Orogenic Link ~41°N–46°N: Collisional Mountain Building and Basin Closure in the Cordillera of Western North America

Research Paper

GEOSPHERE Orogenic link ~41°N–46°N: Collisional mountain building and basin closure in the Cordillera of western North America

1 2 3 4 GEOSPHERE; v. 16, no. 1 Keith D. Gray , V. Isakson , D. Schwartz , and Jeffrey D. Vervoort 1Department of Earth and Atmospheric Sciences, State University of New York at Oneonta, 108 Ravine Parkway, Oneonta, New York 13820, USA 2Department of Math, Science, and Agriculture, North Arkansas College, 1515 Pioneer Drive, Harrison, Arkansas 72601, USA https://doi.org/10.1130/GES02074.1 3Department of Geosciences, Boise State University, 1910 University Drive, Boise, Idaho 83725, USA 4School of the Environment, Washington State University, P.O. Box 642812, Pullman, Washington 99164, USA 16 figures; 4 tables; 1 set of supplemental files

CORRESPONDENCE: [email protected] ABSTRACT ■■ INTRODUCTION

CITATION: Gray, K.D., Isakson, V., Schwartz, D., and Vervoort, J.D., 2020, Orogenic link ~41°N–46°N: Colli‑ Polyphase structural mapping and mineral age dating across the Tectonic settings characterized by steady-state subduction are periodically sional mountain building and basin closure in the Cor‑ Salmon River suture zone in west-central Idaho (Riggins region; ~45°30′N, disrupted by mountain-building events associated with arc-arc or arc-continent dillera of western North America: Geosphere, v. 16, ~117°W–116°W) support a late Mesozoic history of penetrative deformation, collision (Dewey and Bird, 1970; Howell et al., 1985; Clift et al., 2003; accretionary no. 1, p. 136–181, https://doi.org/10.1130/GES02074.1. dynamothermal metamorphism, and intermittent magmatism in response orogens of Cawood et al., 2009). In either case, collision-related orogenesis to right-oblique oceanic-continental plate convergence (Farallon–North initiates with the attempted subduction of a bathymetric high (oceanic arc or Science Editor: David E. Fastovsky Associate Editor: Terry L. Pavlis America). High-strain linear-planar tectonite fabrics are recorded along an basaltic plateau; Livaccari et al., 1981; Cloos, 1993; Liu and Currie, 2016) and unbroken ~48 km west-to-east transect extending from the Snake River continues until plate motions stabilize, e.g., subduction outboard of an accreted Received 15 October 2018 (Wallowa intra-oceanic arc terrane; eastern Blue Mountains Province) arc terrane (Hamilton, 1988; Moresi et al., 2014, and references therein). Andean- Revision received 5 July 2019 over the northern Seven Devils Mountains into the lower Salmon River type orogens involving arc-continent collision over tens of millions of years (e.g., Accepted 14 October 2019 Canyon (ancestral North America; western Laurentia). Given the temporally central North American Cordillera; Fig. 1) typically record pronounced crustal overlapping nature (ca. 145–90 Ma) of east-west contraction in the Sevier thickening, high-pressure silicic magmatism, and pervasive ductile deformation Published online 5 December 2019 fold-and-thrust belt (northern Utah–southeast Idaho–southwest Montana of regional extent (e.g., Oregon/Idaho Blue Mountains Province of Silberling segment), we propose that long-term terrane accretion and margin- et al., 1984; Hamilton, 1969a; Zen, 1985; Avé Lallemant, 1995; Žák et al., 2015). parallel northward translation in the Cordilleran hinterland (~41°N–46°N Late Mesozoic tectonic activity along/across the arc-continent boundary in latitude; modern coordinates) drove mid- to upper-crustal shortening west-central Idaho (Riggins region; Fig. 2) was contemporaneous with oblique >250 km eastward into the foreland region (~115°W–113°W). During subduction of the Farallon plate under ancestral North America (McKenzie and accretion and translation, the progressive transfer of arc assemblages from Morgan, 1969; Engebretson et al., 1985; Giorgis and Tikoff, 2004). subducting (Farallon) to structurally overriding (North American) plates Detailed structural mapping and mineral age dating in the Riggins region was accommodated by displacement along a shallow westward-dipping (~45°30′00″N) support a long-lived history of penetrative deformation, coeval basal décollement system underlying the Cordilleran orogen. In this context, metamorphism, and intermittent calc-alkaline magmatism (Snee et al., 1995; large-magnitude horizontal shortening of passive continental margin strata Gray et al., 2012; McKay et al., 2017). Tectonic activity is recorded across was balanced by the addition of buoyant oceanic crust—late Paleozoic the Salmon River suture zone (SRSZ), which in our study extends from the to Mesozoic Blue Mountains Province—to the leading edge of western northeastern Seven Devils Mountains in the west (~116°30′00″W; Heavens Laurentia. Consistent with orogenic float modeling (mass conservation, Gate fault of Gray and Oldow, 2005) to the Late Cretaceous–Paleocene Idaho balance, and displacement compatibility), diffuse dextral-transpressional batholith in the east (~116°00′00″W; western border zone of Taubeneck, deformation across the accretionary boundary (Salmon River suture: 1971). Across the SRSZ, linear-planar (LS) tectonite fabrics are developed Cordilleran hinterland) was kinematically linked to eastward-propagating in accreted arc assemblages of the eastern Wallowa terrane (late Paleozoic– structures on the continental interior (Sevier thrust belt; Cordilleran Mesozoic Riggins and Seven Devils Groups: Hamilton, 1963a; Vallier, 1977), foreland). As an alternative to noncollisional convergent margin orogenesis, Laurentian metasedimentary units (late Precambrian to early Paleozoic Belt we propose a collision-related tectonic origin and contractional evolution for and/or Windermere Supergroups; Lund et al., 2003), and intervening calc- central portions of the Sevier belt. Our timing of terrane accretion supports alkaline intrusive complexes (Permian–Cretaceous plutons; Kauffman et al., correlation of the Wallowa terrane with Wrangellia (composite arc/plateau 2014). Exceptional exposure and tectonic context allow for the assessment This paper is published under the terms of the assemblage) and implies diachronous south-to-north suturing and basin of disparate rock assemblages metamorphosed and deformed under mid- to CC‑BY-NC license. closure between Idaho and Alaska. upper-crustal conditions (e.g., Zen, 1985; Selverstone et al., 1992).

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Figure 1. Tectonic elements of the central North American Cordillera. (A) The initial strontium 0.706 isopleth (Sri) separates late Paleozoic to mid-Mesozoic accreted oceanic crust (volcanic arc terranes) and Precambrian–early Paleozoic continental margin assemblages (western Laurentia). Ages of faults are cited in the text, Table 4, and Figure 14. Gener- alized cross-sections A–A′, B–B′, and C–C′ are constrained by data from Leeman et al. (1992) and Stanciu et al. (2016); note the coincidence of IDOR seismic line L and section line A–A′. SRSZ—Salmon River suture zone; SFTB—Sevier fold-and-thrust belt; Jura—Jurassic. State abbreviations: CA—California, CO—Colorado, ID—Idaho, MT—Montana, NV—Nevada, OR—Oregon, UT—Utah, WA—Washington, WY—Wyoming. Map compiled from Wheeler and McFeely (1991), Hurlow (1993), Whitney et al. (1999), McClelland et al. (2000), Wyld and Wright (2001), DeCelles (2004), Dickinson (2008), Gaschnig et al. (2010), Gray et al. (2012), and Long et al. (2014). (B) Wallowa terrane exposures in Hells Can- yon, viewing west, with Idaho in foreground (lower Granite Creek drainage). Middle–Late Triassic Wild Sheep Creek Formation (upper Seven Devils Group; maximum thickness >3 km) type section is located ~2.5 km south (Vallier, 1977). (C) Sevier folding in southwestern Montana, Kootenai Formation (Kk4 carbonate unit; Early Cretaceous, Albian).

T vs CHB MAP UNITS ~135- OROFINO Tc KCT Qu alluvial/landslide/glacial deposits [Quat.] 270 Ma ~90 km. ~110 Ma 07005 ~229 Ma SLATE C 31 Tc continental flood basalt [Miocene] 57 ~122 Ma 67 X G T Ptt CK. SCA KT sv 35 60 73 DW-02 Kib granitoid [late Early–Late Cret.] T vs 86 Psv JT ls 24 Kog orthogneiss [Early Cret.] PITTSBURG T i LANDING 78 JKg granitoid [Early Cret.] Psv ~85 Ma ~113 Ma 74 T Ptt Tc G34 KT sv volcaniclastic [Trias.–Cret.] A 64 amphibolite, schist, gneiss 40 KPgs KPgs [late Paleozoic–Cret.?] 35 ultramafic ophiolitic sliver 52 T Ptt KPum Zs [late Paleozoic–Cret.?] ~145 Ma LUCILE SCT Idaho siliciclastic, carbonate Zs JT ls Psv D 45 39 batholith [Trias.–Jura.?] SNAKE T 40 border carbonate [Late Trias.] RIVER RR zone T m 55 volcanogenic [Middle– 60 Chair Point T vs 43 o Late Trias.] T vs plutonic 45 intermediate–mafic ~226 Ma complex 30’ T i intrusive [Trias.] 53 48 I Psv 55 felsic intrusive T vs Psv MRF CHAIR Kib T Ptt POINT [Perm.–Trias.] HGF Tc FLORENCE volcanogenic 34 BASIN Psv [Perm.] GOFF schist/marble BRIDGE T vs Zs OLD TIMER T i SALMON [Precamb.?] T i RIVER X’ MINE 45 Fig. 80 T m RIGGINS ~92 Ma WESTERN ~124 Ma GSA 3b LGp03 67 70 LIMIT OF 5 ID26 ~120 Ma LCA Stop 64 2.7 Zs DUCTILE ~116 Ma JV-003 ~91 Ma 55 60 ~86-93 Ma 85 HEAVENS GATE FABRIC R7 RS SHd-04 58 89 FC-05 RIDGE SRSZ HELLS LAKE CK. GSA CYN. ~115 Ma BRIDGE Stop EASTERN 06KG15 T vs RAPID Kog ~104 Ma type section V-3-86 39 JT ls 2.6 LIMIT OF Fig. R. PMT Cp02 Vallier, JKg~136 Ma KPum DUCTILE 1977 3a C’BALL FABRIC 130 Ma 70 T vs DA MTN. GSA AM-5 KPgs Y Stop FRENCH JKg SEVEN 13 FHS 60 CK. T m 50 Tc ~116 Ma GRANITE GCP 50 PATRICK ID58 BUTTE CK. Qu Sr ~ 0.706 42 DEVILS Tc Idaho HGF 65 batholith ~118 Ma Tc Qu 34 MTNS. 41 35 R30 SNAKE KPgs 10 80 Tc shear 10 RIVER ? zone; T MRF RR 35 Aliberti HAZARD 80 ~135-124 Ma 40 [1988] 85 CK. 598, ID23 Kog ~110-105 Ma 80 ~119 Ma 83z14,99MG 598 50 T vs ~137 Ma PMT POL. 75 STRUCTURAL Psv ~112 Ma Hazard 1003 MTN. KPgs AM-4 Creek BRUIN SYMBOLS ? 76 complex MTN. FRENCH initial 87Sr/86Sr ~0.706 JKg DCS EMS ~92 Ma CK. Fleck and Criss, 2004 68 ~144- T m 135 Ma GSA ~114 Ma 01-53 422, Stop K92-8 Little 70 ID03b 1.1 80 Goose FAULTS ~118 Ma Creek Y’ thrust normal 83z9 Tc 60 complex Zs UPPER JKg PAYETTE low-angle [<30°] 64 Round GRANITE GSA LAKE Valley MTN. Stop FOLDS Riggins pluton 5.1 Kib synform; Payette 55 Devil’s Hamilton, 60 River Arch; LCA tonalite 1969a BRUN- Gray, DA RS Lake Ck. DAGE 2013 Tc antiform; Qu MTN. Onasch, WEST- 1977 45o CENTRAL 75 FABRIC 00’ GSA GSA LITTLE 70 bedding or S-tectonite N IDAHO Stop Stop 80 LS-tectonite SALMON flow top 21 85 5.2 5.3 RIVER 60 GEOCHRONOLOGY Kog Qu Lu-Hf/Sm-Nd garnet NEW GSA MEADOWS PAYETTE Stop 116o LAKE 5.4 U-Pb zircon [magmatic] U-Pb zircon [detrital] MAP LITTLE 30’ o LOCATION PAYETTE 116 Ar-Ar hornblende Ar-Ar muscovite K-Ar biotite W 00’ McCALL 10 km LAKE ~10 km.

Figure 2. Geology of west-central Idaho. (A) Arc-continent boundary transect outlined in white (see Figs. 3A and 3B; Gray, 2013), with 238U/206Pb magmatic zircon sample locality and age reported by Gaschnig et al. (2010): G34 = 85.0 ± 2.1 Ma; Giorgis et al. (2008): 01–53 = 91.5 ± 1.1 Ma, 99MG = 105.2 ± 1.5 Ma; Gray et al. (2012): Cp02 = 103.9 ± 2.7 Ma, LGp03 = 91.7 ± 2.4 Ma; Kauffman et al. (2014): C = 110 ± 4 Ma, D = 145 ± 6 Ma, G = 228 ± 5 Ma, I = 226 ± 10 Ma; Manduca et al. (1993): 83z14 = 110 ± 5 Ma, 83z9 = 118 ± 5 Ma; Mann (2018): AM-4 = 112.2 ± 1.6 Ma, AM-5 = 130.0 ± 1.9 Ma; Unruh et al. (2008): K92–8 = 114.4 ± 2.2 Ma; present study: 06KG15 = 135.84 ± 0.07 Ma, SHd-04 = 90.62 ± 0.03 Ma, FC-05 = 86–93 Ma. 238U/206Pb detrital zircon sample locality and age reported by LaMaskin et al. (2015): 07005 = 135–270 Ma. 147Sm/144Nd garnet sample localities and ages reported by Getty et al. (1993): 598 = 128 ± 3 Ma, 422 = pre–144 Ma (core); McKay et al. (2017): ID03b = 141.4 ± 2.0 Ma and 136.9 ± 3.5 Ma (cores), ID23 = 135 ± 2.4 Ma (core), 123.9 ± 1.3 Ma (rim), ID26 = 124.3 ± 5.8 Ma, ID48 = 112.5 ± 1.5 Ma. 176Lu/177Hf garnet sample localities and ages reported by Kauffman et al. (2014): A = 113 ± 3 Ma; Wilford (2012): JV-001 = 111.0 ± 11 Ma, DW-02 = 121.8 ± 3.2 Ma; present study: JV-003 = 119.8 ± 6.7 Ma. 40Ar/39Ar sample localities and ages reported by Armstrong et al. (1977): 1003–hornblende = 137 ± 4 Ma; Getty et al. (1993): 598–hornblende = 119 ± 2 Ma; Snee et al. (1995): R7–hornblende = 117.0 ± 0.6 Ma, R30–hornblende = 118.1 ± 0.6 Ma. K/Ar sample locality/age of Vallier (1995): V-3–86– hornblende, biotite = 115.3 ± 2.5 Ma. Abbreviations: CHB—Coon Hollow basin; DA—Devils Arch, DCS—Deep Creek stock, EMS—Echols Mountain stock, FHS—Fish Hatchery stock, GCP—Granite Creek pluton, HGF—Heavens Gate fault, KCT—Klopton Creek thrust, LCA—Lake Creek antiform, MRF—Morrison Ridge fault, PMT—Pollock Mountain thrust, RRT—Rapid River thrust, RS—Riggins synform, SCA—Slate Creek antiform, SCT—Slate Creek thrust; SRSZ—Salmon River suture zone. Map compiled from Onasch (1977), Aliberti (1988), Manduca et al. (1993), Vallier (1998), Lund (1984, 2004), Giorgis et al. (2008), Kauffman et al. (2014), and Schmidt et al. (2016b). (Continued on following page.)

felsic schist orthogneiss ca. 113 Ma ca. 244 Ma NW Schmidt et al., 2016a Kauffman et al., 2014 SE X Pittsburg John Day orthogneiss X’ 3 Snake Landing Salmon Mtn. Allison ca. 110 Ma Salmon R. R. Ck. Kauffman et al., 2014 R. T Ptt orthogneiss KPgs km ca. 229 Ma Kog orthogneiss Kib T i gneiss/schist Sr-0.706 0 D [Wallowa arc basement] JT ls ca. 84 Ma undeformed Schmidt et al., 2016b Psv ca. 122 Ma Snee et al., 1995 KT sv KCT Idaho HGF RRT SCT Kauffman deformed tonalite et al., 2014 batholith marine strata ca. 145 Ma [dike] -3 ca. 160–150 Ma Kauffman et al., 2014 LaMaskin et al., 2015 WESTERN EASTERN LIMIT OF LIMIT OF DUCTILE SRSZ DUCTILE FABRIC FABRIC

Y orthogneiss Hazard Y’ 3 Rapid ca. 116 Ma L. Salmon Ck. R. McKay et al., 2017 R. JKg Kib KPgs Kog orthogneiss undeformed km Kog schist orthogneiss orthogneiss ca. 92 Ma quartz diorite JT ls ca. 118–112 Ma ca. 111–105 Ma Giorgis et al., 2008 0 ca. 115 Ma JKg ca. 118 Ma HGF PMT Snee et al., 1995 Manduca et al., 1993 Manduca et al., 1993 Vallier, 1995 Unruh et al., 2008 Giorgis et al., 2008 undeformed deformed tonalite RRT Mann, 2018 Idaho ca. 130 Ma [stock] batholith -3 Schmidt et al., 2016b

SEVEN POLLOCK SUTURE ZONE IDAHO DEVILS MOUNTAIN INTRUSIVE BATHOLITH GP. RIGGINS GP. AMPHIBOLITE SUITE BORDER ZONE [Vallier, 1977] [Hamilton, 1963a] [Aliberti, 1988] [Gaschnig et al., 2010] [Taubeneck, 1971]

Figure 2 (continued ). (B) Simplified cross sections for geographic and chronologic reference, highlighting major faults and mineral ages in west-central Idaho. Line X–X′: northern Snake River–Hells Canyon/lower Pittsburg Landing—lower Salmon River Canyon/Lucile—Florence basin/upper Salmon River Canyon. Line Y–Y′: foothills of northeastern Seven Devils Mountains/lower Rapid River canyon—Little Salmon River/upper Hazard Creek drainages—Bruin Mountain/French Creek headwaters/Upper Payette Lake area.

