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Quickly Erupted Volcanic Sections of the Steens Basalt, Columbia River Basalt
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Group: Secular Variation, Tectonic Rotation, and the Steens Mountain Reversal
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Nicholas A. Jarboe
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University of California, Department of Earth and Planetary Sciences,
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1156 High St., Santa Cruz, CA 95064 USA (njarboe@pmc.ucsc.edu)
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Robert S. Coe
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University of California, Department of Earth and Planetary Sciences,
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1156 High St., Santa Cruz, CA 95064 USA
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Paul R. Renne
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1) Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, CA 94709 USA
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2) University of California, Department of Earth and Planetary Science, Berkeley, CA
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94720 USA
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Jonathan M.G. Glen
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U.S. Geological Survey, MS989, 345 Middlefield Road, Menlo Park, CA 94025 USA
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Edward A. Mankinen
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U.S. Geological Survey, MS937, 345 Middlefield Road, Menlo Park, CA 94025 USA
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Abstract
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The Steens Basalt, now considered part of the Columbia River Basalt Group (CRBG),
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contains the earliest eruptions of this magmatic episode. Lava flows of the Steens Basalt
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cover about 50,000 km2 of the Oregon Plateau in sections up to 1000 m thick. The large
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number of continuously-exposed, quickly-erupted lava flows (some sections contain over
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200 flows) allows for small loops in the magnetic field direction paths to be detected. For
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volcanic rocks, this detail and fidelity are rarely found outside of the Holocene and yield
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estimates of eruption durations at our four sections of ~2.5 ka for 260 m at Pueblo
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Mountains, 0.5 to 1.5 ka for 190 m at Summit Springs, 1-3 ka for 170 m at North Mickey,
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and ~3 ka for 160 m at Guano Rim. That only one reversal of the geomagnetic field
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occurred during the eruption of the Steens Basalt (the Steens reversal at ca. 16.6 Ma) is
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supported by comparing 40Ar/39Ar ages and magnetic polarities to the geomagnetic
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polarity time scale. At Summit Springs two 40Ar/39Ar ages from normal polarity flows
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[16.72 ±± 0.29 Ma (16.61) and 16.92 ±± 0.52 Ma (16.82); ±± equals 2σ error] place their
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eruptions after the Steens reversal, while at Pueblo Mountains an 40Ar/39Ar age of 16.72
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±± 0.21 Ma (16.61) from a reverse polarity flow places its eruption before the Steens
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reversal. Paleomagnetic field directions yielded 50 non-transitional directional-group
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poles which, combined with 26 from Steens Mountain, provide a paleomagnetic pole for
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the Oregon Plateau of 85.7°N, 318.4°E, K = 15.1, A95 = 4.3. Comparison of this new pole
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with a reference pole derived from CRBG flows from eastern Washington and a synthetic
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reference pole for North America derived from global data implies relative clockwise
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rotation of the Oregon Plateau of 7.4 ± 5.0° or 14.5 ± 5.4°, respectively, probably due to
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northward-decreasing extension of the Basin-and-Range.
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Introduction
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The Steens Basalt of the Oregon Plateau may cover as much as 50,000 km2 (Mankinen et
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al., 1987; Carlson and Hart, 1987) of southeastern Oregon, northwestern Nevada, and
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northeasternmost California. Its extent, eruptive timing, and relationship to the Columbia
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River Basalts (CRB) have undergone continued debate. Geochemical and field studies
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have expanded the traditional Columbia River Basalt Group (CRBG) to include the
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Steens Basalt (Hooper et al., 2002; Camp et al., 2003; Camp and Ross, 2004; Brueseke et
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al., 2007) at its base. The relative stratigraphy of the Steens Basalt and the CRBG has
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been traced over 150 km with relationships determined by an intermediate formation, the
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basalt of Malheur Gorge (Camp et al., 2003; Brueseke et al., 2007). The reverse-to-
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normal (R-N) polarity change as recorded at Steens Mountain (Steens reversal) is
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believed to be the only reversal to have occurred during the eruption of the Steens Basalt
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(Mankinen et al., 1987; Camp and Ross, 2004).
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We are studying the location, age, and transitional field behavior of the Steens reversal
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recorded throughout the Oregon Plateau (Figure 1). 40Ar/39Ar age determinations from
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lavas erupted during the transition (Jarboe et al., 2006; Jarboe et al., in preparation) place
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the reversal at 16.69 ±± 0.14 Ma (16.58). See next section “Age Data Presentation” for
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age presentation conventions. Due to Basin-and-Range faulting and little vegetative
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cover, many thick (>500 m) sections of Steens basalt or their possible equivalents are
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well exposed and have been studied by others (Watkins, 1963; Mankinen et al., 1987 and
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references therein; Brueseke et al., 2007). So far over a dozen locations have been
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sampled for paleomagnetic and geochronologic study (Figure 1). A forthcoming paper
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will report on sections studied that record transitional field behavior (Jarboe et al., in
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preparation). In this paper we discuss the paleomagnetic and geochronologic results from
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four sections that record stable polarity secular variation: Summit Springs (60 km
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northeast of Steens Mountain), Pueblo Mountains (65 km south of Steens Mountain),
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North Mickey (25 km east of Steens Mountain) and Guano Rim (95 km southwest of
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Steens Mountain). Except for one lava at Summit Springs, each section erupted during a
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single geomagnetic polarity chron, and 40Ar/39Ar data, stratigraphy, and petrological
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considerations place their eruptions in chrons just before or after the Steens reversal.
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Although some others believe that the Steens Basalt gradually erupted over millions of
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years (Brueseke et al., 2007), we will show that magnetic field behavior, field polarity,
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and geochronology of these sections are consistent with rapid local emplacement (1-3 ka)
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within ~300 ka of the Steens reversal.
