The Topographically Asymmetrical Alaska Range: Multiple Tectonic Drivers Through Space And Time Item Type Thesis Authors Benowitz, Jeffrey Download date 30/09/2021 04:50:02 Link to Item http://hdl.handle.net/11122/9070 THE TOPOGRAPHICALLY ASYMMETRICAL ALASKA RANGE: MULTIPLE TECTONIC DRIVERS THROUGH SPACE AND TIME By Jeffrey Benowitz RECOMMENDED: K Advisory Committee Chai Chair, Department of Geology and Geophysics APPROVED: Dean, College^" Natural Mathematics ■^e A Dean of the Graduate School s7. 'zptt Date THE TOPOGRAPHICALLY ASYMMETRICAL ALASKA RANGE: MULTIPLE TECTONIC DRIVERS THROUGH SPACE AND TIME A DISSERTATION Presented to the Faculty of the University of Alaska Fairbanks in Partial Fulfillment of the Requirements for the Degree of DOCTOR OF PHILOSOPHY By Jeffrey Benowitz, B.S., M.F.A. Fairbanks, Alaska August 2011 UMI Number: 3484665 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. UMI Dissertation Publishing UMI 3484665 Copyright 2011 by ProQuest LLC. All rights reserved. This edition of the work is protected against unauthorized copying under Title 17, United States Code. uestA ® ProQuest LLC 789 East Eisenhower Parkway P.O. Box 1346 Ann Arbor, Ml 48106-1346 Abstract The topographically segmented, ~700 km long Alaska Range evolved over the last -50 Ma in response to both far-field driving mechanisms and near-field boundary conditions. The eastern Alaska Range follows the curve of the Denali Fault strike-slip system, forming a large arc of high topography across southern Alaska. The majority of the topography in the eastern Alaska Range lies north of the Fault. A region of low topography separates the eastern Alaska Range from the central Alaska Range, where most of the high topography lies south of the Denali Fault. To the west, there is a restraining bend in the Fault. Southwest of the bend, the north-south trending western Alaska Range takes an abrupt 90 degree turn away from the Denali Fault. I applied 40Ar/39Ar thermochronology to over forty granitic samples to constrain the thermal history of the western and eastern Alaska Range. I combine the 40Ar/39Ar analyses with available apatite fission track and apatite (U-Th)/He dating. I then inferred the Alaska Range’s exhumation history from the region’s rates and patterns of rock cooling. Periods of mountain building within the Alaska Range are related to Paleocene-Eocene ridge subduction and an associated slab window (-50 Ma to -35 Ma), Neogene flat-slab subduction of the Yakutat microplate (-24 Ma to present), Yakutat microplate latitudinal variation in thickness (-6 Ma to present), block rotation/migration, and fault reorganization along the Denali Fault. However, it is iv clear from basin, petrological and thermochronological constraints that not all of the far-field driving mechanisms affected every segment of the Alaska Range to the same degree or at the same time. Alaska Range tectonic reconstruction is also complicated by near-field structural controls on both the timing and extent of deformation. Fault geometry affects both the amount of exhumation (e.g., -14 km in the Susitna Glacier region of the eastern Alaska Range) and location of topographic development (e.g., north or south of the Denali Fault). The topographic signature we see today is also in part the result of a pre-existing landscape modified by Plio-Quatemary (-3 Ma to present) surface processes. V Table of Contents Page Signature Page .............................................................................................................i Title Page....................................................................................................................ii Abstract......................................................................................................................iii Table of Contents ......................................................................................................v List of Figures........................................................................................................... xi List of Tables .......................................................................................................... xxi Acknowledgements ..............................................................................................xxii CHAPTER 1 SYNOPSIS..........................................................................................1 1.1 Document Format ................................................................................................1 1.2 Motivation ............................................................................................................1 1.3 Research Objectives ............................................................................................5 1.4 Chapter Summaries.............................................................................................6 CHAPTER 2 SPATIAL VARIATIONS IN FOCUSED EXHUMATION ALONG A CONTINENTAL SCALE STRIKE-SLIP FAULT: THE DENALI FAULT OF THE EASTERN ALASKA RANGE.............................................. 10 2.1 Abstract...............................................................................................................10 2.2 Introduction ........................................................................................................11 2.3 Geological Background ....................................................................................13 VI Page 2.3.1 Denali Fault .....................................................................................................13 2.3.2 Uplift History of the Alaska Range ..............................................................14 2.3.3 Physiography of the Eastern Alaska Range .................................................14 2.4 Methods ..............................................................................................................15 2.4.1 Sampling and Analytical Techniques ........................................................ 15 2.4.2 The Geothermal Gradient and Closure Temperatures .................................16 2.5 Results................................................................................................................18 2.6 Discussion ...........................................................................................................19 2.6.1 Along-Strike Exhumation ..............................................................................19 2.6.2 Across-Strike Influence of the Denali Fault ...............................................22 2.6.3 Overall Depth of Exhumation Patterns in the Eastern Alaska Range .....23 2.6.4 Role of Denali Fault Dip in Driving Exhumation ..................................... 24 2.6.5 Climate and Exhumation Rate Trends in the Eastern Alaska Range ......26 2.6.6 Driving Mechanisms of Neogene Exhumation in the Eastern Alaska Range ........................................................................................................................ 28 2.7 Conclusions ....................................................................................................... 31 2.8 Acknowledgements .......................................................................................... 32 2.9 Figures ................................................................................................................33 2.10 References .......................................................................................................46 2.11 Repository Materials 54 , vii Page 2.11.1 Summary of the 40Ar/39Ar Analysis ..........................................................54 2.11.2 Minimum K-Spar Ages ...............................................................................56 2.11.3 MDD Models................................................................................................56 2.11.4 Fission-track Analyses ...............................................................................57 2.11.5 Apatite (U-Th)/He and Fission-track Age Data .......................................58 2.12 Repository Figures .......................................................................................... 60 2.13 Repostory Tables ...........................................................................................102 2.14 Repository References ..................................................................................140 CHAPTER 3 PERSISTENT LONG-TERM (-24 MA) EXHUMATION IN THE EASTERN ALASKA RANGE CONSTRAINED BY STACKED THERMOCHRONOLOGY.................................................................................143 3.1 Abstract .............................................................................................................143 3.2 Introduction ......................................................................................................144 3.3 Tectonics and Exhumation in Southern Alaska ...........................................147 3.3.1 Western Alaska Range ................................................................................148 3.3.2 Central Alaska Range ..................................................................................148 3.3.3 Eastern Alaska Range ..................................................................................149 3.4 Methods and Stacked Thermochronology