Origin and Evolution of the Sierra Nevada themed issue

Oligocene and Miocene arc volcanism in northeastern California: Evidence for post-Eocene segmentation of the subducting Farallon plate

Joseph P. Colgan1,*, Anne E. Egger2, David A. John1, Brian Cousens3, Robert J. Fleck1, and Christopher D. Henry4 1U.S. Geological Survey, 345 Middlefield Road, Mail Stop 973, Menlo Park, California 94025, USA 2Geological and Environmental Sciences, Stanford University, 450 Serra Mall, Stanford, California 94305, USA 3Carleton University, 1125 Colonel By Drive, Ottawa, Ontario, Canada K1S5B6 4Nevada Bureau of Mines and Geology, University of Nevada, Reno, Nevada 89557, USA

ABSTRACT sitionally similar to Oligocene rocks in the Unlike the western Cascades, however, vol­ Warner Range. They are distinctly different canic rocks of the ancestral Cascades are a subset­ The Warner Range in northeastern Cali- from younger (Late Miocene to Pliocene) of a diverse and widespread suite of Cenozoic fornia exposes a section of Tertiary rocks over high-Al, low-K olivine tholeiites, which are volcanic rocks erupted across the Basin and

3 km thick, offering a unique opportunity to more mafic (46%–49% SiO2), did not build Range Province since the Eocene. The ancestral study the long-term history of Cascade arc large edifices, and are thought to be related Cascades samples plotted in Figure 1 are those volcanism in an area otherwise covered ­by to backarc extension. The Warner Range is considered by du Bray et al. (2009) to be plausi­ younger volcanic rocks. The oldest locally ~100 km east of the axis of the modern arc ble constituents of the arc based on a variety of sourced volcanic rocks in the Warner Range in northeastern California, suggesting that criteria, including composition, eruptive style, are Oligocene (28–24 Ma) and include a the Cascade arc south of modern Mount and location (see du Bray et al., 2009, for a sequence of basalt and basaltic andesite lava Shasta migrated west during the Late Mio- complete list), but they acknowledged that “no flows overlain by hornblende and pyroxene cene and Pliocene, while the arc north of clear-cut definition distinguishes constituents of andesite pyroclastic flows and minor lava Mount Shasta remained in essentially the the southern segment of the ancestral Cascades flows. Both sequences vary in thickness same position. We interpret these patterns as magmatic arc” (p. 3), i.e., from similar-age vol­ (0–2 km) along strike and are inferred to be evidence for an Eocene to Miocene tear in the canic rocks related to other tectonic processes,­ the erosional remnants of one or more large, subducting slab, with a more steeply dipping notably the major pulse of mid-Tertiary mag­ partly overlapping composite volcanoes. No plate segment to the north, and an initially matism thought to result from delamination of volcanic rocks were erupted in the Warner more gently ­dipping segment to the south the shallow east-dipping Farallon slab (e.g., Range between ca. 24 and 16 Ma, although that gradually steepened from the Middle Armstrong and Ward, 1991; Best et al., 1989; minor distally sourced silicic tuffs were Miocene to the present. Humphreys, 1995; Henry et al., 2009). Thus, deposited during this time. Arc volcanism although the existence of an ancestral Cascades resumed ca. 16 Ma with eruption of basalt INTRODUCTION arc south of Mount Lassen ­is a straight­forward and basaltic andesite lavas sourced from consequence of Pacific (Farallon)–North Amer­ eruptive centers 5–10 km south of the relict The Cascade volcanic arc in Oregon, Wash­ ican plate interaction, the southern segment of Oligocene centers. Post–16 Ma arc volcanism ington, and northernmost California (Fig. 1) has the arc is sufficiently different from the west­ continued until ca. 8 Ma, forming numerous been established close to its present location ern Cascades that the relationship between eroded but well-preserved shield volcanoes since the Eocene. Eocene to Late Miocene rem­ the two is not straight­forward. Glazner and to the south of the Warner Range. Oligo- nants of the arc consist of a swath of volcanic Farmer (2008), for example, proposed that cene to Late Miocene volcanic rocks in and rocks just west of the modern arc, referred to as no Cascade arc ever existed south of Lassen around the Warner Range are calc-alkaline the western Cascades (Fig. 1) (e.g., McBirney, Peak, although recent studies of the ances­

basalts to andesites (48%–61% SiO2) that 1978; Priest, 1990; Smith, 1993; Sherrod and tral Cascades have concluded otherwise­ (e.g., display negative Ti, Nb, and Ta anomalies Smith, 2000; du Bray et al., 2006). A swath of Putirka and Busby, 2007; Cousens et al., 2008; in trace element spider diagrams, consistent Oligocene to Pliocene volcanic rocks in west­ Busby et al., 2008a, 2008b; Hagan et al., 2009; with an arc setting. Middle Miocene lavas ern Nevada and eastern California (Fig. 1) has Busby and Putirka, 2009). in the Warner Range are distinctly different in been interpreted as a southern continuation of Volcanic rocks of the inferred ancestral age, composition, and eruptive style from this arc, the ancestral Cascades, active during Cascade arc trail off to the north into a poorly the nearby Steens Basalt, with which they subduction of the Farallon plate beneath North mapped area of northern California more than were previously correlated. Middle to Late America prior to northward migration of the 100 km east of the well-defined western Cas­ Miocene shield volcanoes south of the War- Mendocino Triple Junction (Fig. 1) (e.g., Noble, cades, which end to the northwest of Mount ner Range consist of homogeneous basaltic 1972; Christiansen and Yeats, 1992; Putirka and Shasta (Fig. 1). The region of northeastern

andesites (53%–57% SiO2) that are compo- Busby, 2007; Cousens et al., 2008; Busby et al., California between the inferred ancestral Cas­ 2008a, 2008b; Hagan et al., 2009; Busby and cade arc and the undisputed western Cascade *[email protected] Putirka, 2009). arc is mostly covered by Pliocene and younger

Geosphere; June 2011; v. 7; no. 3; p. 733–755; doi:10.1130/GES00650.1; 14 figures; 1 table; 3 appendix tables; 1 supplemental table.

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geology of northeastern California. Although considered part of the Basin and Range Province, the plateau east of the Warner Range in northwest­ ern Nevada is largely unextended (Colgan et al., 2006; Lerch et al., 2008). It is covered by Mio­ cene and younger lava flows and ash-flow tuffs (Middle Miocene bimodal volcanic rocks, Fig. 2); the Middle Miocene tuffs have been inferred to be the earliest eruptions of the Yellowstone hotspot (e.g., Pierce and Morgan, 1992). West of the Warner­ Range, the Modoc Plateau is mostly cov­ ered by flat-lying Pliocene and younger lava flows (Pliocene–Quaternary volcanic rocks, Fig. 2) and has not been broken up by major post-Miocene extension (McKee et al., 1983). Miocene volcanic rocks as old as 14–15 Ma are exposed south of the Warner Range (Middle to Late Miocene volcanic arc rocks, Fig. 2) (Grose, 2000), and similar Mio­ cene volcanic rocks probably extend southeast into northwestern Nevada (Fig. 2), although they have not been mapped in detail. Pre-Tertiary basement rocks are not exposed in the Warner Range. In the Klamath Moun­ tains (~125 km to the west), basement consists of a complex tectonic assemblage of Paleozoic and Mesozoic oceanic-affinity terranes intruded by Mesozoic plutons (e.g., Snoke and Barnes, 2006, and references therein). In northwestern Nevada (100 km to the east), basement consists of abundant Cretaceous and minor Jurassic gra­ nitic plutons intruding Paleozoic and Mesozoic metasedimentary rocks (e.g., Wyld and Wright, 2001). The nearest exposed basement to the south (80–100 km away) consists of Cretaceous gran­ ite (e.g., Grose et al., 1992). A seismic refraction profile across northwestern Nevada and north­ eastern California (including the Warner Range) imaged a low-velocity zone beneath northwestern Nevada (~6.0 km/s) extending to ~16 km depth in the upper crust that Lerch et al. (2007) interpreted as the northern extension of the Mesozoic (Sierra Nevada) batholith beneath Cenozoic cover. Figure 1. Map of western United States showing major plate boundaries and past position Although the western edge of this low-velocity of Mendocino triple junction (Atwater and Stock, 1998), Quaternary Cascade volcanic arc zone is not sharply defined in the seismic data, (Hildreth, 2007), Eocene to Pliocene western Cascade arc (du Bray et al., 2006), and inferred it is well east of the Warner Range. Pre-Tertiary ancestral Cascades in California and Nevada (du Bray et al., 2009). basement beneath the Warner Range and Modoc Plateau most likely consists of accreted crust similar to that exposed in the Klamath Mountains lavas (Fig. 2), and the pre-Pliocene history of REGIONAL GEOLOGIC SETTING and northwestern Nevada, rather than the Creta­ the area is largely obscured. Near the Nevada- ceous Sierra Nevada batholith, which forms the Oregon-California border, however, the Warner­ The Warner Range marks the western bound­ basement to the ancestral Cascades further south. Range (Figs. 2 and 3) exposes nearly 3 km ary of the Basin and Range Province in north­ of Tertiary strata as old as Eocene, offering a ern California and was formed by Miocene and WARNER RANGE VOLCANIC ROCKS window into the pre-Miocene history of this younger slip on the Surprise Valley fault (Russell, area. The purpose of this study is to document 1928; Duffield and McKee, 1986; Carmichael Eocene to Oligocene Volcanic and the history of older (Oligocene and Miocene) et al., 2006; Colgan et al., 2008; Egger et al., Sedimentary Rocks volcanism in this region by mapping and dat­ 2009). Faulting and uplift of the Warner Range ing volcanic rocks, particularly locally sourced have exposed more than 3 km of Tertiary rocks The oldest rocks exposed in the Warner Range rocks, and to establish the tectonic setting of as old as Eocene, providing a well-exposed but are Eocene to Early Oligocene sedimentary this volcanic activity. spatially limited window into the pre-Miocene and sparse volcanic rocks that have no formal

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Figure 2. Simplified geologic map of northeastern Califor- nia and parts of Nevada and Oregon. Modified from state geologic maps of Califor- nia (Jennings­, 1977), Nevada (Crafford,­ 2007), and Oregon (Walker and MacLeod, 1991), with additional data from Helley­ and Harwood (1985), Grose et al. (1991, 1992), and Irwin (1997), and Egger and Miller (2011). Quaternary erup- tive centers are from Hildreth­ (2007). Vol­canoes in eastern part of map are from Grose (2000). Additional geochemical analyses courtesy of V.E. Camp and M.E. Ross (2007, personal commun.).

designation,­ but have been divided into various 31 Ma on interbedded volcanic ash. Rare clasts These deposits have not been directly dated but informal subunits by different authors (Russell, of Mesozoic and Paleozoic basement are pres­ they overlie rocks younger than 33.4 Ma and are 1928; Martz, 1970; Duffield and Weldin, 1976; ent in this unit (Colgan et al., 2008; Egger et al., overlain by rocks as old as 27.5 Ma. They are Duffield and McKee, 1986; Myers, 1998; Egger 2009), but the vast majority of conglomerate interpreted as lahars and indicate volcanic activ­ et al., 2009). We show this unit as undivided Ts clasts are volcanic (predominantly andesite with ity in the general area of northeastern Califor­ in Figure 3, but it can be broadly subdivided conspicuous phenocrysts of hornblende and nia and southern Oregon in the Late Oligocene into three lithologic packages. (1) The basal plagioclase). In Egger et al. (2009), it was sug­ (Duffield and McKee, 1986; Egger et al., 2009). unit is minimally exposed but contains several gested that this unit records erosion and redepo­ hornblende andesite lava flows, one of which sition from a nearby volcanic arc to the south. Oligocene Volcanic Rocks has been dated to 41 Ma (Axelrod, 1966), indi­ (3) The upper unit consists of 500–740 m of cating minor local volcanic activity as early as ­reddish-weathering volcaniclastic rocks called Lake City Basalts (28–26.5 Ma) the Eocene. The base of the Tertiary section is the Lost Woods formation by Martz (1970). North of Cedar Pass, Oligocene vol­cani­ not exposed but presumably rests on Mesozoic These rocks consist primarily of angular clastic rocks are overlain by a sequence of mafic and/or Paleozoic basement. (2) The middle unit unsorted lava blocks to several meters across lava flows that we informally refer to as the consists of as much as 1500 m of Late Eocene to supported by a fine-grained matrix. Blocks con­ Lake City basalts (unit Tlb, Fig. 3). The package­ Oligocene sandstone, conglomerate, and lacus­ sist of dark, generally phenocryst poor lava. The of flows ranges in thickness from a few tens of trine sedimentary rocks, from which Duffield unit contains abundant fragments of petrified meters near Cedar Pass to more than 2 km at and McKee (1986) reported dates of 34 and wood, including whole logs several meters long. Buck Mountain at the north end of the study

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Figure 3. Geologic map of the Warner Range, simplified from Egger and Miller (2011) (legend on following page).

