The Hilina region lies on the seaward south flank of Kilauea, the most active volcano on Earth. Kilauea’s south flank is mobile, moving south at 5-10 cm per year, punctuated by meters of displacement along down-to-the-south normal faults during earthquakes reaching magnitudes historically as great as 7.9. Offshore, the Hilina region contains a prominent bench at ~3000 m depth (the mid-slope bench) with a steep frontal scarp that descends to the ~5000 m seafloor. Large submarine landslides and slope failures ring the Hawaiian Islands, and the mobility and morphology of the Hilina region raises concerns that it might represent such a failure in its early stages. Accordingly, this offshore region has been studies intensively during the joint US – Japan JAMSTEC cruises in 1998, 1999, 2001 (12 dives), and now in 2002 (3 dives).
Important and unexpected discoveries were that the mid-slope bench consists of faulted and gently folded bedded volcaniclastic rocks (volcanic glass sandstones and debris flow breccias) that contain alkalic rocks and glass grains from an early submarine phase of Kilauea volcanism. This refutes earlier interpretations that the mid-slope bench was a large but simple slump of the Kilauea subaerial shield, or that it is debris shed from the modern tholeiitic shield, and agrees with new seismic studies showing well bedded strata in the mid-slope bench. In contrast to the bench, dives at and above its eastern end, as well as at the foot of the Puna Ridge, recovered in-place pillow lavas almost exclusively. Non-degassed samples predominate and are wholly transitional basalts distinct from modern Kilauea tholeiites. These transitional basalts are interpreted as an intermediate stage along the progression from early alkalic to modern tholeiitic magmatism. The view of a monotonic progression from alkalic to tholeiitic magmatism at Kilauea is independently supported by analyses of glass grains in a piston core collected south of Kilauea that includes an interval of alkalic glasses deposited 300 – 200 thousand years ago, based of magnetostratigraphy. Dives in 2001 added the important information that Papa’u Seamount, at the western end of the mid-slope bench, is composed exclusively of debris from subaerial eruptions of Mauna Loa, and that a rock rib directly above the mid-slope bench consists of alkalic, not transitional, pillow lavas.
1.
Relationship between Mauna Loa and early Kilauea
That Papa’u Seamount is derived from Mauna Loa, not Kilauea, and yet forms the western terminus of the mid-slope bench, is evidence that part of Mauna Loa underlay the ancestral Kilauea edifice. If so, then Kilauea may have begun as a relatively shallow water volcano built on Mauna Loa’s shoulder, not one that began at Cretaceous seafloor depths (~5000m). A second implication is that the summit portion of Kilauea is a veneer atop Mauna Loa’s flank and that most of Kilauea’s mass comprises the east rift zone and its submarine extension, the Puna Ridge. Dive S710 was planned to better establish the extent of Mauna Loa materials beneath the poorly examined deep western part of the mid-slope bench. The dive was positioned to ascend the corner region where the east-northeast trending frontal scarp intersects north-northwest trending escarpments that define the bench’s western end. A primary objective was to search for Mauna Loa-derived clasts in breccias, rare farther east along the frontal scarp but common at Papa’u Seamount. A secondary objective was to obtain additional alkalic clasts from early Kilauea to augment the sample base for geochemical and geochronologic studies. Recovered whole rock clasts will be analyzed for major and trace elements to determine if they originated from Mauna Loa or Kilauea. Eruption ages of any suitable alkalic samples will be measured to further establish the time span of Kilauea’s alkalic phase.
2.
Size and age of ancestral alkalic Kilauea
The rib of alkalic pillow lavas extending up from the mid-slope bench, discovered in dive K208, is the first section of in-place lavas from Kilauea’s alkalic phase yet discovered. That dive ended at 2100 m depth, leaving only 2 km of section for the transition to tholeiitic lavas exposed at the shoreline. The rock rib continues to 1600 m depth, and dive S709 was planned to continue sampling up the rib to better establish the size of the early alkalic edifice, provide further samples for radiometric dating that would better establish the duration of the alkalic phase, and hopefully to obtain samples recording the alkalic – transitional boundary. Recovered pillow samples will be analyzed for major and trace elements and Sr, Nd, Pb isotopes, glasses for major elements and volatile contents to establish eruption depths, and eruption ages will be measured on suitable samples by 40Ar/39Ar methods. These results will provide depth-time, composition-time, and source region-time estimates for the early history of Kilauea.
3. Linkages between proximal and distal sedimentation and the structure of the mid-slope bench
Piston core P6 from south of the Hilina region records the onset, peak, and end of Kilauea’s alkalic phase as an interval of abundant high-S alkalic glass grains. This sparse but complete record is not matched in volcaniclastic rocks of the Hilina frontal scarp, although it is suggested by a gross upward decrease in the abundance of alkalic materials. This inadequacy in the volcaniclastic record may be due to the absence of a complete and continuous sample section though the frontal scarp. Dive S708 was planned to complete the section through the frontal scarp begun as dive S508. Recovered clasts will be analyzed for major and trace elements and selected samples for Sr, Nd, Pb isotopes. Glass grains in sandstones will be analyzed for major elements, S, and Cl, by electron probe. The entire Hilina volcaniclastic data set will then be compared with the P6 piston core record to obtain a better understanding of the sedimentary processes that were active to the south of the growing submarine Kilauea edifice.
Obtaining a complete section through the mid-slope bench may also help to resolve an apparent discrepancy regarding the bench structure. Multi-channel seismic lines across the bench region have been interpreted as showing repeated section due to thrust faulting. Repeated section conflicts with the gross, but apparently systematic upward decrease in alkalic materials in the frontal scarp volcaniclastic rocks, but reversals in the compositional trend could have been missed due to inadequate sampling. The nearly complete section through the frontal scarp provided by dives S508 and S708 will assist in searching for evidence of thrust faulting and repeated section.