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The Crustal Structure of the Subglacial Grimsvotn Volcano, Vatnajokull, Iceland, from Multiparameter Geophysical Surveys

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The Crustal Structure of the Subglacial Grimsvotn Volcano, Vatnajokull, Iceland, from Multiparameter Geophysical Surveys The Crustal Structure of the Subglacial Grimsvotn Volcano, Vatnajokull, Iceland, from Multiparameter Geophysical Surveys by Magnus Tumi Gu5mundsson A Thesis submitted in partial fulfilment of the requirements for the degree of Doctor of Philosophy. Department of Geological Sciences University College London The University of London 1992 ProQuest Number: 10608864 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 com plete manuscript and there are missing pages, these will be noted. Also, if material had to be removed, a note will indicate the deletion. uest ProQuest 10608864 Published by ProQuest LLC(2017). Copyright of the Dissertation is held by the Author. All rights reserved. This work is protected against unauthorized copying under Title 17, United States C ode Microform Edition © ProQuest LLC. ProQuest LLC. 789 East Eisenhower Parkway P.O. Box 1346 Ann Arbor, Ml 48106- 1346 2 ABSTRACT The crustal structure of the subglacial Grimsvotn volcano, Vatnajokull Ice Cap, Iceland, has been studied with shallow seismic reflection, magnetics, gravity, seismic refraction and recent data on bedrock topography. Grimsvotn has been highly active in recent times and has developed three calderas, the east (18 km2), north (11 km2) and the main caldera (20 km2). The subglacial topography in the area is characterized by hyaloclastite ridges, formed by subglacial volcanism. These ridges are made of low density nonmagnetic hyaloclastite tuffs with abundant pillows in places. It is inferred that hydrothermal alteration has reduced the magnetization of the rocks in places, especially within the calderas. The floor of the 300-600 m deep main caldera dips gently from south to north and parts of it are covered with lava flows. The caldera fill is at least 100 m thick, made of volcanoclastics interbedded with lava flows and sills. The uppermost 2.5 km of the crust are a region of rapidly increasing P-wave velocity and density. A Bouguer anomaly high over the volcano is caused by a basic intrusive complex below 2 km depth. This complex has a minimum volume of 500 km3. Semicircular Bouguer anomaly and magnetic highs may be the expression of a cone sheet swarm, 8-9 km in diameter. On the basis of the bedrock morphology, the east caldera is considered to be the oldest while the other two appear to be recent formations. Within the main caldera the number of subglacial mounds created by eruptions is roughly equal to that of recorded eruptions over the last 200 years. It is proposed that the main and northern calderas formed when magma was drained laterally some 50 km to the southwest at the time of the eruption of the 12 km3 Laki lava in 1783. The very high heat flow (about 5000 MW) observed in Grimsvotn over the last 100-200 years may be a consequence of the caldera collapse. It is thus considered to be a transient feature and the decline in heat flow observed over the second part of this century is believed to be due to decreased volcanic activity. 3 ACKNOWLEDGEMENTS This thesis is the product of an effort in which many people have taken part. The financing of the field work was provided mainly by the Icelandic Science Fund. A grant from the University of London Central Research Fund is also acknowledged. A grant from the Foreign and Commonwealth Office (FCO) and an Overseas Research Student (ORS) fellowship covered my tuition fees while at UCL. The Icelandic Glaciological Society, the Science Institute at the University of Iceland, Orkustofnun (National Energy Authority) and Landsvirkjun (National Power Company) provided generous support by supplying equipment and manpower out in the field. Special thanks are due to Josef Hdlmjam (NEA) whose expert advice on the practicalities of seismics and whose enthusiasm for the project was of great importance. Ami Snorrason, 6lafur Gu5mundsson, Karl Gunnarsson, Torfi Hjaltason, Georg Gu5ni Hauksson, Hubert N6i Jdhannesson, Bjami Kristinsson and my wife, Anna Lrndal, took active part in the field work. Their stamina, patience and good humour were essential for successful teamwork. At a difficult moment, Astvaldur Gu5mundsson organised an support expedition bringing new supplies and manpower, thereby ensuring the completion of the gravity survey. The support and guidance of John Milsom, my supervisor is