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dant inclusions of melt and/or opaque oxides. Hornblende is conspicuously absent compared to the slightly younger rocks described in the next section. Two samples of the base of the Lake City basalts collected ~3.5 km apart yielded 40Ar/39Ar ages of 27.49 ± 0.33 Ma from plagioclase (sam­ ple AE05-WR03) and 27.83 ± 0.21 Ma from groundmass (sample JC08-WR405) (Table 1; Fig. 5). Plagioclase from a sample (07‑C‑6) mid­ way up the sequence of flows yielded a 40Ar/39Ar age of 27.14 ± 0.08 Ma (Table 1; Fig. 5). Plagio­ clase from a sample of the uppermost flow, just beneath a Late Miocene rhyolite dome at the north end of the study area (WR07-AE40), yielded a disturbed 39Ar release spectrum from which we calculate an isochron age of 25.70 ± 0.94 Ma (Table 1; Fig. 5). This age is consis­ tent with its position at the top of the section, but east of Bald Mountain, the Lake City basalts are overlain by an ash-flow tuff with a sanidine 40Ar/39Ar age of 26.64 ± 0.08 Ma (Fig. 4), which we consider a more precise upper age limit for the Lake City basalts. From these dates we con­ clude that the Lake City basalts were erupted during a relatively brief period (<1–1.5 m.y.) in the Late Oligocene, from ca. 28 to 26.5 Ma. Where exposed, the basal contact of the Lake City basalt sequence is subparallel to bedding in the underlying volcaniclastic rocks, not cut down into them, indicating that the basalt flows primarily built an edifice on top of the older rocks, rather than being deposited in paleo­ topography.­ The map pattern of the basalt flows (Fig. 3) indicates that this edifice was ~2 km high with a southern slope dipping 7°–8°, sug­ gesting a moderate-sized shield volcano.

Cedar Pass Complex (26.6–24.5 Ma) The name Cedar Pass complex is our infor­ mal term for andesitic breccias, lava flows, and hypabyssal intrusive rocks exposed in the vicin­ ity of Cedar Pass in the central Warner Range (Figs. 3, 6A, and 6B). They are exposed as far north as Franklin Creek, where they overlie the Lake City basalts, and grade southward into Duffield and McKee’s (1986) “composite vol­ canic unit” in the South Warner Wilderness (unit Figure 3. (legend). Tcu in Fig. 3), where they overlie older Oligo­ cene volcaniclastic rocks (Fig. 3). Most of the Cedar Pass complex consists area (Fig. 4). The underlying sedimentary rocks interiors where the entire thickness of the flow of andesitic breccias composed of angular are not exposed at this latitude, but they were is exposed. Flows vary from aphyric to mod­ lava blocks ranging from a few centimeters to penetrated by a geothermal well (LCSH‑5, Figs. erately porphyritic with phenocrysts of plagio­ >2 m across in a matrix of more finely broken 3 and 4), putting an upper limit of ~2700 m on clase, olivine, and pyroxene; the olivine is com­ rock fragments. Bedding is occasionally well the thickness of the Lake City basalts. Indi­ monly altered to iddingsite and the plagioclase developed (e.g., deposits on far end of ridge in vidual flows are a few meters thick, dark gray to is usually partly altered to clay or white mica. Fig. 6A), but breccias are more typically mas­ black when fresh, but often reddish weathering. Some flows contain abundant (>50%) large sive and unsorted. Lava blocks range from dark, Flow tops are generally vesicular and flow mar­ (>1 cm) plagioclase phenocrysts; these have phenocryst-poor lava to lighter gray-green and gins are strongly brecciated, with more massive a sieve-like texture in thin section with abun­ abundantly plagioclase-phyric and hornblende-

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TABLE 1. GEOCHRONOLOGIC DATA FROM WARNER RANGE VOLCANIC ROCKS Longitude* Latitude* Sample (°W) (°N) Rock type Method MineralAge (±2σ)† Data source§ Older sedimentary sequence (Ts) D63B 120.15444 41.38333 andesitic ash K-Ar hornblende 33.9 ± 5.4Duffield and McKee (1986) D113B 120.16833 41.42083 andesitic ash K-Ar hornblende 31.1 ± 2.6Duffield and McKee (1986) 699_15A 120.22167 41.50000 andesitic ash K-Ar hornblende 28.8 ± 2.2Duffield and McKee (1986) Lake City basalts (Tlb) AE05WR03 120.22406 41.57833 basalt (basal flow) 40Ar/39Ar plagioclase27.49 ± 0.33 This study (USGS) 07-C-6 120.24034 41.60912 olivine basalt flow 40Ar/39Ar plagioclase27.14 ± 0.08 This study (USGS) WR07AE40 120.25011 41.70415 basalt (upper flow) 40Ar/39Ar plagioclase25.70 ± 0.94 This study (USGS) JC08-WR405 120.23168 41.60900 basalt (basal flow) 40Ar/39Ar groundmass 27.83 ± 0.21 This study (USGS) Cedar Pass complex and ash-flow tuffs (Tcp and Tcu) 07-C-19 120.27277 41.58808 andesite lava flow 40Ar/39Ar plagioclase25.727 ± 0.045This study (USGS) JC07-WR303 120.26150 41.61350 ash-flow tuff 40Ar/39Ar sanidine26.35 ± 0.11 This study (USGS) WR07-AE49 120.22595 41.48698 basalt plug 40Ar/39Ar plagioclase26.86 ± 0.08 This study (USGS) 07-C-10 120.21695 41.43917 andesite lava flow 40Ar/39Ar groundmass 24.47 ± 0.34 This study (USGS) SV96 120.258341.5090 basaltic andesite 40Ar/39Ar groundmass 27.07 ± 0.22 Carmichael et al. (2006) SV70a 120.261241.4920 basaltic andesite 40Ar/39Ar groundmass 30.02 ± 0.52 Carmichael et al. (2006) 966_15 120.271741.5883 andesite K-Ar hornblende 28.7 ± 2.2Duffield and McKee (1986) 433 120.322241.5083 andesite K-Ar hornblende 26.6 ± 2.2Duffield and McKee (1986) D27B 120.216441.4389 ash-flow tuff K-Ar biotite26.3 ± 2.0Duffield and McKee (1986) D79B 120.146741.3039 ash-flow tuff K-Ar biotite25.4 ± 2.0Duffield and McKee (1986) JC08-WR411 120.21417 41.44050 ash-flow tuff 40Ar/39Ar sanidine26.526 ± 0.058 This study (NMT) JC08-WR412 120.21734 41.43877 ash-flow tuff 40Ar/39Ar sanidine25.765 ± 0.061 This study (NMT) H08-57 120.24622 41.60567 ash-flow tuff 40Ar/39Ar sanidine26.642 ± 0.077 This study (NMT) Miocene tuffaceous sedimentary rocks (Tmt) JC08-WR410 120.22570 41.43801 reworked tuff 40Ar/39Ar sanidine19.22 ± 0.27 This study (NMT) 1011 120.243341.4606 ash-flow tuff K-Ar biotite17.3 ± 1.2Duffield and McKee (1986) SV31 120.414041.2315 pumice from tuff 40Ar/39Ar plagioclase14.09 ± 0.05 Carmichael et al. (2006) SV59 120.440341.2295 pumice from tuff 40Ar/39Ar plagioclase13.52 ± 0.06 Carmichael et al. (2006) Miocene Warner Range lavas (Tmb) D302B 120.157841.2811 basalt K-Ar whole rock 15.8 ± 1.0Duffield and McKee (1986) D144B 120.150641.2861 basalt K-Ar whole rock 15.7 ± 1.0Duffield and McKee (1986) D474B 120.182241.2117 basalt K-Ar whole rock 15.7 ± 0.8Duffield and McKee (1986) D418B 120.215041.2633 basalt K-Ar whole rock 14.1 ± 0.8Duffield and McKee (1986) SV142 120.341541.3795 basalt 40Ar/39Ar groundmass 14.57 ± 0.08 Carmichael et al. (2006) SV126 120.064541.1997 basalt 40Ar/39Ar groundmass 15.36 ± 0.08 Carmichael et al. (2006) D173B 120.220041.4039 basalt K-Ar whole rock 14.0 ± 0.8Duffield and McKee (1986) D398B 120.293941.2817 rhyoliteK-Arbiotite 14.5 ± 0.8Duffield and McKee (1986) D235B 120.193341.3111 basalt K-Ar whole rock 14.1 ± 0.8Duffield and McKee (1986) R53B 120.243341.0983 rhyoliteK-Arbiotite 15.5 ± 1.0Duffield and McKee (1986) R54B 120.110641.2383 rhyoliteK-Arbiotite 15.9 ± 1.0Duffield and McKee (1986) BT-1 120.264441.2053 rhyoliteK-Arbiotite 16.0 ± 1.0Duffield and McKee (1986) SV136 120.414841.2305 basaltic andesite 40Ar/39Ar groundmass 12.12 ± 0.05 Carmichael et al. (2006) *Precision of latitude, longitude as originally reported. Locations for this study relative to North American Datum of 1927 (NAD27). Coordinate systems used by Carmichael et al. (2006) and Duffield and McKee (1986) are not specified and assumed to be NAD27. †Precision of ages as originally reported. All 40Ar/39Ar dates from this study calculated relative to Fish Canyon Tuff sanidine = 28.02 Ma. §USGS—U.S. Geological Survey laboratory in Menlo Park, California. NMT—New Mexico Tech.

