The northeast submarine flank of
Hawaii Island, offshore of subaerial Kohala and Mauna Kea, is marked by slump
morphology first recognized by Moore et al. (1989). Recent detailed mapping,
conducted during JAMSTEC and MBARI surveys, shows that two distinct slump
features exist: Laupahoehoe slump to the east and Pololu slump/slide to the
northwest (Smith et al., 2002). Laupahoehoe is marked by 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. Pololu slide impinges upon
(overrides?) the western edge of the Laupahoehoe structures, and is
characterized by hummocky and dissected terrain with poorly developed bench and
basin structure on its north flank.
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.
1. Relationship between Pololu and Laupahoehoe slumps
The relative ages and relationship between
Pololu and Laupahoehoe slumps remain unclear. It appears that the Pololu slump
may be overriding the basins of the Laupahoehoe slump, and therefore be
younger. However, it is unclear why the benches and basins of Pololu, while not
as well developed as those of the Laupahoehoe, trend distinctly east-west. This
is inconsistent with compressional thrusting or gravitational slumping from
Kohala, which would presumably lead to a more NW-SE orientation like that of
Laupahoehoe. This has led to the hypothesis that the Pololu slump is possibly
related to an east rift zone of the older Mahukona volcano (Smith et al., 2002).
This intriguing idea is lent credence by a series of four cones and one fissure
that occur on top of the slump, and appear to be in-place primary volcanic
features (Clague et al. 2002). Compositional comparisons between Pololu,
Laupahoehoe, and Mahukona samples should help determine whether this idea has
merit.
A puzzling feature related to the growth of Kohala (probably at
~0.4-1 Ma) 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.
3. Chemical diversity among Kea trend volcanoes
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.
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.
Thus our work on the Laupahoehoe and Pololu slumps will interface with Hilo
Ridge dives (see Hilo Ridge scientific objectives section for more
information).
Year 2001 results:
Kaiko dive K213
ascended a steep rib on one of the prominent scarps on the deep protion of the
Laupahoehoe slump. Two dominant lithologies were identified: a lower unit of
olivine to picritic basalt, likely originally emplaced as sheet flows, and
upper fine-grain volcanic breccia and sandstone. The basalts from the lower
portion of the dive contain a transition from transitional to tholeiitic
compositions. Glasses from sandstones in the upper portion of the dive are
predominantly subaerially erupted tholeiites.
We have two targets planned for
2002 Shinkai 6500 dives. The first will closely complement K213 and is located
on the outer scarp of a ridge at the same elevation and slightly SE of the 2001
dive, providing additional observations of the structure and lithology of the
Laupahoehoe bench complex. This block appears to be structurally above the one
traversed during K213.
The
second planned dive will take place on the northern deep portion of the Pololu
slump, and will provide the first direct observations and samples of this
feature. It is also on a scarp outboard of closed basin, smaller than the
basins seen on Laupahoehoe. This analogous setting will allow direct comparison
of the structure of the two slumps. Analysis of samples from this dive will
allow determination of the provenance of the slump: is it from Kohala, or
possibly a feature related to an eastern rift zone of the older Mahukona
volcano to the west?
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 and Pololu slide areas has critical implications for understanding the primary depositional growth of the submarine flanks of oceanic volcanic islands, and also for structural evolution of Kohala, Mauna Kea, and Mahukona 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 (obtained in 2001) to interpret the structure of the Laupahoehoe and Pololu benches, sedimentation in their 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 lavas and volcaniclastic rock fragments. 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. Piston core northeast of Hawaii Island (P9, P10, taken in 2001) 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.