Principal Investigator: Mike Coffin

Funded by: Joint Oceanographic Institutions/U.S. Science Advisory Program

Yves Joseph de Kerguélen-Trémarac discovered La France Australe (today's Kerguelen Archipelago) in 1772, and in the same year claimed this 'southern motherland' for France. Upon returning home, his eloquent rhapsodizing to the reigning French monarch about Kerguelen's superb agricultural and mineral potential resulted in the king dispatching three ships and 700 men to colonize Kerguelen in 1773. However, 699 pairs of eyes couldn't lie, and upon Kerguélen's second homecoming to France, he was court-martialed, sentenced to 20 years in prison, and dismissed from the Navy. In contrast, after James Cook landed at La France Australe on Christmas Eve 1776, and one of his men found a bottle containing a parchment inscribed in Latin telling of Kerguélen's visits, his moniker for the locale was the "Isles of Desolation." Successful geophysical surveys on the Kerguelen Plateau were recently undertaken by Australia (1997) and France (1998) over their subsea territories generated by Heard and MacDonald (eponymic for American John J. Heard and Briton William McDonald, who discovered the islands in 1853 and 1854), and the Kerguelen Isles, respectively. The new site survey data from these cruises allowed scheduling of ODP Leg 183, which will venture to those isolated reaches of the Southern Ocean later this year.

Despite the challenging winds and seas of the Kerguelen "Triangle," where two Vendée Globe round-the-world racing yachts capsized early last year, Australia and France have each mounted three multichannel seismic campaigns to the Plateau since the inception of ODP. Thus the Kerguelen Plateau, an enigmatic, heavily structured mafic large igneous province (LIP) second in size only to Ontong Java, has the most extensive geophysical data base of any oceanic plateau worldwide. On the basis of recently acquired seismic data, ODP Leg 183 will address mantle processes resulting in the formation of LIPs, tectono-magmatic development of LIPs themselves, and environmental consequences of voluminous mafic magmatism (Figs. 1, 2).

LIPs, a significant type of planetary volcanism found on the Earth, Moon, Venus, and Mars, represent voluminous fluxes of magma emplaced over relatively short time periods, such as expected from decompression melting of an ascending relatively hot mantle plume. Terrestrial LIPs are dominantly mafic rocks formed episodically in Earth history, perhaps in response to fundamental changes in the processes which control energy and mass transfer from the Earth's interior to its surface. LIPs are important for several reasons. They provide information about mantle compositions and dynamics that are not reflected by volcanism at spreading ridges. For example, today LIPs account for only 5% to 10% of the heat and magma expelled from the earth's mantle, but the giant LIPs may have contributed as much as 50% in Early Cretaceous time, thereby indicating a substantial change in mantle dynamics from the Cretaceous to the present. Since magma fluxes represented by oceanic plateaus are not evenly distributed in space and time, their episodicity punctuates the relatively steady-state production of crust at seafloor spreading centers. These intense episodes of igneous activity temporarily alter the flux of magma and heat from the mantle to the crust, hydrosphere and atmosphere, possibly resulting in global environmental change, such as excursions in the chemical and isotopic composition of seawater. In addition, because oceanic LIPs may resist subduction, they may be future building blocks of continental crust. Yet, despite their huge size and distinctive morphology, oceanic plateaus remain among the least understood features in the ocean basins.

ODP Leg 183 will focus on sampling the ~2 x 10⁶ km² Kerguelen Plateau/Broken Ridge LIP to address four first order problems related to the characterization and quantification of mafic igneous crustal production and its effects during Cretaceous and Cenozoic time. The specific objectives are to: 1) determine the chronology of Kerguelen/Broken Ridge magmatism; 2) constrain mineralogy and composition of mantle sources, melting processes, and post-melting magmatic evolution; 3) evaluate the effects of LIP formation on the environment; 4) identify and interpret relationships between LIP development and tectonism. At each of five primary drilling sites on the Kerguelen Plateau (Fig. 1) and one on Broken Ridge, 200 m of igneous basement penetration is planned. This leg will build on results obtained by basement drilling at four sites on the central and southern Kerguelen Plateau during ODP legs 119 and 120. Studies of basement basalt obtained from dredges and drill cores from Legs 119 and 120 show that much of the southern Kerguelen Plateau formed at 110 to 115 Ma, whereas the central Kerguelen Plateau and parts of Broken Ridge are younger, ~85 Ma. However, ages of basement from major morphological features, such as Elan Bank and the submarine northern Kerguelen Plateau, are unknown because they have not been previously sampled.

Austral summers 1997 and 1998 witnessed three marine geophysical surveys on the Kerguelen Plateau. Two Australian expeditions aboard R/V Rig Seismic in 1997, RS179 and RS180, focused on Elan Bank and the SE Plateau flank, respectively. One French campaign aboard Marion Dufresne in 1998, MD109, investigated the northern Plateau. High quality multichannel seismic (MCS) data acquired during the three research cruises have been used to locate all five primary ODP Leg 183 drill sites. USSAC supported Mike Coffin's participation on RS179, and processing of ODP-relevant seismic data from RS179 and RS180 at the Institute for Geophysics, the University of Texas at Austin.