Despite its tectonic significance (cf. Brown and Ryan, 2011), few have terrane accretion (Selverstone et al., 1992; Lund et al., 2008), (2) postaccretion attempted to map polyphase structures across this collisional orogen (Onasch, margin-parallel terrane translation (McClelland et al., 2000; Tikoff et al., 2001; 1977; Blake, 1991) or relate regional tectonism (SRSZ) to the greater Cordillera Giorgis et al., 2005; Lewis et al., 2014; Schmidt et al., 2016a), and/or (3) collapse (e.g., “tectonic escape” hypothesis of Wernicke and Klepacki [1988], “hit-and- of a North American fringing volcanic arc–back-arc basin assemblage (Gray and run” Laramide model of Maxson and Tikoff [1996]). Since demarcation of the Oldow, 2005). More recent workers attribute intense tectonic activity in west-cen- arc-continent boundary (Armstrong et al., 1977), studies combining structural tral Idaho to hinterland evolution of the Sevier orogeny (LaMaskin et al., 2015). geology and geochronology have focused on Cretaceous plutons exposed In light of previous modeling, we have assessed the spatial distribution and along the boundary (Manduca et al., 1993; Giorgis et al., 2008; Benford et al., development of mesoscopic structures in west-central Idaho. New across-strike 2010; Braudy et al., 2017; Montz and Kruckenberg, 2017), with little attention structural mapping (scale = 1:24,000; 70 localities) and mineral crystallization to country rock units involved in the contractional orogen. As a consequence, ages (3 U-Pb zircon, 1 Lu-Hf garnet) refine the space-time evolution of LS tec- timing relations are inferred between structures recorded in accreted island- tonites across the arc-continent boundary (ca. 136–91 Ma fabric development).

arc assemblages (F1–5 fold elements of Onasch, 1977, 1987), autochthonous Given the coeval nature of east-west contraction in the Sevier fold-and-thrust

continental metasedimentary rocks (S1r, Lm-1 fabrics; Blake et al., 2009, 2016), belt (SFTB), midcrustal metamorphic tectonites (SRSZ) evolved together with

and intervening metaplutonic complexes (D1–4 events; Manduca et al., 1993; more eastern structures of the SFTB: e.g., Paris, Willard, Basin-Elba, Meade, “overprinting” of Giorgis et al., 2008; Gray, 2015). Nevertheless, Cordilleran-scale and Medicine Lodge thrust systems (DeCelles, 2004) and fabric elements along tectonic syntheses have been proposed to explain (1) arc-continent collision/ the Lewis and Clark Line (Sears et al., 2004). In this context, we propose that

late Mesozoic terrane accretion and northward translation in the Cordille- Cordilleran margin (Coney et al., 1980; Saleeby, 1983; Jones et al., 1986; Kelley ran hinterland (~41°N–46°N, present coordinates) drove shortening eastward and Engebretson, 1994; Johnston, 1999, 2001, 2008; Wyld and Wright, 2001; into the continental interior (~116°W–113°W). This history of kinematically Housen and Dorsey, 2005; Lee et al., 2007; Schmandt and Humphreys, 2011). linked hinterland-foreland contraction (SRSZ–SFTB orogens) followed Mid- dle Jurassic collapse of late Paleozoic fringing volcanic arc assemblages, e.g., Intermontane composite terrane belt of British Columbia, Canada (Monger et Composite Terrane Belts al., 1982; cf. Martini et al., 2013). As an alternative to conventional modeling (noncollisional mountain building of Dewey and Bird, 1970; Burchfiel and Davis, The Intermontane belt includes island arcs, marginal marine basins, and 1975; Burchfiel, 1980; Price and Carmichael, 1986; Edelman, 1992; Taylor et al., subduction-related accretionary complexes that formed proximal to North 2000; Hyndman et al., 2005; Wells et al., 2008), we offer a collision-related America in mid-Paleozoic to early Mesozoic time (e.g., Rusmore et al., 1988, tectonic origin and contractional evolution for central portions of the SFTB 2013). Fringing arc–accretionary prism pairs include the Quesnellia–Cache (Gray, 2016). Our hypothesis is compatible with cross-orogen structural, geo- Creek terranes of southeast British Columbia (Gabrielse, 1991; Johnston and physical, and geochronological data derived from and/or applied to the North Borel, 2007), Rattlesnake Creek–Stuart Fork terranes of northern California American Cordillera (Potter et al., 1986; Allmendinger et al., 1987; Oldow et al., (Hacker et al., 1993; Ernst et al., 2017), and Olds Ferry–Baker(?) terranes of 1989, 1990; Brown et al., 1992). the southern Blue Mountains Province (Fig. 1A; Oldow et al., 1989; Kurz et al., 2012). Posttectonic sedimentary overlap assemblages indicate attachment to ancestral North America during the Middle Jurassic (e.g., Bowser basin of ■■ TECTONIC SETTING northern British Columbia [Ricketts et al., 1992; Logan et al., 2000]). Regional high-pressure metamorphism (>7 kbar: Ghent et al., 1979; Webster et al., 2017), Late Mesozoic Plate Interaction pervasive ductile deformation (ca. 172–167 Ma fabric; Gibson et al., 2005), and intermittent magmatism (ca. 159 Ma anatectic plutons; Sevigny and Parrish, 1993) Long-standing convergent plate boundaries may experience significant across southern portions of the Omineca belt are best explained by east-west periods of intense metamorphism and deformation leading to widespread contraction and crustal thickening associated with terrane accretion (Monger contractional strains, particularly when the angle of convergence is high (e.g., et al., 1972, 1982, 1994; Price, 1994; Murphy et al., 1995; Evenchick et al., 2007). Dewey, 1980; Tikoff and Teyssier, 1994). This type of margin characterized The Insular belt includes volcanic island arcs, oceanic plateaus, and overly- ancestral western North America, where eastward subduction of the Farallon ing carbonate platforms—Wrangellia, Alexander, Peninsular terranes—which plate persisted between at least Late Jurassic and early Cenozoic time (Hamilton, formed in deep-marine settings during early Paleozoic to middle Mesozoic time 1969b; Colpron and Nelson, 2009; Pavlis et al., 2019; for opposite polarity, (Jones et al., 1977; Gehrels and Saleeby, 1987; Umhoefer and Blakey, 2006; Israel see Johnston, 2008). Plate motion reconstructions between the northern et al., 2014). This composite extends from southwestern Alaska through western Sierra Nevada and northwest Washington State (Atlantic and Pacific hotspot British Columbia into the Blue Mountains region of Washington-Oregon- Idaho reference frames; Engebretson et al., 1985) indicate high-angle convergence (Figs. 1A and 1B; Hillhouse et al., 1982; Oldow et al., 1989; Dickinson, 2004; Kurz and subduction in the latest Jurassic (ca. 150 Ma) progressing into right-oblique et al., 2012) and was incrementally attached to the Intermontane belt during latest motion through late Early Cretaceous time (ca. 100 Ma). Analysis of the North Jurassic to Paleocene time (Monger et al., 1982; Crawford et al., 1987; Hansen American apparent polar wander path, however, shows left-oblique interaction et al., 1989; Plafker et al., 1989; Rubin et al., 1990; Burchfiel et al., 1992; Char- with the Pacific basin ca. 150–135 Ma (May and Butler, 1986; May et al., 1989). Žák don et al., 1999; Cole et al., 1999; Bergh, 2002; Ridgway et al., 2002; Evenchick et al. (2015) interpreted magnetic fabric in northeast Oregon (Wallowa batholith; et al., 2007; Trop and Ridgway, 2007; Schwartz et al., 2010, 2011). Integrated Fig. 1A) as recording dextral translation of the Blue Mountains Province ca. geophysical, geochronological, and mesoscopic structural data support a gen- 140–125 Ma. Regional structural and stratigraphic relations to the south suggest eral model of early high-angle arc-continent collision with a progression toward northward terrane transport ca. 110 Ma (western Nevada shear zone; Wyld dextral-oblique plate convergence and margin-parallel translation of outboard and Wright, 2001). In either case (sinistral or dextral motion), sustained plate terranes, e.g., Wallowa arc of the northern Blue Mountains Province (Beck et convergence over the late Mesozoic (also Seton et al., 2012) resulted in closure al., 1981; Lund, 1984; Engebretson et al., 1985; Umhoefer, 1987; Wernicke and of marginal marine basins and collapse of fringing volcanic arc assemblages, Klepacki, 1988; Hansen et al., 1989; Manduca et al., 1993; Irving et al., 1995; Wyld e.g., Stikinia of the Intermontane composite terrane belt (Monger et al., 1994; and Wright, 2001; Giorgis and Tikoff, 2004; Housen and Dorsey, 2005; Rusmore Hammer and Clowes, 2004). More outboard oceanic arc/plateau terranes et al., 2013). Timing estimates on terrane accretion in the Pacific Northwest (Wrangellia of the Insular belt; Jones et al., 1977; Nokleberg, 2005) and offshore (arc/plateau assemblages between Idaho and Alaska; i.e., >45°N) suggest south- tectonometamorphic assemblages (Franciscan forearc; Hsü, 1991; Dumitru et to-north closure of oceanic tracts separating the Wrangellian composite and al., 2010) were subsequently attached to and/or translated northward along the Laurentian continental margin (e.g., Pavlis, 1982; Hampton et al., 2010).

■■ REGIONAL SETTING Lower-greenschist- to upper-amphibolite-facies volcanogenic, siliciclastic, and carbonate rocks in west-central Idaho (Riggins Group, Lucile Slate of Blue Mountains Province Hamilton, 1963a, 1969a) are commonly correlated with Wallowa terrane exposures in the Seven Devils Mountains (Vallier and Fredley, 1972; Detra, The Blue Mountains region of northeastern Oregon, west-central Idaho, and 1980; Gray and Oldow, 2005), Snake River Canyon (Vallier, 1967; Goldstrand, southeastern Washington State includes oceanic fragments of the Wallowa 1987; White and Vallier, 1994), and Wallowa Mountains of northeast Oregon and Olds Ferry island-arc terranes (Vallier and Batiza, 1978; Kays et al., 2006; (Nolf, 1966; Whalen, 1988; Follo, 1994). Tropical marine fauna in the Kurz et al., 2012, 2017), Baker terrane subduction-accretionary complex (Mullen, Martin Bridge Formation (reef carbonate facies; Stanley et al., 2008) and 1985; Schwartz et al., 2010, 2011), and Izee overlap basin assemblage (Mesozoic paleomagnetic data from the Wild Sheep Creek Formation (upper Seven clastic terrane of Dickinson and Thayer, 1978; Dorsey and LaMaskin, 2007). Devils Group; Vallier, 1977; Hillhouse et al., 1982; Fig. 1B) indicate Middle– Concealed largely by Miocene flood basalt (Columbia River Basalt Group; Late Triassic paleolatitudes ~18° north of the equator. Wilson and Cox (1980) e.g., Hooper and Swanson, 1990), east-to northeast–trending terrane belts are estimated ~60° clockwise rotation of Early Cretaceous plutons (e.g., Wallowa discontinuously exposed over an area exceeding 50,000 km2 (map coverage batholith; Walker, 1986; Johnson et al., 2011; Fig. 1A) with respect to North of Silberling et al., 1984; cf. Jagoutz and Schmidt, 2012, Kohistan arc complex: America prior to Eocene time. Intra-oceanic rocks of the Wallowa terrane ~55,000 km2). Controlled-source seismic refraction and wide-angle reflection may represent a southern portion of Wrangellia (Jones et al., 1977; Saleeby, profiling across the Baker, Izee, and Olds Ferry belts indicates a crustal thick- 1983; Hillhouse and Gromme, 1984; McGroder et al., 1989; Dickinson, 2004; ness of ~24–36 km (IDOR seismic line L of Stanciu et al., 2016; Davenport et al., Insular belt Epigondolella sp. of Orchard et al., 2006; Johnston, 2008; Kurz 2017; Fig. 1A), and thus of sufficient age/buoyancy to have caused subduction et al., 2017) or Stikinia (Sarewitz, 1983; Mortimer, 1986; Oldow et al., 1989) of zone jamming and collision-related orogenesis (Cloos, 1993; Dickinson, 2008). the Alaskan and Canadian Cordillera. According to Vallier (1995), much of the Wallowa arc is missing. Explanations include westward-directed overthrusting of the Laurentian margin during terrane accretion and translation into northwest Washington, western British Western Laurentia Columbia, and southern Alaska, i.e., south-to-north segmentation of Wrangellia (Jones et al., 1977; Saleeby, 1983; Johnston, 2001, 2008; figure 8b reconstruc- In the Riggins region, westernmost exposures of ancestral North America tion of Dickinson, 2004). (Laurentia) consist of garnet-sillimanite-biotite schist, calc-silicate schist, The Olds Ferry terrane contains weakly metamorphosed Middle Triassic– marble, and minor quartzite of unknown pre-Cretaceous age (Hamilton, 1969a; Middle Jurassic arc-volcanogenic rocks of the Weatherby and Huntington Lund, 1984; Kelly Mountain Schist of Blake, 1991). Generally speaking, fine- to Formations (Brooks and Vallier, 1978; Mann and Vallier, 2007; Tumpane and medium-grained phyllosilicate minerals distinguish deeply eroded continental Schmitz, 2009). Intermediate-composition volcanic flows and hypabyssal metasedimentary rocks on the east from coarsely crystalline hornblende- intrusive rocks are interpreted to represent a deep-marine island arc (Pes- biotite schists and gneisses of island-arc origin to the west (eastern Blue sagno and Blome, 1986; Vallier, 1995) or continent-fringing arc assemblage Mountains Province; e.g., Blake et al., 2009). Along-strike correlations include (Miller, 1987; Dickinson, 2004; isotopic enrichment of Kurz et al., 2017). In high-grade units of the Mesoproterozoic Belt-Purcell (e.g., Winston, 1986) and/ contrast, the Baker terrane includes variably metamorphosed ocean-floor or Neoproterozoic Windermere Supergroups (Ross, 1991; Lewis et al., 2005), fragments—pillowed basalt, radiolarian chert, serpentinite-matrix mélange— which extend southward from British Columbia, Canada, into northern and and arc-derived rocks of Middle Devonian to Early Jurassic age (e.g., Elkhorn central Idaho (Fig. 1A). At this latitude (~45°N), late Precambrian–Permian Ridge Argillite and Burnt River Schist; Blome and Nestell, 1991; Schwartz passive-margin strata are sparse due to exhumation and unroofing of the et al., 2010). Early Permian faunas recovered from the Elkhorn Ridge Argil- Idaho batholith ± truncation of the Laurentian continental margin (Lund et al., lite support an exotic origin (Tethyan, McCloud affinity; e.g., Bostwick and 2003, and references therein). Koch, 1962). Paleomagnetic data from turbidite deposits of the Ochoco basin (Cenomanian Gable Creek Formation; e.g., Wilkinson and Oles, 1971) suggest late Early Cretaceous proximity to the southern Sierra Nevada (~39°N) and Intrusive Rocks vertical-axis clockwise rotation (~37°) with respect to stable North America (Fig. 1A; Housen and Dorsey, 2005). Overlying the Olds Ferry–Baker terrane Middle Jurassic to late Early Cretaceous magmatism in the Blue Mountains couplet are fault-bounded clastic units of the Izee assemblage (Late Trias- Province is recorded by three populations of intrusive rocks (synthesis of sic Carnian–Norian Vester Formation: Dickinson and Thayer, 1978), which Schwartz et al., 2011). The oldest plutons recognized are gabbroic to quartz contain abundant detrital zircon of Precambrian to Paleozoic age (LaMaskin dioritic in composition (ca. 162–154 Ma; Parker et al., 2008) and intrude et al., 2011). southern portions of the Baker and Wallowa terranes. Intermediate ages are

represented by tonalite-trondhjemite-granodiorite plutons within the same Riggins (e.g., Blake et al., 2009) before entering the Orofino-Ahsahka area in terranes (ca. 148–140 Ma; Tulloch and Kimbrough, 2003), e.g., syntectonic north-central Idaho (Figs. 1A and 2A; McClelland and Oldow, 2007; Lewis et Pole Bridge pluton of the northwestern Wallowa batholith (Fig. 1A; Johnson et al., 2014; Schmidt et al., 2016a). According to Giorgis et al. (2008), the main al., 2011; Žák et al., 2015). Youngest plutons include variably metamorphosed phase of deformation overlapped ca. 105–90 Ma magmatism and overprinted and/or deformed tonalitic bodies that intrude all terranes and the Izee basin accretion-related contractional structures of the SRSZ (Lund and Snee, 1988): assemblage (ca. 130–120 Ma; Johnson and Schwartz, 2009), e.g., ca. 130 Ma unspecified thrust/reverse faults, upright to overturned folds, and pervasive Fish Hatchery stock of the eastern Wallowa terrane (Aliberti, 1988; Gray and LS tectonite fabric. Isakson, 2016; figures 20 and 21 of Schmidt et al., 2016b; locality AM-5 of Mann, Following the language of Lund et al. (2008), “Salmon River suture” refers 2018; GSA Rapid River Stop 13 of Fig. 2A). to the ancient boundary separating accreted terranes of the Blue Mountains

Calc-alkaline magmatism across the eastern Wallowa terrane and western Province and western Laurentia as approximated by the Sri 0.706 isopleth. In Laurentian margin is recorded by variably deformed granitic rocks of the contrast, “Salmon River suture zone” terminology considers the broad belt Hazard Creek complex (ca. 137–112 Ma; Armstrong et al., 1977; Manduca et of metamorphism, magmatism, and penetrative deformation overlapping the

al., 1993; Unruh et al., 2008; Schmidt et al., 2016a; Mann, 2018), Little Goose Sri 0.706 isopleth and related to arc-continent collision (Lund, 1984; Giorgis et Creek complex (ca. 111–101 Ma; Manduca et al., 1993; Giorgis et al., 2008; Gray al., 2007). Although boundaries separating accretion- and non-accretion-re- et al., 2012; Kauffman et al., 2014; McKay et al., 2017; Montz and Kruckenberg, lated tectonic elements are not defined (field mapped at 1:24,000-scale), and 2017), and Payette River tonalite (ca. 93–85 Ma; Manduca et al., 1993; Snee superposition relations remain undocumented (overprinting shear zone fabric), et al., 1995; Gray et al., 2012; Idaho batholith border zone plutonic suite of structures assigned to the SRSZ and Western Idaho shear zone are explained Gaschnig et al., 2010). Magmatic epidote-bearing orthogneiss bodies (Zen, by right-oblique oceanic-continental plate convergence (Giorgis et al., 2005; 1985) are elongated subparallel to the arc-continent boundary, with oldest Lund et al., 2008). In addition to sharing a mid-Cretaceous kinematic history plutons emplaced into volcanic arc assemblages in the west (e.g., ca. 114 Ma, (dextral transpression), both elements are described as north-south–striking Granite Mountain locality K92–8; Unruh et al., 2008), the youngest plutons (McCall–New Meadows–Riggins–Slate Creek segment) and as comprising emplaced into Laurentian metasedimentary rocks in the east (e.g., ca. 92 east-dipping LS tectonite fabric (figure 5 synoptic plot of McClelland et al., Ma, Salmon River Canyon locality LGp3; Gray et al., 2012), and plutons of 2000; Tikoff et al., 2001). In this paper, we define through-going boundaries intermediate age in between (e.g., ca. 110 Ma, Slate Creek locality C; Kauff- of the SRSZ based on across-strike structural mapping, polyphase fabric anal- man et al., 2014; Figs. 2 and 3B). Middle Cretaceous magmatism recorded ysis (superposition relations), and the east–west limits of pervasive ductile by ca. 114–92 Ma plutons (sample localities K92–8, LGp3, and C) was coeval deformation. with east-west contractional deformation/crustal thickening in the SRSZ as indicated by westward-directed thrust/reverse faulting, tight-to-isoclinal folding, and tectonite fabric development (Snee et al., 1995; Gray et al., 2012; ■■ METHODS McKay et al., 2017). Structural Analysis

Arc-Continent Boundary Fabric and fold element data were collected from 70 outcrops studied along an unbroken ~48 km transect (Table 1; Fig. 2A) extending from the Across the lithospheric boundary separating arc terranes of the Blue Moun- Oregon-Idaho border (Snake River: elevation ~1400 ft [427 m]; ~45°30′20″N, tains Province and western Laurentia (Stanciu et al., 2016), initial strontium ~116°40′00″W) east over the northern Seven Devils Mountains (He Devil Peak: 87 86 Sr/ Sr ratios (Sri) recorded in Permian–Cretaceous calc-alkaline plutons elevation ~9340 ft [2847 m]) into the lower Salmon River Canyon (French Creek:

show a change from Sri ≤ 0.704 in the west (oceanic crust) to Sri ≥ 0.708 elevation ~2000 ft [~619 m]: ~47°27′30″N, ~116°00′00″W) near the village of in the east (continental crust; Armstrong et al., 1977; Manduca et al., 1992; Riggins, Idaho. Geologic strip maps and structural sections (1:24,000 scale,

Fleck and Criss, 2004). Width of the Sri isotopic transition (≤30 km, west-to- paper base mapping; Gray, 2013) were simplified to show major faults (Fig. 2B), east; e.g., Fleck and Criss, 1985; Benford et al., 2010) has been attributed to representative fold geometries, and polyphase fabric elements with emphasis post-accretion dextral-transpressional deformation along the Western Idaho placed on regional synmetamorphic structures (S1–L1; Gray et al., 2012). Maps shear zone (e.g., McClelland et al., 2000; Tikoff et al., 2001). East-west short- and sections are accompanied by stereographic projection plots compiled ening is estimated between ~28 and ~112 km, with ~50 km of right-lateral across four fault-bounded domains (I–IV; Fig. 3) bridging accreted island-arc offset (Giorgis and Tikoff, 2004; Giorgis et al., 2005). This structure is pro- rocks of the eastern Blue Mountains Province (Wallowa terrane + correlative

posed to follow the Sri 0.706 contour (i.e., best-fit curve) from the Owyhee units; Hamilton, 1963a; Vallier, 1977; Schmidt et al., 2016b) and the western Mountains in southwestern Idaho (Benford et al., 2010) through McCall and margin of Laurentia (Hamilton, 1969a; Blake, 1991). From structurally lowest

Figure 3. Simplified strip maps and structural sections along the arc-continent boundary transect: Snake River–Seven Devils Mountains–Salmon River Canyon (modified from Gray, 2013). Base map: U.S. Department of the Interior (Geological Survey) 7.5′ topographic series. (A) West map—Squirrel Prairie, He Devil, Heavens Gate, Kessler Creek, Pollock. (Continued on following page.)

