Structure of the Arc—Continent Transition

STRUCTURE OF THE ARC—CONTINENT TRANSITION

IN THE RIGGINS REGION OF

WEST-CENTRAL IDAHO

By KEITH D. GRAY

A dissertation submitted in partial fulfillment of the requirements for the degree of

DOCTOR OF PHILOSOPHY

WASHINGTON STATE UNIVERSITY Department of Geology

December 2012

To the faculty of Washington State University:

Members of the Committee appointed to examine the dissertation of KEITH D. GRAY find it satisfactory and recommend that it be accepted.

______

A. John Watkinson, Ph.D. [Chair]

______

Jeffrey D. Vervoort, Ph.D.

______

Reed S. Lewis, Ph.D.

ACKNOWLEDGEMENTS

The author acknowledges Drs. A.J. Watkinson, R.S. Lewis, and J.D.

Vervoort (committee members) for overseeing this project. Thank you for encouragement and patience throughout. Early research was funded by Dr. J.S. Oldow (UI, UTD), the Belt Association, and Tobacco Root

Geological Society. C. Scott and E. O’Fallon are thanked for office support. The work of zircon gurus V. Isakson and D. (‘SHG’) Schwartz is deeply appreciated. Thanks to A. Jansen for laboratory assistance, and Dr. K. Wilke and K. Felt for help w/ the 11th floor plotter. Dr. D.

Blake (UNCW) is thanked for time and travels related to preliminary exams. Thanks to Dr. T.L. Vallier for visiting Pullman on his 76th birthday; also, for the maul provided in June 1997. The Spencer’s are recognized for their early +’ve influence. 1000 thanks to KG’s family for years of understanding. A special acknowledgement is also extended to my fiancé K. Palmeira for making everything possible. Finally, the author wishes to acknowledge former geoscientists of the SRSZ:

To all who have come before, yet left some ground untrammeled. –R.F.B.

iii

STRUCTURE OF THE ARC—CONTINENT TRANSITION

IN THE RIGGINS REGION OF

WEST-CENTRAL IDAHO

Abstract

By Keith D. Gray, Ph.D. Washington State University December 2012

Chair: A. John Watkinson

New U–Pb zircon geochronology from the Riggins region of west- central Idaho refines the timing of synmetamorphic deformation across the Salmon River suture zone, a broad NNE-striking belt (>25 km-wide) of high strain recording Jura-Cretaceous arc—continent collision.

Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-

MS) yields mid-Cretaceous crystallization ages on formerly undated intrusive rocks sampled from the northeastern Seven Devils Mountains and Salmon River canyon. Ages range from ~136 Ma (Heavens Gate stock) in accreted oceanic crust to ~92 Ma (Looking Glass pluton) along the western margin of Laurentia. This study integrates existing U-Pb, Ar-

Ar, and Sm-Nd isotopic data from the region in efforts to constrain fabric development across the arc-continent transition. L-S tectonites are mapped along a 50-km, E-W transect extending from the Snake River

(Hells Canyon) into the Salmon River canyon (Crevice area). From west- to-east, a systematic decrease in the age of penetrative deformation is observed. Although modified by late-stage folding, L1-S1 fabrics are continuous across the arc-continent boundary and record a progressive history of accretion-related tectonism (post-136 Ma to post-92 Ma).

TABLE OF CONTENTS

Page

ACKNOWLEDGEMENTS...... iii

ABSTRACT...... iv

LIST OF TABLES...... vii

LIST OF FIGURES...... viii

DEDICATION...... x

CHAPTER

I. INTRODUCTION

Why here, why now?...... 1

Previous work in the Riggins region...... 5

Chapter organization...... 7

II. MESOZOIC TECTONICS OF WESTERN NORTH AMERICA

Relative plate motions...... 8

Cordilleran superterranes...... 10

Terrane accretion in west-central Idaho...... 11

III. METHODOLOGY

Structure...... 13

Geochronology...... 14

IV. GEOLOGY OF THE CREVICE PLUTON AND SALMON RIVER CANYON

(modified from Gray et al., 2012)

Abstract...... 16

Introduction...... 17

Tectonic setting...... 22

Geochronology...... 28

Structures of the Riggins region...... 31

Page

Ages of deformation...... 47

Discussion and tectonic implications...... 51

Conclusions...... 61

V. ADDITIONAL GEOCHRONOLOGY, STRUCTURE, & DEFORMATION CONSTRAINTS

Relevance of data...... 64

Heavens Gate stock (U–Pb; zircon)...... 64

Lake Creek dike (U–Pb; zircon)...... 73

Sm–Nd (garnet); from McKay et al. (in review)...... 80

VI. STRUCTURAL SYNTHESIS OF THE ARC—CONTINENT TRANSITION...... 91

VII. CONCLUDING REMARKS...... 102

BIBLIOGRAPHY...... 105

APPENDICES

1. ANALOG TO WALLOWA TERRANE?: KOHISTAN ISLAND-ARC, PAKISTAN

2. TRIAXIALLY-DEFORMED LITHIC CLASTS FROM HEAVENS GATE RIDGE

3. XRF CHEMISTRY FROM INTRUSIVE ROCKS ON HEAVENS GATE RIDGE

(WSU GeoAnalytical Laboratory)

4. LINE SCAN AND BACKSCATTER ELECTRON IMAGES, TWO-STAGE GARNET

(from McKay et al., in review).

5. TECTONIC HISTORY AND HIKING TOUR OF THE NORTHERN SEVEN DEVILS

MOUNTAINS, WEST-CENTRAL IDAHO (from Gray, 2012).

PLATES

1. STRIP MAP (WEST: HELLS CYN.-SEVEN DEVILS MTNS.-L. SALMON R.)

2. STRIP MAP (EAST: L. SALMON R.-RIGGINS-SALMON RIVER CYN.)

3. STRUCTURAL SECTIONS (WEST: A-A′-A′′-A′′′; EAST: B-B′, C-C′)

4. TOPOGRAPHY OF TRANSECT (USGS: 7.5-MINUTE QUADRANGLES)

LIST OF TABLES

Page

1. Table 1. U–Pb geochronology; Crevice, Looking Glass plutons....29

2. Table 2. U–Pb geochronology; Heavens Gate stock...... 68

3. Table 3. U–Pb geochronology; Lake Creek dike...... 76

vii

LIST OF FIGURES

Page

1. Figure I-1. Location of the ocean-continent boundary...... 1

2. Figure I-2. Location of the study area...... 3

3. Figure I-3. Areas of study along the transect...... 4

4. Figure I-4. Hornblende tectonites in Riggins Group...... 6

5. Figure T-1. Plate motions along the North American margin...... 9

6. Figure T-2. Early Jurassic setting of western North America.....9

7. Figure M-1. Crosscutting planar fabric elements...... 14

8. Figure M-2. Mechanical separation of zircon...... 15

9. Figure 1. Tectonic sketch map of the northern Cordillera...... 18

10. Figure 2. Geology of west-central Idaho...... 19

11. Figure 3. Geology of the Salmon River canyon...... 24,25

12. Figure 4. CL images; Crevice, Looking Glass plutons...... 28

13. Figure 5. U–Pb data; Crevice, Looking Glass plutons...... 30

14. Figure 6. Stereonets; structures in the Riggins region...... 32

15. Figure 7. Photos of structures in the Riggins region...... 33

16. Figure 8. Fabric element map of the Lake Creek area...... 37

17. Figure 9. Folds east of the Lake Creek antiform...... 38

18. Figure 10. Stereonets; structures of Salmon River canyon...... 39

19. Figure 11. S1 foliation of the Crevice pluton...... 41

20. Figure 12. Textural variation in the Crevice pluton...... 42

21. Figure 13. S2 structures of the Crevice pluton...... 44

22. Figure 14. Systematic jointing in the Crevice pluton...... 46

23. Figure 15. Photomicrographs; garnet of Salmon River...... 49

24. Figure 16. Lake Creek antiform; structures on east limb...... 57

viii

Page

25. Figure 17. Steep zones of high shear strain...... 58

26. Figure 18. Schematic section through western SRSZ...... 59

27. Figure 19. Heavens Gate Ridge; topographic divide...... 65

28. Figure 20. Heavens Gate fault; upper/lower plate rocks...... 66

29. Figure 21. CL images; Heavens Gate stock...... 67

30. Figure 22. U–Pb data; Heavens Gate stock...... 67,68

31. Figure 23. Geology of Heavens Gate Ridge...... 70

32. Figure 24. Photomicrographs; Heavens Gate stock...... 71

33. Figure 25. Photomicrograph; R7 amphibolite...... 72

34. Figure 26. Field photograph of the Lake Creek dike...... 74

35. Figure 27. CL images; Lake Creek dike...... 75

36. Figure 28. U–Pb data; Lake Creek dike...... 75,76

37. Figure 29. Field photographs; Lake Creek dike area...... 78

38. Figure 30. Photomicrographs; Lake Creek dike...... 79

39. Figure 31. Sm–Nd locality #07a; Berg Creek amphibolite...... 81

40. Figure 32. Photomicrographs; garnet from locality #07a...... 82

41. Figure 33. Sm–Nd locality #48; Pollock Mtn. amphibolite...... 83

42. Figure 34. Photomicrographs; garnet from locality #48...... 84

43. Figure 35. Field photographs; Pollock Mountain area...... 86

44. Figure 36. Geologic map of the Pollock Mountain area...... 87

45. Figure 37. Sm–Nd locality #03b; Pollock Mtn. amphibolite...... 89

46. Figure 38. Exposures along the Salmon River...... 91

47. Figure 39. Deformed tonalite of the Looking Glass pluton...... 96

48. Figure 40. Location of WISZ in greater McCall area...... 97

49. Figure 41. Island-arc rocks of the Riggins region...... 99

Dedication

For Mom and our moon.

Our communicator. Photograph by Diane P. Gray (March 3, 1947 — May 27, 2011)

CHAPTER I. INTRODUCTION

Why here, why now?

In the northwestern U.S., isotopic compositions of granitic plutons change abruptly across a boundary separating oceanic and continental crust (Armstrong et al., 1977; Criss and Fleck, 1987; Manduca et al.,

1992), which formed in response to Mesozoic accretion of oceanic arc terranes to western North America (Jones et al., 1972; Fig. I-1).

Figure I-1. Location of the ocean-continent boundary in west-central Idaho. Accreted terranes in the west form part of the Blue Mountains province (Silberling et al., 1984). North American rocks in the east are intruded by the Late Cretaceous to early Tertiary Idaho batholith (Taubeneck, 1971; Armstrong et al., 1977; Gaschnig et al., 2010). State abbreviations: ID—Idaho, MT—Montana, NV—Nevada, OR—Oregon, WA— Washington, WY—Wyoming. Modified from Giorgis et al. (2008).

Terrane accretion is evidenced in west-central Idaho, where late

Paleozoic-Mesozoic volcanogenic rocks of island-arc affinity

(Hamilton, 1963b) are juxtaposed with Precambrian-lower Paleozoic[?] continental margin units (Lund, 1984; Blake, 1991). In the Riggins region, Jura–Cretaceous metamorphism and deformation linked to arc— continent collision is manifest across the north-northeast-striking

Salmon River suture zone (SRSZ: Lund and Snee, 1988). Since its conception, the SRSZ has played a fundamental role in models proposed for the tectonic evolution of the central North American Cordillera

(Oldow et al., 1989; Burchfiel et al., 1992; McClelland et al., 2000;

Giorgis et al., 2005; Gray and Oldow, 2005).

This study investigates the spatial-temporal relationships between high-strain metamorphic tectonites (e.g., linear and planar fabrics; cf. Turner and Weiss, 1963) developed across the SRSZ at the latitude of Riggins, Idaho (~45.3ON; Fig. I-2). While polyphase structures are documented locally (Onasch, 1977; Manduca et al., 1993) and across western portions of the suture (Gray and Oldow, 2005), a complete fabric element analysis spanning the arc—continent crustal transition does not exist. Moreover, the timing of suture zone deformation in this region is poorly constrained due to the lack of crystallization ages on deformed plutons and garnet-bearing volcanogenic rocks. Where

U–Pb constraints are well established (Orofino and McCall areas to the north and south of Riggins, respectively; Fig. I-2), map coverage and zircon sampling is limited to metaplutonic rocks exposed along the arc—continent boundary (Manduca et al., 1993; McClelland and Oldow,

2007; Unruh et al., 2008). As a consequence, the timing of regional deformation in arc-supracrustal and continental margin units has been only approximated by Ar–Ar geochronology (e.g., Snee et al., 1995).

Figure I-2. Index map of Idaho showing the location of study area and structural transect (Plates 1 and 2). Modified from Hamilton (1963b).

Goals of the present study are two-fold: (1) to place timing constraints on metamorphic tectonites developed across the SRSZ, and

(2) to provide a three-dimensional analysis of suture zone structures via strip maps and structural sections across the island-arc—continent transition. Item one incorporates new U–Pb geochronology and fabric element analyses in the context of existing radiometric and structural data synthesized for the region (Chapter IV; Gray et al., 2012, and references therein). Item 2 produces a 50 km, west-to-east structural transect which extends from Hells Canyon of the Snake River, through

the northern Seven Devils Mountains, into the Salmon River canyon due east of Riggins, Idaho (Fig. I-3; Plates 1, 2, and 3). This transect is significant in that crosses the easternmost exposures (~116OE) of marine Permian and Triassic volcanic rocks in western North America

(Vallier, 1977), intersects the arc—continent boundary at ~900, (Fleck and Criss, 2004), and terminates in continental rocks of westernmost

Laurentia (Blake, 1991). In total, these elements combine to form the most complete, mid-crustal section analyzed through the SRSZ to date.

Figure I-3. Areas of study along the structural transect. (a.) Hells Canyon, viewing west from Dry Diggins L.O. (b.) Northern Seven Devils Mtns.; Heavens Gate Ridge in foreground. (c.) Salmon River canyon, viewing west across Spring Ck. (d.) Manning Bridge over Salmon River, Crevice area. Total relief of transect ~8000′ [He Devil, Seven Devils = 9393′; Snake R. = 1400′; for topography of transect, see Plate 4].

Previous work in the Riggins region

Volcanogenic rocks of oceanic affinity have been the subject of ongoing research in the region since Warren Hamilton’s pioneering work in the early 1960s. However, the first structural investigation was not conducted until the late 1970s when Onasch (1977) identified a series of ‘critical outcrops’ across the Rapid River thrust fault

(Hamilton (1963b, 1969). Consistent with earlier interpretations

(Hamilton 1963a, b), deformation and accompanying metamorphism were linked to emplacement of the Idaho batholith (southern Atlanta lobe and outliers; e.g., McDowell and Kulp, 1969). Aliberti (1988) followed with greater map coverage through penetratively deformed volcanic, carbonate, and siliciclastic rocks correlated with the Seven Devils island-arc terrane (Brooks and Vallier, 1978), which subsequently became the Wallowa terrane (Silberling et al. 1984, 1987). By this time, Lund (1984) had explored both sides of the arc—continent boundary in the Slate Creek—Gospel Hump Wilderness area ~25 km northeast of Riggins, and formulated a dextral-transcurrent model for terrane accretion. The Salmon River suture zone acquired its full identity a few years later, when Lund and Snee (1988) addressed the timing of regional metamorphism and deformation using 40Ar/39Ar age- spectrum data obtained from suture zone plutonic rocks and metamorphic tectonites in the Riggins Group (Fig. I-4).

Suture zone structures in the Salmon River canyon have been assessed only by Blake (1991), who documented a variety of fabric elements ~15-

20 km east of Riggins. In that study, an alternative, high-angle plate convergence model (as compared to strike-slip; Lund and Snee, 1988)

Figure I-4. Squaw Creek schist [Riggins Group] exposed along the Salmon River ~1 km north of Riggins, Idaho. (a.) Metamorphic rocks dip gently to the northeast. A.J. Watkinson provides scale in lower right. (b.) Hornblende-bearing mica schist from the locality on left. Argon cooling ages from these and other metamorphic rocks in the region [Lund and Snee, 1988; Snee et al., 1995] are discussed in Ch. 4.

was proposed to explain arc—continent collision. Since then, great controversy has arisen over the kinematics of suture zone deformation

(e.g., 2006 Geological Society of America Field Forum; Giorgis et al.,

2007). In short, this problem stems from the lack of evidence in the

Riggins region supporting a strike-slip component of displacement. In the areas surrounding McCall (Fig. I-2), dextral motion is recorded by offset mafic layering (cm-scale) and rare shear sense indicators

(winged feldspar porphyroclasts) developed on the lineation normal face (McClelland et al., 2000; Giorgis and Tikoff, 2004; Giorgis et al., 2008). These structures are attributed to formation of the western Idaho shear zone, which according to McClelland et al. (2000), projects >50 km north from McCall into the Riggins region. The present study was initiated by attempting to locate shear zone boundaries (cf.

Ramsay and Graham, 1970) inside the Salmon River canyon; specifically, the western boundary near Lake Creek, as reported by Blake et al.

(2009). Geologic arguments in support of and refuting the western

Idaho shear zone are described in Chapter IV (Gray et al., 2012).

Chapter organization

Following brief discussions of Cordilleran geology (Chapter II:

Mesozoic tectonics of western North America) and research methods used in this study (Chapter III: Methodology), the author’s principal findings are presented (Chapter IV: Geology of the Crevice pluton &

Salmon River canyon). This content represents all published doctoral work to date (Gray et al., 2012), and is followed by an account of more recent data collected from the Salmon River suture zone (Chapter

V: Additional geochronology, structure, & deformation constraints).

Specifically, new U–Pb zircon ages are reported from Heavens Gate

Ridge in the northeastern Seven Devils Mountains (western margin of

SRSZ) and the Salmon River canyon (central SRSZ); therein, the structural significance of each age determination is described and timing constraints on suture zone deformation are provided. What follows next are excerpts from McKay et al. (in review), primarily microstructural in nature, which build on the garnet geochronometry of

Getty et al. (1993). Sm–Nd garnet data and structural interpretations discussed here are from the Pollock Mountain area (western SRSZ) and two localities along the Salmon River corridor (central and eastern

SRSZ). This section precedes a summary of the author’s collective research findings (Chapter VI: Structural synthesis of the arc– continent transition) and final comments on the island-arc–continent transition in west-central Idaho (Chapter VII: Concluding remarks).

CHAPTER II. MESOZOIC TECTONICS OF WESTERN NORTH AMERICA

Relative plate motions

Long-lived collisional plate boundaries (active over tens of millions of years) may experience significant periods of intense crustal deformation and metamorphism while accumulating finite contractional strains, particularly when the angle of convergence is high (Dickinson, 1977; Dewey, 1980; Monger et al., 1982). This type of active margin is evidenced in the North American Cordillera, where subduction of oceanic crust under western Laurentia occurred between at least Triassic and early Cenozoic time (e.g., Burchfiel et al.,

1992; Dickinson, 2004). During this interval, westernmost ancestral

North America occupied a tectonic environment analogous to that which exists today in the central Chilean Andes (Andean-type continental margin of Hamilton, 1969b). The early plate motion reconstructions of

Engebretson et al. (1985), based on seafloor magnetic anomalies tied to a hot-spot reference frame, show the Farallon plate subducting obliquely beneath North America in a sinistral sense ca. 175 Ma, and then orthogonally ca. 150-125 Ma (Fig. T-1). According to Blake et al.

(1988), Early Cretaceous subduction resulted from normal convergence which persisted until ~90 Ma (compare with dextral motion through 100

Ma; Fig. T-1a). High-angle plate convergence and widespread shortening across the Cordilleran margin resulted in closure of marginal marine basins and accretion of rafted island-arc assemblages (e.g., Saleeby and Busby-Spera, 1992; Smith et al., 1993; Fig. T-2). Consequently, large tracts of the Alaskan and Canadian Cordillera are underlain by

suspect tectonostratigraphic terranes (Coney et al., 1980) that were incrementally added to the westward expanding continental borderland.

Figure T-1. Jura–Cretaceous plate motions. Arrows emanate from points of tectonic interest along the continental margin and show convergence velocities for the adjacent oceanic plate. This model assumes that mantle hotspots in the Atlantic region have remained fixed relative to Pacific basin hotspots. Note the change in relative motions from left- oblique to right-oblique ca. 125-100 Ma. Abbreviations: FP—Farallon plate, NAP—North American plate. From Engebretson et al. (1985).

Figure T-2. (a.) Paleogeographic map of western North America [~37-55 ON] constructed for the Early Jurassic, ca. 180 Ma. Fringing volcanic arcs are palinspastically restored to show their presumed locations at this time. Velocity vector shown on the Farallon plate [FP; Fig. T-1d] indicates high-angle ocean-continent convergence. Map by R.C. Blakely. Sources include Burchfiel et al. [1992] and Saleeby and Busby-Spera [1992], plus others [http://cpgeosystems.com/wnampalgeog.html]. (b.) An active volcanic arc within a plate tectonic setting similar to the map reconstruction shown on left. Artwork by J.E. Kauffman.