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Age Data Presentation
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Ages in early literature were usually reported with one sigma uncertainty, while two
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sigma is commonly reported today. We suggest (and herein adopt) using ± to represent
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exclusively one sigma, ±± to indicate two sigma and ±x to indicate an x% confidence
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interval. For example ±95 would represent an uncertainty at the 95% confidence interval.
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This convention is used for ages throughout this paper, with standard paleomagnetic
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conventions used elsewhere. When citing values with uncertainties from other work, we
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prefer to present the uncertainty given in the original and convert to other uncertainties if
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needed for clarity.
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We present ages here using the Fish Canyon sanidine (FCs) age of 28.201 ±± 0.214 Ma
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determined by astronomical calibration (Kuiper et al., 2008) and the 40K decay constant
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of 5.463 ±± 0.107 × 10-10/a (Steiger and Jager, 1977; updated by Min et al., 2000). To
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ease comparison to other 40Ar/39Ar ages in the literature, the 40Ar/39Ar ages determined
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using the Earthtime (An NSF supported international scientific initiative;
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http://www.earth-time.org) conventions of 28.02 Ma for the FCs (Renne et al., 1998) and
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5.543 ±95 0.089 × 10-10/a for the 40K decay constant (Steiger and Jager, 1977) are
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included in parenthesis after each age.
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Paleomagnetic Procedures
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All paleomagnetic procedures and analyses were performed at the University of
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California, Santa Cruz unless otherwise noted. Paleomagnetic cores were sampled with a
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2.5 cm diameter, water-cooled, diamond-studded, hollow core bit using a hand-held
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gasoline powered drill. The cores were drilled 5-10 cm deep, oriented to an accuracy of
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1º-2º while still attached to the outcrop using an orienting stage and a Brunton compass.
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Sun sites, sun shadows, and site points of known direction were used to correct for local
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magnetic anomalies. Flow bottoms were generally drilled to minimize the chance of
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remagnetization by overlying flows. The orientation angles were recorded to the nearest
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degree and time to the nearest minute. Cores were later cut into 2.5 cm long specimens
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back at the laboratory. In general the deepest, least weathered specimens from each core
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were used when determining the paleomagnetic field directions.
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The natural remanent magnetization (NRM) of the specimens was stepwise-
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demagnetized in a decaying alternating field (AF) of up to 200 mT and magnetizations
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were measured in a 2G superconducting magnetometer. Twelve-position measurements
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were made using custom built hardware and software. An Agico JR-5 calibration sample
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was measured at least daily and kept within 1.2º of the expected direction with an
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estimated error no greater than 1.2º. The characteristic remanent magnetization (ChRM)
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direction of each specimen was determined with straight-line-to-the-origin fits
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(Kirschvink, 1980) and occasional great circles (McFadden and McElhinny, 1988) using
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PMGSC42 software (Enkin, 2005). Most specimens were well-behaved upon stepwise
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AF-demagnetization (Figure 2a). Any viscous component was typically removed by 2 to
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15 mT (Figure 2b). A few specimens taken from near lightning strikes required greater
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demagnetization fields to reveal the ChRM, but in most cases a well defined direction
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was determined (Figures 2c,d). In areas of unusually strong lightning remagnetization,
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some specimens were overprinted with magnetizations that were not removed even at the
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highest (~200 mT) demagnetization steps (Figure 2e). In such cases the magnetic
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direction during AF-demagnetization usually followed a great circle toward the ChRM
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direction (Figure 2e).
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Generally, at least eight samples were taken from each flow and the mean flow directions
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were determined using Fisher (1953) or McFadden and McElhinny (1995) statistics
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(Figure 3). Directions, virtual geomagnetic poles (VGPs), and other data for the flows are
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shown in Table 1. Three flows at Summit Springs yielded too few ChRMs or great circle
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fits to determine mean flow directions. In the remaining flows 658 cores were measured,
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and of these 570 had resolvable characteristic directions, 72 yielded acceptable great
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circle fits, and 16 directions were rejected. Of the rejected directions 8 had lightning
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overprints so strong as to completely overwhelm the ChRM, 1 had an unstable
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demagnetization path, and 7 had resolvable characteristic directions but with outlying
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directions far (> 40°) from the flow mean direction. These rare outlying directions are
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likely due to mis-orientation of the core, undetected post-eruptive movement of the
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sampled outcrop, or complete overprinting.
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Sampling Strategy and Grouping Flow Directions
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The volcanic sections presented here were sampled as part of our search for lava flows
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erupted during the Steens reversal to shed light on transitional field behavior. For this
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reason we chose flow-on-flow sections, where exposure is high, stratigraphy is
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straightforward, and cover between flows that might conceal a long eruption hiatus or
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other geological complexity is minimal (see Appendix A1.1 and A1.2 for photos). If
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measurements in the field with a hand held-fluxgate magnetometer suggested lava flows
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with intermediate polarity, then we sampled almost every flow unit. If not, we still
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sampled the section in case overprints obscured a transition zone, usually skipping some
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lava flows that could be aquired on a return visit if a transition were found, so that we
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could cover a greater interval. For studies of secular variation in flow-on-flow sections
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such as these, skipping some flows is common practice because the episodic nature of
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volcanism results in packets of successive lavas that span little time and have directions
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that are the same or very similar. To test that this is the cause rather than extended
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intervals of unchanging field direction, geochemical analyses of some of these packets
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are being performed under the assumption that little magma differentiation occurs during
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a burst of frequent eruptions.