Figure 3 (continued). (B) East map—Pollock, Riggins, Riggins Hot Springs, Kelly Mountain. GPS—global positioning system, rx—rocks.

to highest levels (west to east across the SRSZ), major domain-bounding subjected to a modified version of the chemical abrasion method of Mattinson structures include the Heavens Gate fault (Gray and Oldow, 2005; Kauffman (2005), consisting of a single step with concentrated HF (190 °C for 12 h). et al., 2014; Schmidt et al., 2016b), Rapid River thrust/Riggins normal fault The U and Pb isotopic measurements were made on an IsotopX Isoprobe-T (Hamilton, 1969a; Onasch, 1977, 1987; Hooper, 1982; Lund and Snee, 1988), multicollector TIMS equipped with an ion-counting Daly detector and 9 Faraday and Pollock Mountain thrust (Aliberti, 1988; Blake, 1991; Selverstone et al., cups. Analytical protocols, standard materials, and data reduction methods 1992; McKay et al., 2017). are outlined in the Supplemental Material1. Finite strain was evaluated on clast-supported volcanic conglomerate along the western edge of domain II in the northeastern Seven Devils Mountains (Heavens Gate Ridge; Fig. 2A). On a single rectangular block (~25 m3), the long Lu-Hf Garnet Analysis and short axes of pebbles and cobbles were measured across three intersecting faces (10 clasts/face). Mutually perpendicular faces and corresponding strain Sample locality JV-003 in the lowermost Riggins Group (Figs. 2A and planes (cf. Ramsay and Huber, 1983) were defined by the S1 foliation (top 3B; structural domain III; undated Fiddle Creek Schist; Hamilton, 1963a) was surface of exposure: x-y plane) and its intersection with spaced cleavage (x-z selected given its proximity to the Salmon River suture zone–Western Idaho plane) and systematic jointing (y-z plane). Lengths of long (x), intermediate shear zone boundary customarily assumed by previous workers (McClel- (y), and short (z) principal axes were used to calculate axial ratios and con- land et al., 2000; Tikoff et al., 2001; Giorgis et al., 2005, 2008; GSA Stop 2.6: struct a Flinn diagram. Pilot data were collected to (1) show changes in bulk “transition” of Blake et al., 2009). At this location, felsic volcanic rocks of strain across the Heavens Gate fault (located ~1.5 km west), and (2) compare the Wallowa terrane (Early Permian Hunsaker Creek Formation equivalent lithic clast geometries with porphyroclast shape fabrics determined along the of Kauffman et al., 2014) lie between the Rapid River and Pollock Mountain arc-continent boundary (Giorgis and Tikoff, 2004). thrusts, i.e., east-west contractional structures bounding domains II/III and III/IV, respectively (Fig. 2B). Consistent with U-Pb zircon localities described above (06KG15: upper Seven Devils Group; SHd-04: Pollock Mountain U-Pb Zircon Analysis Amphibolite), Lu-Hf garnet locality JV-003 represents a major lithotectonic assemblage (lower Riggins Group) and provides context for the continued Our westernmost sample locality (06KG15; 11KGHG01 of Gray, 2013) was assessment of overlapping(?) orogenic belts in west-central Idaho (Gray selected due to its proximity to the Heavens Gate fault (~75 km trace length), et al., 2012). which represents both a domain boundary (I/II; Fig. 3A) and regional discon- Several 200–250 mg garnet fractions were handpicked from our tinuity (metamorphism, bulk strain) in the eastern Wallowa terrane (Fig. 2A). mechanically crushed and separated field sample. Fragments containing the Sample localities SHd-04 and FC-05 were selected from central and eastern least amount of mineral inclusions (e.g., quartz and ilmenite) were selected portions of the transect (domains III and IV, respectively; Fig. 3B), where for analysis. Other inclusion types can possess different initial Hf isotopic higher-grade volcanic arc and continental margin assemblages are exposed compositions and were avoided (e.g., zircon; Scherer et al., 2000). Garnet (Hamilton, 1969a; Blake, 1991). These localities occupy critical areas of the fractions and one 200–250 mg whole-rock powder fraction were dissolved

SRSZ emphasized by previous workers (SHd-04: two-stage garnet of Selver- on a hotplate (~110 °C) in a concentrated HF and HNO3 acid mixture (10:1

U-Th-Pb isotopic data Compositional Parameters Radiogenic Isotope Ratios Isotopic Ages stone et al., 1992; GSA Stop 2.7 shear zone flattening of Blake et al., 2009) ratio) for 2–3 d for garnet and 5–7 d for whole rock. A second 200–250 mg 206 206 208 207 207 206 207 207 206 Th Pb* mol % Pb* Pbc Pb Pb Pb Pb Pb corr. Pb Pb Pb -13 206 204 206 206 235 238 206 235 238 Sample U x10 mol Pb* Pbc (pg) Pb Pb Pb % err U % err U % err coef. Pb ± U ± U ± (a) (b) (c) (c) (c) (c) (d) (e) (e) (f) (e) (f) (e) (f) (g) (f) (g) (f) (g) (f) and introduced in the present study (FC-05; Idaho batholith border zone of whole-rock powder fraction was dissolved in a high-pressure, steel-jacketed 06KG15 z1 0.341 0.2591 98.99% 28 0.22 1787 0.109 0.048679 0.428 0.142904 0.483 0.021291 0.081 0.723 132.38 10.06 135.63 0.61 135.812 0.109 z2 0.392 0.0783 96.61% 8 0.23 532 0.125 0.048840 1.303 0.143380 1.391 0.021292 0.138 0.669 140.14 30.58 136.05 1.77 135.815 0.186 z4 0.304 0.0661 95.46% 6 0.26 397 0.097 0.048762 2.040 0.143183 2.143 0.021296 0.153 0.693 136.42 47.91 135.87 2.73 135.844 0.205 z5 0.339 0.1186 97.78% 13 0.22 812 0.108 0.048692 0.834 0.143009 0.903 0.021301 0.098 0.731 133.04 19.60 135.72 1.15 135.874 0.132 Taubeneck, 1971). Teflon capsule (~160 °C; 5–7 d) in concentrated HF and HNO3 mixture (10:1 z3 0.353 0.0829 96.46% 8 0.25 510 0.113 0.048565 2.092 0.142756 2.225 0.021319 0.256 0.560 126.90 49.23 135.50 2.82 135.986 0.344

SHd-04 z1 0.821 0.1188 96.05% 8 0.41 457 0.263 0.047685 1.135 0.093045 1.227 0.014152 0.120 0.781 83.68 26.93 90.34 1.06 90.588 0.108 Zircon was isolated from oriented field samples using standard ratio). Following dissolution, samples were dried and converted from fluorides z2 0.241 0.3036 99.35% 43 0.16 2791 0.077 0.047771 0.238 0.093267 0.289 0.014160 0.075 0.746 87.94 5.65 90.54 0.25 90.641 0.068 z3 0.457 0.2240 99.18% 36 0.15 2200 0.146 0.047713 0.413 0.093097 0.455 0.014151 0.093 0.528 85.08 9.80 90.38 0.39 90.585 0.083 z4 0.454 0.0520 95.46% 6 0.20 398 0.145 0.047648 2.298 0.092985 2.411 0.014154 0.164 0.706 81.84 54.52 90.28 2.08 90.600 0.148 z5 0.231 0.1167 97.65% 12 0.23 767 0.074 0.047767 0.840 0.093175 0.910 0.014147 0.104 0.707 87.73 19.90 90.46 0.79 90.561 0.093 z6a 0.100 0.3789 99.18% 32 0.26 2193 0.032 0.047772 0.252 0.093234 0.301 0.014155 0.075 0.731 87.99 5.96 90.51 0.26 90.607 0.068 gravimetric and magnetic separation techniques at Boise State University to chlorides using a H3BO3 and 6 M HCl mixture. Each sample was dried z6b 0.087 0.3668 99.53% 57 0.14 3866 0.028 0.047749 0.250 0.093258 0.299 0.014165 0.078 0.706 86.86 5.92 90.53 0.26 90.673 0.070

FC-05 (Boise, Idaho). Handpicked grains from sample separates were mounted on again, dissolved in 6 M HCl, spiked with a mixed Lu-Hf tracer, and allowed z1 0.004 0.9348 99.81% 135 0.15 9348 0.001 0.047761 0.124 0.089585 0.180 0.013604 0.072 0.857 87.42 2.95 87.12 0.15 87.105 0.062 z2 0.005 0.2173 99.17% 31 0.15 2173 0.002 0.047801 0.326 0.093493 0.375 0.014185 0.078 0.690 89.44 7.73 90.75 0.33 90.802 0.070 z3 0.004 0.6304 99.72% 93 0.15 6470 0.001 0.047804 0.130 0.089324 0.185 0.013552 0.072 0.841 89.58 3.09 86.87 0.15 86.775 0.062 z4 0.006 0.5099 98.30% 15 0.73 1070 0.002 0.047769 0.320 0.088496 0.369 0.013436 0.078 0.697 87.87 7.59 86.10 0.30 86.038 0.066 z5 0.003 0.6843 99.42% 45 0.33 3105 0.001 0.047775 0.167 0.091516 0.217 0.013893 0.072 0.776 88.13 3.96 88.91 0.18 88.944 0.064 epoxy disks, polished to expose zircon centers, carbon-coated, and imaged to equilibrate on a hotplate for at least 24 h. Chemical separations of Lu z6 0.005 7.5207 99.97% 986 0.16 68308 0.002 0.047863 0.063 0.096295 0.130 0.014592 0.073 0.957 92.50 1.50 93.35 0.12 93.384 0.068 z7 0.003 0.5824 99.58% 61 0.21 4262 0.001 0.047769 0.188 0.090530 0.234 0.013745 0.074 0.718 87.83 4.45 88.00 0.20 88.003 0.065 by cathodoluminescence (CL) to determine spot locations for U-Pb isotopic and Hf were carried out in clean laboratory facilities located at Washington (a) z1, z2 etc. are labels for single zircon grains or fragments annealed and chemically abraded after Mattinson (2005). (b) Model Th/U ratio iteratively calculated from the radiogenic 208Pb/206Pb ratio and 206Pb/238U age. (c) Pb* and Pbc represent radiogenic and common Pb, respectively; mol % 206Pb* with respect to radiogenic, blank and initial common Pb. (d) Measured ratio corrected for spike and fractionation only. Fractionation estimated at 0.18 +/- 0.03 %/a.m.u. for Daly analyses, based on analysis of NBS-981 and NBS-982. (e) Corrected for fractionation, spike, and common Pb; up to 1 pg of common Pb was assumed to be procedural blank: 206Pb/204Pb = 18.042 ± 0.61%; 207Pb/204Pb = 15.537 ± 0.52%; 208Pb/204Pb = 37.686 ± 0.63% (all uncertainties 1-sigma). analyses. Laser ablation–inductively coupled plasma–mass spectrometry State University (Pullman, Washington). Chemically separated Lu and Hf Excess over blank was assigned to initial common Pb, using the Stacey and Kramers (1975) two-stage Pb isotope evolution model at the nominal sample age. (f) Errors are 2-sigma, propagated using the algorithms of Schmitz and Schoene (2007). (g) Calculations are based on the decay constants of Jaffey et al. (1971). 206Pb/238U and 207Pb/206Pb ages corrected for initial disequilibrium in 230Th/238U using Th/U [magma] = 3. (LA-ICP-MS) and chemical abrasion–isotope dilution–thermal ionization mass were dissolved in 2% HNO3 and analyzed on a ThermoFinnigan Neptune spectrometry (CA-ID-TIMS) analyses were also conducted at Boise State multicollector (MC) ICP-MS at Washington State University. Our single Lu-Hf 1 Supplemental Material. Analytical protocols, stan- University. LA-ICP-MS used a New Wave Nd:Yag ultraviolet 213 nm laser isochron age was calculated using Isoplot (Ludwig, 2003) and a 176Lu decay dard materials, and data reduction methods. Please coupled to a Thermo Scientific X-Series II quadrupole ICP-MS. Zircon dates constant of 1.867 × 10-11 yr-1 (Scherer et al., 2001; Söderlund et al., 2004). Cheng visit https://doi.org/10.1130/GES02074.S1 or access the full-text article on www.gsapubs.org to view the were obtained using CA-ID-TIMS (on select grains) based on LA-ICP-MS results et al. (2008) and Zirakparvar et al. (2010) discussed the dissolution, spiking, Supplemental Material. and morphology determined from CL. Zircons removed from mounts were and chemical separation methods used in this study.

TABLE 1. ESOSCOPIC STRCTRE OF THE ARC‑CONTINENT BONDARY TRANSECT, EST‑CENTRAL IDAHO Outcrop ap unit Latitude Longitude Elev. S0 S1 L1 S2 S3 Other Comment °N ° ft.

Domain I 1 Trvs 45°20.502 116°31.447 7950 N88E42SE ndeformed felsic dike; island‑arc magmatism 2 Trvs 45°20.471 116°31.443 8016 N3545S N0553NE Fracture below Goat Pass col; normal fault 3 Trvs 45°20.521 116°31.412 8026 N4035S Sandstone interbeds massive volcanic flows 4 Trvs 45°20.907 116°30.882 7617 N4830NE N04E70N Systematic/longitudinal oint local folding Domain II 5 Trvs 45°20.950 116°30.574 7559 N6430NE N3543NE Heavens Gate fault zone fabric; deeply eroded 6 Trvs 45°21.696 116°29.957 8246 N04E56N N0736NE N2944NE N6672S N70/30SE Crenulation associated with S3 fabric; argillite 7 Trvs 45°21.665 116°29.907 8174 N0548NE 10° rake egacrystic feldspar alignment; fault zone 8 Trvs 45°21.638 116°29.900 8170 N5573NE N0544NE N60/55SE Ais of N‑vergent fold; S2: aial‑planar fabric 9 Trvs 45°22.026 116°29.740 8358 N1938NE 19° rake S1 flattening/transposition fabric S0 S1 10 Trvs 45°22.068 116°29.700 8395 N30E50SE 0° rake S1 S0; transposed bedding in volcaniclastic rocks 11 JKg 45°20.541 116°29.339 7578 N17E53SE 0° rake N0558NE ‑Pb locality 06KG15 ca. 136 a; argillite screen 12 Trvs 45°23.460 116°27.225 6106 N1235NE 20° rake S1 S0; transposed bedding in volcaniclastic rocks 13 Trm 45°23.170 116°26.627 5795 N0775NE 7° rake S1 S0; intense L‑tectonite carbonate clasts Domain III 14 Trvs 45°22.229 116°21.340 ~1985 N37E23SE N7482NE ~N30 Ais of isoclinal fold; S0 transposed S1 15 Trvs 45°22.378 116°21.526 1978 N10E33SE 20° rake N7583S S1 S0 transposed sedimentary layering 16 JTrls 45°24.463 116°20.484 2086 N3828NE N47E43SE N63E/23NE Pervasive mineral lineation on S2; tourmaline 17 JTrls 45°23.859 116°20.123 1904 N67E60N N50E20SE N35E35SE Top of sheeted intrusion w/in Suaw Creek Schist 18 JTrls 45°26.197 116°18.545 ~1700 N1239NE N4505NE Syn‑ and postkinematic amphibole growth 19 JTrls 45°25.284 116°16.852 1793 N3078NE N20/11SE Hinge of NE‑vergent mesoscopic fold; centimeter‑scale 20 JTrls 45°25.264 116°16.750 1816 N4352S 12° rake S1 S0; bedding transposed on upper limb of fold 21 Trb 45°24.708 116°15.170 1763 N0580NE 40° rake Near contact w/ Suaw Creek Schist; folded thrust 22 Trb 45°24.086 116°13.884 1846 N3475S 45° rake S1 S0; transposed sedimentary layering 23 Trb 45°24.160 116°13.790 1845 N4069S Felsic boudinage; top‑down‑to‑the‑S shear 24 Trvs 45°24.207 116°13.367 2296 N3868S 47° rake Top‑to‑the‑SE shear sinistral; L1 normal face 25 Trvs 45°24.094 116°13.073 2175 N50E50SE Basal conglomerate; agglomerate Hamilton, 1963b 26 Psv 45°23.969 116°13.059 1813 N0576S 0° rake Fiddle Creek Schist; east limb of fold, south of river 27 TrPtt 45°24.029 116°13.022 1765 N0685S Intrusive body; pervasive country rock fabric sill 27 sill 45°24.029 116°13.022 1765 N08E75SE Attitude of fine‑grained sheeted intrusion felsic sill 28 Psv 45°24.031 116°13.022 1765 N10E59SE N0636NE Lu‑Hf locality JV‑003 ca. 120 a; footwall of fault 29 Psv 45°24.113 116°13.007 1836 N60E55SE N06E48SE Fault: 10° rake; minor intraformational thrust 30 Trvs 45°23.952 116°12.943 1804 N19E59SE N25E74SE Vein orientation; crosscuts S1 31 Trvs 45°23.940 116°12.942 1779 N5545S 20° rake N25/33SE Fold ais of, or related to, Lake Creek antiform 32 Trvs 45°24.012 116°12.616 2533 N35E65SE 27° rake S1 S0 33 Trvs 45°24.111 116°12.095 1839 N1866NE N1340NE Top‑to‑the‑S shear: C‑S fabric; chlorite mats define S2 34 Trvs 45°24.153 116°12.004 1908 N12E63SE Top‑to‑the‑N shear; rotated garnet porphyroblast 35 Trvs 45°24.155 116°11.990 1908 N35E65SE 20° rake Offset dike; minor N‑directed thrust‑ramp/flat geometry Domain IV 36 KPgs 45°24.783 116°11.688 1793 N08E60SE N3570NE Spaced cleavage; aial‑planar to local fold 37 KPgs 45°24.763 116°11.208 1844 N25E60SE 17° rake N4589NE Dissolution cleavage; aial‑planar to local fold 38 KPgs 45°24.760 116°11.204 1865 N16E59SE 10° rake N6367NE Dike/amphibolite contact; base of dike lower left 38 dike 45°24.760 116°11.204 1865 N3068NE ‑Pb locality SHd‑04 ca. 91 a; obliue to S1 38 dike 45°24.760 116°11.204 1865 N16E84N Fanning fracture cleavage; obliue to ductile fabric 39 KPgs 45°24.808 116°11.116 1872 N28E60SE 15° rake oderate dip approaching intrusive contact Trk continued

TABLE 1. ESOSCOPIC STRCTRE OF THE ARC‑CONTINENT BONDARY TRANSECT, EST‑CENTRAL IDAHO continued Outcrop ap unit Latitude Longitude Elev. S0 S1 L1 S2 S3 Other Comment °N ° ft.

Domain IV 40 Trk 45°25.131 116°10.284 1841 N10E85SE N10E85SE Aial surface of vertical fold; top‑to‑the‑NE shear 40 Trk 45°25.131 116°10.284 1841 N15E85N N25E/25S Fold hinge; steeply inclined, N‑vergent antiform 41 Trk 45°25.130 116°10.277 ~1900 N12E83SE 0° rake uartz‑rich intrusive; emplaced into amphibolite 42 Trk 45°25.117 116°10.155 1835 N58E78SE N25E40SE Shallow limb of N‑vergent antiform overturned 43 Trk 45°25.115 116°10.145 1849 N02E85SE N20E49SE Interleaved mafic schist: fault sliver of Riggins Group 44 Kog 45°25.153 116°09.914 1869 N37E90 Symmetric K‑feldspar boudin; Allison Creek campsite 45 TrPv 45°25.397 116°09.442 1846 N16E80SE 0° rake N15E/40NE Ais of minor fold; biotite‑rich metasedimentary rocks 46 TrPv 45°25.418 116°09.424 1869 N22E75SE Vertical S‑fold; top‑to‑NE/detral shear; L1 normal 46 TrPv 45°25.418 116°09.424 1869 N6863NE N40E43N Limbs of synform; ~fold ais: N30/35N 46 TrPv 45°25.418 116°09.424 1869 N28E85SE Feldspar porphyroclasts: top‑down‑to‑the‑SE shear 47 TrPv 45°25.429 116°09.419 1850 N10E74SE 28° rake Country rock fabric near dike 48 TrPv 45°25.673 116°08.366 1915 N12E80SE 0° rake S, , folds; opposing shear on L1 normal face 49 s 45°25.585 116°08.117 1970 N25E74SE 17° rake Deformed intrusive rock within Kelly ountain Schist 50 s 45°25.470 116°07.895 1990 N57E87N 10° rake S1 S0 51 Kog 45°24.746 116°07.652 2025 N13E64SE Intrusive rock of Partridge Creek Gneiss 51 s 45°24.746 116°07.652 2025 N00E75 27° rake N2458S Fold limbs of steeply inclined synform screen 51 s 45°24.746 116°07.652 2025 N11E18N Hinge of upright open antiform; local fold train in S1 screen 51 s 45°24.746 116°07.652 2025 N0233NE N20/45SE Ais of southwest‑vergent antiform/synform pair screen 52 Kib 45°24.508 116°07.630 2068 N1588NE 30° rake Ca. 92 a; Looking Glass pluton Gray et al., 2012 53 Kog 45°24.047 116°07.122 1902 N0873NE N6715NE top‑S shear Ca. 105 a; Crevice pluton Gray et al., 2012 54 s 45°24.087 116°06.258 1899 N12E64SE 30° rake Near contact w/ Crevice pluton; garnet flattened on S1 55 s 45°24.114 116°06.244 1862 N1518NE N1555NE Aial surface of isoclinal fold; S‑vergent 56 Kog/s 45°24.188 116°05.668 2118 N2072NE Intercalated country rock/intrusive rock 57 Kog 45°24.661 116°04.741 1986 N2060NE N2064NE s screen Kelly ountain Schist 58 s 45°25.100 116°03.648 1980 N2989NE 17° rake N1068NE Satellite intrusion or sliver of Crevice pluton 59 Kib 45°25.368 116°02.650 1993 N16E70SE Centimeter‑scale mafic screen; Kelly ountain Schist 60 Kib 45°25.368 116°02.416 1951 N20E80N Gneissic fabric; 1 cm mafic screens 61 Kib 45°25.354 116°02.080 1533 N05E84SE N4035S ndeformed pegmatite dike; crosscuts S1 62 Kib 45°25.635 116°01.687 2028 N75E35N afic enclave 63 Kib 45°25.398 116°01.662 1823 N55E54N igmatitic teture/magmatic foliation 64 Kib 45°25.573 116°01.550 ~2000 N85E55N Aligned biotite mats; magmatic fabric/migmatite 65 Kib 45°25.542 116°01.139 1599 N2834S Dike; crosscuts fabric oriented N4062NE 66 Kib 45°25.611 116°00.844 2017 N00E70E Felsic intrusive rock containing small mafic selvages 67 Kib 45°25.617 116°00.020 1990 N13E60SE ‑Pb locality FC‑05 ca. 86–93 a; pervasive fabric 68 Kib 45°25.581 115°59.832 1998 N6048NE eak magmatic fabric 69 Kib 45°25.688 115°59.626 2044 N0565NE eak biotite alignment; eastern margin of suture zone 70 Kib 45°25.839 115°59.225 1955 Nonpenetratively deformed Idaho batholith Outcrop locations are provided on Figures 3A 1–15, 3B 16–55, and 2A 56–70. Rake angles measured from downdip direction 0° downdip lineation. ncertain structural generation.

