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Gravity Measurements in Bolivia and Their Implications for the Tectonic Development of the Central Andean Plateau

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Gravity Measurements in Bolivia and Their Implications for the Tectonic Development of the Central Andean Plateau Gravity Measurements in Bolivia and their Implications for the Tectonic Development of the Central Andean Plateau DISSERTATION Presented in Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy in the Graduate School of The Ohio State University By Kevin M. Ahlgren Graduate Program in Geodetic Science The Ohio State University 2015 Dissertation Committee: Michael Bevis, Advisor Alan Saalfeld Ralph von Frese Copyright by Kevin M. Ahlgren 2015 ABSTRACT A geodetic network of gravity stations in Bolivia was established, reduced, adjusted, and analyzed. This network consists of more than 1300 relative stations with estimated precisions of less than ±0.15 mGal. The network is supported and adjusted using 15 absolute gravity stations. The motivation for observing a completely new and large gravity network in Bolivia is to contribute to the global geopotential model (GGM), which will eventually succeed EGM2008, to provide Bolivia with a much-improved height system, and to use these results to place new geophysical constraints on geodynamic models of the Central Andes. The Central Andes comprise an area of South America associated with a widening of the Andean mountain belt. The evolution of this portion of the range is not completely understood. Traditionally, models with large amounts of steady crustal shortening and thickening over a period of time from 30 Ma - 10 Ma have been hypothesized. Recently, an alternative hypothesis involving a rapid rise of the Central Andes between 10 - 6.8 Ma has been proposed. A tectonic model of this rapid rise hypothesis is developed using locally-compensated isostatic assumptions and delamination of a dense layer of eclogite in the lithospheric mantle. The model is ultimately evaluated for consistency with the observed isostatic gravity anomaly field. Consistency between the model and the isostatic anomaly field is not supported though and fails to support the rapid rise delamination hypothesis. ii DEDICATION This document is dedicated to my daughters: Madeline and Ava iii ACKNOWLEDGMENTS I wish to thank my advisor, Dr. Michael Bevis, for supporting this thesis, fruitful discussion, and guidance throughout my studies. I would also like to thank my committee members and instructors: Dr. Alan Saalfeld and Dr. Ralph von Frese. I also wish to acknowledge Dr. von Frese for the extensive use of his gravimeter. I also wish to acknowledge the National Geospatial-Intelligence Agency (NGA), who supported much of this project. I also would like to thank a number of individuals related to this project. First, Dana Caccamise and Eric Kendrick for their help in everything. Second, Colonel Arturo Echalar Rivera for your leadership and guidance with the realization of this network and numerous other endeavors. Third, to all the sergeants, drivers, and other individuals who collected data that was used in this thesis. Finally, I also wish to thank fellow and former graduate students Abel Brown, Jacob Heck, and David Raleigh for data collection, comments, and suggestions. My parents, John and Sally; sister, Erica; and brother, Keith have provided me so much throughout this endeavor, and I greatly appreciate their support, love, and guidance. Finally, I wish to thank my wife, Christina, who has had to deal with my numerous trips to South America, weekends at work, long nights, and having to explain to people what her husband does. iv VITA 2005................................................................B.C.E. Civil Engineering, University of Minnesota – Twin Cities 2011................................................................M.S. Geodetic Science and Surveying, The Ohio State University 2007 to 2012 ..................................................Graduate Research Associate, Department of Earth Sciences, The Ohio State University 2012 to present ..............................................Assistant Professor, Department of Geography and Planning, St. Cloud State University Publications Jekeli, C., Yang, H. J., & Ahlgren, K. (2013). Using isostatic gravity anomalies from spherical harmonic models and elastic plate compensation to interpret the lithosphere of the Bolivian Andes. Geophysics, 78(3), G41-G53. Fields of Study Major Field: Geodetic Science v TABLE OF CONTENTS ABSTRACT ........................................................................................................................ ii DEDICATION ................................................................................................................... iii ACKNOWLEDGMENTS ................................................................................................. iv VITA ................................................................................................................................... v TABLE OF CONTENTS ................................................................................................... vi LIST OF TABLES ........................................................................................................... viii LIST OF FIGURES ............................................................................................................ x CHAPTER 1: INTRODUCTION ....................................................................................... 1 CHAPTER 2: OPERATIONAL GRAVITY .................................................................... 13 CHAPTER 3: GRAVITY ANOMALIES ......................................................................... 64 CHAPTER 4: DELAMINATION .................................................................................... 82 CHAPTER 5: DISCUSSION AND FUTURE WORK .................................................. 108 REFERENCES ............................................................................................................... 113 APPENDIX A: ADDITIONAL GRAVITY DATA IN BOLIVIA REGION ................ 119 APPENDIX B: LACOSTE AND ROMBERG MODEL G GRAVIMETERS .............. 129 APPENDIX C: NETWORK ADJUSTMENT WITH CONDITION EQUATIONS ..... 130 vi APPENDIX D: ISOSTATIC CORRECTIONS WITH SPHERICAL TESSEROIDS .. 133 APPENDIX E: LITHOSPHERIC MODEL PARAMETERS AND VALUES .............. 136 vii LIST OF TABLES Table 1: Gravity raw readings and corrections ................................................................. 31 Table 2: Gravity observation 's with elapsed time for gravimeter unit: g142 and survey line: K016 - UYNI ............................................................................................................ 33 Table 3: Gravity observation 's with elapsed time for gravimeter unit: g1025 and survey line: K016 - UYNI ............................................................................................................ 34 Table 4: Absolute gravity differences – A10 in 2011 and FG5 in 1997 ........................... 40 Table 5: Relative gravity misclosure statistics at absolute gravity stations ...................... 55 Table 6: Relative gravity statistics - RMS of network adjusted residuals ........................ 58 Table 7: Relative gravity statistics - maximum of network adjusted residuals ................ 59 Table 8: Relative gravity statistics - standard error of estimated gravity ......................... 61 Table 9: Gravity statistics [mGal] ..................................................................................... 70 Table 10: Isostatic Crustal Parameters .............................................................................. 80 Table 11: Ranges of Acceptable Outcome Values ........................................................... 92 Table 12: Pop-up model critical parameters ..................................................................... 93 Table 13: Scenario parameters – See Figure 35 ................................................................ 95 Table 14: Gravity amplitude [mGal] at varying distances. Tabulated values associated with Figure 38. .................................................................................................................. 98 Table 15: Observation statistics and high-velocity zone (HVZ) .................................... 103 Table 16: Individual Gravimeter Observation Statistics ................................................. 129 viii Table 17: Non-controlling model parameters ................................................................. 136 ix LIST OF FIGURES Figure 1: Geodetic Surfaces: Terrestrial, Geoid, and Ellipsoid .......................................... 2 Figure 2: Central Andes ...................................................................................................... 5 Figure 3: LaCoste and Romberg gravimeter .................................................................... 14 Figure 4: Static GPS results - K169 – Height Residual [cm] computed from individual CGPS stations as shown in map view in lower left plot. Plots on right hand side from top to bottom: Northing/Easting estimated position, height residual and baseline length, and height residual and station height difference. ................................................................... 20 Figure 5: Single survey line / single gravimeter analysis overview ................................. 21 Figure 6: Lacoste & Romberg Model G - Serial Number G801 Calibration Curve ......... 22 Figure 7: Survey line with Heaviside measured with 2 gravimeters (g1025 & g142). Upper figure showing the mean gravity difference along the line. Lower figure showing the discrepancy between the two gravimeters’ observations. ..........................................
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