Cordilleran superterranes

Recognition of the ‘suspect’ nature (allochthoneity) of terranes forming the Cordilleran collage (Helwig, 1974; Davis et al., 1978) led to a first-order distinction between the western accreted oceanic fragments and more eastern elements indigenous to North America (e.g.,

Jones et al., 1983). Generally speaking, ancestral North America

(Laurentia) includes its Precambrian cratonal margin, miogeoclinal platforms and basins (Precambrian to Paleozoic), and pericratonic fringing volcanic arc complexes (late Paleozoic to Mesozoic). In contrast, allochthonous western terranes consist of late Paleozoic to

Mesozoic magmatic arcs, micro-continents, and intervening ocean basins that were accreted to the Laurentian margin in the late Mesozoic

(e.g., Colpron et al., 2007). Although the northern Cordillera has been subdivided into numerous terranes (>50; Coney et al., 1980), each of which recording its own pre-accretionary history, it can also be viewed as consisting of ‘superterranes’; for example, the Intermontane and Insular Superterranes (Superterranes I and II, respectively, of

Monger et al., 1982). These larger tectonic entities contain sets of previously amalgamated terranes which have evolved together since their collective emplacement onto the western Laurentian margin.

The Intermontane Superterrane (I) represents one interrelated set of arcs, marginal seas, and continental fragments that once formed a late

Paleozoic-early Mesozoic fringe to North America. Examples of these fringing-arc–accretionary prism couplets include the Quesnellia–Cache

Creek, Olds Ferry–Baker, and Rattlesnake–Stuart Fork terranes. Middle

Jurassic accretion resulted in two northeasterly trending, oppositely-

verging compressional belts exposed in northeastern Washington and adjacent British Columbia (Miller, 2000; Cheney and Zieg, in press).

According to Cheney and Buddington (2012), major fold and thrust belts formed when Quesnellia was inserted eastward between Laurentian supracrustal rocks and their basement. In southeastern British

Columbia, the north-south-striking Omineca Belt (Monger et al., 1982;

Monger and Price, 2002) overlaps this accretionary boundary.

By contrast, the Insular Superterrane (II) consists of intra-oceanic island-arcs, offshore marine basins, and deep-marine plateaus (e.g.,

Wrangellia, Alexander, Stikinia, and Wallowa terranes) that originated from separate sites in late Paleozoic time (e.g., Jones et al., 1977;

Monger and Price, 1979). This amalgamation stretches from southeastern

Alaska to northeastern Oregon (Blue Mountains) and was united with

Superterrane I in the Cretaceous, as evidenced by structures in the

Coast Plutonic Complex (Dickinson, 1976; Monger et al., 1982). On the basis of crosscutting plutons (e.g., Monger and Berg, 1987), it is clear that the entire width of the southern Canadian Cordillera was tied to North America in the late Mesozoic (Oldow et al., 1989).

Terrane accretion in west-central Idaho

In the northwestern U.S., a major late Mesozoic compressional belt associated with convergent margin tectonism and terrane accretion is defined by the north-northeast-trending SRSZ (Lund and Snee, 1988).

Developed in late Paleozoic to mid-Mesozoic oceanic crust of the Blue

Mountains province (Silberling et al., 1984, 1987) and Precambrian[?] continental rocks of westernmost Laurentia (Lund, 1984; Blake, 1991),

the suture zone straddles fundamentally different lithosphere. Across

87 86 west-central Idaho, initial Sr/ Sr (Sri) isotope ratios determined from Cretaceous plutonic rocks indicate a sharp break between oceanic values (Sri = 0.704 or less) and continental values (Sri = 0.706 or greater; Armstrong et al., 1977; Manduca et al., 1992; Fleck and

Criss, 2004). In the Riggins region, plutons stitching the arc— continent boundary record this change in isotopic values over a distance of <5 km (Criss and Fleck, 1987; Gray et al., 2012).

Recent geologic mapping and U–Pb zircon dating across western portions of the suture zone place Permian and Triassic plutonic rocks of the Wallowa terrane (i.e., Superterrane II of Monger et al., 1982) directly against westernmost Laurentia (Lewis et al., 2011). While supported in the present study, this interpretation contrasts with early studies in the Blue Mountains province which typically show intervening accreted terranes (e.g., Vallier, 1995; Avé Lallemant,

1995). The local juxtaposition of Superterrane II with ancestral North

America may relate to Early Cretaceous clockwise rotation (~65O) of the

Blue Mountains block (Wilson and Cox, 1980). In this context, rocks comprising the fringing arc–accretionary prism (i.e., Olds Ferry-Baker terranes; Superterrane I) were rotated away from their original inboard position acquired during Late Jurassic-Early Cretaceous accretion (e.g., Avé Lallemant, 1995). Regardless of the mechanism, the SRSZ marks an important along-strike structural transition between allochthonous arc terranes of the northern and southern Cordillera; specifically, the Quesnellia terrane of southern British Columbia and

Black Rock terrane of northwestern Nevada (Wyld and Wright, 2001).

CHAPTER III. METHODOLOGY

Structure

Structural analyses were conducted on lower greenschist to upper amphibolite facies metamorphic tectonites across the Salmon River suture zone (Plates 1, 2). At each field locality, fabric element orientation data were recorded and plotted on geologic base maps compiled at 1:24,000-scale (Onasch, 1977; Aliberti, 1988; Blake,

1991). Other maps were utilized to place mesoscopic structures into a regional tectonic framework (Hamilton, 1969a; Manduca et al., 1993;

Giorgis et al., 2008; Gray et al., 2012). Field studies were combined with microstructural analyses in efforts to determine timing relations between mineral growth and fabric development. The terminology used follows Passchier and Trouw (2005) in describing porphyroblast-fabric relationships. Thin section slides were prepared at Washington State

University and viewed under a petrographic microscope. Backscattered electron (BEI) images were prepared at the University of Alabama.

The term fabric is the accepted English translation of the German word , used by Sander (1930) to denote the internal ordering of an aggregate. According to Turner and Weiss (1963), the component parts in aggregates are crystalline grains, and their spatial arrangement and mutual relations constitute the internal order of the fabric. This concept of fabric extends the notion of internal spatial order to non-lattice bodies such as crystalline aggregates, geologically familiar as rocks. The following shorthand notation is utilized when discussing linear-planar (L-S) tectonite fabrics:

Sn – refers to a planar fabric element of secondary origin (e.g., schistosity or gneissosity)

Ln – refers to a linear fabric element of secondary origin (e.g., mineral or clast alignment)

A subscript (n, above) is included with each symbol, which defines the relative age of the fabric element. Wherever possible, relative ages were determined using crosscutting fabric relationships (Fig. M-1).

Figure M-1. Schematic illustration of fanning axial-planar cleavage (S2; subvertical) crosscutting an earlier planar fabric (S1; folded). After L.U. De Sitter; printed in Turner and Weiss (1963).

Geochronology

Rock samples were collected in quantities of 5-10 kg, and zircons were isolated using standard crushing, milling, and mineral separation procedures (Fig. M-2). Grains handpicked under a binocular microscope were mounted in epoxy with standards, polished to expose centers, carbon-coated, and imaged with scanning electron microscopy- cathodoluminescence (SEM–CL) at the University of Idaho (UI). Analyses were conducted at Washington State University’s Radiogenic Isotope and

Geochronology Laboratory (RIGL) with a New Wave Nd:YAG UV 213 nm laser coupled to a ThermoFinnigan Element2 single collector, double- focusing, magnetic sector inductively coupled plasma–mass spectrometer

(ICP–MS) with a 30 um laser spot diameter and 10 Hz repetition rate

Figure M-2. (a.) Jaw-crusher used to produce rock chips during the early mechanical separations stage of zircon [UI]. (b.) Rock chips.

following the methods of Chang et al. (2006). Time-dependent fractionation was corrected using the intercept method, and time- independent fractionation was corrected by normalizing unknown analyses to bracketing standards (Peixe = 564 Ma; Dickinson and

Gehrels, 2003; FC1 = 1099 Ma; Paces and Miller, 1993), with Peixe serving as the primary zircon standard.

Final zircon ages quoted later in the text represent weighted means of pooled 206Pb/238U ages calculated using Ludwig’s (2003) Isoplot.

Calculated weighted mean ages incorporate all analyses in the main data clusters shown in Tera–Wasserburg diagrams to avoid biasing results with potentially subjective data filtering. Calculated weighted mean ages (along with mean square weighted deviations and probabilities of fits) were calculated using internal uncertainties only, estimated by the standard errors of sample analyses. Standard deviations of the analyses of standards that bracket the unknowns were then added quadratically to the weighted mean errors to calculate the total uncertainties.

CHAPTER IV. GEOLOGY OF THE CREVICE PLUTON AND SALMON RIVER CANYON Modified from Gray, K.D., Watkinson, A.J., Gaschnig, R.M., and Isakson, V.H. (2012) Age and structure of the Crevice pluton: overlapping orogens in west- central Idaho? Canadian Journal of Earth Sciences

Abstract

New U–Pb zircon geochronology from the Riggins region of west- central Idaho refines the timing of contractional deformation across the Salmon River suture zone (SRSZ), a broad north-northeast-striking belt (>25 km-wide) of high strain recording Jura-Cretaceous island- arc—continent collision. Laser Ablation-Inductively Coupled Plasma-

Mass Spectrometry (LA-ICP-MS) yields mid-Cretaceous crystallization ages on formerly undated plutonic rocks sampled from the Salmon River canyon. In the Crevice pluton (~105 Ma), the development of steep to moderate northerly striking gneissic foliation (S1) was followed by tops-to-the-west slip on shallow mylonitic shear zones (S2) and brittle overprinting via systematic joints (Jn) of regional extent. Together, these structures form the pluton’s internal architecture. Subvertical gneissic foliation in the adjacent Looking Glass pluton (~92 Ma) indicates ductile deformation was ongoing in the Late Cretaceous.

Prior to this investigation, penetrative fabrics in local arc volcanogenic, plutonic, and continental rocks have been unequivocally linked to post-collisional dextral transpression on the narrow (<10 km-wide) western Idaho shear zone (WISZ). As an alternative to this model which requires spatially overlapping and temporally distinct orogenic belts (SRSZ\WISZ), we consider a protracted history whereby regional synmetamorphic structures accumulated over the pre-118 Ma to post-92 Ma interval without an overprinting orogen-scale ductile shear

zone. In our view, a progressive deformation history more accurately accounts for the time-transgressive nature and structural continuity of fabrics observed across the arc—continent transition. This tectonic history proposed for western Idaho may be analogous to other long- lived accretionary margins in the North American Cordillera (e.g., the

Omineca Belt of southeastern British Columbia).

Introduction

In the North American Cordillera, initial 87Sr/86Sr isotope ratios record the transition from plutons emplaced into oceanic lithosphere on the arc (western) side to those intruded into continental lithosphere on the craton (eastern) side; a ratio of 0.706 is interpreted to mark the arc-craton boundary (e.g., Armstrong et al.,

1977). Across west-central Idaho, the Sri-0.706 isopleth separates late

Paleozoic to Mesozoic accreted crust of oceanic affinity and

Precambrian[?] continental crust along the western Laurentian margin

(Fig. 1). Near the alpine village of McCall (Fig. 2), mid-Cretaceous

87 86 calc-alkaline plutons recording the arc-craton transition ( Sr/ Sri ratios <0.7045 to >0.707; Manduca et al., 1992) are deformed by steep northerly striking mylonitic fabrics attributed to dextral transpression on the post-accretionary western Idaho shear zone

(McClelland et al., 2000; Tikoff et al., 2001; Giorgis and Tikoff,

2004). The timing of deformation in this area (ca. 120-90 Ma) is based on U–Pb zircon ages of syntectonic plutons emplaced into the terrane boundary (Manduca et al., 1993; Giorgis et al., 2008).

Figure 1. Simplified tectonic map of the North American Cordillera showing allochthonous arc terranes (Wrangellia, Stikinia, Wallowa) and assemblages related to the Laurentian margin (e.g., Baker, Olds Ferry, Izee of Blue Mountains province; Silberling et al., 1984, 1987). Gray- shaded belts of concentrated metamorphism, magmatism, and deformation in west-central Idaho (SRSZ; Lund and Snee, 1988) and southeastern British Columbia (Omineca Belt; Monger et al., 1982) are discussed in the text. State abbreviations: CA—California, ID—Idaho, MT—Montana, NV—Nevada, OR—Oregon, WA—Washington, WY—Wyoming. Area covered in Fig. 2 shown by bold red outline. Modified from McClelland et al. (2000).

Figure 2. Geology of west-central Idaho. Sri-0.706 isopleth runs north from McCall and enters the Salmon River canyon ~15 km east of Riggins. Arc volcanogenic rocks of the Wallowa terrane, Riggins Group, and Pollock Mountain amphibolite are juxtaposed along east-southeast- dipping thrust faults which crosscut metamorphic isograds (Hamilton, 1960, 1969) and foliation (Gray and Oldow, 2005) in the region. Fault abbreviations: HGF—Heavens Gate fault, RRT—Rapid River thrust, PMT— Pollock Mountain thrust. 40Ar/39Ar localities (R, D) and plateau ages of

Snee et al. (1995): R7-hbl=116±0.6 Ma; R11-hbl=84.9±0.3 Ma; R12- bio=82.6±0.4 Ma; R16-hbl=Ar loss/no plateau; R17-hbl=106.8±0.5 Ma; R30-hbl=118.1±0.6 Ma; D81-1-hbl= 145.1±1.5 Ma. 147Sm/144Nd garnet localities of Getty et al. (1993) shown for single-stage (#598=128±3 Ma; also 40Ar/39Ar-hbl=119±2 Ma) and two-stage (#422=pre-144 Ma; i.e., from apparent “ages” of ~144, 141, 136 Ma) growth. 147Sm/144Nd garnet localities #48 (112.5±1.5 Ma), #07a (113±35 Ma), #03b (136.9±3.5 Ma), and #23 (core=135±2.4 Ma; rim=123±1.3 Ma) are from McKay (2011). 238U/206Pb zircon locality HG-01 (136±1.0 Ma) and LC-02 (111.9±0.9 Ma) are shown from this study (Ch. 5), and K92-8 (114.4±2.2 Ma) is shown from Unruh et al. (2008). Metamorphic isograds are redrawn from Hamilton (1960, 1969). Location of map areas covered in Figs. 3a, 8, 23, 36, and 40 are indicated by red boxes. Geologic map compiled from Hamilton (1969), Aliberti (1988), Blake (1991), Vallier (1998), Gray and Oldow (2005), Giorgis et al. (2006), and Blake et al. (2009).

In the Riggins/Salmon River canyon region ~50 km north of McCall

(Fig. 2), the timing of deformation is weakly constrained due to the lack of age control on volcanogenic and plutonic rocks spanning the arc-craton transition (e.g., Hamilton, 1969a; Blake, 1991). There, the interval of penetrative deformation is approximated by 40Ar/39Ar cooling data on amphibolite facies metamorphic tectonites (Snee et al., 1995) exposed west-northwest of the Sri-0.706 isopleth. These data, combined with new U–Pb (zircon) ages reported here on deformed plutonic rocks in the Salmon River canyon, refine the timing of contractional deformation along the Riggins segment of the arc-craton boundary.

With a depth exceeding 1.5 km, the ~east-west Salmon River canyon

o intersects the Sri-0.706 isopleth at ~90 (Fleck and Criss, 2004) and exposes an otherwise inaccessible portion of the arc-craton boundary.

Despite excellent exposure, little information exists on the age and structural character of local plutonic rocks; as such, deformation constraints have relied principally on U–Pb ages furnished from the

McCall area. Early investigations in the Salmon River canyon described

40Ar/39Ar thermal histories of tonalitic plutons (Snee et al., 1995) and

mesoscopic structures therein (Blake, 1991), but studies uniting both

U–Pb geochronology and detailed structural observations have yet to emerge from the region. By integrating these disciplines, this study documents the internal architecture and age of deformation for the late Early Cretaceous Crevice pluton, exposed along the Salmon River

~15 km east of Riggins (Fig. 2). For regional context, we synthesize existing geochronological and structural data from west-central Idaho in an attempt to correlate tectonite fabrics in the Crevice pluton

(Gray et al., 2010) with mesoscopic structures formed west of the Sri-

0.706 isopleth (Onasch, 1977, 1987; Gray, 2001; Gray and Oldow, 2005).

Laser ablation-inductively coupled plasma-mass spectrometry (LA–ICP–

MS) was used in this study to determine zircon ages from the Crevice pluton and a neighboring intrusion, the Looking Glass pluton (Blake,

1991). Both plutons are deformed by steep to moderate northerly striking gneissic foliations typical of arc-craton boundary segments in the north (Slate Creek—Gospel Hump Wilderness; Lund and Snee, 1988) and south (McCall area; Manduca et al., 1993). To date, mesoscopic structures in the Salmon River canyon have been linked to formation of the western Idaho shear zone (e.g., Blake et al., 2009), described by

Giorgis et al. (2005) as a steep north-south-striking mylonitic belt spatially coincident with the arc-craton isotopic transition.

We emphasize that steep north-south-striking zones of high shear strain are not restricted to mid-Cretaceous plutonic rocks emplaced into the arc-craton boundary, a fundamental tenet of tectonic models proposed for west-central Idaho (e.g., Tikoff et al., 2001; Gray and

Oldow, 2005; Giorgis et al., 2008; Blake et al., 2009). Our structural

analysis in the Riggins region shows that similarly oriented high- strain fabrics exist in arc volcanogenic and carbonate rocks 10-25 km west of the boundary. Although reoriented by late-stage regional folding, these structures are continuous across the arc-craton transition and show no evidence of overprinting related to an orogen- scale ductile shear zone (cf. Ramsay and Graham, 1970; e.g., Coast shear zone of Klepeis et al., 1998). As such, we suggest that post-105

Ma tectonites in the Crevice pluton record part of a progressive deformation history possibly extending back into the Late Jurassic.

Tectonic Setting

Accreted terranes

Lower greenschist to upper amphibolite facies volcanogenic and carbonate rocks west of the arc-craton boundary (Plates 1, 2) are typically correlated with the Wallowa terrane (Onasch, 1977; Sarewitz,

1982; Aliberti, 1988), an allochthonous Permo-Triassic intra-oceanic assemblage exposed in the Seven Devils Mountains (Vallier and Fredley,

1972; Gray and Oldow, 2005), Snake River canyon (Vallier, 1977;

Goldstrand, 1987), and Wallowa Mountains of northeastern Oregon (Nolf,

1966; Follo, 1994). Greenschist volcanic flows exposed in the Rapid

River drainage ~15 km southwest of Riggins (Fig. 2) show trace-element geochemistry supporting formation in a calc-alkaline island-arc environment (Sarewitz, 1983). Tropical marine fauna (Stanley and

Whalen, 1989) and paleomagnetic data (Hillhouse et al., 1982) from

Hells Canyon suggest Middle and Upper Triassic paleolatitudes of ~18o north of the equator. The Wallowa terrane has been interpreted as

representing either the southernmost extension of Wrangellia (e.g.,

Jones et al., 1977) or Stikinia (e.g., Oldow et al., 1989) of the

Alaskan and Canadian Cordillera (Fig. 1).

Volcanogenic rocks in the Salmon River canyon comprise multiple north-northeast-trending belts elongated subparallel to the arc-craton boundary (Blake et al., 2009)(Fig. 3a). Lithologies include mafic orthogneiss (hornblende ± biotite ±garnet ± epidote), garnet amphibolite, and local kyanite-bearing schist (Blake, 1991; Bruce,

1998). Fine- to medium-grained mafic gneisses are commonly associated with boudinaged pegmatite veins and felsic stringers which both parallel and crosscut metamorphic fabric. Approximately 1 km east of

Lake Creek (Fig. 2), the lithologically heterogeneous Pollock Mountain amphibolite (Aliberti, 1988) and enigmatic rocks of the Riggins Group

(Hamilton, 1963b) are juxtaposed across the southeast-dipping Pollock

Mountain thrust fault (Blake, 1991; Selverstone et al., 1992).

According to Hamilton (1963b), volcanic rocks of the Riggins Group represent a deep marine island-arc assemblage of the andesite- keratophyre association (Schermerhorn, 1973) and share petrologic similarities with submarine rocks of the Aleutian Islands. Vallier

(1977) correlated widespread metapelitic rocks (Squaw Creek schist) in the Riggins region with disrupted argillaceous strata of the Baker terrane (Fig. 1), a subduction-accretionary complex exposed in northeastern Oregon (e.g., Vallier, 1995; Schwartz et al., 2011).