■■ RESULTS rocks are penetratively deformed with small (<100 m2) intermediate- to mafic- composition hypabyssal intrusive bodies of unknown age (T. Vallier, 1997, Structural Analysis personal commun.). Coarsely crystalline feldspar porphyroclasts record a downdip (90° from strike azimuth) to steeply plunging mineral lineation and West (Domains I and II) top-to-the-west/reverse sense of shear (Figs. 5D and 5E). Southeast of Windy Saddle (~1.5 km), deeply eroded argillaceous rocks enclose a small tabular Western foothills of the northern Seven Devils Mountains (Figs. 2A and 4A) intrusion (locality 06KG15 of this study; outcrop 11) containing a moderately rise out of Hells Canyon and Little Granite Creek drainage (45°30′20″N latitude) east-dipping gneissic foliation (S1; ~50°) and downdip mineral lineation (L1) along a shallow (≤30°) west-facing dip slope in the Wallowa terrane (Fig. 3A, defined by aligned hornblende (Figs. 3A, 5B, 5F, and 5G). Smooth to moderately cross-section A–A′). Lower greenschist–facies (albitized) basaltic-andesite flows, rough disjunctive foliation (microlithon spacing ≤5 mm: S2) cuts LS tectonite interbedded volcaniclastic rocks, and carbonate lenses of the Middle to Late fabric at a shallow angle (≤30°; Fig. 5F), as described for second-generation Triassic Wild Sheep Creek Formation (Figs. 1B and 4B-4E; Vallier and Batiza, structures in the Riggins region (figures 7C and 7D of Gray et al., 2012). 1978; Vallier, 1998) are deformed in north-south–trending upright symmetric In clast-supported pebble-cobble conglomerate, S1 dips variably east- open-to-close folds (outcrops 3 and 4, Table 1; cf. Fleuty, 1964). Broad-wave- ward (~20°–60°) and is defined by triaxially deformed volcanic (mostly basaltic length structures (l ≥ 1 km; e.g., Devil’s Arch) are associated with steep, easterly andesite) and minor carbonate clasts. Analysis of x/y and y/z axial ratios (cf. dipping (≥70°) longitudinal fractures locally displaying strike-slip kinematics Flinn, 1962; Ramsay and Huber, 1983) revealed both flattening and constric- (Figs. 4B and 4G). In the far west, subvertical, north-south–striking fractures tional strains (Figs. 6A and 6B), consistent with mixed oblate/prolate shape crosscut a quartz diorite pluton of late Early Cretaceous age (115.3 ± 2.5 Ma: fabric reported from middle Cretaceous granitoids of the Little Goose Creek hornblende, biotite; K-Ar locality V-3–86 of Vallier, 1995). Along this segment complex (Fig. 2A; Giorgis and Tikoff, 2004). Long-to-short axes viewed on of the transect (domain I), arc supracrustal and plutonic rocks (e.g., V‑3–86) the x-z strain plane reach 10-to-1 (Figs. 6C and 6D); aligned lithic clasts form preserve original igneous and sedimentary textures—glomeroporphyritic a pervasive downdip to southeast-plunging stretching lineation on S1 (L1: fabric, volcanic flow banding, lithic clast imbrication, graded bedding (Figs. ≤20° rake; Table 1). Shear sense indicators are rare. In most outcrops, clasts 3A, 4C-4F)—and show no evidence of dynamic recrystallization or pervasive are symmetrically flattened on S1 (Fig. 6E), and the sense of shear is unclear ductile strain (Vallier and Batiza, 1978). (Figs. 6D and 6F); however, sigmoidal strain markers identified ~1.5 km north In the Windy Saddle area (Figs. 5A and 5B), shallowly to moderately of Heavens Gate lookout show top-to-the-west shear (Figs. 5C and 5E), as northeast-dipping volcanic sandstone and breccia are structurally overlain evidenced by stretched pebbles in structurally overlying Late Triassic/Norian by upper-greenschist-facies (locally hornblende- and/or oligoclase-bearing) carbonate rocks (outcrop 13, Fig. 3A; Martin Bridge Limestone of Hamilton, metamorphic tectonites across the Heavens Gate fault (outcrops 5–12, sec- 1963a, 1969a; Epigondolella conodont sp. of Sarewitz, 1982). Asymmetric lithic tion A′–A″). This structure separates penetrative and nonpenetrative domains clasts recording top-to-the-west/thrust kinematics (east-west crustal shorten- (II and I, respectively) and marks the limit of high-strain ductile deformation ing; Figs. 2A, 3A, and 5C) combined with local strike-slip indicators (north-south west of the arc-continent boundary (Fig. 2A, ~116°30′00″W, “western limit of subvertical shearing; Fig. 4G) suggest post-Norian transpressional deformation ductile fabric”). Three-point solutions determined on fault segments between in the eastern Wallowa terrane (Žák et al., 2012, 2015). Cannon Ball Mountain and Old Timer’s Mine suggest a shallow east- to south- Primary textures east of the Heavens Gate fault—flow banding, phenocryst east-dipping (≤30°) surface cutting synmetamorphic fabric in the hanging wall clustering, clast imbrication, and graded bedding (S0)—are largely obscured (outcrops 5, 11) and sedimentary layering in the footwall (outcrop 4). Local by the S1 transposition foliation (S0 = S1; Fig. 7A) and overprinting postmeta- fault kinematics are unclear; however, the juxtaposition of greenschist tec- morphic deformation (e.g., small-scale duplexing; Fig. 7B). Where recognized, tonites (Figs. 5C, 5D, and 5E) with lower-grade rocks recording original textures S1 crosscuts S0 at a high angle (>70°); bedding/cleavage intersection relations (Figs. 4C, 4D, and 4E) supports local east-side-up/thrust motion. Schmidt et suggest tight-to-isoclinal folding of S0, where S1 is axial-planar to overturned al. (2016b) interpreted normal displacement to the north (Lucile area; Fig. 2A). west-vergent nappe structures (southern Heavens Gate Ridge, section A′–A″; The Heavens Gate fault/tectonic front has a protracted deformation history Fig. 3A). Depending on lithology, lithic clast size, and phyllosilicate content, that may also include strike-slip movement related to mid-Cretaceous dextral superposed structures include penetrative spaced or crenulation cleavage (S2, transpression in the lower Slate Creek–Riggins–northern McCall areas, i.e., S3 fabric) and associated intersection or crenulation lineations (L2, L3). Typically oblique arc-continent collision (Lund and Snee, 1988; McClelland et al., 2000; oriented subparallel to S1, S2 crosscuts synmetamorphic fabric at a shallow cf. Murphy, 1997, transpressive thrusting, Yukon Territory). angle (<30°). In contrast, S3 strikes orthogonal to S1 and crosscuts at a high Upper-plate protoliths include greenish-gray volcanic conglomerate, angle (Fig. 7C). Pencil cleavage formed by the intersection of S1 and S3 (argil- argillite, and minor basaltic-andesite flows of the Wild Sheep Creek Formation laceous rocks) occupies an axial-planar position to shallow, easterly plunging, (Fig. 5B). Across Heavens Gate Ridge (Figs. 4B and 5A), supracrustal cover upright symmetric open-to-close map-scale folds (D3 of Gray and Oldow, 2005).

HEAVENS GATE FAULT NE 1 cm

Dry Diggins T W. Fork Sheep FRON Creek

Heavens Gate Ridge DEVILS ONIC ? ARCH T Wild Sheep

EC Creek Fm. T

Wild Sheep HEAVENS f. Creek Fm. d. GATE FAULT 0.5 cm e.

c. g. TE GA 06KG15 ~136 Ma G Mirror Lake

A -?- T

S VEN S1 trace

Sheep Lake ? HEA 1 km W

West limb of East limb of SW DEVIL’S ARCH DEVIL’S ARCH SE NW

Figure 4. Footwall rocks of the Heavens Gate fault. (A) Northeastern Seven Devils Mountains, viewing to the southwest. He Devil on far right (elevation: 9339 ft./2863 m). (B) Google Earth™ image looking north along the axis of Devil’s Arch (antiform). Note outcrop locations C–G. (C) Volcaniclastic rocks exposed below/east of Goat Pass, west limb of antiform; Middle–Late Wild Sheep Creek Formation; outcrop 2 (Fig. 3A; Table 1). (D) Clastic beds on east limb, Windy Saddle; outcrop 4. (E) Porphyritic basalt between outcrops 2 and 4; Wild Sheep Creek Formation. (F) Early Cretaceous quartz diorite (ca. 115 Ma; Vallier, 1995) from lower Granite Creek drainage (Fig. 1B). (G) Mineral streaking on high-angle fault south of Seven Devils Lake (southwest corner of Fig. 5B); attitude: N35W (trend), 20NW (plunge).

Windy B Saddle 06KG15 ~136 Ma

HEAVENS GATE FAULT

S0 S1 western flanks of Cannon Ball Mountain 1 km E W

GEOLOGY OF HEAVENS GATE Z argillite 45 46 contact low-angle fault XY 50 m overturned antiform? D1 XZ Y of Gray and Oldow, 2005 s L1 ? YZ STRAIN 25 ANALYSIS Fig. 6A,B post-metamorphic fold [D3] 53 volcani- X clastic argillite 31 lamination in argillite [S0] SEDIMENTARY STRUCTURES 56 Fig. 7C 45 metamorphic foliation [S1] w/ mineral ()m or lithic clast s 36 m 34 stretching ()s lineation [L1] TOPS-WEST s 20 S1 1 km STRUCTURES Fig. 5E 40 m hypa- s W byssal 37 E intrusive DUPLEX ? STRUCTURES Fig. 7B 40 51 52 m 49

S2 S1

volcaniclastic volcanic felsic 48 46 49 intrusive argillite

hornblende G 30 42 diorite 37 50 volcani- m clastic argillite 50 S1 LOCALITY volcanic 35 06KG15 marble breccia Fig. 5F,G massive volcanic m HEAVENS GATE flows FAULT 70 STRIKE-SLIP Fig. 4B,G

Figure 5. Hanging-wall rocks of the Heavens Gate fault. (A) South end of ridgeline viewed from western flanks of Cannon Ball Mountain. (B) 1:24,000 scale mapping in the Wild Sheep Creek Formation. (C) Deformed volcanic clasts ~2 km north of U.S. Department of Agriculture Forest Service lookout (Fig. 6C); stacking patterns/asymmetry indicate tops-to-the-west shear. Dashed and solid lines placed along the trace of bedding (S0) and synmetamorphic foliation (S1). In this area, a strong shape-preferred orientation of lithic clasts defines linear- planar tectonite fabric. (D) Hypabyssal intrusive rocks north of Fire Camp Saddle (southwestern Kessler Creek quadrangle; Quarcoo and Gray, 2016). Aligned feldspar megacrysts define L1 (rake ~30° from down-dip direction). Across the Salmon River suture zone, down-dip (0° rake) to obliquely-plunging (≤45° rake) mineral and/or stretching lineations are developed in accreted oceanic crust of the Blue Mountains Province, Laurentian continental crust, and intervening calk-alkaline intrusive rocks (Figures 2a, 3, Table 1). (E) Sigma structures in feldspar porphyroclasts support tops-west shear. (F) U–Pb zircon sample locality 06KG15: N45°20.535’, W116°29.333’; hornblende diorite. Dashed/solid lines along traces of S1/S2. (G) Hornblende fabric of U–Pb locality 06KG15. Early Cretaceous tectonic activity—ca. 136 Ma magmatism and deformation—obfuscates the Salmon River belt (Gray and Oldow, 2005; Gray, 2015).

8 Z LITHIC CLAST STRAIN ANALYSIS k= KEY this study 5 S1 XY XY strain planeYZstrainplane Aliberti XZ Y X-axis [cm] Y-axis [cm] X/Y[X/Ycm]Y-axis[cm]Z-axis[cm]Y/ZY/[cZm] YZ [1988] 6.53.9 1.67 5.94.1 1.44 S3 18.6 10.71.744.5 1.92.37 X Jn 18.47.2 2.56 2.51.0 2.50 CONSTRICTION 1.20.4 3.00 1.81.0 1.80 X/Y 3 k =1 41.27.5 5.49 2.01.0 2.00 16.04.0 4.00 10.53.5 3.00 FLATTENING b. 3.61.2 3.00 5.42.6 2.08 4.21.9 2.21 8.51.9 4.47 20.1 10.31.955.1 1.82.83 7.22.1 3.43 3.51.9 1.84 3 5k=0 Y/Z

Z X Z S1 XZ X

S1 X

S0 Y S1

L1 S1 Y Wild Sheep SE Creek Fm. XY Z YZ

Figure 6. Lithic clast strain analysis in the Seven Devils Group–Wild Sheep Creek Formation. (A) Lengths of maximum (x), intermediate (y), and minimum (z) principal stretch axes with ratios calculated for x/y and y/z. (B) Flinn (1962) diagram: x/y (ordinate), y/z (abscissa). Black dots—data from this study. Open circles—Aliberti (1988); note subvertical north-south shear zone in lower Rapid River canyon (Fig. 2A). Mutually intersecting faces (gray rectangular block) upon which axial data were collected: S1—synmetamorphic foliation; S3—spaced cleavage; Jn—joint set. (C) Data collection site on southern Heavens Gate Ridge, ~100 m south of fire lookout. Dashed line along the trace of S1. (D) x–z plane. (E) x–y plane. Aligned volcanic clasts define L1; outcrop 10, Figure 3A/Table 1. (F)y–z plane. White ruler = 6 inches (~15.25 cm).

Central (Domain III) metasedimentary rocks (Lightning Creek Schist), S3 is associated with a shallow (≤20°) south-plunging crenulation lineation on S1 (L3: outcrop 24). Crenulations Greenschist-facies volcanic, volcaniclastic, and carbonate rocks in the west are orthogonal to steep downdip (~75° west) chlorite mineral lineations (L1: out- (Wild Sheep Creek and Martin Bridge units) are structurally overlain by high- crop 26) in the underlying Fiddle Creek Schist, i.e., silica-rich volcanic tuffs and er-grade (biotite- to andesine-bearing) metamorphic tectonites across the Rapid flows defining the basal unit of the Riggins Group (Hamilton, 1963a). River thrust fault (Figs. 2A and 3B, cross-sections A′′–A′′′ and B–B′; Hamilton, West of Rough Creek (~100 m; Fig. 3B), S1 dips steeply to the east (outcrop 1963b; Onasch, 1977; Lund and Snee, 1988). This east- to southeast-dipping, 28: Lu-Hf locality JV-003; Figs. 9A and 9B). Early fabric is subtle here and follows syn- to postmetamorphic structure (Hamilton, 1963a; Onasch, 1977; Aliberti, the trace of aligned phyllosilicates. Augen-shaped aggregate clusters of quartz 1988; Snee et al., 1995) separates the Permian–Triassic Seven Devils and Rig- ± feldspar enclosing millimeter-scale garnet are elongated in the downdip gins Groups (domains II and III, respectively). Along our transect, the Rapid direction (L1; Fig. 9C). In this area, S2 is dominant and relates(?) to moderately River thrust is cut out by a major north-south–striking brittle normal fault east-dipping imbricate faults disrupting the antiformal hinge (Figs. 3B and (Riggins fault of Hooper, 1982; Hamilton, 1969a; Fitzgerald, 1984; Aliberti, 1988; 9A, inset). Approaching Ruby Rapids and the Berg Creek Amphibolite (Ham- Gray and Oldow, 2005) carrying gently west-tilted (≤25°) continental flood ilton, 1963a), counterclockwise-rotated (viewing north) garnet porphyroblasts basalt in its hanging wall (Miocene Columbia River Basalt Group; Hooper and record top-to-the-northwest/reverse shear (uppermost Lightning Creek Schist; Swanson, 1990). The Riggins fault dips steeply east (≥75°), slices through ridge- outcrops 33, 34, and 35). Deeply eroded biotite-chlorite schist containing ca. and-valley topography at ~90° (e.g., Papoose Saddle–Squaw Creek segment; 112 Ma synkinematic garnet (Sm-Nd sample locality ID48; McKay et al., 2017) Fig. 2A; Quarcoo and Gray, 2016), and relates to basin-and-range extensional crops out in footwall exposures of the Pollock Mountain thrust (Blake, 1991; deformation (Capps, 1941; Tikoff et al., 2001; Giorgis et al., 2006; Schmidt et Selverstone et al., 1992), which strikes north to northeast across the Riggins al., 2016b) offsetting the crust-mantle boundary (Fig. 1A, cross-section A–A′; region (>100 km trace length; Aliberti, 1988) and separates domains III and IV seismic line L of Stanciu et al., 2016). along our transect (Figs. 2A and 3B). Amphibolite-facies metamorphism and attendant ductile deformation South of the Salmon River, the Pollock Mountain thrust places upper-am-

obscure original igneous and sedimentary textures in the Riggins Group (S1R phibolite-facies metamorphic tectonites over lower-grade pelitic rocks of the transposition foliation of Onasch, 1977, 1987). Bedding/cleavage intersection Squaw Creek Schist (Fig. 2A; e.g., Ar-Ar locality R30, pre–118 Ma synkinematic relations recorded in deep-marine metasedimentary rocks (Squaw Creek hornblende of Snee et al., 1995; Aliberti and Manduca, 1988). Along our transect, Schist; Hamilton, 1963a; Jurassic metaflysch of Lund et al., 2007) support west-northwest–directed thrusting cuts out the Squaw Creek Schist and carries early tight-to-isoclinal map-scale folding of S0 (outcrop 17; Table 1; Figs. 3B the Pollock Mountain Amphibolite over the Berg Creek Amphibolite (eastern and 8A), where S1 is both axial-planar (fold hinges; Fig. 8B) and parallel to limb of Lake Creek antiform; section C–C′, outcrops 36–39). Metamorphic fabric S0 (attenuated limbs; Fig. 8C). High-strain fabric (S1) is broadly deformed dips variably southeast (~45°–85°) in both the hanging wall and footwall (Gray (l ≥ 1.5 km) across upright symmetric to overturned southeast-plunging (~20°– et al., 2012; Blake et al., 2016); in the former, aligned hornblende needles define 40°) postmetamorphic folds. Map-scale structures spanning lower portions of a downdip to obliquely (≤20° rake) southeast-plunging mineral lineation on S1

the Salmon River Canyon (Riggins town site to Ruby Rapids; Fig. 3B) include (L1: outcrops 37 and 39; Lm-1 of Blake et al., 2009). At locality SHd-04 (Fig. 10A), the upright Riggins synform and overturned Lake Creek antiform (outcrops LS tectonites are deformed in west-northwest–vergent tight-to-isoclinal folds 16–35, sections B–B′ and C–C′; Hamilton, 1969a; Onasch, 1977, 1987; Bruce, 1998; (SHd-04 host rocks). Mesoscopic structures plunge gently west-southwest figure 10 of Blake et al., 2009; figure 16 of Schmidt et al., 2016b). (~20°–30°) and possess a steep southeast-dipping (≥70°) axial-planar spaced East of Berg Creek Ranch (Fig. 8A), synmetamorphic foliation is wrapped and crenulation cleavage (tonalite dike and amphibolite host, respectively; around the Lake Creek antiform (west end of section C–C′; Fig. 3B). Attitudes (S1) Figs. 10B and 10C). Counterclockwise-rotated (viewing north) subhedral garnet change abruptly across the hinge (~1.5 km wide), where upright west-vergent porphyroblasts recording top-to-the-northwest shear (Fig. 10D), combined with mesoscopic folds and moderately east-dipping brittle reverse faults disrupt lower isoclinally folded felsic stringers (Fig. 10E), support dextral-transpressional units of the Riggins Group (outcrops 25–33). Basal metaconglomerate of the deformation in easternmost exposures of the Pollock Mountain Amphibolite. Lightning Creek Schist defines a prominent marker horizon (Hamilton, 1963a); Magmatic epidote-bearing plutons east of the Pollock Mountain Amphi- axial ratios reach 8-to-1 in triaxially deformed felsic volcanic clasts (outcrop 25; bolite (outcrops 40–47) are generally characterized by a northeast-striking Fig. 8D). South of the Salmon River, arc-volcanogenic rocks are crosscut by an gneissic foliation (S1: ~45°–85° dip; Fig. 11A) and downdip to obliquely south- undated intermediate-composition pluton related(?) to the Chair Point plutonic east-plunging hornblende ± biotite lineation (L1: ≤45° rake; Fig. 11B). South complex, Permian–Triassic crystalline basement of the Wallowa terrane (Fig. of the Salmon River, however, S1 attitudes vary from north-south–striking 2A; Walker, 1986; Kurz et al., 2012; Kauffman et al., 2014). Local plutonic rocks (~15°–50° west-dipping) to east-west–striking (~35° north-dipping) and thus show a steeply east-dipping (~80°) gneissic foliation that is axial-planar to the effectively span a complete range of strike orientations (Onasch, 1977; Blake Lake Creek antiform (S3: outcrop 27; Figs. 3B, 8E, and 8F). In adjacent mica-rich et al., 2016). Mineral and stretching lineations on S1 are rare (L1: ≤40° oblique

S0 volcaniclastic argillite 46 argillite S1 ?

