2-7. Kohala-Laupahoehoe landslide �@[P. Lipman]

Dives on the deep ridges and elongate basins of the Laupahoehoe slump area (Smith et al., in press), newly identified by the JAMSTEC SeaBeam surveys (Fig. 2-7-1), would provide especially intructive counterparts to the Hilina and South Kona benches, as well as potential samples of ancestral Kohala volcano. The Hilo Ridge, previously interpreted as the submarine extension of an east rift zone of Mauna Kea, may alternatively be the east rift zone of a highly elongate large Kohala edifice (Holcomb et al, 2000; Kauahikaua et al., 2000); as such it could also provide important samples of early Kohala growth.

The Laupahoehoe slump appears to be overridden by the Pololu debris avalanche, located farther NW and previously interpreted to have its headwall in the large subaerial valleys on NE Kohala (Moore et al., 1989. Characterized by an ENE failure direction and hummocky terrain containing blocks and/or cones 2-5 km in diameter of 50-200 m relief, the Pololu slide impinges upon structures at ~3000 m water depth (wd) that are proposed to constitute the Laupahoehoe slump or compressional bench. These structures are NE-oriented scarp-and-bench topographic features analogous to the Hilina bench on the mobile SE flank of Kilauea; they may have formed similarly by mechanisms of volcano spreading at the toe of a slump structure initiated higher on the island. Six enclosed basins (100-400 m deep, 4-10 km long, 1-5 km wide) lie at 3000-5000 m wd, fronted by 50-200 m high ridges on their seaward sides. The basins may result from local rotational slumps or from uplift above discontinuous thrust faults in the bench.

Prominent slope breaks at ~1100 and ~400 m wd mark the end of tholeiitic shield building at Kohala and Mauna Kea respectively; the Laupahoehoe benches are overlain by both shield margins and most likely were derived from an elongate Kohala edifice. The smoothly dipping slope of northeastern Mauna Kea at 1200-3000 m wd appears to have grown over the older slide terrane; it resembles the upper Kilauea flank of the Hilina slump, which is known to be well-sedimented and composed of primary volcanics. The two-slide complex is 90 km wide and abuts the distal Haleakala east rift zone. An outer debris apron continues 95 km from the base of the island, as recorded by GLORIA side-scan sonar images.

A puzzling feature for a feature related to the growth of Kohala (probably at ~0.4-1 Ma), however is the presence of closed basins, incompletely filled with sediment, behind the Laupahoehoe benches. Alternatively to a purely Kohala-related origin, could the closed basins record continued volcano spreading due to proximity to the high Mauna Kea edifice, but without development of upslope extension and slump structures? Critical observations would be presence or absence of recent deformation structures and/or fresh talus on the deep scarps below the Laupahoehoe benches.

Another important problem is the degree of similarity/differences among tholeiites from the successive Kea-trend volcanoes on Hawaii Island (Kilauea, Mauna Kea, Kohala) that have been potential sources of volcaniclastic deposits sampled in the Hilina bench, Hawaii Scientific drill hole, and the distal turbidites. Reliable compositional data are now available for Kilauea (young lavas, marine samples) and Mauna Kea (drill hole), but not from Kohala because of the ubiquitous weathering of its subaerial tholeiite. Basalt samples from the deep-water lower slope of the Laupahoehoe benches and/or the Hilo Ridge will have far better quality for petrologic study than subaerial tholeiite material from Kohala, and should provide some of the first reliable materials to characterize this large Kea-trend volcano. These sites also offer the possibility of recovering transitional or alkalic basalt from early growth stages of Kohala, analogous to those recently found along the lower scarp of the Hilina bench offshore of Kilauea volcano.

Year 2001 dive targets: The attached map (Fig. 2-7-1) show two recommended dive sites on the Laupahoehoe slump targeting steep slopes along the lower benches, based mainly on the available JAMSTEC bathymetry, geologic interpretation (Smith et al., in press), and analogies with successful sampling strategies along the lower south flank of Hawaii Island. Especially promising is the western site, because it is likely closest to the intrusive core of Kohala, as demarked by gravity data, and thus is the best candidate to sample debris from the ancestral alkalic edifice. If initial dive(s) in the Laupahoehoe area are successful, they will provide a framework for a more detailed future study, including relations with deeper parts of the Pololu slide. A third potential pilot dive site is along the base of the distal Hilo Ridge. This ridge has never been studied by diving, and compositional date for only a few (4?) rock samples have been reported from dredging.
(Fig. 2-7-1)
Work plan: The new dives on the north submarine flank of Hawaii Island will closely interface with the abundant existing subaerial 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 Laupahoehoe slide area, for which questions far 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 Kohala and Mauna Kea and development of large slumps elsewhere in the Hawaiian chain and on other oceanic islands. Thorough study of the geologically young, but seemingly inactive Laupahoehoe benches and basins area should permit instructive comparisons with the new dive data on geometrically similar slump and compressional structures at Hilina (Kilauea volcano), Waianae (Oahu), and South Kona (Mauna Loa).

We will use submersible visual/video data and marine seismic profiling data to interpret the structure of the Laupahoehoe benches, sedimentation in its basins, the distribution of more distal slide blocks, and their relation to volcano spreading along a basal the detachment. 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 trace elements for bulk-rock samples by XRF, INAA, and solution-based ICP-MS methods, glass compositions by electron-probe (EPMA), laser-ablation ICP-MS, and ion-probe analyses, and volatile contents by EPMA and FTIR measurements. Radiogenic and stable isotopic compositions of pillow- glass, glass-sand, and whole-rock samples will be compared to analogous data from other Hawaiian volcanoes, especially with the detailed data emerging for Mauna Kea from the Hawaii Scientific Drilling Project. We will use a combination of dating techniques, including K-Ar, and 40Ar/39Ar methods, to determine the eruption ages of any high-K basalt samples. New piston core northeast of Hawaii Island (P10, P11) will also test the important stratigraphic, paleomagnetic, and geochronologic correlations among basalt-glass turbidites and permit comparisons with the landslide record on the north flank of Kohala and Haleakala volcanoes.