phyric lava, and multiple types of lava blocks Payne Peak. At least one small (<1 km2) por­ 0.08 Ma (sample H08–57, Fig. 7) and is capped are often found within the same outcrop. Some phyritic ande­site body intrudes the breccias by a basalt flow with a40 Ar/39Ar age of 24.47 ± blocks are glassy and some have well-preserved at Cedar Pass (Fig. 3). This intrusion con­ 0.34 Ma (sample 07-C10, Fig. 5). A por­phyritic, radial fractures (Fig. 6B), indicating that they tains abundant pheno­crysts of hornblende and plagioclase- and hornblende-bearing lava flow were hot at the time of emplacement and have plagioclase­ in a fine-grained matrix. on the summit of Cedar Mountain (Fig. 3) been minimally disturbed since. We interpret Duffield and McKee (1986) interpreted one yielded a 40Ar/39Ar age of 25.73 ± 0.05 Ma these deposits as block-and-ash flows that range vent area in the Cedar Pass complex centered (sample ­07-C19, Fig. 5). Duffield and McKee from minimally reworked (glassy lava blocks on Dry Creek Basin (Fig. 3), on the basis of (1986) reported a K-Ar age of 28.7 on the with well-preserved radial fracturing) to signifi­ radial dips in the surrounding deposits. This is same outcrop, and a K-Ar age of 26.6 Ma from cantly reworked (e.g., the well-bedded deposits consistent with our new mapping (Fig. 3; Egger another sample to the southwest. Carmichael­ shown in Fig. 6A). and Miller, 2011), and we suggest that another et al. (2006) reported 40Ar/39Ar ages of ca. 27 Ma Porphyritic lava flows containing pheno­ vent may exist between Cedar Pass and Cedar and ca. 30 Ma from two samples in the upper crysts of plagioclase, hornblende, and lesser Mountain, where reworked block-and-ash flow part of the section in the Deep Creek drainage pyroxene are interbedded with the andesitic deposits contain lava blocks as much as 2 m (Fig. 3). We regard our new data, particularly breccias at the summit of Cedar Mountain and across and are intruded by a porphyritic ande­ the sanidine dates from the ash-flow tuffs, to be on the north edge of the South Warner Wilder­ site body that is exposed in roadcuts at Cedar more accurate estimates of the age of the Cedar ness (Fig. 3). Dark, gray-weathering, pheno­ Pass (Fig. 3). Pass complex than previous dates. The tuffs in cryst-poor lava flows with a medium-grained East of Bald Mountain (Fig. 3), the Cedar Pass the Cedar Pass complex were thus erupted in groundmass of plagioclase and pyroxene (but complex overlies a densely welded rhyolite ash- <2 m.y., between ca. 26.6 and 24.5 Ma, and pos­ no hornblende) overlie andesitic breccias on flow tuff with a sanidine 40Ar/39Ar age of 26.64 ± sibly between 26.6 and 25.7 Ma.

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Figure 5. 40Ar/39Ar step-heating diagrams for samples analyzed at the U.S. Geological Survey. Unfilled steps omitted from plateau age calculation. Dates recalculated using an assumed age of 28.02 for Fish Can- yon Tuff (Renne et al., 1998). Original data (Table A2) col- lected using a TCR-2 sanidine monitor with an assumed age of 27.87 Ma corresponding to an age of 27.84 for Fish Canyon Tuff sanidine. MSWD—mean square of weighted deviates.

Hays Volcano (ca. 24 Ma) exposed and they may be more extensive in the dates of ca. 24 Ma from the Hays volcano, simi­ On the east side of Surprise Valley in the subsurface. Individual outcrops consist of layers lar in age to the youngest dated lava flow in the southern Hays Canyon Range, Oligocene ash- of spatter, agglutinated lavas, and scoria blocks Cedar Pass complex. flow tuffs are overlain by the remnants of an as thick as several meters (typically ~1 m). These Oligocene spatter volcano called the Hays vol­ layers ­dip as much as 20° radially away from an Ash-Flow Tuffs (26.6–25.8 Ma) cano by Carmichael et al. (2006) (Fig. 3). These inferred central vent that is highly altered and Several densely welded ash-flow tuffs are deposits now cover an area ~12 × 12 km and more deeply eroded than the surrounding hills. interbedded with the Cedar Pass complex and are as thick as 550 m, although their base is not Carmichael et al. (2006) reported two 40Ar/39Ar locally separate it from the Lake City basalts.

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Valley are a >300-m-thick (base not exposed) rhyolite ash-flow tuff that Carmichael et al. (2006) called the Forty-nine ash-flow deposit. It is unknown if this tuff correlates with those described here, but Carmichael et al. (2006) reported a poorly defined biotite40 Ar/39Ar age of ca. 26 Ma from it, so it is at least of similar age. The sanidine- and/or anorthoclase- and bio­ tite-bearing ash-flow tuffs are compositionally distinct from the locally sourced Oligocene lava flows and breccia deposits of the Cedar Pass complex. They were most likely generated by large caldera-forming eruptions and may have traveled significant distances from their sources, as first noted by Duffield and McKee (1986). The thickness of the tuffs (to 160 m in the Warner­ Range; >300 m in the Hays Can­ yon Range) and the limited lateral extent of individual tuff beds are consistent with deposi­ tion in paleovalleys or other low spots in the Oligocene paleotopography; ash-flow tuffs elsewhere in the Basin and Range have been documented to have traveled hundreds of kilo­ meters down such channels (e.g., Henry, 2008, Henry and Faulds, 2010). We cannot corre­ late these tuffs with any known calderas, but caldera-forming eruptions were ongoing dur­ ing the Oligocene in northwestern and central Nevada (e.g., Noble et al., 1970; Best et al., 1989; Christiansen and Yeats, 1992), and these areas are plausible sources. Figure 6. (A) View north of well-bedded Oligocene Cedar Pass complex near Bald Mountain Miocene Volcanic Rocks (Fig. 3), indicating significant reworking of portions of the complex. Beds in foreground are about 5 m thick. (B) Radially fractured lava block in Cedar Pass complex along Parker Tuffs and Tuffaceous Sedimentary Rocks Creek (Fig. 3), indicating minimal reworking of portions of the complex. Hammer is 27 cm. (ca. 19–13 Ma) (C) View south of Oligocene and Miocene volcanic rocks east of Warren Peak. Note angular­ South of Squaw Peak, Miocene mafic lava conformity of homoclinally west-dipping section. Exposed section is about 1200 m thick. flows are separated from the Cedar Pass com­ (D) View southwest of inferred Miocene volcano at Emerson Peak; note radial dike and plex by ~100 m of poorly exposed tuffaceous west-dipping lava flows in foreground; lava flows on skyline dip south and west away from sandstone and siltstone containing small the viewer. Dike in foreground is about 2 m thick. (E) Panoramic view of Shinn Mountain (<2 mm) biotite and feldspar crystals; some (Fig. 2), a ca. 12 Ma basaltic andesite shield volcano ~650 m high. feldspars are distinctly iridescent. Plagioclase from a reworked tuff in the upper part of this deposit yielded a 40Ar/39Ar age of 19.22 ± Ash-flow tuffs also make up the bulk of the tion includes several ash-flow tuffs in addition 0.27 Ma (Table 1; Fig. 4). Duffield and McKee Oligocene­ section south of Parker Creek (Fig. 3), to distal material from the Cedar Pass complex (1986) reported a K-Ar sanidine age of 17.3 Ma and in the Hays Canyon Range (Fig. 3). East of (Fig. 4). The base of the section here consists from a thin (<5 m) densely welded ash-flow tuff Bald Mountain (Fig. 3), the Lake City basalts and of ~70 m of poorly welded, lithic- and pumice- at the top of this deposit ~3 km to the north. Cedar Pass complex are separated by a densely poor tuff with abundant biotite and lesser pheno­ Biotite- and sanidine-bearing tuffaceous sedi­ welded, pumice-rich, crystal- and lithic-poor crysts of sanidine, plagioclase, and quartz. This mentary rocks are also present south of Parker tuff containing sparse phenocrysts of biotite and tuff yielded a 40Ar/39Ar sanidine age of 26.53 ± Creek, between andesitic breccias of the Cedar anorthoclase, which yielded a 40Ar/39Ar anortho­ 0.06 Ma (sample JC08‑WR11, Fig. 7). Higher in Pass complex to the north and mafic lava clase age of 26.64 ± 0.08 Ma (sample H08–57, the section there is a thick (~160 m) lithic- and flows (described in the following) to the south Fig. 7). A few meters upsection,­ a single ­out­ pumice-rich tuff with abundant biotite and sparse (Fig. 3). Like the ash-flow tuffs in the Oligo­ crop of a moderately crystal- and pumice-rich, feldspar phenocrysts (Fig. 4). The lower 30–40 m cene section, the tuffaceous sedimentary rocks reddish-weathering tuff with pheno­crysts of are densely welded with a well-developed basal and sanidine- and biotite-bearing ­tuffs (espe­ anorthoclase and biotite yielded a 40Ar/39Ar vitrophyre. This tuff yielded a 40Ar/39Ar sanidine cially the densely welded tuff) in the lower part anorthoclase­ age of 26.35 ± 0.11 Ma (sample age of 25.77 ± 0.06 Ma (sample­ JC08‑WR412, of the Miocene section are distinctly different JC07-WR303, Table 1). On the ridge north of Fig. 7). The oldest exposed rocks in the Hays from the locally derived mafic lava flows above Squaw Peak (Fig. 3), the Oligocene volcanic sec­ Canyon Range on the east side of Surprise and below them.

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100 % Radiogenic argon 100 % Radiogenic argon 80 60 98 1 10 K/Ca K/Ca 0.1 1

0.01 0.1 Individual analyses Individual analyses and uncertainties and uncertainties

JC08-WR410 Data at 1σ, results at 2σ H08-57 Data at 1σ, results at 2σ plagioclase anorthoclase

19.2 ± 0.3 26.64 ± 0.08 MSWD 1.60 MSWD 2.52 n = 16 n = 14

14 16 18 20 22 24 26 24.0 25.0 26.0 27.0 28.0 Age (Ma) Age (Ma)

100 100.0 % Radiogenic argon % Radiogenic argon 99.5 98 90.0 96 100 98.5 100 K/Ca K/Ca

10 10 Individual analyses Individual analyses and uncertainties and uncertainties

JC08-WR412 Data at 1σ, results at 2σ JC08-WR411 Data at 1σ, results at 2σ sanidine sanidine

25.77 ± 0.86 26.53 ± 0.05 MSWD 1.38 MSWD 3.50 n = 11 n = 15

24.0 25.0 26.0 27.0 28.0 24.0 25.0 26.0 27.0 28.0 Age (Ma) Age (Ma)

Figure 7. 40Ar/39Ar single-crystal age probability diagrams for samples analyzed at New Mexico Tech University. Unfilled circles not used in age calculations. Dates relative to Fish Canyon Tuff sanidine with an assumed age of 28.02 (Renne et al., 1998). MSWD—mean square of weighted deviates.