W ~100 m

S0 trace

S1 36

S3 trace

W N

Figure 7. East-west contractional structures in the Wild Sheep Creek Formation. (A) Imbricated metasedimentary sequence below Heavens Gate lookout; note parallelism of S0 and S1 in upper left (east-dipping limb) and orthogonal relations in lower right (hinge zone). (B) Small-scale duplexing in argillaceous rocks exposed south of lookout. Granite Mountain on skyline; trees on ridge are ~5–10 m tall. (C) Bedding/cleavage intersection relations in hinge of isoclinal(?) map-scale fold. Laminated argillite between outcrops 7 and 10 (Fig. 3A; Table 1). Pencil tip in upper right for scale.

plunge). Variably oriented metamorphic tectonites are mapped across the Van (outcrop 48; Fig. 11C), where gneissic foliation (S1) strikes northeast (~N10–15E), Ridge Gneiss (south of section B–B′ in Blake et al., 2016), which lies directly dips steeply southeast (~80°), and contains a strong downdip stretching lin-

west of the arc-continent boundary (Sri 0.706 isopleth) akin to ca. 118–112 Ma eation (L1) defined by rodded quartz (Fig. 11B). From northeast to southwest granitic rocks of the Hazard Creek complex (Blake, 1991; Manduca et al., 1993; across the exposure, minor folds display sinistral shear (S), dextral shear (Z), Mann, 2018; Unruh et al., 2008). North of the Salmon River, tight-to-isoclinally and neutral (M) shapes. Local ductile deformation/fold patterns revealing a folded pegmatite stringers record opposing shear kinematics. Mesoscopic combination of S, Z, and M geometries are consistent with east-west short- structures are observed across a single surface exposing the y-z strain plane ening and flattening perpendicular to S1/x-y (cf. Ramsay and Huber, 1983), as

Lightning Creek Schist

Squaw S1 Creek trace EASTERN LIMB OF Schist RIGGINS SYNFORM [UPRIGHT] Berg Creek Amphibolite

FOLDED WESTERN LIMB OF LAKE CREEK ANTIFORM [OVERTURNED]

Lightning Creek Schist

S1 Z X

Squaw Creek Berg Creek NW XZ Schist S0 1 mm Amphibolite SE S1

E Lightning Creek Schist

S3 S1

NW Chair Point plutonic cpx.?

Figure 8. Contractional structures in the Riggins Group. (A) Map-scale folding of the Squaw Creek Schist and Berg Creek Amphibolite, lower Salmon River Canyon, showing the Riggins synform and Lake Creek antiform (Berg Creek Ranch in lower right). East-west contractional structures deform linear-planar fabric recording ca. 124–107 Ma synkinematic mineralization (hornblende/garnet-bearing tectonites reported by Snee et al. [1995], McKay et al. [2017], and this study [Lu-Hf garnet locality JV-003; Fig. 9C]. (B) Bedding/cleavage intersection relations in hinge of west-vergent tight-to-isoclinal(?) fold, southwest limb of Riggins synform, outcrop 17 (Fig. 3B; Table 1). Notebook lies on axial-planar foliation (S1). (C) Photomicrograph of ca. 112 Ma garnet, uppermost Berg Creek Amphibolite (Wilford, 2012; McKay et al., 2017). Thin section is oriented parallel to L1 and perpendicular to S1; crossed polarized light. Note trace of S1 wrapping elongate subhedral garnet porphyroblast; outcrop 21. (D) Limb of minor fold in lowermost Lightning Creek Schist (basal conglomerate); dashed ellipse definesx-z strain plane; outcrop 25. (E) Hinge of Lake Creek antiform; outcrop 31. Dashed lines along S1 trace; dashed/dotted line along L1. Pack for scale. (F) Axial-planar foliation (S3 [Lake Creek antiform]) in undated intrusive rocks; outcrop 27. Brunton compass sighting arm for scale; cpx.—complex.

POLLOCK MOUNTAIN AXIAL THRUST SURFACE Pollock SHd-04 TRACE Mountain ~91 Ma Amphibolite ? Berg ? Creek Amphibolite Fiddle Creek Lightning Schist Rough Creek S2 Creek Schist B ? S2 S3 JV-003 ? ~119 Ma Lake S1 Creek E ~150 m S1

JV-003 ~119 Ma ~25 m E

Figure 9. Structures of the Lake Creek bridge area. (A) Google Earth™ image of Salmon River Canyon ~10 km east of Riggins. Upright antiformal structures contain steep east-dipping axial surfaces approximated by the S3 foliation (Figs. 3B (section C–C′], 8E and 8F). Second-generation structures include S2 slip surfaces and associated brittle shear zones (inset). Sample localities indicated by mineral habit symbols: Lu-Hf garnet JV-003: 45°24.031′N, 116°13.022′W; U-Pb zircon SHd-04: 44°24.760′N, 116°11.204′W; box covers area shown in part B. (B) Locality JV-003 shown in the context of structures obscuring Lake Creek antiform. (C) In situ field photograph (JV-003) highlighting S1/S2 superposition relations and millimeter-scale garnet porphyroblasts enclosed by augen clusters of quartz and minor feldspar. Compare S1/S2 relations with superposed structures on Heavens Gate Ridge (Table 1, Domain II; Fig. 3A, section A’-A’’; Fig. 5F).

recorded by structures ~30 km west on Heavens Gate Ridge (Wild Sheep Creek portions of the Crevice pluton (figure 12b of Gray et al., 2012) and Payette Formation; Figs. 6A and 6E). River tonalite (Fig. 2A; GSA Stop 5.3 of Giorgis et al., 2006). At this latitude (~45°25′00″N), outcrop 69 marks the eastern limit of penetrative ductile deformation; farther east, massive medium-grained hornblende tonalite of East (Domain IV) the Idaho batholith is encountered (outcrop 70, Fig. 12H; peraluminous suite of Gaschnig et al., 2010). Our approximate boundary (Fig. 2A, “eastern limit of Near the mouth of Kelly Creek (Fig. 3B), medium-grained calc-silicate ductile fabric,” ~116°00′00″W) lies ~55 km north of the undeformed two-mica metasedimentary rocks (undated Kelly Mountain Schist; Blake, 1991) mark granite locality (GSA Stop 5.4) described by Giorgis et al. (2006). the western edge of ancestral North America (Fig. 12A, cross-section C–C′, outcrop 49; Table 1). In this area, schistosity (S1) strikes subparallel to the arc-continent boundary (~N10–25E trend). South of Kelly Creek, however, S1 U-Pb Zircon Analysis orients ≤45° oblique to the boundary (e.g., outcrop 50). East of the Crevice pluton (figures 11 and 12c of Gray et al., 2012), tight-to-isoclinal folds deform Sample 06KG15 fine-grained garnet-bearing mica schist (Fig. 12B, section D–D′, outcrops 54, 55, and 56). Second-order (parasitic) structures observed on shallow (~20°– Sample 06KG15 was collected from the southern terminus of Heavens Gate 30°) east-northeast–dipping limbs support west-southwest tectonic transport Ridge, ~1 km east of Windy Saddle in the northeastern Seven Devils Mountains (Fig. 12C), consistent with structures in accreted island-arc assemblages to the (Figs. 2A, 3A, 4A, 4B, 5A, and 5B). Medium-grained (≤5 mm) biotite-bearing west (Seven Devils Group [Figs. 5B, 5C, 5E, and 7B], Riggins Group [Figs. 8A, hornblende diorite was sampled for 238U/206Pb zircon analysis (Figs. 5F and 5G). 8B, 9A, and 9B], and Pollock Mountain Amphibolite [Figs. 10A, 10B, and 10D]). The majority of grains recovered are subhedral to anhedral with rounded edges, Mineral and stretching lineations (L1) are variably developed on S1, range from resulting in ellipsoidal geometries; prismatic and acicular crystal habits are less downdip to ~45° obliquely plunging (Figs. 2A, 3B, and 12D, outcrops 49–54; common. Most lack growth zoning and xenocrystic core-rim textures; however,

Blake et al., 2016), and consist of aligned muscovite + biotite ± sillimanite (Lm-1 many grains display possible remnant sector zoning in CL (Fig. 13A). LA-ICP-MS of Blake et al., 2009). analysis yielded U-Pb dates ranging from ca. 150 to 127 Ma, and a few older

Granitic orthogneiss units east of the Sri 0.706 isopleth (undated Partridge dates between ca. 677 and 292 Ma. CA-ID-TIMS analysis of five zircon grains Creek Gneiss; Blake, 1991) include tightly folded country rock screens of possessing rounded, prismatic, and acicular habits yielded a weighted mean western Laurentia (Kelly Mountain Schist). Upright structures show steeply U-Pb date of 135.84 ± 0.07 Ma and is interpreted here as a magmatic crystallization east-dipping axial surfaces (e.g., figure 9b of Gray et al., 2012). Across eastern age (Figs. 13B and 13C; Table 2). Early Cretaceous zircon crystallization in the portions of the Crevice pluton, east-northeast–dipping (≤45°) mylonitic shear Heavens Gate area overlapped with syntectonic emplacement of ca. 145–120 Ma zones (S2) offset S1 in the Kelly Mountain Schist (Fig. 12E). Local truncation calc-alkaline plutons in northeastern Oregon, e.g., Wallowa and Bald Mountain relations are reminiscent of polyphase structures in the Riggins Group (e.g., batholiths, Cornucopia stock (Johnson et al., 1997; Žák et al., 2012, 2015). Given Lightning Creek Schist), where shallow east-dipping brittle shears cut sub- its age, location, and bulk chemical composition, sample 06KG15 may represent vertical synmetamorphic fabric (Fig. 9A, inset). Ductile deformation is tracked a small satellite body associated with weakly metamorphosed (greenschist) eastward through continental metasedimentary rocks (outcrops 54–58) into and deformed (local magmatic fabric) hornblende-bearing tonalite of the Fish border zone plutons of the Idaho batholith (outcrops 59, 60, and 61; Figs. 1A, 2A, Hatchery stock (130.0 ± 1.9 Ma; 238U/206Pb locality AM-05 of Mann, 2018; Aliberti, and 3B; Taubeneck, 1971; Manduca et al., 1993; Gaschnig et al., 2010). Across 1988; Snee et al., 1995; Gray and Isakson, 2016). Both intrusions are hosted by the lower French Creek drainage, moderately developed north-northwest–dip- volcanogenic rocks of the Wild Sheep Creek Formation (Sarewitz, 1983; Schmidt ping (~50°–60°) gneissic fabrics are observed (outcrops 62–65); aligned biotite et al., 2016b) and are proximal to east-northeast–dipping imbricate faults (U-Pb mats, elongate mafic masses (schlieren), and leucosomal layers combine to locality 06KG15, upper plate of Heavens Gate fault [Fig. 7B], and U-Pb locality form migmatitic textures previously unrecognized along our transect. AM-05, footwall of Morrison Ridge–Rapid River fault system [Fig. 2A]). At this Approximately 2 km east of French Creek, a strong north-south–striking latitude (~45°21′00″N), locality 06KG15 provides the westernmost zircon data solid-state fabric is observed in coarsely crystalline granitic orthogneiss (U-Pb from intrusive rocks assigned to the SRSZ. locality FC-05, outcrop 67; Fig. 12G). Gneissic fabric is marginal to the French Creek migmatite zone, shares attitudes with S1 in the southeastern Crevice

pluton/Kelly Mountain Schist, and resides >10 km east of the Sri 0.706 isopleth. Sample SHd-04 Upriver (~0.5 km), aligned mica flakes mark the trace of an east-dipping (~65°) spaced foliation in medium-grained biotite tonalite (outcrop 69). Compared Shrunken Head dike rises above the Salmon River ~4 km west of the

with locality FC-05, rocks record lower strain and resemble mildly deformed Sri 0.706 isopleth (Gus Creek confluence; Figs. 3B, 10A, and 10B). Fine- to

Figure 10. Contractional structures in the Pollock Mountain Amphibolite. (A) Contact relations between island arc-volcanogenic rocks and Shrunken Head dike. Zircon symbol marks U-Pb locality SHd-04: 44°24.760′N, 116°11.204′W; outcrop 38 (Fig. 3B; Table 1). Note fanning spaced cleavage at base of exposure; structures are axial-planar to northwest-vergent mesoscopic folds in S1. (B) Overturned fold in amphibolite, cut by spaced cleavage and dike; notebook is oriented parallel to S1. (C) Dike fabric; coin for scale. (D) Microstructures in Shrunken Head host rock. Sublinear inclusion trail patterns (si: e.g., plagioclase, quartz, rutile) oblique to matrix fabric (se: S1 trace) indicate top-to-the-northwest rotation (si and se elements of Sander, 1930). Thin section photomicrograph orients parallel to L1 and perpendicular to S1; crossed polarized light. Sample collected from GSA Stop 2.7 of Blake et al. (2009), west of locality SHd-04. (E) Vertical S-fold in felsic stringer suggests right-lateral shear; ~25 m east of locality SHd-04. T—toward observer, A—away from observer. Combined top-to-the-northwest and dextral shear kinematic indicators (shown in parts C/D and E, respectively) support local transpressional deformation, consistent with mesoscopic structures ~30 km west (Figs. 4B, 4G, and 5C; Wild Sheep Creek Formation).

Sr 0.706

WESTERN BLUE MTNS. LAURENTIA PROVINCE Kelly Van Mountain Ridge Schist Gneiss

L1 S1 Van Ridge NE ~1 km Gneiss

NE Y SW Z

Figure 11. Contractional structures in the Van Ridge Gneiss, outcrop 48 (Fig. 3B; Table 1). (A) Easternmost exposures of the Blue Mountains Province, showing approximate

location of Sri = 0.706 contour. (B) Downdip stretching lineation (L1) on S1. (C) Lineation-normal face/y-z strain plane reveals tight-to-isoclinal folds in granitic pegmatite of the Salmon River intrusive suite (F2 structures of Blake et al., 2009). From left to right, long- to short-limb relations define S, Z, and M shapes; subvertical fold hinges orient parallel to L1. Epidote-bearing tonalitic orthogneiss (undated pegmatite host rock) possibly belongs to the eastern Hazard Creek complex (e.g., ca. 114 Ma locality K92–8 of Unruh et al., 2008; ca. 112 Ma locality AM-4 of Mann, 2018, see Fig. 2A) and/or the western Little Goose Creek complex (e.g., ca. 110 Ma locality C of Kauffman et al., 2014, Fig. 2A).

medium-grained (≤5 mm) biotite tonalite was collected for 238U/206Pb zircon injection/zircon crystallization (ca. 91 Ma) overlapped with silicic magmatism analysis (Fig. 10C). Most grains are prismatic; however, crystal habits range in the Looking Glass pluton (91.7 ± 2.4 Ma; 238U/206Pb locality LGp03 of Gray et from acicular to equant. Internal structures vary from simple/unzoned to al., 2012), Payette River tonalite (91.5 ± 1.1 Ma; 238U/206Pb locality 01–53; Giorgis showing well-developed growth rings; some grains possess inherited cores et al., 2008; see Fig. 2A), and other syntectonic plutons in west-central Idaho with remnant and resorbed sector zoning (Fig. 13A). LA-ICP-MS analysis (e.g., 90 ± 5 Ma; 238U/206Pb locality 83z11 of Manduca et al., 1993; Idaho batholith yielded the following U-Pb zircon dates: ca. 94–83 Ma, ca. 120–97 Ma, and border zone suite of Gaschnig et al., 2010). To our knowledge, all previously ca. 390–140 Ma. The youngest and most consistent population produced published dates within this age range (U-Pb zircon: 90 ± 5 Ma) were derived

a weighted mean date of 90.1 ± 1.2 Ma. We interpret older populations as from calc-alkaline intrusive rocks located east of the Sri 0.706 isopleth (Fig. 2A). xenocrysts incorporated from enclosing island-arc–supracrustal rocks, i.e., late Paleozoic to mid-Mesozoic volcanic cover and/or associated hypabyssal intrusions (Vallier, 1977; Schmidt et al., 2016b). CA-ID-TIMS analysis on six Sample FC-05 zircon grains showing unzoned to well-developed growth rings yielded a weighted mean U-Pb date of 90.62 ± 0.03 Ma, which is interpreted here as a Sample locality FC-05 resides in the western gneissic border zone of

magmatic crystallization age (Figs. 13B and 13C; Table 2). Late Cretaceous dike the Idaho batholith (Taubeneck, 1971), ~10 km east of the Sri 0.706 isopleth

Kelly Mountain Schist S1

S1 SW

HINGE SE SW

E Crevice S1 G pluton

S1 FC-05 ~86-93 Ma

H S2 Kelly Mountain Schist

Idaho batholith W SE border zone Idaho batholith

Figure 12. Contractional structures east of Sri 0.706 isopleth. (A) Calc-silicate metasedimentary rocks (Kelly Mountain Schist; western Laurentia) near the arc-continent boundary; outcrop 49 (Fig. 3B; Table 1). Dashed line lies along trace of S1. (B) Southeast-plunging isoclinal fold in the Kelly Mountain Schist, east of Crevice pluton; outcrop 55; note minor fold in upper right. Notebook for scale. (C) Folded quartzite boudinage; outcrop 55. (D) Stretching lineation (L1; ~45° rake); outcrop 54. Note flattened garnet porphyroblasts on S1. Southwest-vergent folds (shown in C) combined with oblique L1 stretching lineations suggest local transpressional deformation, consistent with structures in accreted arc assemblages to the west (Figs. 4B, 4G, 5C, 5E, 9, and 10). (E) Mesoscopic shear zone in the Crevice pluton (S2 structures of Gray et al., 2012); note offset country rock screen. Hammer on shear plane for scale. (F) U-Pb sample locality FC-05, ~7 km east of Crevice pluton; outcrop 67, 45°25.617′N, 116°00.020′W. (G) Gneissic fabric of locality FC-05; dashed line along trace of S1. Coin in lower right for scale. (H) Undeformed Idaho batholith; outcrop 70. Coin in upper left for scale.

data‑point error ellipses are 2σ 206 238

Pb/ random tracer λ

238 136.4 ean 135.84 0.07 0.10 0.17 677 a

Pb/ td by data‑pt errs only, 0 of 5 re.

0.02136 206 SD 0.33, probability 0.86 z3 136.2 136.3

134 a 0.02132 136 136.1 z4 z2 z5 z1 135.8 135.9 Date (Ma) 136 a 0.02128

135.6 135.7 06KG15 207Pb/235 0.02124 06KG15 06KG15 bo heights are 2σ 0.138 0.140 0.142 0.144 0.146 135.5

data‑point error ellipses are 2σ 206Pb/238 random tracer λ ean 90.62 0.03 0.05 0.11 90.8 90.76 td by data‑pt errs only, 0 of 7 re. 0.01418 238 SD 0.92, probability 0.48

Pb/ 90.72 102 a 238 a 206 90.7 90.68 90.64 0.01416 93 a 90.60 90.6 z6b 88 a 90.56 Date (Ma) z2 0.01414 90.52 z6a 92 a 90.5 88 a 90.48 z3 z1 90.44 z5 207 235 z4 SHd‑04 90.4 Pb/ SHd‑04 bo heights are 2σ SHd‑04 0.01412 90.40 0.089 0.091 0.093 0.095 CA-ID-TIMS analyses

data‑point error ellipses are 2 0.0150 σ

Figure 13. U-Pb zircon geochronology for samples

238 06KG15, SHd-04, and FC-05. (A) Representative 94 139 a 86 a scanning electron microscope–cathodolumines- 94 a 0.0146 Pb/ cence (SEM-CL) images; circles indicate 25 μm laser 206 140 a 92 spots with corresponding 206Pb/238U laser ablation– 147 a inductively coupled plasma–mass spectrometry 0.0142 (LA-ICP-MS) dates. (B) Wetherill concordia diagrams 90 of chemical abrasion–isotope dilution–thermal 91 a 139 a ionization mass spectrometry (CA-ID-TIMS) anal- 0.0138 88 yses; filled error ellipses represent analyses used in weighted mean calculation. (C) Weighted mean 155 a 206Pb/238U CA-ID-TIMS dates. Errors are given as 128 a 86 0.0134 ±x/y/z, where x represents internal error based FC‑05 92 a on analytical uncertainties only, y includes tracer 93 a calibration, and includes 238U decay constant as 84 z FC‑05 207Pb/235 outlined in supplemental text (see text footnote 1). 0.0130 MSWD—mean square of weighted deviation. 0.086 0.088 0.090 0.092 0.094 0.096 0.098

TABLE 2. ‑Pb IRCON SARY Sample Lithology Location Age a

Latitude °N Longitude ° LA‑ICP‑S 2σ CA‑ID‑TIS 2σ 06KG15 Hornblende diorite 45°20.535′ 116°29.333′ Ca. 150–127 2.2 135.84 0.07 SHd‑04 Biotite tonalite dike 45°25.617′ 116°00.020′ 90.1 1.2 90.62 0.03 FC‑05 Biotite tonalite 45°24.760′ 116°11.204′ Ca. 97–85 Ca. 93–86 ‑Th‑Pb isotopic data are provided in Data Repository files see tet footnote 1. LA‑ICP‑Slaser ablation–inductively coupled plasma–mass spectrometry; CA‑ID‑TISchemical abrasion–isotope dilution–thermal ionization mass spectrometry.

