, possibly due to cool- 0.1 ing, could result in increased potassium and Molar K/Ca decreased calcium in the lavas. Migration of 0.0 melting into different source materials could also 0123result in compositional evolution of the lavas. For Age (Ma) example, Church et al. (1986) suggested that geo-

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122° 30’ 122° 00’

Lewis Fork East River ColumC

bia

45° 45’ 45° 45’

Fork

iver West R

River

VANCOUVERV R Washougalshou

Willamette Camas Riverier Columbia TROUTDALE a PORTLANDORTLT CORBETT

BEAVERTON 45° 30’ River 45° 30’

BORING Little Sandy Tualat River in River TUALATIN SANDY Sandy River y OREGONO ON CITY River Clackamas amette ack Estacada Will ma 45° 15’ 45° 15’

Rive e 0 20 40 km r Higher K2O Lower K2O Molalla

122° 30’ 122° 00’

Figure 14. The geographic distribution of the 50% of Boring volcanic ﬁ eld (BVF) samples with above-median K2O (higher K2O) is not

substantially different from that of the 50% with K2O below the median. Comparison of the centers of these distributions, however, shows

that the higher K2O group center is ~5.3 km due north of that of the lower K2O group, consistent with the almost imperceptible northward migration of volcanic activity in the BVF of ~1 cm/yr determined from age- and activity-grouped results.

graphic changes in the age and structure of the with the course of the Columbia River through evidence of nuclear recoil effects of 39Ar and/or crust in this region are responsible for differences the Cascades. Additional chemical and isotopic 37Ar that are related to the fi ne-grained character in Pb isotopic compositions of sulfi de minerals studies of the Boring lavas are clearly needed of the lavas. The age effects due to recoil of 39Ar in Tertiary ore deposits emplaced in the Cascade to evaluate their total compositional variation exceed those of 37Ar even in these high-Ca rock mineral belt. They showed signifi cantly lower and petrogenesis, but K/Ca results reported here types, but most samples show less than ~5% net 206Pb/204Pb between 44°N and 45°30′N, the zone docu ment a trend within the BVF that is consis- recoil loss of either of these species. Age control referred to as the Oregon Embayment. The BVF tent with current plate motion studies. from paleomagnetic polarity, stratigraphy, and spans the north edge of this zone and the southern available plateau ages from the large data set of edge of the higher K/Ca, 200–0 ka age group and CONCLUSIONS Boring volcanic rocks documents the validity the northern edge of the lower K/Ca, 2.7–2.1 Ma of recoil model ages calculated as the integrated age group meet at precisely 45°30′N (Fig. 9). Volcanism in the BVF occurred during late ages of gas fractions in the central portions of This coincidence between Pb isotope and erup- Pliocene and Pleistocene time. With few excep- age spectra. Use of recoil model ages has broad tive (age) group boundaries is also coincident tions, 40Ar/39Ar age spectra in this study exhibit implications for 40Ar/39Ar studies of volcanic

