NATURE AND EVOLUTION OF CONDUIT FAULTS IN THE 2004-2008 MOUNT ST. HELENS LAVA DOME ERUPTION by ELIZABETH ANNE FRIEDLANDER B.Sc. Honours, Colorado College, 2007 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE in The Faculty of Graduate Studies (Geological Sciences) THE UNIVERSITY OF BRITISH COLUMBIA (Vancouver) January 2012 © Elizabeth Anne Friedlander, 2012 Abstract Mount St. Helens reawakened 24 years after erupting in the 1980’s. This effusive eruption produced 95 million cubic meters of dacite in the form of 7 discrete, competent spines or domes of lava between September 2004- June 2008. The spines comprise low- porosity dacite that is inferred to have crystallized at a depth of about 1 km and are enveloped by a 1-3 meter carapace of fault gouge. The rate of linear extrusion of the spines peaked at 11 m/day in November 2004 and subsequently slowed to < 0.5 m/day. Dome growth was accompanied by a “drumbeat” seismicity that was sourced from 1-0.5 km below the vent. Here, field, petrographic, and microstructural observations on the nature of deformation attending the extrusion of Spines 4, 5 and 7 at Mount St. Helens (2004-2006) are presented. The enveloping fault zones provide a static view of the cumulative strain produced by shear along the conduit wall. The conduit faults narrow from Spine 4 to Spine 7 and exhibit fewer macroscopic brittle features. Strain accommodation is achieved through a scale-dependent ductility. The subsurface ascent velocities for each packet of magma are reconstructed using surface observed extrusion rates. Computed shear strain rates for the margins of the conduit range from 1x10-4 to 7.9x10-5 s-1. As ascent rate decreases, fault zone width also decreases maintaining an average shear strain rate of 4.3x10-5 s-1. Intense strain localization within each fault zone is expressed by 0.001 m thick slickensides implying very high (co-seismic) transient shear-strain rates of 1x10-1s-1 (Spines 4-5) to 2.2x10-2s-1 (Spine 7). I conclude with a time and space model for the evolution of the fault zone as magma ascends the conduit, and how the fault zones evolve through time. The factors that ii contribute to the differences in conduit fault zone width and nature throughout the eruption are: 1) differences in ascent rates at the onset and origin of brittle failure, 2) variations in shear strain rates, 3) and the increasing residence time throughout the eruption that the damaged fault rocks experienced at high temperatures within the conduit. iii Preface This thesis is written as a manuscript written for publication in a peer reviewed journal. I am senior author and my co-authors are L.A. Kennedy and J.K.Russell. The project was equally supervised by L.A. Kennedy and J.K. Russell with critical initial guidance in the field from John Pallister from the USGS at the Cascade Volcano Observatory. I was responsible for executing the entire field campaign into the crater of Mount St. Helens in the summer of 2010 that involved other research groups from University College London and Ludwig-Maximillians-Universitat Munchen but did not collaborate with them on this research. I carried out the initial fieldwork, hand sample and microstructural observations with direct guidance from L.A. Kennedy and J.K. Russell. I developed the reconstruction of the spines in time and space model with critical guidance from J.K. Russell. Additionally, the structural reconstruction and fault mechanics was developed with L.A. Kennedy and J.K. Russell. iv Table of Contents Abstract ........................................................................................................................................... ii Preface ............................................................................................................................................ iv Table of Contents .......................................................................................................................... v List of Tables ................................................................................................................................ vii List of Figures ............................................................................................................................ viii List of Symbols and Abbreviations .......................................................................................... x Acknowledgments ....................................................................................................................... xi 1. Introduction .............................................................................................................................. 1 1.1 Dome Growth and Conduit Shear Zones .................................................................................. 2 2. Conduit Faults in Mount St. Helens 2004‐2008 Lava Dome ...................................... 8 2.1 Introduction ...................................................................................................................................... 8 2.2 Methodology ...................................................................................................................................... 8 2.3 Structure of the Mount St. Helens 2004‐08 Lava Dome ................................................... 10 2.3.1 Spine 4 .......................................................................................................................................................... 10 2.3.2 Spine 5 .......................................................................................................................................................... 14 2.3.3 Spine 7 .......................................................................................................................................................... 17 2.3.4 Comparison of Spines 4, 5, & 7 .......................................................................................................... 20 2.4 Microstructural Observations .................................................................................................. 22 2.4.1 Undeformed Dacite ................................................................................................................................. 22 2.4.2 Spine 4 Microstructures ....................................................................................................................... 22 v 2.4.3 Spine 7 Microstructures ....................................................................................................................... 24 2.5 Summary of Conduit Fault Features ....................................................................................... 27 2.5.1 Mode of Failure / Deformation Mechanisms .............................................................................. 27 2.5.2 Strain Variation within Each Spine and ThrouGhout the Eruption ................................... 29 3. Extrinsic conditions attending MSH lava dome growth ............................................ 52 3.1 Introduction ................................................................................................................................... 52 3.2 Surface Observations of Dome Growth from September 2004‐June 2008 ............... 52 3.3 A Top‐Down Model: Putting Magma Back into the Conduit ........................................... 55 3.4 Results of Calculated Velocity Path and Shear Strain Rate ............................................. 57 3.5 Ascent Velocities and Shear Strain Rates ............................................................................. 58 4. Discussion ................................................................................................................................ 65 4.1 Origin and Evolution of the 2004‐2008 Mount St. Helens Conduit Faults ................. 65 4.2 Geometric Variability of the Shear Zones in Spines 4, 5 and 7. ..................................... 68 4.3 Seismic Insights to Mount St. Helens Conduit Faults ........................................................ 72 5. Conclusions .............................................................................................................................. 78 References .................................................................................................................................... 79 Appendices ................................................................................................................................... 84 APPENDIX A: Structural Data ........................................................................................................... 84 Table A.1. Structural Data. ................................................................................................................ 85 APPENDIX B: Grid Maps ..................................................................................................................... 86 APPENDIX C: List of Samples ............................................................................................................ 90 APPENDIX D: Geochemistry ............................................................................................................. 93 vi List of Tables Chapter 2: Nature of Conduit Fault Table 2.1: Structural Data …………………………………………………………. 11 Table 2.2: Field Summary of Fault Zone Thickness……………………………….. 21 Chapter 3: Extrinsic Conditions attending ascent Table 3.1: Surface Observations of Eruption ……………………………………… 61 Table 3.2: Summary of Calculated