(southern banks of Salmon River; Figs. 2A, 3B, and 12F). Medium to coarsely Amphibolite-facies metamorphism/ductile deformation across the central SRSZ crystalline biotite tonalite (Fig. 12G) was collected for 238U/206Pb zircon analysis. (e.g., locality JV-003) is also compatible with syntectonic magmatism reported Crystal habits range from prismatic to subequant, and internal structures are along strike to the north (ca. 120–110 Ma tonalite-trondhjemite plutonic suite variable. Most grains show convoluted, embayed, and/or rounded xenocrystic of Lee, 2004; McClelland and Oldow, 2007; Snee et al., 2007; 115.8 ± 1.3 Ma cores or are CL dark with bright overgrowth textures; others exhibit well- 238U/206Pb zircon locality 06RL401 of Schmidt et al., 2016a) and to the south (118 developed growth zoning (Fig. 13A). LA-ICP-MS zircon analysis yielded ± 5 Ma 238U/206Pb locality 83z9 of Manduca et al., 1993; 114.4 ± 2.2 Ma locality distinct populations of ca. 97–85 Ma (equant/CL-dark) and ca. 165–123 Ma. K92–8 of Unruh et al., 2008; 112.2 ± 1.6 Ma locality AM-4 of Mann, 2018; Fig. 2A). CA-ID-TIMS analysis of seven grains selected from our younger population yielded concordant results between ca. 93 and 86 Ma (Figs. 13B and 13C; Table 2), with no resolvable age of zircon crystallization. If dates reflect magmatic ■■ DISCUSSION crystallization, then FC-05 overlaps in age with SHd-04, situated ~15 km west— in addition to LGp03, 01–53, and 83z11—and also belongs to the border zone Tectonism over Space and Time suite (e.g., Gaschnig et al., 2010). At this latitude (~45°25′00″N), locality FC-05 provides the easternmost zircon data from intrusive rocks assigned to the SRSZ. In this section, we discuss late Mesozoic tectonic activity in west-central Idaho (post-150 Ma, Table 3), which followed ca. 170 Ma terrane accretion in the central Klamath Mountains (Rattlesnake Creek suture belt: Irwin and Lu-Hf Garnet Analysis Wooden, 1999; figures 1 and 3 of Dickinson, 2008; Fig. 1A) and offshore(?) amalgamation of the Blue Mountains Province (ca. 160–155 Ma; Schwartz et al., Sample JV-003 2011). The discussion proceeds in a time-sliced fashion, with emphasis placed on mesoscopic structural, coeval metamorphic, and intermittent magmatic Sample JV-003 was collected from siliceous metavolcanic rocks (lowermost activity east of the Heavens Gate fault. Our tectonic model linking the SRSZ Riggins Group; Hamilton, 1963a) exposed along the Salmon River ~6 km west and SFTB orogens is introduced (Fig. 14).

of the Sri 0.706 isopleth (Lake Creek bridge area; Figs. 2A, 3B, 9A, and 9B). Medium-grained garnet porphyroblasts are subhedral to anhedral in form, surrounded by augen-shaped clusters of quartz ± feldspar (Ca-diffusion halos), Ca. 144–130 Ma and elongated subparallel to stretching in the downdip direction (Figs. 3B and 9C). Linear regression using three garnet fractions and two whole-rock Along central portions of the Cordilleran margin (~40°N–48°N: northern analyses yielded a date of 119.8 ± 6.7 Ma (Table 3). Regression including all Sierra Nevada to northwest Washington; Fig. 1A), latest Jurassic–middle Cre- garnet fractions yielded an overlapping date of 117.9 ± 7.0 Ma, with a higher taceous high-angle convergence (ca. 150–125 Ma; Figs. 14A and 14B) resulted mean square of weighted deviation (MSWD). Linear regressions result in part in underthrusting of oceanic lithosphere beneath western North America from the spread of points (176Lu/177Hf >8) combined with small uncertainties, but (Engebretson et al., 1985; Burchfiel and Davis, 1975). Over this interval, east- may also represent complexities in the ages. Early Cretaceous crystallization west contractional strains likely accumulated along low-angle imbricate faults falls within analytical error of 121.8 ± 3.2 Ma garnet growth in the upper dipping eastward into the continental margin. Progressive thrust stacking Slate Creek drainage (hornblende gneiss; 176Lu/177Hf locality DW-02 of Wilford, and tectonic burial of underplated arc-volcanogenic rocks—Blue Mountains 2012; Fig. 2A) and 124.3 ± 5.8 Ma growth in lower Squaw Creek drainage Province—conceivably setup the pressure-temperature-depth conditions nec- (biotite schist; 147Sm/144Nd locality 26 of McKay et al., 2017; Figs. 2A and 3B). essary for achieving upper-amphibolite-facies metamorphism (~7–11 kbar,

TABLE 3. Lu‑Hf GARNET GEOCHRONOLOGY: JV‑003 45°24.031′N, 116°13.022′ Sample 176Lu/177Hf 176Hf/177Hf Lu Hf ppm ppm 14JV003GA 8.723 0.044 0.301100 0.000015 9.00 0.147 14JV003GB 12.24 0.06 0.310459 0.000006 9.72 0.113 14JV003GC 10.87 0.05 0.308106 0.000016 9.40 0.123 14JV003GD 12.08 0.06 0.309592 0.000007 9.52 0.112 14JV003RA 0.0279 0.0001 0.283067 0.000007 0.734 3.735 14JV003RB 0.0310 0.0002 0.283211 0.000006 0.763 3.492 Note: Lu and Hf concentrations were determined by isotope dilution with uncertainties estimated to be better than 0.5. The 176Lu/177Hf uncertainties on regressions and age calculations were estimated at 0.5. 176Hf/177Hf ratios were corrected for instrumental mass bias using 179Hf/177Hf 0.7935 and normalized relative to 176Hf/177Hf 0.282160 for JC‑475 Vervoort and Blichert‑Toft, 1999. Ages were calculated using the 176Lu decay constant value of Sderlund et al. 2004. Errors reported for 176Hf/177Hf represent within‑ run uncertainty epressed as 2σ, standard error. Total uncertainty on individual 176Hf/177Hf measurements was estimated at 0.005. These uncertainties were added to within‑run uncertainties in uadrature for regressions and age calculations.

~550–675 °C, ~15–20 km; e.g., Zen, 1985; Selverstone et al., 1992). In this sce- in the northern Seven Devils/Heavens Gate area (eastern Wallowa terrane) nario, loading of buoyant oceanic crust (mature arc of Hamilton, 1988; Cloos, overlapped with calc-alkaline magmatism and coeval dextral-transpressional 1993; Stancin et al., 2016) was possibly associated with structurally overriding deformation in northeast Oregon, e.g., ca. 140 Ma emplacement of the Pole (undocumented thrust?) Neoproterozoic–Paleozoic passive-margin strata. Bridge and associated syntectonic plutons (Wallowa batholith; Fig. 1A; Vallier, Westward-vergent deformation across the incipient Salmon River suture was 1995; Johnson et al., 1997, 2011; Žák et al., 2015). coeval with intracontinental contraction in the Sevier hinterland (e.g., Albion Mountains, ≤250 km east), including ca. 140 Ma thrust-loading of stratal assemblages above and west of the Basin-Elba fault (Miller, 1983; Kelly et al., 2015). Ca. 130–112 Ma In the Riggins region of west-central Idaho, midcrustal metamorphic conditions are represented by ca. 144–135 Ma core garnet growth in the Pollock Sustained high-angle plate convergence (Fig. 14B; ~41°N–44°N), crustal

Mountain Amphibolite, i.e., structurally highest thrust sheet (locality 422 of thickening, and uplift/cooling west of the Sri 0.706 isopleth are recorded by Getty et al., 1993; ID03b/23 of McKay et al., 2017). Along our transect, high- synkinematic mineral growth in the Pollock Mountain Amphibolite (Fig. 2; strain synmetamorphic deformation of probable Early Cretaceous age is ca. 128 Ma/119 Ma garnet/hornblende from sample locality 598 of Selverstone recorded along southern Heavens Gate Ridge (Fig. 3A). As described, ca. 136 et al., 1992, Getty et al., 1993; ca. 124 Ma garnet ID23 of McKay et al., 2017) Ma intrusive rocks (locality 06KG15; Figs. 5F and 5G) were deformed together and structurally underlying rocks of the Riggins Group (Fig. 2; ca. 124 Ma/ with volcanic arc cover of the Wild Sheep Creek Formation (Figs. 5B, 5C, and 112 Ma garnet ID26/48 of McKay et al., 2017; ca. 120 Ma garnet JV-003 of this 6C–6F). Based on regional mapping and U-Pb age data (northern Heavens study; ca. 117 Ma hornblende R7/30 of Snee et al., 1995). In each case, meta- Gate Ridge–Lucile area; Kauffman et al., 2014; Quarcoo and Gray, 2016), we morphism was accompanied by pervasive flattening deformation (Figs. 8D interpret low-volume silicic magmatism (06KG15) as late syntectonic to local and 9C), linear-planar fabric development (outcrops 14–39; Table 1; Fig. 3B), fabric development (Figs. 5C–5E) and possibly related to ca. 130 Ma imbricate and/or top-to-the-west porphyroblast rotation (Fig. 10D; see figure 15b of Gray thrust stacking/anatectic melting across western portions of the SRSZ (Figs. 2A, et al., 2012; figure 4d of McKay et al., 2017). Dynamic recrystallization persisted 7A, and 7B; figures 19, 20A, and 20B of Schmidt et al., 2016b). Tectonic activity through late Early Cretaceous time, as volcanogenic and overlying carbonate

platform assemblages of the eastern Wallowa terrane—Riggins/Seven Devils magmatism/contractional deformation in the Keating Ridge Gneiss, Spring Groups, Martin Bridge Formation—were progressively overridden by major Creek Gneiss, and/or Van Ridge Gneiss (Fig. 3B; units of Blake, 1991) tem- westward-advancing thrust sheets: ca. 144–124 Ma thrusting of continental porally overlapped with thrust-related garnet growth in the Lightning Creek margin strata onto the Pollock Mountain Amphibolite (cryptic miogeoclinal Schist (ID48; McKay et al., 2017). In this context, dextral transpression west

structure proposed here), followed by ca. 124–112 Ma thrusting of the Pollock of the Sri 0.706 isopleth was expressed by horizontal shortening in island-arc Mountain Amphibolite over the Riggins Group (Hamilton, 1969a; Aliberti, 1988; crust (westward-vergent folding, thrusting, fabric development; Figs. 7, 8, 9, Selverstone et al., 1992; McKay et al., 2017; Figs. 14B and 14C). and 10A–10D) and vertical lengthening/extrusion of adjacent intrusive rocks

Syntectonic mineral growth overlapped with emplacement of magmatic (eastern Van Ridge Gneiss; Fig. 11). East of the Sri 0.706 isopleth, right-oblique epidote-bearing tonalitic plutons east of the Pollock Mountain Amphibolite convergence was accompanied by ca. 105–90 Ma extrusion (localities Cp02, (Round Valley pluton of Zen, 1985; ca. 118–112 Ma Hazard Creek complex of LGp03, FC-05 granitoids; Figs. 2A, 3B, and 12G) and westward-vergent folding, Manduca et al., 1993; Unruh et al., 2008 Schmidt et al., 2016a; Mann, 2018). and tectonite fabric development (western Laurentia: Kelly Mountain Schist + High-pressure granitic magmatism was coeval with east-west shortening correlative units; Figs. 12A–12D; Blake, 1991; Lund and Snee, 1988). across the arc-continent boundary, as evidenced by contractional structures As described, Laurentian metasedimentary rocks were deformed together at localities AM-6 (ca. 130 Ma), JV-003 (ca. 120 Ma), 83z9 (ca. 118 Ma), ID58 with Cretaceous granitoids east of the arc-continent boundary (orthogneiss (ca. 116 Ma), K92–8 (ca. 114 Ma), AM-4 (ca. 112 Ma), and 83z14 (ca. 110–105 screens; Fig. 12E). Local parallelism of solid-state fabric (S1: southeastern Ma). Structures are broadly time-transgressive and track ductile deformation Crevice pluton/Kelly Mountain Schist; cross-section D–D′ in Fig. 3B) supports through the Riggins Group, Pollock Mountain Amphibolite, and suture zone syn– to post–104 Ma penetrative deformation (Gray et al., 2012; West Mountain plutonic suite (Fig. 2A). By 112 Ma, high-angle ocean-continent (Farallon–North locality 10NB376 of Braudy et al., 2017). Timing constraints are compatible America) convergence had transitioned into right-oblique plate motion (Fig. with ca. 111–105 Ma structures documented north and south of our transect 14C), thus adding a strike-slip component of deformation to the Cordilleran (e.g., locality C of Kauffman et al., 2014; 83z14/99MG of Manduca et al., 1993; margin, i.e., 120–90 Ma interval of McClelland et al. (2000). As a consequence, Giorgis et al., 2008; Fig. 2A). Age-equivalent calc-alkaline magmatism and boundary-normal shortening/crustal thickening (Pollock Mountain, Rapid east-west contractional deformation in the Partridge Creek Gneiss/Little Goose River, Ahsahka thrust systems; Fig. 10A) and associated high-pressure silicic Creek complex (Blake et al., 2009, 2016) would extend ca. 111–104 Ma structures magmatism (Hazard Creek complex + equivalent units; e.g., Manduca et al., westward into the Van Ridge Gneiss/Hazard Creek complex (Figs. 2A and 1993) were accompanied by right-lateral transcurrent shearing (Figs. 4B, 4G, 11), i.e., easternmost plutons of the Blue Mountains Province. Subsequent and 10E) and margin-parallel northward translation of the Blue Mountains block magmatism and deformation within the Idaho batholith border zone— (Lund and Snee, 1988; Wyld and Wright, 2001; Giorgis et al., 2005). syntectonic emplacement ca. 93–85 Ma—are recorded by the Looking Glass pluton (Fig. 3B) and U-Pb sample localities of this study (Figs. 10A and 12F). Based on the available age constraints and unit correlations reported from Ca. 112–90 Ma the Salmon River Canyon, we propose that LS tectonite fabric (strain) accu-

mulated ca. 115–86 Ma east of the arc-continent boundary (Sri 0.706 isopleth). Over this interval, dextral-oblique consumption of the Farallon plate (~45° Otherwise stated, ductile deformation persisted between the onset of Partridge convergence angle: Giorgis and Tikoff, 2004; Fig. 14D) continued producing Creek Gneiss/Little Goose Creek complex magmatism (ca. 115 Ma; upper age magma of the suture zone suite (ca. 112 Ma: locality AM-4 of Mann, 2018), and bound of Manduca et al., 1993; 110–120 Ma tectonism of McClelland et al., 2000; was possibly associated with early construction of the Idaho batholith (ca. Lee, 2004; Snee et al., 2007; Kauffman et al., 2014) and the end of intrusive 100 Ma metaluminous phase; Gaschnig et al., 2010). Mid-Cretaceous tectonic activity recorded by locality FC-05 (ca. 86 Ma: lower age-bound of present

activity included emplacement/deformation of the Crevice and age-equivalent study; Fig. 13). This interval overlapped with tectonism west of the Sri 0.706 plutons (ca. 110–104 Ma; locality Cp02 of Gray et al., 2012; locality C of Kauffman isopleth, as supported by ca. 112 Ma synkinematic garnet growth (Riggins et al., 2014), Looking Glass pluton/Payette River tonalite (ca. 93–85 Ma; e.g., Group locality ID48; McKay et al., 2017) and ca. 118–100 Ma magmatism in the LGp03 of Gray et al., 2012), and smaller calc-alkaline bodies dated herein (ca. 91 Hazard Creek–Little Goose Creek complexes, i.e., age-equivalent portions of Ma: SHd-04; ca. 93–86 Ma: FC-05; Figs. 10A–10C, 12F, 12G, and 13C). Pervasive the Keating Ridge Gneiss, Spring Creek Gneiss, and/or Van Ridge Gneiss (unit ductile deformation recorded in easternmost exposures of the Riggins Group correlations of Blake et al., 2009, 2016). West of locality ID48, late Early Cre- (ca. 112 Ma; locality ID48 of McKay et al., 2017) may relate to westward-directed taceous shortening was accommodated by east-dipping imbricate structures movement along the Pollock Mountain thrust and/or syntectonic magmatism of the Rapid River thrust–Morrison Ridge fault system (small-scale duplexing in the western Hazard Creek complex (ca. 114–112 Ma; AM-4 of Mann, 2018; in Schmidt et al., 2016b; Shingle Creek structures of Onasch, 1977), which K92–8 of Unruh et al., 2008). If previous whole-rock geochemical correlations crosscut ca. 113–111 Ma garnet-bearing metamorphic tectonites (Wilford, 2012; are correct (Manduca et al., 1993; Blake et al., 2009), then ca. 114–112 Ma silicic McKay et al., 2017; see truncated isograd of Hamilton, 1963b, 1969a; figure 2

Figure 14. Late Mesozoic convergence along the North American margin. Velocity vectors show oceanic plate motions relative to fixed locations along the continental margin. Note progressive change in convergence angle from ~90° (ca. 150–125 Ma) through 60° dextral-oblique (ca. 100 Ma). Abbreviations: IB—Idaho batholith; CHB—Coon Hollow basin; FP—Farallon plate; I-W-U—Idaho-Wyoming-Utah salient; LCL—Lewis and Clark Line; NAP—North American plate; SRP—Snake River Plain; SRSZ—Salmon River suture zone. Specu- lative model depiction of terrane accretion, northward translation, and clockwise vertical-axis rotation: (A) Ca. 150 Ma: impact of Blue Mountains Province; arc-continent collision (terrane accretion) is dominated by east-west compression (Selverstone et al., 1992). Northernmost segment of the western Nevada shear zone accommodates collision (strain ac- cumulation/stress transmittal). Note: Contractional structures are partially obscured by the western Snake River Plain. (B) Ca. 125 Ma: continued accretion of Blue Mountains block; margin-parallel northward translation, clockwise rotation, and dextral transpression under way (cf. Tikoff and Teyssier, 1994). Note: Structures partially obscured by Snake River Plain/ southern Idaho batholith. (C) Ca. 112 Ma: continued accretion, translation, and rotation (pre-Eocene; Wilson and Cox, 1980). Change from frontal- to oblique-collision: compressional to strike-slip tectonism. Entrance of Blue Mountains into Syringa embayment (Schmidt et al., 2016a). Note: Transpressional structures obscured by northern Idaho batholith. (D) Ca. 100 Ma: arc–continent suturing in Syringa embayment; ~30° clockwise rotation (post-90 Ma; Lewis et al., 2014). Early metaluminous intrusive activity in Idaho batholith (Clarke, 1990; Gaschnig et al., 2010). Modified from Engebretson et al. (1985). We acknowledge conflicting kinematic models—sinistral- versus dextral-oblique—and uncertainties associated with ca. 150–100 Ma plate interactions along the North American margin. However, our incorporation of Engebretson et al. (1985) showing orthogonal to dextral-oblique convergence is compatible with dextral-transpressional strains recorded in the Wallowa terrane of northeast Oregon (Žák et al., 2015) and granitic rocks of west-central Idaho (Giorgis and Tikoff, 2004).

