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Shallow Crustal Structure of the Caldera of Axial Seamount, Juan De Fuca Ridge

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Shallow Crustal Structure of the Caldera of Axial Seamount, Juan De Fuca Ridge AN ABSTRACT OF THE THESIS OF Marijke van Heeswijk for the degree of Master of Science in Geophysics presented on June 25. 1986. Title:_Shallow crustal Structure of the Caldera of Axial Seamount, Juan de Fuca Ridge Abstract approved: Redacted for privacy L. Dale Bibee An airgun refraction line along the length of the caldera of Axial Seaniount as recorded by three Ocean Bottom Seismometers has been analyzed using the tau-zeta inversion technique (Dorman and Jacobson, 1981). Five resulting velocity profiles show that the seismic velocities of the upper 1.4 km of the crust are low and similar to Mid-Atlantic Ridge upper crustal velocities. The low velocities and failure to observe shear waves are thought due to a combination of a thick section of pillow basalts and sheet flows, and a high porosity. The porosity is believed mostly due to a large fracture density. Hammond (pers. comm., 1986) has proposed the caldera is the site of overlapping spreading centers. A high fracture density in the upper crust of the caldera could be due to stresses generated in the area of overlap (Macdonald et al., 1984). No systematic variation in velocity structure along the length of the caldera could be resolved. Shallow water (about 1560 m), smooth bathymetry and the absence of sedimentsalloweddirect measurement of the surface velocities of the caldera floor. An average surface velocity of 3.05 kmls is observed. Assuming a highly simplified crustal model, this velocity translates into a minimum porosity of 30% near the surface of the caldera floor. This minimum porosity is predicted to gradually decrease to about 0% at 1.4 km depth. Upper limits on the porosity can not be found with the available information. No magma chamber has been observed to a depth of 1.4 km. A compressional wave attenuation source in the northwest corner of the caldera below a depth of 1.4 km, however, might be a small magma body or alternatively an anomalously highly fractured area. Shallow crustal Structure of the Caldera of Axial Seamount, Juan de Fuca Ridge by Marijke van Heeswijk A THESIS submitted to Oregon State University in partial fulfillment of the requirements for the degree of Master of Science Completed June 25, 1986 Commencement June 1987 I; Redacted for privacy Assistant Professor of Geophysics in charge of major Redacted for privacy Dean of Co)4ge of Oceanography Redacted for privacy Dean of Graduate Date thesis is presented June 25. 1986 Typed by researcher for Marijke van Heeswijk ACKNOWLEDGEMENTS Many people helped to see this project to completion. I especially thank Dale Bibee, for his advice, patience, and willingness to keep acting as an advisor, even while on a long deserved vacation. Without Dale and Paul O'Neil the data for this study would never have been collected; they kept unwilling OBS's working under all conditions, even if it meant many nights without sleep. I thank Bob Embley, who made sure science did get done while on board the Surveyor, inspite of numerous problems. Bob managed to keep everybody laughing, no matter what went wrong. Sue Hanneman was a great help; without Sue important details would not have been recorded or would have been lost somewhere in a big stack of papers. I also wish to thank the crew of the Surveyor. Back on terra firma discussions with Bill Menke and Randy Jacobson kept me honest and on my toes. Steve Hammond and Bob Embley were always willing to discuss the latest on Axial Seamount and generously provided maps. Randy Jacobson, Mitch Lyle, and Bill Menke read this thesis and provided helpful comments. Writing this thesis was greatly facilitated by the use of an Apple computer, which was generously donated to the geophysics students by Bill Menke. Many people made that the time at Oregon State will be a memorable one. To name a few: my officemate Bruce Dubendorff, for being a great comrade, and being as argumentative as myself, BryndIs Brandsdottir and Brandur, for their friendship and hospitality while I was trying to decide whether I lived in Newport or Corvallis, Shu-Fa Dwan, for always being so cheerful and being a good friend, Haraldur Audunsson, for his friendship and wisdom, Miguel Alvarado-Omaña, for teaching me about Mexico, Gina Frost, for reminding me how easy graduate student life is and Pordur Aras son, Pierre Doguin, Yanmck Duroy, Steve Jaumé, Michel Poujol, and Osvaldo Sanchez-Zamora for being good friends. We'll have to have a reunion some time equidistance from everybody. How about Spitsbergen? My family deserves special thanks; my parents, Ineke and Jeroen for moral supportfrom across the Atlantic and Jane and Henry from across the United States. The person I owe most of my gratitude to is Tom: without his patience and understanding