On the west side of the Warner Range, Car­ same geologic unit as the ones described in the and to >1000 m at Emerson Peak (Figs. 4 and michael et al. (2006, p. 1198–1199) dated previous paragraph, which would then consti­ 6D). Duffield and McKee (1986) divided these several “rhyolitic ash-flow deposits of various tute a package of primarily biotite-bearing, vari­ flows into two map units (Tvm and Tvb) sepa­ ages”; they did not describe these deposits in ably reworked tuffs derived from distal sources rated by a layer of tuff, but they are indistin­ detail, but mentioned “notable biotite” (p. 1207) that accumulated in the Warner Range area from guishable from each other in outcrop and are and reported ages of 14.0 and 13.4 Ma (Table 1). 19 to 13 Ma and possibly more recently. part of the same volcanic edifice or edifices, so These tuffaceous deposits are overlain by Mio­ we show them as a single unit (Tmb) in Figure 3. cene basalt flows at the south end of our Fig­ Miocene Lavas of the Warner and Hays Miocene lava flows of similar appearance and ure 3, and by Pliocene sedimentary rocks and Canyon Ranges (16–14 Ma) composition cap the Hays Canyon Range on the lava flows in the vicinity of Alturas (Carmichael Middle Miocene basaltic to andesitic lava east side of Surprise Valley (Fig. 3), where they et al., 2006). Because of their proximity, similar flows are exposed in the southern Warner Range unconformably overlie the eroded remnants of position in the section, and similar lithology, we as far north as Parker Creek (Fig. 3), and thicken Hays volcano (Carmichael et al., 2006). Indi­ tentatively suggest that these tuffs belong to the southward to ~500 m at Warren Peak (Fig. 6C) vidual Miocene lava flows are generally 2–5 m

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thick, occasionally as thick as 20 m, with scoria­ 6°–7°, and summit vent complexes with well- lava flows, intrusive rocks, and lava blocks), and ceous tops and massive interiors. The interiors preserved craters. Their flanks are composed analyses of the Hays volcano southeast of the of some flows exhibit such coarse, gabbro-like of homogeneous basaltic andesite lava flows, Warner Range (from Carmichael et al., 2006). textures in hand specimen and thin section that with scoria and spatter exposed in the summit Miocene samples are divided into two groups: they may actually be sills injected into the stack vents and/or craters. Extant K-Ar dates from the Miocene lavas in the Warner and Hays Canyon of flows. Flows often have prominent columnar volcanoes shown in Figure 2 range from 14.5 to Ranges, which range from ca. 16 to 14 Ma, and joints (Fig. 6C), but are locally glassy and flow 8 Ma; most are in the 10–12 Ma range (Grose, slightly younger Miocene lava flows, including banded. Phenocrysts are variably abundant and 2000; Grose et al., 1991, 1992). They are thus the basaltic andesite shield volcanoes south of include plagioclase, pyroxene, and olivine. slightly younger than the Miocene volcanoes the Warner Range, which range from ca. 14 to Miocene lava flows in the Warner Range are in the southern Warner Range, but can be con­ 8 Ma. Pliocene low-K olivine tholeiites are plot­ locally separated by a section of volcaniclastic sidered part of the same volcanic episode with ted separately. rocks as much as 100 m thick (unit Tmt, Fig. 4; respect to their composition and eruptive style. Oligocene and Miocene Warner Range lavas equivalent to Duffield and McKee’s Tvt unit range in composition from basalt to andesite, (1986). South of Parker Creek, these volcani­ Late Miocene to Pliocene Tholeiitic Basalts the Miocene lavas on average being slightly clastic rocks are sporadically exposed in road­ (8–3 Ma) more alkalic (Fig. 8). (Duffield and McKee also cuts on the densely forested western slope of mapped rhyolites in the Warner Range [Fig. 3] the range. They consist of graded, cross-bedded Late Miocene to Pliocene high-alumina, low- that we did not analyze in this study.) Oligocene coarse sandstones with abundant plagioclase potassium olivine tholeiite lava flows crop out volcanism began with the more mafic Lake City crystal fragments and dark lava chips, inter­ extensively on both sides of the Warner Range basalts and progressed to the more andesitic bedded with massive inversely-graded deposits of (Figs. 2 and 3); the lava was referred to as high- Cedar Pass complex. The youngest Oligocene angular mafic lava blocks supported by a brown alumina olivine tholeiite by Hart et al. (1984), lavas, i.e., the ca. 24 Ma Hays volcano (Fig. 2), sandy matrix. Below the summit of Warren Peak and as low-potassium olivine tholeiite by Car­ are similar to the andesitic parts of the Cedar (Fig. 3), the volcaniclastic section consists of a michael et al. (2006). In outcrop and hand speci­ Pass complex but slightly more alkalic. Mio­ layer of tuff with a fine-grained gray ashy matrix men the flows are usually easy to distinguish cene lavas in the Warner Range consist primar­ supporting a mixture of small (<1 cm) white from the older lavas by their light to medium ily of basalt, whereas slightly younger lavas to pumice lapilli and angular black lava fragments. gray, nonporphyritic, diktytaxitic texture. In the the east, west, and particularly south (Fig. 2) Based on published K-Ar and 40Ar/39Ar dates area of Figure 3, they range from ca. 8 to 3 Ma, consist mostly of basaltic andesite and andesite (Table 1), Miocene mafic lava flows in the but most are 4–3 Ma (Carmichael et al., 2006). (Fig. 8). Compared to rocks in the western Cas­ Warner ­and Hays Canyon Ranges were erupted West of the Warner Range, Pliocene lavas (the cades, Warner Range lavas are overall relatively during a relatively narrow interval between ca. Devil’s Garden basalt of McKee et al., 1983) are enriched in incompatible elements (K, Ba, Sr, 16 and 14 Ma; most dates are between 15 and flat lying and overlie Middle to Late Miocene La), the Miocene lavas being somewhat more 16 Ma (Duffield and McKee, 1986; Carmichael lava flows and tuffaceous sedimentary rocks. enriched than the Oligocene (Fig. 9). In this et al., 2006) (Table 1). Duffield and Weldin They crop out partway up the western slope respect the Warner Range lavas are more simi­ (1976) suggested the presence of several vents of the range to an altitude of almost 1900 m, lar to rocks of the ancestral Cascades in Cali­ for the Miocene lava flows at the south end of ~450 m above the surrounding plain to the west fornia and Nevada, possibly because both suites our Figure 3. Our mapping supports this sugges­ (Fig. 3). In northern Surprise Valley, Pliocene were built on thicker, less mafic crust than the tion, and one of their inferred vents, at Emer­ lava flows (the Vya Group of Carmichael et al., western Cascades. son Peak, is pictured in Figure 6D; here, lava 2006) are cut and tilted ~15° west by many Both Oligocene and Miocene Warner lavas flows on Emerson Peak dip west, while flows in small-offset normal faults (Fig. 3). are relatively enriched in the light REEs; the the foreground dip to the east, and a prominent Miocene lavas are somewhat more enriched than mafic dike strikes toward the inferred eruptive GEOCHEMISTRY the Oligocene ones (Fig. 10). They are distinctly center in the low topography below the high different from the Pliocene low-K, high-Al peaks. This relationship, and the overall north­ We obtained major, trace, and rare earth basalts, which have much flatter REE patterns ward-thinning of the Middle Miocene lava flows element (REE) data for samples of Oligocene (Fig. 10). Oligocene and Miocene andesites and (Fig. 4), is consistent with them being erupted and younger volcanic rocks (Supplemental basaltic andesites have the prominent depletions from one or more overlapping shield volcanoes Table 11), and Sr and Nd isotopic analyses of in high-field-strength elements (Nb, Ta, Ti, Zr) in the southern Warner Range, similar in size a few representative samples (Table A3). Addi­ (Fig. 10) characteristic of magmas generated and shape to the intact volcanoes of slightly tional analyses of Warner Range samples from in a subduction setting. Lavas from the Warner younger age described in the next section. Carmichael et al. (2006) and V.E. Camp and Range and nearby areas are relatively nonradio­ 87 86 M.E. Ross (2007, personal commun.) are com­ genic, ranging from Sr/ Sri = 0.7035 to 0.7045 Miocene Volcanoes South of the Warner piled in Supplemental Table 1 (see footnote 1) (Fig. 11), consistent with their position well to Range (14 to as Young as 8 Ma) and included in the following discussion (sym­ the west of inferred North American continen­ Several dozen Miocene volcanoes are exposed bols are the same in Figs. 8–13). Oligocene tal crust. The Miocene lavas are distinctly more south of the Warner Range, extending from samples are plotted in three groups: Lake City radiogenic than the Oligocene lavas, however, 87 86 near Likely south to Honey Lake Valley (Fig. 2) basalt flows, Cedar Pass complex (analyses of with Sr/ Sri > 0.7039, whereas the Oligocene 87 86 (Grose, 2000). They average ~8–10 km in diam­ lavas plot in a more restricted range at Sr/ Sri eter and are 500–600 m above the surround­ 1Supplemental Table 1. Excel file. If you are view­ < 0.7039 (Fig. 11). The Pliocene low-K basalts ing the PDF of this paper or reading it offline, please 87 86 ing landscape. Shinn Mountain (Fig. 6E) is visit http://dx.doi.org/10.1130/GES00650.S1 or the have distinctly lower Sr/ Sri than either the typical of one of these volcanic edifices, which full-text article on www.gsapubs.org to view Supple­ Miocene or Oligocene lavas (Fig. 11). With exhibit radial drainage patterns, flank slopes of mental Table 1. the exception of the younger Miocene lavas,

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Oligocene volcanic rocks, and we interpret them to be part of the arc formed by subduction of the Farallon plate beneath northern California, which was ongoing in the Oligocene (e.g., Atwater ­and Stock, 1998). Relative to the Miocene lavas, Oligocene­ Warner Range lavas have lower 87 86 Sr/ Sri (Fig. 12A) and Ce/Pb (Fig. 12B) ratios, higher primitive-mantle-normalized Sr/P (Fig. 12C), and are more depleted in Nb (Fig. 12D). Since there is little correlation between Sr/P

ratio and SiO2 content (Fig. 12E), we interpret these patterns to indicate that that the Oligocene lavas were derived from mantle with a relatively large (compared to the Miocene) fluid flux from the subducting slab. These fluids had relatively 87 86 87 86 low Sr/ Sri ratios, similar to Sr/ Sri ratios in modern southern Cascade arc mafic lavas (Borg 87 86 et al., 1997). The overall low Sr/ Sri ratios from the Oligocene samples are consistent with variable fluid flux from subducted basaltic crust and minor (if any) contribution from subducted sediments or continental crust.