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Nonetheless, we still encountered some repetitions of the same or very similar directions
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in stratigraphically ordered flows. Following the practice of earlier studies at Steens
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Mountain, these flow packet directions are combined into directional groups (DGs) by
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the method described by Mankinen et al., (1985). Specifically, lava-flow mean directions
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that are in sequence and whose α95’s overlap are combined unless they trend in a
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consistent direction, in which case the flows are not grouped. After this procedure there
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are 50 remaining directions, of which 13 represent averages of more than one flow and 37
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that are individual flow directions. All the directions for individual lava flows, as well as
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the grouped-flow directions, are given in Table 1. The mean direction for each of the four
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sections do not differ significantly whether computed from the directional groups or from
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the individual flows, but the confidence circles are a little larger when flows are grouped
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because N is smaller. In general, we expect that the grouped data provide a more
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representative sampling of secular variation, and thus the directions and VGPs shown in
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the figures are for the DGs unless otherwise noted.
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40Ar/39Ar
Geochronology Procedures
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All sample preparation and analyses for 40Ar/39Ar geochronology were done at the
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Berkeley Geochronology Center (BGC). Plagioclase, sanidine, or groundmass aliquots
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were prepared from either alteration-trimmed rocks from the same flows as the
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paleomagnetic cores or the paleomagnetic cores themselves. These samples were
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crushed, washed, and sieved into size fractions. Each size fraction used was magnetically
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separated with a Frantz Isodynamic Separator, washed ultrasonically in a dilute (3-4%)
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HF solution for 3-5 minutes, and rinsed in a purified-water sonic-bath for 20-40 minutes.
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The samples were then hand-picked under a microscope. For plagioclase and sanidine
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aliquots, clear grains were selected and any grains with visible inclusions or surface
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alteration were discarded. Individual groundmass grains were selected to exclude any
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containing phenocryst fragments. These aliquots and Fish Canyon sanidine (FCs) grains
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were then placed into separate pits in aluminum disks, wrapped tightly in aluminum foil,
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and irradiated for 5 hours in the CLICIT facility of the TRIGA reactor at Oregon State
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University. The neutron fluence (J-parameter) experienced by each aliquot was calculated
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using an age of 28.02 Ma (Renne et al., 1998) from the FCs standards which had been
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placed in the center and around the edge of the disk. After waiting typically 4-6 months
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for 37Ar to decay to optimal measurement levels, samples were degassed with a CO2 laser
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and the argon isotopes were analyzed with an online MAP 215C mass spectrometer.
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Samples were then heated in steps for plagioclase and groundmass samples and to total
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fusion or in steps for single grains of sanidine. Analysis of the empty chamber and
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atmospheric argon were run often to determine the blank correction and the
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spectrometer’s mass discrimination, respectively. Parabolic or linear curves were fit to
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the individual ion beam intensity versus time data to determine the relative abundances of
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the 40Ar, 39Ar, 38Ar, 37Ar, and 36Ar isotopes found in the sample. The plateau ages were
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then determined with the program Mass Spec version 7.621 (Deino, 2001) using 95%
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indistinguishability confidence criterion applied to at least 50% of the 39Ar released
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comprising at least three contiguous steps unless otherwise stated. Weighted (by inverse
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variance) mean ages from multiple single-grain plateau ages were determined with
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Isoplot 3.66 (Ludwig, 2003).
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Volcanic Sections: Geology and Paleomagnetism
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Pueblo Mountains
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The reverse polarity Steens-like Pueblo Mountains section (42.1ºN, 118.7ºW) is 60 km
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south of Steens Mountain at the southern end of the Steens Mountain escarpment (Figure
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4; photos and larger scale map in Appendix A1.1). The Steens Basalts were first
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described at the type section at Steens Mountain by Fuller (1931): “The rock is distinctive
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in the field both from a peculiar porous texture, which is quite characteristic, and from its
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local content both of labradorite phenocrysts ranging from 1 to 4 cm. in length, and of
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olivine grains, which are predominantly under 2 mm. in diameter.” (photos Appendix
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A1.3) Unlike the Steens type section, which is underlain by mid-Miocene volcanics, the
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Pueblo Mountains section is unconformably underlain by crystalline Middle Cretaceous
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intrusive and metamorphic basement (Hart et al., 1989). We sampled 11 of about 20
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flow-on-flow lavas from a continuous section extending across 2.3 km and spanning 260
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m of elevation. Four other flows located to the south of the main section were also
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sampled in an unsuccessful attempt to find normal polarity lava flows from the overlying
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normal polarity chron. Flows in the Pueblo Mountains are tilted 20ºW about a strike of
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180º and our paleomagnetic field directions have been corrected accordingly. The attitude
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of the beds was determined by field measurements and are in good agreement with
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1:24,000 scale mapping of the area by Rowe (1971).
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The mean direction for each flow at Pueblo Mountains is given in Table 1. To estimate
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how long the changes in magnetic directions took, we compare the record of the
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continuous section to a high resolution historical record from Germany (Schnepp and
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Lanos, 2005). Their record encompasses the last 2600 years from sites with similar
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latitudes and geographical extent as our study area (Figure 5a). The directions span about
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30º east-to-west and 15º north-to-south with the whole area traversed in about 2000 years.
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Smaller loops of the field are also made during the main traverse. The lower resolution
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record at Pueblo Mountains is comparable (Figure 5b). It makes a little over one large
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loop with some internal complexity that is suggestive of a small loop. The secular
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variation behavior of the field is similar to that observed in other high-resolution records
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(Ohno and Hamano, 1992; Hagstrum and Champion, 2002). Assuming that the
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geomagnetic field at 16.6 Ma behaved similarly to the modern field, the record suggests
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that the Pueblo Mountains section erupted in about 2500 years. Also supporting a short
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eruption duration is the low dispersion of VGPs (14.6°), which is significantly less than
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the 21.2° estimated for full secular variation during this period of geologic time
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(McFadden et al., 1991). Thus we conclude that the upper ~250 m of the section of
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Steens-like lavas at Pueblo Mountains erupted in about 2.5 ka.
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Summit Springs
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The section at Summit Springs (43.1ºN, 118.3ºW) is 60 km northeast of Steens Mountain
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at the northern end of the Steens Mountain escarpment (Figure 4, photos Appendix A1.2).
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It consists of approximately 50 normally magnetized flow-on-flow lavas in a well-
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exposed 190 m thick section. Many of these Steens Basalts are plagioclase-rich with
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plate-like crystals up to 3 cm in length. The section is covered at the bottom by the much
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younger Devine Canyon Tuff, which flowed south out of the Harney Basin over some
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existing topography (Vic Camp, pers. comm., 2005). Under the tuff, 1 km east of the
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bottom of the section, a reverse polarity flow is exposed in a road cut adjacent to State
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Highway 78. This basalt is not Steens-like in appearance and does not contain the large
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plagioclase crystals found in many Steens basalt flows, setting it apart from the other
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flows at Summit Springs. Given its uncertain stratigraphic relationship with the main
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section and its problematic 40Ar/39Ar age determination, this flow could have erupted
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before or after the Steens reversal. Even with this stratigraphic uncertainty, we include its
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magnetic direction in the mean pole calculation as it almost certainly erupted within 1 Ma
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of the Steens reversal.
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Eighteen horizontal flows were sampled, fifteen of which gave directions consistent
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enough to calculate flow mean directions (Table 1). Flow ss17 is the younger Divine
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Canyon Tuff and is not discussed further. Comparison of the Summit Springs directional
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path to that of Steens Mountain is done without correcting for latitude because the
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geocentric axial dipole (GAD) field inclination differs by less than half a degree between
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the two sites. This is much less than other errors such as those related to bedding
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corrections or core orientation.
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The beginning and end of the directional path as recorded at Summit Springs (Figure
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6a,b) has some similarities to the path at the end of the reversal at Steens Mountain
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(Figure 6c,d) (Mankinen et al., 1985), but the sense of movement in the middle of the two
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records is different. The first five field directions (DGs 1-5) from the bottom of the
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section at Summit Springs span a large angle representing an unknown amount of time.
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In the upper part of the section (DGs 5-10) a small loop of the field suggests an eruption
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duration of about 500-1500 years when compared to Holocene records. In conclusion,
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based on 40Ar/39Ar ages and paleomagnetic arguments, the upper 150 m of lavas at
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Summit Springs is likely to have erupted over 500-1500 years, possibly immediately after
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the Steens reversal.
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North Mickey
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The section at North Mickey (42.8ºN, 118.3ºW) is 25 km east of Steens Mountain at the
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north end of the Mickey Basin (Figure 4). The section is on a down-dropped block of
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Steens Basalt as mapped by Hook (1981) and “correlation is established on the basis of a
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paleomagnetic reversal observed in the two areas, and supported by petrologic
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observations and chemical analysis.”(Hook, p.2). He found two reverse polarity flows
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below seven normal polarity flows at Mickey Butte 7.5 km southwest of the North
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Mickey section. Two 40Ar/39Ar dates from groundmass separates [a reverse lava 16.69 ±±
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0.18 Ma (16.58), a normal lava 16.59 ±± 0.30 Ma (16.48)] by Brueseke et al. (2007)
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suggest the reversal at Mickey Butte is the Steens reversal. On inspection the Mickey
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Butte section has extensive cover between outcropping flows. Only a few flows are
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exposed in the 60 m section between the reverse polarity flow at 1470 m and the normal
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polarity flow at 1530 m. Looking for transitional lavas erupted during the Steens reversal,
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the better exposed flow-on-flow section at North Mickey was drilled. While only normal
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polarity lavas were found at the North Mickey location, stratigraphic continuity and the
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Steens-like character (large 2-4 cm plagioclase crystals) of the lower flows suggest that
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these lavas erupted soon after the Steens reversal.
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The 320 m North Mickey section consists of approximately 35 normal polarity flow-on-
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flow lavas in a well-exposed section. Every flow of the first 16 from the bottom was
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sampled for paleomagnetic analysis. Some flows at the top of the section were skipped as
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we believed to have already sampled through any potential reversal. The last two flows
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near the top were not Steens-like: aphyrhic, fine grained, and weathering to a reddish
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brown color. The AF-demagnetization data for all samples from this section were very
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well-behaved (Fig 2a). Only 8 great circle fits were used from a total of 128 samples and
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only one sample direction was discarded. Paleomagnetic directions are given in Table 1.
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Once again movement of the magnetic field during the eruption of the section suggests
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that the flows were erupted over a short period of time. The first 7 directions (DGs 1-7)
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make one counter-clockwise loop of the magnetic field, which represents an estimated
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500-1500 years based on modern analogues (Figure 7a). The youngest flows at Steens
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(Figure 7b) also loop in this way (DGs 10-16), suggesting that the North Mickey flows
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may have erupted during the same period of time as the uppermost flows found at Steens
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Mountain. Provided this correlation is correct, the North Mickey section preserves flows
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younger than the youngest flows found at Steens Mountain. From near the last directions
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recorded at Steens, the directions at North Mickey move to the northwest (DGs 7-12)
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(Figure 7c). This could imply about half the previous loop duration of about 750 years.
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The last four directions (DGs 13-16) record a movement back to slightly east of the GAD
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field direction before returning to the northwest (Figure 7d). Because this behavior
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appears less continuous, the eruption rate may have been much lower for this upper part
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of the section. The number of skipped flows and the change in petrology for the upper
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two flows precludes estimating the duration of eruption for this part of the section based
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on secular variation. In conclusion the bottom 17 flows (170 m) at the North Mickey
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location erupted in about 1000-3000 years and may overlap in time with the end of the
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Steens record. The next two flows are Steens-like and probably erupted within a few tens
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of thousands of years after the Steens reversal. The top two flows are not Steens-like and
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probably erupted within 1 My of the Steens reversal based on mapping by Hook (1981)
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and lava ages from Brueseke et al. (2007).
