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Tectonic Analysis of Northwestern South America from Integrated Satellite, Airborne and Surface Potential Field Anomalies

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Tectonic Analysis of Northwestern South America from Integrated Satellite, Airborne and Surface Potential Field Anomalies TECTONIC ANALYSIS OF NORTHWESTERN SOUTH AMERICA FROM INTEGRATED SATELLITE, AIRBORNE AND SURFACE POTENTIAL FIELD ANOMALIES DISSERTATION Presented in Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy in the Graduate School of The Ohio State University By Orlando Hernandez, B.S., M.S. ***** The Ohio State University 2006 Dissertation Committee: Approved by Dr. Ralph R. B. von Frese, Adviser Dr. Hallan C. Noltimier Adviser Dr. Michael Barton Graduate Program in Dr. Douglas E. Pride Geological Sciences °c Copyright by Orlando Hernandez 2006 ABSTRACT Northwestern South America is one of the most populated regions of the Americas with more than 80 million people concentrated along the Andes Mountains. This region includes a complex and dangerous mosaic of tectonic plates that have produced dev- astating earthquakes, tsunamis, volcanic eruptions and landslides in the last decades. The region’s economic development has also seriously suffered because the region is poorly explored for natural resources. To more effectively assess the tectonic hazards and mineral and energy resources of this region, we must improve our understanding of the tectonic setting that produced them. This research develops improved tectonic models for northwestern South America from available satellite, airborne and surface gravity and magnetic data integrated with global digital topography, seismic, and GPS plate velocity data. Reliable crustal thick- ness estimates that help constrain tectonic stress/strain conditions were obtained by inverse modeling of the magnetic anomalies and terrain compensated gravity anoma- lies. Correlated positive Terrain Gravity Effects (TGE) and Free Air Gravity Anoma- lies (FAGA) suggest that the crust - mantle interface under the northwestern Andes is closer to the surface than expected, indicating that these mountains are not iso- statically compensated. Correlated negative FAGA anomalies observed along western South America and the Greater and Lesser Antilles islands are associated with iso- statically disturbed mantle displaced by subducting oceanic plates. Subtracting TGE ii from the terrain-correlated FAGA (TCFAGA) yield compensated terrain gravity ef- fect (CTGE) anomalies that characterize the Andes Mountains with deep roots of low density crust displacing denser underlying mantle and thickening the local crust. FAGA and TGE correlate at all levels of compensation, but the correlations are especially strong where the compensation is less than 100%. Correlated first vertical derivative FAGA (FVD(FAGA)) and differentially reduced−to-pole total field (DRTP) magnetic anomalies show crustal thickness variations and states of magneto-isostatic compensation. Continental crustal thickness estimates for the North Andes are in the range from roughly 34 km to 55 km, conforming well to and extending regional seis- mic constraints. The analysis highlights crustal deformation from plate collision and subduction in Northwestern South America. Inversely correlated FVDFAGA and DRTP magnetic anomalies suggest thickness variations in the lower crust and thermal effects in terms of the Curie isotherm. Directly correlated FVDFAGA and DRTP magnetic anomalies indicate thickness variations of the upper crust due to the formation of recent topography. iii To Olga Lucia, Camilo Andres, Carolina and Alejandra iv ACKNOWLEDGMENTS I thank the Government of The Unites States of America, through the FUL- BRIGHT Fellowship Program and The Ohio State University for this opportunity to pursue my doctorate at OSU. I also thank the Government of Colombia and The Universidad Nacional de Colombia for endorsing my application and for granting me authority to study abroad. I am deeply and indebted grateful to my advisor Pr. Dr. Ralph R. B. von Frese for his tutelage and academic advice over the years. I also thank Drs. Hallan Noltimier, Michael Barton, Douglas Pride and Terry Wilson for their guidance, encouragement, review and criticism of my research work. I want to extend my thanks to Mohammad Asgharzadeh, Timothy Leftwich, Laramie Potts, Luis Gaya-Pique and Hyung Rae Kim for their assistance and cooperation dur- ing my stay at the Ohio State University. This work has been partially supported by grants from FULBRIGHT - LASPAU, COLFUTURO and Sociedad Minera Kedahda S.A. I also thank the Planetary Geo- dynamics Branch at NASA’s Goddard Space Flight Center for providing access to satellite gravity and magnetic data. v VITA October 26, 1962 . Born - Pacho, Colombia December, 1988 . ..B.S. Geological Sciences, Universidad Nacional de Colombia August, 1995 . .M.S. Exploration Geophysics ITC, Delft, The Netherlands. 