Alternatively, the Riggins Group succession may represent a higher-

Figure 3, continued. Geology of the Salmon River canyon. (a.) Fabric element map along the Salmon River corridor between Allison Ck. and Kelly Mtn.; see Fig. 2 for area covered. U–Pb zircon ages are shown for the Crevice and Looking Glass plutons. Note 40Ar/39Ar hornblende cooling age from the Looking Glass pluton [84.9±0.3 Ma: Snee et al., 87 86 1995]. Initial strontium [ Sr/ Sri] sample localities plotted from Criss and Fleck [1987]. Geologic map modified from Blake [1991] and Blake et al. [2009]. (b.) Schematic section drawn through the ~105 Ma Crevice pluton. See Fig. 3a for surface location of section line A—A’.

grade metamorphic equivalent of volcanogenic rocks in the easternmost

Wallowa terrane (e.g., Onasch, 1977; Lund and Snee, 1988).

Western Laurentia

Mid- to upper amphibolite facies metasedimentary rocks exposed along the ancestral North American margin (Plate 2) consist of garnet- sillimanite-biotite schist, calc-silicate gneiss, and minor biotite- rich quartzite; leucosomes of quartz, plagioclase, and potassium- feldspar result in local migmatitic textures (Blake, 1991; Blake et al., 2009). Between Van and Kelly Creeks (Fig. 3a), the presence of sillimanite and fine-grained phyllosilicate minerals distinguish continental metasediments on the east (Kelly Mountain schist) from coarse-grained hornblende gneisses of volcanogenic origin to the west

(Van Ridge gneiss; Blake, 1991). Given its geographic location, the

Kelly Mountain schist may correlate with high-grade metasedimentary rocks of the Mesoproterozoic Belt-Purcell Supergroup (e.g., Winston,

1986b) or Neoproterozoic Windermere Supergroup (e.g., Lund et al.,

2003), both of which extend southward from British Columbia, Canada, into northern and central Idaho (Fig. 1).

Intrusive rocks

Late Mesozoic plutonism in the Riggins region is less voluminous than in areas surrounding McCall (Fig. 2), where elongate granitic complexes are predominant (e.g., Manduca et al., 1993). Between Kelly

Mountain and Allison Creek in the Salmon River canyon (Fig. 3a), Blake

(1991) mapped a variety of undated intrusive rocks within the more prevalent arc volcanogenic and continental metasedimentary rocks described above. In this study, we investigated the Crevice and

Looking Glass plutons (U-Pb ages previously unknown) emplaced into the western margin of Laurentia.

The Crevice pluton includes biotite granodiorite and granite that locally contain deformed country rock screens and elongate xenoliths derived from the Kelly Mountain schist (pCk unit, Fig. 3b; Blake et al., 2009). Grain size varies from medium to coarsely crystalline; within pegmatitic rocks, potassium-feldspar megacrysts exceed 15 cm in length. Millimeter-scale, sub- to euhedral garnet dodecahedra and aggregate clusters ~1 cm in diameter are randomly disseminated throughout the pluton. The Looking Glass pluton consists of medium to coarsely crystalline melanocratic tonalite and tonalite orthogneiss.

Along its margins, aligned biotite and hornblende form a magmatic crystallization foliation which anastomoses through a quartz and plagioclase dominated matrix (Blake, 1991). Other undated granitoid units in the Salmon River canyon include the Spring Creek gneiss,

Partridge Creek gneiss, and Keating Ridge gneiss, which form ~1 km- wide belts oriented subparallel to the arc-craton boundary (Fig. 3a).

Jura-Cretaceous tectonism

Garnet whole-rock Sm–Nd ages from tectonized amphibolite ~25 km southwest of Riggins suggest collision of the Wallowa terrane with western Laurentia prior to 128±3 Ma (Getty et al., 1993; sample locality #598, Fig. 2). Late Jurassic[?] to late Early Cretaceous terrane accretion led to formation of the Salmon River suture zone

(SRSZ: Lund and Snee, 1988), a broad north-northeast-striking deformed belt (>25 km-wide) straddling the arc-craton juncture (Fig. 1).

According to McClelland et al. (2000), suturing was followed by mid-

Cretaceous transpressional deformation on the narrower (<10 km-wide) western Idaho shear zone (WISZ). Although the superposition of mylonitic fabric on suture zone structures is not documented, the WISZ and SRSZ are viewed as spatially overlapping but temporally distinct tectonic elements in west-central Idaho (McClelland et al., 2000;

Tikoff et al., 2001; Giorgis et al., 2007; Blake et al., 2009). This concept of ‘overlapping Jura-Cretaceous orogens’ follows the early premise of Hamilton (1963a), and plays a fundamental role in tectonic models proposed for the central North American Cordillera (e.g., Gray and Oldow, 2005; McClelland and Oldow, 2007; Giorgis et al., 2005).

Geochronology

Crevice pluton

Medium-grained biotite granodiorite of the Crevice pluton was sampled from riverside exposures north and south of Manning Bridge

(Figs. I-3d, 3a; Plate 2), which crosses the Salmon River ~2 km east of the Sri-0.706 isopleth (Criss and Fleck, 1987; Fleck and Criss,

2004). Zircons from both sample localities show core-to-rim zoning in

CL, with many displaying bright cores surrounded by dark rims (Fig.

4a). In most cases, cores and rims are indistinguishable in age, but

Figure 4. SEM–CL images of representative zircons analyzed. (a.) Crevice pluton: 11KGCp01. (b.) Looking Glass pluton: 11KGLGp03. Individual analytical spots and ages are shown for selected grains.

we also identified a few distinctly older inherited cores (Table 1), which primarily yielded Mesozoic ages (ca. 140-250 Ma) along with two individual ages of 479 and 1451 Ma. Calculated weighted mean ages

(Fig. 5a, b) based on the analyses of rims and age-equivalent cores are 105.3±3.0 Ma (11KGCp01) and 103.9±2.0 Ma (11KGCp02). These ages are within error of one another and provide an approximate late Early

Cretaceous crystallization age for the Crevice pluton.

TABLE 1. Summary of U–Pb zircon geochronology. SAMPLE 238U 1 sigma 207Pb 1 sigma Th 206/238 1 sigma 207/206 1 sigma NUMBER 206Pb % error 206Pb % error U age abs err age abs err

11KGCp01 (Kc) Crevice pluton: biotite granodiorite; N45.24112, W116.07031 11KGCp01_1a 59.72 1.2% 0.0492 0.7% 0.16 107.0 1.3 155.7 15.5 11KGCp01_2a 58.66 2.4% 0.0477 2.8% 0.37 109.0 2.6 84.8 65.1 11KGCp01_3a 60.05 1.5% 0.0488 0.9% 0.05 106.5 1.5 140.3 21.7 11KGCp01_4a 61.03 1.4% 0.0502 1.7% 0.34 104.8 1.5 204.7 39.4 11KGCp01_5a 61.01 1.3% 0.0490 1.1% 0.04 104.8 1.3 147.1 24.9 11KGCp01_6a 61.40 1.3% 0.0487 1.1% 0.15 104.1 1.3 133.0 26.4 11KGCp01_6b 61.09 1.7% 0.0477 1.6% 0.13 104.7 1.7 84.0 37.1 11KGCp01_7a 27.66 1.5% 0.0512 1.3% 0.56 229.0 3.3 248.9 28.9 11KGCp01_8a 41.87 1.9% 0.0487 1.8% 0.25 152.2 2.8 133.2 41.6 11KGCp01_10 60.44 1.8% 0.0508 2.0% 0.29 105.8 1.8 233.4 45.1 11KGCp01_11a 61.08 1.8% 0.0472 1.1% 0.06 104.7 1.8 59.4 26.2 11KGCp01_11b 62.79 2.0% 0.0506 2.5% 0.43 101.9 2.1 222.4 57.0 11KGCp01_12a 52.48 1.6% 0.0484 1.6% 0.68 121.7 1.9 116.9 37.2 11KGCp01_13a 44.76 1.4% 0.0501 1.4% 0.97 142.4 2.0 200.5 31.8

11KGCp02 (Kc) Crevice pluton: biotite granodiorite; N45.40116, W116.11521 11KGCp02_1a 62.39 1.3% 0.0480 1.0% 0.13 102.5 1.3 99.0 24.2 11KGCp02_2a 61.44 1.6% 0.0467 2.0% 0.38 104.1 1.7 32.5 47.5 11KGCp02_4a 55.49 0.9% 0.0483 0.4% 0.15 115.1 1.1 114.2 9.9 11KGCp02_4b 12.97 1.2% 0.0569 0.7% 0.27 478.7 5.5 488.9 14.8 11KGCp02_6a 61.49 1.6% 0.0492 0.7% 0.12 104.0 1.7 155.4 16.9 11KGCp02_7a 45.18 1.5% 0.0484 1.5% 0.21 141.1 2.1 118.2 34.6 11KGCp02_8b 61.93 1.7% 0.0499 2.2% 0.47 103.3 1.7 190.3 49.7 11KGCp02_9a 60.00 1.1% 0.0491 0.5% 0.09 106.6 1.2 154.6 11.4 11KGCp02_10a 58.84 1.1% 0.0485 0.5% 0.16 108.6 1.2 122.3 12.9 11KGCp02_10b 27.28 1.9% 0.0517 2.2% 0.51 232.1 4.3 271.0 50.2 11KGCp02_11a 62.99 1.5% 0.0495 1.8% 0.33 101.5 1.5 173.2 40.5

11KGLGp03 (Klg) Looking Glass pluton: hbl-bio tonalite; N45.40876, W116.12582 11KGLGp01_1a 72.88 1.9% 0.0467 1.7% 0.78 87.8 1.7 34.2 40.1 11KGLGp01_2a 67.58 1.5% 0.0466 1.4% 0.28 94.7 1.4 31.3 33.5 11KGLGp01_3a 68.82 1.6% 0.0497 1.5% 0.27 93.0 1.5 181.0 34.6 11KGLGp01_4a 69.00 1.5% 0.0469 1.5% 0.40 92.8 1.3 44.2 35.8 11KGLGp01_5a 68.20 1.2% 0.0511 1.3% 0.68 93.8 1.2 245.0 30.4 11KGLGp01_6a 68.22 1.8% 0.0466 2.2% 0.27 93.8 1.7 30.1 52.6 11KGLGp01_7a 70.03 1.6% 0.0488 1.7% 0.30 91.4 1.4 138.3 39.3 11KGLGp01_8a 71.18 1.6% 0.0489 2.0% 0.25 89.9 1.4 143.4 45.7 11KGLGp01_8b 70.53 1.2% 0.0477 0.9% 0.36 90.8 1.1 83.6 21.7 11KGLGp01_9a 69.90 1.3% 0.0474 1.3% 0.34 91.6 1.2 70.9 30.1 11KGLGp01_11a 70.43 2.0% 0.0476 1.2% 0.30 90.9 1.8 79.2 28.9 11KGLGp01_12a 68.89 1.7% 0.0467 2.0% 0.21 92.9 1.6 35.3 46.3 11KGLGp01_13a 73.41 1.9% 0.0495 1.4% 0.65 87.2 1.7 171.9 33.2 11KGLGp01_14a 70.19 2.0% 0.0481 2.7% 0.44 91.2 1.8 106.2 63.4

Crevice pluton

Looking Glass pluton

Figure 5. U–Pb age systematics of zircons analyzed; WSU–RIGL. (a.) Crevice pluton: 11KGCp01. (b.) Crevice pluton: 11KGCp02. (c.) Looking Glass pluton. Tera–Wasserburg concordia diagrams are shown on the left, and weighted mean 206Pb/238U age plots are shown on the right.

Looking Glass pluton

Medium-grained tonalitic orthogneiss of the Looking Glass pluton was sampled opposite the mouth of Partridge Creek on the north side of the Salmon River (Fig. 3a; Plate 2). Zircons generally show faint oscillatory zoning in CL, and most grains lack distinct cores (Fig.

4b). Analyses are clustered within a few million years of 92 Ma and yield a weighted mean early Late Cretaceous crystallization age of

91.7±2.4 Ma (11KGLGp03; Fig. 5c).

Structures of the Riggins region

Mesoscopic structures

Across western portions of the SRSZ (Fig. 2; Plate 1), the sequential development of mesoscopic structures is recorded in metamorphic rocks of the eastern Wallowa terrane and correlative[?] units. Polyphase fabric elements designated S1, S2, and S3 (foliation) and L1, L2, and L3 (lineation) in order of decreasing age are described here, together with structures of the Salmon River canyon, to place deformation in the Crevice pluton into a regional framework.

S1-L1. The most conspicuous structure in the Riggins region consists of a northerly striking (northwest to northeast), variably dipping

(shallow to steep), penetrative synmetamorphic foliation (S1; Figs. 6a;

7a, b; Plate 1) that is commonly defined by aligned chlorite or biotite. Relict igneous and sedimentary textures are preserved in some rocks of low metamorphic grade (e.g., Rapid River drainage), but are generally obscured by this early fabric element. In westernmost exposures along Heavens Gate Ridge (Fig. 2), S1 contains a down-dip to

Figure 6. Lower-hemisphere, equal-area stereographic projections of mesoscopic structures in the Riggins region. (a.) S1-L1: regional synmetamorphic fabric elements. (b.) S2-L2: penetrative; commonly observed. (c.) S3-L3: penetrative; locally observed.

Figure 7. Photo-montage of fabric elements in the Riggins region. (a.) Greenschist tectonites of Heavens Gate Ridge. Elongate volcanic clasts along the trace of S1; axial ratios locally exceed 10:1 [long-to-short axes]; pencil for scale. (b.) Trace of S1 on slump block below Heavens Gate; penny for scale. (c.) Hornblende schist of Riggins Group [Squaw Creek schist] displaying mineral lineation [L1] on S1; note coplanar, non-lineated amphibole needles overprinting earlier syntectonic growth; coin for scale. Slab prepared from outcrops near Salmon River– Race Creek confluence; Fig. 2. (d.) S2 spaced cleavage cutting S1 in mafic volcanic flows exposed on Heavens Gate Ridge; coin for scale. (e.) S2 ‘slip’ cleavage showing apparent offset [± pressure solution?] in compositional layering and tops-west shear [white arrow] in Squaw Creek schist; Salmon River–Race Creek confluence. (f.) penetrative S3 crenulation cleavage and shallow southeast-plunging crenulation lineation [L3] on S1; Squaw Creek schist, Race Creek; coin for scale.

southeast-plunging stretching lineation (L1) defined by elongate volcanic and carbonate clasts (Figs. 6a, 7a). Strong linear-planar fabric is also present in overlying Late Triassic carbonate rocks, which locally are mylonitized. Whereas foliation in underlying volcaniclastic rocks shows moderate easterly dips, S1 in carbonate ranges from subhorizontal to vertical due to late-stage folding; as such, stretching lineations on S1 plunge variably (shallow to steep).

Near the Little Salmon—Salmon River confluence (Fig. 2), stretching lineations are expressed by aligned muscovite, biotite, and hornblende in the Squaw Creek schist (Fig. 7c). L-S tectonites are traced >10 km east of Riggins into the Salmon River canyon, where higher-grade schists and gneisses of the Riggins Group are encountered.

S2-L2. At many localities, a northerly striking penetrative

o foliation (S2; Fig. 6b) cuts S1 at a shallow angle (generally <20 ;

Fig. 7d). Foliation morphology is dependent on bulk composition and texture of the host rock (cf. Tobish and Paterson, 1988), and changes are noted accordingly. Within fine-grained micaceous rocks of the

Squaw Creek schist, S2 is defined by crenulation cleavage; in coarse

volcaniclastic rocks farther west (Wild Sheep Creek Fm.; Vallier,

1977), S2 forms a spaced cleavage. Across Heavens Gate Ridge, S2 spacing is dictated by the size of triaxially-deformed lithic clasts lying in the flattening plane of S1. Along the Salmon River ~5 km north of Riggins, spaced cleavage is axial-planar to outcrop-scale isoclinal folds developed in the Squaw Creek schist; there, displacement along S2 shear domains records tops-southwest motion (Fig. 7e). This fabric element corresponds to S1b of Gray and Oldow (2005) and the oblique

‘slip cleavage’ of Hamilton (1963b), which varies from incipient to well developed. Shallow to moderately southeast-plunging intersection or crenulation lineations (L2) are locally observed on S1 (Fig. 6b).

S3-L3. Steep north-northwest-striking spaced or crenulation cleavage

(S3; Fig. 6c) is locally superimposed on S1 and S2, consistent with the

S3 foliation of Gray and Oldow (2005). This fabric element occupies an axial-planar position to local upright, symmetric, outcrop-scale folds in the Riggins Group. Aligned chlorite flakes are locally observed inside S3 cleavage domains (Onasch, 1977). Within fine-grained schist, southeast-plunging crenulation lineations (L3; Fig. 7f) are oriented subparallel to fold axes of major post-metamorphic structures in the region (e.g., Riggins synform; Onasch, 1977, 1987; Fig. 2). In the

Lake Creek area of the Salmon River canyon, steep east-dipping spaced and crenulation cleavages are axial-planar to the southeast-plunging

(~45O) Lake Creek antiform (Onasch, 1977, 1987; Bruce, 1998).

Salmon River canyon

The first detailed account of mesoscopic structures in the Salmon

River canyon was completed by Blake (1991), who described steep to moderate northerly striking gneissic foliation/schistosity and down- dip stretching lineations in volcanogenic and continental rocks across the arc-craton boundary. According to Blake (1991), compositional layering (S0) is transposed by a pervasive flattening fabric (S1) and only where S0 is deformed in tight-to-isoclinal folds are original textures preserved. Blake (1991) further noted that most units contain symmetrical boudinage and porphyroclasts which lie in S1; tops-to-the- east shear sense is recorded in the Van Ridge gneiss, Keating Ridge gneiss, and Kelly Mountain schist (Fig. 3).

As described earlier, L1-S1 tectonites are traced continuously from

Heavens Gate Ridge in the northeastern Seven Devils Mountains >20 km east into the Salmon River canyon (Fig. 2; Plate 2). Synmetamorphic foliation (S1) changes orientation around the Lake Creek antiform (Fig.

8), where steep southwest-dipping S1 in the Riggins Group (western limb) becomes moderately southeast-dipping (eastern limb). Moderately southeast-dipping gneissic foliation on the eastern limb is maintained within Riggins Group exposures >1.5 km east of Lake Creek before steepening through the overlying Pollock Mountain amphibolite to vertical near the Sri-0.706 isopleth (Figs. 3, 8). On both sides of the isotopic boundary, metamorphic foliation is deformed by northerly trending upright folds (F2 structures; Blake 1991) which result in shallower dip angles locally (Figs. 3b, 9); albeit on a smaller scale, these post-metamorphic structures warp synmetamorphic foliation in the

Figure 8. Fabric element map of the Lake Creek area. Synmetamorphic foliation (S1) traced from the west (Heavens Gate Ridge) is deformed around the post-metamorphic Lake Creek antiform (e.g., Blake et al 2009). See Fig. 2 for area covered. 40Ar/39Ar sample locality R16 (87.9±0.4 Ma; biotite plateau age) plotted from Snee et al. (1995). 147Sm/144Nd garnet localities (#07a: 113±35 Ma; #48: 112.5±1.5 Ma) shown from McKay (2011). Map unit patterns omitted to highlight structural data; Pollock Mountain amphibolite (pma) shows gray-shading consistent with map in Fig. 3a. Modified from Hamilton (1969) and Blake (1991).

Figure 9. Folded synmetamorphic fabric east of Lake Creek antiform. (a.) Arc metavolcanogenic rocks; pcg unit, Fig. 3a; hammer for scale. (b.) Continental metasedimentary rocks; pCk unit, Fig. 3a; coin for scale. Note felsic veins oriented subparallel to steep east-dipping axial surfaces [red dashed line]. View is to the north in both.

same manner that late-stage map-scale folds deform S1 farther west

(e.g., Riggins synform/Lake Creek antiform; Figs. 2, 8; Plate 2).

Mesoscopic structures east of Lake Creek are characterized by moderate to steep northeast-striking gneissic foliations and southeast-plunging mineral stretching lineations (Figs. 3a, 10).

Strike azimuths reach N60E, and shallow dips are noted in outcrop- scale folds with close geometries. Granitic pegmatite veins containing

Figure 10. Lower-hemisphere, equal-area projections of synmetamorphic structures in the Salmon River canyon. (a.) L-S tectonites external to the Crevice pluton; see Figs. 3a, 8 for data collection areas. (b.) Steep northeast-striking foliation [N2E70SE] with characteristic southeast-plunging mineral stretching lineation; pencil is parallel to lineation. Inset: ~30o angular discordance between oblique lineation plunge [dashed red arrow] and down-dip direction [90o from strike]; Van Ridge gneiss exposed along Salmon River above Spring Bar [Fig. 3a].

strong internal fabric are mostly parallel to foliation, but also crosscut at shallow angles. Stretching lineations in orthogneiss rake

20-30o from the down-dip direction; true down-dip plunges (90o from strike) are observed in fine-grained schist, but are relatively minor when compared to the oblique lineations described above. Across the

Salmon River suture zone, mesoscopic structures are remarkably symmetrical; i.e., from triaxially-deformed lithic clasts on Heavens

Gate Ridge to boudinaged veins, garnet porphyroblasts, and feldspar porphyroclasts in the Salmon River canyon (Fig. 2). In all of these

features, the maximum stretching direction (i.e., X-axis of strain ellipsoid; Ramsay and Huber, 1983) is oriented parallel to the local mineral or lithic clast stretching lineation (Figs. 7a, c, and 10).