of Gray et al., 2012) and ca. 118–107 Ma hornblende-defined fabric in the upper older arc basins in the western U.S. Cordillera that do record proximity to—and Riggins Group (Squaw Creek Schist, Berg Creek Amphibolite; Figs. 2A, 3B). host sediment from—the Laurentian continental margin (e.g., northern Sierra Nevada and Klamath Mountains), readers are referred to LaMaskin (2012). In our view, the Coon Hollow basin (Pittsburg Landing area) contains no Origin of LS Tectonites direct evidence of sediment sourced outside of the Wallowa terrane (mud rock

eNd = 1.9–5.0; LaMaskin et al., 2015). The absence of a clear North American Given the orogen-scale continuity of LS tectonite fabric (~25–40 km across source (i.e., detrital zircon population) is difficult to reconcile with Coon Hol- strike; Fig. 2A; Table 1; cf. continuity concept of Sander, 1930), intensity of low sedimentation subsequent to terrane accretion (versus a precollisional ductile deformation (Figs. 5C, 5D, 5G, 6D, 8C, 9C, 10A, 11B, 12B, and 12G), and marine setting). In a postcollisional scenario, basin fill would include mixed involvement of island-arc–intrusive rock–continental margin assemblages detritus shed from the collisional orogen (~25–40-km-wide SRSZ) and break- along our transect, we attribute the formation of ca. 136–91 Ma synmetamor- down of shielding topographic barriers, i.e., clastic material sourced from the phic structures (Table 4) to arc-continent collision and associated evolution of island-arc assemblage and ancestral North America (e.g., Boghossian et al.,

the dextral-transpressional Salmon River suture. Although the age and extent 1996). Given this basin’s proximity to the Sri 0.706 isopleth (Laurentia) and of deformation differ (see chronology of Snee et al., 1995; map compilation of the Idaho batholith (~35 km east; Fig. 2A), an influx of terrigenous sediment Lund, 2004), our interpretation of fabric (origin) is consistent with Lund and would be expected in response to arc accretion, midcrustal exhumation, and Snee (1988) and others supporting post–160 Ma terrane accretion in west-cen- unroofing of proximal continental margin strata (Lund et al., 2003; Giorgis et tral Idaho. More recent workers suggest otherwise. al., 2008; Gaschnig et al., 2010). Amato et al. (2013) presented a similar argu- ment for terrane accretion in southern Alaska. Detrital zircon dating in the Mesozoic Chugach accretionary complex (e.g., Berg et al., 1972) suggested Sevier Orogeny that Wrangellia was isolated from Laurentian sources until middle Late Cre- taceous time (ca. 89–85 Ma). According to Amato et al. (2013), the lack of In the model of LaMaskin et al. (2015), Early Cretaceous (ca. 144–128 Ma) continentally derived sediment in the oldest accretionary unit (ca. 164 Ma upper-amphibolite-facies metamorphism, thrust-related crustal thickening, Potter Creek flysch assemblage; “mesomélange” of Amato and Pavlis, 2010) and pervasive ductile deformation (Hamilton, 1963a; Onasch, 1977, 1987; Lund is incompatible with a mid-Jurassic collision event (McClelland et al., 1992a) and Snee, 1988; Blake, 1991; Selverstone et al., 1992; Getty et al., 1993; Snee et because detritus shed from that orogen should have been carried into the al., 1995; Gray et al., 2012; McKay et al., 2017) postdates terrane accretion and coeval trench. Unlike the outboard Chugach accretionary complex (shielded relates to noncollisional tectonism of the Sevier orogeny (Armstrong, 1968; by Peninsular-Alexander-Wrangellia terranes), the Coon Hollow basin occupied Burchfiel, 1980). Tectonic activity occurred in response to a through-going an inboard (intra-arc) position and was protected from western Laurentia by Andean-type subduction zone system emerging along the Cordilleran margin. the easternmost Wallowa terrane. In this context, postcollisional sedimenta- The geochemical analysis of Late Jurassic volcanogenic rocks in the Pittsburg tion in west-central Idaho (160–150 Ma interval of LaMaskin et al., 2015) would Landing area (Fig. 2A; ca. 160–150 Ma Coon Hollow Formation) led authors to have transected a narrow (~35 km-wide), north-south tract of pre-Cenozoic conclude that arc-continent collision is not recorded by penetrative structures topography separating the Pittsburg Landing area and ancestral North America. in the Riggins region (outcrops 5–38; Fig. 3). According to LaMaskin et al. (2015), Orogenic components in west-central Idaho are characteristic of major exposed rocks of the Wallowa terrane occupied a fluvial to deep-marine setting arc-arc and arc-continent collision zones in the North American Cordillera during pre–160 Ma terrane accretion. Because the sedimentary assemblages at and other well-studied accretionary orogens (Moores, 1970, 1998; Brown and Pittsburg Landing have not produced detrital zircon of pre–late Paleozoic age Ryan, 2011; Xiao and Santosh, 2014), e.g., Trans-European suture zone/Avalo- (3% of grains older than 260 Ma; LaMaskin et al., 2015) and were sourced from nia composite arc accretion (Pharoah, 1999; Nance et al., 2002), Kohistan arc crystalline basement/volcanic cover rocks of the Wallowa arc (Walker, 1986; obduction/pre-Oligocene collision in northern Pakistan (Bard, 1983; Khan et Vallier, 1995; Schmidt et al., 2016a, 2016b; Kurz et al., 2017), we question terrane al., 2009), and Grampian-Taconic orogeny of western Ireland–New England/ accretion prior to 160 Ma sedimentation (Coon Hollow basin [CHB] of Figs. episodic island-arc–ophiolite accretion (van Staal et al., 2007; Hollis et al., 2A and 14). Furthermore, Middle–Late Triassic rocks are strongly deformed in 2012). As described for these orogens, lithotectonic assemblages in west-cen- northwest-vergent fold nappes and imbricate faults (Klopton Creek fold-thrust tral Idaho record the products of oblique plate convergence culminating with system; White and Vallier, 1994; Kauffman et al., 2014) reminiscent of contrac- post–160 Ma arc-continent collision: thrust-induced regional metamorphism, tional structures across Heavens Gate Ridge (Fig. 3A, section A′–A″; Fig. 7B). intermittent high-pressure silicic magmatism, and pervasive ductile deforma- More importantly, the sedimentary response supporting pre–160 Ma accretion tion. In this context, the Seven Devils–Riggins–Salmon River Canyon transect is simply not recorded by this fluvial to deep-marine depocenter (Morrison, (Figs. 2 and 3; Gray, 2013) provides a basis for comparison with other high- 1963; Vallier, 1977; Goldstrand, 1987; Lewis et al., 2014). For age-equivalent and strain zones involving terrane accretion.

TABLE 4. LATE ESOOIC TECTONIC ACTIVITY: SALON RIVER STRE ONE VERSS SEVIER FOLD‑AND‑THRST BELT Age of deformation Dating methods Lithologic units and structures involved Data source

Salmon River suture zone Post–150 Ma ‑Pb detrital zircon Klopton Creek thrust: crosscuts Coon Hollow Fm.; Pittsburg Landing hite and Vallier 1994; Laaskin et al. 2015 Ca. 145 a ‑Pb magmatic zircon Emplacement/deformation of felsic intrusive rocks; Heavens Gate fault zone Kauffman et al. 2014 Ca. 144 to 124 a Sm‑Nd garnet Pollock ountain Amphibolite: cut by ca. 118 a Hazard Creek comple Getty et al. 1993; anduca et al. 1993; cKay et al. 2017 Syn– to post–136 Ma ‑Pb magmatic zircon etamorphic tectonites: deformed intrusive rocks; Heavens Gate Ridge 06KG15 Gray 2013 Syn– to post–130 Ma ‑Pb magmatic zircon orrison Ridge fault: crosscuts intrusive rocks of Fish Hatchery stock deformed Aliberti 1988; Schmidt et al. 2016b; ann 2018 Ca. 120 to 90 a ‑Pb magmatic zircon Foliated intrusive rocks emplaced along and west of island‑arc–continent boundary cClelland and Oldow 2007 Pre–118 and 109 Ma Ar‑Ar hornblende Lineated amphibole: Suaw Creek/Lightning Creek Schist, Berg Creek Amphibolite Hamilton 1963a; Snee et al. 1995; Gray et al. 2012 Syn–118 Ma ‑Pb magmatic zircon etamorphic tectonites: Hazard Creek Comple, Pollock ountain Amphibolite Aliberti 1988; anduca et al. 1993 Syn– to post–114 Ma ‑Pb magmatic zircon etamorphic tectonites: Hazard Creek Comple, Pollock ountain Amphibolite Aliberti 1988; nruh et al. 2008; ann 2018 Syn– to post–113 Ma Lu‑Hf, Sm‑Nd garnet Flattened/top‑to‑southwest rotated garnet porphyroblasts: Berg Creek Amphibolite cKay et al. 2017 Post–113 Ma ineral isograd Rapid River thrust system: imbricate fault crosscuts garnet isograd Hamilton 1969b; cKay et al. 2017 Syn–/post–111–105 Ma ‑Pb magmatic zircon etamorphic tectonites: Little Goose Creek comple anduca et al. 1993; Giorgis et al. 2008 Syn– to post–105 Ma ‑Pb magmatic zircon etamorphic tectonites, west‑to‑east strain gradient: Crevice pluton Gray et al. 2012; Blake et al. 2016 Ca. 100 a ‑Pb magmatic zircon etamorphic tectonites: Coolwater culmination Lund et al. 2008 Pre–98 Ma to post–90 Ma ‑Pb magmatic zircon etamorphic tectonites across 0.706 isopleth; pre–Chipmunk eadow intrusive rocks Taubeneck 1971; Benford et al. 2010 Syn–93 to 90 Ma ‑Pb magmatic zircon etamorphic tectonites: Payette River tonalite/early Idaho batholith SHd‑04, FC‑05 anduca et al. 1993; Giorgis et al. 2008; Gaschnig et al. 2010 Sevier fold‑and‑thrust belt Ca. 153 to 144 a K‑Ar biotite, hornblende etamorphic tectonites: Newfoundland, central Ruby tns.; N tah, NE Nevada Allmendinger and Jordan 1984; iller and Gans 1989; Hudec 1992 Ca. 146 a Apatite fission track Canyon Range thrust; central tah Stockli et al. 2001 Ca. 145 to 140 a Ar‑Ar muscovite, K‑Ar illite illard thrust; NE tah, eastern Idaho Burtner and Nigrini 1994 Ca. 142 to 102 a Ar‑Ar illite, slate, phyllite Luning‑Fencemaker thrust belt; western Nevada yld et al. 2003 Ca. 139 to 132 a Lu‑Hf garnet Basin‑Elba thrust; Albion tns., south‑central Idaho Kelly et al. 2015 Pre–128 Ma Ar‑Ar hornblende etamorphic tectonites: Ruby tns., East Humboldt Range; NE Nevada Dallmeyer et al. 1986 Ca. 120 a Stratigraphic Pavant thrust; central and S tah DeCelles et al. 1995; itra 1997 Ca. 115 to 110 a Stratigraphic eade thrust: west‑central tah, SE Idaho DeCelles et al. 1993; itra 1997 Ca. 100 a Stratigraphic Lewis and Clark fault system sinistral‑slip: northern Idaho, S ontana allace et al. 1990; Sears et al. 2010 Ca. 95 a Stratigraphic asatch Range faults: north‑central tah Yonkee 1992 Ca. 92 to 90 a Stratigraphic edicine Lodge thrust: SE Idaho, S ontana Schmitt et al. 1995 Ca. 90 to 84 a Fission track, stratigraphic Crawford thrust: north‑central tah, S yoming Burtner and Nigrini 1994; DeCelles 1994 Ca. 88 to 75 a Stratigraphic Tendoy and Sapphire thrusts; S ontana Schmitt et al. 1995 Represented in tectonic model of Figure 14.

Omineca Crystalline Belt Evenchick et al., 2007). Early phases of fan development (ca. 172–163 Ma; Gibson et al., 2005, 2008) are represented by kilometer-scale bivergent con- The Omineca belt of southeastern British Columbia overlaps the lithospheric tractional structures (e.g., Carnes nappe; Brown and Lane, 1988), outcrop-scale boundary separating easternmost allochthonous terranes of the Canadian Cor- tight-to-isoclinal folds, and high-strain transposition foliation defined by peak dillera and the Laurentian continental margin (Monger et al., 1972, 1982, 1994; metamorphic mineral alignment. Early fabric elements (D1 and D2) are over- Archibald et al., 1983; Wheeler and McFeely, 1991). Positioned along strike of printed by lower-strain coaxial folds (D3) constrained to ca. 104–84 Ma (U-Th-Pb the SRSZ (Fig. 1A), the belt is characterized by widespread transpressional zircon/monazite; Gibson et al., 2008). According to Monger et al. (1982), regional deformation, granitic magmatism, and greenschist- to upper-amphibolite-fa- synmetamorphic structures evolved in response to the progressive collision cies metamorphism (Gabrielse and Reesor, 1974; Price, 1981; Avé Lallemant of composite arc terranes with ancestral North America (Crawford et al., 1987; and Oldow, 1988; Carr, 1991, 1992; Parrish, 1995). In the south (~51°N), the Sel- Rubin et al., 1990; Journeay and Friedman, 1993; Chardon et al., 1999; McClel- kirk fan records multiple generations of structures (Brown and Tippett, 1978; land and Mattinson, 2000; Gehrels et al., 1990, 2009; Simony and Carr, 2011).

Coast Plutonic Complex Grampian-Taconic Orogeny

Dextral-transpressional deformation west of the Omineca belt is recorded Western Ireland is possibly the most well-studied region exposing the late along the ~1500 km-long, arc-arc collisional boundary separating the Insular Early–Middle Ordovician Grampian orogeny (Fig. 15; Lambert and McKerrow, and Intermontane composite terranes (Coast plutonic complex of Monger et 1976; Soper et al., 1999; Dewey and Ryan, 2015), which is broadly equivalent al., 1982; Coast Mountains orogen of Gehrels et al., 1990; see also Journeay to the Taconic arc-continent collisional event in eastern North America (Dewey and Friedman, 1993). As described by Crawford et al. (1987), syntectonic mag- and Shackleton, 1984). In the North Mayo tectonic zone, Neoproterozoic strata matism along this east-west compressional regime involved underthrusting of of the Dalradian Supergroup (Condon and Prave, 2000) are interpreted to the Alexander-Wrangellia (Insular) composite belt beneath the Intermontane record ca. 720–595 Ma passive-margin sedimentation related to the rifting/ belt (Stikinia/Yukon-Tanana terranes). West-directed folding/thrusting (Sum- breakup of Rodinia (Strachan and Holdsworth, 2000). To the south, continentally dum-Fanshaw fault system; McClelland et al., 1992b) was accompanied by derived mélange and dismembered ophiolite (Cambrian–Ordovician Clew Bay emplacement of magmatic epidote-bearing plutons at mid- to lower-crustal Complex; e.g., Dewey and Mange, 1999) combine to form the Northwestern levels (e.g., ca. 100 Ma syntectonic Ecstall pluton; Hutchison, 1982; Zen, 1985). composite terrane (e.g., Murphy et al., 1991). High-pressure metasedimentary According to McClelland et al. (2000), metamorphism increases from sub- rocks of the Dalradian and Clew Bay assemblages are separated by a strong greenschist- to upper-amphibolite-facies conditions across the contractional magnetic lineament (Fair Head–Clew Bay Line; e.g., Max et al., 1983) marking belt (west to east). Mesoscopic structures associated with regional-scale thrust the westward continuation of the Highland Boundary fault system (Scotland). faults and folds (nappes) are dominated by east-dipping transposition fabric In both Britain and Ireland, this northeast-striking zone of contractional/strike- observed in arc-supracrustal rocks (Crawford and Hollister, 1982; Stowell and slip deformation overlaps the boundary separating autochthonous Laurentian Crawford, 2000). When taken together, the overall structural style (imbricate margin strata and accreted island-arc rocks of the Grampian orogeny (e.g., thrusts, tight-to-isoclinal folds, linear-planar fabrics), kinematics of contrac- van Staal et al., 1998). Polyphase structural mapping in the Irish Caledonides tional deformation (westward-vergent), metamorphic trends (easterly increase shows that the Dalradian Supergroup and Clew Bay Complex shared a in grade), and deep-seated silicic magmatism (epidote-bearing plutons) are Middle Ordovician tectonic history. According to Chew (2003), contractional remarkably similar to orogenic components of the SRSZ. In both regions, Early deformation is represented by shallow east-plunging isoclinal folds (nappes) Cretaceous (120–90 Ma) calc-alkaline magmatism and prograde metamorphism and tectonite fabric (S1–L1) recording pronounced flattening/stretching record pronounced crustal thickening historically attributed to terrane accretion strains, i.e., triaxially deformed lithic clasts (cf. Heavens Gate; Figs. 3A, 6, 7, (Davis et al., 1978; Monger et al., 1972, 1982, 1994; Zen, 1985; Crawford et al., and 8D). Regionally developed synmetamorphic structures are attributed to 1987; Blake, 1991; Selverstone et al., 1992; Getty et al., 1993; Manduca et al., oblique plate convergence—northwest-directed sinistral transpression—and 1992; Avé Lallemant, 1995; Snee et al., 1995; McClelland and Mattinson, 2000; collision involving ancient volcanic arcs of the Iapetus Ocean (Fig. 15, inset) Stowell and Crawford, 2000; Gray et al., 2012; McKay et al., 2017). and Laurentian continental margin (Harris, 1995; Ryan and Dewey, 2011). The North Cascade Mountain system and magmatic arc of north-central Washington mark the southern continuation of the Coast Mountains orogen, i.e., Insular-Intermontane suture of Whitney and McGroder (1989) and North Implications for Ocean Closure Cascades–southeastern Coast belt of Hurlow (1993), Miller and Paterson (2001), and Gehrels et al. (2009). Late Paleozoic to Cretaceous arc assemblages In their pioneering study, Getty et al. (1993) attributed ca. 144 Ma garnet occupying the Cascades metamorphic core (Fig. 1A; Chelan Mountains, Nason, growth in west-central Idaho (locality 422; Fig. 2A) to outboard amalgamation and Swakane terranes; Tabor et al., 1989; Journeay and Friedman, 1993; Wyld et of the Blue Mountains Province or its onset of suturing to ancestral North al., 2006) were intruded ca. 115–85 Ma synchronous with regional contraction America. McKay et al. (2017) refined this date (ca. 141 Ma; locality ID03a) (e.g., transpressional Pasayten fault zone of Hurlow, 1993; Miller et al., 2016). Over and reported additional Sm-Nd data (ca. 135–112 Ma crystallization) from the this interval, the northern Cascades experienced westward-directed thrusting, Pollock Mountain Amphibolite and Riggins Group (localities ID23 and ID48, dextral strike-slip displacement, and pervasive ductile deformation together with respectively). These workers interpreted garnet growth as recording progres- amphibolite-facies metamorphism (Brandon et al., 1988; Hurlow, 1993; Paterson sive thrust stacking associated with prolonged arc-continent collision (~30 and Miller, 1998). According to Miller and Paterson (2001), the magmatic arc m.y.; McKay et al., 2017). In this context, suturing was under way by ca. 141 and core regions of the orogen reached a crustal thickness of ≥55 km (cf. SRSZ: Ma and followed closure of an intervening oceanic tract (syncollisional flysch >50 km; Zen and Hammarstrom, 1984; Selverstone et al., 1992). Mid-Cretaceous basin of Pavlis, 1982; ca. 155–50 Ma suture of Sigloch and Mihalynuk, 2017) tectonism is interpreted to record major intra-arc shortening and final suturing separating the Wallowa/Baker/Izee/Olds Ferry terranes and western margin of the Insular composite terrane to western Laurentia (Whitney and McGroder, of Laurentia. We interpret long-lived tectonic activity in the SRSZ (ca. 145–90 1989; McGroder, 1991; van der Heyden, 1992; Dickinson, 2004). Ma) as recording orthogonal to right-oblique terrane accretion, margin-parallel

translation, and clockwise rotation of the Blue Mountains block. The onset of region of the Cordilleran orogen (Fig. 1A; e.g., Misch, 1960; Allmendinger et al., suturing followed ca. 160 Ma amalgamation (Schwartz et al., 2010, 2011) and 1984). While the location of initial thrusting is unclear (southern Idaho—Royse coincided with final closing of the basin (minimum age ca. 141 Ma); subse- et al., 1975; Miller, 1980; northeast Nevada—Hodges et al., 1992; Hudec, 1992; quent metamorphism, contractional deformation, and strike-slip displacement west-central Utah—Lawton et al., 1997), compressional deformation propa- were associated with sustained arc-continent collision/convergence between gated eastward with local hindward-directed and out-of-sequence events over the Farallon and North American plates. Syntectonic magmatism dominated late Mesozoic time (Morley, 1988; DeCelles, 1994, 2004; Long et al., 2014). Kine- late-stage, accretion-related activity in the SRSZ (Manduca et al., 1992, 1993; matic reconstructions reported from the Sevier type area in west-central Utah Giorgis et al., 2005, 2008; Figs. 2A, 10A, 10C, 12F, and 12G) and was focused (Canyon and Pavant Ranges; Fig. 1A) indicate east-west shortening exceeded along the thermally weakened arc–continent collisional boundary (Tikoff et 160 km (DeCelles and Coogan, 2006). Displacement estimates are lower in the al., 2001; Giorgis et al., 2006). north (~100–160 km: eastern Idaho–western Wyoming–northern Utah salient Basin closure by ca. 141 Ma suggests that outboard terranes (Insular belt) [e.g., Royse, 1993]; ~50 km: southwest Montana–southeast Idaho segment accreted from south to north between Idaho and Alaska (>45°N; present coor- [e.g., Skipp, 1988]), but exceed 200 km along the northwestern Montana– dinates) over latest Jurassic to Paleocene time (Getty et al., 1993; Ridgway et Alberta, Canada, segment (Price et al., 2000). Here, we discuss the timing of al., 2002; Gehrels et al., 2009). In this scenario, an ocean tract of indeterminate tectonic activity across the SFTB (hinterland-foreland regions; ~116°W–113°W) width—narrow backarc or broader basin?—closed incrementally northward as and recall coeval accretion-related events in the SRSZ (west-central Idaho; transpressional terranes were successively attached to the Laurentian margin. ~117°W–116°W). Initial collision of the Blue Mountains block occurred east-northeast of the Sierra Nevada–Franciscan subduction zone system (Fig. 1A; Hamilton, 1969b; Anczkiewicz et al., 2004; suture belt of Dickinson, 2008). Final accretion and Ca. 150–120 Ma

