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Crustal Structure of the Northern Rocky Mountains Based on Gravity Interpretation

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Crustal Structure of the Northern Rocky Mountains Based on Gravity Interpretation University of Montana ScholarWorks at University of Montana Graduate Student Theses, Dissertations, & Professional Papers Graduate School 1994 Crustal structure of the northern Rocky Mountains based on gravity interpretation Catherine L. Gaskin 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 Gaskin, Catherine L., "Crustal structure of the northern Rocky Mountains based on gravity interpretation" (1994). Graduate Student Theses, Dissertations, & Professional Papers. 7518. https://scholarworks.umt.edu/etd/7518 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]. Cat HY Maureen and Mike MANSFIELD LIBRARY TheMontana University of Permission 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. * * Please check "Yes " or "No " and provide signature Yes, I grant permission No, I do not grant permission Author’s Signature Date: : 2 Any copying for commercial purposes or financial gain may be undeifaken only with the author’s explicit consent. Cruslal Struclurc of The Northern Rocky Mountains Based on Gravity Interpretation By Catherine L. Gaskin B.A., University of Montana, 1990 Presented in partial fulfillment of the requirements for the degree of Master of Science University of Montana 1994 Approved by CRairman, Board of Examiners an. Graduate School 3 /, m i- Date ^ UMI Number: EP38319 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 OisMTtatk»f Publishing UMI EP38319 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 ProOuesf ProQ uest LLC. 789 East Eisenhower Parkway P.O. Box 1346 Ann Arbor, Ml 48106 - 1346 Gaskin, Catherine L., M.S., May 1994 _ , _ Geology Crustal Structure of the Northern Rocky Mountains based on Interpretation of Gravity Data Director: Steven D. Sheriff ^ Previous crustal studies in the northern Rocky Mountains have shown various directions of dip on the crust/mantle boundary and a wide range of values for crustal thickness. The previous estimates of crustal thickness range from 20 to 70 kilometers thus indicating a need for a consistent crustal model for this region. A study of the Bouguer gravity field from eastern Washington to western Montana provides new constraints on drawing accurate crustal sections and insights into the tectonic history of the northern Rocky Mountains. A forward gravity model along a 500 kilometer profile reveals an undulating Moho, a crustal root beneath the Idaho batholith, a sliver of continental crust beneath the accreted terranes and a steeply dipping fault along the western Idaho suture zone. This model indicates that the crust/mantle boundary rises to the northwest and southeast from a depth of 42 kilometers beneath the Idaho batholith to about 32 kilometers in eastern Washington and western Montana. This crustal root is expressed as a regional gravity low of about 3(K) kilometers wavelength and several kilometers amplitude . West of the western Idaho suture zone, a sliver of continental crust is wedged beneath accreted terranes. During Late Cretaceous to Paleocene shortening, the lower continental crust detached from its cover and was overridden by exotic terranes. Examination of the free air and Bouguer anomalies plotted versus elevation clearly show the distinction between the two lithosphcric plates along the suture zone. These plates appear to be approximately in isostatic equilibrium. The present-day Andes serve as a geotec tonic model of the Late Cretaceous Northern Rocky Mountains. 11 Acknowledgments I thank Michael Baxter, who provided much support throughout this project. Also, thanks to my committee chairman, Steve Sheriff whose enthusiasm for the study and enlightening discussions made this project exciting. The other members of my committee, Don Hyndman, Jim Sears and Randy Jeppeson, provided insightful comments throughout the study. For providing the gravity, well log and seismic data which made this study possible, I thank Sara Poland and Amoco Production Company, Denver, Colorado. For asking thought provoking questions early in the study, I thank my fellow graduate students especially D. Jay Johnson. A special thanks to the following for providing financial support: Foland/Amoco fellowship and Bertha Morton Scholarship funds through the University of Montana, Society of Professional Well Log Analysts and Geological Society of America. Ill TABLE OF CONTENTS Abstract ; ; Acknowledgments j I List of T a b le s ........................................................................................................................ List of Figures..................................................................................................................... vi INTRODUCTION.................................................................................................................. GEOLOGIC SETTING ...................................................................................................... 3 PASSIVE CONTINENTAL MARGIN .................................................................... 3 ACTIVE CONTINENTAL MARGIN ...................................................................... 3 Shortening and Thickening of the c r u s t ..........................................................4 Stretching and thinning of the crust .................................................................. 5 PREVIOUS GEOPHYSICAL INTERPRETATIONS..................................................... 8 SEISMIC STUDIES ..................................................................................................g GRAVITY STUDIES..................................................................................................9 GRAVITY MODELING .............................................................................................14 GRAVITY D A T A ...................................................................................................... 16 MODELING PROCEDURES............................................................................17 INTERPRETATIONS OF M O D E L S .....................................................................I 8 A CLOSER LOOK AT MODEL C .........................................................................21 Interpretation of Moho Topography ..............................................................22 Presence of a crustal r o o t .............................................................................23 Suture Zone formation and plate compensation ........................................... 24 Attenuated c ru s t ..............................................................................................27 A modem analog ............................................................................................29 CONCLUSIONS...............................................................................................................30 REFERENCES................................................................................................................. 80 IV LIST OF TABLES Table 1 Model A input parameters ...................................................................................32 Table 2 Model B input parameters ...................................................................................34 Table 3....Model C input parameters ...................................................................................36 LIST OF FIGURES Figure 1. Study area location map 38 Figure 2. Tectonic map of the western U.S ................................................................... 39 Figure 3. Missing miogeocline m ap............................................................................ Figure 4. General geology map.................................................................................... Figure 5. WISZ location map.......................................................................................... z*2 Figure 6. Cross-section of study area from Montana to Idaho ...................................... 43 Figure 7. Eocene extensional system map ...................................................................... 44 Figure 8 . Previous seismic studies ................................................................................ ks Figure 9. Compilation map of U.S. crustal thickness ......................................................46 Figure 10. Compilation of study area crustal thickness. ..................................................47 Figure 11. Regional Isostaiic Gravity map of U.S ............................................................48 Figure 12. Mohl's (1989) study area .................................................................................49 Figure
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