Crevice pluton

Field and petrographic observations indicate the late Early

Cretaceous (~105 Ma) Crevice pluton (Fig. 11a) is deformed in two generations of mesoscopic structures, designated S1 and S2 in order of decreasing age. Brittle overprinting is represented by a joint set of regional extent, Jn (Plates 1, 2).

S1. A steep northwest- to northeast-striking penetrative foliation

(S1) is defined in medium-grained granodiorite by subparallel plates of biotite (U–Pb sample locality 11KGCp01; Figs. 3a, 11b). In biotite- rich zones, alternating mafic/felsic compositional layering forms a moderately developed gneissic foliation. Coarser textures in megacrystic granite show weak alignment of potassium-feldspar, biotite, and quartz on the trace of S1. Steep northeast-striking country rock screens (pCk unit; Figs. 3b, 11d) and thin felsic veins parallel to S1 (Fig. 11e) crop out locally. In thin section, the S1 foliation is expressed by clusters of muscovite and biotite which tightly wrap augen-feldspar porphyroclasts; grain-size reduction in quartz occurs along porphyroclast margins, where subgrain development and ribbon structures are observed (Fig. 11c). Optical substructures in feldspar also include undulatory extinction and deformation twinning. Collectively, microstructures are indicative of dynamic

Figure 11. S1 foliation of the Crevice pluton. (a.) The Crevice pluton exposed at Manning Bridge. (b.) In situ outcrop photograph showing aligned biotite plates which define the main tectonite fabric [N10W79NE] at U–Pb sample locality 11KGCp01; pencil tip for scale. (c.) Thin section photomicrograph under crossed-polarized light showing mica sheets wrapping augen feldspar; note sub-grain development along porphyroclast margins, and finer-grained matrix material as compared to feldspar porphyroclast. (d.) Country rock screen west of Manning Bridge oriented parallel to S1; hammer handle [~1 m] lies parallel to foliation. (e.) Lower-hemisphere, equal-area stereographic projections of S1 and felsic veins.

recrystallization associated with early solid-state deformation in the

Crevice pluton (Gray et al., 2010).

Highest strains are recorded by pervasive gneissic fabric southeast of Manning Bridge (U–Pb sample locality 11KGCp02) and within a small satellite[?] body ~0.25 km east of the main intrusion (Fig. 3a). Along the pluton’s western margin near Partridge Creek, igneous textures are massive to weakly foliated (minor biotite alignment). On its eastern margin, tectonite fabric is subparallel to moderately east-dipping schistosity in adjacent continental rocks (pCk unit, Fig. 3b).

Qualitatively, textural variations ranging from massive on the west to penetratively deformed in the east suggest a west-to-east strain gradient may exist inside the Crevice pluton (Fig. 12).

Figure 12. Textural variation in the Crevice pluton. (a.) Massive equigranular granite exposed along western contact with the Kelly Mountain schist; pCk unit in Fig. 3; penny for scale. (b.) Weakly foliated biotite granodiorite of pluton’s interior, viewing to north; note alignment of biotite and feldspar along the trace of S1; coin for scale (c.) Gneissic foliation in the southeast at U–Pb sample locality 11KGCp02, viewing to south; pencil tip aligned parallel to trace of S1.

S2. Shallow east-dipping mylonitic shear zones (1-5 m thick) crosscut the main pluton fabric (S1) and record second generation structures (S2) in the Crevice pluton (Fig. 13a). Mesoscopic shear

zones are variably oriented but consistently show tops-to-the-west

(i.e., reverse) motion as indicated by S-C fabric relations and sigma

(σ) and delta (δ) augen microstructures (Fig. 13b, c). Asymmetric porphyroclasts recording west-over-east kinematics are generally expressed by σ-structures, with fewer δ-type geometries represented.

In contrast to S1 where medium-grained biotite is concentrated along feldspar margins, grains are finer inside the mesoshears and form penetrative mm-scale bands which delineate S2. Together with ribbon quartz and feldspar stringers, these subparallel horizons mark penetrative C-surfaces. Porphyroclast shape fabrics define gently inclined S-surfaces, which form 10-15o angles with bounding C-surfaces

(Fig. 13d). The pre-existing main fabric (S1) transected by narrow mesoscopic shear zones (S2) in the pluton correspond to Type I S-C mylonites (cf. Berthé et al., 1979).

While concentrated locally in the Crevice pluton, late crosscutting features are also recognized in intrusive rocks west of the Sri-0.706 isopleth (e.g., pegmatite west of Allison Creek; Fig. 3a). Giorgis et al. (2007) reported similar ‘small-scale shear zones’ in the Little

Goose Creek complex north of McCall (Fig. 2), which deflect mylonitic fabric there but are relatively minor (B. Tikoff, 2010; pers. com.) compared to the abundant S2 structures described here.

Figure 13. S2 structures of the Crevice pluton. (a.) Tops-to-the-west ductile shear zone [N25W14NE] exposed at U–Pb sample locality 11KGCp01. Note the trace of S1 sweeping into S2 from below [follow red dashed lines]; coin for scale. (b.) Polished slab from shear zone [N35E22SE] west of Manning Bridge; coin for scale; note C-S composite fabrics. Photograph courtesy of R. Conger-Best. (c.) δ-structure inside S2 mesoshears [N67W28NE] east of Manning Bridge; pencil-tip for scale. (d.) σ-structure in subhorizontal shears west of Manning Bridge; coin for scale; microstructures show east over west kinematics. (e.) Lower-hemisphere, equal-area projection of S2.

Jn. Latest structures in the pluton consist of steep northwest- striking fractures (Jn; Fig. 14a) which geometrically correspond to a systematic joint set of regional extent (Gray, 2001). Plumose structures developed on smooth surfaces (Fig. 14b) record a mode-I fracture origin (cf. Irwin, 1959). Joint spacing ranges from cm-scale to ~1 m. Systematic joints transecting the Crevice pluton and surrounding country rocks control northwest-trending segments of the

Salmon River (Fig. 3a) and other major drainages in the region. This strong northwest-southeast structural grain (Fig. 14c) is also reflected by normal fault (half-graben) systems developed in Middle

Miocene volcanic rocks of the western Snake River plain (e.g., Malde,

1959; Bonnichsen and Godchaux, 2002), which project across the Snake

River canyon into the Wallowa Mountains of northeastern Oregon (Smith and Allen, 1941; Hamilton, 1962; Fig. 14d).

Figure 14. Jointing (Jn) in the Crevice pluton. (a.) Systematic joints [N45W85NE] exposed along the Salmon River at Manning Bridge, viewing to the southeast. (b.) Plumose structure documents mode-I fracture nucleation and propagation from northwest to southeast [left to right in photograph]; joint surface is ~5 m across. (c.) Lower-hemisphere, equal-area projection of Jn. (d.) Steep northwest-striking joints crossing the northern Seven Devils Mountains and Hells Canyon of the Snake River (area of gray-shaded parallelogram) share orientations with Jn of the Crevice pluton. Rose diagram shows strike orientations; data weighted by number of joints across the 7, 14, and 21 contours. Joints plotted in the context of Late Cenozoic extensional structures of western Idaho and eastern Oregon. Modified from Hamilton (1962).

Macroscopic structures

Map-scale folds warping S1 west of the Sri-0.706 isopleth include the Riggins synform and Lake Creek antiform (Fig. 2). These broad wavelength (>2 km) structures deform volcanogenic rocks of the Riggins

Group which locally contain ~200 Ma detrital zircons (Lund et al.,

2007). North of the Salmon River canyon, the Slate Creek antiform

(Lund, 1984) deforms the contact separating metavolcanic rocks of the

Seven Devils Group and Riggins Group (Rapid River thrust; Hamilton,

1963b). This west-northwest-vergent imbricate fault system locally cuts metamorphic rocks with ~118 to 109 Ma cooling ages (Snee et al.,

1995). According to Lund and Snee (1988), younger northwest-vergent faults disrupt the Slate Creek antiform. Specifically, the North Fork reverse fault truncates thrusts on the antiform’s eastern limb and juxtaposes 109- and 101-Ma metamorphic belts (Lund et al., 1993).

Clearly, a protracted history of post-metamorphic deformation is recorded in the Riggins region. Shallow east-dipping faults that truncated early metamorphic fabrics (and isograds of Hamilton, 1969a;

Fig. 2) were subsequently folded and then crosscut by later thrust and reverse faults. As Hamilton (1963a) described early on, ‘cross-folds are superimposed on tight folds which deform isoclinal folds, but structural continuity of large units is, in general, maintained.’

Ages of Deformation

Constraints from the Riggins region

Ar–Ar. Age determinations on mesoscopic structures in the Riggins region are based largely on 40Ar/39Ar thermal histories of amphibolite

facies metamorphic tectonites (e.g., Snee et al., 1995). From west-to- east, cooling ages on hornblende, biotite, muscovite, and microcline systematically decrease from ~118 to 88 Ma across subparallel, fault- bounded, northeast-trending belts approaching the arc-craton juncture

(Lund and Snee, 1988; Lund et al., 1993). These ages indicate distinct episodes of metamorphism accompanied by synkinematic hornblende growth, most notably in lineated rocks with 118.1±0.6 Ma, 116.1±0.6

Ma, and 106.8±0.5 Ma cooling ages (Snee et al.; 1995-sample localities

R30, R7, and R17, respectively; Fig. 2). Samples exhibiting two growth patterns (lineated and non-lineated) yield ages between ~101 and 89

Ma; in these rocks, non-lineated (post-kinematic) overprinting of the lineated (synkinematic) population was linked to emplacement of suture zone plutons (Lund and Snee, 1988). 40Ar/39Ar hornblende spectra from deformed tonalitic plutons in the Salmon River canyon indicate cooling through ~82 Ma (Snee et al., 1995; localities R11 (Looking Glass pluton) and R12; Figs. 2 and 3a, respectively).

An older age (Jurassic) of synmetamorphic deformation was suggested by Gray and Oldow (2005) based on field observations in the Rapid

River drainage ~15 km southwest of Riggins. In this area, a post- kinematic tonalite pluton with a 145.1±1.5 Ma 40Ar/39Ar hornblende cooling age (Snee et al. 1995—locality D81; Fig. 2) intruded massive volcanic flows of the eastern Wallowa terrane (Sarewitz 1982). Gray and Oldow (2005) included these rocks in a succession of isoclinally- folded greenschist tectonites exposed on Heavens Gate Ridge (Plate 3).

The Jurassic age of deformation was further supported by pre-144 Ma

core garnet growth in the structurally higher Pollock Mountain amphibolite (Getty et al. 1993—147Sm/144Nd sample locality #422; Fig. 2).

Sm–Nd. Integrated 147Sm/144Nd isotopic, petrographic, and thermo- barometric studies on single-stage garnet from the Rapid River thrust plate indicate tectonic loading, burial, and metamorphism (peak

≤655ºC, 8.5 kbar) in the Riggins region ca. 113±35 Ma (McKay, 2011;

Stowell et al., 2011—sample locality #07a, Fig. 2). According to McKay

(2011), the high uncertainty is not due to analytical error, but poor separation between garnet and whole rock in Sm/Nd space; furthermore,

~113 Ma is compatible with other garnet ages in the Salmon River canyon (112.5±1.5 Ma—locality #48, McKay (2011); Figs. 2, 8; 111±11

Ma; Wilford, 2012). In our structural analysis, we determined that inclusion-rich garnet porphyroblasts at locality #07a overgrow and are partially wrapped by the regional schistosity (S1). Given that linear inclusion trails (Si) are semi-continuous with the external matrix fabric (Se), late Early Cretaceous garnet growth was likely late synkinematic with formation or continuing development of S1 (Fig. 15).

Figure 15. Photomicrograph of single-stage garnet west of the Lake Creek antiform. Subhedral, inclusion-rich garnet porphyroblasts in the Berg Creek amphibolite [147Sm/144Nd sample locality #07a of McKay (2011); see Figs. 2, 8]. (a.) Evenly distributed, linear- to weakly-sigmoidal mineral inclusion trails are semi-continuous with trace of external matrix fabric [S1] across porphyroblast margins. (b.) Inclined trails

suggest tops-to-the-west rotation with respect to enclosing fabric; same rotation sense is preserved in two-stage garnet on the antiform’s eastern limb [locality #48, Fig. 8] and at Pollock Mountain [locality #422, Fig. 2; Getty et al. (1993)]. Sections are cut parallel to L1.

U–Pb. Foliated granodiorite plutons ~25-50 km northeast of Riggins yielded mean 206Pb/238U zircon ages of 113.1±0.6 Ma and 99.4±3.2 Ma, respectively (Unruh et al., 2008—localities 87KL017 and MC22-91). Age of the former pluton, which intruded arc volcanogenic rocks exposed in the Slate Creek drainage, corroborates Sm–Nd results on syntectonic garnet sampled from the Salmon River canyon (~113 Ma; McKay 2011— localities #07a, 48; Fig. 8). Age of the latter pluton, which intruded the arc-craton boundary along the South Fork of the Clearwater River, is within error of sample 11KGCp02 dated from the Crevice pluton

(103.9±2.7 Ma). McClelland and Oldow (2007) reported zircon ages of

90.2±2.7 Ma (tonalite) and 111.0±1.6 Ma (trondhjemite) from deformed plutons exposed near locality MC22-91 of Unruh et al. (2008).

Constraints from the McCall region

U–Pb. Metaplutonic rocks comprising the syntectonic Hazard Creek complex, Little Goose Creek complex, and Payette River tonalite

(Manduca, 1988; Fig. 2) yielded 206Pb/238U crystallization ages of 118±5

Ma, 105.2±1.5 Ma, and 91.5±1.1 Ma, respectively (Manduca et al., 1993;

Giorgis et al., 2008). All three complexes are penetratively deformed by moderate to steep northerly striking solid-state foliations and steeply plunging mineral stretching lineations; eastern portions of the Payette River tonalite contain strong magmatic fabrics (e.g.,

Giorgis et al., 2007). According to Giorgis et al. (2008), a weakly

foliated pegmatite dike (U–Pb age = 90.0±1.4 Ma) in the Little Goose

Creek complex indicates ductile deformation was waning by ~90 Ma.

Ar–Ar. 40Ar/39Ar hornblende plateau ages on deformed plutonic rocks in the McCall area range from ca. 88-79 Ma (Snee and Kuntz, USGS, unpublished data in Snee et al., 1995). These Late Cretaceous ages— errors not provided—were later corroborated by the results of Giorgis et al. (2008), in which biotite from a shallow east-dipping mesoscopic shear zone (equivalent to S2 structure of Crevice pluton?) in the

Little Goose Creek complex yielded an 81.8±0.6 Ma cooling age.

Discussion and Tectonic Implications

Pluton ages, relations, and deformation constraints.

U–Pb zircon ages determined for the late Early Cretaceous Crevice pluton (105.3±3 Ma, 103.9±2.7 Ma) are within error of the Little Goose

Creek complex (105.2±1.5 Ma; Giorgis et al., 2008), which contains the

Sri-0.706 isopleth near McCall (Fig. 2) and extends >50 km north towards the Salmon River canyon (Manduca, 1988; Manduca et al., 1993).

Based on its age, bulk composition, and along-strike projection to the south, the Crevice pluton is likely a continuation of biotite- granodiorite and megacrystic orthogneiss observed in the Little Goose

Creek complex; as such, we interpret the Little Goose Creek complex as projecting north-northeast from McCall and crossing the Salmon River

~0.25 km east of Partridge Creek (Figs. 2, 3a; Plate 2).

The majority of inherited zircon cores in Crevice pluton yielded

Mesozoic ages (ca. 140-250 Ma; Fig. 4, Table 1), although 479 Ma and

1451 Ma ages were also identified. Early Mesozoic ages are unknown

from this immediate portion of autochthonous western Laurentia but are common in the Wallowa terrane of northeast Oregon (e.g., Schwartz et al., 2010, 2011; Kurz et al., 2011). Early Ordovician ages (ca. 479

Ma) are found in detrital zircon populations of the Baker terrane and

Izee basin (Alexander and Schwartz, 2009; LaMaskin et al., 2011).

Possible Laurentian sources for our 1451 Ma zircon include the early

Paleozoic-Neoproterozoic plutonic belt of central Idaho (Lund et al.,

2003) and the Mesoproterozoic Belt Supergroup in northern Idaho

(Winston, 1986b; Ross and Villeneuve, 2003)(Fig. 1). Although the

Crevice pluton lies east of the Sri-0.706 isopleth and was emplaced into continental crust, the interaction of its magmas with oceanic crust (containing Mesozoic zircons) could have been accomplished if accreted terrane rocks were mechanically underplated beneath or tectonically wedged into the Laurentian margin, as described for the

Orofino, Idaho area (Strayer et al., 1989; Lund et al., 2008).

Our Late Cretaceous U–Pb zircon age determined for the Looking Glass pluton (91.7±2.4 Ma) is within error of the petrographically and compositionally similar Payette River tonalite (91.5±1.1 Ma; Giorgis et al., 2008), which intruded the Little Goose Creek complex and continental crust east of McCall along the western border zone of the

Idaho batholith (e.g., Taubeneck, 1971; Manduca et al., 1993)(Fig. 2).

The Late Cretaceous Looking Glass pluton also overlaps in age with weakly deformed tonalite (90.2±2.7 Ma) exposed along-strike to the north (McClelland and Oldow, 2007). Local parallelism between magmatic fabric in the pluton and solid-state foliation in adjacent country rocks (pcg unit; Fig. 3a) suggests that pluton emplacement was

syntectonic (Blake et al., 2009) and represents late-stage orogenic magmatism along the arc-craton accretionary boundary.

New U–Pb (zircon) ages for the Crevice and Looking Glass plutons, combined with existing Ar–Ar (hornblende, biotite) and Sm–Nd (garnet) data from the Salmon River canyon, refine the timing of ductile deformation along the Riggins segment of the boundary. As demonstrated earlier, solid-state fabrics in the Crevice pluton (S1 and S2) post- date ~105 Ma and thus earlier penetrative structures in the Riggins region (S1-L1: pre-118 Ma). Pervasive fabric in the syntectonic Looking

Glass pluton (Blake, 1991; Blake et al., 2009) indicates ductile deformation was ongoing at ~92 Ma. Collectively, these data record a

25+ million year history (pre-118 Ma to post-92 Ma) of contractional tectonism across the Salmon River suture zone (Plates 1, 2).

Overlapping orogens?

An important question arises as to what relation, if any, exists between S1-S2 fabrics in the Crevice pluton and penetrative structures west of the arc-craton boundary. To date, structures east of Lake

Creek (Figs. 2, 8, 10; Plate 2) have been unequivocally linked to mid-

Cretaceous transpressional deformation on the WISZ (Blake 1991; Bruce

1998; Giorgis et al., 2005, 2007, 2008; Blake et al., 2009). This interpretation is based on the apparent transposition of L1-S1 suture zone tectonites across the eastern limb of the Lake Creek antiform

(Blake et al., 2009). Earlier studies in the Riggins region have reported similar overprinting relationships, and bear directly on the

‘overlapping orogens’ issue at hand.

Lund and Snee (1988) initially noted a 10 km-wide ‘high-grade overprinted zone’ parallel to the arc-craton boundary within which garnet porphyroblasts were rolled during formation of a ‘secondary schistosity.’ In the same study, it was further stated that during emplacement and deformation of tonalitic plutons, amphibolite facies country rocks were deformed ‘into a new fabric’ inside the suture zone. Snee et al. (1995) later referred to lineation dominated structures as ‘completely transposing older fabrics’. Although previous studies document multiple metamorphic events represented locally by syn- and post-tectonic mineral growth (Snee et al. 1995—

40Ar/39Ar locality R16; Figs. 2, 8), crosscutting relations supporting fabrics of different ages are not established. At R16, mylonitized rocks of the Riggins Group crop out >2 km west of the WISZ’s proposed western margin (Blake et al., 2009) and ~10 km east of the original tectonic boundary (Rapid River thrust fault) separating Hamilton’s

(1963b) proposed overlapping Jura-Cretaceous orogenic belts.

