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AN ABSTRACT of the THESIS of Involves Obduction of An AN ABSTRACT OF THE THESIS OF Jonathan D. Williams for the degree of Master of Science in Geolo presented on May 23, 2000. Title: Reconstructing Northern Alaska: Crustal-Scale Evolution of the Central Brooks Range. Abstract approved: Robert J. Lillie KinematictectonicmodelsconstrainedbyAiryisostaticequilibrium demonstrate the crustal-scale evolution of the Brooks Range during ocean basin closure, arc-continent collision, and exhumation of the orogen. The Bouguer gravity anomaly low that develops across the orogen is related by wavelength to the amount of shortening during collision, and by amplitude to the combined effects of erosional unroofing and isostatic rebound. Three collision models test a range of pre-collision crustal geometries and investigate a variety of evolution histories. The preferred solution comprises the best aspect of all three models and involves obduction of an oceanic arc onto a passive continental margin with sedimentary cover 250 km wide. Two distinct periods of convergence and unroofing are identified, separated by strike-slip faulting that influences the hinterland. This model involves -200 km of shortening by crustal overlap and up to 17.5 km of erosional unroofing and isostatic rebound; it results in a symmetric, 40 mGal Bouguer gravity low that is consistent with the observed anomaly across the Brooks Range. The Brooks Range can thus be described as a relatively hard collision that is deeply exhumed compared to other orogens. East of the modeled profile a reversal in asymmetry of the Bouguer gravity low across the Northeastern Brooks Range can be attributed to continuing Tertiary contraction. In the central Brooks Range, thick- skinned thrusting that formed the Doonerak antiform characterized this period of convergence. ©Copyrightby Jonathan D. Williams May 23, 2000 All Rights Reserved ReconstructingNorthernAlaska: Crustal-ScaleEvolution of the CentralBrooks Range by Jonathan D. Williams A THESIS submitted to Oregon State University in partial fulfillment of the requirements for the degree of Master of Science PresentedMay 23, 2000 CommencementJune 2001 Master ofScience thesis of Jonathan D. Williams presentedon May 23, 2000 APPROVED: Major Professor, representingGeology Chair of Department of Geosciences Dean of Graduate School I understand that my thesis will become part of the permanent collection of Oregon State University libraries. My signature below authorizes release of my thesis to any reader upon request. ACKNOWLEDGMENTS Many thanks to the following for support during this project: 0 Professor Robert Lilliefor providing me with a challenging project that investigated a phenomenal part of Alaska Dr. Andrew Meigs for his open door to discussions on Alaska, cycling and life in general. Professor Wesley Wallace and Dr. Catherine Hanks for developing the structural models that formed the basis of this project, supporting my trips to Alaska, and giving excellent advice regarding the Alaskan aspects of the project. 0 Industry sponsors of the University of Alaska Fairbanks (UAF) Tectonics and Sedimentation Research Group- BP Exploration, ARCO Alaska Inc, Chevron, Phillips, Petrofina, and Union Texas. 0 My wife, Janelle, for living through the last few months of thesis writing. TABLE OF CONTENTS Page INTRODUCTION ...................................................................... 1 Motivation for KinematicModeling ......................................... 1 Quantifying Collision: The Value of Bouguer GravityAnomalies ...... 4 GEOLOGY OF THE BROOKS RANGE ........................................... 9 Tectonic History of the BrooksRange ....................................... 9 Constraints on the Evolution of the CentralBrooks Range ............... 12 Geothermometry and Cooling History ............................. 12 GeophysicalImaging ofCrustal Structure ......................... 13 METHODOLOGY ...................................................................... 14 Description of Model Units ................................................... 14 InitialModel Starting Geometry ............................................. 16 Calculation Procedure .......................................................... 18 Assessmentof the Initial Model .............................................. 22 ALTERNATIVE TECTONIC MODELS ........................................... 25 Model A -Introductionof CrustalDuplexing.............................. 25 Concepts Under Test ................................................. 25 Kinematic Evolution .................................................. 26 Evaluation of Model A ............................................... 26 Model B -WidePassive Margin andThin Overriding Crust ............. 30 Concepts Under Test ................................................. 30 Kinematic Evolution ................................................. 31 Evaluation of Model B ................................................ 33 TABLE OF CONTENTS(Continued) Model C - Oceanic Arc and Strike-Slip Involvement ............................. 35 Concepts Under Test................................................ 35 KinematicEvolution................................................. 37 Evaluationof Model C ............................................... 38 DISCUSSION ........................................................................... 40 Assessment of Alternative Tectonic Models .............................. 40 Composite Model .............................................................. 42 Flexure, Isostasy and the Colville Basin .................................... 46 Along-Strike Variations in Evolution of the Brooks Range ............. 48 GlobalContext ofthe Brooks Range Orogeny ............................ 51 CONCLUSIONS ....................................................................... 54 Results Summary .............................................................. 54 Implications of Results ....................................................... 55 BIBLIOGRAPHY ..................................................................... 57 APPENDICES .......................................................................... 62 INTRODUCTION TO APPENDICES .............................................. 63 Appendix 1 -Initial Model Spreadsheet .................................. 65 Appendix 2 -Model A Spreadsheet ....................................... 71 Appendix 3 - Model B Spreadsheet ....................................... 75 Appendix 4 - Model C Spreadsheet ....................................... 81 Appendix 5 -Composite Model Spreadsheet ............................ 87 LIST OF FIGURES Figure 1. Selected tectonic units of northern Alaska ................................. 2. Previous results from kinematic modeling of continental collisions ... 3. Soft versus hard collisions................................................... 4. Erosion and isostatic rebound ................................................ 5. TACT interpretation of crustal structure................................... 6. Starting geometry for kinematic modeling and arrangement of 1-D mass columns .................................................................. 7. Initial kinematic model ....................................................... 8. Initial model exhumation step ............................................... 9. Model A ........................................................................ 10. Detailed comparison of present-day from kinematic models A, B and C ................................................................................. 11. Model B........................................................................ 12. Model C ........................................................................ 13. Composite model starting geometry ........................................ 14. Composite kinematic model ................................................. 15. Composite model interpretation of present-day crustal structure....... 16. Schematic representation of Airy isostasy and regional isostasy ....... 17. Along-strike variations in crustal structure of the Brooks Range ....... 18. Comparison of the Brooks Range to theAlps ............................. 19. Example 1D mass column................................................... RECONSTRUCTINGNORTHERNALASKA: CRUSTAL-SCALE EVOLUTION OF THE CENTRALBROOKS RANGE INTRODUCTION Motivation for Kinematic Modeling The Brooks Range is thought to have developed initially through arc- continent collision, but has since been modified by post-collisional extension, convergence, and erosion. The central Brooks Range has undergone much investigation because it is near the trans-Alaska pipeline and the Dalton Highway (Fig. 1) (Mull and Adams, 1989; Plafker and Mooney, 1997; Oldow and Ave Lallement, 1998; and references therein). However, many questions regarding the crustal geometry of the orogen and its tectonic evolution remain unresolved. For example: 1. How much crustal overlap occurred as a result of collisional shortening? 2. How much exhumation by erosional unroofing and accompanying isostatic rebound took place during and since the collision? 3. What was the geometry and width of the south-facing passive continental margin of the northern Alaska (NAK) crust prior to collision? 4. What is the tectonic affinity of crustal units within the core of the mountain range? 5. How extensive were processes such as extensional unroofing and strike-slip emplacement of crustal bodies? An attempt to answer these questions is made by modeling the evolution of crustal
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