1989-1997 . ..Geologist, Geophysicist INGEOMINAS, Colombia 1997-2006 . .Associate Professor Universidad Nacional de Colombia PUBLICATIONS Hernandez, O., R. R. B. von Frese, Crustal modeling of Northwestern South Amer- ica from spectrally correlated free-air and terrain gravity, Journal of South American Earth Sciences (in review). Acosta J. E., Hernandez, O., Analisis neotectonico al Este de Pasca, Cundinamarca. Geologia Colombiana, Vol 23. 2000 Romero, O. F., Hernandez, O., Exploracion geologica y analisis mineralogico de los depositos de esmeraldas de San Antonio de Yacopi, Cundinamarca. Geologia colom- biana, Volumen 22, 1999. vi FIELDS OF STUDY Major field: Geological Sciences Studies in: Geomathematics Dr. Ralph. R. B. von Frese Geophysics Drs. Hallan Noltimier and Ralph. R. B. von Frese Petrology Drs. Michael Barton and Phil Westerhoff Historical Geology Drs. Michael Barton and Loren E. Babcock Structural Geology Drs. Terry Wilson and Hans Diederix Exploration Geophysics Drs. Colin V. Reeves Sally Barritt and Rob Sporry Geographic Information Systems Drs. Ekkehard Jordan and Cees van Westen vii TABLE OF CONTENTS Page Abstract . ii Dedication . iv Acknowledgments . v Vita . vi List of Tables . xi List of Figures . xii Chapters: 1. INTRODUCTION . 1 1.1 Nature and Significance of the Problem . 2 1.2 Objectives . 6 1.3 Description of procedures . 6 1.3.1 Data Compilation . 7 1.3.2 Data processing . 7 1.3.3 Data interpretation and integration with geology . 9 2. THEORETICAL GRAVITY AND MAGNETIC MODELING . 11 Abstract . 12 2.1 Introduction . 13 2.2 Forward Modeling of Gravity and Magnetic Anomalies . 14 2.2.1 Isostatic models of mountainous topography . 15 2.2.2 Isostatic model of depressed topography . 20 2.2.3 Isostatic model of an oceanic ridge . 20 2.2.4 Local body gravity effects . 23 viii 2.2.5 Sedimentary basin gravity effects . 23 2.2.6 Dipping crustal interface . 26 2.2.7 Generalized oceanic - continental subduction . 30 2.2.8 Estimating the angle of subduction . 30 2.3 Conclusions . 32 3. ISOSTATICALLY DISTURBED TERRAINS FROM SPECTRALLY COR- RELATED FREE-AIR AND TERRAIN GRAVITY DATA . 34 Abstract . 35 3.1 Introduction . 35 3.2 Spectrally correlated free-air and terrain gravity . 37 3.3 Analysis of anomaly correlations . 42 3.4 Discussion of results . 50 3.4.1 Digital terrain model (DTM) . 53 3.4.2 Free-air gravity anomalies (FAGA) . 53 3.4.3 Terrain gravity effects . 54 3.4.4 Terrain-correlated free-air gravity anomalies . 55 3.4.5 Terrain-decorrelated free-air gravity anomalies . 57 3.4.6 TCFAGA and FCTGE anomaly correlations . 57 3.5 Conclusions . 59 4. CRUSTAL THICKNESS AND DISCONTINUITY ESTIMATES . 61 Abstract . 62 4.1 Introduction . 63 4.2 Crustal modeling . 63 4.3 Comparison with Airy MOHO estimates . 64 4.4 Comparison with seismic MOHO estimates . 68 4.5 Crustal cross-sections . 71 4.5.1 Pacific subduction zone - Andes Mountains - Guiana Craton 71 4.5.2 Guiana Craton-Andes Mountains-Caribbean subduction zone 74 4.5.3 Middle American Trench - Caribbean Plate - North American Plate . 77 4.6 Crustal discontinuities of the North Andes Microplate . 81 4.7 Discussion . 90 4.8 Conclusions . 93 5. CRUSTAL MODELING FROM MAGNETIC DATA . 95 Abstract . 96 5.1 Introduction . 97 5.2 Core Field and external field reductions . 99 ix 5.3 Differential Reduction to the Pole (DRTP) . 101 5.4 First vertical derivatives of FAGA (FVD(FAGA)) . 105 5.5 Spectral correlations of CHAMP DRTP (TMFA) and FVD (FAGA) estimates . 105 5.6 Aeromagnetic anomalies of the North Andes Microplate . 109 5.7 Discussion . 114 5.8 Conclusions . 117 6. CRUSTAL THICKNESS VARIATIONS AND SEISMICITY . 119 Abstract . 120 6.1 Introduction . 121 6.2 Zero Curvature of crustal thickness variation . 122 6.3 Seismic data compilation . 123 6.3.1 ANSS Catalog . 123 6.3.2 RSNC historical and instrumental seismic catalog . 125 6.3.3 Recent seismicity in the RSNC catalog . 129 6.4 Discussion . 133 6.4.1 Ocean-continent collision zones . 133 6.4.2 Ocean-ocean collision zones . 138 6.4.3 Continent-continent collision zones . 139 6.4.4 Oceanic spreading ridges . 139 6.4.5 Transform faults . 139 6.4.6 Cratonic zone . 139 6.5 Focal mechanism solutions . 140 6.6 Conclusions . 142 7. CONCLUSIONS AND RECOMMENDATIONS . 143 Appendices: A. TWO DIMENSIONAL GRAVITY AND MAGNETIC ANOMALIES DUE TO A POLYGON BY GM-SYS . 151 A.1 Gravity Anomaly due to a Polygon . 153 A.2 Magnetic Anomaly due to a Polygon . 155 B. FRACTIONAL VERTICAL DERIVATIVES . 160 BIBLIOGRAPHY . 162 x LIST OF TABLES Table Page 1.1 Compiled geographical, geological and geophysical databases and sources of information. 8 2.1 Correlation coefficients of gravity effects for an isostatically compen- sated mountain range, for observations at the surface, 20 km, 50 km, 100 km, 150 km and 200 km altitude, and 100% and 50% isostatic com- pensation . 16 2.2 Densities (gm/cm3) and magnetic susceptibilities (cgs) for intrusive bodies and host rocks modeled in Figures 2.6 and 2.7. 26 2.3 Correlation coefficients between FAGA and TFMA for an intrusive body with different rock composition affecting a sedimentary host rock . 28 2.4 Correlation coefficients between FAGA and TFMA for a sedimentary basin hosted within mafic and metasedimentary basements . 28 2.5 Correlation coefficients between FAGA and TFMA for a layer dipping at 15o, 30o, 45o,.
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