Miocene Volcanism

No volcanic rocks were erupted in the Warner­ Range between ca. 24 and 16 Ma, although minor distally sourced tuffs accumulated in at least one locality. However, volcanic rocks between 24 and 16 Ma are exposed in north­ western Nevada and southeastern Oregon, sug­ gesting that active volcanism may have shifted to nearby areas in the Early Miocene rather than ceasing in the region altogether. The nearest examples to the Warner Range are volumetri­ Figure 8. Total alkali-silica classification diagram for Warner Range volcanic rocks (after cally minor mafic lava flows erupted between LeBas et al., 1986). Oregon and Washington Cascades from du Bray et al. (2006); inferred 30 and 21 Ma in the Pine Forest and Black California and Nevada arc from du Bray et al. (2009). Warner Range volcanic rocks are Rock Ranges in Nevada, ~100–120 km to the mildly trachyitic and range from basalt to andesite, similar to both western Cascades and east (Colgan et al., 2006; Lerch et al., 2007), ancestral Cascades lavas. and a 23–21 Ma complex of rhyolite domes and andesitic volcanoes in Oregon, ~100 km to the north (Scarberry et al., 2010). Abundant, domi­ Warner ­Range lavas are isotopically more simi­ or more overlapping volcanic edifices mostly nantly andesitic rocks with lesser basaltic ande­ lar to rocks of the Washington and Oregon Cas­ composed of andesitic block-and-ash flows and site and rhyolite also erupted between 24 and cades than they are to the ancestral Cascades in lesser lava flows (Fig. 14B). The youngest lava 18 Ma in Nevada north of Reno (Garside et al., California and Nevada. flow in the Cedar Pass complex is ca. 24 Ma, 2000; 2003). Volcanic activity in the Warner­ although peak eruptive activity may have ceased Range resumed ca. 16 Ma and formed one or DISCUSSION earlier. The full duration of magmatism at Hays more overlapping basalt to basaltic andesite Volcano on the east side of Surprise Valley shield volcanoes. Subsequent eruptions formed Oligocene Volcanism is less clear, but by 24 Ma it had built a spat­ more than 12 basaltic andesite shield volca­ ter cone (or several overlapping cones) at least noes to the south and west of the Warner Range Andesitic volcanism in northeastern Califor­ 550 m high (Fig. 3). Local Oligocene volcanism (Fig. 2). Published K-Ar dates from these vol­ nia and southern Oregon was ongoing by the thus took place between 28 and 24 Ma, although canoes are somewhat younger than the lavas in Oligocene, as indicated by the thick sequences the life span of any individual eruptive center the Warner Range (mostly 12–11 Ma); this pat­ of lahars in the lower part of the Warner Range was probably much less. Volcanism ceased in tern may indicate an age progression in volcanic section. Local volcanic activity began with the Warner­ Range between ca. 24 and 16 Ma, eruptions from the Warner Range to the south eruption of the Lake City basalts, which built but was probably active during this time else­ and/or west, but more systematic and precise a broad basaltic shield volcano between ca. where in the region (see following discussion). dating is necessary to confirm this. 28 and 26.5 Ma. Subsequent eruptions in the Major, trace element, and isotopic data are Similar to the Oligocene lavas, major, trace Warner Range were more silicic and built one most consistent with a subduction source for the element, and isotopic data are most consistent

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1500

Pliocene high-Al olivine tholeiite Miocene lava flows; northeastern California (14–7 Ma) Miocene lava flows; Warner Range (16–14 Ma) 1000 Oligocene Hays volcano Oligocene Cedar Pass complex Ba (ppm) Oligocene Lake City basalts 500 Oregon and Washington Cascades 45–4 Ma (n = 2130) Inferred California and Nevada arc 35–4 Ma (n = 558)

0 45 50 55 60 65 15 1500

10 1000 CaO (wt%) Sr (ppm) 5 500

0 0 45 50 55 60 65 45 50 55 60 65 4 50

40 3

30 2 O (wt% ) 2 La (ppm)

K 20

1 10

0 0 45 50 55 60 65 45 50 55 60 65 400

10 300

200

5 V (ppm) MgO (wt% )

100

0 0 45 50 55 60 65 45 50 55 60 65

SiO2 (wt%)SiO2 (wt%)

Figure 9. Variation diagrams for Warner Range volcanic rocks. Oregon and Washington Cascades from du Bray et al. (2006); inferred California and Nevada arc from du Bray et al. (2009). Warner Range volcanic rocks are consistently more similar to the inferred California and Nevada arc than they are to the Oregon and Washington Cascades.

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Figure 10. Plots of chondrite and primitive-mantle–normalized (Sun and McDonough, 1989) rare earth element (REE) data from Warner Range volcanic rocks. Oligocene and Miocene lavas are depleted in Nb, Ta, Zr, and Ti, and enriched in the light REEs, distinctly different from the Pliocene basalt flows.

with a subduction source for the Miocene vol­ (Fig. 12A), smaller negative Nb anomalies (Fig. Oligocene to the Miocene mantle sources is also canic rocks, and we also interpret them to be part 12D; lower Sr/P, Fig. 12C), and slightly higher inconsistent with a larger sedimentary compo­ of the arc formed by subduction of the Farallon­ Ce/Pb (Fig. 12B) than the Oligocene lavas. Sr nent in the Miocene source. plate beneath northern California, which was isotope ratios are not negatively correlated with Miocene lavas in the Warner Range and ongoing in the Miocene (e.g., Atwater­ and Ce/Pb (Fig. 12B), and Ce/Pb does not corre­ nearby to the south have a distinctly different

Stock, 1998). Neither the Oligocene nor the late at all with SiO2 content (Fig. 12F), so we composition and eruptive style from volcanic Miocene lavas show obvious evidence of crustal attribute these patterns to the Miocene lavas rocks of overlapping age erupted in the Basin 87 86 contamination based on their similar Sr/ Sri having a different mantle source than the Oligo­ and Range Province to the east, notably the

over the entire range of SiO2 (Fig. 12A). This cene suite. The Miocene source may include a Steens Basalt, with which they have some­ does not rule out some component of crustal smaller fluid component derived from the sub­ times been included on regional maps (e.g., contamination, however, since the isotopic sig­ ducting slab, but one with more radiogenic Sr. Camp and Ross, 2004; Brueseke et al., 2007; nature of the accreted crust that underlies the The more radiogenic Sr is accompanied by less Camp and Hanan, 2008). Miocene lavas in region would be minor and hard to trace. The radiogenic Nd, so this component cannot simply northeastern California erupted to form a chain 87 86 Miocene lavas have more radiogenic Sr/ Sri be seawater. The shift to higher Ce/Pb from the of shield volcanoes, while the Steens Basalt

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80–100 km and 1500 °C, but no detailed work has been done on the mantle source conditions of the Pliocene HAOT lavas there. The bulk of the Pliocene flows on both sides of the Warner Range were erupted ca. 4–3 Ma (Carmichael et al., 2006), and in Colgan et al. (2008), limited apatite (U-Th)/He data were interpreted to record rapid cooling of the Warner­ Range during slip on the Surprise Valley fault ca. 4–3 Ma. Pliocene lavas in the Warner Range thus appear to have been erupted during Plio­ cene extensional faulting, consistent with some flows being cut and tilted by normal faults (Fig. 3). Late Miocene and Pliocene lava flows of similar composition crop out widely in north­ ern Nevada, southern Oregon, and eastern Cali­ fornia, where they are temporally and spatially associated with Basin and Range extension (Hart et al., 1984). Volcanism of fundamentally arc-like character had probably ceased in the Warner Range by 14 Ma, certainly by 8 Ma, and we surmise that the pulse of HAOT volcanism in the Warner Range at the time was caused by extension forming pathways for the lavas to rap­ idly ascend from the mantle to the surface. The same mechanism was suggested by Jordan et al. (2004) for Late Miocene to Pliocene basaltic volcanism in southern Oregon.

Implications for Evolution of the Cascade Arc

We interpret Oligocene (28–24 Ma) and Figure 11. Plot of Sr and Nd isotopic data for Warner Range volcanic rocks. Pliocene and Middle Miocene (16–8 Ma) volcanism in the younger Oregon and Washington Cascades from GEOROC (2007) database; inferred Cali- Warner Range and nearby to be a direct con­ fornia and Nevada arc from du Bray et al. (2009). Rocks of the inferred California and sequence of subduction of the Farallon plate Nevada arc are overall distinctly more radiogenic than the Oregon and Washington Cas- beneath northern California. In map view, these cades; Warner Range volcanic rocks plot in the middle and overlap the more radiogenic end volcanic centers are continuous with the north­ of the Oregon and Washington Cascades. NE CA—northeastern California. ern extent of the ancestral Cascades (Fig. 1), but are more than 100 km east of the Eocene to Miocene western Cascades, which extend as far was erupted from roughly north- to northeast- Latest Miocene to Pliocene Volcanism south as Mount Shasta (Figs. 2 and 14). Does trending dikes in eastern Oregon (e.g., Camp this offset in arc segments reflect the geometry and Ross, 2004). Middle Miocene lavas in the Pliocene high-alumina olivine tholeiite of the subducting slab, post-Eocene tectonic Warner Range are younger (mostly 16–15 Ma, (HAOT) lavas in the Warner Range have a dis­ deformation, or a combination of the two? If it Table 1) than the ca. 16.6 Ma (Jarboe ­et al., tinctly different source than the subduction- is related to the subducting slab, why are the arc 2008) Steens Basalt, although they do overlap related Oligocene and Miocene lavas. Simi­ segments apparently offset in such a manner? in time with the most voluminous (Grande lar Pliocene HAOT lavas at Medicine Lake All or part of the apparent offset of the two Ronde) phase of the Columbia River Basalts Volcanic­ field (Fig. 2) were sourced from hot arc segments could be the result of significant (e.g., Hooper et al., 2002). Origin of the Steens (1200–1300 °C) dry mantle just beneath the Eocene or younger westward translation of the Basalt and related Columbia River Basalts is crust-mantle boundary (~35 km) (Bartels et al., Klamath Mountains relative to the northern much debated, particularly their relationship 1991; Bacon et al., 1997; Donnelly-Nolan et al., Sierra Nevada (Fig. 14), accommodated by east- to a deep mantle plume, but they are clearly 2008). Elkins Tanton et al. (2001) determined west extension in the region shown in Figure 2. not ordinary subduction-related lavas (e.g., that the depth of melting and mantle tempera­ Geologic studies of northern California indi­ Chesley­ and Ruiz, 1998; Hooper et al., 2002; ture for HAOT lavas increases eastward to cate that such translation took place, but con­ Camp and Ross, 2004; Camp and Hanan, ~66 km and ~1500 °C at a point roughly equi­ sistently conclude that it happened during the 2008). Compared to the Steens Basalt, Mio­ distant between the Warner Range and Medi­ Cretaceous (Jones and Irwin, 1971; Constenius cene mafic lava flows in the Warner Range cine Lake Volcano (Fig. 2). Carmichael et al. et al., 2000; Wyld et al., 2006). Although some are distinctly depleted in Nb, Ta, and Ti, for a (2006) surmised that this trend continued east reconstructions have suggested that such motion given MgO content (Fig. 13). to the Warner Range, where it would equate to was Eocene or younger (e.g., Dickinson, 2002;

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0.7046 0.7046 Egger et al., 2009; Egger and Miller, 2011; this A B study). Helley and Harwood (1985) mapped 0.7044 0.7044 no angular unconformity between the Pliocene

0.7042 Tuscan Formation and the Eocene Montgomery i i 0.7042 Sr Sr Creek Formation on the east edge of the Klamath 86 86 0.7040 0.7040 Mountains (Fig. 2). No evidence was found Sr/ Sr/ (Colgan et al., 2006) for significant extension 87 87 0.7038 0.7038 in northwestern Nevada prior to ca. 12 Ma, and Scarberry et al. (2010) documented only minor 0.7036 0.7036 extension (<5% strain) since the Early Miocene

0.7034 0.7034 north of the Warner Range in southern Oregon. 45 50 55 60 65 0 2 4 6 8 The western Cascades (Fig. 14) have undergone SiO2 (wt %) Ce/Pb significant clockwise rotation since the Eocene, 0.7046 0.7046 but this motion was accommodated relative C D to the unrotated Sierra Nevada by a pivot in 0.7044 0.7044 northern California, not by substantial west­ ward motion of the Klamath block relative to

0.7042 i 0.7042 i

Sr the northern Sierra Nevada (Wells et al., 1998; Sr 86 86 0.7040 0.7040 Wells and Simpson, 2001). Overall, because Sr/ Sr/ there is geologic evidence for Cretaceous sepa­ 87 87 0.7038 0.7038 ration of the Klamath Mountains and northern Sierra Nevada, and no geologic evidence for 0.7036 0.7036 such motion in the Cenozoic, we conclude that