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Guano Rim
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The section at Guano Rim (42.1ºN, 119.5ºW) is 95 km southwest of Steens Mountain in
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the middle of a 20 km long escarpment. It consists of about 50 flow-on-flow reverse
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polarity lavas sampled 160 m up a small canyon (Figure 4). These samples were taken
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much earlier than the other sections, in 1986 and 1988, and their paleomagnetic
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directions determined at the U.S. Geological Survey (Menlo Park, California). Test
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samples were progressively AF-demagnetized in peak fields up to 80 mT. Each flow was
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magnetically cleaned at a peak alternating field chosen on the basis of behavior during
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the progressive demagnetization experiments (the stable endpoint method) to remove
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viscous and lightning strike overprints. Directions for each flow are given in Table 1. The
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samples are very well-behaved with k values for the flows varying from 103 to 1900 and
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only three flows with k < 200.
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Once again based on secular variation considerations these Steens-like lavas seem to have
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erupted over a short period of time. The first six directions (DGs 1-6) make one small
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loop to the west of the expected GAD field direction, suggesting 500-1500 year duration
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(Figure 8a). Then the field moved to a southwest and shallower direction before returning
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to another tight group (DGs 8-12) in a similar direction to the first (Figure 8b). This
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second movement suggests a duration of about 1000-2000 years, so the duration of the
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eruptions at Guano Rim may be ~3000 years.
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Locality Means, Stability Tests, and Averaging of Secular Variation
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Table 2 contains mean directions and VGPs for the four localities. Both polarities are
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well-represented, and the directions pass reversal tests. At the two northern localities the
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flows are all normal polarity except for one stratigraphically isolated flow (ss18, Table 1)
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at Summit Springs discussed earlier, whereas at the two southern localities the flows are
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entirely reverse. The means of the opposite polarity flows differ from antiparallel by 5.4°,
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well under the critical value of 8.4° for distinguishability at 95% confidence (McFadden
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and McElhinny, 1990). Because only one-fifth of the flows required tilt correction (the
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Pueblo Mountains section, dipping only 20°), a strong fold test is precluded, but even so
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the improvement in clustering upon untilting (k1/k2=1.19) is significant at 76%
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confidence. In light of these stability tests and the straightforward behavior of the great
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majority of samples during progressive demagnetization, these lava flows have almost
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certainly preserved reliable directions of the geomagnetic field at the times they cooled.
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Figure 9 shows the grouped flow directions that constitute the means in Table 2 for the
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four new localities and also Steens Mountain. For reverse directions the antipodes are
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plotted and all the directions have been rotated so that the mean of the entire distribution
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is at the center of the equal area projection. Although secular variation is not well
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averaged at any one of the four localities, due to rapid eruption rates, a compilation of all
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four sections should do a much better job. The total eruptive time for the continuous parts
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of the four sections is probably 6-10 ka, distributed on both sides of a polarity transition
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estimated to have taken 4400  900 years (Mankinen et al., 1985). In addition, directions
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are included from the nine flows that are not demonstrably part of the ‘continuously’
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emplaced sections, but for which age and polarity indicate that they were erupted within a
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million years of the transition, most of them probably much closer. The shape of the
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distribution, which does not depart greatly from circular symmetry, resembles that
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expected for the time-averaged field at the latitude (43°N) of Steens Mountain (e.g.,
380
Tauxe and Kent, 2004, Fig. 5). This is true for our four new localities combined, for the
381
Steens Mountain locality itself, and for all of them together (Figure 9). Moreover, the
382
angular dispersions of VGPs for all three data sets (Table 2) agree well with the expected
383
range of 20.0-22.6° from analyses of 5-22.5 Ma global lava-flow data (McFadden et al.,
384
1991).
385
386
These VGPs, corresponding to the directions in Figure 9, are plotted in Figure 10. Note
387
that the transformation to VGPs maps the approximately circular distribution of
388
directions into a distribution that is distinctly elongated. Although differing amounts of
389
tectonic rotation about a vertical axis could also produce elongation in this same
390
orientation, the agreement with expected shape and amount of secular variation of
391
directions described above suggests that little or no such movement has occurred between
392
the localities. Even stronger testimony against substantial differential tectonic rotation is
393
the comparable elongation in VGP distribution of the pre- and post-transitional flows of
394
Steens Mountain itself (Figure 10), which are from a single structural block. In
395
conclusion, all evidence indicates that the distribution of directions and associated VGPs
396
provides a representative time-average of middle Miocene secular variation sufficient to
397
yield a good estimate of the geocentric axial dipole field for the region.
398
399
Mid-Miocene Pole and Rotation of the Oregon Plateau
400
Our new data from the four localities invite a reexamination of the question whether the
401
Oregon Plateau has rotated relative to cratonic North America. The mean paleomagnetic
402
pole for our 50 directional groups and the 26 non-transitional directional groups at Steens
403
Mountain defines a high-quality paleomagnetic pole (Table 2) that represents a large
404
portion of the central Oregon Plateau (Figure 1). Omitted are several much older, smaller
405
scale studies that likely were not performed to the same standards. This Oregon Plateau
406
paleomagnetic pole is 5.1° from the High Plains pole used by Mankinen et al. (1987),
407
which consists almost entirely of VGPs from Steens Mountain and is very close to the
408
Steens pole in Table 2. Those authors also compiled results from studies published in the
409
nineteen sixties and seventies for 59 presumably unrotated flows of the Columbia River
410
Basalt Group (CRBG pole in Table 2). These flows are on average only one-million years
411
younger than the Steens Basalt and lie farther north in Washington and northernmost
412
Oregon. The CRBG and High Plains poles are almost identical, and so they concluded
413
that no significant rotation (0.4°  7.6°) occurred between the south-central Oregon
414
Plateau and the CRBG block of southeast Washington since mid-Miocene time. Our new
415
Oregon Plateau pole, however, implies clockwise rotation of 7.5°  5.9° relative to the
416
CRBG.