Possible evidence[?] supporting temporally distinct tectonic events may — or may not — lie in the mineral inclusion trail patterns of rotated garnet porphyroblasts on the eastern limb of the Lake Creek antiform (locality #48; Fig. 8), which are commonly oriented oblique to external matrix fabric (Bruce, 1998; Blake et al., 2009). As stated earlier, linear- to weakly-sigmoidal inclusion trails oblique to S1 are also present in synkinematic garnet west of the antiform (locality

#07a, Figs. 8, 15). Furthermore, snowball garnet recovered from

Pollock Mountain >25 km west of the Sri-0.706 isopleth (locality #03b;

Fig. 2; see also Chapter V, Figs. 36, 37) shows that rotational

patterns are not confined to a narrow north-south-striking belt centered on the Sri-0.706 isopleth. Until internal foliation (Si) and external foliation (Se) are individually dated to establish temporally distinct fabric-forming events, the oblique Si–Se microstructural relationships discussed here are not evidence — on their own — of tectonic overprinting related to a superimposed shear zone.

More recently, Gray and Oldow (2005) stated that pre-145 Ma synmetamorphic structures in the Riggins region are ‘overprinted and transposed’ by a steeply east-dipping mylonitic shear zone. Curiously, this superposition of structural fabrics has yet to be demonstrated directly via overprinting relations observable on meso- or microscopic scales; specifically, the transposition of Late Jurassic[?] to late

Early Cretaceous synmetamorphic structures (western SRSZ) by mid-

Cretaceous mylonites of the WISZ—for example, as shown by Klepeis et al. (1998) along the western boundary of the Coast shear zone ~50 km north of Prince Rupert (Fig. 1). In that study, shear zone development was supported by clear superposition relationships established between penetrative structures of the western thrust belt (>92 Ma schistosity;

S1-2) and crosscutting shear zone fabrics (~57-55 Ma; S3-4). Klepeis et al. (1998) also mapped trondhjemite pegmatites in this area, which from west-to-east become progressively more deformed and rotated into parallelism with the Coast shear zone.

Progressive deformation

Our structural analysis across the arc-craton boundary reveals no evidence of spatially overlapping tectonic belts in the Riggins region

of west-central Idaho. We cannot distinguish between high-strain metamorphic tectonites west of the Sri-0.706 isopleth, and do not recognize any characteristic features or patterns associated with an overprinting ductile shear zone (e.g., external fabric sweeping into a higher strain zone; cf. Ramsay and Graham, 1970). Between Heavens Gate

Ridge and the Crevice pluton area (Fig. 2), we do not observe structural elements supporting temporally distinct orogenic events; i.e., an initial suturing event followed by deformation on a post- accretionary mylonitic shear zone (McClelland et al., 2000).

From west-to-east across the Lake Creek antiform, changes in the attitude of S1 are achieved without any obvious increase in strain or overprinting mylonitic fabric. Instead, superimposed on gneissic foliation of the eastern limb are (1) shallow to moderately northeast- dipping spaced cleavage [S2] defined locally by aligned chlorite, and

(2) subvertical northerly striking boudinaged veins of granitic pegmatite (Fig. 16). The west-to-east steepening of fabrics east of the Lake Creek antiform is largely gradational (Fig. 8), as described for penetrative structures across the syntectonic Hazard Creek complex northwest of McCall (Manduca, 1988; Manduca et al., 1993)(Fig. 2).

On the local scale (i.e., east of Lake Creek), bands of moderately dipping gneissic foliation are observed within more steeply dipping zones (and vice-versa: steep within moderate dips). This variation in attitude is common across the SRSZ, and reflects post-metamorphic folding in the Riggins region (e.g., Onasch, 1987; Gray and Oldow,

2005). Late-stage folds warping S1 result in steep northerly striking high-strain zones (Fig. 17) reminiscent of fabric-fold relations in

Figure 16. Structures on eastern limb of the Lake Creek antiform. (a.) Synmetamorphic foliation [S1=N13W73NE] in Riggins Group [Lightning Creek schist] is crosscut by spaced cleavage [S2=N13W38NE] ~0.3 km east of Lake Creek; see Fig. 8; coin for scale. (b.) Symmetrically- boudinaged pegmatite vein [N26E63SE] cuts synmetamorphic foliation [S1=N40E79SE] obliquely, 0.1 km east of Lake Creek; ~1 m hammer for scale. (c.) Foliation trace [black dashed line] at a low angle to deformed pegmatite shown above; coin for scale.

Figure 17. Subvertical zones of high shear strain west of the arc- craton boundary. (a.) Late Triassic carbonate rocks [S1=N12W76NE] exposed below Heavens Gate Ridge, northeastern Seven Devils Mountains [Fig. 2]; width of view ~10 m. (b.) Hand sample showing mylonitic texture in Late Triassic carbonates; alternating light/dark bands define S1; coin for scale. (c.) Berg Creek amphibolite along steep western limb [S1=N25W86NE] of Lake Creek antiform [locality #07a of McKay (2011); Fig. 8]. Note deformed felsic stringers oriented subparallel to the trace of S1 [black dashed line]; compare with structures shown in Fig. 16b, c. Hammer for scale.

the Salmon River canyon (Fig. 9). Similar overprinting is observed in the western Alps where large-scale nappe refolding and backthrusting severely modified early fabric element orientations (Pfiffner et al.,

2000; Bucher et al., 2003), the Brooks Range collisional orogen of northern Alaska (Vogl, 2002), and in the southeastern Canadian

Cordillera where west-vergent backfolding reorganized early obduction- related mylonitic structures of a composite oceanic terrane (Brown et al., 1986). This Middle Jurassic deformation in the Canadian hinterland (Quesnel Lake area, Fig. 1) evolved in response to oblique convergence between allochthonous oceanic terranes and the ancestral

North American continental margin (Monger and Price, 1979).

Based on our structural observations and synthesis of regional geochronology, we view post-105 Ma fabrics in the Crevice pluton as recording part of a time-transgressive history of ductile deformation operative over the pre-118 Ma to post-92 Ma interval (Fig. 18).

Figure 18. Schematic cross-section illustrating the west-to-east, time-transgressive nature of synmetamorphic deformation across the Salmon River suture zone (SRSZ). See Fig. 2 and text section ‘Ages of deformation’ for field localities where individual age constraints were determined. Fault abbreviations: HGT—Heavens Gate thrust, RRT— Rapid River thrust, PMT—Pollock Mountain thrust. The thrust faulting sequence and timing relationships between faults are uncertain.

In this model, L-S fabric elements were progressively added to the broadening suture zone as mid- to Late Cretaceous contractional deformation migrated eastward across accreted arc terranes onto the western margin of Laurentia. Middle Jurassic to Paleocene regional metamorphism, granitic magmatism, and intense deformation in the

Omineca Belt of southeastern British Columbia (Monger et al. 1982) records a similar history. In both regions, synkinematic metamorphism is attributed to collisions between volcanic arc terranes and the ancestral North American margin (Quesnellia in B.C., Ross et al.,

1985; Wallowa in U.S., Lund and Snee, 1988). Like the SRSZ, this high- strain zone straddles the tectonic boundary separating allochthonous terranes and western Laurentia (Fig. 1), and its characteristic structure and metamorphism are superimposed on both. As described by

Monger et al. (1982), the Omineca Belt is one of the ‘most conspicuous manifestations of Mesozoic orogeny in the Canadian Cordillera.’

Given that we do locally observe subvertical ~north-south-striking fabrics (Fig. 17), it is conceivable that spaced partitioning of transpressional origin (Sanderson and Marchini, 1984) manifests across western portions of the suture zone. In this context, narrow zones of strike-slip[?] displacement may have partitioned strain within a broad fold and thrust belt extending from the northeastern Seven Devils

Mountains >25 km eastward into the Salmon River canyon. Incidentally,

Aliberti (1988) described a prominent zone of flattening oriented

N15E80SE in volcaniclastic rocks along the upper Rapid River drainage

(eastern Wallowa terrane; Fig. 2). Whether these subvertical zones accommodated strike-slip[?] motion between blocks dominated by east-

west shortening is equivocal at present, as local down-dip to steeply southeast-plunging stretching lineations (L1) are consistent with both contractional and transpressional deformation (e.g., Green and Tikoff,

1997). This scenario is further complicated by the lack of (1) field evidence supporting strike-slip motion in the SRSZ, and (2) age control on structures potentially associated with spatial partitioning; specifically, the timing of deformation within steep high-strain zones (Fig. 17) in comparison to that inside adjacent blocks. In our view, late-stage back[?] folding in the Riggins region accounts for the subvertical mylonitic fabrics questioned here.

Conclusions

The ~105 Ma Crevice pluton records three important structural elements in the Riggins region of west-central Idaho: (1) moderate to steep, northwest- to northeast-striking penetrative foliation, (2) shallow tops-to-the-west ductile shear zones, and (3) subvertical northwest-striking mode-I fractures of regional extent. Together, these structures form the pluton’s internal architecture.

U–Pb zircon data from the Crevice and Looking Glass plutons carry local significance in that these are the first crystallization ages reported from the Salmon River canyon, which in our view exposes the most complete east-west crustal section through western Idaho’s arc- craton transition. Together with existing Ar–Ar and Sm–Nd data, new zircon ages serve in constraining deformation along the Riggins segment of the arc—continent collisional boundary; i.e., pre-118 Ma

hornblende cooling (e.g., Snee et al., 1995) to post-92 Ma crystallization of the Looking Glass pluton (reported in this study).

We contend that S1 in the Crevice pluton is genetically related to

S1 of the Riggins region (Gray and Oldow, 2005) and that these structures formed during a protracted tectonic event recorded across the Salmon River suture zone. Late Jurassic[?] to early Late

Cretaceous deformation is preserved along a ~25 km, west-to-east transect extending from Heavens Gate Ridge in the northeastern Seven

Devils Mountains to the Crevice pluton area of the Salmon River canyon

(Fig. 18). Tops-to-the-west mylonitic shear zones (S2), hallmark structures of this pluton, developed after major east-west shortening

(estimated at 30-110 km; Giorgis et al., 2005). Brittle overprinting of ductile fabrics by a subvertical, northwest-striking systematic joint set (Jn) controlled later topographic development of the Salmon

River canyon and vicinity.

In the absence of evidence supporting temporally distinct orogenic belts, we submit that variably oriented high-strain fabrics west of the arc-craton boundary accumulated during a progressive history of contractional deformation. Although our preferred model does not account for the western Idaho shear zone in its traditional overprinting sense (e.g., McClelland et al., 2000; Giorgis et al.,

2008; Blake et al., 2009), we consider the possibility that strain partitioning related to transpressional deformation could manifest across western portions of the SRSZ; that is, as discrete high angle strike-slip fault zones[?] bounding age-equivalent[?] folds in adjacent blocks. In the case that regional L-S tectonites are cut by a

younger transpressional shear zone (WISZ), we posit that the superposition relations documenting such are unsubstantiated to date.

Our structural analysis in the Salmon River canyon suggests that significant along-strike changes occur between Riggins and areas to the north (e.g., McClelland and Oldow, 2007) and south (e.g., Giorgis et al., 2008). The ideas presented in this paper were developed following efforts to identify structures related to overlapping orogenic belts in west-central Idaho; new U–Pb geochronology led to this revised interpretation of the arc-craton boundary. We hope this perspective on western Idaho tectonics will initiate a focused discussion on the diachronous evolution of this and other important transitional zones in the North American Cordillera.

Acknowledgements

Reconnaissance studies in the SRC were supported by RAs to KG under

J.S. Oldow at the University of Idaho (2008) and Texas-Dallas (2009).

Research grants to KG from the Belt Association, Inc. and Tobacco Root

Geological Society provided additional early support (2008). The

Crevice project was funded by Praetorius-Exxon (2010) and LeClerc

(2011) scholarships awarded to KG at Washington State University. A.

Jansen, D. Killingsworth, and D. Wilford are gratefully acknowledged for laboratory assistance, and D.E. Blake for ‘stimulating scientific discussions’. We especially thank R.S. Lewis and J.D. Vervoort for early comments on the manuscript, and appreciate thorough reviews by

T. A. LaMaskin and an anonymous individual.

CHAPTER V. ADDITIONAL GEOCHRONOLOGY, STRUCTURE, & DEFORMATION CONSTRAINTS

Relevance of data

In this section, additional analytical data are presented from the

Salmon River suture zone. First, new zircon U–Pb ages are reported from intrusive rocks exposed on Heavens Gate Ridge and in the Lake

Creek bridge area (Fig. 2). Second, garnet Sm–Nd ages from a separate study are discussed (excerpts from McKay et al., in review). This newer garnet geochronology is derived from amphibolite units mapped on

Pollock Mountain and in the Salmon River canyon (Aliberti, 1988;

Blake, 1991). For regional context, fabric element maps and analyses are provided. Together with structural observations, this additional geochronology is utilized to further constrain the timing of suture zone deformation. Whereas most data presented in Chapter IV relate to eastern parts of the suture, the ages discussed here pertain to more western areas which record an older tectonic history.

Heavens Gate stock (U-Pb zircon)

Setting

The Heavens Gate stock crops out near the southern terminus of

Heavens Gate Ridge, between the elevations of 7500 and 7700 feet, ~1 km east of Windy Saddle in the northeastern Seven Devils Mountains

(Fig. 2; Plate 1). The stock is small (<0.25 km x <0.25 km), poorly exposed, and roughly elliptical in surface form. Topographically, the

Heavens Gate ridgeline forms part of the high north-south drainage divide separating the Snake and Salmon River canyons (Fig. 19).

Figure 19. Heavens Gate Ridge. This prominent topographic feature rises high above the Salmon River [~10 km east of Heavens Gate; left of photograph] and the Snake River [~10 km west of Heavens Gate; right of photograph] in west-central Idaho. Rocks in the foreground crop out along the western margin of the SRSZ. Northern Seven Devils Mountains appear on the skyline in distance. View is to the south-southwest.

Geologically, the Heavens Gate area contains the westernmost exposures of metamorphic tectonites in the Salmon River suture zone

(Gray et al., 2012). At this latitude (45O21’15’’; Fig. 2), the western boundary of the suture zone is defined by the moderately east- northeast-dipping Heavens Gate fault, which carries upper greenschist facies metamorphic tectonites (Fig. 7a, b) over massive to weakly deformed volcanogenic rocks (Fig. 20). Upper- and lower-plate rocks both correlate with the Middle to Upper Triassic Wild Sheep Creek

Formation (T.L. Vallier, 2012, pers. com.), the most widespread unit in the eastern Wallowa terrane (Vallier, 1977, 1995, 1998).

Figure 20. Heavens Gate fault. (a.) Traced from the north, the fault crosses Windy Saddle [upper left of photograph] and swings southeast into a forested area [center] below Cannonball Mountain [northwest flank shown in grassy area, lower right]. Approximate location of the Heavens Gate stock is indicated by yellow star. (b.) Lower-plate volcanic [inset; Dog Lake, Seven Devils Mtns.] and volcaniclastic [Lower Cannon Lake] rocks. (c.) Upper-plate volcaniclastic rocks.

Geochronology

Medium-grained, biotite-bearing hornblende diorite was sampled from the Heavens Gate stock (Fig. 21a). Most zircons recovered are subhedral in form and have rounded edges which result in ellipsoidal geometries. Less common prismatic and rare acicular grains show well- developed growth zoning in CL (Fig. 21b). Core-rim textures are present in a few grains (Fig. 21b; Table 2: e.g., 11KGHG01-6a, 6b).

Figure 21. (a.) Hornblende diorite sampled from the Heavens Gate stock, coin for scale. (b.) SEM–CL images of representative zircons analyzed in this study, with select analytical spots and ages shown.

Dark, xenocrystic cores are differentiated from rims by color contrast

(rims are generally light to colorless). Analysis of both core and rim material resulted in a weighted mean zircon age of 136.0±1.0 Ma (Fig.

22b), which is interpreted as the stock’s crystallization age.

Figure 22. U–Pb age systematics for the Heavens Gate stock [11KGHG01]. (a.) Tera–Wasserburg concordia diagram.

Figure 22, continued. U–Pb age systematics for the Heavens Gate stock [11KGHG01]. (b.) Calculated weighted mean 206Pb/238U age plot.

TABLE 2. Summary of U-Pb zircon geochronology; N45o20’53.5, W116o29’33.3. SAMPLE 238U 1 sigma 207Pb 1 sigma 206/238 1 sigma 207/206 1 sigma NUMBER 206Pb % error 206Pb % error age abs err age abs err 11KGHG1_3a 47.13 1.4% 0.0490 2.2% 135.3 1.8 150.1 49.9 11KGHG1_3b 46.19 1.2% 0.0482 2.3% 138.1 1.6 108.9 53.3 11KGHG1_4a 46.68 1.7% 0.0492 2.9% 136.6 2.3 158.3 67.3 11KGHG1_5b 47.03 1.2% 0.0516 1.9% 135.6 1.6 265.6 43.3 11KGHG1_6a 46.33 1.5% 0.0495 2.9% 137.7 2.1 172.3 67.3 11KGHG1_6b 47.24 1.4% 0.0524 2.2% 135.0 1.8 304.3 50.3 11KGHG1_7b 47.69 1.0% 0.0498 2.1% 133.8 1.4 184.4 47.9 11KGHG1_9a 47.11 0.9% 0.0479 1.5% 135.4 1.2 94.5 36.0 11KGHG1_10a 45.75 1.2% 0.0496 2.4% 139.4 1.7 175.7 54.3 11KGHG1_11a 47.62 1.2% 0.0497 2.0% 134.0 1.6 180.0 46.5 11KGHG1_12a 47.03 1.1% 0.0492 1.9% 135.6 1.5 156.1 43.2 11KGHG1_13a 46.61 1.3% 0.0511 1.7% 136.9 1.8 243.5 39.8 11KGHG1_14a 47.38 1.3% 0.0497 2.0% 134.6 1.7 179.6 46.5 11KGHG1_15a 46.50 0.9% 0.0489 1.8% 137.2 1.3 141.2 41.5 11KGHG1_17a 46.40 1.3% 0.0504 2.0% 137.5 1.8 213.0 46.2 11KGHG1_18a 45.27 1.1% 0.0486 1.9% 140.9 1.6 126.2 43.6 11KGHG1_19a 46.23 1.1% 0.0487 2.0% 138.0 1.5 131.4 45.6 11KGHG1_20a 47.58 1.0% 0.0487 1.4% 134.1 1.3 135.2 32.0 11KGHG1_21a 48.15 1.0% 0.0468 1.9% 132.5 1.3 40.9 46.0

Structure

The Early Cretaceous (~136 Ma) Heavens Gate stock intruded metasedimentary and volcanic rocks along the western margin of the Salmon

River suture zone (Figs. 2, 23a). In this area, upper-plate rocks of the Heavens Gate fault are deformed by penetrative structures described in Chapter IV (Figs. 6, 7). Intrusive rocks contain a strong linear-planar tectonite fabric, consistent with country rocks (S1;

Figs. 21a; 23a, b). Along the ridge, the S1 foliation strikes northerly

(i.e., northwest-to-northeast) and maintains moderate easterly dips

(southeast-to-northeast; Fig. 23c). Systematic changes in the attitude of S1 reflect post-metamorphic folding in the Riggins region (D3 event of Gray and Oldow, 2005). Synmetamorphic structures locally record tops-westerly shear (Fig. 23d, e). However, kinematic indicators in the Heavens Gate area are rare. Strain markers are typically symmetric, as described for other parts of the suture zone (e.g.,

Blake, 1991; Gray et al., 2012). Nevertheless, asymmetric lithic clasts are supported by stepped actinolite fibers observed near the conglomerate–argillite contact below the USFS fire lookout (Fig. 23c, f). In this area, slicken-fibers and elongate lithic clasts define northwest-trending lineations which plunge moderately southeast.

In thin section, the Heavens Gate stock shows evidence of pervasive ductile deformation and associated high strains (Fig. 24). Original igneous textures are nowhere recognized; feldspar grains have been extensively replaced (albitization?), amphiboles are shattered, and matrix minerals have experienced a significant reduction in grain size

Figure 23. Geology of Heavens Gate Ridge. (a.) Map location of the Heavens Gate stock [~136 Ma], indicated by yellow star in lower right. Volcanogenic rocks in this area correlate with Middle and Upper Triassic Wild Sheep Creek Fm. For explanation of map symbols, see Fig. 3a. HGF—Heavens Gate fault. (b.) U–Pb zircon sample locality. The trace of S1 is shown by dashed red line. Hammer and pack for scale. (c.) Macroscopic structural interpretation of the local geology. O O Bedding [S0] in argillite dips ~45 west, and is cut by ~45 east- dipping/axial-planar cleavage [S1.]. To explain local juxtaposition of argillite and conglomerate, a west-vergent thrust fault is inferred; note question marks along fault. (d.) Asymmetric volcanic clasts along the trace of S1 [N2W26NE] suggest tops-to-the-NW shear; outcrop is located along the ridge-top above Papoose Lake; notebook for scale. (e.) Imbricated feldspar porphyroclasts suggest tops-to-the-SW shear; undated mafic hypabyssal[?] intrusion exposed ~1 km NW of the Heavens Gate stock. (f.) Aligned actinolite fibers ornament a fault surface [N12W48NE] below the Heavens Gate fire lookout; slicken-fibers trend N37W, plunge 35SE, and step down to the NW [i.e., direction of black arrow] suggesting tops-to-the-NW slip; approximate location of this conglomerate outcrop is indicated by the black box shown in (c.).