entrapment of amalgamated terranes took place in the right-angle bend (Sri 0.706 isopleth)/Syringa embayment region of north-central Idaho (Fig. 1A; Latest Jurassic–middle Early Cretaceous contractional deformation, Yates, 1968; Schmidt et al., 2016a). Timing estimates for terrane accretion in greenschist- to upper-amphibolite-facies metamorphism, and granitic west-central Idaho (McKay et al., 2017; this study), the Coast Mountains orogen magmatism are recorded in northeastern Nevada (Pilot, Ruby, East Humboldt (Monger et al., 1982, 1994; Miller et al., 2016), and central/eastern Alaska Range Ranges; e.g., Miller, 1984; Hudec and Wright, 1990), northwestern Utah (Trop and Ridgway, 2007) are compatible with basin closure and suturing in (Silver Islands, Newfoundland Mountains; e.g., Miller and Allmendinger, the south (Idaho: ca. 144–141 Ma garnet growth) followed by more northern 1991; Allmendinger and Jordan, 1984), east-central Nevada/west-central events over time (Alaska: end of suturing ca. 60 Ma; e.g., Ridgway et al., 2002). Utah (Snake Range; e.g., Miller et al., 1988), and south-central Idaho (Albion Overlapping ages of middle Cretaceous tectonism (115–90 Ma) in west-central Mountains; Snoke and Miller, 1988; Wells et al., 1997). While many of these Idaho (SRSZ), north-central Washington (Cascades), and westernmost Can- areas (~41°N–42°N) record Cenozoic extensional deformation associated with ada (Coast Mountains) support north-south fragmentation and subsequent core-complex development (Coney and Harms, 1984; MacCready et al., 1997), addition of the Wrangellian composite to the Cordilleran collage (Coney et al., an earlier history of east-west contraction is also recognized (Fig. 1A; ca. 150– 1980). Our correlation of the Wallowa terrane with Wrangellia (Jones et al., 145 Ma metamorphic tectonites, central Ruby Mountains [e.g., Hudec, 1992]; 1977; Dickinson, 2004; Kurz et al., 2017) is based on similarities in structural east-southeast–vergent folds/penetrative fabric, northern Albion Mountains style, rock associations, and the timing of accretion along the SRSZ–north- [e.g., Miller, 1980][DeCelles, 2004]). Smith et al. (1993) noted the coincidence ern Cascades–Coast Mountains orogen (e.g., Selverstone et al., 1992; Vallier, of ca. 165–150 Ma contraction with an increased rate of plate convergence 1995; Crawford et al., 1987; Whitney and McGroder, 1989; Miller and Paterson, (Farallon–North America, >8 cm/yr; Gordon et al., 1984; Engebretson et al., 2001; McKay et al., 2017). Having established along-strike connections with 1985), as compared to the ca. 145–130 Ma interval (~5 cm/yr; Humphreys, the northern Cordillera (>46°N; Fig. 1A), and compared orogenic components 1995; Madsen et al., 2006). This apparent decrease in plate convergence rate with western Ireland (Fig. 15), we call attention to areas east of the SRSZ and overlapped in time with westward-directed thrusting/crustal thickening/core consider terrane accretion in the context of age-equivalent mountain building garnet growth (Pollock Mountain Amphibolite; Getty et al., 1993), pronounced on the continental interior (cf. Burchfiel, 1980; Edelman, 1992; DeCelles, 2004). flattening deformation (Wild Sheep Creek Formation; Fig. 6; Aliberti, 1988), and pervasive linear-planar fabric development (figure 3A of McKay et al., 2017) across western portions of the SRSZ (Figs. 2A, 14A, and 14B). Coeval calc- Temporally Overlapping Orogen alkaline magmatism and contractional deformation are recorded by ca. 145– 130 Ma stocks, sills, and satellite plutons emplaced into the eastern Wallowa Areas extending from northwestern Utah through northeastern Nevada into terrane (Heavens Gate Ridge U-Pb zircon localities D and 06KG15 of Kauffman south-central Idaho (~38°N–43°N, ~113°W–116°W) occupy the Sevier hinterland et al., 2014; this study; Figs. 5 and 13; Tables 1 and 4), magmatic epidote-bearing

GRAMPIAN OROGENY: ca. 475–460 Ma HIGHLAND [intraoceanic arc–continent collision] BOUNDARY FAULT Clew Bay Cpx. [Early Ordovician] arc, mélange, and ophiolitic rocks Dalradian Spgp. [Neoproterozoic] passive continental margin strata

thrust strike-slip Glasgow fault fault Figure 15. Early Paleozoic Grampian orogeny of the Irish Cale- donides, showing transpressional oceanic arc–continent collision Donegal granite zone (>75 km wide) with associated contractional/strike-slip [Silurian] Scotland structures spanning the Fair Head–Clew Bay Line (Iapetus suture, inset map; e.g., Dewey and Shackleton, 1984). According FAIR HEAD– to Chew (2003), an early high-strain event (D1) was shared by CLEW BAY LINE Laurentian cover rocks and accreted oceanic elements; subse- collisional suture County quent deformation across the suture zone included crustal-scale Donegal nappe development (D2) and dextral shearing (D3). Compare Belfast 0° with the oblique convergence setting and zone of broad sutur- DEVONIAN ing in west-central Idaho (~25–40 km wide; Fig. 2A); orogenic SUTURE ZONE components are strikingly similar. Modified from Chew (2003) and Dewey and Ryan (2015). Cpx—complex; Spgp—Supergroup.

Dublin ~15°S Laurentian rifted marginmap

IAPETUS South Ireland SUTURE Connemara N Avalonia arc 30°S 100 km REMNANT ARCS

plutons south of Riggins (Armstrong et al., 1977; Zen, 1985; Manduca et al., of contractional deformation in the arc edifice and frontal portions of the 1993; Unruh et al., 2008), and syntectonic granitoids in northeastern Oregon orogen (Wyld and Wright, 2001). In the latter, contractional strains accumu- (Johnson et al., 1997, 2011; Žák et al., 2012, 2015). lated along major north-northeast–striking transpressional belts (e.g., western According to DeCelles (2004), evidence of Early Cretaceous (142–112 Ma) Nevada shear zone; Fig. 1A) and more eastern thin-skinned structures of magmatism, metamorphism, and contractional deformation is sparsely scat- the SFTB (DeCelles, 2004), i.e., stage 1 deformation of Yonkee and Weil tered across the Sevier hinterland; geochronological data constrain eastward (2015). We attribute tectonism in the Western Nevada shear zone (northern displacement on aerially extensive thrust sheets to ca. 145–132 Ma (Can- segment, ca. 140–130 Ma) to dextral-oblique collision and translation of the yon Range thrust [apatite fission track]—Stockli et al., 2001; Willard thrust Blue Mountains Province. [40Ar/39Ar muscovite]—Yonkee et al., 1989, [fission track]—Burtner and Nigrini, 1994; Paris thrust [K-Ar illite]—Burtner and Nigrini, 1994; Basin-Elba thrust [40Ar/39Ar muscovite]—Wells et al., 2008, [176Lu/177Hf garnet]—Kelly et al., 2015). Ca. 120–90 Ma Wyld and Wright (2001) emphasized the lack of subduction-related magmatism and regional shortening ca. 140–120 Ma, as compared to Late Jurassic Middle Cretaceous dextral transpression in the Cordilleran hinterland (pre– (Camilleri et al., 1997) and post–120 Ma time (e.g., Burchfiel et al., 1992). 115 Ma to ca. 108 Ma: Pueblo Mountains, southeast Oregon; Wyld and Wright, Orogenic quiescence is explained by (1) extension in the Cordilleran mag- 2001) coincided with emplacement of the Meade thrust sheet (onset ca. 115 Ma: matic arc and forearc regions (e.g., Smith et al., 1993), and (2) partitioning DeCelles et al., 1993; Mitra, 1997) and layer-parallel shortening in the foreland

region (e.g., pressure-solution cleavage; Fig. 1A, cross-section C–C′; Protzman fragments are progressively transferred to the overriding (accommodating) and Mitra, 1990). Deformation was contemporaneous with west-southwest– plate, satisfying mass conservation/balance requirements, i.e., hinterland dis- directed thrusting in the Orofino-Ahsahka belt (onset ca. 116 Ma: Schmidt placement within newly conjoined crustal units (Struik, 1988; Oldow et al., 1990; et al., 2016a; Yates, 1968; Strayer et al., 1989; McClelland and Oldow, 2007) Wilson, 1990). Tectonic stresses are transmitted away from the subduction zone, and displacement on the Pollock Mountain–Rapid River–Morrison Ridge across an emergent arc-continent collision zone (Brown and Ryan, 2011), into fault systems in the SRSZ (ca. 116–111 Ma; Figs. 2, 3, 14B, and 14C; Hamilton, the continental interior (Dickinson et al., 1978). Sustained high-angle conver- 1963b, 1969a; Aliberti, 1988; Gray et al., 2012; McKay et al., 2017). Subsequent gence results in widespread transpressional deformation, thrust-related crustal activity in the SFTB included ca. 100–85 Ma slip along the Medicine Lodge, thickening, and intermittent magmatism across the contractional orogen (e.g., Tendoy, and Sapphire thrusts (southwest Montana–east-central Idaho segment; Trans-European suture zone; Pharoah, 1999). Given that the SRSZ and SFTB are Fig. 1A, section A–A′; Schmitt et al., 1995) and sinistral transpression across spatially (117°W–113°W latitude; Fig. 1), temporally (ca. 145–90 Ma tectonism; the northwest-southeast–trending Lewis and Clark Line (Fig. 1A, section B–B′; Figs. 2 and 14; Table 4), and kinematically linked (shared basal-décollement; e.g., flexural-slip fabric of Sears et al., 2004). Late Early Cretaceous tectonism cf. Bally, 1984; Fig. 16), a causal relation is considered here. north of the Snake River Plain (SFTB: ca. 100–85 Ma; Idaho and Montana) Most geologists accept that formation of the Cordilleran foreland fold-and- overlapped with contraction in the Orofino-Ahsahka belt (active through ca. thrust belt (Sevier orogen; Armstrong, 1968) followed accretion of composite 90 Ma: McClelland and Oldow, 2007; Lewis et al., 2014; Schmidt et al., 2016a) terranes in the absence of major collisional events (Allmendinger and Jor- and ductile deformation in the Coolwater culmination (SRSZ: onset ca. 100 Ma; dan, 1984; Burchfiel et al., 1992; DeCelles, 1994; Taylor et al., 2000; English “orogenic welt” of Lund et al., 2008; Fig. 14D). and Johnston 2004; ribbon continent alternative of Johnston, 2008). Models As the angle of dextral-oblique plate convergence steadily increased (ca. favoring noncollisional mountain building include (1) rapid oceanic-continental 120–90 Ma; ~30°–60°; see inset Fig. 14D; Engebretson et al., 1985; Giorgis and plate convergence with a thermally weakened back-arc region (Hyndman et Tikoff, 2004), and the magnitude of boundary-normal shortening decreased al., 2005), (2) flat-slab subduction of a seafloor spreading center, seamount, (>100–50 km; Skipp, 1988; Royse, 1993; DeCelles, 2004), contractional and/or oceanic plateau (Bird, 1988; Murphy et al., 2003), and (3) dextral strike- deformation shifted from north-south–striking structures along the Idaho- slip movement along the Northern Rocky Mountain–Tintina trench fault system Wyoming-Utah salient to northwest-southeast–striking thrusts (e.g., Meade with an eastward component of displacement (Price and Carmichael, 1986). and Medicine Lodge) and sinistral-transpressional elements (Lewis and Clark Regardless of the mechanism, large-magnitude crustal shortening across the Line) of southwestern Montana (Fig. 1A). Based on the age relations (northward central SFTB (Fig. 1A) was coeval with collision-related tectonism in the SRSZ diachroneity) and attitudes of structures outlined above, this transition (Figs. 2A and 3). Although their structural styles and conditions of deforma- was possibly related to early high-angle arc-continent collision (east-west tion differ (thick-skinned mid/lower-crustal levels in SRSZ versus thin-skinned, compression ca. 150–125 Ma; ~41°N–43°N) followed by northward translation shallow/midcrustal levels in SFTB; cf. hinterland-foreland of Mexican orogeny; and clockwise rotation (dextral transpression ca. 125–90 Ma; ~43°N–46°N) Fitz-Díaz et al., 2017), overlapping ages of contraction argue for kinematic of the Blue Mountains block (ca. 150–90 Ma accretion-related tectonism in coordination of respective structures (southern Canadian Rocky Mountains the SRSZ [Lund and Snee, 1988; Selverstone et al., 1992; Lund et al., 2008; [Brown et al., 1992]; northern U.S. Rocky Mountains [McClelland and Oldow, cf. Fossen and Tikoff, 1998]). In our tentative model, age-equivalent tectonic 2004]). Moreover, seismic reflection data collected across northern Idaho activity in the SFTB (Fig. 1; Table 4; ca. 140–90 Ma; ~41°N–46°N) was associated and Washington (~33–35 km depth, 48.5°N, COCORP seismic line WTF-82–1; with oceanic-continental (Farallon–North American) plate coupling enhanced Fig. 1A) show prominent west-dipping sequences (crust-penetrating thrusts) by terrane accretion: subduction zone jamming (positive buoyancy; Cloos, linking accretion‐related shortening with contractional structures of western 1993; Ranalli et al., 2000), oceanward-jumping (plate reorganization; Hamilton, Montana (Potter et al., 1986, 1987). Accordingly, we propose that long-lived 1988), and resultant end-loading of the Laurentian continental margin (Oldow terrane accretion, translation, and rotation in the Cordilleran hinterland—i.e., et al., 1990; Yonkee and Weil, 2015; Blakey and Ranney, 2018). tectonic evolution of the SRSZ—drove crustal shortening eastward into the foreland region. In our model (Fig. 14), contact between arc-volcanogenic rocks of the Blue Orogenic Link ~41°N–46°N Mountains Province and passive-margin sedimentary strata initiated north of the Mesozoic marine province (Speed, 1978) near the juncture of Idaho, Oregon, In oblique subduction settings where thick oceanic tracts are attached and Nevada (Fig. 1A, schematic cross-section C–C′). We suggest that northern to the structurally overriding plate, addition of buoyant crust is balanced by portions of the Western Nevada shear zone (southern counterpart of SRSZ?; shortening in the colliding mass and continental margin assemblage (e.g., figure 1 of Wyld and Wright, 2001) accommodated terrane accretion and Quesnellia arc/Neoproterozoic strata of the Omineca belt; Monger et al., 1982; served in transmitting early eastward-directed tectonic stresses. Late Meso- Scholl et al., 1986; Ross, 1991). As subduction/accretion continue, terrane zoic contractional structures of the SRSZ and SFTB (bivergent thrust faults,

tight-to-isoclinal folds, tectonite fabrics; Figs. 1–3; Tables 1 and 4) evolved east-west contractional structures in the Sevier orogen: mid- to upper-crustal together as arc terranes impacted (~41°N: high-angle collision; Fig. 14A), thrust faults, regional folds, and related fabric elements between the modern migrated northward along (dextral translation/clockwise rotation; Figs. 14B latitudes of Riggins, Idaho (~45°N), and Winnemucca, Nevada (~41°N). Early and 14C), and were ultimately incorporated into the continental margin (46°N, structures associated with late Mesozoic island-arc–continent collision Syringa embayment; Fig. 14D; Schmidt et al., 2016a). Large-scale horizontal include the Western Nevada shear zone (southeast Oregon), Willard–Basin- shortening of miogeoclinal strata was compensated (volumetrically mass-bal- Elba–Paris–Meade thrust systems of southeastern Idaho, and midcrustal anced) by the addition of buoyant oceanic crust (Scholl et al., 1986; Cloos, 1993; metamorphic tectonites of northeastern Nevada: contractional structures Stanciu et al., 2016) to the leading edge of ancestral North America. Hinterland south of the Snake River Plain. Later structures include the Medicine Lodge tectonites that equilibrated under midcrustal metamorphic conditions (SRSZ thrust, Sapphire thrust, and sinistral-transpressional elements along the [Selverstone et al., 1992]; SFTB [Kelly et al., 2015]) were kinematically linked to Lewis and Clark Line north of the Snake River Plain (southwestern Montana, upper-crustal foreland structures along a gentle westward-dipping basal-décol- east-central Idaho). In response to right-oblique oceanic-continental plate lement system underlying the Cordilleran orogen (seismic profiling—Bally et convergence, subduction zone jamming, terrane transfer, and accretion (end- al., 1966; Allmendinger et al., 1987; Cook et al., 1992; orogenic float concept— loading of Laurentian margin), compressional deformation propagated >250 Oldow et al., 1989, 1990; hinterland-foreland linkage—Brown et al., 1992). In km eastward across the SRSZ (hinterland: ~117°W–116°W) into the SFTB this framework, latest Jurassic (ca. 145 Ma) to late Early Cretaceous (ca. 90 Ma) (foreland: ~115°W–113°W). This long-lived history of temporally overlapping composite terrane accretion, translation, and rotation in the Cordilleran hin- (ca. 145–90 Ma tectonic activity) and kinematically linked (shared basal terland played a dynamic role in transmitting displacements across the SRSZ décollement) hinterland-foreland contraction followed pre–145 Ma collapse into the SFTB (~41°N–46°N, ~117°W–113°W; Gray, 2016). of fringing arc assemblages and stabilization of the North American margin. In our view, orogenic components in west-central Idaho—pervasive ductile deformation (>25-km-wide tectonite belt), pronounced thrust-related crustal ■■ CONCLUSIONS thickening (>50 km: Pollock Mountain–Rapid River–Morrison Ridge–Heavens Gate fault systems), and high-pressure syntectonic magmatism (~8–11 kbar: Structural and geochronological studies conducted in west-central Idaho suture zone plutons)—fit a collisional model explanation for intracontinental indicate cross-orogen linkages between tectonic elements of the western mountain building in the Cordillera of western North America (~41°N–46°N). U.S. Cordillera: Salmon River suture zone and Sevier fold-and-thrust belt. A Late Mesozoic terrane accretion supports inclusion of the Wallowa arc with causal link with suturing of volcanic arc terranes (Blue Mountains Province) to Wrangellia and implies progressive south-to-north ocean closure between ancestral North America (western Laurentia) is proposed for development of Idaho and Alaska (ca. 145–60 Ma).

Figure 16. Tectonic model illustrating the addition of oceanic crust (Blue Mountains Province) to ancestral North America (western Laurentia) between Riggins, Idaho, and Winnemucca, Nevada (ca. 135 Ma, ~42°N–43°N). During arc-continent collision, the Wallowa–Baker–Izee–Olds Ferry terranes were transferred to Laurentia along a shallow west-dipping basal décollement emanating from the convergent boundary and underlying the Cordilleran orogen (Bally, 1984; Oldow et al., 1990; Brown et al., 1992; active continent–back-arc postcollisional geometry of Moores and Twiss, 1995, p. 213). Mid- to upper-crustal contractional structures in the foreland region (~115°W–113°W: e.g., Albion Mountains; Kelley et al., 2015) accommodated the addition of buoyant oceanic crust and evolved together with hinterland tectonites across the Salmon River suture zone (~117°W–116°W; McKay et al., 2017). Later strike-slip deformation associated with right-oblique terrane accretion/ margin-parallel translation was concentrated in calc-alkaline plutons emplaced into the boundary (ca. 120–90 Ma: Manduca et al., 1993; figure 6b schematic of Johnston, 2008). Age-equivalent deformation in adjacent country rock assemblages (Snee et al., 1995; Gray et al., 2012; this study) was principally of contractional nature. Dextral transpression viewed in this manner relaxes the need for a spatially coincident and temporally overlapping tectonic element in west-central Idaho (Western Idaho shear zone of McClelland et al., 2000; Tikoff et al., 2001, 2017; Giorgis et al., 2005–2008; Blake et al., 2009, 2016; Braudy et al., 2017; Davenport et al., 2017; Schmidt et al., 2016a; Montz and Kruckenberg, 2017; ipso facto Humphrey, 1992).

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(2015) for stimulating part of this research, p. 133–144, https://doi.org/10.1016/0012-821X(83)90195-4. which has its origins in academic studies providentially guided by J.S. Oldow at the University Beck, M.E., Jr., Burmester, R.F., Engebretson, D.C., and Schoonover, R., 1981, Northward transla- of Idaho (1996–1997). Structural mapping in the south-central Kessler Creek quadrangle was tion of Mesozoic batholiths, western North America: Paleomagnetic evidence and tectonic funded by U.S. Geological Survey EDMAP Program award G16AC00102 (2016) to Gray. This paper significance: Geofísica Internacional, v. 20, no. 3, p. 144–162. is dedicated to Tracy L. Vallier. Benford, B., Crowley, J., Schmitz, M., Northrup, C., and Tikoff, B., 2010, Mesozoic magmatism and deformation in the northern Owyhee Mountains, Idaho: Implications for along-zone variations for the western Idaho shear zone: Lithosphere, v. 2, no. 2, p. 93–118, https://doi .org/10.1130/L76.1. 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