greatly acknowledged and the trust he has shown by allowing me to undertake this project. Helgi Bjomsson introduced me to Grimsvotn and his help and advice has been of fundamental importance. By giving me a job he has also offered me the chance to continue work on the fascinating subject of subglacial volcanoes. Professor Sveinbjom Bjomsson gave advice on cooling of intrusions and many other matters. At UCL Steve Kaye, David Harrison, Jim Hu and Sheila Gairioch were good company and our office was despite everything a good place to work in. I benefitted greatly from our countless discussions on geophysics and other matters. The assistance of technical staff at UCL is greatly acknowledged. Finnur Pdlsson has helped me in many ways, not least by giving good advice on how to make maps. Finally, my greatest and dearest thanks go to my wife, Anna, which has encouraged me in every way from the very beginning. Without her support and patience over the last year this thesis would not have been completed. 4 CONTENTS ABSTRACT 2 ACKNOWLEDGEMENTS 3 CONTENTS 4 1. INTRODUCTION 11 2. GEOLOGICAL AND GEOPHYSICAL FRAMEWORK 16 2.1. Tectonics and geology of Iceland 16 2.1.1. General outlines 16 2.1.2. Structure of the crust 19 2.1.3. Central volcanoes 24 2.2. Active volcanoes 25 2.2.1. Krafla 26 2.2.2. J>eistareykir 28 2.2.3. Hengill 30 2.2.4. Summary 31 2.3. Geology of Western Vatnajokull 32 2.3.1. Topography and tectonics 32 2.3.2. Evidence from regional geophysical data 37 2.3.3. Volcanic systems within Vatnajokull 40 2.4. Grimsvotn 42 2.4.1. Geography and geology 42 2.4.2. Jokulhlaups 46 2.4.3. Volcanic activity 48 2.4.4. Geothermal activity 51 2.4.5. Possible relations between Grimsvotn and the 1783 Laki eruption 53 3. GEOPHYSICAL SURVEYS 55 3.1. Aims and methods 55 3.1.1. Exploration methods 56 3.1.2. Exploration in ice covered areas 58 5 3.1.3. Design of program 59 3.2. Seismic reflection 61 3.2.1. Field procedures 61 3.2.2. Data reduction 63 3.2.2.1. Velocity analysis 63 3.2.2.2. Processing 68 3.2.3. Results 70 3.3. Magnetics 70 3.3.1. Field methods 72 3.3.2. Data processing 73 3.3.2.1. Coordinate transformation and corrections 74 3.3.2.2. Diurnal corrections 74 3.3.2.3. Adjustment to a common base 75 3.3.2.4. Map compilation 76 3.3.3. Magnetic field map 77 3.4. Gravity 78 3.4.1. Field methods 79 3.4.2. Data reduction 81 3.4.2.1. Free air anomalies 81 3.4.2.2. Topographic reductions 82 3.4.2.3. Digital terrain models 86 3.4.2.4. Reduction densities 90 3.4.2.5. Accuracy of Bouguer anomaly values 92 3.4.2.6. Map compilation 96 3.4.3 Bouguer anomaly map 97 3.5. Seismic refraction 99 3.5.1. Field methods 99 3.5.2. Results 100 4. SHALLOW STRATIGRAPHY AND STRUCTURE OF MAIN CALDERA 108 4.1. Bedrock topography 108 4.2. Morphology of caldera floor 111 4.3. Deeper reflections 113 4.4. Comparison with previous surveys 114 6 4.5. Discussion 116 5. CRUSTAL STRUCTURE 117 5.1. Analysis of magnetic data 118 5.1.1. Magnetic properties of rocks 118 5.1.2. Qualitative analysis 120 5.1.3. Forward modelling 124 5.1.3.1. Main caldera 126 5.1.3.2. Southwest margin of north caldera 130 5.1.3.3. Subglacial peak 132 5.1.4. Discussion 134 5.2. Gravity interpretation 135 5.2.1. Constraints for gravity modelling 136 5.2.2. Forward modelling 138 5.2.2.1. Model I 140 5.2.2.2. Model II 146 5.3. Summary 149 5.3.1. Bedrock morphology 149 5.3.2. Crustal structure 150 5.3.3. Evolution of volcano 155 6. HEAT FLOW AND THE GRIMSVOTN CALDERAS 157 6.1. Relations between geothermaland volcanic activity 157 6.2. The calderas 165 6.2.1. Icelandic calderas 168 6.2.2. Calderas and gravimetry 168 6.2.3. Effect of vertical density gradient on gravity anomalies in calderas 170 6.2.4. Characteristics of the Grimsvotn calderas 173 6.3. Time of caldera formation 177 6.4. Discussion 187 7. CONCLUSIONS 192 REFERENCES 195 APPENDIX A. Field expeditions 211 APPENDIX B. Barometric levelling 216 APPENDIX C. Digital tenrain models 222 APPENDIX D. Gravity stations 226 7 Figures 1-1. Western Vatnajokull. 12 2-1. Plate margins, volcanic zones and fissure swarms in Iceland. 18 2-2. P-wave velocity and density structure of the Icelandic crust. 20 2-3. Depth to a partially molten layer under the crust in NE-Iceland. 23 2-4. Topographic maps of a) Krafla, b) Peistareykir and c) Hengill. 27 2-5. Gravity and magnetic maps of Krafla (a,b), Peistareykir (c,d) 29 and Hengill (e,f). 30 2-6. Vatnajokull, central volcanoes and the volcanic systems. 33 2-7. ERTS-1 image of Vatnajokull from January 31 1973. 34 2-8. Bedrock map of the areas in Vatnajokull mapped with radio-echo soundings until 1991.
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