0.7034 0.7034 the position of the arc segments in Figure 14 is a 0 1 2 3 0 0.1 0.2 0.3 0.4 function of where they formed. Sr/Ppmn Nb* North of Mount Shasta, Oligocene volcanic 2.8 8.0 rocks of the western Cascades are west of the E F Quaternary arc (Figs. 1 and 2). South of Mount 2.4 7.0 Shasta, Oligocene and Miocene rocks of any

2.0 6.0 kind are absent; Pliocene volcaniclastic rocks of the Tuscan Formation (ca. 3.4–1.8 Ma) are

pm n 1.6 5.0 deposited directly on pre-Tertiary basement and Ce/Pb

Sr/P Eocene sedimentary rocks of the Montgomery 1.2 4.0 Creek Formation (Helley and Harwood, 1985; Irwin, 1997). The westernmost volcanic arc 0.8 3.0 rocks of possible Late Miocene age are undated

0.4 2.0 rocks along a ridge south of Medicine Lake 45 50 55 60 65 45 50 55 60 65 (Fig. 2), and the westernmost dated Miocene SiO2 (wt%) SiO2 (wt%) volcanic centers are between Alturas and Susan­ ville (Fig. 2). Fundamentally arc-related vol­ Pliocene high-Al olivine tholeiite canic activity was ongoing in the Warner Range Miocene lava flows; northeastern California (14–7 Ma) in the Oligocene, and from 16 to ca. 8 Ma in Miocene lava flows; Warner Range (16–14 Ma) the Warner Range and the belt of Miocene vol­ Oligocene Cedar Pass complex canoes extending another 100 km south (Fig. 2). Oligocene Lake City basalts With a few minor exceptions, post-Miocene vol­ Figure 12. Plots of select trace element and isotopic data for Warner Range volcanic rocks. canism in the area of these older volcanic rocks has been limited to HAOT lavas with a distinctly Ppmn is primitive-mantle–normalized P (after Borg et al., 1997). Oligocene lavas have lower 87 86 different mantle source than the subducting slab Sr/ Sri and Ce/Pb ratios, higher primitive–mantle-normalized Sr/P, and are more depleted in Nb than the Miocene lavas. With the exception of one Miocene sample, both suites have (e.g., Guffanti et al., 1990; Carmichael­ et al., overall low 87Sr/86Sr ratios. 2006). No Quaternary vents have been mapped i in this area (east of ~120°30′W, Fig. 2), and the axis of the Quaternary Cascade arc is 100– 150 km to the west. The eastern edge of the arc Humphreys, 2009), geologic evidence for this the Warner Range and Klamath Mountains must therefore have migrated west between the is absent. Late Miocene and Pliocene rocks in (Fig. 2), but where they are exposed they are Middle Miocene and the present (e.g., Guffanti northeastern California are locally cut by many conformable with overlying units. In the Warner and Weaver, 1988; Guffanti et al., 1990; Muffler­ small normal faults, but these have only minor Range, no angular unconformity separates rocks et al., 2009). The absence of pre-Pliocene vol­ offset and indicate very little post-Miocene ranging from older than 34 Ma to 14 Ma, con­ canic rocks beneath the Pliocene–Quaternary extension. Older rocks that would record pre- sistent with no significant extension at the time arc south of Mount Shasta indicates that vol­ Miocene extension are mostly covered between (Duffield and McKee, 1986; Colgan et al., 2008; canism migrated west into this area after the

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Figure 13. Geochemical data from Middle Miocene (16– 14 Ma) Warner Range volcanic rocks (this study) plotted with analyses from the Steens Basalt (Johnson et al., 1998). Warner Range volcanic rocks are promi- nently depleted in Nb, Ta, Zr, Rock/Primitive Mantle and Ti relative to Steens Basalt, with consistently lower Ti for a given Mg than Steens Basalt.

Figure 14. Map of western United States (same area as Fig. 1) showing location of proposed tear in Eocene to Miocene sub- ducting slab. Siletzia block is from Wells et al. (1998), arc segments are from this study (Fig. 1), and proposed northern slab tear is from Humphreys (2009).

Late Miocene.­ Together, these patterns suggest accreted block, with a tear along its southern CONCLUSIONS that the segment of the Cascade arc south of margin separating it from the still gently dip­ Mount Shasta migrated ~100 km west between ping slab to the south (Fig. 14). Based on the Arc volcanism in the Warner Range began the Middle Miocene and the Quaternary, while pattern of offset arc segments described in this in the Oligocene, and volcanic centers were the Cascade arc north of Mount Shasta remained study, we suggest a more southern location for active locally from 28 to 24 Ma and from 16 in place or migrated east. the slab tear (Fig. 14) that is more consistent to 14 Ma. Oligocene and Middle Miocene If the Oligocene and Miocene arc in north­ with a major change in slab dip across it; how­ volcanic rocks both have clear subduction eastern California was active ~100 km east ever, we agree with the mechanism proposed sources, but involved different degrees of of its present location and not elsewhere, by Humphreys (2009) for its formation. Once fluid in the melting. From 14 Ma, arc vol­ why and how did it move to where it is now? formed, the northern segment of the slab sub­ canism migrated south and west, and Plio­ A simple explanation is that the subduct­ ducted at the same angle, or perhaps shallowed cene eruptions around the Warner Range ing slab south of present-day Mount Shasta slightly to the present, while the southern seg­ were limited to HAOT lavas with a differ­ dipped more shallowly in the Oligocene and ment initially remained attached to the gently ent mantle source than the subducting slab. Miocene, then steepened either gradually or dipping Laramide slab beneath the western The Oligocene and Middle Miocene arc abruptly sometime after the Middle Miocene. United States interior. The transition from south of Mount Shasta was continuous with North of Mount Shasta, however, the arc either Basin and Range ignimbrite volcanism to the the inferred ancestral Cascades in west­ remained in place (albeit more diffuse, e.g., ancestral Cascades remains poorly understood, ern Nevada and eastern California, but was Taylor, 1990) or actually migrated east since but recent work indicates that normal subduc­ 100 km east of both the modern arc and the the Eocene (e.g., Verplanck and Duncan, 1987; tion was established beneath eastern Califor­ Eocene to Pliocene western Cascade arc Wells, 1990). We reconcile these observa­ nia and western Nevada by the Miocene, if north of Mount Shasta. We conclude that tions by proposing that the Eocene to Middle not earlier (Putirka and Busby, 2007; Cousens separation of these arc segments is the result Miocene subducting slab along the western et al., 2008; Busby et al., 2008a, 2008b; Busby of a northeast-trending tear in the subduct­ North American plate margin was broken by a and Putirka, 2009; Hagan et al., 2009; Vikre ing slab that formed in the Eocene, at which northeast-trending tear located south of Mount and Henry, 2010). The southern segment of time the western Cascades began to form. Shasta and north of the Warner Range (red dot­ the slab continued subducting at a shallower The ancestral Cascades began to form by ted line in Fig. 14). Humphreys (2009) pro­ dip than the northern portion until the Middle the Oligo­cene, as the southern segment of posed that Early Eocene accretion of the Silet­ Miocene, before steepening and moving the the slab progressively steepened and the arc zia block in central Oregon (Fig. 14) caused a arc westward to its present location by the late migrated west, reaching its present position new subduction zone to form outboard of the Pliocene. by the late Pliocene.

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40 39 APPENDIX 1. Ar/ Ar binocular microscope. Samples were irradiated at vidual sanidine grains were fused using a CO2 laser ANALYTICAL METHODS Texas A&M University for 7 h and analyzed at the operating at 10 W for 5 s. Extracted gases were puri­ New Mexico Geochronology Research Laboratory fied with SAES GP-50 getters. Argon was analyzed Samples were coarsely crushed and sieved to (New Mexico Institute of Mining and Technology), with a Mass Analyzer Products model 215–50 mass ~20–40 mesh, then run through a magnetic separa­ using procedures described in McIntosh et al. (2003). spectrometer operated in static mode. Variance- tor to concentrate sanidine, quartz, and plagioclase. Neutron flux was monitored using interlaboratory weighted mean ages of the 9–23 grains reported in These were leached with 5% HF for ~1 h to remove standard Fish Canyon Tuff sanidine FC-1 with an Table A1 were calculated by the method of Sam­ any matrix. Sanidine was then handpicked under a assigned age of 28.02 Ma (Renne et al., 1998). Indi­ son and Alexander (1987), using decay constants of

