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. ii 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). iv 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 v 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) vi 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 ix Dedication For Mom and our moon.