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Volcanotectonic Evolution and Characteristic Volcanism of the Neovolcanic Zone of Iceland Ruth Ella Beatrice Andrew

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Volcanotectonic Evolution and Characteristic Volcanism of the Neovolcanic Zone of Iceland Ruth Ella Beatrice Andrew Volcanotectonic Evolution and Characteristic Volcanism of the Neovolcanic Zone of Iceland Dissertation zur Erlangung des Doktorgrades der Mathematisch-Naturwissenschaftlichen Fakultäten der Georg-August-Universität zu Göttingen vorgelegt von Ruth Ella Beatrice Andrew aus Abingdon, Oxfordshire, England Göttingen 2008 D 7 Referentin: Jr. Prof. S. Philipp Abteilung Struktur Geologie und Geodynamik, Georg-August-Universität Göttingen Korreferent: Prof. A. Gudmundsson Department of Earth Sciences, Royal Holloway, University of London Tag der mündlichen Prüfung: 8 Juli 2008 Abstract The thesis focuses on three aspects of the volcanotectonic activity in the Neovolcanic Zone of Iceland: (1) effects of deglaciation, (2) mechanical interaction between central volcanoes, and (3) effects of volcanoes as soft, elastic inclusions on the propagation of volcanic fissures and rift zones. The Neovolcanic Zone contains rocks belonging to the Brunhes normal magnetic epoch, dating back to 0.78 Ma, and represents the on-land expression of the Mid-Atlantic Ridge. This zone, composed of three segments, is the location of most of the volcanotectonic activity in Iceland. Within the Neovolcanic Zone, the Holocene volcanism is primarily confined to the volcanic systems, essentially large groups or (within the rift zone) swarms of volcanic and tectonic features. Most volcanic systems contain a central volcano, many of which have a collapse caldera, and (within the rift zone) a fissure swarm. The volcanic systems are fairly evenly distributed throughout the Neovolcanic Zone. In addition to the polygenic central volcanoes, the Neovolcanic Zone contains numerous monogenic basalt volcanoes. These include table mountains and hyaloclastite ridges, formed in subglacial eruptions, as well as lava shields and volcanic fissures, formed in subaerial eruptions. The retreat of the ice at the close of the Weichselian, and the associated unloading of the crust and isostatic uplift, has long been held accountable for the increase in volcanic activity in the late glacial and early postglacial periods. Here I present conceptual and numerical models to explain the formation and location of subglacial table mountains and hyaloclastite ridges as, as well as subaerial lava shields and central volcanoes, within the Neovolcanic Zone as a result of the ice retreat. The ice retreat reduced compression in the crust, and created tensile stresses around the deep-seated magma reservoirs, thereby explaining how, where, and when the table mountains and lava shields formed. During the Weichselian, the ice load generated compression that encouraged the development and expansion of shallow crustal magma chamber. Subsequently, the unloading encouraged increased volcanic activity Numerical models show that there is strong mechanical interaction between central volcanoes within clusters, such as in the central part of Iceland. This interaction encourages shared dykes and seismogenic faulting between nearby volcanoes, as is supported by observations. Central and subglacial volcanoes function as soft, elastic inclusions in the stiffer host-rock (basaltic) lava pile. Stress-modelling results indicate two main points. First, that soft subglacial mountains may hinder the propagation of volcanic fissures, as is supported by models of the 1783 Laki Volcanic Fissure. Second, that large central volcanoes may temporarily arrest the propagation of large parts of or entire rift-zone segment, as is supported by models of the Torfajökull Volcano in South Iceland. Diese Arbeit beschäftigt sich mit drei Aspekten der vulkanotektonischen Aktivität der Neovulkanischen Zone Islands: (1) den Auswirkungen der Deglaziation, (2) der mechanischen Wechselwirkung zwischen Zentralvulkanen und (3) dem Einfluss von Vulkanen als weiche, elastische Einschlüsse auf die Ausbreitung von vulkanischen Spalten und Riftzonen. Die Neovulkanische Zone Islands enthält Gesteine der Brunhes normalmagnetischen Epoche, jünger als 0,78 Millionen Jahre, und repräsentiert den Mittelatlantischen Rücken an Land. In dieser Zone, die aus drei Segmenten aufgebaut wird, findet der Großteil der vulkanischen Aktivität auf Island statt. Innerhalb der ii Neovulkanischen Zone ist der holozäne Vulkanismus vor allem auf die Vulkansysteme beschränkt, die im wesentlichen große Gruppen oder (innerhalb der Riftzone) Schwärme von vulkanischen und tektonischen Strukturen darstellen. Die meisten Vulkansysteme beinhalten einen Zentralvulkan, von denen viele eine Kollapscaldera aufweisen sowie (innerhalb der Riftzone) einen Spaltenschwarm. Die Vulkansysteme sind recht gleichmäßig über die Neovulkanische Zone verteilt. Zusätzlich zu den polygenen Zentralvulkanen enthält die Neovulkanische Zone zahlreiche monogene Basaltvulkane. Diese umfassen Tafelberge und Hyaloklastitrücken, die in subglazialen Eruptionen gebildet wurden, sowie Lavaschilde und vulkanische Spalten, die in subaerischen Eruptionen gebildet wurden. Der Rückzug des Eises am Ende der Weichseleiszeit, und die damit verbundene Entlastung der Kruste und der isostatische Aufstieg werden seit langem für die Zunahme der vulkanischen Aktivität im spätglazialen und frühen postglazialen Zeitraum verantwortlich gemacht. Hier präsentiere ich konzeptuelle und numerische Modelle, die die Bildung und Verteilung subglazialer Tafelberge und Hyaloklastitrücken sowie subaerischer Lavaschilde und Zentralvulkane innerhalb der Neovulkanischen Zone als Ergebnis des Eisrückzuges erklären. Der Rückzug des Eises reduzierte die Kompression in der Kruste und rief Zugspannungen in der Umgebung der tiefen Magmenreservoire hervor, wodurch erklärt wird, wie, wo und wann sich die Tafelberge und Lavaschilde bildeten. Während des Weichselglazials führte die Belastung durch das Eis zu Kompression, die die Entwicklung und Ausdehnung flacher krustaler Magmenkammern begünstigte. In der Folge wurde durch die Entlastung die Steigerung der vulkanischen Aktivität hervorgerufen. Numerische Modelle zeigen, dass es eine starke mechanische Wechselwirkung zwischen Zentralvulkanen in Clustern, wie sie im Zentralteil Islands vorkommen, gibt. Diese Wechselwirkung begünstigt Gänge, die in mehreren Vulkanen aufdringen sowie seismogene Störungsaktivität zwischen benachbarten Vulkanen, wie auch Beobachtungen bestätigen. Zentralvulkane und subglaziale Vulkane fungieren dabei als weiche, elastische Einschlüsse innerhalb der steiferen Nebengesteine des (basaltischen) Lavastapels. Ergebnisse von Spannungsmodellierungen weisen auf zwei Hauptpunkte hin. Erstens, dass die weichen subglazialen Berge die Ausbreitung vulkanischer Spalten behindern, wie Modelle der Laki-Vulkanspalte von 1783 zeigen. Zweitens, dass große Zentralvulkane temporär die Ausbreitung ganzer Riftzonensegmente stoppen können wie durch Modelle des Torfajökull-Vulkans in Südisland bestätigt wird. iii Acknowledgments Writing my PhD thesis is something that I couldn‟t imagine three years ago, and there are a number of people I would like to thank for helping me reach this point. First of all I would like to thank the University of Göttingen for allowing me the opportunity to carry out this research. Similarly, I thank the members of the Department of Structural Geology and Geodynamics for helping me ease into life in Göttingen. Special thanks go to Prof. Agust Gudmundsson, my supervisor, for giving me the position. Agust‟s help and support has been invaluable, and I credit him with large leaps in my education over these past three years. I thank him for the opportunity to study Iceland further, and for showing me places in Iceland I would never have found by myself. I also thank him for his patience with my often repeated questions on rock mechanics, and my ideas on Iceland. Secondly I would like to thank Jr. Prof. Sonja Philipp for stepping in when Agust moved to London. I thank her for helping me with the tedious formalities, and general support, at the end of my PhD. I also thank her for introducing me to numerical modelling, particularly to Ansys and Beasy, and offering support in this when needed. Special thanks go to Ludovic Letourneur for putting up with me! I thank Ludo for being extremely supportive throughout, offering welcome distractions, and taking me away from Germany for short breaks. I especially want to thank Ludo for talking me through aspects of geology previously foreign to me, for fixing my models when they stump me, and for his invaluable help on fieldwork. My thanks go to my family for their support, particularly my mum, my sister and Abbie and Alec for raising my mood. I would like to thank Emma Howley for being all I could hope for in a friend, supporting me when I was low, proof reading, and offering welcome distractions when needed. I thank Chris Gross for letting me share his office, and providing me with distractions and conversation. I thank Marie-France Hesse for her help with many forms and formalities. I also thank Ines Galindo for useful discussions and help with fieldwork and other logistics. A special mention goes to Dr. Thorvaldur Thordarson, for locating us along the Laki cone row (and the logistics involved with getting there), and extremely helpful and interesting discussions. Also for making me feel welcome in a visit to Edinburgh and giving me sometimes much needed inspiration. Thanks is extended to Gudrun Larsen, Jay Miller and Magnus Tumi Gudmundsson for help and input when requested. iv Contents 1 Introduction ....................................................................................................................................
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