U.S. Dept. of Commerce / NOAA / OAR / PMEL / Publications
Conclusions
1. The northern Cleft segment has experienced two recent episodes of seafloor spreading. The extrusive activity associated with these episodes is bimodal; a sheet flow (high extrusion rate) probably erupted first, then at least 7 months later a more voluminous eruption of pillow lavas (slow extrusion rate) occurred along the northward extension of the same fissure system over a line extending 20 km to the north.
2. The eruptive fissure for the YSF is also the primary source of hydrothermal activity, a relationship that has also been observed at the southern Cleft segment. The YSF erupted along several kilometers of a fissure system, but its northernmost section appears to have been the longest-lived eruptive site. The lava pillars along the eastern and western margins of the YSF were probably formed along the slower moving margins of the flow and their tops preserve the original level of the upper flow surface before it drained out to the south. This eruption probably lasted only hours or days.
3. The suite of sulfide chimneys found within 100 m of the currently hydrothermally active fissure system indicates that there was a lower temperature, presheet flow hydrothermal system at the northern Cleft site.
4. The low level of hydrothermal activity over the NPM relative to the YSF and the continuity of rift structures connecting the volcanic vents suggest that the NPM were fed by a dike injected laterally to the north that subsequently cooled rapidly. There are some indications from plume measurements and bottom time series observations that the hydrothermal vents associated with the sheet flow area are also decreasing in intensity.
Acknowledgments. This work was supported through the NOAA Vents Program, and we thank Steve Hammond, the Vents Program manager, for his support through the course of this study. We are also particularly grateful to the NOAA Undersea Research Program and their Program Manager, David Duane, for support of the Alvin dive programs. Many persons contributed to the program at sea, but we particularly thank Susan Hanneman, Todd Schattgen, Dan Clapp, Bob Dziak, Kim Murphy, Bruce Appelgate, Roy Newman, Annette deCharon, Dennis Seem, Floyd Mader, and Andra Bobbitt. The skill and dedication of the Alvin Group should never be taken for granted, and we gratefully acknowledge their performance in providing a safe and effective research submersible. We thank the officers and crew of the research vessels Atlantis II and Discoverer for their dedication to deep-ocean research. Many colleagues contributed to various stages of this study, in particular, Ian Jonasson, Chris Fox, Gary Massoth, Mike Perfit, Dave Butterfield, Kim Murphy, Dick Feely, Ed Baker, Randy Koski, and Matt Smith. Finally, we thank Jessica Waddell for her attention to detail in editing the final manuscript. We thank Bill Normark, Ellen Kappel, and Mike Perfit for their careful and constructive reviews of the manuscript. PMEL contribution 1429.
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