this work would never have been accomplished. This research was supported by the National Oceanic and Atmospheric Administration VENTS program under contract NA-84-ABH-0030. TABLE OF CONTENTS INTRODUCTION 1 TECTONIC SEllING OF AXIAL SEAMOUNT 2 DATA ACQUISITION 13 DATA ANALYSIS 17 RESULTS 39 Inverse Model 39 Forward Model 47 DISCUSSION AND INTERPRETATION OF RESULTS 59 Petrologic interpretation of results 59 Porosity and crack density of the upper crust 63 Comparison of results with other studies 67 CONCLUSIONS 72 BIBLIOGRAPHY 74 LIST OF FIGURES Figure Page 1. Location map of the Juan de Fuca ridge and surrounding areas 3 2. Seabeam bathymetry of Axial Seamount and the northern and 8 southern Juan de Fuca ridge 3. Seabeam bathymetry of the caldera of Axial Seamount 9 4. Major morphological features in the caldera of Axial Seamount 11 5. Positions of Ocean Bottom Seismometers and airgun lines 3 and 5 14 6. Flowchart displaying all data reduction steps 18 7. Reduced record section (vertical channel) for profile 14-3N 19 8. Reduced record section (vertical channel) for profile 14-3S 21 9. Reduced record section (vertical channel) for profile 3-3N 23 10. Reduced record section (vertical channel) for profile 3-3S 25 11. Reduced record section (hydrophone channel) for profile 20-iN 27 12. Corrected and uncorrected travel time versus range for profile 30 14-3N 13. Corrected and uncorrected travel time versus range for profile 14-3 S 31 14. Corrected and uncorrected travel time versus range for profile 3-3N 32 15. Corrected and uncorrected travel time versus range for profile 3-3S 33 16. Corrected and uncorrected travel time versus range for profile 20-iN 34 17. Velocity profiles resulting from tau-zeta inversion of travel time data 40 18. Velocity gradient profiles resulting from tau-zeta inversion of travel 41 time data 19. Map showing the range extents of travel time data used in the 44 tau-zeta inversions 20. Schematic cross-section illustrating extent of redundant sampling 45 between airgun sections 3-3N, 20-iN and 14-3S 21. Projection of velocity profiles onto cross-section below airgun line 3 46 22. Three velocity models forwhichWKBJ synthetics have been generated49 23. Comparison of the data with WKBJ results generated from model 3 50 24. Comparison of the data with WKBJ results generated from model 1 52 25. Comparison of the data with WKBJ results generated from model 2 53 26. Reflectivity and WKBJ synthetic record sections for velocity model 2 56 27. Cross-section of an idealized ophiolite sequence 60 28. Transmission coefficients for P to S conversion as a function of 65 horizontal phase velocity 29. Comparison of representative velocitydepth profiles of three 69 mid-oceanic ridge systems and the results of this study SHALLOW CRUSTAL STRUCTURE OF THE CALDERA OF AXIAL SEAMOUNT, JUAN DE FUCA RIDGE INTRODUCTION In July of 1985 a seismic refraction experiment was carried out on board the Surveyor, to obtain a velocity profile for the shallow crust of the caldera of Axial Seamount. In addition, the caldera was surveyed with SEABEAM, sidescan sonar and bottom photography. All work is part of the VENTS program, an ongoing multidisciplinary program of the National Oceanic and Atmospheric Administration which aims to understand the impact of hydrothermal vents on the world's oceans. Hydrothermal vents have been observed in the caldera of Axial Seamount. This report describes the results of the seismic experiment, which was carried out to gain understanding of the structure of the crust through which hydrothermal fluids move. The seismic source was a surface towed airgun, and three Ocean Bottom Seismometers acted as receivers. One refraction line was shot along the length of the caldera and divided into five separate airgun sections. Each section was analyzed using the tau-zeta inversion method (Dorman and Jacobson, 1981), and one of the five resulting velocity profiles was constrained in more detail by synthetic modeling. Surface velocities of the caldera floor could be measured directly, because of smooth bathymetry, absence of sediments, and relatively shallow water depth (about 1560 m). An average surface velocity of 3.05 krnls was found. Seismic velocities of the caldera crust to a depth of 1.4 km were obtained; the velocities are low, presumably due to a combination of a thick section of pillow basalts and sheet flows, and a high porosity. No large magma chamber was observed to a depth of 1.4 km. The high porosity of the caldera crust is probably due to a high fracture density. Hammond (pers. comm., 1986) has proposed the caldera ofAxialSeamount is the site of overlapping spreading centers. The high fracture density in the upper crust of the caldera could be due to stresses generated in the area of overlap (Macdonald et al., 1984). Near the caldera floor surface the minimum porosity is estimated to be 30%, and it is expected to decrease to about 0% at 1.4 km depth.
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