417
418
In addition, the paleomagnetic pole for the CRBG block might not be strictly
419
representative for the North American craton. There is a paucity of other useful data of
420
appropriate age from North America itself, but by reconstructing the relative positions of
421
the plates using sea-floor magnetic anomalies, mid-Miocene data from other continents
422
become available. Using this method, Besse and Courtillot (2002) provide a 15 Ma
423
synthetic pole for the North American plate (Table 2). Relative to it the Oregon Plateau
424
pole is rotated 14.5°  5.4° clockwise.
425
426
These new estimates for Oregon Plateau rotation reopen an early suggestion by Magill
427
and Cox (1980 and 1981) that south-central Oregon rotated about 10° clockwise relative
428
to southeast Washington since 20 Ma because of E-W extension of the Basin-and-Range
429
that decreases to zero northward. The High Plains result of Mankinen et al. (1987)
430
appeared to rule out that hypothesis. Nonetheless, several later studies revived the idea of
431
a Basin-and-Range-extension contribution to clockwise rotation of the Oregon Coast
432
Range and pushed the boundary of the rotated block to western Oregon so as to respect
433
the conclusion that Steens Mountain had not rotated (e.g., Wells and Heller, 1988). Our
434
new pole for the Oregon Plateau, however, indicates 7.5 degrees clockwise rotation
435
relative to the CRBG pole of southeast Washington and 14.5 degrees relative to the
436
synthetic pole for North America. It is derived from almost three times the number of
437
directional groups, including those for Steens Mountain, and represents a much larger
438
area of the Oregon Plateau. Northward decreasing Basin-and-Range extension still
439
appears to be the most likely mechanism for explaining such clockwise rotation. The
440
opening of the Oregon-Idaho graben is one well-studied example, its extension dated
441
between 15.3 Ma and 10.5 Ma and dying out to the north in central Oregon (Cummings et
442
al., 2000).
443
40K
444
Decay Constant and FCs Age Discussion
445
The conventional age of the Fish Canyon sanidine (FCs) agreed upon by the Earthtime
446
community is 28.02 ± 0.28 Ma (1σ as per the uncertainty convention used in this paper,
447
including the uncertainty in the decay constant; Renne et al, 1998). A more precise age
448
for the FCs (28.201 ±± 0.046 Ma, decay constant uncertainties included) has been
449
determined by intercalibration with the astronomical timescale (Kuiper et al., 2008). The
450
determination of this age is insensitive to the 40K decay constant value and any more
451
accurate age of the FCs determined in the future is unlikely to fall outside of the above
452
uncertainties. Other uncertainties in determining an 40Ar/39Ar age are now likely to
453
dominate. This FCs age is also in close agreement with an independent determination of
454
the FCs age of 28.28 ±± 0.06 Ma (Mundil et al., 2006) by intercalibration with the U/Pb
455
dating system.
456
457
The conventional 40K decay constant value of 5.543 × 10-10/a (sum of λβ- and λЕ ) stated
458
in Steiger and Jager, (1977) is from Beckinsale and Gale (1969) with a 40K half-life(T1/2)
459
of 1.265 ±95 0.0020 for “young” ages ( Beckinsale and Gale state “.. the error in T as a
460
result of errors in λЕ and λβ- is never greater than about ± 1.6% at the 95% confidence
461
level.”) adjusted for 40K abundance in Garner et al., (1976). Using the 1.6% uncertainty,
462
we get a decay constant of 5.543 ±95 0.089 × 10-10/a. This decay constant was updated to
463
5.463 ±± 0.107 × 10-10/a by Min et al., (2000) using new values for various physical
464
constants and a statistically rigorous analysis of the underlying activity data. This 40K
465
decay constant has much higher uncertainties than the reproducibility afforded by current
466
analytical techniques and methods. Motivated in part by the geochronological
467
community, two experiments to directly measure the 40K half-life by liquid scintillation
468
counting (LSC) techniques give T1/2= 1.248 ±95 0.004 × 109 a (Malonda and Carles,
469
2002) and T1/2= 1.248 ±95 0.003 × 109 a (Kossert and Gunther, 2004). Converting these
470
half-lives to decay constants and taking the weighted mean (although the two
471
experiments have some correlated error), we calculate a 40K decay constant of 5.5541 ±±
472
0.010 × 10-10/a. This value is in agreement with the geologically determined value of
473
5.530 ±± (7%) × 10-10/a (Mundil et al., 2006).
474
475
Nominal ages in this study were determined using 28.02 Ma (Renne et al., 1998) for FCs
476
and the Steiger and Jager (1977) decay constant. A more accurate representation of our
477
results, we believe, is given by the astronomically-calibrated age of Kuiper et al. (2008)
478
for FCs and the Steiger and Jager updated by Min et al., (2000) value for the 40K decay
479
constant. These preferred ages are most appropriate for comparison with the GTS2004
480
time scale, as discussed below. For a summary of the ages using different 40K decay
481
constants and FCs ages, see Table 3. See Appendix A2 for additional plateau and
482
isochron plots (A2.1-3), an isotopic data summary table (A2.4), an age adjustment
483
calculation spreadsheet (A2.5), and a spreadsheet with extended isotopic data and plots
484
(A2.6).
