Southwest Oahu Volcanic Field & Kauai Channel

By M. Nakagawa, M. Coombs and E. Takahashi

Introduction

GLORIA image showed that there exist two areas west of Oahu Island with high reflectivity, southwest Oahu and Kauai Channel, predicting the presence relatively young lava flows on the ocean bottom. However, detail topography and sample collecting have not been done yet. During the 2001 and 2002 Hawaiian cruise of the JAMSTEC research ship Kairei and Yokosuka, Seabeam mapping, SCS survey, and several dives have been carried out in these areas.

Newly performed Seabeam mapping revealed a low dome-shaped structure 120 km southwest of Oahu Island. The volcanic field is approximately 100 km in diameter and 500 m high with shallowest point at 3950 m depth. Such quite gentle shield-shaped topography has not been recognized yet. There exist numerous flat-topped cones, steep ridges or cones, and sheet flows on the gentle shield (or dome) like structure. Two ROV (Kaiko) dives and three manned submersible dives (Shinkai 6500) have been performed in 2001 and 2002 cruises. In addition, three single channel seismic profiles have been also done in NE-SW direction on the eastern half of the volcanic field. In Kauai Channel area, Seabeam mapping and back-scattered imaging have indicated the presence of numerous flat-topped cones and lava flows. These are distributed in the area approximately 50 km in diameter. Dome-shaped structure such as the Southwest Oahu volcanic field was not present. One ROV Kaiko dive and one Shinkai 6500 dive have been carried out in 2001 and 2002 cruise. Based on these results of 2001 and 2002 cruises, we would like to mention the objectives of scientific research.

Scientific significance

SW Ohahu Volcanic Field

Hawaiian Ridge has produced mainly huge shield-building magmatism, but thin submarine lava fields (Arch lavas) have been found far from the shield. The shield volcanoes have been formed along the axis of Hawaiian plume, and are composed mainly of tholeiitic magma, whereas the Arch lavas are alkalic, similar to so-called rejuvenated-stage magma. Considering spatial distribution in Hawaiian Ridge, the SW Oahu volcanic field locates apparently between the shield volcanoes and Arch lavas fields. In addition, preliminary estimation of magma of the SW Oahu volcanic field is about 3000 km3, three times of that of North Arch field. Thus, magmatism of the SW Oahu volcanic field could be a link between shield and Arch lava. In addition, the relationship between Arch type and rejuvenated-stage lava must be investigated to understand off axis magmatism of the Hawaiian plume.

Kauai Channel Volcanic Field

Many volcanic centers of rejuvenated-stage are distributed on both Oahu and Kauai islands, called as Honolulu and Koloa volcanics respectively. The Kauai Channel volcanic field is situated between the two volcanics. In order to reveal time and space development of these rejuvenated-stage magmatism, active age and chemical affinity of magmatism of the field would provide important information. In addition, numerous volcanic centers are distributed around Kauai Island and western off Oahu Island. If we investigate the Kauai Channel volcanic filed, we have to consider both subaerial and submarine rejuvenated-stage volcanics (Koloa and Honolulu volcanics). These studies could reveal time and space evolution and/or structure of Hawaiian plume.

Work plans
A research team has been established for the research of both the SW Oahu and the Kauai Channel volcanic field. On the basis of the results of the 2001 cruise combined with preliminary results of the 2002 cruise, the first paper for the SW Oahu volcanic field will be submitted soon by E. Takahashi et al. Further research will be conducted by each group as follows:

Petrology and Gochemistry (major, trace, REE and Sr-Nd isotopes): M. Nakagawa, N. Noguchi and D. Clague (Note. Pb isotopes will be added by a suitable person)

Study on pre-eruption volatile contents (EPMA & FT-IR) and glass chemistry (EPMA): M. Coombs and D. Clague

Study of noble gas isotopes: T. Hanyu

Magma genesis and eruption model (based on petrology and experiments): E. Takahashi

K-Ar isochron dating: N. Noguchi, M. Nakagawa, T. Itaya

Ar-Ar dating: N. Hirano