2-3. Waianae landslide �@[M. Coombs and J. Moore]

Submarine landslides are significant in the history of Hawaiian volcanoes, as are also an important geologic hazard. Sidescan surveys of the Hawaiian Exclusive Economic zone have identified 70 landslides over 20 km in length (Moore et al., 1989). Two categories of landslides are recognized: debris avalanches and slumps. Slumping is the result of gravitational spreading and dike injection along rift zones. In general, studies on these features have had two broadly defined purposes. The first is to understand the structure and emplacement mechanisms for such huge, catastrophic events. The second is to understand the evolution of Hawaiian volcanoes by studying the deep-seated rocks that are exposed on landslide blocks.

The Hilina slump area, off the south flank of Kilauea, has been studied in detail during the 1998 and 1999 JAMSTEC cruises, and followup studies are planned for this cruise. The results of this work show that such these features are rich sources of information regarding the geologic and geochemical evolution of island volcanoes because they provide a window into the early phases of Hawaiian eruptive activity (i.e., Lipman et al, 2001; Sisson et al., 2001). Another landslide that has been the target of recent study is the Nuuanu/Wailau landslide complex, northeast of Oahu and north of Molokai. These debris avalanches produced many blocks that traveled hundreds of kilometers eastward (Moore et al., 1989). Reconstruction of the blocks from Nuuanu provides a picture of the pre-slide Koolau Volcano on Oahu (Yokose 2001). Geochemical analysis of landslide blocks from Wailau and Nuuanu slides may provide information of the source regions of the slides (Shinozaki et al. 2001; Clague et al. 2001).

Waianae landslide is a slump-type landslide, southwest of Oahu, and provides a second opportunity (along with the Hilina slump south of the island of Hawaii) for study of a large ocean island slump. It consists of several coherent blocks spaced 5-20 km apart, separated by transverse scarps (Moore et al. 1989). The bulk of the slump is believed to consist of material from Waianae volcano, although the 130-km-wide slump may comprise material from Kaena ridge to the west and Penguin bank to the east (Moore et al., 1989). Previous research on land has suggested that the catastrophic movement of the Waianae slump triggered a major compositional change in the post-shield lavas of Waianae Volcano (Pressley, et al., 1998). Investigation of this little known landslide should provide new information regarding the processes of volcano deformation and collapse, as well as possibly revealing the early magmatic history of Waianae volcano. Proposed studies on Waianae are similar to those described above for Hilina and Nuuanu/Wailau.

The results obtained during this cruise may be used in conjunction with a dive on an upper Waianae slump block (158?28'10" W, 21?23'15", -2560 to -1920 m), performed earlier this summer by Dave Clague at MBARI, to provide a more complete picture of the evolution of Waianae. The MBARI dive sampled a series of lava whose compositions are tholeiitic.

Our objectives for study of the Waianae slump are twofold. The first is to combine geologic observations made during ROV dives, SeaBeam mapping, and single channel seismic data to better understand the structure and emplacement of the Waianae landslide blocks. Our second objective is to use material collected from deep landslide blocks to understand the early history of Waianae volcano. The tilted fault blocks that comprise the slump likely expose rocks older than those seen on land. The steep toe of the slump (the outer face of the outermost block), meets the seafloor abruptly at a depth of ~4700m. Deep sites potentially provide access to very early Waianae volcano, and thus may provide information regarding the geochemical and geologic evolution of Waianae, in much the same way that volcaniclastic rocks from the Hilina slump are derived from early Kilauea (e.g., Lipman et al. 2001). If the parallel to Hilina/Kilauea holds true, the rocks collected may be from the early alkalic stage of the volcano's history, and thus dateable.

References
Clague DA, Moore JG, Davis AS, 2001. Volcanic breccia and hyaloclastite in blocks from the Nuuanu and Wailau landslides, Hawaii. AGU Monograph, in press.
Lipman, PW, Sisson TW, Ui T, Naka J, Smith JR, 2001. Ancestral submarine growth of Kilauea volcano and instability of its south flank. In AGU Monograph, in press.
Moore JG, Clague DA, Holcomb RT, Lipman PW, Normark WR, Torresan ME, 1989. Prodigious submarine landslides on the Hawaiian ridge. Jour Geophys Res 94, 17465-17484.
Pressley, T., J. Sinton, N, Pringle, 1998, Post-shield volcanism and catastrophic mass wasting of the Waianae Volcano, Oahu, Hawaii: Bull. Vol., v. 58, p. 597-616.
Shinozaki K, Ren Z-Y, Takahashi E, 2001. Geochemical and petrological characteristics of Nuuanu and Wailiau landslide blocks. AGU Monograph, in press.
Sisson TW, Lipman PW, Naka J, 2001. Submarine alkalic through tholeiitic shield-stage development of Kilauea Volcano, Hawaii. AGU Monograph, in press.
Yokose H, 2001. Landslides on the windward flanks of Oahu and Molokai, Hawaii: SHINKAI 6500 Submersible investigations. AGU Monograph, in press.
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