Figure 24. Thin section photomicrographs of the Early Cretaceous [~136 Ma] Heavens Gate stock. (a.) Altered porphyroclasts enveloped by aligned matrix minerals, which define the trace of S1. Note the heavy replacement of feldspar, fractured nature of hornblende, and overall mylonitic texture. (b.) Hornblende ‘fish’ surround augen feldspar. Notice mineral inclination and stacking [also present in (a.)].

(mylonitization). Porphyroclasts are wrapped by penetrative foliation

(S1). Hornblende imbrication suggests tops-to-the-west shear.

Deformation constraints

The lower age boundary on regional synmetamorphic deformation (and thus, the formation of S1–L1 suture zone fabrics) is not established.

To reiterate, the timing of penetrative deformation in the Riggins region (i.e., with respect to arc volcanogenic rocks) has only been approximated using Ar–Ar thermal data (Lund and Snee, 1988; Snee et al., 1995). As stated in Chapter IV, the oldest cooling age associated with synkinematic mineral growth is 118.1±0.6 Ma (e.g., Pollock

Mountain amphibolite, locality R30; Fig. 2). Comparable late Early

Cretaceous 40Ar/39Ar ages obtained from lower-grade rocks of the Riggins

Group fall along the structural transect investigated in this study

(Figs. 2, 25). While these data have served in constraining the uplift

(cooling) history of west-central Idaho, 40Ar/39Ar ages on their own do not directly date the formation of suture zone tectonites. It can only be stated that the onset of synmetamorphic deformation occurred prior to ca. 118 Ma, the time when hornblende (in this case) became closed to significant diffusion of argon gas (cf. Dodson, 1973).

Figure 25. Photomicrograph of amphibolite from the Squaw Creek schist (R7; Fig. 2). Note moderately aligned hornblende (dark prismatic mineral among lighter feldspar and quartz grains). This sample was dated by the 40Ar/39Ar age-spectrum technique and yielded an Early Cretaceous age of 116±0.6 Ma. Modified from Snee et al. (1995).

In contrast to the Ar–Ar dating technique, the U–Pb zircon method

(Chapter III) has valuable applications not only for determining the age of crystallization but also for evaluating the tectonic evolution of complex regions (e.g., Northern Guangxi, South China; Wang et al.,

2006). Together with the hornblende cooling age described above (~118

Ma), zircon crystallization ca. 136 Ma constrains the early history of suture zone deformation (post-136 Ma, and pre-118 Ma). Prior to this investigation, synmetamorphic structures in the Riggins region were thought to be (1) Jurassic in age, (2) unrelated to island-arc

accretion, and (3) overprinted by Late Cretaceous deformtion to the east (Gray and Oldow, 2005). While pre-Cretaceous tectonism in the

Seven Devils region is permissible (e.g., Klamath Mountains orogenic belt; Irwin, 1960; Hamilton, 1963a), the new U–Pb age constraints and structures discussed here refute these earlier interpretations.

Lake Creek dike (U-Pb zircon)

Setting

The Lake Creek dike crops out along the south bank of the Salmon

River, between the elevations of 1800 and 1850 feet, ~0.3 km west of the Lake Creek confluence (Figs. 2, 8, 26; Plate 2). For the reasons outlined in Chapter IV (Overlapping orogens?), this part of the Salmon

River canyon has received notice in recent years (Blake et al., 2009;

Gray et al., 2012). Earlier studies also emphasized field relations in this area (e.g., Onasch, 1977; Blake, 1991; Bruce, 1998), and the reconnaissance geologic map of Hamilton (1969) calls attention to it by noting a ‘gradational change between arbitrarily designated units.’

Geochronology

Fine-grained, biotite-hornblende-quartz diorite was sampled from the

Lake Creek dike (Fig. 27a). Most zircons are subhedral in form and display growth zoning in CL (Fig. 27b). Core-rim textures are present in a few grains (Fig. 27b- laser spots #4a, #4b; Table 3). Cores are typically lighter in color than rims. Analysis of both core and rim material resulted in a weighted mean zircon age of 111.9±0.9 Ma (Fig.

28b), which is interpreted as the dike’s crystallization age.

Figure 26. Field photograph of the Lake Creek dike, looking south. Kimathi and Kathure Nkanata-Gray [left and right] provide scale.

Figure 27. (a.) Biotite-hornblende-quartz diorite from the Lake Creek dike, coin for scale. (b.) SEM–CL images of representative zircons analyzed in this study, with analytical spots and ages shown.

Figure 28. U–Pb age systematics for the Lake Creek dike [11KGLC02]. (a.) Tera-Wasserburg concordia diagram.

Figure 28, continued. U–Pb age systematics for the Lake Creek dike [11KGLC02]. (b.) Calculated weighted mean 206Pb/238U age plot.

TABLE 3. Summary of U-Pb zircon geochronology; N45o23’59.8”, W116o13’15.3”. SAMPLE 238U 1 sigma 207Pb 1 sigma 206/238 1 sigma 207/206 1 sigma NUMBER 206Pb % error 206Pb % error age abs err age abs err 11KGLC2_2a 56.79 0.9% 0.0485 1.7% 112.5 1.1 122.5 39.1 11KGLC2_2b 57.28 0.8% 0.0490 1.3% 111.6 0.9 148.5 29.0 11KGLC2_3a 57.90 1.0% 0.0483 1.3% 110.4 1.1 111.6 29.4 11KGLC2_3b 56.18 1.3% 0.0478 1.8% 113.7 1.5 90.3 41.2 11KGLC2_4a 58.43 0.9% 0.0478 1.2% 109.4 0.9 89.9 27.1 11KGLC2_4b 56.58 0.7% 0.0481 0.7% 112.9 0.8 103.6 17.6 11KGLC2_7a 56.52 3.0% 0.0467 6.4% 113.1 3.4 36.3 147.0 11KGLC2_7b 56.87 3.1% 0.0526 5.7% 112.4 3.5 311.6 124.6 11KGLC2_11a 56.73 0.9% 0.0494 0.9% 112.6 1.0 166.0 21.7 11KGLC2_11b 57.08 0.8% 0.0499 0.6% 112.0 0.9 188.8 13.5

11KGLC2_12b 56.96 2.3% 0.0524 4.6% 112.2 2.6 301.6 102.2

Structure

The late Early Cretaceous (~112 Ma) Lake Creek dike intruded deformed metaplutonic rocks of unknown Mesozoic–Paleozoic age (Figs.

8, 26), which are emplaced into basal conglomerate of the Lightning

Creek schist (Fig. 29a; ‘agglomerate’ of Hamilton 1963b). Consistent with other intrusive rock/country rock contacts observed in this area

(Figs. 16, 17), the dike cuts subvertical fabric at a shallow angle

(<100; Fig. 29b). Macroscopically, rocks reside in the hinge zone of the upright, southeast-plunging Lake Creek antiform (Fig. 8). The hinge zone is disrupted by a series of second-order antiform-synform pairs, which deform L-S tectonites over a distance of ~0.5 km. Post- metamorphic folds are cut by steep northwest-striking brittle[?] surfaces which form narrow (~1 m-wide) anastomosing zones of high shear strain (Fig. 29c). Superimposed on all structures are left- stepping en echelon veins recording clockwise shear (Fig. 29d).

In outcrop, fine-grained leucocratic intrusive rocks in the dike appear weakly deformed (Figs. 27a, 29b). Alignment of hornblende and biotite form a subtle lineation. On the microscopic scale, mineral alignment is more pronounced (Fig. 30a). However, deformation is most clearly expressed by the dominant felsic constituents. Feldspar porphyroclasts, wrapped by mafic laths and needles, are highly fractured and fragmented; in addition, subrounded, elongate quartz shows a granular texture and reduction in grain size (Fig. 30b.).

Figure 29. (a.) Basal conglomerate of the Lightning Creek schist— S1 dips southeast on west limb of meso-fold [hinge zone of Lake Creek antiform]; hammer for scale. (b.) Lake Creek dike attitude: N8E81SE, host rock attitude: N15W88NE; pocket transit [Brunton] for scale. (c.) Northwest-striking shears in hinge; hammer for scale. (d.) En echelon tension gashes [N12E64SE] here are oblique to the high-strain zone.

Figure 30. Thin section photomicrographs of the late Early Cretaceous [~112 Ma] Lake Creek dike. (a.) Aligned mafics wrap feldspar [pinkish- grey minerals]. (b.) Grain comminution in quartz [light grey + gold]; note grain elongation and alignment, from upper right to lower left.

Deformation constraints

Zircon crystallization in the Lake Creek dike ca. 112 Ma places a significant timing constraint on structures formed deep inside the

Salmon River suture zone. This U–Pb age determination derives from rocks intruding the hinge zone of the Lake Creek antiform, which occupies a central position along the transect (Plate 2; Fig. 2). As described in the previous section, and illustrated in Figure 8, the dike is slightly oblique to penetrative foliation wrapped around the antiform. Internally, the Lake Creek dike shows evidence of solid- state deformation (Fig. 30) likely associated with formation of S1-L1 tectonites. However, the dike’s orientation (~north-south strike, steep east-dip) is also subparallel to second-generation fabric linked to antiform development (axial-planar S2; Fig. 8). Whether fabric in the dike relates to S1 or S2 deformation is equivocal at present. Of greater importance is to note that deformation in the Lake Creek area was ongoing ca. 112 Ma. This timing constraint is compatible with late

Early Cretaceous syntectonic garnet growth in the Salmon River canyon

(McKay, 2011), the details of which are elaborated on below.

Garnet microstructures McKay, M.P., Stowell, H.H., Gray, K.D., and Schwartz, J.J. (in review). Metamorphism and pressure-temperature-time paths related to thrusting: Salmon River suture zone, west-central Idaho; Tectonics

This section contains excerpts from an article under review (McKay et al.; citation above). For details on Sm–Nd geochronology included therein (micro-milling methodology and analytical results), the reader is referred to McKay (2011). New garnet ages determined in that study are combined with structural analyses (this study) in efforts to elucidate the timing relationships between garnet growth and fabric development across the arc-continent transition. These data supplement the U–Pb (zircon), Ar–Ar (hornblende), and Sm–Nd (garnet) spatial- temporal deformation constraints synthesized in Chapter IV, and the additional U–Pb (zircon) constraints provided earlier in this chapter.

Salmon River canyon

Locality #07a. An Sm-Nd age of 113±35 Ma was obtained from garnet in the Berg Creek amphibolite exposed along the Salmon River (Figs. 2,

8). According to McKay (2011), a poor isochron resulted from low garnet 147Sm/144Nd ratios and small spread between these ratios and the whole-rocks. The small range in 147Sm/144Nd ratios and resulting high uncertainty are likely due to inclusions that were not removed during partial dissolution. Although the uncertainty is large, ~113 Ma is compatible with the high precision age of 112.5±1.5 Ma determined for sample #48 (McKay 2011; Figs. 8, 33c), which produced a 6-point isochron (whole-rock, matrix, hornblende, and 3 garnet separates).

Furthermore, synkinematic garnet dated from locality #07a using the

Lu-Hf decay system yielded an age of 111±11 Ma (Wilford, 2012).

Along the subvertical western limb of the Lake Creek antiform (Figs.

2, 8, 17c, 31), inclusion-rich garnet porphyroblasts overgrow and are

Figure 31. Sample #07a. (a.) Field locality sampled for Sm–Nd garnet dating in the Berg Creek amphibolite [113±35 Ma; McKay, 2011]. The subvertical trace of S1 [N38W85NE] is indicated by a dashed red line; hammer and orange notebook provide scale. Outcrop shown in Fig. 17a is located <100 m east of this location. (b.) Hand samples showing garnet porphyroblasts along the trace of S1; i.e., aligned parallel to pencil.

partially wrapped by northwest-striking synmetamorphic foliation (S1) penetrating the Berg Creek amphibolite. Medium-grained (1-5 mm), highly fractured, sub- to euhedral garnet dodecahedra and aggregate clusters elongated in the plane of schistosity are characteristic

(Fig. 32). Microfractures in garnet are oriented at high angles to S1, which is locally defined by mm- to cm-scale layering composed of plagioclase + quartz + hornblende ± calcite in a ferromagnesian

Figure 32. Thin section photomicrographs of ca. 113 Ma garnet growth at sample locality #07a. (a.) Garnet poikioblast with rim overgrowth wrapped by external matrix fabric, S1. (b.) Subhedral garnet elongated in the flattening plane of S1. Note microfractures oriented at high angles to wrapping fabric. Both thin sections are cut parallel to the steep southeast-plunging stretching lineation [L1] at this locality.

sedimentary or volcanic protolith (Bruce, 1998). Southeast-plunging stretching lineations (L1; Fig. 8) formed by aligned hornblende and zoisite needles are developed on S1 (Blake, 1991; Gray et al., 2012).

Evenly distributed mineral inclusions (quartz + plagioclase + rutile) form linear to weakly sigmoidal patterns that merge with the trace of

S1 across porphyroblast margins (Fig. 15a). Inclusion trails are typically oriented subparallel to the external matrix fabric (S1); however, subfabrics are also observed oblique to S1 and suggest that some grains underwent ~450 counterclockwise (i.e., tops-to-the-west) rotational strain with respect to enclosing fabric (Fig. 15b).

Locality #48. The moderately southeast-dipping limb of the Lake

Creek antiform (Figs. 8, 33a) contains a heterogeneous sequence of interlayered biotite schist, calc-silicate gneiss, amphibole-bearing biotite gneiss, and garnet amphibolite (Blake et al., 2009). Prior investigations in the Riggins region (Blake, 1991; Selverstone et al.,

1992) placed penetratively deformed rocks at this locality in the

Figure 33. Sample #48. (a.) Field locality sampled for microstructural analysis in the Pollock Mountain amphibolite. Trace of S1 [N24E44SE] indicated by dashed red line; green vegetation on outcrop is ~2m high. (b.) Hand sample of garnet amphibolite obtained from the outcrop at left; coin for scale. (c.) Hand samples of chlorite-biotite schist sampled for Sm–Nd geochronology [112.5±1.5 Ma; McKay, 2011]; coin for scale. In situ exposures were not identified for structural analysis.

hanging wall of the Pollock Mountain thrust fault (Aliberti, 1988), which projects >25 km northeast from Pollock Mountain into the Salmon

River canyon (Fig. 2). In the present study, fine- to medium-grained garnet-biotite amphibolite cropping out ~0.75 km northeast of Ruby

Rapids (Fig. 8) was sampled for microstructural analysis (Fig. 33b).

Here, synmetamorphic foliation (S1) strikes northeast, dips moderately southeast, and contains a down-dip stretching lineation defined by aligned amphibole needles. Felsic intrusions of the Salmon River dike swarm (Blake, 1991) are abundant in this area (Fig. 33a).

On the microscopic scale, subhedral garnet porphyroblasts partially truncate aligned matrix minerals defining the external foliation trace

(Fig. 34a), consistent with garnet/S1 overprinting relations observed

Figure 34. Thin section photomicrographs from garnet amphibolite at structural locality #48. (a.) Subhedral garnet porphyroblast showing inclusion-free rim[?] overgrowth texture, partially wrapped by aligned matrix minerals defining the trace of S1; subcircular inclusion trails occupy the core. (b.) Augen poikiloblasts wrapped by S1. (c.) Linear inclusion trails oblique [~450] to external fabric; arrows indicate counterclockwise rotation with respect to enclosing matrix fabric; compare with rotation in Fig. 15b. (d.) Sigmoidal trail patterns at right angles to external fabric terminate at grain margins. Sections are cut parallel to the local mineral stretching lineation [Fig. 8]. Note: garnet at this locality was not dated in the McKay (2011) study.

to the west (locality #07a). However, elliptical and augen-shaped garnet porphyroblasts are more common east of the antiformal axis.

Linear and sigmoidal inclusion trails oriented at high angles (~45-900) to external matrix fabric show evidence of counterclockwise rotation; inclined trails and porphyroblast trains wrapped by aligned matrix material suggest tops-to-the-west rotation (Fig. 34b, c).

Deformation constraints

At least two distinct timing relationships between porphyroblastic garnet growth and fabric development are recognized in amphibolite

units of the Salmon River canyon (localities #07a and #48; Figs. 2,

8). At both localities, sub- to anhedral garnet poikioblasts (1) truncate aligned matrix minerals defining external fabric, S1=Se, and

(2) are partially wrapped by Se. However, changes in the orientation of included trails (Si) with respect to external fabric (Se) are noted. At locality #07a, Si is typically oriented subparallel to Se and can be traced across crystal margins into the surrounding matrix (Fig. 15a).

0 Si is also observed oblique to Se (~45 ; 15b). At locality #48, Si is generally oblique to Se but can also be traced across crystal margins into the matrix (Fig. 34c). In some grains, Si is orthogonal to Se

(~900) and is truncated by the wrapping matrix mineralogy (Fig. 34d).

Given that linear inclusion trails are semi-continuous with external foliation, garnet growth ca. 112-113 Ma (McKay, 2011) was likely synkinematic with formation or continuing development of the regional schistosity (S1). However, the abrupt discontinuity between spiral trail patterns (Si) and S1 (Se) in some grains suggests that tops-to- the-west rotational strain followed the cessation of garnet growth.

Pollock Mountain

Locality #03b. Sm-Nd isotope data for “two-stage garnet” from the

Pollock Mountain/Cold Springs Saddle area (near locality #422 of Getty et al., 1993; Fig. 2) were collected by McKay (2011) for three garnet and three whole-rock and matrix splits. Isotope ratios combined with whole-rock and matrix material produced a core age of 141.4±2.0 Ma.

According to McKay et al. (in review), this result is based on two points because matrix and whole-rock samples are indistinguishable.

Two additional garnet core separates define a three-point isochron with an age of 136.9±3.5 Ma. Overlapping ages indicate initial garnet growth (stage I) was between ca. 143 and 133 Ma (errors considered).

A steep southeast-dipping homoclinal (or isoclinal?) sequence of alternating garnet-biotite-plagioclase-hornblende amphibolite and felsic hornblende-biotite gneiss extends across the southwest-trending ridge below Pollock Mountain (Fig. 35). Sm-Nd isotopic systematics

Figure 35. Pollock Mountain, west-central Idaho; elevation = 8048′. (a.) Continental flood basalts of Miocene age [Columbia River Basalt Group; Tcrb] cap the summit region. Early Cretaceous metaplutonic rocks [Hazard Creek complex, Khcc] crop out in the foreground. View is to the northeast. (b.) Pollock Mountain amphibolite [pma] occupies the central forested area between Khcc and Permo–Triassic volcanogenic rocks of the Seven Devils Group [PTrsd]. View is to the southwest.

suggest that amphibolite here represents metamorphosed Triassic[?] island-arc volcanic rocks that were altered by sea water (Aliberti,

1988). High-grade metamorphic rocks exposed in this area occupy the upper-plate of the Pollock Mountain thrust fault (Fig. 2). Local mapping (Hamilton, 1969a; Aliberti, 1988) shows the fault cutting lithologic contacts and penetrative structures in the hanging wall

(Fig. 36). These timing relationships are consistent with other west-

Figure 36. Geologic map showing the location of 147Sm–144Nd garnet sample locality #03b (137.0±3.5 Ma; McKay, 2011) in the Cold Springs Saddle- Pollock Mountain area. Northeast-striking belts of garnet-amphibolite and biotite-hornblende orthogneiss (Hazard Creek complex?) occupy the hanging wall block of the Pollock Mountain thrust fault. The late Early Cretaceous (~118.0±5 Ma; Manduca et al., 1993) Hazard Creek complex intruded penetratively deformed amphibolite <0.25 km east of locality #03b. Lithologic contacts modified after Aliberti (1988).

northwest-vergent faults in the region (Heavens Gate fault- Gray and

Oldow, 2005; Rapid River thrust system- Hamilton 1960, 1963b, 1969).

Sub- to euhedral, medium- to coarse-grained (5-15 mm) garnet porphyroblasts characterize the Pollock Mountain amphibolite at this locality (Fig. 37b). In hand sample, elliptical porphyroblasts are wrapped by southeast-dipping synmetamorphic foliation (S1; Fig. 37c) defined by mm-scale hornblende, biotite, and plagioclase layers; aligned hornblende needles form an east-southeast-plunging stretching lineation (L1) on S1. Local L–S tectonites share orientations with penetrative structures in the adjacent Hazard Creek complex (Aliberti,

1988), which intruded amphibolite at Cold Springs Saddle (Fig. 36).

