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A Structural History of the Garnet Stock and Its Relationship to Deformation Along the Lewis and Clark Line, Western Montana

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A Structural History of the Garnet Stock and Its Relationship to Deformation Along the Lewis and Clark Line, Western Montana University of Montana ScholarWorks at University of Montana Graduate Student Theses, Dissertations, & Professional Papers Graduate School 2012 A STRUCTURAL HISTORY OF THE GARNET STOCK AND ITS RELATIONSHIP TO DEFORMATION ALONG THE LEWIS AND CLARK LINE, WESTERN MONTANA Zackary Steven Wall 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 Wall, Zackary Steven, "A STRUCTURAL HISTORY OF THE GARNET STOCK AND ITS RELATIONSHIP TO DEFORMATION ALONG THE LEWIS AND CLARK LINE, WESTERN MONTANA" (2012). Graduate Student Theses, Dissertations, & Professional Papers. 1372. https://scholarworks.umt.edu/etd/1372 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]. A STRUCTURAL HISTORY OF THE GARNET STOCK AND ITS RELATIONSHIP TO DEFORMATION ALONG THE LEWIS AND CLARK LINE, WESTERN MONTANA By Zackary Steven Wall B.S. West Virginia University, Morgantown, WV, 2009 Thesis Presented in partial fulfillment of the requirements for the degree of Master of Science The University of Montana Missoula, MT September, 2012 Approved by: Sandy Ross, Associate Dean of The Graduate School Johnnie Moore, Department Chair Geosciences James Sears Geosciences Julia Baldwin Geosciences Andrew Ware Physics i Wall, Zackary S., M.S., September, 2012 A Structural History of the Garnet Stock and its Relationship to Deformation Along the Lewis and Clark Line, Western Montana Director: James W. Sears The Lewis and Clark line experienced sinistral transpressive shear during the Late Cretaceous- late Paleocene Laramide orogeny along the boundary between the massive Lewis-Eldorado-Hoadley thrust slab to the NE and the Sapphire and Lombard thrust slabs to the SW. The transpression extruded SE trending, en echelon flower structures along a 40 km-wide shear zone. Late Cretaceous satellite stocks of the Boulder batholith intruded the shear zone and interfered with folding and faulting. One of these, the 83-Ma Garnet stock, invaded a narrow NE-trending fracture zone that straddled the shear zone at deeper levels in the Proterozoic Belt Supergroup but mushroomed in a SE-trending, SW-verging syncline in the Paleozoic section, imprinting a compressional fabric in its aureole of E-W striking cleavage planes and top-to-the-SW rotated porphyroblasts. The stock and its family of sills did not cross the axial plane of the neighboring anticline, and the contact aureole was generally confined to the syncline and the steep limb of the anticline-syncline pair. Maximum pressure and temperature of contact metamorphism is estimated at 2.8 kbar and 650°C from thermodynamic modeling of andalusite- sillimanite bearing hornfels. Magmatic stoping led to downward shear being applied to rock of the contact aureole, creating a down-dip stretching lineation and layer-parallel boudins. Stratigraphic units thin to ~1/2 of their normal thickness within the aureole due to metamorphism as well as shear associated with stoping. Emplacement was followed by a second phase of transpression that began after 76 Ma and ended during the early Cenozoic, when the Lewis and Clark Line underwent dextral transtensile shear evidenced by a newly-mapped right-lateral extensional accommodation zone within the field area. i ii ACKNOWLEDGEMENTS A thank you goes out to my committee members James Sears, Julia Baldwin, and Andrew Ware. Thanks to Johnnie Moore for essential guidance in the late stages of this project. Thanks also to John Childs, a patient and encouraging boss. Victor Guevara helped me in the field and with edits. Access to many outcrops on private land was provided by the Charletons of Garnet, MT. This project was partially funded by a Tobacco Root Geological Society scholarship and a Geological Society of America grant. ii iii Contents I. Introduction .............................................................................................................................. 1 Present Study ........................................................................................................................... 1 Geologic Setting ....................................................................................................................... 3 Previous Work in the Garnet Area ......................................................................................... 11 II. Fabric Analysis ..................................................................................................................... 12 Fabric Analysis Methods ............................................................................................................ 12 Fabric Analysis Results ............................................................................................................... 13 Fabric Analysis Discussion .......................................................................................................... 17 III. Volume Strain Analysis ........................................................................................................ 26 Volume Strain Analysis Methodology ........................................................................................ 26 Volume Strain Analysis Results .................................................................................................. 29 Volume Strain Analysis Discussion ............................................................................................. 36 IV. Thermodynamic Modeling .................................................................................................. 40 Thermodynamic Modeling Methodology .................................................................................. 40 Thermodynamic Modeling Results ............................................................................................ 41 Petrography ........................................................................................................................... 42 Model Results ........................................................................................................................ 42 Thermodynamic Modeling Discussion ....................................................................................... 51 V. Conclusions ......................................................................................................................... 55 Further Research .................................................................................................................... 56 Appendix I .................................................................................................................................. 57 Stratigraphy................................................................................................................................ 57 Cambrian Silver Hill and Flathead Formations (Єsh, Єf)........................................................ 57 Cambrian Hasmark Group (Єh; Єm, Єpa, Єpi) ................................................................... 57 iii iv Cambrian Red Lion Formation (Єrl) ....................................................................................... 58 Devonian Maywood Formation (Dm) .................................................................................... 61 Devonian Jefferson Formation (Dj) ........................................................................................ 61 Peripheral Units ..................................................................................................................... 61 Appendix II ..................................................................................................................................... 63 Bulk Rock Composition of Major Elements in Weight Percent .............................................. 63 Bulk Rock Composition of Trace Elements in ppm. ............................................................... 64 Bulk Rock Composition of Trace Element Oxides in ppm ...................................................... 65 XRF Precision .......................................................................................................................... 66 Appendix III .................................................................................................................................... 67 Theriak-Domino Modeling Composition Data ....................................................................... 67 References ..................................................................................................................................... 68 iv v Figures Figure 1 Geologic map of the Missoula East Quadrangle. ............................................................................ 5 Figure 2 Geologic cross section A-A’ through the Cave Gulch Syncline and Deep Creek Anticline . ............ 7 Figure 3 Geologic cross section B-B’ along Deep Creek. ............................................................................... 8 Figure 4 Mining districts along the Lewis and Clark Line.. .......................................................................... 10 Figure 5 Structure within and beyond the LCL (Stereonets) ...................................................................... 14 Figure 6 Equal-area rose diagram of slickenside
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