Ocean Drilling Program (ODP) Leg 183 is the first thematic leg to investigate the history of the Kerguelen Plateau/Broken Ridge, an example of unique Cretaceous emplacement of a large igneous province (LIP) in the southern Indian Ocean (Fig. F1). Understanding the origin and evolutionary history of the Kerguelen Plateau is of particular importance because it has been suggested to link to the onset of the Kerguelen-Heard mantle plume (e.g., Storey et al., 1989). The structurally complex Kerguelen Plateau has been divided into southern, central, and northern domains. Preliminary interpretations of the results of Leg 183 suggest that the age of the Southern Kerguelen Plateau, only hundreds of kilometers from Antarctica, formed 119 m.y. ago. To the north, the Central Kerguelen Plateau and the once-contiguous Broken Ridge and Elan Bank formed between 94 and 110 m.y. ago. In contrast, the Northern Kerguelen Plateau is much younger, having formed <35 m.y. ago (Coffin, Frey, Wallace, et al., 2000; Frey et al., 2000; Pringle and Duncan, 2000). These results indicate that several intense episodes of volcanism formed this large plateau over a long period of time, in contrast with the case for the Ontong Java Plateau in the Pacific, where the plateau appears to have formed mainly in a single massive volcanic event at ~122 Ma (Parkinson et al., 2001).
During Leg 183, the changes in magnetic and physical properties of basement rocks have been investigated by whole-core measurements of magnetic susceptibility, natural remanent magnetization (NRM), and compressional wave velocity. In general, we observed that for subaerial basalts the brecciated flow tops have higher susceptibilities and stronger NRM intensities than the massive interiors. Submarine lava flows display the opposite behavior; altered pillow rims have lower susceptibilities and NRM intensities than the massive pillow interiors. On the other hand, changes in compressional wave velocity show the same general trend for both subaerial and submarine basaltic rocks: the least-altered massive interior samples have velocities greater than the flow tops, which are highly altered (Coffin, Frey, Wallace, et al., 2000). Although these trends are consistent with observations reported in previous studies (e.g., Audunsson et al., 1992; Planke et al., 1999), further shore-based rock magnetic studies are needed to correctly identify the magnetic minerals, to explore the relationship between magnetic properties and lithology and alteration, and to evaluate the fidelity of the natural magnetic memory in the basalt rocks from the Kerguelen Plateau/Broken Ridge.
The purpose of this paper is to present the results of such a rock magnetic investigation carried out on basement cores recovered from Leg 183 sites. Additional work on the sediments of these sites is presented in Antretter et al. (this volume). We will first describe the rock magnetic results, focusing on the cores that provided the most readily interpretable data. We will then discuss and interpret the data in conjunction with lithology, alteration, and oxidation.