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INVESTIGATIONS OF MAGMATIC END-MEMBERS: SILICIC MAGMA CHAMBERS AND MAFIC DIKES A DISSERTATION SUBMITTED TO THE DEPARTMENT OF GEOLOGICAL AND ENVIRONMENTAL SCIENCES AND THE COMMITTEE ON GRADUATE STUDIES OF STANFORD UNIVERSITY IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY Gwyneth Retta Hughes May 2010 © 2010 by Gwyneth Retta Hughes. All Rights Reserved. Re-distributed by Stanford University under license with the author. This work is licensed under a Creative Commons Attribution- Noncommercial 3.0 United States License. http://creativecommons.org/licenses/by-nc/3.0/us/ This dissertation is online at: http://purl.stanford.edu/cf090yt6229 Includes supplemental files: 1. Caldera references for Chapters 2 and 3 (Caldera_index_ref.pdf) 2. Bayes Classifier Code for Chapter 3 (bayes_classifier.zip) 3. Caldera data for Chapter 2 (Arc_caldera_data.csv) 4. Caldera data for Chapter 3 (All_caldera_data.csv) ii I certify that I have read this dissertation and that, in my opinion, it is fully adequate in scope and quality as a dissertation for the degree of Doctor of Philosophy. Gail Mahood, Primary Adviser I certify that I have read this dissertation and that, in my opinion, it is fully adequate in scope and quality as a dissertation for the degree of Doctor of Philosophy. David Pollard I certify that I have read this dissertation and that, in my opinion, it is fully adequate in scope and quality as a dissertation for the degree of Doctor of Philosophy. Paul Segall Approved for the Stanford University Committee on Graduate Studies. Patricia J. Gumport, Vice Provost Graduate Education This signature page was generated electronically upon submission of this dissertation in electronic format. An original signed hard copy of the signature page is on file in University Archives. iii Abstract Approximately 10% of the global population, some 550 million people, live within 100 km of an active volcano, making it imperative that the causes of magma accumulation in the crust and the factors affecting the scale of subsequent volcanic eruptions are investigated and modeled. This PhD dissertation examines magmatic processes at two scales: silicic calderas represent the large but infrequent end of the continuum, whereas mafic dikes are small in scale but common. Many previous authors have presented hypotheses for how large silicic systems develop. In order to test and refine these hypotheses, I undertook a global compilation that empirically examines how the characteristics of 140 young silicic calderas reflect their crustal-tectonic setting. Results indicate that the size and geochemistry of silicic calderas are affected by the nature of the underlying crust, the tectonic setting, and the local stress regime. For example, large, rhyolitic calderas tend to occur in continental settings under extension. There are, however, few true “rules,” and exceptions may prove useful in analyzing how silicic magma chambers form. Based on this compilation, I present a probabilistic method for determining the tectonic-crustal setting of a given caldera from its diameter, and eruption geochemistry. Focusing specifically on arc settings, this study demonstrates that (1) the abundance of silicic calderas in a given arc is proportional to the trench-normal convergence rate, except in arcs with back-arc spreading; and (2) silicic calderas in continental arcs tend to occur farther behind the volcanic front than do more typical iv arc volcanoes, possibly because of the abundance of pre-existing structures in continental margins. At the opposite end of the volcanic spectrum, this dissertation examines the intrusion of two mafic dikes. The first lies beneath Mammoth Mountain, California, and was associated with a 1989 seismic swarm. Based on an inversion of leveling data constrained by relocated earthquakes, I propose that a dike 2 km long, 8 km high, with 1 m of opening was intruded at 9 km depth beneath the south side of Mammoth Mountain. The second dike investigation focused on the eruption of Miyakejima, Japan, in 2000, associated with more than 10,000 earthquakes, several small eruptions and progressive caldera collapse. Displacements recorded by GPS stations and pre- and syn- event seismicity were used to determine a geological explanation of the event. In the proposed model, a shallow dike propagated ~30 km away from Miyakejima for one week, stopped propagating laterally after intersecting a pre- existing fault zone, and then continued to open and grow vertically for nearly two months. v Acknowledgements I would like to thank my advisors Gail Mahood and Paul Segall for leading me into interesting branches of research and providing extensive academic support over the past five years. In addition, as committee members, David Hill and David Pollard have been invaluable as sources of feedback and encouragement. A great deal of this thesis involved reading papers in other languages. While I was able to cope with those written in romance languages, Uwe Martens, Yo Fukushima, and Curran Hughes helped translate papers in German, Japanese, and Russian. Members of the Segall research group were a valuable resource of advice on programming and data analysis. The Stanford Earth Sciences library staff also helped me in obtaining a variety of data and resources. My officemates Karen Knee and Matt Coble provided much helpful feedback and camaraderie. A special thanks is extended to my parents Sue Cornish and Steve Hughes and all family members who have unendingly encouraged (and financially supported) my academic endeavors. Finally, I could not have come this far without the support, help, advice and encouragement of my wonderful husband, Michael Cardiff. vi Table of Contents Abstract ................................................................................................................... iv Acknowledgements ............................................................................................. vi Table of Contents.................................................................................................vii List of Tables.......................................................................................................... xi List of Figures .......................................................................................................xii 1 Introduction..................................................................................................... 1 2 Silicic Calderas in Arc Settings: Characteristics, Distribution and Tectonic Controls.............................................................................................. 7 2.1 Abstract ................................................................................................................. 7 2.2 Introduction......................................................................................................... 8 2.3 Background: Choosing the Examined Parameters ...............................10 2.4 Data and Methods ............................................................................................13 2.4.1 Data Specific to Each Caldera............................................................................ 13 2.4.2 Data Specific to Arcs and Arc Sections .......................................................... 16 2.4.3 The Control Group: “Normal” Arc Volcanoes ............................................. 17 2.4.4 Across‐arc Distribution of Silicic Calderas .................................................. 18 2.5 Results and Discussion...................................................................................18 2.5.1 Caldera Attributes.................................................................................................. 19 2.5.2 Crustal Attributes................................................................................................... 19 vii 2.5.3 Tectonic Controls on Silicic Caldera Formation........................................ 22 2.5.4 Distance from Volcanic Front............................................................................ 29 2.5.5 Association With Structural Features............................................................ 31 2.6 Conclusions and ImPlications......................................................................32 2.6.1 Conclusions............................................................................................................... 32 2.6.2 Further Implications............................................................................................. 33 2.7 Tables...................................................................................................................37 2.8 Figures .................................................................................................................40 3 Tectonic settings of silicic calderas: analysis of a global compilation.......................................................................................................57 3.1 Abstract ...............................................................................................................57 3.2 Introduction.......................................................................................................59 3.3 Previous Work ..................................................................................................61 3.4 Data.......................................................................................................................63 3.4.1 Caldera‐specific Parameters.............................................................................
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