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fi elds of fi ne-grained basaltic rocks, as these rock ogy samples from nearby outcrops of the same fl ow and abundances recommended by Steiger and Jager types offer fewer options for high-quality mineral that were correlated by physical continuity of outcrop (1977), including atmospheric Ar. Sanidine from the (where possible), petrography, and geochemistry. All Taylor Creek Rhyolite (TCs), studied by Dalrymple separates that are available in more silicic rock samples used for geochronology were selected by and Duffi eld (1988), was used as the neutron fl ux types. As demonstrated with the BVF, sample petrographic examination of the rocks in thin sec- monitor for neutron activation in these studies, but coverage within the volcanic fi eld was expanded tion. Vesicular rocks and those exhibiting alteration values of J, the dimensionless irradiation parameter, by almost 40% through the use of this approach. of either the groundmass or any mineral phase with were adjusted for all samples to yield ages of 28.02 Grouped by eruptive episode, the geographic suffi cient potassium to affect the age were rejected. Ma on sanidine from the Fish Canyon Tuff (FCs) Use of samples with groundmass plagioclase having (Renne et al., 1998). In this study the age relationship distributions of Boring lava samples defi ne a minimum dimensions <15–20 μm was restricted, and between these two sanidine monitors is defi ned by: series of elongate, northeast-southwest trends those of <10 μm were generally avoided. Analyses λt λt that appear to progress irregularly in a clock- for this study were performed over a period of 10 yr, R = (e – 1)TCs/(e – 1)FCs = wise sense as their centers migrate north- during which experimental procedures evolved from 1.0088089 ± 0.0004648 (0.046%). using crushed whole-rock samples to groundmass ward with time. Age groups based on equal separates and some extraction and data acquisition Uncertainties are reported at one standard error of the numbers of samples show a similar pattern procedures were modifi ed. The 40Ar/39Ar incremental- mean except where stated. Uncertainties in J are typi- of northward migration, demonstrating that heating experiments (e.g., Merrihue and Turner, 1966; cally <±0.2%. activity-based groupings have no effect on the Dalrymple and Lanphere, 1974; Fleck et al., 1977) The argon extraction system for releasing argon geographic trend. Periods of greatest volcanic were performed on whole-rock samples and ground- and removing reactive gases utilizes a temperature- mass separates of the Boring lavas. In both cases sam- activity occurred from 2.7 to 2.2 Ma, from 1.7 controlled, molybdenum-shielded resistance furnace ples of 250–1000 g were fi rst crushed in a jaw crusher with a molybdenum crucible and SAES ST-172 get- to 0.5 Ma, and from 350 ka to 50 ka, whereas and then in a roller mill to the appropriate grain size. ters. The gas is exposed to a 4 A tungsten fi lament no record of volcanism was found between 2.2 Initially, a size range of 250–355 μm was used, but and 125 K cold fi nger during purifi cation. Argon and 2.0 Ma and between 500 and 350 ka. Begin- in more recent preparations the roller mill output was is extracted from the samples in temperature steps reduced to a size range of 180–250 μm for ease in ning ca. 2.7 Ma, volcanic activity in the BVF selected to distribute the total released over a mini- eliminating phenocryst phases. Use of a disk grinder mum of ~8 steps and extend to temperatures where migrated northward at an average rate of ~9.3 ± was avoided due to the greater frictional heating that the potassium-derived 39Ar drops to small fractions of 1.6 mm/yr, with the greatest migration on the might occur. The crushed materials were washed the total released, but some extractions may exceed 15 western side of the fi eld resulting in an apparent with tap water and rinsed with deionized water. More steps. Early in this study stepwise heating included all clockwise rotation of the age-group trends. recent groundmass separates have also been treated argon remaining in each sample following a 200 °C, in an ultrasonic bath to remove loose and fi ne par- K O and CaO values in volcanic rocks of the 12–24 h bakeout of the samples. This included large 2 ticulates, as well as breaking up and removing some amounts of atmospheric Ar in the fi rst steps, as release BVF are well correlated with age. Grouped only fi ne glassy selvages within the groundmass. Although of radiogenic 40Ar and potassium-derived 39Ar is not by concentration, geographic centers of the low uncommon, interstitial carbonate was removed when observed from basalts and andesites under routine (older) and high (younger) K O groups sup- present with cold 1N HCl followed by a minimum of laboratory heating times of 5–15 min until tempera- 2 3 rinses with deionized water in a sonic bath and resiz- port a northward migration of volcanic activity, tures of ~500 °C are reached. Because these low-tem- ing. Groundmass separates were prepared by remov- perature steps may introduce large amounts of atmo- showing an overall rate similar to that calculated ing the most highly magnetic grains with a hand mag- spheric argon, hydrocarbons, N2, O2, H2O, and CO2 for age groupings of the lavas. Together with net or by Frantz isodynamic separator at the lowest to the mass spectrometer without adding additional the elongate northeast-southwest distributions setting. Depending on the sample mineralogy, olivine information to the analysis, degassing of the samples and clockwise rotation of age-group trends, the and/or coarse plagioclase was removed in a less mag- to the pumping system at ~500 °C was adopted. Argon netic fraction with the Frantz. Where magnetic separa- calculated migration rates are consistent with isotopic analyses were carried out using an MAP216 tions were ineffective, generally due to the abundance single-collector mass spectrometer with a Bauer- recent GPS measurements of relative motion of of fi nely dispersed magnetite, heavy liquid separation Signer source, a Johnson MM1 electron multiplier, crustal blocks within the U.S. Pacifi c Northwest. with appropriate densities of acetone-diluted methy- and peak stepping. Analyses are automated, using lene iodide was employed. Groundmass concentrates machine- and BASIC-language software developed ACKNOWLEDGMENTS were then hand-picked for purity. by Brent Dalrymple for the earlier samples and Lab- All samples for 40Ar/39Ar incremental heating were View software developed by Andrew Calvert since We thank Ray Wells for support and patience as this packaged in Cu-foil cylinders and packed compactly 2003. Data reduction was done using routines devel- study progressed and the extent of the Boring volcanic in quartz vials for neutron irradiation. Positions of the oped by these workers, and with Isoplot (developed by fi eld (BVF) and its variations were recognized. The samples in the vials were measured precisely from Ludwig, 2003, with updates through 2009). technical assistance of James Saburomaru, Charles fl atbed scanner images of the vials after evacuation Holdsworth, Honore Rowe, Dean Miller, Donald and sealing. Heights of the top and bottom surfaces of APPENDIX 2. PALEOMAGNETIC Shamp, and Blair Bridges was critical to this project each sample cylinder in the vial were measured at the TECHNIQUES and is greatly appreciated. Ian Madin participated in vial centerline with digital graphics software using 2 mapping and sampling of the BVF. Ray Wells, Mike images of the vial taken 180° opposite each other. In Oriented samples of Boring lavas and shallow Clynne, Laura Webb, Jan Lindsay, and an anonymous this manner the average height of any two diametri- intrusions were collected for paleomagnetic analysis reviewer provided insightful reviews that led to sig- cally opposite points at the edge yields an accurate primarily from artifi cial (e.g., road cut, quarry) and nifi cant improvements to the manuscript. We thank height for the center of the upper and lower surfaces of stream-bank exposures in order to obtain the freshest science editors Carol Frost and Shanaka de Silva and the cylindrical sample packet, regardless of any slope possible rock. As previously mentioned, 40Ar/39Ar and associate editor Rebecca Flowers for advice and edi- on these surfaces relative to the long axis of the vial. paleomagnetic samples were collected within the same torial assistance. Any use of trade, fi rm, or product Samples were irradiated in the central thimble of units, and every attempt was made to collect both sam- names is for descriptive purposes only and does not the U.S. Geological Survey TRIGA reactor in Den- ple sets from the same outcrop. This was not always imply endorsement by the U.S. Government. ver, Colorado, at a constant power level of 1 MW possible, however, due to the differing rock proper- for 2 h (Dalrymple et al., 1981), and analyzed in the ties advantageous to each technique (see Appendix 1). APPENDIX 1. 40Ar/39Ar TECHNIQUES U.S. Geological Survey laboratories in Menlo Park, Between 8 and 12 paleomagnetic core samples 2.5 cm California. Uncertainties in Ar analyses are reported in diameter were drilled at each site using a portable Insofar as possible, we have obtained samples for at the 1σ level with weighted means calculated using gasoline-powered drill, and were oriented and marked, 40Ar/39Ar dating from the same outcrops drilled for the inverse variance as the weighting factor. Quartz prior to removal, using an orienting tool consisting of paleomagnetic analyses. However, many samples vials containing the samples were cadmium shielded magnetic and solar compasses and a clinometer. Ori- from drilled outcrops were less suitable for 40Ar/39Ar for irradiation to reduce thermal neutron reactions, ented hand samples were also collected at remote and dating due to weathering, glassy groundmass, or fi ne especially 40K(n,p)40Ar, that produce isotopic interfer- inaccessible sites, to determine a unit’s polarity, from grain size. In these cases we selected the geochronol- ences. Ages were calculated using the decay constants which core samples were drilled in the laboratory.

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