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Paleomagnetism of Eocene Dikes from the Bitterroot Metamorphic Core Complex: Clockwise Crustal Rotation During Tertiary Extension

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Paleomagnetism of Eocene Dikes from the Bitterroot Metamorphic Core Complex: Clockwise Crustal Rotation During Tertiary Extension University of Montana ScholarWorks at University of Montana Graduate Student Theses, Dissertations, & Professional Papers Graduate School 1990 Paleomagnetism of Eocene dikes from the Bitterroot metamorphic core complex: Clockwise crustal rotation during Tertiary extension Paul Theodore Doughty 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 Doughty, Paul Theodore, "Paleomagnetism of Eocene dikes from the Bitterroot metamorphic core complex: Clockwise crustal rotation during Tertiary extension" (1990). Graduate Student Theses, Dissertations, & Professional Papers. 8338. https://scholarworks.umt.edu/etd/8338 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]. Mike and Maureen MANSFIELD LIBRARY Copying allowed as provided under provisions of the Fair Use Section of the U.S. COPYRIGHT LAW, 1976. Any copying for commercial purposes or financial gain may be undertaken only with the author’s written consent. MontanaUniversity of PALEOMAGNETISM OF EOCENE DIKES FROM THE BITTERROOT METAMORPHIC CORE COMPLEX: CLOCKWISE CRUSTAL ROTATION DURING TERTIARY EXTENSION by Paul Theodore Buerkle Doughty B. A., Washington University in St. Louis, 1986 Presented in partial fulfillment of the requirements for the degree of Master of Science UNIVERSITY OF MONTANA 1990 Approved by ^ ^ IX — Chairman, BoarŒ or Exay^prers Dëan, Graduate School ^ Date UMI Number: EP39139 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 Oi«Mirtation P ubliaN ^ UMI EP39139 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 uesf ProQuest LLC. 789 East Eisenhower Parkway P.O. Box 1346 Ann Arbor, Ml 48106-1346 Doughty, Paul T., M.S., June 1990 Geology Paleomagnetism of Eocene dikes from the Bitterroot metamorphic core complex: clockwise crustal rotation during Tertiary extension (248 pp.) Director: Steven D. Sheriff In the midst of Tertiary extension in western Montana and within the Bitterroot metamorphic core complex, an Eocene dike swarm diverges in strike across the Bitterroot mylonite. It strikes N21°E on the west side and N 4 7 ° E on the east side. The 26° change in strike suggests either a refracted stress field or rotation during Eocene or younger extension. To measure such extensional motion across the Bitterroot mylonite, standard paleomagnetic techniques were used to acquire paleomagnetic data from 19 dikes at three localities. The Sapphire tectonic block locality, with 8 dikes, has a mean declination of 46.4 ° and inclination of 69.1°, its 95% cone of confidence ) is 13.4°. The other two localities are within the Bitterroot dome. The Lolo locality has Dec = 336-8°, Inc = 66.5°, = 9.^ (6 dikes). The Selway locality has Dec = 331.6°, Inc = 60.5°, o^g = 6.9^ (5 dikes). Comparison with the expected Eocene— direction (Dec = 348.5°; Inc = 65.5°) reveals these rotational parameters: Sapphire locality, rotation = 58° ± 3 ^ (95% confidence limit); Bitterroot localities, rotation = —12° ± 2CP and —17° ± 12° , respectively. No locality shows significant flattening. At the 95% confidence level, the Bitterroot dome localities display 0° and —5° of significant rotation, respectively. The southern Sapphire tectonic block displays a clockwise rotation of 26° at the 95% confidence level. These data, as well as local and regional geological relationships, indicate that the Bitterroot dome was drawn out from under the Sapphire tectonic block and transported, with little rotation, toward the northwest during Eocene extension. Concurrently, the Sapphire tectonic block rotated in a clockwise direction around a vertical axis near its north edge. The 5 7 — 43.5 Ma age of extension in the Bitterroot metamorphic core complex coincides with extension in British Columbia and south—central Idaho. Such similarities in the age and style of extension suggest that a regional system of Eocene extension extends across much of the northern Cordillera. As a result, highly metamorphosed regions of the crust like Boehl's Butte and the Salmon River Arch may be Eocene core complexes. 