During the two six-week R/V Rig Seismic cruises last year, Australian, American, and Norwegian scientists acquired ~3,500 km of MCS and other geophysical data. The MCS acquisition system consisted of a 20-airgun, 3000 in³ source, and a 3000 m, 240-channel digital streamer. RS179 concentrated on Elan Bank, a prominent salient extending westward from the main Kerguelen Plateau, and completed a long transect across the entire Plateau from the Enderby Basin in the west to the Australia-Antarctic Basin in the east. RS180 focused on the Plateau's eastern flank. The MCS data show dipping intrabasement reflections on Elan Bank, suggesting subaerial basalt flows. A deep MCS transect of the entire Plateau between 53°S and 58°S documents major tectonic events following plateau construction, including Cretaceous faulting and Tertiary reactivation of Labuan Basin flanking the Plateau to the east. Several MCS lines show thick sediment and sediment drifts in the Labuan Basin, where Antarctic Bottom Water flows along the Plateau's eastern flank.

The primary objective of both R/V Rig Seismic cruises was to define the extent of Australia's claim to 'legal' Continental Shelf (LCS, i.e., beyond the 200-nautical-mile Australian Exclusive Economic Zone (EEZ)) generated by Heard and McDonald islands, both active volcanoes (and inscribed on the World Heritage list last year on the basis of their outstanding examples of biological and physical processes in an essentially undisturbed environment). Under the 1982 United Nations Convention on the Law of the Sea (UNCLOS), beyond the EEZ countries may claim LCS which UNCLOS grants a nation sovereign rights to explore and exploit the resources of the seabed and subsoil. To define the outer limit of Australia's LCS on the Kerguelen Plateau, information is required on the physiography and sediment thickness of 'continental' margins that extend beyond the EEZ. Data to support Australia's LCS claim need to be collected, interpreted, and presented to the recently elected U.N. Commission on the Limits of the Continental Shelf within ten years of UNCLOS entering into force, i.e., by 2004 for Australia. The Kerguelen Plateau surveys form one important part of Australia's program of collecting data necessary for Australia to meet its commitments under UNCLOS.

A secondary objective of the two cruises was to carry out geophysical site surveys for ODP Leg 183. Three dedicated site surveys, for KIP-6, KIP-7, and KIP-13, were completed (Fig. 1). The KIP-6 site will sample Elan Bank, a major morphotectonic component of the Kerguelen Plateau province, for the first time. The site will constrain Elan Bank's age, composition, construction environment, and tectonic history. The KERIMIS (KERguelen, Imagerie Multifasceau et Imagerie Sismique) campaign aboard Marion Dufresne (MD109) earlier this year acquired ~2,000 km of MCS data and dredged >1000 kg of rocks from the northern Kerguelen Plateau in investigating the feature's structure and geodynamics and carrying out site surveys for ODP Leg 183. The seismic acquisition system consisted of a 2400 m, 96-channel digital streamer, and a 10-Generator-Injector (GI) airgun (G=1050 in³; I=960 in³) source. Two major transects across the entire northern Plateau from the Enderby Basin to the Australia-Antarctic Basin were completed, and three dedicated site surveys, for KIP-1, KIP-2, and KIP-3 were undertaken (Fig. 1). The KIP-2 site will sample the Plateau north of the Kerguelen Archipelago for the first time to establish its age, composition, and construction environment, as well as to document tectonic events and test plate reconstructions.

The MCS and other geophysical data acquired during the recent Rig Seismic and Marion Dufresne cruises have been critical to site selection for ODP Leg 183, and those countries' allocations of significant resources in support of ODP objectives on the Kerguelen Plateau are gratefully acknowledged. Australia's site surveys aboard >i>Rig Seismic were funded and conducted by the Australian Geological Survey Organisation (Project Leader Phil Symonds; Cruise Leaders Doug Ramsay and George Bernadel), in collaboration with the Institute for Geophysics, The University of Texas at Austin, and the Department of Geology, University of Oslo, Norway, with financial support provided by the U.S. Science Support Program (USSSP) of JOI. [n.b., The Rig Seismic was retired from AGSO service in last month, representing a significant loss to the non-industrial geophysical research fleet worldwide, and currently operates in the commercial sector.] France's site survey aboard Marion Dufresne was conducted by the Ecole et Observatoire des Sciences de la Terre (EOST) of the UniversitŽ Louis Pasteur (Strasbourg 1) (Chief Scientist Roland Schlich), in collaboration with the DŽpartement des Sciences de la Terre et de l'Environnement de l'UniversitŽ Libre de Bruxelles (ULB), with the assistance of the Institut Franais de la Recherche et de l'Exploitation de la Mer (IFREMER-GENAVIR) with multichannel seismic reflection, and the logistical and financial support of the Institut Franais pour la Recherche et al Technologie Polaires (IFRTP).

Cruise: R/V Rig Seismic 179/180 (1997)

UTIG and affiliated staff: M. Coffin and T. Gladczenko (shipboard); S. Saustrup, A. Lopez, and L. Gahagan (shore-based)

Publications:

Coffin, M.F., Pringle, M.S., and Storey, M., submitted. Kerguelen Hot Spot Output since 130 Ma, Science. . Könnecke, L.K., Coffin, M.F., and Charvis, P., 1998. Early development of the Southern Kerguelen Plateau (Indian Ocean) from shallow wide-angle ocean bottom seismometer and multichannel seismic reflection data, Journal of Geophysical Research, 103, 24,085-24,108.