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Structural Styles of the Southern Boundary of the Sapphire Tectonic Block Anaconda-Pintlar Wilderness Area Montana

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Structural Styles of the Southern Boundary of the Sapphire Tectonic Block Anaconda-Pintlar Wilderness Area Montana University of Montana ScholarWorks at University of Montana Graduate Student Theses, Dissertations, & Professional Papers Graduate School 1976 Structural styles of the southern boundary of the Sapphire tectonic block Anaconda-Pintlar Wilderness Area Montana Charles Gilbert Wiswall 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 Wiswall, Charles Gilbert, "Structural styles of the southern boundary of the Sapphire tectonic block Anaconda-Pintlar Wilderness Area Montana" (1976). Graduate Student Theses, Dissertations, & Professional Papers. 7118. https://scholarworks.umt.edu/etd/7118 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]. STRUCTURAL STYLES OF THE SOUTHERN BOUNDARY OF THE SAPPHIRE TECTONIC BLOCK ANACONDA-PINTLAR WILDERNESS AREA, MONTANA by Gil Wiswall B .A ., Colgate U n iv e rs ity , 1973 Presented in partial fulfillm ent of the requirements for the degree of Master of Science UNIVERSITY OF MONTANA 1976 Approved by: hairman. Board o f Examiners Dean^G radu hool 5 Date ^ Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. UMI Number: EP37919 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 OisMTtation Pubtiahing UMI EP37919 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 P roQ ^ st ProQuest LLC. 789 East Eisenhower Parkway P.O. Box 1346 Ann Arbor, Ml 48106 -1346 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Wiswall, Gil, M.S., Summer, 1976 Geology S tru c tu ra l Styles o f the Southern Boundary o f the Sapphire Tectonic Block, Anaconda-Pintlar Wilderness Area, Montana (62 pp.) Director: Donald W. Hyndman / ic o 2> i Structural analysis of a portion of the southern boundary of the Sapphire Tectonic Block shows that rocks of the upper Belt Supergroup and early- to mid-Cambrian units have experienced three distinct deformational events. Standard structural mapping and s ta tis tic a l analysis were employed in order to determine whether the concept of the Sapphire Tectonic Block could explain the observed structural relationships in the Falls Fork drainage, Anaconda-Pintlar wilderness area, Montana. The deformational events may be divided into F] and post-F] on the basis of structural style. The F-| event is represented by a large scale, westward-verging, recumbent anticline. Deformation was by plastic flow resulting in a well-developed axial plane s c h is to s ity accompanied by strong transpo sition o f bedding. Post-F] events involve progressive deformation at shallower tectonic levels. ^ 2 superimposed concentric-style mesoscopic structures on the F-) fold. Fg was coaxial with F 2 , folding the inherited fabric into a macroscopic syncline. Continued application of stress resulted in thrusting and subsequent folding of the thrust planes. A model involving an existing step in the basement surface is employed to explain the observed sequence of deformation. F] is thought to represent decollement deformation at the base of the sliding block. The F-j fold resulted from a buttress effect of the basement step. Progressive deformation raised the F-j structure to higher tectonic levels. Post-F^ structures are consistent with the characteristic structural style and eastward transport of the Sapphire Tectonic Block. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ACKNOWLEDGMENTS Special thanks go to Dr. Don Hyndman for untiring guidance both in the field and in the preparation of the manuscript. Drs. Bob Weidman and Jim Talbot also provided valuable assistance. F ield work was supported in part by a G rant-in-A id of Research from the Society of Sigma Xi. m Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. TABLE OF CONTENTS Page ACKNOWLEDGMENTS .......................................................................................... i i i LIST OF FIGURES.......................................................................................... vi LIST OF PLATES ......................................................................................... v il LIST OF TABLES ......................................................................................... v ii CHAPTER I. INTRODUCTION............................................................................... 1 Tectonic Setting ................................................................. 1 Regional Structure ............................................................ 3 Stratigraphy ......................................................................... 6 Metamorphism ......................................................................... 7 Igneous Rocks ......................................................................... 8 Timing of the Sapphire Tectonic Block Movement . 10 Location and Present Study .............................................. 12 II. STRATIGRAPHY............................................................................... 13 Wallace Formation ................................................................. 13 Flathead Quartzite ............................................................. 14 U n d iffe ren tiated Cambrian ................................................ 15 I I I . STRUCTURAL GEOLOGY ................................................................... 18 General Statement ................................................................. 18 F-| F a b r ic ................................................................................... 18 Planar Structures .................................................................. 19 IV Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CHAPTER Page F o ld in g ...................................................................................... 25 Wallace Formation ............................................................ 25 Flathead Quartzite ........................................................ 27 U n d iffe re n tia te d Cambrian ........................................... 30 F-| Movement P ic t u r e ............................................................ 30 Post-F-| Deformation ............................................................ 33 ^2 F a b r ic .................................................................................. 38 F2 E v e n t .................................................................................. 40 Thrust Faults ......................................................................... 42 Post-F-j Movement P i c t u r e ................................................ 45 IV. DYNAMIC INTERPRETATION ........................................................ 48 V. SUMMARY AND CONCLUSIONS........................................................ 55 REFERENCES CITED ....................................................................................... 59 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. LIST OF FIGURES F ig u re Page 1 Map o f Sapphire Tectonic Block showing s tru c tu ra l zones ......................................................................... 4 2 Location map showing present area and previous related studies .......................................................... 11 3 Diagrammatic sketch showing va ria tio n o f Dip of F-| axial planes from east to w est ...........................20 4 Comparison o f F-i fa b ric elements in the Wallace Formation ....................................................................... 23 5 Sketch of folds in strongly transposed Wallace Form ation .................................. ..................................26 6 Complete folds in zones of less intense transposition in Wallace Formation ................................... 26 7 Sketch showing calcareous layers compressed into cores o f fo ld in more competent sandy layers .... 28 8 Comparison o f bedding attitu d e s in Flathead Quartzite to attitudes of schistosity .......................... 28 9 Sketches showing F] folds in the Flathead Q u a r t z i t e ...................................... 29 10 P-T stability limits of minerals discussed in t e x t ................................................................................................ 29 11 Diagram showing fie ld s o f folding .................................... 32 12 Comparison o f F-| and F2 fo ld d a t a .........................................36 13 Examples o f Fg f o l d s ....................................................................... 39 14 Orientation diagram of poles to bedding in the Georgetown th ru s t p late ...........................................
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