TABLE A1. 40Ar/39Ar ANALYTICAL DATA (NEW MEXICO TECH UNIVERSITY) Sample: H08-57 anorthoclase Lab no. 58640 Laser-fusion experiment J = 0.0008649 ± 0.05% 36Ar/39Ar 39ArK Age ±1σ ID 40Ar/39Ar 37Ar/39Ar (× 10–3) (×10–15 mol) K/Ca % 40Ar* (Ma) (Ma) 08A 17.14 3.4557 2.1492 0.6570.1 98.0 0.1480.358 10A 17.33 0.1915 1.1867 1.8622.7 98.1 2.6640.139 14A 17.35 0.1474 1.1256 1.9393.5 98.2 3.4620.129 09A 17.40 0.1434 1.1501 4.5943.6 98.1 3.5580.082 07A 17.20 0.1411 0.3206 4.8743.6 99.5 3.6170.074 11A 17.39 0.1429 0.8971 2.2243.6 98.5 3.5710.126 06A 17.41 0.1633 0.8759 2.6283.1 98.6 3.1250.114 05A 17.30 0.1146 0.4594 3.6694.5 99.3 4.4530.080 12A 17.24 0.1405 0.2021 5.6323.6 99.7 3.6310.066 02A 17.56 0.1411 1.2012 6.2213.6 98.0 3.6160.061 13A 17.18 0.1521 –0.12721.651 3.4100.3 3.3540.138 03A 17.41 0.1894 0.5801 3.9272.7 99.1 2.6930.077 04A 17.22 0.1539 –0.14692.532 3.3100.3 3.3160.093 01A 17.32 0.1306 0.1385 8.7263.9 99.8 3.9070.054 15A 17.61 0.1858 0.8192 1.3522.7 98.7 2.7470.155 Sample: JC08-WR410 plagioclase Lab no. 58674 Laser-fusion experiment J = 0.0008571 ± 0.06% 01 20.45 4.0463 31.33620.250 0.13 56.4 17.784 1.291 14 14.94 4.8037 11.59291.083 0.11 79.7 18.388 0.281 12 15.27 5.6012 12.28830.747 0.09 79.3 18.690 0.449 03 18.64 3.7165 22.66091.143 0.14 65.7 18.891 0.362 02 13.32 4.7696 4.5699 0.6200.1192.819.0790.519 16 18.94 4.6378 23.09571.237 0.11 66.0 19.288 0.320 11 21.09 5.2670 30.56580.250 0.10 59.2 19.291 1.272 07 14.30 5.8282 7.5523 1.0280.0987.819.3820.306 13 14.89 4.6121 8.9638 1.3430.1184.819.4730.240 05 14.16 4.7172 6.4760.616 0.11 89.2 19.489 0.511 04 15.67 6.6472 12.159 0.6780.0880.619.5080.496 08 15.69 5.2745 11.75310.684 0.10 80.6 19.526 0.475 15 16.18 5.8688 13.440.936 0.09 78.5 19.594 0.408 10 21.48 5.3539 29.53950.346 0.10 61.4 20.357 0.798 09 14.54 4.5372 3.3207 0.2650.1195.821.4901.055 06 20.66 6.8319 24.53580.165 0.07 67.6 21.573 1.819 Sample: JC08-WR412 sanidine Lab no. 58695 Laser-fusion experiment J = 0.000852 ± 0.05% 09 16.83 0.0105 0.5564 2.60448.699.025.4430.096 04 16.87 0.0112 0.2315 2.50045.799.625.6480.106 03 16.90 0.0083 0.2556 10.94461.299.625.6740.049 08 17.42 0.0110 1.9095 4.21446.696.825.7260.080 05 16.92 0.0095 0.2165 4.27853.599.625.7270.075 02 17.07 0.0087 0.6748 4.02758.698.825.7470.081 06 16.98 0.0077 0.2355 5.47866.499.625.8070.063 07 17.00 0.0101 0.2978 3.60950.699.525.8150.082 13 16.99 0.0082 0.2482 5.01162.299.625.824 0.071 11 17.07 0.0092 0.3285 2.08355.799.425.9000.131 10 17.04 0.0102 0.1707 2.36150.099.725.9340.108 12 17.22 0.0080 0.4508 1.62963.599.226.0770.164 01 17.41 0.0097 0.16813.72252.799.726.4940.052 Sample: JC08-WR411 sanidine Lab no. 58734 Laser-fusion experiment J = 0.0008306 ± 0.05% 06A 17.90 0.0103 0.5625 12.79049.799.126.3820.050 07A 17.91 0.0100 0.4886 18.23251.299.226.4250.050 10A 17.87 0.0098 0.3501 17.29351.999.426.4360.049 09A 17.97 0.0106 0.6462 16.709 48.3 98.9 26.451 0.049 04A 17.86 0.0103 0.1943 17.596 49.4 99.7 26.481 0.049 15A 18.03 0.0110 0.7347 10.300 46.4 98.8 26.491 0.050 02A 17.88 0.0102 0.2155 19.283 49.9 99.6 26.500 0.045 01A 17.97 0.0298 0.4833 29.70417.199.226.5190.039 05A 17.90 0.0124 0.1809 10.59641.299.726.5460.052 03A 17.97 0.0171 0.368 13.73629.999.426.5730.053 14A 17.92 0.0105 0.1428 7.30148.499.826.5960.055 12A 17.93 0.0109 0.1615 8.05046.999.726.5990.051 08A 17.97 0.0115 0.111112.90244.599.826.6820.048 11A 18.01 0.0307 0.1512 10.810 16.6 99.8 26.720 0.053 13A 18.07 1.3048 0.6610.931 0.39 99.5 26.766 0.274 Note: Data are from the New Mexico Tech University (New Mexico Institute of Mining and Technology, Socorro, New Mexico). J is the irradiation parameter (e.g., McDougall and Harrison, 1999) determined by analyses of Fish Canyon Tuff sanidine with an assumed age of 28.02 Ma (Renne et al., 1998).

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Steiger ­and Jäger (1977). Individual analyses were rado), with irradiation times between 10 and 16 h. defined as the weighted mean age of contiguous gas discarded if they were obvious xenocrysts or had low Plagio­clase and groundmass 40Ar/39Ar ages (Table fractions representing more than 50% of the 39Ar 39Ar contents or radiogenic yields. A2) were obtained by incremental-heating analy­ released for which no difference can be detected sis; i.e., sequential extraction of the argon from the between the ages of any two fractions at the 95% APPENDIX 2. 40Ar/39Ar sample at progressively higher temperatures until level of confidence (Fleck et al., 1977). Where no ANALYTICAL METHODS the sample was fused. Incremental-heating analyses plateau is defined by the ages, the40 Ar/39Ar isochron utilized a low-blank, tantalum and molybdenum, age or the integrated age of all increments may be Samples collected for 40Ar/39Ar geochronol­ resistance-heated furnace, commonly releasing all of used. The sanidine age for sample JC07-WR303 was ogy were crushed and sieved to sizes appropriate the Ar in 9–13 temperature-controlled heating incre­ obtained by laser-fusion analysis in which multiple

for mineral separation of each sample (methods of ments. Ages from incremental-heating experiments grains were fused with a CO2 laser in a single heating U.S. Geological Survey, Menlo Park, California). are determined by an evaluation of the 40Ar/39Ar step. The reported age for laser-fusion analyses rep­ Samples were irradiated in the U.S. Geological age spectrum and isochron diagrams of the data. resents the weighted mean of six or more replicate Survey TRIGA Reactor Facility (Denver, Colo­ Many of the ages are defined by a 40Ar/39Ar plateau, analyses, with the inverse variance of propagated,

TABLE A2. 40Ar/39Ar ANALYTICAL DATA (USGS SURVEY LABORATORY, MENLO PARK, CA) Sample: 07-C-10 groundmass Lab no. IRR266-36 Step-heating experiment J = 0.002673929 Step Age 39 40 40 39 37 39 36 39 (°C) % ArK Rel% Ar* Ar/ Ar Ar/ Ar Ar/ Ar K/Ca Cl/K (Ma) 550 3.2430 95.253 5.347890.51109 0.000971.02628 0.0005124.416 ± 0.107 625 18.490898.4375.15439 0.353870.00034 1.482400.00017 24.317 ± 0.081 650 13.593998.7135.09163 0.355710.00029 1.474740.00008 24.090 ± 0.080 675 9.6789 98.540 5.096370.44362 0.000341.18241 0.0001024.072 ± 0.080 725 6.4058 97.978 5.149080.65575 0.000500.79981 0.0001724.185 ± 0.083 775 5.1043 97.397 5.167090.73495 0.000630.71358 0.0003324.127 ± 0.084 840 7.7527 97.792 5.105960.65833 0.000530.79667 0.0004723.938 ± 0.082 800 1.7458 99.835 5.067080.60456 0.000160.86755 0.0005324.249 ± 0.101 900 8.4479 97.946 5.078720.65409 0.000500.80184 0.0007523.849 ± 0.081 950 7.7645 96.426 5.114210.74151 0.000790.70726 0.0010723.645 ± 0.083 1000 10.396395.2335.13722 1.293270.00116 0.405370.01174 23.467 ± 0.083 1050 6.5585 94.658 5.163546.67332 0.002740.07828 0.0014923.529 ± 0.089 1125 0.8175 92.831 5.38338 15.03452 0.005420.03455 0.0023824.187 ± 0.190 Sample: JC08-WR405 groundmass Lab no. IRR266-48 Step-heating experiment J = 0.002344283 Step Age 39 40 40 39 37 39 36 39 (°C) % ArK Rel% Ar* Ar/ Ar Ar/ Ar Ar/ Ar K/Ca Cl/K (Ma) 550 5.5866 78.324 9.526821.73351 0.007440.30233 0.0014531.321 ± 0.714 625 15.084256.55712.565792.59557 0.019160.20180 0.0010329.860 ± 0.583 630 6.1304 80.431 8.368672.33084 0.006150.22476 0.0003628.288 ± 0.292 675 9.5608 85.116 7.745872.10048 0.004450.24945 0.0005627.709 ± 0.140 725 14.549088.3937.49387 1.779630.00340 0.294490.00089 27.832 ± 0.149 775 12.480587.9417.44417 1.923840.00354 0.272390.00151 27.511 ± 0.128 825 8.2313 86.060 7.576512.11855 0.004130.24732 0.0025427.406 ± 0.135 875 6.7535 88.282 7.318871.68067 0.003330.31185 0.0049727.152 ± 0.121 925 8.2794 85.092 7.694081.93784 0.004380.27042 0.0081527.514 ± 0.124 975 7.9472 77.377 8.389115.93968 0.008030.08799 0.0545327.353 ± 0.133 1025 4.7824 70.481 9.03032 14.83917 0.013070.03501 0.0059926.982 ± 0.173 1075 0.4734 59.955 11.25541 14.31843 0.019150.03630 0.0048428.585 ± 1.791 1150 0.1413 35.761 18.72379 13.79688 0.044520.03768 0.0026528.355 ± 7.095 Sample: AE05WR03 plagioclaseLab no. IRR266-38 Step-heating experiment J = 0.002626108 Step Age 39 40 40 39 37 39 36 39 (°C) % ArK Rel% Ar* Ar/ Ar Ar/ Ar Ar/ Ar K/Ca Cl/K (Ma) 550 3.8209 2.069163.25409 78.249130.56249 0.006360.22975 16.800 ± 34.037 600 7.2916 7.55671.70566 106.83624 0.253660.00456 0.1291627.414 ± 4.633 675 17.869221.36024.83480118.38713 0.098620.00408 0.0219027.065 ± 1.353 750 22.713370.6357.67623 116.721180.03970 0.004150.00249 27.627 ± 0.513 825 20.842165.9688.09148 116.121730.04123 0.004170.00921 27.189 ± 0.551 900 12.263228.55618.61155110.95241 0.075490.00438 0.0405126.973 ± 0.904 985 5.9499 17.630 25.95670 97.946220.09929 0.005011.42026 23.036 ± 1.693 1085 2.1914 13.713 39.32596 86.996500.13869 0.005680.59056 26.910 ± 3.818 1200 1.7169 1.70798.29517 70.910380.34635 0.007050.08023 8.323 ± 7.790 1400 5.3414 0.102267.90479 105.054440.93453 0.004650.04336 1.392 ± 8.230 Sample: 07-C-19 plagioclase Lab no. IRR266-40Step-heating experiment J = 0.002561241 Step Age 39 40 40 39 37 39 36 39 (°C) % ArK Rel% Ar* Ar/ Ar Ar/ Ar Ar/ Ar K/Ca Cl/K (Ma) 550 0.4424 51.760 11.22553 19.948810.02373 0.025960.00220 27.003 ± 2.956 625 1.8866 62.462 8.67826 21.50875 0.016900.02405 0.0001525.232 ± 1.093 700 4.8044 81.111 6.77415 22.190060.01040 0.023300.00026 25.585 ± 0.667 775 9.0968 87.715 6.23086 22.31545 0.008690.02317 0.0021425.452 ± 0.224 850 13.848988.6796.12627 22.59742 0.008530.02287 0.0002625.306 ± 0.172 925 17.342293.0345.83461 22.81220 0.007620.02265 0.0004525.288 ± 0.154 1000 13.289380.1986.82465 23.00961 0.010870.02246 0.0005525.500 ± 0.198 1075 11.241194.6035.79041 22.98090 0.007350.02248 0.0010125.521 ± 0.198 1150 7.3812 88.149 6.2574822.179080.00858 0.023310.00296 25.683 ± 0.254 1250 13.645690.9276.25712 21.00064 0.007670.02464 0.0097026.465 ± 0.176 1350 7.0215 89.013 6.55638 23.712210.00892 0.021780.00363 27.191 ± 0.243 (continued)

