South Arch Volcanic Field

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). Another lava flows were then surveyed by the ROV Kaiko during the Kairei cruise in 2001. The two lava flows had different lava morphology. The thickness of sedimentary cover was also different, suggesting age difference for the two flows. The collected 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. The Shinkai dive is planned to survey the eastern lava flow that have not been well investigated, as well as traverse central flow area that might be a possible vent area of the flows. The Shinkai 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.