HR: 1340h
AN: V53A-0613    [Abstracts]
TI: SHINKAI 6500 Observations of the Volcanic-Tectonic Relationships on the SEPR Rise Crest and Western Slope at 14$^{o}$10'S to 14$^{o}$13'S
AU: * Hilde, T W
EM: geodynamics@tamu.edu
AF: Texas A&M University, Depatment of Geology & Geophysics, College Station, TX 77843 United States
AU: Kisimoto, K
EM: kiyo.kisimoto@aist.go.jp
AF: Geological Survey of Japan , AIST, 1-1-1 Higashi, Tsukuba, 305-8567 Japan
AB: Two SHINKAI 6500 submersible dives, numbers 829 and 830, were made on the Southern East Pacific Rise on July 15 and 16, 2004 during R/V YOKOSUKA cruise YK04-07. Both dives started at mid-slope, 2727 m and 2744 m depth, respectively, traversed up slope to the rise crest where dive 829 made 4 crossings of the rise crest working southward and dive 830 made 3 crossings of the rise crest working northward. Basalt samples were collected at 6 sites. The upper western slope is totally composed of young rounded and elongate pillows. Elongate pillows dominate the steeper slopes. There is little or no sediment on the pillows which display abundant glassy surfaces. Many of the elongate pillows contain numerous eruptive 10-15 cm diameter buds. There are no large vertical displacements or crevasses on the upper flank slope of the rise. Locally, 1-3 m near vertical relief exists that appears to be flow fronts, displaying suspended and draped pillow faces. At the edge of the rise crest a rapid transition occurs from the abundant elongate pillows on the slope, to more ronded pillows and then oblate pillows and sheet flows. The rise crest is generally flat at 2610-2620 m, the surface being made of oblate pillows and sheet flows across the 400-600 m width of the crest. The crest surface is broken by numerous collapsed lava tubes and is dissected by crevasses and axial graben structures which have a ridge parallel strike. The crevasses and collapsed lava tubes are more abundant near the margn of the axial grabens. The axial grabens, bounded by step escarpments and ranging from 80-120 m wide, were crossed on all transects of both dives. Their depth ranges from 5-15 m deep and the bounding escarpments cut across pillows, sheet flows, lava tubes and massive rock. Their long, straight, rise parallel strike, mapped previously with [TAMU]2 sonar imagery, was confirmed by SHINKAI 6500's 330 kHz side-scan sonar and CTFM navigational sonar systems. The graben escarpments and crevasses appear to be faults, and the axial graben a rift structure. While lava withdrawal and collapse likely contribute to the rift development, the axial rift escarpments' long, straight, rise parallel strike and their 10m plus displacement are consistent with ruptures formed in the extensional environment of a divergent plate boundary. The axial rift structures are apparenly later covered by rise flank flows, while faulting to produce off-ridge, rise parallel spreading fabric occurs along the lower flanks and base of the rise.
DE: 8414 Eruption mechanisms
DE: 9355 Pacific Ocean
DE: 8150 Plate boundary--general (3040)
DE: 3035 Midocean ridge processes
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting