Hilina slump area, 2001

2-4. Hilina slump area, 2001 @[P Lipman, T. Sisson, and J. Kimura]

A key part of the first JAMSTEC Hawaii project has been the submersible investigation of deep-water portions of the Hilina slump area. The Hilina slump, south flank of Hawaii Island, is currently the most actively deforming flank of any oceanic island worldwide. This is an actively deforming area at least 40 km wide on the southeast flank of the currently active Kilauea volcano that is presently moving seaward at rates up to 10 cm/yr, which has been mapped by recent detailed multi-beam bathymetric surveys. The Hilina region consists of an assemblage of on-land slump blocks bounded by the Hilina fault system, a steep upper submarine slope down to about 300 m water depth that represents the primary constructional slope of the Kilauea shield, a mid-slope bench including a closed basin that has formed by compressional uplift along the toe of the slump, and a steep lower scarp that has generated large outlying slide block (Figure 2-4-1).
(Fig. 2-4-1)
In 1868 and 1975 this region moved abruptly several to tens of meters during major earthquakes (M7.9 and M7.2, respectively) with attendant destructive tsunamis. The tsunami generated in both 1868 and 1975 resulted in extensive damage and fatalities on Hawaii, and the 1975 tsunami produced minor damage in California. The possibility exists that future detachments of this type, or far more extensive and catastrophic debris avalanches, will occur in the future. The entire south flank of the island shows evidence for slumping and collapse. This proto-slump has now broken into two slumps that are buttressed in the middle by Loihi Seamount. These slumps are the Punaluu slump west of Loihi and the Hilina slump east of Loihi. The presence of debris-avalanche deposits along adjacent island flanks indicates the potential for catastrophic failure of such unstable slopes. The continuous creep and incremental movement associated with the large earthquakes are apparently driven by both magmatic processes within the active volcanoes and gravity. However, the mechanisms by which these slowly creeping slumps fail catastrophically are unknown, as are the precursors to such activity.

Previous results: Submersible observations and samples from the 1998-1999 cruises show that the lower south flank of Hawaii, offshore from Kilauea volcano and the active Hilina slump system, consists entirely of compositionally diverse volcaniclastic rocks; pillow lavas are confined to shallow slopes. Submarine-erupted basalt clasts have strongly variable alkalic and transitional basalt compositions (to 41% SiO2, 10.8% alkalis), contrasting with present-day Kilauea tholeiites. The volcaniclastic rocks provide a unique record of ancestral alkalic growth of an archetypal hotspot volcano, including transition to its tholeiitic shield stage, and associated slope-failure events. The geometry and diverse constituents of the slump terrain require initial slumping during submarine alkalic volcanism. A conspicuous NW-trending scarp, aligned with Papafu Seamount, bounds the slump terrain on its SW side but has no subaerial expression. In contrast, a parallel scarp, which bounds the Punalufu slump from Mauna Loa, merges on land with a conspicuous fault zone that moved 2-3 m laterally during the 1868 earthquake. This contrasting subaerial expression, along with the dominance of alkalic clasts in the lower scarp, suggests that the distal slump and its western bounding scarp initially developed prior to shield growth at Kilauea and inception of the active Hilina faults; size and volume of the tholeiitic shield are smaller than previously inferred. Such early structures may have localized younger Hilina slumping. Many questions remain, but perhaps the active Hilina slump structures on Kilaueafs south flank are in an early growth stage, thus posing greater future potential for larger-scale landsliding and devastating tsunamis.

The results of the 1998-99 dives onto the deformed flanks and adjacent seafloor of Hawaii provided critical motivation to return to this area with the ROV KAIKO in 2001. Many of the original questions relating to the mechanics and history of deformation along the flanks are still unanswered, and in addition, new ones have been raised. For example, what is the volcanic flank really composed of? The presence of indurated volcanic sandstones throughout the deep portions of the south flank of Kilauea suggest that the distal slopes of Hawaiian volcanoes are largely composed of sediment. This possibility has implications for the mechanical strength and long-term stability of the deforming flanks. However, the occurrence of primary volcanic rocks upslope of the sedimentary strata on Kilauea suggests that the transition from volcanic to clastic environment may be relatively complicated. In order to interpret the evolution of the islands, and the kinematics of the deforming flanks, it is important to know the location and nature of this transition.

Year 2001 dive targets: Three additional dives were carried out in the Hilina area, in order to test the interpretations of early hotspot magmatic evolution and concurrent slope failures developed so far (Figure 2-4-1). The priority targets were two sites along the lower flank of Papafu Seamount and the aligned transverse structural trend (including completing a dive lost to bad sea conditions in 1999), and a dive to test boundaries of the compositional transition from submarine-erupted alkalic to tholeiitic basalt (Fig. 2-4-1). The three 2001 dives together make a linked package that explore the previously unknown western boundary of the Hilina bench.

Dive #1 (K207) along the transverse structure was considered (unsuccessfully) the best potential opportunity to sample primary alkalic lavas of ancestral Kilauea, as the deep-water steep slope most proximal to the Kilauea eruptive center. Volcaniclastic materials collected along this slope should provide additional controls on the early growth history and petrologic evolution of Kilauea This dive site also provides an opportunity to evaluate deformation features related to the transverse boundary.

