University of Montana ScholarWorks at University of Montana Graduate Student Theses, Dissertations, & Professional Papers Graduate School 1997 Paleomagnetic and structural analysis of the Wallowa terrane northeast Oregon and west-central Idaho : a reevaluation of the Salmon River suture zone Joshua N. Distler The University of Montana Follow this and additional works at: https://scholarworks.umt.edu/etd Let us know how access to this document benefits ou.y Recommended Citation Distler, Joshua N., "Paleomagnetic and structural analysis of the Wallowa terrane northeast Oregon and west-central Idaho : a reevaluation of the Salmon River suture zone" (1997). Graduate Student Theses, Dissertations, & Professional Papers. 7077. https://scholarworks.umt.edu/etd/7077 This Thesis is brought to you for free and open access by the Graduate School at ScholarWorks at University of Montana. It has been accepted for inclusion in Graduate Student Theses, Dissertations, & Professional Papers by an authorized administrator of ScholarWorks at University of Montana. For more information, please contact [email protected]. Maureen and Mike MANSFIELD LIBRARY The University ofIVLONTANA Pennission is granted by the author to reproduce this material in its entirety, provided that this material is used for scholarly purposes and is properly cited in published works and reports. ** P lease check "Yes" or "No" and provide signature** Yes, I grant permission VT No, I do not grant permission____ Author's Dale 3 / ^ / 9 ^ ____________________________ Any copying for commercial purposes or financial gain may be undertaken only with the author's explicit consent. PALEOMAGNETIC AND STRUCTURAL ANALYSIS OF THE WALLOWA TERRANE, NORTHEAST OREGON AND WEST-CENTRAL IDAHO: A REEVALUATION OF THE SALMON RIVER SUTURE ZONE ly Joshua N. Distier B.A. Amherst College, 1993 presented in partial fulfillment of the requirements for the degree of Master of Science The University of Montana 1997 Approved by: *^teve?i D. Sheriff, Co-Committ@^<^hair Dean, Graduate School Date UMI Number: EP37878 All rights reserved INFORMATION TO ALL USERS The quality of this reproduction is dependent upon the quality of the copy submitted. In the unlikely event that the author did not send a complete manuscript and there are missing pages, these will be noted. Also, if material had to be removed, a note will indicate the deletion. UMT OtaMrtition UMI EP37878 Published by ProQuest LLC (2013). Copyright in the Dissertation held by the Author. Microform Edition © ProQuest LLC. All rights reserved. This work is protected against unauthorized copying under Title 17, United States Code ProQuest LLC. 789 East Eisenhower Parkway P.O. Box 1346 Ann Arbor, Ml 48106 -1346 Distier, Joshua N., M.S., May, 1997 Geology Paleomagnetic and Structural Analysis of the Wallowa Terrane, Northeastern Oregon and West-Central Idaho: A Réévaluation of the Salmon River Suture Directors: Steven D. Sheriff and James W. Sears Paleomagnetic evidence indicates that many Cordilleran terranes have experienced significant clockwise rotation since the Cretaceous. The rotation is often coupled with major dextral faults and evidence of coeval northward translation. Beck (1976, 1980) proposed a "ball-bearing model" to explain this data trend, whereby terranes are rolled northward along the North American cratonic margin after docking due to oblique subduction of the Farallon plate. The Wallowa terrane of northeastern Oregon and west-central Idaho provides an intriguing example of the allochthonous terranes which sutured onto the North American craton at this time. It experienced 60° of clockwise rotation since the middle Cretaceous and exhibits a well-exposed suture zone against rocks of continental affinity along the Salmon River Canyon. However, because the terrane accreted in the restrictive Columbia embayment and shear sense on the suture has been unresolved, its tectonic history is unclear. In order to test the ball-bearing model, paleomagnetic and structural examinations of the Salmon River suture zone (SRSZ) were conducted to determine whether strain evidence indicates transcurrent motion along the fault plane, and if so, whether it was dextral or sinistral. Results indicate that the SRSZ was indeed a left-lateral strike-slip fault at one point. Paleomagnetic site means from the SRSZ locality near Slate Creek show pronounced streaking relative to an undeformed sampling locality on the Imnaha River (well outboard of the suture zone). The plane of streaking coincides with the strike of the foliation plane and suggests shear strain in that plane. East-west macroscopic fold axes measured in the Salmon River Canyon and the Seven Devils Mountains indicate large-scale left-lateral shear strain along the SRSZ. These results reveal that the Wallowa terrane did not undergo significant northward translation and associated dextral shearing after suturing. Such behavior appears anomalous among the Cordilleran terranes, and might be a result of this terrane s early entrapment in the Columbia embayment. 11 TABLE OF CONTENTS ABSTRACT..................................................................................................... ii LIST OF TABLES.............................................................................................iv LIST OF FIGURES...........................................................................................v ACKNOWLEDGMENTS............................................................................. vii INTRODUCTION............................................................................................1 GEOLOGIC BACKGROUND ............................................................ 6 Structure ..............................................................................................12 Metamorphism ................................................................................. 13 METHODS......................................................................................................15 Field ..................................................................................................... 15 Locality 1: Imnaha River........................................................ 16 Locality 2: Slate Creek........................................... 19 Laboratory...........................................................................................22 RESULTS.........................................................................................................24 Paleomagnetic results....................................................................... 24 Locality 1: Imnaha River........................................................ 24 Locality 2: Slate Creek.............................................................32 Origins of remanent magnetism ....................................................42 Structural results ................................................................................44 DISCUSSION..................................................................................................48 Evidence for shear .............................................................................48 CONCLUSIONS............................................................................................ 55 APPENDIX A - Paleomagnetic data ............................................................. 59 APPENDIX B - Structural data ...................................................................... 78 REFERENCES................................................................................................ 86 111 LIST OF TABLES TABLE 1. Paleomagnetic site information ................................................ 17 TABLE 2. Site-mean paleomagnetic results ................ 26 TABLE 3. Paleomagnetic locality m e^s and statistics ...........................42 IV LIST OF FIGURES FIGURE 1. Geologic map of the NW United States ...................................... 2 FIGURE 2. Ball-bearing model .......................................................................... 5 FIGURE 3. Geologic map of the Wallowa terrane.........................................7 FIGURE 4. Sampling Locality 1......................................................................... 18 FIGURE 5. Sampling Locality 2.........................................................................20 FIGURE 6 . Paleomagnetic reversal data ........................................................27 FIGURE 7. Zijderveld diagram for sample 2A, site .1................................... 29 FIGURE 8. Zijderveld diagram for sample 3A, site .3...................................30 FIGURE 9. Unblocking temperature curves ...................................................31 FIGURE 10. Zijderveld diagram for sample 4A, site 8................................ 33 FIGURE 11. Zijderveld diagram for sample lA, site 11 .............................. 34 FIGURE 12. Locality 1 site-means.....................................................................35 FIGURE 13. Zijderveld diagram for sample 6 A, site 17 .............................. 37 FIGURE 14. Zijderveld diagram for sample lA, site 23...............................38 FIGURE 15. Zijderveld diagram for sample 3B, site 26 ............................... 39 FIGURE 16. Zijderveld diagram for sample 5B, site 28 ............................... 40 FIGURE 17. Locality 2 site-means .....................................................................41 FIGURE 18. Foliatioa data. Slate Creek Road......................... 45 FIGURE 19. Foliation data, Salmon River .....................................................