HR: 1340h
AN: V53A-0612    [Abstracts]
TI: Largest Off-axis Lava Flow Field From the Southern East Pacific Rise at 14\deg S - Preliminary Results of NIRAI-KANAI Cruise leg 1, YK04-07 Yokosuka/Shinkai6500 Dives
AU: * Kisimoto, K
EM: kiyo.kisimoto@aist.go.jp
AF: Geological Survey of Japan, AIST, AIST Central 7, 1-1-1 Higashi, Tsukuba, 305-8567 Japan
AU: Umino, S
EM: sesumin@ipc.shizuoka.ac.jp
AF: Shizuoka University, 836 Ohya, Shizuoka, 422-8529 Japan
AU: Geshi, N
EM: geshi-nob@aist.go.jp
AF: Geological Survey of Japan, AIST, AIST Central 7, 1-1-1 Higashi, Tsukuba, 305-8567 Japan
AU: Hilde, T W
EM: hilde@geo.tamu.edu
AF: Texas A&M University, College Station, Texas, TX 77843 United States
AU: Kumagai, H
EM: kumagai@jamstec.go.jp
AF: JAMSTEC, 2-15 Natsushima, Yokosuka, 237-0061 Japan
AU: Sinton, J
EM: sinton@soest.hawaii.edu
AF: SOEST/University of Hawaii, 1680 East-West Rd, Honolulu, HI 96822 United States
AU: White, S M
EM: swhite@geol.sc.edu
AF: University of South Carolina, 700 Sumter St., 617 EWS Bldg., Columbia, SC 29205 United States
AB: Knowledge of the mechanism of off-axis thickening of layer 2A and its petrological-geochemical characteristics is essential to comprehensive understanding of the evolution of oceanic crust and the magma plumbing system of fast-spreading ridges. NIRAI-KANAI Cruise, Leg 1, was aimed to address this issue by using R/V Yokosuka and Shinkai 6500 submersible of JAMSTEC. Although we could carry out only 7 Shinkai dives out of 15 planned dives due to bad weather, we have achieved our primary objectives of the cruise. These are; 1) Complete mappings of 8\deg S and 14\deg S large off-axial flow fields by the SeaBeam of R/V Yokosuka, which showed the 14\deg S flow field has 342 km$^{2}$ and 19 km$^{3}$ , the world largest among the known submarine lava flows; 2) Complete mapping of "Sojourn Ridge", an unusual volcanic ridge with a total volume up to 700-km$^{3}$ centered at 14\deg S, 115\deg W, which shows the transition from spreading axis through a seamount chain to the E-W-striking sub-continuous volcanic ridges; 3) A side-scan sonar system and a sub-bottom profiler equipped on Shinkai depicted detail volcanotectonic structures, sedimentary and lava flow morphologies of the axial and off-axial flow fields, and enabled us to estimate ages of the 14\deg S flow ranging from 34 to 18 kyr; 4) Four dives on the 14\deg S off-axial flow field, two dives on the nearby ridge axis and one dive on an off-axis seamount at 16\deg S provided the first direct observations of a large off-axis lava flow field and seamount and detailed insight into volcanotectonic features of the ridge axis. Eastern half of the 14\deg S flow was first mapped by TAMU$^{2}$ 12-kHz side scan sonar imagery and bathymetry in 1995 during the Ridge Flux Project. During this cruise, we have completed mapping of the entire flow field using the SeaBeam 2112 multibeam sonar system on R/V Yokosuka. These mappings show that 14\deg S flow extends from 13\deg 41'S, 112\deg 26'W to 14\deg 07'S, 112\deg 38'W and approximately in an area of 49 km X 16 km. Total area covered by the flow field amounts to 342 km$^{2}$ . The volume of the flow field is calculated by estimating average thicknesses of lava in the topographical domains classified, which were multiplied by the individual area: 100 m for the eastern cone, 50 m for the lobate terraces, 50 m for the lowlands and highland and 60 m for the northern flow lobes. The estimated volume of the entire flow field + eastern cone is 19 km$^{3}$ , among which the western fissure and the eastern vent extruded 13 and 4.4 km$^{3}$ , respectively. Thickness of the sediments on the basement was measured to be 2-2.5m by the sub-bottom seismic profiler attached to Shinkai on dive834. As the western half spreading rate at 14\deg S is known to be 7.5 cm/yr, the age of the basement is calculated to be 113,700 yr. With this sedimentation rate of 1.76-2.20 cm/kyr (200-250 cm/113.7 kyr), the ages of the flow field were estimated to be 34.1-18.2kyr(for average sediment thickness of 40-60cm).
DE: 8400 VOLCANOLOGY
DE: 8414 Eruption mechanisms
DE: 3000 MARINE GEOLOGY AND GEOPHYSICS
DE: 3035 Midocean ridge processes
DE: 3040 Plate tectonics (8150, 8155, 8157, 8158)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting