THE INTERACTION BETWEEN TECTONICS, TOPOGRAPHY, AND CLIMATE IN THE SAN JUAN MOUNTAINS, SOUTHWESTERN COLORADO by Ryan Edward McKeon A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in Earth Sciences MONTANA STATE UNIVERSITY Bozeman, Montana January 2009 ©COPYRIGHT by Ryan Edward McKeon 2009 All Rights Reserved ii APPROVAL of a thesis submitted by Ryan Edward McKeon This thesis has been read by each member of the thesis committee and has been found to be satisfactory regarding content, English usage, format, citation, bibliographic style, and consistency, and is ready for submission to the Division of Graduate Education. Dr. David R. Lageson Approved for the Department of Earth Sciences Dr. Stephan G. Custer Approved for the Division of Graduate Education Dr. Carl A. Fox iii STATEMENT OF PERMISSION TO USE In presenting this thesis in partial fulfillment of the requirements for a master‟s degree at Montana State University, I agree that the Library shall make it available to borrowers under rules of the Library. If I have indicated my intention to copyright this thesis by including a copyright notice page, copying is allowable only for scholarly purposes, consistent with “fair use” as prescribed in the U.S. Copyright Law. Requests for permission for extended quotation from or reproduction of this thesis in whole or in parts may be granted only by the copyright holder. Ryan Edward McKeon January 2009 iv ACKNOWLEDGEMENTS I would like to begin by thanking my advisor David Lageson and my thesis committee members Cathy Whitlock and Bill Locke for guiding me through this project with many helpful conversations and suggestions along the way. Shari Kelley of New Mexico Tech graciously assisted me with my thermochronologic investigation, which would not have been possible otherwise. Eric Leonard of Colorado College is thanked for the use of his ELA data for the San Juans. In addition, I acknowledge the help of the following people: Ryan Bergstrom, Stuart Challender, Beth Helmke, Al Parker, and Colin Shaw. I would also like to thank the following institutions for providing funding for this work: The Colorado Scientific Society – William G. Pierce Memorial Fund The American Alpine Club – Research Grant The Wyoming Geological Association – J.D. Love Fellowship Sigma Xi - Grant-in-Aid of Research # G20078231218196960 National Science Foundation – CREST Project to Shari Kelley v TABLE OF CONTENTS 1. INTRODUCTION ...........................................................................................................1 2. STUDY AREA ................................................................................................................4 3. METHODS ....................................................................................................................10 Apatite Thermochronology ...........................................................................................10 Topographic and Climatic Characteristics ....................................................................12 Cirque Analysis .............................................................................................................13 Topographic Swath Profiles ..........................................................................................15 Erosion Models .............................................................................................................16 Incision and Isostatic Rebound .....................................................................................18 4. RESULTS ......................................................................................................................19 Apatite Thermochronology ...........................................................................................19 Topographic and Climatic Characteristics ....................................................................21 Cirque Analysis .............................................................................................................22 Swath Profiles ...............................................................................................................23 Erosion Models .............................................................................................................24 Incision and Isostatic Rebound .....................................................................................25 5. DISCUSSION ................................................................................................................27 Driving Mechanism for Exhumaiton ............................................................................27 Landscape Evolution .....................................................................................................31 The Glacial Buzzsaw in the San Juan Mountains .........................................................32 6. CONCLUSIONS............................................................................................................35 APPENDICES ...................................................................................................................37 APPENDIX A: Additional Discussion For Topics Discussed in the Introduction .......38 APPENDIX B: Additional Description of Study Area .................................................45 APPENDIX C: Additional Description of Methods .....................................................51 APPENDIX D: Additional Data From Results .............................................................61 REFERENCES CITED ......................................................................................................69 vi LIST OF TABLES Table Page 1. Apatite (U-Th)/He Results ......................................................................................21 2. Apatite Fission Track Data from Shari Kelley .......................................................21 3. Topographic and Climatic Characteristics ..............................................................22 4. Erosion and Incision Modeling Results ..................................................................27 vii LIST OF FIGURES Figure Page 1. Aerial Photograph of Central San Juan Mountains ..................................................4 2. Map of Location and Tectonic Setting .....................................................................5 3. Generalized Geologic Map of the San Juan Mountains ...........................................6 4. Map and Cross Section of the Rio Grande Rift ........................................................7 5. Map Showing the Extent of LGM Glaciation and ELA ...........................................8 6. Map Showing the Distribution of Precipitation ........................................................9 7. Map Showing the Location of the Analysis Zones .................................................11 8. Steps of Cirque Analysis Methodology ..................................................................14 9. Map and Graph of Apatite (U-Th)/He Results .......................................................20 10. Map Showing the Location of Northeast-Facing Cirques ....................................22 11. Graphs showing the Results of the Cirque Analysis ............................................24 12. Graph of Cirque Relief vs. Lithology ...................................................................25 13. Topographic and Climatic Swath Profiles ............................................................26 14. Graphs of Erosion Potential vs. Cooling Age .......................................................28 15. Map Showing the Distribution of Geophysical Relief .........................................29 16. Map Showing the Location of the Aspen Anomaly and the Drainage Pattern of the San Juan Mountains ........................................................30 17. Conceptual Model of the Glacial Buzzsaw ..........................................................34 viii ABSTRACT Alpine glaciers have been referred to as „buzzsaws” on the grounds that they control the topographic development of actively deforming mountain ranges; however, the nature of the linkage between glacial erosion and topography in different tectonic and climatic settings remains unclear. In the San Juan Mountains of southwestern Colorado, an intracontinental mountain range with dramatically lower annual precipitation than previously studied ranges, distinct spatial variations in morphology resulting from Quaternary glaciation coincide with different exhumation histories that were derived using apatite (U-Th)/He thermochronology. The northwestern region had cooling ages of 3-10 Ma over an elevation range of 1300 m, moderate correlation between mean elevation and glacial thresholds, and regionally high values for relief and slope above cirque floors. The southern region, by contrast, had cooling ages of 19-32 Ma over an elevation range of 800 m, no correlation between mean elevation and glacial thresholds, and low values for relief and slope above cirque floors. The average magnitude of incision into a reconstructed maximum topography surface is nearly equal for the two study regions suggesting that the effects of glacial erosion are localized to high topography. The northwestern and southern regions show little variation in climate and fluvial and hillslope erosive potential, which implies that erosionally induced isostatic rebound is an unlikely source for the difference in cooling ages. Instead, I infer that active tectonism (possibly related to the