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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),
16,50116,525 (2000).
Copyright ©2000 by the American Geophysical Union. Further electronic distribution
is not allowed.
11. Conclusions
- 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.
- 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.
- 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.
- 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.
- 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.
- The fact that hydrothermal activity lasted 12 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.
- The Source site was apparently a preexisting high-temperature hydrothermal
system and was unaffected by the 1993 dike intrusion.
- The source area of the dike(s) that fed the 1993 eruption on the CoAxial
segment (and possibly the two 198191 eruptions) appears to be located
somewhere between the Source and Floc sites (~46°12N),
the latitude at which the T wave swarm started.
- 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.
- The available observations of the patterns of extrusion and hydrothermal
cooling and of the geochemistry of the lavas and vent fluids venting for these
19811993 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 JuneJuly 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 19931994.
The NOAA Undersea Research Program and the National Science Foundation provided
generous support for the Alvin dives in 19931995. 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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