UTIG Research Projects Archive

Principal Investigator: Mike Coffin

Funded by: National Science Foundation

The Ontong Java Plateau (OJP) was the focus of geophysical surveying during February 1998 by scientists from Japan, the United States, Norway and Poland led by A. Taira aboard the University of Tokyo's R/V Hakuho Maru. (Fig. 1). The expedition had several objectives, the one of most interest to ODP-oriented scientists being to obtain seismic data to support proposed ODP drilling, hopefully by JOIDES Resolution during its upcoming campaign in the Indian and Pacific Oceans. The OJP is easily the largest of Earth's large igneous provinces (LIPs), being at least five times more voluminous than the Deccan Traps in western India. It is suggested to have formed initially by a very large mantle plume in mid-Cretaceous time, a hypothesis with profound geodynamic and geochemical implications. This time (~120 Ma) also coincides with the transition into mid-Cretaceous "Greenhouse" climates, elevated sea levels, reduced strontium ratios, black shale events and severe fluctuations of the paleobiota, and the OJP is the prime suspect in the minds of many investigators as the cause of these drastic changes in the global environment. However, its igneous basement has been sampled at only three oceanic drill sites and on two of the Solomon Islands. These basalts have radiometric dates that so far outline two distinct volcanic episodes, an initial and probably main volcanic pulse from 124-120 Ma and a subsequent pulse at about 90 Ma. These few dates and their associated geochemistries are clearly a preliminary and inadequate characterization of this giant plateau.

Geophysical site surveying was accomplished with multichannel reflection and refraction seismology on the main plateau, on the transition from the main plateau to the Nauru Basin, within the Nauru Basin, and on the "Eastern Salient" of the plateau just north of Stewart Basin (Figure 1). In general this multichannel reflection work was done with 2 or 3 large airguns (17 and 20 liter) fired to a 48-channel seismic streamer. The refraction experiments were done by shooting to sonobuoys throughout the area and to a crossed array of 15 ocean-bottom seismometers (OBSs) on the crest of the main plateau. The OBS array was emplaced to record deeper crustal arrivals, both from our airgun sources and from more distant earthquakes. These data will be processed at several shore-based labs (OBSs at the University of Tokyo, multichannel reflection and most of the sonobuoys at the Institute for Geophysics, The University of Texas at Austin, and the remaining sonobuoys at University of North Carolina, Wilmington).

Preliminary results from shipboard are encouraging. Basement was generally apparent throughout the survey area on the shipboard monitor records and on preliminary stacks of the multichannel data. Although refractions received by sonobuoys were difficult to obtain in the notoriously reverberant Nauru Basin, good refractions were obtained over critical areas there and excellent refractions were recorded over the main OJP. Reflection records over the main plateau reveal smoothly-layered, undistorted sediments overlying a relatively smooth basement surface. This combined morphology is in marked contrast to the pervasive faulting and erosion present on the crest of Earth's second largest LIP, the Kerguelen Plateau in the Indian Ocean. Moving down the OJP towards the Nauru Basin the basement surface remains relatively smooth and unfaulted. This simplifies tracing crustal structure from the main plateau to the adjacent basin, a critical goal for model discrimination. Much of the western Nauru Basin has a strongly reverberant acoustic character that proved by drilling in the northern Nauru Basin to be mid-Cretaceous basalts at least 600 m thick. However, a "seismic window" through these basalts exists near the eastern end of our long east-west seismic line that probably reveals original, Earliest Cretaceous basement (140-145 Ma). In contrast to the main OJP, the crest of the Eastern Salient just north of Stewart Basin is characterized by faulting of the basement surface. The overlying sediments, whose thickness is only a few hundred meters, show angular unconformities that suggest block rotation and erosion. This suggests that the history of the Eastern Salient is very different from the main part of the OJP and that this area experienced major deformation subsequent to formation of the main plateau.

The R/V Hakuho Maru is a 100-m long, well-equipped research vessel that transits routinely at 15 knots and is staffed with highly professional officers and crew. All of the non-Japanese scientists were impressed with the efficiency of operations and with the hospitality of the Japanese crew and scientific parties. These characteristics and good weather combined to allow more than 90% of the pre-expedition goals to be accomplished.

Cruise: R/V Hakuho Maru KH98-1 Leg 2 (1998)

UTIG and affiliated staff: M. Coffin, M. Wiederspahn, K. Mochizuiki, O. Eldholm, T. Gladczenko (shipboard); S. Saustrup, J. Jiao, L. Gahagan (shore-based)

Publications:

Coffin, M.F., and Gahagan, L.M., 1995. Ontong Java and Kerguelen Plateaux: Cretaceous Icelands? Journal of the Geological Society, London, 152, 1047-1052.

Gladczenko, T.P., Coffin, M.F., and Eldholm, O., 1997. Crustal structure of the Ontong Java Plateau: modeling of new gravity and existing seismic data, Journal of Geophysical Research, 102, 22,711-22,729.