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Recent eruptions on the CoAxial segment of the Juan de Fuca Ridge: Implications for mid-ocean ridge accretion processes

R. W. Embley,1 W. W. Chadwick,2 M. R. Perfit,3 M. C. Smith,3 and J. R. Delaney4

1Pacific Marine Environmental Laboratory, National Oceanic and Atmospheric Administration, Hatfield Marine Science Center, Newport, Oregon 97365
2Cooperative Institute for Marine Resources Studies, Oregon State University, Hatfield Marine Science Center, Newport, Oregon 97365
3Department of Geology, University of Florida, Gainesville, Florida, 32611
4School of Oceanography, University of Washington, Seattle, WA 98195

Journal of Geophysical Research, 105(B7), 16,501–16,525 (2000).
Copyright ©2000 by the American Geophysical Union. Further electronic distribution is not allowed.

11. Conclusions

  1. The 1993 CoAxial eruption on the Juan de Fuca Ridge is the third well-documented eruption to occur on this segment since 1981. These closely spaced events probably released decades of accumulated stress along this segment.
  2. These three eruptions were generated by dike injections from within the CoAxial segment. The alternate hypothesis that the 1993 dike propagated from Axial Volcano appears unlikely, both because the Sr isotopes of the most recent extrusives from Axial Volcano and the CoAxial segment are different and because the surface manifestations of the 1993 dike at the Floc and Flow sites (short-term hydrothermal venting along dike-related structures) clearly line up along the axis of the CoAxial segment.
  3. The epicenters from the early, southern part of the 1993 T wave swarm must either be systematically biased to the west or were related to faulting along the western fault block ridge induced by the dike intrusion on-axis.
  4. On the other hand, the 1993 lava flow and the along-axis extent of intrusion-related venting at the Flow site coincide very well with the diameter of the cluster of T wave epicenters that occurred during the later, northern part of the seismic swarm.
  5. The venting at the Floc site occurred along a preexisting, 5 to 6-km-long fissure system between about 46°16N and 46°19.5N. This fissure system did not produce eruptions during the 1993 event, but sheet flows had been erupted from the system in at least two locations in the recent past.
  6. The fact that hydrothermal activity lasted 1–2 years longer at the Floc site than at the Flow site means that the Floc site had a larger source of heat to mine, probably due to the 1993 dike being wider and/or extending deeper beneath the site.
  7. The Source site was apparently a preexisting high-temperature hydrothermal system and was unaffected by the 1993 dike intrusion.
  8. The source area of the dike(s) that fed the 1993 eruption on the CoAxial segment (and possibly the two 1981–91 eruptions) appears to be located somewhere between the Source and Floc sites (~46°12N), the latitude at which the T wave swarm started.
  9. The three diking events that occurred along the CoAxial segment between 1981 and 1993 must have relieved a significant amount of strain along at least a portion of the segment and the lack of any more events since 1991 makes a comparison to the behavior of Icelandic central volcanos tempting.
  10. The available observations of the patterns of extrusion and hydrothermal cooling and of the geochemistry of the lavas and vent fluids venting for these 1981–1993 CoAxial eruptions are consistent with that predicted from a dike having a significant lateral component of injection.

Acknowledgments. Without the goodwill of the U.S. Navy in opening up their SOSUS system to the scientific community, the June–July 1993 eruption would almost certainly have gone undetected and the detailed studies reported in this paper would never have been carried out. The NOAA VENTS Program provided support for onshore data analysis and a good portion of the seagoing effort. We particularly thank the ROPOS and Alvin groups and the crew and scientific parties of the research vessels Discoverer and Atlantis II for their excellent support. Williamson and Co. of Seattle, Washington, provided excellent support for the 1996 AMS-60 side-scan survey, and we thank Stuart Sides and Miguel Valasco of the U.S. Geological Survey for their excellent at-sea processing of the data. H.P. Johnson generously included the senior author on the 1995 Alvin expedition and allowed us use of some of the dive data. R. Koski and Hank Chezar generously provided data collected by the U.S. Geological Survey's deep-towed camera system in 1993–1994. The NOAA Undersea Research Program and the National Science Foundation provided generous support for the Alvin dives in 1993–1995. We especially thank D. Kelley, J. Getsiv, T. Gregg, S. Hanneman, R. Greene, A. Bobbitt, and D. Wright for their many long hours on watch during the Discoverer and Atlantis II expeditions. S. Merle and Paul D. Johnson provided excellent technical and graphics support. We are also grateful to Ralph Currie, H. P. Johnson, and Mark Holmes for providing tapes of the SeaMARC II data, and to V. Tunnicliffe for Plate 3d. PMEL contribution 2063.

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