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Geodynamics of Terrane Accretion Within Southern Alaska Benjamin Patrick Hooks

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Geodynamics of Terrane Accretion Within Southern Alaska Benjamin Patrick Hooks The University of Maine DigitalCommons@UMaine Electronic Theses and Dissertations Fogler Library 2009 Geodynamics of Terrane Accretion within Southern Alaska Benjamin Patrick Hooks Follow this and additional works at: http://digitalcommons.library.umaine.edu/etd Part of the Tectonics and Structure Commons Recommended Citation Hooks, Benjamin Patrick, "Geodynamics of Terrane Accretion within Southern Alaska" (2009). Electronic Theses and Dissertations. 98. http://digitalcommons.library.umaine.edu/etd/98 This Open-Access Dissertation is brought to you for free and open access by DigitalCommons@UMaine. It has been accepted for inclusion in Electronic Theses and Dissertations by an authorized administrator of DigitalCommons@UMaine. GEODYNAMICS OF TERRANE ACCRETION WITHIN SOUTHERN ALASKA By Benjamin Patrick Hooks B.S. Allegheny College, 2001 M.S. University of Maine, 2003 A THESIS Submitted in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy (in Earth Sciences) The Graduate School The University of Maine August, 2009 Advisory Committee: Peter O. Koons, Professor of Earth Sciences, Advisor Phaedra Upton, Faculty Associate in Earth Sciences Scott E. Johnson, Professor of Earth Sciences Daniel R. Lux, Professor of Earth Sciences Terry L. Pavlis, Professor of Geological Sciences, University of Texas at El Paso Library Rights Statement In presenting this thesis in partial fulfillment of the requirements for an advanced degree at The University of Maine, I agree that the Library shall make it freely available for inspection. I further agree that permission for "fair use" copying of this thesis for scholarly purposes may be granted by the Librarian. It is understood that any copying or publication of this thesis for financial gain shall not be allowed without my written permission. Signature: Date: GEODYNAMICS OF TERRANE ACCRETION WITHIN SOUTHERN ALASKA By Benjamin P. Hooks Thesis Advisor: Dr. Peter O. Koons An Abstract of the Dissertation Presented in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy (in Earth Sciences) August, 2009 The subduction and accretion of an exotic terrane at the southern margin of Alaska is driving uplift of the St. Elias and Alaska Ranges, and is responsible for some of the largest strain releases in history. Here are presented results from numerical models conditioned by geological observations that reproduce the tectonic landscape, deformation, and strain patterns at macro- (1000-km) and meso- (<100 km) scales. These models utilize completely coupled thermal and mechanical solutions that account for the development of heterogeneities to both the thermal and rheological structure of the lithosphere. Perturbation to the thermal structure related to flattening of the buoyant down-going slab offsets the hot mantle wedge flow, cooling the fore-arc region of the orogen developing a thin sliver of material that behaves frictionally. This frictional sliver provides a primary control on the transfer of strain to the over-riding crust and influences the observed deformation patterns. Strengthening of the fore-arc causes a large-scale discontinuous jump in the deformation front. Initial deformation consists of the development of the Alaska Range orogenic wedge and dextral Denali Fault system. The deformation pattern reorganizes most of the strain captured by the St. Elias orogenic wedge forming above the down-dip limit of the frictional sliver. These model results are consistent with the observed slip on the Denali Fault indicating the partitioning of northwestward translation of the accreting terrane into the fold-thrust belt of the Alaska Range, relatively fast uplift within the St. Elias Range, and the temporal shift in deformation patterns observed within the thermochronological and stratigraphic records. The mesoscale model strain patterns, including the effects of evolving topography and erosion, are consistent with the geological observations; the St. Elias Range thin-skinned fold-thrust belt develops with uplift reaching a maximum within the kinematic tectonic corner. The basic strain pattern is controlled by the tectonic geometry, with the surface conditions providing a secondary influence on the rates and magnitudes of deformation. The results of this study indicate that the non-linear feedback between rheology, temperature, and geometry provide a primary control on strain patterns during orogenesis. DISSERTATION ACCEPTANCE STATEMENT On behalf of the Graduate Committee for Benjamin P. Hooks, I affirm that this manuscript is the final and accepted dissertation. Signatures of all committee members are on file with the Graduate School at the University of Maine, 42 Stodder Hall, Orono, Maine. ________________________________________ Dr. Peter O. Koons, Professor of Earth Sciences ii © 2009 Benjamin Patrick Hooks All Rights Reserved iii Acknowledgments This project has benefitted from the support of the St. Elias Erosion and Tectonics Project and a research grant from the National Science Foundation Continental Dynamics Program (EAR #0409162; principle investigators - P.O. Koons and P. Upton). iv Table of Contents Acknowledgments.............................................................................................................. iv List of Figures .................................................................................................................... ix List of Tables .................................................................................................................... xii Chapter 1 : Introduction ...................................................................................................... 1 1.1 Purpose ...................................................................................................................... 1 1.2 ST. Elias Erosion/Tectonics Project .......................................................................... 2 1.3 Geologic Background ................................................................................................ 3 1.4 Theoretical Background .......................................................................................... 11 1.5 General Geological Implications ............................................................................. 14 1.6 Introduction to the Project ....................................................................................... 16 Chapter 2 : Numerical Methods ........................................................................................ 17 2.1 Introduction ............................................................................................................. 17 2.2 Thermal Solutions ................................................................................................... 23 2.3 Mechanical Solutions .............................................................................................. 26 2.4 Model Megathrust ................................................................................................... 29 2.5 Sensitivity Analysis and Limitations....................................................................... 30 v Chapter 3 : The Influence of Thermal Perturbations on the Characteristic Signal of Terrane Accretion ................................................................................................. 34 3.1 Introduction ............................................................................................................. 34 3.2 Numerical Methods ................................................................................................. 37 3.3 Results and Discussion ............................................................................................ 38 3.3.1 Thermal Evolution ............................................................................................ 38 3.3.2 Thermo-Mechanical Evolution ......................................................................... 43 3.4 Petrology and Strain Transfer ................................................................................. 46 3.5 Petrological Implications......................................................................................... 54 3.6 Conclusions ............................................................................................................. 59 Chapter 4 : Macroscale Geodynamics of Southern Alaska .............................................. 60 4.1 Introduction ............................................................................................................. 60 4.2 Geodynamics of Southern Alaska ........................................................................... 60 4.3 Numerical Modeling Methods ................................................................................ 65 4.4 Results ..................................................................................................................... 67 4.4.1 Reference Model Results .................................................................................. 67 4.4.2 Tectonic Model Results .................................................................................... 72 4.4.3 Strain-Softening Model Results........................................................................ 78 4.4.4 Erosional Tectonic Model Results .................................................................... 84 4.4.5 Erosional Strain-Softening Model Results ....................................................... 87 vi 4.5 Discussion ..............................................................................................................
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