485
40Ar/39Ar
486
Results
487
Summit Springs
488
The second lava flow from the top of the section (ss02, Figure 4) has a normal polarity
489
and an 40Ar/39Ar plateau age of 16.72 ±± 0.28 Ma (16.61) from a plagioclase separate
490
(Figure 11a). The 15-step plateau encompasses all of the degassing steps and is well-
491
behaved. Inverse isochron analyses of Summit Springs lavas give ages that generally
492
agree with the plateau ages and have atmospheric 36Ar/40Ar ratios (Figure 11b). Our
493
determination of the age of the lava from flow ss09 was less straightforward. Plagioclase
494
separates of two different size fractions from the same sample were dated (Figure 11c,d).
495
The smaller size fraction (63-180 μm) was analyzed first because smaller grains generally
496
have higher potassium and lower calcium concentrations and therefore lower age
497
uncertainties. A single degassing analysis gave two plateau ages with the criteria
498
described in the “40Ar/39Ar Geochronology Procedures” section. A younger plateau
499
consisting of the last six degassing steps gave an age of 17.14 ±± 0.36 Ma (17.03) while
500
an older plateau consisting of the first eight degassing steps gives an age of 17.39 ±± 0.38
501
Ma (17.28). The younger age plateau has more radiogenic gas released, smaller
502
uncertainties, and higher probability so we choose it as the age of this degassing analysis.
503
The age was older than expected, given its normal polarity and Steens-like appearance, so
504
the larger size fraction (180-300 μm) was analyzed (Figure 11d). This analysis again gave
505
a good plateau age [16.71 ±± 0.38 Ma (16.60)]. The larger uncertainty in the Ca/K ratio is
506
due mostly to the larger uncertainties in the 37Ar (half life 35.0 days) abundances because
507
this analysis was performed nine months after the smaller size fraction. This age is 0.43
508
Ma younger than the previous age but their 2σ errors overlap by 0.31 Ma. While we
509
suspect that the plateau age of the larger size fraction is more likely correct, the weighted
510
mean of the two ages [16.92 ±± 0.26 Ma (16.83)] is the most objective 40Ar/39Ar age for
511
flow ss09 although using a weighted mean may underestimate the uncertainty.
512
513
Given the issues described above we also analyzed some groundmass separates from this
514
flow. The generally lower Ca/K of groundmass separates allows for greater precision on
515
each heating step, but often alteration and/or recoil effects can prevent good plateaus
516
from being produced. Two groundmass step-heating analyses yielded disturbed spectra
517
likely due to recoil effects. In both cases (one spectrum shown in Figure 11e) the step
518
ages start low and increase to a maximum at about 1/3 of the 39Ar released and then
519
slowly decrease in age. When only groundmass samples are available and the age spectra
520
are disturbed, the total gas integrated age may define a valid eruption age, provided that
521
alteration is insignificant and recoil effects are limited to argon isotope redistribution
522
within, rather than net loss from, the sample. Our integrated ages for two groundmass
523
analyses are 16.63 ±± 0.16(2σ) (16.52) Ma and 16.68 ±± 0.16 Ma (16.57). These ages are
524
consistent with the weighted mean of the two plagioclase separate ages but, given our
525
inability to verify the abovementioned criteria for validity, we do not include the ages in
526
the weighted mean age of the lava.
527
528
Two other eruptive units from the Summit Springs section were also dated. The first is
529
the Devine Canyon Tuff (V. Camp, pers. comm., 2005), which covers the flat area at the
530
bottom of the section (ss17, Figure 4). Ages of ten single-crystal sanidine grains were
531
determined by step-heating (Figure 11h) and have a weighted mean age of 9.781 ±±
532
0.022 Ma (9.718) that is much younger than the Steens lavas. These very precise ages fall
533
into two groups (Figure 5i,j,k) with ages of 9.756 ±± 0.020 Ma (9.693) and 9.810 ±±
534
0.022 Ma (9.747), suggesting an eruption age for the Devine Canyon Tuff of 9.756 ±±
535
0.020 Ma (9.693) and xenocrystic contamination of the tuff with a population of crystals
536
only 54 ka older. The second is from a basaltic lava of reverse polarity about 1 km east of
537
the main section (ss18, Figure 4). Age determinations from a plagioclase separate were
538
inconclusive (not shown), and a groundmass age spectrum is disturbed with an integrated
539
age of 17.3 ±± 0.6 Ma (17.2) (Figure 11f). Its isochron age is better defined at 16.16 ±±
540
0.16 Ma (16.06) (Figure 11g). Summarizing the most pertinent points of the Summit
541
Spring ages, the two lavas dated in the upper section erupted near the 16.6 Ma Steens
542
reversal while the more isolated reverse polarity lava erupted within 1 My of the Steens
543
reversal.
544
545
Pueblo Mountains
546
An 40Ar/39Ar age of 16.72 ±± 0.21 Ma (16.61) was determined from a plagioclase
547
separate of the reverse polarity flow pm10 (Figure 12a). As found at Summit Springs, this
548
age is indistinguishable from that of the 16.6 Ma Steens reversal. The dated sample lies
549
stratigraphically below the continuous section. The age of the section is also constrained
550
by a 16.511 ± 0.042 Ma (16.404) age from the weighted mean of ten step-heated sanidine
551
grains from a reverse polarity rhyolite located at the top of the section (Figure 12b).