Dive #2 (K208) along a prominent rib above the western margin of the mid-slope bench was intended to constrain the three-dimensional distribution and compositions of pillow lavas, found at these depths farther to the east (dives K95, S504), versus the volcaniclastic rocks that dominate below the bench. Pillow lavas sampled during this dive should provide an important control on the compositional transition from alkalic to tholeiitic lavas during early growth of Kilauea.

Dive #3 (K209) along the southwest flank of Papafu Seamount was intended to test alternative interpretations of this enigmatic feature. Based largely on bottom photographs, this rounded submarine topographic high has previously been interpreted as a massive sand-rubble landslide deposit derived from an embayment near the Kilauea shoreline at the west margin of the active on-land Hilina fault system (Fornari and Moore, 1978). Alternatively, recent seismic-reflection profiling suggests that the seamount is a broad anticlinal structure, containing reflective bedded sediments of the mid-slope bench that have been compressionally upwarped along the transverse boundary of the Hilina bench during seaward thrusting (J. Morgan and G. Moore, written commun., 2000).

Geochemical objectives
The 1998 and 1999 JAMSTEC cruises to Hawaii revealed a nearly complete record of the geochemical and petrologic evolution of Kilauea volcano in rocks of the submarine Hilina region. This is the first case for any Hawaiian volcano where development can be traced from inception-stage strongly alkalic magmas, through transitional basalts, to fully mature shield-stage tholeiites. Interpretations based on existing samples are that both increasing degrees of partial melting and zonation in the composition of the Hawaiian plume contribute to the progression from alkalic to tholeiitic magma types. Results also point to stagnation of early alkalic magmas in the lithosphere where many underwent fractional crystallization, in contrast to the mature shield stage where primitive magmas are delivered to the bases of the volcano edifices.

Much of the evidence for the early alkalic phase of Kilauea consists of high-S alkalic glass grains in volcanic glass sandstones. These include nephelinites, basanites, phonotephrites, hawaiites, and alkalic basalts. Their high sulfur contents establish that they were erupted at great water depths and were not subaerial magmas erupted during the waning stages of mature Hawaiian volcanoes. They must represent products of the inception of volcano growth. The glass-bearing sandstones, as well as interstratified debris flow breccias containing alkalic clasts, are exposed along the frontal scarp of the 3000 m depth Hilina bench. Previous JAMSTEC dives have recovered only three nephelinite clasts suitable for conventional geochemical studies. The suite of basanite clasts is only slightly larger with four samples. A primary objective was therefore to expand the number of clast samples derived from ancestral Kilauea to better understand its geochemical diversity and the processes that led to its inception. Samples of primary alkalic eruptive deposits, such as pillow lavas, dikes, or sheet flows would further strengthen the case that the alkalic Hilina compositions are indeed products of Kilauea. Alkalic clasts or lavas are also suitable for 40Ar/39Ar dating that would augment the existing suite of six dated ancestral Kilauea samples.
Pillow lavas of the post-alkalic transitional-basalt phase have previously been sampled in only one area at the northeastern end of the Hilina bench. It is presently unknown if the suite of samples from that area is representative. Another stratigraphically controlled suite of pillow lavas with transitional compositions would improve understanding of this intermediate magmatic stage, and potentially could record either the cessation of alkalic magmatism or the onset of mature tholeiitic magmatism. More comprehensive pillow lava samples could also help to reveal the size and depth of the ancestral Kilauea edifice through analyses of volatiles quenched in pillow rind glasses, combined with known pressure - volatile solubility relations. Together, these studies will provide a much clearer and definitive picture of the development of Kilauea and other oceanic-island volcanoes worldwide.

Work plan: These continuing marine studies on the south flank slump 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. The Hawaiian Volcano Observatory of the USGS monitors earthquake activity in Hawaii, including the active Hilina slump area. Scripps Institute of Oceanography recently has installed a transect of GPS-measurable survey points across the Hilina bench; these will document the current motion of the underwater region adjacent to the actively deforming coastline. A joint Japanese-US research program is underway to study the seismicity of Kilauea Volcano. The University of Hawaii is completing interpretation of a seismic reflection survey of the Hilina bench that should allow first-order predictions about the types of materials that may outcrop at the seafloor along the flanks and in the slide blocks; these data were used to guide siting of the 2001 dives, particularly across the southwest flank of Papafu Seamount and the incised flank above the bench. Results of the new KAIKO dives will provide important constraints on the seismic interpretations and models. In addition the dive results provide an incomplete three-dimensional sampling of the submarine flank of Hawaii Island, apparently containing interfingering volcaniclastic debris derived from several volcanoes, that should make for instructive comparisons and contrasts with the complete one-dimensional section being provided by the in-progress Hawaii Scientific Drilling Project at Hilo.

Successful interpretation of the stratigraphic, structural, and petrologic complexities of the Hilina slump 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 the Hilina slump system and development of large slumps elsewhere in the Hawaiian chain and on other oceanic islands. Thorough study of the active Hilina slump area should permit fascinating comparisons with the new dive data on geometrically similar slump and compressional structures at Waianae, South Kona, and Laupahoehoe.

We will use submersible visual/video data and marine seismic reflection data to interpret the structure of the west margin of the Hilina bench, 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 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. Two new piston cores south of Hawaii Island (P12, P13) will also test the important stratigraphic, paleomagnetic, and geochronologic correlations among basalt-glass turbidites and fill gaps between cores P5 and P6 collected in 1998.