Mineral inclusions (plagioclase + quartz + ilmenite; Getty et al.,

1993; McKay, 2011) form pronounced trails through the cores of two- stage garnet porphyroblasts (Fig. 37e, f). Spiral rotational patterns in deformed cores (stage I; Fig. 37f) suggest tops-to-the-west shear, consistent with kinematic indicators in the Salmon River canyon

(localities #07a, #48; Figs. 8, 15a). Semi-continuous inclusion trails are oblique to external matrix fabric and terminate along core margins where rim overgrowths are encountered (stage II; Fig. 37f); however, some sigmoidal trails extend across crystal margins[?] and appear to merge with the trace of S1 (Fig. 37e, upper left margin of grain).

Figure 37. Sample #03b [a-d] and 10IDMM23 [g]. (a.) Pollock Mountain amphibolite exposed at Cold Springs Saddle. Trace of S1 [N40E47SE] is indicated by dashed red line; pack for scale. (b.) Garnet amphibolite in hand sample, viewing down on S1; coin for scale. (c.) Deformed porphyroblasts wrapped by S1; note strain shadow along right edge of top grain. (d.) Porphyroblasts deformed by S1 and S2[?] foliations. (e.) Backscatter electron [BEI] image captures spiral inclusion trail patterns. (f.) BEI image highlighting tops-west rotation; i.e., counterclockwise motion with respect to the enclosing matrix fabric. (g.)BEI image from sample locality #23 [i.e., #598 of Getty et al., 1993; Fig. 2], collected ~5.5 km northeast of locality #03b; lithology there is kyanite-garnet-biotite schist. Sm-Nd ages from McKay [2011]: Core = 135±2.4 Ma [3-pt. isochron: core, rock matrix, whole rock]; Rim = 123±1.3 Ma [4-pt. isochron: core, rock matrix, whole rock, rim]. Line scan data through two-stage garnet [B-B’] provided in Appendix.

Deformation constraints

At Cold Springs Saddle (Sm-Nd locality #03b; #422 of Getty et al.,

1993), core garnet growth ca. 137 Ma may have predated formation of S1, as evidenced[?] by sigmoidal trail patterns which terminate against inclusion-free rim overgrowths (Fig. 37e; e.g., lower left of garnet) and at high angles to external matrix fabric (Fig. 37f; grain margin in upper right). In this context, remnants of an earlier foliation

(Si=pre-S1?) preserved in garnet cores were overgrown (statically?) and then rotated counterclockwise during later development of the regional schistosity (S1). However, Aliberti (1988) interpreted synkinematic growth here using similar rotational patterns and Si-Se relationships.

Approximately 5.5 km northeast of Cold Springs Saddle, ~123 Ma rims

(McKay, 2011) are wrapped by S1 and appear syn-tectonic (Fig. 37g).

Be that as it may, two-stage garnet porphyroblasts at locality #03b are clearly flattened along the S1 foliation plane (Fig. 37b, c, d).

As alluded to earlier, penetrative deformation in the Cold Springs

Saddle area is recorded by both the Pollock Mountain amphibolite

(Triassic?) and the late Early Cretaceous Hazard Creek complex (Fig.

36). Across the north slopes of Pollock Mountain, screens of deformed amphibolite crop out within gneissic granitic rocks; both map units contain pervasive northeast-striking foliations (S1) and moderately southeast-plunging lineations (L1). Given that S1–L1 fabric elements are continuous across (and thus deform) the lithologic contact

(pma\Khcc; Fig. 36), ductile deformation in the Pollock Mountain area was active ca. 118±5 Ma (i.e., U–Pb age of the syntectonic Hazard

Creek complex; Manduca et al., 1993; Giorgis et al., 2007, 2008).

CHAPTER VI. STRUCTURAL SYNTHESIS OF THE ARC–CONTINENT TRANSITION

Deciphering the tectonic evolution of ancient plate boundaries (and associated mountain belts) requires understanding of the formative environments and sequential development of penetrative structures. As introduced in Chapter I, the Hells Canyon—Seven Devils Mountains—

Salmon River canyon transect is ideal for such, given its tremendous topographic relief and exposure (Fig. I-3). The concentration of erosional activity along the Salmon River, in particular, has revealed ductile deformation features formed at mid-crustal levels (Fig. 38).

Figure 38. Subvertical cliff exposures above the Salmon River. View is to the south, from a ridgeline above Spring Bar campground (Fig. 3a).

The following discussion attempts to synthesize time-transgressive structural fabrics formed across the arc—continent transition, as supported by structural and geochronological data collected in this study and compiled for the greater Riggins region (Fig. 2; Ch. IV).

Ca. 141-118 Ma

Early Cretaceous, high-angle convergence between the Farallon and

North American plates (e.g., Engebretson et al., 1985; Fig. T-1b, c) resulted in ~east-directed underthrusting of the Wallowa terrane beneath the Laurentian margin (Selverstone et al., 1992). During the initial stages of island-arc accretion, contractional strains likely began accumulating along structures dipping eastward into the continental margin. Continued ~east-west shortening led to crustal thickening within the incipient arc—continent collision zone (suture).

Progressive burial of accreted volcanogenic rocks (protoliths of the

Pollock Mountain amphibolite; Aliberti, 1988) set up the mid-crustal pressure-temperature (P-T) conditions necessary for amphibolite facies metamorphism. In the case of garnet sampled at Sm-Nd locality #03b

(Figs. 2, 36, 37), P= 4.25-7.00 kbar and T= 625-6750 C (McKay, 2011).

Core garnet growth ca. 141 to 135 Ma (locality #03b, #23) preceded and overlapped with zircon crystallization in the Heavens Gate stock

(136±1.0 Ma, Fig. 22; U-Pb locality HG-01, Fig. 2). Pervasive ductile deformation and flattening strains recorded by fabrics in both areas

(Figs. 7a, 23, 36, 37) were possibly related to pre~118 Ma deformation in both the Riggins Group (e.g., locality R7, Snee et al., 1995) and

Pollock Mountain amphibolite (locality R30, Snee et al., 1995). As described in Chapter IV, L1-S1 fabrics documented at Heavens Gate Ridge

(Fig. 7a, b) are traced easterly through overlying mylonitic carbonate rocks (Figs. 2; 17a, b) into hornblende-bearing schists of the Riggins

Group (Figs. I-4, 7c). Between ca. 136 and 118 Ma, tectonites had

developed in greenschist and amphibolite facies metamorphic rocks across western portions of the Salmon River suture zone (Fig. 18).

Ca. 118-92 Ma

Sustained ~east-west shortening (Fig. T-1a, b) and boundary-normal strain accumulation across the widening suture zone is evidenced by structures developed in the Hazard Creek complex (Aliberti, 1988;

Manduca et al., 1993). As described in Chapter V, tectonism ca. 118±5

Ma in the Cold Springs Saddle-Pollock Mountain area resulted in northeast-striking gneissic foliations, steeply plunging stretching lineations, and subparallel lithologic contacts (transposed intrusive contacts separating Khcc, pma, and bho map units; Fig. 36). Similar deformation-imposed, meso- to macroscale features are observed in volcanogenic, argillaceous, and carbonate units exposed along and below Heavens Gate Ridge (Gray and Oldow, 2005; Figs. 2, 23). From locality #03b, high-strain fabrics are traced eastward through the

Pollock Mountain amphibolite and Hazard Creek complex into ~105 Ma granitoids of the Little Goose Creek complex (Giorgis et al., 2008).

Approximately 30 km northeast of Pollock Mountain (locality #03b), post-118 Ma deformation is recorded by late syntectonic garnet growth in the Berg Creek amphibolite (locality #07a- McKay, 2011; Wilford,

2012; Figs. 2, 15). Pervasive mylonitic deformation and metamorphism here ca. 113 Ma is compatible with local 106.8±0.5 Ma hornblende cooling (locality R17, Fig. 2; Snee et al., 1995). As demonstrated by

Lund and Snee (1988) and discussed in Chapter IV, argon cooling ages on metamorphic tectonites systematically decrease from west-to-east

(~118 to 101 Ma) across subparallel belts approaching the Sri-0.706 isopleth. Garnet crystallization ca. 113 Ma (locality #07a) and hornblende cooling ca. 107 Ma (locality R17) are consistent with this regional trend, and support a progressive decrease in the age of synmetamorphic deformation across the suture zone (Fig. 18).

The time-transgressive nature of synmetamorphic deformation is also supported by microstructures developed in the 111.9±0.9 Ma Lake Creek dike (U-Pb locality LC-02; Figs. 2, 28, 30), which crops out along the

Salmon River ~2 km southeast of locality #07a (Fig. 8). The dike cuts metaplutonic[?] rocks (Figs. 26, 29b) which experienced an earlier history of penetrative deformation, together with adjacent rocks of the Riggins Group (L1-S1 tectonites in Lake Creek area; Figs. 8, 16,

17c). Ongoing ductile deformation ca. 112 Ma may relate to rotational strains observed in garnet porphyroblasts studding the Berg Creek amphibolite (locality #07a). As previously discussed (Chs. 4, 5), formation of the enveloping matrix foliation overlapped with local

~113 Ma garnet growth (McKay, 2011). Inclusion trail obliquity to external matrix fabric (~45O; Fig. 15b) may have resulted from tops- westerly shear strains associated with continuing development of S1.

Progressive eastward migration of suture zone structures may also

O explain Si-Se relationships (<45 to 90 obliquity) observed in garnet porphyroblasts of the Pollock Mountain amphibolite, ~2 km northeast of

Lake Creek (structural locality #48; Figs. 2, 23a, 34c, d). There, synmetamorphic foliation dips moderately southeast in hanging wall rocks of the Pollock Mountain thrust (Fig. 8; Aliberti, 1988; Blake,

1991). Although this structure was not identified in the present

study, or that of McKay (2011), garnet porphyroblasts rotated through

~900 (Fig. 34d) may relate to counterclockwise rotation associated with west-directed thrusting. At present, movement on the Pollock Mountain thrust is poorly constrained; McKay et al. (in review) propose that

112-113 Ma garnet growth in underlying rocks of the Riggins Group may have resulted from crustal loading/burial associated with the fault.

Age constraints on structures east of locality #48 approaching the

Sr1-0.706 isopleth are provided by intrusive rocks discussed in Chapter

IV (Crevice and Looking Glass plutons; Fig. 3). The Early Cretaceous

Crevice pluton (~105 Ma) marks the easternmost igneous body analyzed in this study (Fig. 2). As previously described, plutonic rocks are deformed in two generations of mesoscopic structures. Early fabric (S1;

Fig. 11) is manifest in adjacent continental metasedimentary rocks

(pCk unit, Fig. 3); superposed mylonitic shear zones (S2; Fig. 12), while concentrated locally, are recognized elsewhere in the Salmon

River canyon. Although bulk strain varies across the Crevice pluton

(Fig. 12), post-105 Ma deformation in this area supports the overall west-to-east evolution and age transgression of structural fabrics.

The Late Cretaceous Looking Glass pluton (91.7±1.2 Ma) represents the youngest intrusion analyzed in this study. Gneissic foliation

(Fig. 39) in outcrops along the Salmon River (sample 11KGLG03; Fig.

3a) offers evidence that ductile deformation was active ca. 92 Ma.

According to D. Blake (pers. comm., 2011), 92 Ma might indicate the time of ductile extension and uplift based on preliminary petro-fabric orientation data from the southwest side of the Crevice. The 40Ar/39Ar

hornblende age of 84.9±0.3 Ma reported from the Looking Glass pluton

(Figs. 2, 3a; R11- Snee et al., 1995) supports this interpretation.

Figure 39. Foliated metatonalite of the ~92 Ma Looking Glass pluton. (a.) Hand sample of 11KGLGp03 [Figs. 3, 5c, Table 1]; note gneissic banding; coin for scale. (b.) Thin section photomicrograph; 11KGLGp03.

western Idaho shear zone

The foregoing discussion offered a synthesis of structural fabrics developed across the arc—continent boundary ca. 141 to 92 Ma. All data utilized in this construction were derived from the Riggins region, with emphasis on fabric element—mineral age relationships established along the transect (Plates 1, 2). Regional tectonic studies, however, must also integrate and (whenever possible) account for data collected in adjacent regions. In the case of the Salmon River suture zone, this requires consideration of structures documented in areas along strike to the north and south. The following discussion considers the south.

Since the publication of McClelland et al. (2000), research in the

McCall area has focused on structures attributed to formation of the western Idaho shear zone (WISZ: e.g., Tikoff et al., 2001; Giorgis and

Tikoff, 2004; Giorgis et al., 2005, 2007, 2008). According to Giorgis et al. (2008), WISZ-related strain is concentrated in the 5-10 km wide

Little Goose Creek complex (~111 to 105 Ma), which centers on the Sri-

0.706 isopleth north of McCall (Fig. 2). However, pervasive mylonitic fabrics extend into adjacent rocks of the Hazard Creek complex (~118

Ma) and Payette River tonalite (~90 Ma). Loosely-defined shear zone margins are typically drawn subparallel to north-south-striking normal faults, which separate metaplutonic complexes and reactivate mid- to

Late Cretaceous mylonite (Giorgis et al., 2006, 2007, 2008; Fig. 40).

Figure 40. Location of the western Idaho shear zone (WISZ) in the McCall area of west-central Idaho; from Giorgis et al. (2008). As illustrated here, the shear zone is contained primarily within the fault-bound Little Goose Creek complex, with minor overlap into the Hazard Creek complex (stippled pattern to the west) and Payette River tonalite (hachured pattern to the east). See Fig. 2 for area covered.

Based on the map symbolism used in Figure 40 (brackets enclosing the

WISZ), the shear zone’s western boundary runs ~north-south through the eastern Hazard Creek complex, and crosses over Granite Mountain ~25 km

northwest of McCall (Fig. 2). In this framework, ~114 Ma orthogneiss at locality K92-8 (Fig. 2; Unruh et al., 2008) lies a few kilometers west of the WISZ. It follows that more western exposures fall outside of the shear zone as well; e.g., garnet-bearing amphibolite and mylonitic orthogneiss in the Cold Springs Saddle-Pollock Mountain area

(Figs. 2, 36). Other high-strain assemblages excluded from the WISZ include the tonalitic sheets and layered mafic gneisses in central- western portions of the Hazard Creek complex (Manduca et al., 1993).

When extrapolated north towards Riggins, the western boundary of the

WISZ (as entertained here; Fig. 40) crosses Patrick Butte before entering the Salmon River canyon near Lake Creek (Figs. 2, 8). This north-south line of projection requires truncation, transposition, or an alternative superposition mechanism whereby older synmetamorphic structures traced from the west (Figs. 6a, 7a, b; 8, 17, 18) are overprinted by the younger deformation belt. As deliberated in Chapter

IV (Progressive deformation), the characteristic structural elements associated with an overprinting shear zone (cf. Ramsay and Graham,

1970; e.g., Coast shear zone of Klepeis et al., 1998) are simply not recognized in the Lake Creek area. Rather, the pervasive northeast- striking fabrics in variably metamorphosed arc-volcanogenic, plutonic, and continental rocks are continuous across the accretionary boundary.

Given that structural overprinting (SRSZ\WISZ) is not recognized in the Salmon River canyon, how do mylonitic fabrics of the McCall region relate in space and time to high-strain tectonites to the north? A key to understanding this relationship is to realize that structures in these regions are developed in fundamentally different crust. For

example, the voluminous calc-alkaline granitic rocks around McCall

(>575 km2; Manduca et al., 1993; Fig. 2) are not present in the Riggins region (Blake et al., 2009). While plutonic rocks do exist (Fig. 3), supracrustal rocks of the Wallowa terrane are predominant (Fig. 41).

Figure 41. Island-arc rocks of the Wallowa terrane, northern Seven Devils Mountains. In the Riggins region, Triassic volcanogenic rocks are abundant across western portions of the Salmon River suture zone. In the McCall region, Cretaceous metaplutonic rocks are predominant.

In the McCall area, Cretaceous plutonism obscures the tectonic contact separating oceanic and continent basement blocks (Manduca, 1988;

Manduca et al., 1992). In the Salmon River canyon, coarse-grained amphibolite gneiss (arc affinity) lies in direct contact with fine- grained sillimanite-bearing schist (continental origin; Blake, 1991).

Despite major compositional differences, both regions record widespread contractional strains which accumulated from west-to-east across the arc—continent boundary. East-west contraction in metasedimentary and volcanic rocks of the Riggins region was accommodated

by folding (Figs. 8, 9), faulting (Fig. 23), and the development of pervasive flattening fabrics (Figs. 7, 17; Appendix 2). Ductile deformation under greenschist and amphibolite conditions transposed bedding, compositional layering, and intrusive contacts in many areas

(e.g., Fig. 36). Early isoclinal folds developed with axial-planar cleavage (Gray and Oldow, 2005). In the McCall region, early isoclinal folding and foliation developed under amphibolite facies conditions in the western Hazard Creek complex (HCC); younger structures in the eastern HCC indicate shortening and vertical flow in response to east- west compression (Manduca et al., 1993). According to Manduca et al.

(1993), most fabrics in the McCall area share a common orientation, making the traditional analysis of overprinting fabrics impossible.

As described in Chapter IV (Ages of Deformation), the timing of penetrative deformation in the McCall region is constrained by mid-

Cretaceous syntectonic plutons emplaced into and west of the arc— continent boundary (Figs. 2, 36, 41). Syntectonic emplacement and dynamic metamorphism in the western HCC ca. 118±5 Ma (Manduca et al.,

1993) is within error of synkinematic garnet growth in the Pollock

Mountain amphibolite (123±1.3 Ma; Sm-Nd locality #23, McKay, 2011;

Figs. 2, 37g). This early deformation in the HCC may relate to post-

136 Ma tectonism in the Heavens Gate area (Fig. 23) and regional pre-

118 Ma synkinematic hornblende growth (Ar-Ar localities R7 and R30-

Snee et al., 1995; locality #598- Getty et al., 1993; Figs. 2, 25,

39). In this scenario, granitic magmas of the HCC were intruded ca.

123 Ma; i.e., near the lower error bound on zircon crystallization

(Manduca et al., 1993). Alternatively, deformation in the HCC may

100

relate to synkinematic garnet growth ca. 113 Ma (locality #07a, McKay,

2011; ~111 Ma, Wilford, 2012; Figs. 2, 8). In this scenario, granitic magmas of the HCC were intruded ca. 113 Ma; i.e., near the upper error bound on zircon crystallization (Manduca et al., 1993; see also Fig.

2, U-Pb locality K92-8 (114.4±2.2 Ma) of Unruh et al., 2008). In either timeframe, penetrative deformation in the HCC was allowably coeval with high-strain structures developing in the Riggins region.

With respect to ~111-105 Ma mylonitic deformation recorded by the

Little Goose Creek complex (LGCC: Manduca et al., 1993; Giorgis et al., 2008), age-equivalent deformation in the Riggins region may manifest in the Pollock Mountain amphibolite (Fig. 8, locality #48;

Fig. 34) and/or the ~112 Ma Lake Creek dike (Figs. 26, 30). As demonstrated in Chapter IV, post-105 Ma ductile deformation fabrics recorded in the late Early Cretaceous Crevice pluton (Figs. 3, 5, 11,

12) overlap in age with pervasive mylonitic structures of the LGCC.

Late-stage deformation in the McCall region is represented by ~90 Ma fabrics in the Payette River tonalite (Manduca et al., 1993; Snee et al., 1995; Giorgis et al., 2008). Coeval deformation in the Riggins region is supported by fabrics in ~age-equivalent intrusive rocks of the syntectonic Looking Glass pluton (91.7±1.2 Ma; Figs. 3, 5, 39).

These Late Cretaceous granitic plutons were emplaced into the

Laurentian margin during the waning stages of east-west contraction

(Giorgis et al., 2008; Blake et al., 2009). Whether penetrative structures developed in response to island-arc accretion (Gray et al.,

2012) or post-accretion tectonism (McClelland et al., 2000; Tikoff et al., 2001; Giorgis et al., 2007, 2008) remains open to debate.

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CHAPTER VII. CONCLUDING REMARKS

Regional geochronologic and structural studies indicate high-strain linear-planar tectonite fabrics accumulated across the island-arc— continent transition between at least ~137 Ma and 92 Ma. Synkinematic garnet growth ca. 137 Ma (Pollock Mountain amphibolite) may represent the onset of penetrative suture zone deformation. Gneissic fabric in the Looking Glass pluton indicates ductile deformation was active ca.