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TABLE A2. 40Ar/39Ar ANALYTICAL DATA (USGS SURVEY LABORATORY, MENLO PARK, CA) (continued) Sample: WR07-AE49 plagioclase Lab no. IRR266-50 Step-heating experiment J = 0.002284517 Step Age 39 40 40 39 37 39 36 39 (°C) % ArK Rel% Ar* Ar/ Ar Ar/ Ar Ar/ Ar K/Ca Cl/K (Ma) 550 0.8775 36.688 17.47286 13.846620.04119 0.03754-0.0000426.471 ± 6.670 625 2.1047 60.080 10.99838 13.963940.01869 0.037230.00047 27.280 ± 0.797 700 6.4858 77.692 8.23487 14.41359 0.010140.03606 0.0001726.428 ± 0.179 775 5.8394 88.559 7.21550 14.938800.00688 0.034780.00012 26.405 ± 0.414 850 8.2472 92.317 6.95964 14.42862 0.005750.03602 0.0000726.539 ± 0.174 925 13.353992.2576.97018 14.81898 0.005870.03506 0.0001226.569 ± 0.125 1000 14.258687.7747.35616 14.46968 0.006990.03591 -0.00010 26.671 ± 0.249 1075 16.891695.3316.74250 14.67075 0.005070.03542 -0.0000226.555 ± 0.126 1150 12.480796.6466.66326 14.38482 0.004680.03613 0.0000626.599 ± 0.130 1225 7.5982 93.619 6.80873 14.529310.00544 0.035770.00026 26.333 ± 0.194 1400 11.862492.5656.93284 13.842400.00552 0.037560.00027 26.498 ± 0.275 Sample: WR07AE40 plagioclase Lab no. IRR266-44Step-heating experimentJ = 0.002439847 Step Age 39 40 40 39 37 39 36 39 (°C) % ArK Rel% Ar* Ar/ Ar Ar/ Ar Ar/ Ar K/Ca Cl/K (Ma) 550 10.07506.953 55.94744 69.843140.19535 0.007170.16247 17.863 ± 1.221 625 19.971624.33821.74895 72.270200.07554 0.006910.05958 24.305 ± 0.545 700 22.151415.04734.21411 70.522740.11773 0.007090.00887 23.615 ± 0.716 775 16.99104.279 91.35055 66.56444 0.314190.00754 0.0027717.911 ± 1.672 820 9.9378 3.94195.5430762.221920.32767 0.008090.00550 17.204 ± 1.863 875 7.8526 0.780261.76782 55.31123 0.894100.00914 0.011129.305 ± 5.205 925 5.7056 0.792304.35182 53.77897 1.036560.00941 0.0456010.962 ± 7.952 975 3.6981 1.685376.86279 52.91219 1.268380.00957 0.2281128.737 ± 11.276 1025 3.6170 3.079330.28443 55.11492 1.098400.00917 0.5922345.875 ± 8.987 Sample: 07-C-6 plagioclase Lab no. IRR266-42 Step-heating experimentJ = 0.002506046 Step Age 39 40 40 39 37 39 36 39 (°C) % ArK Rel% Ar* Ar/ Ar Ar/ Ar Ar/ Ar K/Ca Cl/K (Ma) 550 1.6336 60.632 10.05269 16.42473 0.017860.03160 0.0209927.647 ± 2.355 625 4.9166 80.859 7.51154 16.727840.00943 0.031020.00793 27.556 ± 0.756 700 10.225887.0226.72620 17.108450.00763 0.030320.00137 26.570 ± 0.212 775 17.176190.1906.52065 17.162980.00685 0.030220.00061 26.696 ± 0.147 850 17.796389.9856.57178 17.277960.00695 0.030020.00240 26.845 ± 0.157 925 11.622984.0787.06368 17.250080.00852 0.030070.00811 26.959 ± 0.192 1000 6.3171 70.100 8.31372 16.30165 0.012870.03184 0.2457726.441 ± 0.280 1075 2.4023 74.297 8.06667 15.750300.01131 0.032970.02144 27.177 ± 0.607 1150 3.9203 84.756 7.02928 15.450120.00784 0.033610.01952 27.010 ± 0.537 1400 23.988992.0716.45840 15.991590.00610 0.032460.00240 26.969 ± 0.131 Sample: JC07-WR303 Sanidine-IRR253-88Laser-fusion experiment 07L0305J = 0.002071645 Age Run # 40Ar* (mol/g )% 40Ar* 40Ar/39Ar 37Ar/39Ar 36Ar/39Ar K/Ca Cl/K (Ma) 07L0305A 3.6460E-1498.2517.11870 0.31584 0.000481.66094 0.0007125.958 ± 0.112 07L0305B 3.0870E-1497.8047.14536 0.30923 0.000581.69646 0.0008425.937 ± 0.120 07L0305C 3.3157E-1498.8617.12637 0.29048 0.000321.80596 0.0004526.145 ± 0.116 07L0305D 2.6224E-1497.2007.01449 0.28143 0.000711.86406 0.0004325.308 ± 0.127 07L0305E 2.7700E-1497.6397.16840 0.39514 0.000651.32754 0.0002825.978 ± 0.127 07L0305F 2.8506E-1498.8517.13443 0.29685 0.000331.76725 0.0003526.172 ± 0.128 Weighted Mean Age (Ma) (n=5) (MSWD = 0.83)26.04 ± 0.05 Total gas age (Ma)25.929 ± 0.045 Note: J is the irradiation parameter (e.g., McDougall and Harrison, 1999) determined by analyses of Taylor Creek Rhyolite sanidine (TCR-2) with an assumed age of 27.87 Ma.

within-run (i.e., internal) errors used as the weight­ (87Sr/86Sr = 0.710245 ± 16) and the Eimer and Amend trace elements by X-ray fluorescence (XRF), using 87 86 ing factors (Taylor, 1982). Sanidine from the Taylor (E&A) SrCO3 ( Sr/ Sr = 0.708022 ± 10). Nd isotope procedures described by Johnson et al. (1999). Creek Rhyo­lite (TCR-2) was used for calculation of ratios are normalized to 146Nd/144Nd = 0.72190. Analy­ Samples were analyzed at the USGS for 10 major the neutron flux in all irradiations. Ages in Table A2 ses of the U.S. Geological Survey standard BCR-1 elements by XRF, and for 55 major, trace, and rare are relative to TCR-2, and ages in the text (Table 1; yield 143Nd/144Nd = 0.512668 ± 20 (n = 4), and 30 earth elements ­by inductively coupled plasma–atomic Fig. 5) were recalculated relative to an age of runs of the La Jolla standard average 143Nd/144Nd = emission spectrometry (ICP-AES) and ICP–mass 28.02 Ma for the Fish Canyon Tuff sanidine standard 0.511848 ± 8. All quoted uncertainties in Table A3 are spectrometry (MS). (Renne et al., 1998). Decay and abundance constants 2s standard deviations of the mean. Major element data reported in Supplemental are those recommended by Steiger and Jäger (1977). Table 1 (see footnote 1) are XRF analyses from either APPENDIX 4. GEOCHEMICAL WSU or the USGS. Trace element data reported are APPENDIX 3. Sr/Nd ISOTOPE ANALYTICAL METHODS USGS ICP-MS and/or ICP-AES analyses if avail­ ANALYTICAL METHODS AND DATA able, and WSU XRF analyses if not. Rare earth ele­ Major, trace, and rare earth element analyses ment data are USGS analyses. Specific data sources Samples were analyzed for Sr and Nd isotopic ratios reported in this study were obtained from two facili­ are noted in Supplemental Table 1 (see footnote 1) for at Carleton University (Ontario, Canada), using tech­ ties, Washington State Geoanalytical Lab at Washing­ each sample; for example “WSU XRF” means all data niques described in Cousens (1996). Samples were ton State University (WSU, Pullman, Washington), reported are XRF analyses from WSU, while “WSU run on a Thermo-Scientific Finnigan Triton T1 thermal and the U.S. Geological Survey (USGS, Denver, XRF, USGS ICP” means major element data reported ionization mass spectrometer running in static mode. Colorado), which at the time of this study contracted are XRF analyses from WSU and trace and/or rare Sr isotope ratios are normalized to 86Sr/88Sr = 0.11940. analyses out to SGS Laboratories (Toronto, Canada). earth element data are ICP-MS and ICP-AES data Two Sr standards are run at Carleton, NIST SRM987 Samples were analyzed at WSU for 27 major and from the USGS contract lab.

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TABLE A3. Sr AND Nd ISOTOPIC DATA 87 86 87 86 Age Rb Sr Sr/ Sr Sr/ Sri Sample (Ma) (ppm) (ppm) (present) (±2σ) 87Rb/86Sr (initial) 143Nd/144Nd (±2σ) JC08-WR4163016.7453 0.703887 0.000014 0.10660.7038420.5128430.000009 07-C-62724.3465 0.703867 0.000080 0.15110.703809 0.512855 0.000007 07-C-52631.2506 0.703757 0.000013 0.17830.7036910.5128250.000006 07-C-20 26 27.3 510 0.703767 0.0000120.1548 0.703710 0.5128350.000011 WR07-AE115 15 44.5 4330.7039550.0000060.2973 0.703892 0.512826 0.000008 JC08-WR4131545.6388 0.704058 0.000013 0.33990.7039860.5128320.000012 08-C-21241.7548 0.704202 0.0000150.2201 0.704164 0.512743 0.000008 08-C-18 12 40.5 6040.7040430.000013 0.19390.7040100.5127960.000008 WR07-AE9645.9394 0.703678 0.0000180.0433 0.7036760.512890 0.000008 08-C-91134.6552 0.7040290.0000110.1813 0.704001 0.5128140.000008 08-C-11026.7589 0.704510 0.000009 0.1311 0.704491 0.512684 0.000008 08-C-11 14 10.1 626 0.7039530.0000110.0467 0.703944 0.512838 0.000006 WR08-AE081416.2557 0.704016 0.000014 0.08410.7039990.5128010.000009 WR07-AE40 27 22.6 451 0.703882 0.0000100.14490.703826 0.512847 0.000008

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Walker, G.W., and MacLeod, N.S., 1991, Geologic Map Wells, R.E., Weaver, C.S., and Blakely, R.J., 1998, Fore-arc Cretaceous and Cenozoic strike-slip faults: Implica­ of Oregon: U.S. Geological Survey, scale 1:500,000, migration in Cascadia and its neotectonic significance: tions for the Baja British Columbia hypothesis and 2 sheets. Geology, v. 26, p. 759–762, doi: 10.1130/0091-7613 other models, in Haggart, J.W., et al., eds., Paleogeog­­ Wells, R.E., 1990, Paleomagnetic rotations and Cenozoic (1998)026<0759:FAMICA>2.3.CO;2. raphy of the North American Cordillera: Evidence tectonics of the Cascade arc, Washington, Oregon, and Wyld, S.J., and Wright, J.E., 2001, New evidence for Creta­ for and against large-scale displacements: Geological California: Journal of Geophysical Research, v. 95, ceous strike-slip faulting in the United States Cordillera, Association of Canada Special Paper 46, p. 277–298. p. 19,409–19,417, doi: 10.1029/JB095iB12p19409. and implications for terrane-displacement, deformation Wells, R.E., and Simpson, R.W., 2001, Northward migra­ patterns, and plutonism: American Journal of Sci­ tion of the Cascadia forearc in the northwestern U.S. ence, v. 301, p. 150–181, doi: 10.2475/ajs.301.2.150. Manuscript Received 14 October 2010 and its implications for subduction deformation: Earth, Wyld, S.J., Umhoefer, P.J., and Wright, J.E., 2006, Recon­ Revised Manuscript Received 25 February 2010 Planets, and Space, v. 53, p. 275–283. structing northern Cordilleran terranes along known Manuscript Accepted 28 February 2010

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