11 ACKNOWLEDGEMENTS The late Ruth Doughty Bradshaw generously provided financial support for my academic studies from 1982 to 1989. This thesis is dedicated to her memory. The National Science Foundation also contributed indirect support for this project. Dr. Steven D. Sheriff, who remained calm through the destruction of two Echo drills, a laboratory drill chuck, and a 6 channel seismograph, provided excellent guidance and philosophical insights throughout the study. Other committee members. Dr. James W. Sears and Dr. Len E. Porter, offered helpful comments. Jay Gunderson introduced me to many of the intricacies of paleomagnetic lab work. Selway River Outfitters provided logistical support in the Selway- Bitterroot Wilderness. iii TABLE OF CONTENTS CHAPTER_______ PAGE ABSTRACT ................................................ ii ACKNOWLEDGEMENTS ....................................... iii LIST OF T A B L E S ......................................... vi LIST OF ILLUSTRATIONS..................................... vii INTRODUCTION ............................................ 1 REGIONAL GEOLOGY ....................................... 7 LEWIS AND CLARK FAULT Z O N E .......................... 7 BITTERROOT MYLONITE ................................... 8 BITTERROOT D O M E ..................... 13 SAPPHIRE TECTONIC BLOCK .............................. 16 EOCENE MAGMATISM ..................................... 18 DIKE S W A R M ........................................... 19 PALEOMAGNETIC FIELD AND LABORATORY WORK ............... 24 LOCALITIES ........................................... 24 METHODS ................................................ 26 PALEOMAGNETIC RESULTS ................................... 31 PALEOMAGNETIC BEHAVIOR ...................... 31 SITE-MEAN DIRECTIONS ................................. 42 LOCALITY-MEAN DIRECTIONS 4 3 DISCUSSION .............................................. 49 TILT CORRECTION ..................................... 49 RELIABILITY OF THE PALEOMAGNETIC DIRECTIONS ......... 53 EXPECTED DIRECTIONS ................................... 54 iv CHAPTER PAGE DISCORDANT PALEOMAGNETIC DIRECTIONS . 55 TECTONIC IMPLICATIONS ................. 58 TECTONIC INTERPRETATION AND SPECULATION 63 REGIONAL SPECULATIONS I ................. 69 CONCLUSIONS ............................... 77 REFERENCES CITED ........................ 79 APPENDICES .............................. 86 APPENDIX A PALEOMAGNETIC LOCALITIES . 86 APPENDIX 6 LABORATORY TESTS ............... 91 APPENDIX C IRON OXIDE PETROGRAPHY ......... 118 APPENDIX D STABILITY OF MAGNETIZATIONS . 120 APPENDIX E EFFECTS OF THERMAL DEMAGNETIZATION 129 APPENDIX F HEMATITE MAGNETIZATIONS ......... 139 APPENDIX G PALEOMAGNETIC DATA ............. 143 LOLO LOCALITY ............................ 145 SELWAY LOCALITY .......................... 169 SULA LOCALITY ............................ 207 APPENDIX H: TILT CORRECTION ................. 246 APPENDIX I: SITES OF FURTHER INTEREST . 247 LIST OF TABLES TABLE____________________ PAGE 1. Site-mean paleomagnetic data ........................ 45 2. Locality-mean paleomagnetic data .................... 56 VI LIST OF ILLUSTRATIONS FIGURE________ PAGE 1. Regional geology of the Cordillera ............... 2 2. Geology of western Montana and Idaho ............. 5 3. Eocene geology of western Montana and Idaho . 6 4. Reference m a p ....................................... 17 5. Eocene dike s w a r m ................................... 20 6. Rose diagram-Eocene d i k e s .......................... 21 7. Paleomagnetic localities .......................... 25 8. Vector end-point diagram .......................... 32 9. Photomicrograph-magnetic mineralogy ............... 33 10. Vector end-point diagram .......................... 35 11. Vector end-point diagram .......................... 36 12. Intensity decay-thermal ............................ 38 13. Intensity decay-alternating field .......... 39 14. I RM acquisition..................................... 40 15. Selway locality site-mean directions ............. 46 16. Lolo locality site-mean directions ............... 47 17. Sula locality site-mean directions ......... 48 18. Tilt correction-Sula locality ...................... 51 19. Tilt correction-Selway locality ................... 52 20. Locality-mean directions .......................... 57 21. Paleomagnetic rotation map ........................ 59 22. Geologic map-Sapphire tectonic block ............. 62 23. Geologic map-Eocene extension ...................... 65 vi i INTRODUCTION The Bitterroot metamorphic core complex in western Montana and east-central Idaho is part of an Eocene extensional system which stretches from southern British Columbia to southern Idaho
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