Earth history is punctuated by massive magmatic events. Resultant mafic large igneous provinces (LIPs) provide the strongest evidence that at certain times in the past, energy transfer from the Earth's interior to its surface has occurred in a manner substantially different from modern plate tectonic processes. Cretaceous time, in particular, is marked by voluminous and episodic basaltic magmatic events generated from the mantle, and these events appear to correlate with extreme states or rapid changes in the oceans, atmosphere, and biosphere. The Kerguelen Plateau/Broken Ridge, one of two giant oceanic plateaus formed in Cretaceous time, is a prime target for investigating (1) mantle processes resulting in LIPs; (2) mechanisms of growth, emplacement, and post-constructional deformation of LIPS; and (3) environmental consequences of voluminous mafic magmatism.

Ocean Drilling Program (ODP) Leg 183 will penetrate igneous basement to depths of ~150 to 200 m at several morphologically and tectonically diverse locations on the ~2 x 106 km2 LIP formed by the Kerguelen Plateau/Broken Ridge in the Southeast Indian Ocean. This leg will build on results obtained by basement drilling at four ODP sites on the Central and Southern Kerguelen Plateau during Legs 119 and 120. A major objective of Leg 183 is to determine the magmatic chronology of the Kerguelen Plateau/Broken Ridge LIP by determining the eruption ages of the uppermost igneous crust at several locations. 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.

During evolution of a LIP, it is likely that hydrothermal and metamorphic processes differ from those occurring in a spreading ridge environment. Therefore, another objective is to use cores of the basement and overlying sediments to assess the interaction between LIP magmatism and the surficial environment. Episodes of high magma flux during formation of a LIP may have significant impact on the Earth's hydrosphere, atmosphere, and biosphere. Additional goals of Leg 183 are to determine the mechanism of LIP growth and the tectonic history of the plateau by integrating seismic data with studies of the sedimentary and igneous cores (i.e., seismic volcanostratigraphy). Specifically, these cores will be used to address the following issues: the timing and extent of initial uplift, the relative roles of subaerial and submarine volcanism, the cooling and subsidence into a submarine environment, and the multiple episodes of post-emplacement deformation.

A unique aspect of this LIP is its clear association with a long linear volcanic ridge, i.e., the Ninetyeast Ridge. Dating of basement basalt from the seven Deep Sea Drilling Project (DSDP) and ODP drill holes that penetrated the igneous basement of the Ninetyeast Ridge established a systematic south to north progression of ages from 38 to ~82 Ma along this hot spot track. In addition, the Kerguelen Archipelago and Heard Island, constructed on the Northern and Central Kerguelen Plateau, respectively, have a volcanic record from ~38 Ma to the present. Studies of subaerial lavas from these islands and submarine lavas recovered by drilling provide a 115 m.y. record of volcanism that can be used to evaluate the hypothesis that the Kerguelen Plateau/Broken Ridge system is related to decompression melting of a plume head and that the subsequent Ninetyeast Ridge and oceanic island volcanism are related to partial melting of the following plume tail. The Kerguelen plume is particularly important because it is a source of an "enriched isotopic component" that forms an end-member in the isotopic arrays defined by ocean island basalts, and it may have been important in creating the distinctive isotopic characteristics of Indian Ocean ridge basalts. Determination of spatial and temporal variations in geochemical characteristics of the basalts forming the Kerguelen Plateau and Broken Ridge are essential for understanding the early history of the Kerguelen plume.

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