2-9. North Kona bench complex �@[P. Lipman]

Studies on the west (Mauna Loa-Hualalai) side of Hawaii Island, where there are multiple young landslide features (especially South Kona. Alika, and North Kona), provide counterparts to compare and contrast with the existing detailed Hilina and Nu'uanu results. The North Kona bench complex (Fig. 2-9-1), first recognized by the USGS GLORIA side-scan surveys (Lipman et al., 1988) and then in more detail from SeaBeam bathymetry (Chadwick et al., 1993) involves multiple geometrically intricate proximal mid-slope benches and basins, and a a distal region containing scattered small blocks and hummocks(up to 1 km across and 100 m high). In contrast to the Hilina slump, no extensional block fault structures are preserved on land; any such faults originally present may have been completely buried on the steep slopes draped by young lavas between the northwest rift zone of Hualalai and the Kealakekua fault that roughly coincides with the northem extent of lavas from Mauna Loa.
(Fig. 2-9-1)
The North Kona benches are likely entirely related to Hualalai volcano: in addition to the well defined northwest rift zone that was active during the tholeiitic shield stage of this volcano, a south rift zone has been traced 10-15 kn south of the Kealakekua fault by on-land gravity data (Kauahikaua and Hildenbrand, 2000), indicating that Hualalai extends well to the south of its present surface exposures (which are overlapped by Mauna Loa lavas just north of the Kealakekua fault. Because tholeiitic volcanism ceased at Hualalai at >100 ka, it seems unlikely that lavas from Mauna Loa will interfinger at deep ocean depths of the lower North Kona benches. Although Hualalai has been in the post-shield alkalic stage for > 100,000 years, it continues to erupt on average every few hundred years (most recently in 1801) and is among the potentially most hazardous of Hawaiian volcanoes because of the substantial population living on its slopes and the likely high flow velocities likely from its low-viscosity alkalic lavas.

The large benches of the North Kona slump have never been visually imaged, focus of dive targets, or even sampled by dredging. The deeper parts of several of them provide exceptional places to search for lavas that may extend our view of Hualalai magmatism back in time. This would provide new views on the evolution of the Hawaiian plume. Evidence may also be obtained on the relative proportions of pillow lava vs hyaloclastite involved in construction of the submarine flanks of large oceanic-island volcanoes, a current topic of controversy. Several dives by the Monterey Bay Aquarium Research Institute (MBARI) were carried out in May 2001, just south of the
Alika landslide that bounds the south side of the North Kona slump, but no results are yet available.

The lithologic sequence of lavas and volcaniclastic rocks on the mid-slope benches, if studied in detail, may provide critical information on (1) the early growth of Hualalai, (2) frequency of large submarine slumps from another active Hawaiian volcano, (3) further information on the relative role of extensional slumping versus distal compression and uplift in generating the submarine benches on the offshore flanks of several Hawaiian volcanoes, and (4) any possible involvement of rocks from Mauna Loa volcano.

Year 2001 dive targets: The 2001 KAIKO dives were designed to provide pilot information on the feasibility of such studies. One dive (K218) was designed to traverse a 900 m section up the steep lower bench scarp downslope from the northwest rift zone of Hualalai, and a second dive (K219) to traverse a similar 800-m scarp on a bench directly downslope from the summit region of this volcano. The resulting video images and in-places samples, when adequately analyzed, should provide important information on the primary volcanic sources (compositional comparisons with young Hualalai lavas) and eruption environment (subaerial vs. submarine).

Work plan: The new data on the North Kona terrain will closely interface with the abundant existing data and several in-progress studies on Hawaiian volcanoes and with ongoing slope-stability studies in Hawaii and elsewhere. Successful interpretation of the stratigraphic, structural, and petrologic complexities of the North Kona area, for which questions still outnumber answers, has critical implications for understanding the primary depositional growth of the submarine flanks of oceanic volcanic islands, and also for structural evolution of Hualalai and development of large slumps elsewhere in the Hawaiian chain and on other oceanic islands. Thorough study of the geologically young, but seemingly inactive North Kona area should perrnit instructive comparisons with the new dive data on geometrically similar slump and compressional structures at Hilina (Kilauea volcano), South Kona (Mauna Loa), Laupahoehoe
(Kohala), and Waianae (Oahu) .

We will use submersible visual/video data and marine seismic profiling data to interpret the structure of the North Kona bench and more distal slide blocks, and their relation to volcano spreading along a basal detachrnent. Samples will be analyzed chemically and petrographically in order to determine compositions and eruption depths of pillow lavas and the fragments in volcaniclastic rocks. Analytical methods will include major and minor elements for bulk-rock samples by XRF and INAA methods, glass compositions by electron-probe, and ion-probe analysis, and volatile contents by FTIR measurement. Radiogenic and stable isotopic compositions of pillow-glass, glass-sand, and whok~rock samples will be compared to analogous data from other Hawaiian volcanoes. Results could also permit informative comparisons on the subaerial-submarine, and tholeiitic alkalic transitions, especially with the detailed data emerging from the Hawaii Scientific Drilling Project for Mauna Kea. We will use a combination of dating techniques, including K-Ar, and 40Arl39Ar methods, to determine the eruption ages of any high-K basalt samples. A new piston core west of Hawaii Island (P13, P14) will also test the important stratigraphic, paleomagnetic, and geochronologic correlations among basalt-glass turbidites and permit comparisons with the landslide record on the west flank of Hualalai.

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