552
553
554
Comparing Ages to the Geomagnetic Polarity Timescale
555
The boundaries for geomagnetic polarity chrons for much of the Neogene have been
556
determined from the astronomical tuning of deep-sea ocean cores that have magnetic
557
signatures. These studies have been compiled into a composite geomagnetic polarity time
558
scale (GPTS) in Gradstein et al. (2004). For Neogene times before the C5Ar.3r chron
559
(12.878 Ma) the chron boundaries of Gradstein et al. (2004) were determined using a
560
seafloor-spreading-rate history model of the Australia-Antarctic plate pair with
561
astronomically tuned tie points at the top of C5Ar.3r from Abdul-Aziz et al. (2003), the
562
top and bottom of C5Br from Shackleton et al. (2001), and a recalculated 23.03 Ma
563
Oligocene-Miocene boundary from Shackleton et al. (2000) using the La2003 orbital
564
model. A potentially better GPTS for the Early and Mid-Miocene is was determined by
565
Billups et al. (2004) from orbitally-tuned Ocean Drilling Program (ODP) Site 1090 cores
566
that have a magnetic signature. Their study should give more accurate astronomical time
567
boundaries to Early Neogene chrons, but the chrons relevant for this study are at the very
568
top of the cores and have low sedimentation rates. Both of these factors tend to make the
569
chron boundaries less reliable and we choose to compare to the Gradstein et al. (2004)
570
GPTS (Figure 13), which generally agrees with Billups et al. (2004) to one precession
571
cycle (~20 ka).
572
The younger normal polarity lava at Summit Springs (ss02) likely erupted in the C5Cn.3n
573
chron after the Steens reversal. Its errors (at two sigma confidence) encompass all of the
574
C5Cn.3n normal chron, reaches 1/3 into the younger C5Cn.2n normal chron, and does
575
not reach the older C5Dn normal chron (Figure 13). The two sigma errors of the weighted
576
mean 40Ar/39Ar age for the normal polarity lava ss09 encompass most of the reversed
577
C5Cr chron and one third of the of the normal C5Cn.3n chron. Barring any unknown
578
normal cryptochron during the reverse C5Cr chron and considering the younger
579
groundmass integrated ages, the lava is most likely to have erupted in the C5Cn.3n chron
580
after the Steens reversal. This interpretation is also supported by the similarities between
581
the Summit Springs magnetic field path and Steens Mountain post-reversal magnetic
582
field path. At Pueblo Mountains the reverse polarity flow pm10 likely erupted during the
583
C5Cr chron before the Steens reversal, although the two sigma errors of the 40Ar/39Ar age
584
reach into the short C5Cn.2r after the Steens reversal. The younger and more precise age
585
of the reverse polarity capping rhyolite (pma) at Pueblo Mountains unequivocally places
586
its eruption during the C5Cn.2r chron, indicating that no eruptions took place at this
587
section during the 170 ka-long C5Cn.3n (Gradstein et al., 2004) normal polarity chron.
588
589
Conclusions
590
Paleomagnetic analyses and 40Ar/39Ar ages from Pueblo Mountains, Summit Springs,
591
North Mickey, and Guano Rim suggest that the bulk of these sections were erupted in
592
short bursts (~1-3 ka) within ~300 ka of the Steens reversal. Comparison to the
593
geomagnetic polarity time scale (Figure 7) and two 40Ar/39Ar ages (Table 3) show that the
594
Pueblo Mountains reverse section erupted near and likely before the Steens reversal.
595
Comparing the simple directional path of the remanent magnetization with secular
596
variation of the recent field indicates that the continuously sampled section erupted in
597
about 2.5 ka. At Summit Springs two 40Ar/39Ar ages, dominantly normal polarity (all but
598
one stratigraphically isolated flow), and the simple directional path suggest that the
599
section erupted in the chron after the Steens reversal within about 0.5 to 1.5 ka. At North
600
Mickey geologic mapping and dates by others place the section near the Steens reversal,
601
and secular variation analysis indicates that the lower part erupted in 1-3 ka. At Guano
602
Rim the low (14.1°) dispersion of VGPs and the directional path suggest a ~3 ka eruption
603
duration. The rapid eruption of these sizeable sections near the time of the Steens reversal
604
suggests that the Steens Basalts were all emplaced within a few hundred thousand years
605
on the Oregon Plateau at around 16.6 Ma.
606
607
Although the flows at each individual locality do not average out secular variation, by
608
combining all of the sections enough time is sampled to obtain a meaningful average
609
paleomagnetic pole. From our four sections there are a total 26 normal and 24 reverse
610
directions (Table 2) that yield a positive reversal test. Combining our VGP directions
611
with the non-transitional VGPs at Steens Mountain yields a new pole for the Oregon
612
Plateau that indicates clockwise rotation of 7.4°  5.9° with respect to the CRBG and
613
14.4°  5.4° with respect to cratonic North America. This implies some extension to the
614
east of the study area since 16.6 Ma that dies out rapidly to the north.
615
616
617
Acknowledgements
618
We would like to extend special thanks to Kim Knight for assistance with 40Ar/39Ar
619
sample preparation and analysis methods and Chris Pluhar for assistance with
620
paleomagnetic procedures and methods. Laurie Brown, Andy Calvert, Jim Gill,
621
Johnathan Hagstrum, Peter Hooper, and Brad Singer provided helpful reviews which
622
improved the clarity of this paper. We thank Eli Morris and Walter Schillinger for UCSC
623
paleomagnetic instrumentation and software support, Tim Becker for BGC lab support,
624
and Fred Jourdan for assistance with determining plagioclase alteration. For highly
625
competent field work assistance we would like to thank Mike Dueck, Bijan Hatami, Peter
626
Lippert, and Andy Daniels. This work was funded by NSF grant EAR-0310316 and -
627
0711418 to RSC and JMG, minigrants from the UCSC Committee on Research and
628
Institute of Geophysics and Planetary Physics, and support to the BGC from the Ann and
629
Gordon Getty Foundation.
630
631
632
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