92 Ma. Intervening ages of deformation are constrained by previously undated intrusive rocks (U-Pb zircon; this study), new Sm-Nd garnet ages determined in a collaborative study (McKay et al., in review), and existing Ar-Ar data (Snee et al, 1995) derived from the Riggins region. Prior to this investigation, the sequential development of suture zone structures - in both space and time - was unknown.

Synmetamorphic structures (L1-S1) are continuous across the arc— continent transition. Although modified by late regional deformation

(post-metamorphic folds, faults), L1-S1 fabrics are traced from the

Heavens Gate fault (western boundary of suture zone) eastward through metamorphosed arc-volcanogenic, carbonate, and siliciclastic rocks of the Wallowa terrane into metasedimentary rocks of western Laurentia.

Penetrative fabrics are also recorded by intrusive rocks emplaced into oceanic crust (Heavens Gate stock, Lake Creek dike) and continental crust (Crevice and Looking Glass plutons). Prior to this study, structures of the Riggins region were presumed to be overprinted by post-accretionary fabrics of the western Idaho shear zone (WISZ).

Across the arc—continent boundary segment considered in this study, there is no evidence of tectonic overprinting related to an orogen-

102

scale ductile shear zone. In the Salmon River canyon, L-S tectonites

87 86 extend well beyond the arc-craton isotopic transition ( Sr/ Sri ratios from <0.7045 to >0.707; Fig. 3a) into adjacent areas of the suture zone. This contrasts with mylonitic fabrics described for the McCall region, which according to Giorgis et al. (2005, 2007, 2008) follow

87 86 18 the sharp Sr/ Sri and δ O isotopic gradients recorded by Cretaceous plutonic rocks (Manduca et al., 1992). These along-strike changes in structure warrant explanation; specifically, what accounts for the broad (>25 km) high-strain zone identified in the Riggins region (Gray et al., 2012) in comparison to the narrower[?] deformation belt (~5-10 km) proposed for the south (Giorgis et al., 2005, 2007, 2008)?

When comparing the Riggins and McCall segments of the arc—continent boundary, two fundamental differences are noted: (1) bulk lithology, and (2) extent of mapping. As discussed in Chapters IV and VI, the

McCall region is largely underlain by granitic intrusive material.

With the exception of local country rock screens and pendants (Manduca et al., 1993; Giorgis et al., 2007), metaplutonic rocks are dominant

(orthogneiss). Due to widespread Cretaceous magmatism, the tectonic contact separating accreted oceanic crust and the Laurentian margin no longer exists. Moreover, the supracrustal rocks involved in arc- continent collision are missing. As such, structural mapping in the greater McCall region has taken place within granitic rocks (+ minor screens and pendants) along the disrupted accretionary boundary.

In the Riggins region, island-arc supracrustal units are juxtaposed with ancestral North America (e.g., Blake, 1991; Fig. 3). Granitic intrusions are minor compared to the oceanic and continental rocks

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exposed across the accretionary boundary. Furthermore, penetrative structures that formed in response to Jura-Cretaceous docking of the

Wallowa terrane are abundant. As a consequence, structural mapping is not limited to a narrow belt (5-10 km) of syntectonic plutons centered on the Sri-0.706 isopleth. Owing to the admirable quality of exposures, fabrics are mapped across the entire arc—continent collisional orogen

(~50 km). Thus, the Riggins region offers an unrivalled opportunity to investigate structures across a complete section of the boundary.

The Pollock Mountain area may reveal details addressing the fabric overprinting issue questioned in this study. Located midway between

McCall and Riggins (Fig. 2), this area includes elements of both arc— continent boundary segments discussed above. Near Cold Springs Saddle, garnet-amphibolite interlayers with injection gneiss derived from the

Hazard Creek complex (Aliberti, 1988). Arc-volcanogenic and granitic units are deformed by steep to moderate southeast-dipping gneissic fabrics typical of arc-continent boundary segments to the northeast

(Riggins) and southwest (McCall). In this area, L-S tectonites are 15-

25 km west of the proposed western boundary of the WISZ (Figs. 2, 41).

From Pollock Mountain, gneissic fabrics are traced several kilometers to the east into Cretaceous granitic rocks (Hamilton, 1969a; Manduca,

1988). In this context, synmetamorphic structures of the Salmon River suture zone (Gray et al., 2012) extend directly into mylonitic fabrics assigned to the WISZ (Giorgis et al., 2008). The question remains: On what basis are these structures distinguished? Geologic mapping in areas northwest of McCall may reveal overprinting fabric; at present, evidence points toward a single broad zone of protracted deformation.

104

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Vallier, T.L. 1998. Islands & rapids; a geologic story of Hells Canyon. Confluence Press, Lewiston, Idaho.

Vogl, J.J. 2002. Late orogenic backfolding and extension in the Brooks Range collisional orogen, northern Alaska. Journal of Structural Geology 24: 1753–1776.

Wang, X., Zhou, J., Qiu, J., Zhang, W., Liu, X., and Zhang, G. 2006. LA–ICP–MS U-Pb zircon geochronology of the Neoproterozoic igneous rocks from Northern Guangxi, South China: Implications for tectonic evolution. Precambrian Research 145: 111–130.

Wilford, D. 2012. Lu-Hf geochronology of the Salmon River suture zone, west-central Idaho. M.S. thesis, Washington State University.

Winston, D. 1986b. Sedimentology of the Ravalli Group, Middle Belt Carbonate and Missoula Group, Middle Proterozoic Belt Supergroup, Montana, Idaho and Washington. In Belt Supergroup: A guide to the Proterozoic rocks of western Montana and adjacent areas. Edited by S. Roberts. Montana Bureau of Mines and Geology Special Pub. 94: 85–124.

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APPENDICES

1. ANALOG TO THE WALLOWA TERRANE?: KOHISTAN ISLAND-ARC, NE PAKISTAN

The Kohistan terrane of northern Pakistan (Figs. 1, 2) represents an intra-oceanic island-arc that was obducted onto the Indian plate ca.

75 to 55 Ma (e.g., Searle et al., 1999). From north to south, the arc displays a coherent 30-40 km thick section of metamorphosed volcanic, sedimentary, and plutonic rocks (Gaetani et al., 2004). Widespread arc-supracrustal and basement assemblages in this region are possibly analogous to broad exposures of the Wallowa terrane in west-central

Idaho (Lewis et al., 2012). Although Hamilton (1963b) demonstrated the island-arc chemical signature of local volcanic rocks (Riggins and

Seven Devils Groups), additional geochemistry is needed to show that a complete arc sequence is represented across the Salmon River suture.

Figure 1. Tectonic sketch map of northern Pakistan and surrounding regions. The Cretaceous Kohistan Arc Complex lies between the Main Mantle Thrust [MMT] and Shyok Suture [SS]. From Gaetani et al. (2004).

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Figure 2. Section through the western Himalaya, Hindu Kush, and Pamir. Subduction and closure of the Tethys produced thrust faulting in the Kohistan arc along the Indus Suture Zone. From Searle et al. (2001).

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BIBLIOGRAPHY

Gaetani, M., Burg, J.P., Zanchi, A., and Jan, Q.M. 2004. A geological transect from the Indian plate to the east Hindu Kush, Pakistan. Field Trip Guide Book PR-01, Florence, Italy. Vol.no 1, 32nd International Geological Congress.

Searle, M.P, Hacker, B.R., and Bilham, R. 2001. The Hindu Kush seismic zone as a paradigm for the creation of ultrahigh-pressure diamond- and coesite-bearing continental rocks. Journal of Geology 109: 143-153.

Searle, M.P., Asif Khan, M., Fraser, J.E., Gough, S.J., and Jan, M.Q. 1999. Tectonic evolution of Kohistan-Karakoram collision belt along the Karakoram Highway transect, north Pakistan. Tectonics 18: 929-949.

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2. TRIAXIALLY-DEFORMED LITHIC CLASTS FROM HEAVENS GATE RIDGE

Strain ratios determined from principal strain axes [X = long, Y = intermediate, Z = short]; Trwsc unit, Plate 1. See also Figs. 7a, 23d.

X-Z plane X-Y plane Y-Z plane Ln Ln X Z X/Z X Y X/Y X/Y Y Z Y/Z Y/Z 14.7 3.7 3.97 6.5 3.9 1.67 0.51 5.9 4.1 1.44 0.36 24.0 5.5 4.36 18.6 10.7 1.74 0.55 4.5 1.9 2.37 0.86 11.1 2.8 3.96 18.4 7.2 2.56 0.94 2.5 1.0 2.50 0.92 22.2 5.7 3.89 1.2 0.4 3.00 1.10 1.8 1.0 1.80 0.59 8.1 1.9 4.26 41.2 7.5 5.49 1.70 2.0 1.0 2.00 0.69 2.6 0.7 3.71 16.0 4.0 4.00 1.39 10.5 3.5 3.00 1.10 10.5 3.5 3.00 3.6 1.2 3.00 1.10 5.4 2.6 2.08 0.73 18.6 5.4 3.44 4.2 1.9 2.21 0.79 8.5 1.9 4.47 1.50 6.2 1.7 3.65 20.1 10.3 1.95 0.67 5.1 1.8 2.83 1.04 5.5 4.2 1.31 7.2 2.1 3.43 1.23 3.5 1.9 1.84 0.61

2 FLINN DIAGRAM [1962] K = ∞ PLANE STRAIN

PROLATE 1.5 K = 1

[constriction]

Series1 Ln [X/Y] Ln

0.5 OBLATE [flattening]

0 K = 0 0 0.5 1 1.5 2 Ln [Y/Z]

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3. XRF ANALYSIS OF INTRUSIVE ROCKS SAMPLED FROM HEAVENS GATE RIDGE Data from WSU GeoAnalytical Laboratory

UTM coordinates 10KGHG01 [mafic; deformed]: 0539335-northing, 5023184-easting; NAD83 10KGHG02 [felsic; undeformed]: 0538684-northing, 5022262-easting; 11T

Sample 10KGHG01 10KGHG02

Date 28 -Jan-11 28 -Jan-11

Unnormalized Major Elements (Wt. %): SiO2 53.08 68.63 TiO2 0.937 0.387 Al2O3 18.54 15.68 FeO* 7.52 2.76 MnO 0.156 0.049 MgO 2.74 1.37 CaO 6.61 3.69 Na2O 4.79 4.50 K2O 3.00 1.45 P2O5 0.782 0.126 Sum 98.17 98.64

Normalized Major Elements (Wt. %): SiO2 54.07 69.58 TiO2 0.955 0.392 Al2O3 18.89 15.89 FeO* 7.66 2.80 MnO 0.159 0.049 MgO 2.79 1.39 CaO 6.73 3.74 Na2O 4.88 4.56 K2O 3.06 1.47 P2O5 0.797 0.127 Total 100.00 100.00

Unnormalized Trace Elements (ppm): Ni 15 10 Cr 12 23 Sc 16 6 V 239 54 Ba 396 471 Rb 71 31 Sr 549 426 Zr 129 116 Y 27 9 Nb 6.2 3.8 Ga 18 17 Cu 330 20

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Zn 97 47 Pb 4 7 La 27 16 Ce 55 28 Th 4 2 Nd 28 11 U 1 2

sum tr. 2023 1301 in % 0.20 0.13 sum m+tr 98.37 98.77 M+Toxides 98.42 98.80

Major elements are normalized on a volatile - free basis, with total Fe expressed as FeO. NiO 19.1 13.0 Cr2O3 17.8 34.1 Sc2O3 24.2 9.2 V2O3 350.9 79.7 BaO 442.6 525.8 Rb2O 77.1 34.1 SrO 649.5 503.8 ZrO2 174.1 157.0 Y2O3 34.7 11.8 Nb2O5 8.9 5.4 Ga2O3 24.7 22.9 CuO 412.5 25.2 ZnO 121.4 58.4 PbO 4.0 7.4 La2O3 31.2 19.0 CeO2 67.5 34.2 ThO2 4.3 2.4 Nd2O3 33.1 13.3 U2O3 0.8 1.8 Cs2O 0.0 0.0 As2O5 0.0 0.0 W2O3 0.0 0.0 sum tr. 2498 1558 in % 0.25 0.16 1 0KGHG01

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4. LINE SCAN AND BACKSCATTER ELECTRON IMAGES OF TWO-STAGE GARNET; from McKay et al., in review [Tectonics]

Garnet line scan (left) and BEI (right) displaying spessertine-rich, inclusion-rich cores encased in lower spessertine, inclusion-poor rims. Data obtained from Sm-Nd locality 10IDMM23 (below). Simplified geologic map from McKay (2011). After Lund (2004) and Hamilton (1969).

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5. Tectonic History and Hiking Tour of the Northern Seven Devils Mountains, West-Central Idaho

Modified from Gray (2012); GeoNote45 - Idaho Geological Survey

This extinct volcanic terrain was formed by island-arc eruptions in the ancestral Pacific Ocean, plate tectonic activity, and alpine glaciation. It is accessed by U.S. Forest Service Road #517 off US-HWY

95, approximately one mile south of Riggins, Idaho. Located between the Snake and Salmon Rivers, the northern Seven Devils rise above 9300 feet and comprise the westernmost mountain belt in the central Rocky

Mountains. Glimpses into Hells Canyon, the deepest river-carved gorge in North America, and an occasional mountain goat sighting (Oreamnos americanus; de Blainville 1816) add to this dynamic landscape.

Tectonic History

Island-arc volcanism is represented in the northern Seven Devils

Mountains by a thick sequence of metamorphosed lava flows of Middle and Upper Triassic age (Fig. 1). Massive iron- and magnesium-rich volcanic rocks dominate this part of the Wallowa terrane1, an exotic oceanic assemblage now residing in eastern Oregon and western Idaho.

These rocks were once part of an island chain that existed far off the coast of ancient North America, similar to today’s Mariana Islands in the northwestern Pacific Ocean (Vallier, 1998). During the Late

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Jurassic to Early Cretaceous interval, rocks of the Seven Devils region were accreted to the western edge of North America, which existed at that time ~10 miles east of Riggins, Idaho (Fleck and

Criss, 2004). This slow process of terrane accretion (and continental growth) shifted the Pacific coastline westward towards its present location. In west-central Idaho, the collisional event is recorded across a broad zone of deformation known locally as the Salmon River

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suture (Blake et al., 2009). The island-arc—continent suturing event was followed by vertical uplift along steep northerly trending faults which offset Miocene flood basalts. Geologically speaking, these relatively young structures have played an important role in raising the Seven Devils Mountains to their current elevations (Fig. 2).

Hiking Tour

Stop #1: H av ns at Vista (8429′). From the parking area, follow the footpath ~¼ mile east up onto the ridgeline for spectacular panoramic views into Oregon, Washington, and Montana. At the forest fire lookout (built ca.1978), signposts inform of the surrounding peaks, rivers, and canyons. Below the lookout are moderately inclined, olive-green metasedimentary rocks containing rounded pebbles, cobbles, and boulders. These mixed volcanic ‘clasts’ were likely sourced from underlying lava flows exposed in the Seven Devils Mountains. Many clasts display symmetrical cigar or pancake shapes acquired during metamorphism and deformation related to island-arc—continent collision. Strong [linear-planar] deformation fabrics characterize western portions of the Salmon River suture zone.

Stop #2: Windy Saddl (7606′). Through this narrow wind gap runs the shallow east-northeast-dipping Heavens Gate fault (Gray and Oldow,

2005), which separates deformed rocks viewed at Stop #1 and basaltic lava flows comprising the northern Seven Devils Mountains (Fig. 2).

______

1In this cont xt, th t rm “t rran ” [vs. “t rrain”] refers to a displaced crustal fragment with a geologic history that differs from surrounding areas.

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Only remnants of the fault are preserved here; deeply eroded, reddish- brown volcanic rocks near the kiosk show evidence of high-temperature fluids moving into the fault zone (veins). Most recent motion along the fault postdates[?] the formation of Early Cretaceous linear-planar fabrics on Heavens Gate Ridge and predates the eruption of Miocene basalts that once covered this area (Gualtieri and Simmons, 1978).

Stop #3: Snak Riv r Ov rlook (~8000′). From Windy Saddle, descend on Trail #124, which crosses the East Fork of Sheep Creek (Fig. 2) and passes through an old burn area up onto the ridge overlooking Hells

Canyon. This point is >6000 feet above the Snake River, and offers views into the Wallowa Mountains of northeastern Oregon. On the west side of Hells Canyon, a sharp angular unconformity separates tilted strata of the Wallowa terrane (below) and subhorizontal flood basalts

(above). This prominent erosional surface represents a 200+ million year time gap in local Earth history. Mountain goats are sometimes encountered along this winding trail segment.

Stop #4: H ll Hill (~7100′). In this area, Trail #124 skirts across a steep talus slope consisting of fractured grayish-green volcanic rocks. Look for lighter colored blocks of limestone in the rubble, which may contain the flat-clam Daonella, a shallow marine fossil species (Fig. 1) used to date rocks in the Wallowa terrane. Downslope movement of mechanically weathered material has formed this vast apron of loose, unconsolidated rock. From here, continue hiking towards the

West Fork of Sheep Creek (Fig. 2). Devils Tooth and other peaks rise in the distance (Fig. 3). After crossing the creek, ascend ~1000 feet onto a grassy basalt plateau. Watch for Trail #123 on your left.

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Figure 3. Devils Tooth. Photograph courtesy of T.L. Vallier.

Stop #5: Alpine Lakes (~7600′). Trail #123 branches southeast

(left) near Lily Pad Pond and passes several small cirque basins perched below the northern Seven Devils. Volcanic bedrock hosting these lakes was scoured by mountain glaciers that existed here

~25,000-12,000 years ago; linear grooves carved into local rock surfaces (e.g., Basin Lake; Fig. 2) attest to glacial activity

(scouring) in this area. Perennial springs and snowfields feed into the lakes, some of which are planted seasonally with juvenile rainbow or cutthroat trout by the Idaho Fish & Game (Jones, 2003).

Stop #6: Foot o th D vils (~8200′). Unobstructed views of He

Devil (9393′) and She Devil (9360′), the two highest peaks of this range, compliment the magnificent geology surrounding Sheep Lake. This area exposes massive volcanic rocks of the Middle and Upper Triassic

Wild Sheep Creek Formation. Here, a thick sequence of basaltic lava flows and sills define the western limb of a broad anticline; in cross-section, an upright symmetric fold emerges (‘Devils Arch’, Fig.

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2). Local topographic highs reflect upward arching of thick (>5000 feet) volcanic rocks and block uplift along steep northerly trending normal faults. Subparallel faults in this area are inferred to follow tributary streams of the upper Sheep Creek drainage network (W. Fork), and may represent the southern continuation of a high-angle fault identified along the lower Sheep Creek drainage (Vallier, 1998).

Further Information

This GeoNote is intended to provide a general geologic overview of the northern Seven Devils Mountains. Technical reports and detailed geologic maps are found in the reference list provided.

Acknowledgements

The author thanks R.S. Lewis, J.D. Kauffman, G.W. Grader, and A.

Prazenica (‘Fishtank’) for helpful reviews.

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References Cited

Blake, D.E., K. D. Gray, S. Giorgis, and B. Tikoff, 2009, A tectonic transect through the Salmon River suture zone along the Salmon River Canyon in the Riggins region of west-central Idaho, in J.E. O’Connor, R.J. Dorsey, and I.P. Madin, eds., Volcanoes to Vineyards: Geologic Field Trips through the Dynamic Landscape of the Pacific Northwest: Geological Society of America Field Guide 15, p. 345–372.

Blainville, H. de, 1816, Sur plusieurs espèces d'animaux mammifères, de l'ordre des ruminans: Bulletin des Sciences, par la Société Philomathique de Paris, v. 8: p. 73-82.

Fleck, R.J., and R.E. Criss, 2004, Location, age, and tectonic significance of the western Idaho suture zone (WISZ): U.S. Geological Survey Open-File Report 2004-1039, 48 p.

Gray, K.D., and J.S. Oldow, 2005, Contrasting structural histories of the Salmon River belt and Wallowa terrane: Implications for terrane accretion in northeastern Oregon and west-central Idaho: Geological Society of America Bulletin, v. 117, no. 5/6: p. 687–706.

Gualtieri, J.L., and G.C. Simmons, 1978, Preliminary Geologic map of the Hells Canyon area, Idaho County, Idaho, and Wallowa County, Oregon: U.S. Geological Survey Open-File Report 78-805, scale 1:48,000, 2 sheets.

Jones, G.D., 2003, Hiking Idaho’s Seven Devils—The Complete Guide to Every Trail, Lake, and Peak: Middleton, Idaho, CHJ Publishing, 125 p.

Vallier, T.L., 1998, Islands & Rapids— A Geologic Story of Hells Canyon: Lewiston, Idaho, Confluence Press, 151 p.

Walker, J.D., and J.W. Geissman, 2009, Revised geologic time scale: GSA Today, v. 19, no. 4/5, p. 60.

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