2-8. The South Arch Volcanic Field �@[T.Kani and T. Hanyu]

Introduction
The Hawaiian Arch is a broad swell in the Cretaceous sea floor surrounding the Hawaiian Islands; its axis is 500-1000 m shallower than the adjacent Hawaiian Trough, which separates the Arch from the Hawaiian Ridge (Hamilton, 1957). Numbers of Tertiary/Quaternary volcanoes occur off axis of the Hawaiian Ridge, as discovered in 1986 during surveys around the Hawaiian Islands using the GLORIA sonar system (Lipman et al, 1989; Clague et al., 1990). Since most of these volcanics are located on the Hawaiian Arch, the origin of the arch lavas may be somewhat related to the structure of the arch itself. The total area of identified arch lavas exceeds that of the subaerialy exposed Hawaiian Islands; therefore, volcanism on the Hawaiian Arch now becomes one of the major and previously unappreciated problems among Hawaiian hot spot magmatism (Clague et al., 2001).

Previous study on South Arch lavas
The South Arch Volcanic Field is one area of the arch volcanism located 200km south of Hawaii Island. The South Arch volcanic field consists of flat sheet flows & pillows in a 35 by 50 km area. It was first studied by dredging, seismic reflection and seafloor photography in 1988 (Clague et al., 1988). It is reported that South Arch lavas erupted at 1~10 ka according to the palagonite thickness on lava surfaces (Lipman et al., 1989), demonstrating South Arch volcanism is contemporaneous to the presently active volcanism at Kilauea and Loihi. Two dredged samples are alkalic basalts with similar chemical composition to rejuvenated stage alkalic lavas, as well as North Arch lavas (Lipman et al., 1989; Clague et al., 2000). Chemical affinity of these alkalic lavas indicates that this type of magmatism occurs more broadly than previously thought. Their compositions, and those of North Arch lavas (Detrick et al., 1988), suggest partial melting of a source that is more MORB-like than that of the main stage Hawaiian volcanism.

Research Plan
The geochemical and petrological study of the South Arch is of great importance to understand overall alkali volcanism of the Hawaiian Volcanism. Limited numbers of samples have been so far analyzed due to the difficult access to the volcanic field on the deep ocean floor (~4900m) nearly 200km away from the Hawaii Island. Moreover, geologic occurrence of the dredged samples is ambiguous. A ROV-Kaiko dive is planned to the young lava field first recognized by GLORIA (Lipman et al., 1989). The KAIKO dive will aim for systematic sampling together with detailed geologic observations on structure of the volcanic field. Comparative study of South Arch to North Arch and alkalic lavas of the arch-type volcanoes will provide better understanding for the following aspects:
(1) Why South Arch and North Arch are significantly different in volume?
(2) What kind of melting process causes the arch type alkalic volcanism?
(3) Temporal and special distribution of mantle components beneath Hawaiian hot spot.

References:
Clague, D. A., Moor, J. G., Torreson, M., Holcomb, R. T., and Lipman, P. W., 1988, Shipboard report for Hawaii GLORIA ground-truth cruise F2-88-HW, 25 Februaly-9 March. 1988: U.S. Geological Survey Open-File Report 88-292.
Clague, D. A, Holcomb, R. T., Sinton, J. M., Detrick, R. S., and Torresan, M. E., 1990, Pliocene and Pleistocene alkalic flood basalts on the seafloor north of the Hawaiian islands: Earth and Planetary Science Letters 98, p. 175-191.
Clague, D. A., and Dixon, J. E., 2000, Extrinsic controls on the evolution of Hawaiian ocean island volcanoes. Geochemistry, Geophysics, Geosystems 1, paper number 1999GC000023.
Detrick, R., Sinto, J., and Clague, D. A., 1988, Volcanism on the Hawaiian flexural arch: Results from seabeam, seismic reflection surveying and dredging: EOS (America Geophysical Union Transactions), v. 69, p. 1444.
Hamilton, E. L., 1957, Marine geology of the southern Hawaiian Ridge: Geological Society of America Bulletin, v. 68. p. 1011-1026.
Lipman, P. W., Clague, D. A., Moor, J. G., Holcomb, R. T., 1989, South Arch volcanic field- Newly identified young lava flows on the sea floor south of the Hawaiian ridge: Geology, v. 17, p. 611-614.