Reviews of the application of magnetic fabric analysis to various rock types including igneous rocks have been provided by Tarling and Hrouda (1993), Jackson and Tauxe (1991), and Rochette et al. (1992). Magnetic fabric studies of various kinds of igneous bodies have been used extensively to examine their structure and emplacement history. Examples of fabric studies of igneous bodies include those involving dikes (e.g., Knight and Walker, 1988; Archanjo et al., 2002), sills (e.g., Ferré et al., 2002; Liss et al., 2002), plutonic complexes (Lopez-de-Luchi et al., 2002; Steenken et al., 2000; Talbot et al., 2000), and lava flows (e.g., Herrero-Bervera et al., 2002; Morris, 2000). Although there are many magnetic fabric studies of igneous bodies, those involving oceanic basalts are much less numerous, in part because of their complex extrusion and cooling histories (Ellwood, 1975, 1978; Ellwood and Watkins, 1976).
The primary purpose of these studies has been to improve our understanding of emplacement mechanisms for igneous bodies. Magnetic fabrics have been used to provide important constraints on the flow regimes associated with the formation of these bodies, especially dikes (Callot et al., 2001; Aifa and Lefort, 2001) and flows (Glen et al., 1997; Canon-Tapia et al., 1995). Recent work on the magnetic anisotropy of lava flows has focused more attention on their internal structure and the character of the boundaries between successive flows (e.g., Canon-Tapia et al., 1997; Canon-Tapia and Coe, 2002). These studies have examined individual flows in considerable detail to identify and characterize within-flow variations that can be interpreted in terms of their spatial position and cooling history.
In this study we present the results of magnetic fabric measurements on basalts from four sites drilled during Ocean Drilling Program Leg 192. Magnetic properties of both pillowed and more massive units obtained at Sites 1183, 1185, 1186, and 1187 on the Ontong Java Plateau (Fig. F1) are examined in an attempt to characterize systematic differences. Our sampling of the basalt cores is not sufficient to allow us to identify and characterize small-scale (approximately a few centimeters) within-flow variations for the Ontong Java Plateau. However, several of the within-flow characteristic features reported by Canon-Tapia and Coe (2002) in their study of the Birkett lava flow are apparent, if not well documented, in our results. To obtain a rough measure of within-unit variations in magnetic fabric, we made a detailed study of a single 1.3-m-thick complete pillow basalt from Site 1187. Although it is not known how typical this pillow may be and therefore whether other pillows would display similar variability, its overall dimensions and level of alteration are directly comparable to many others drilled during Leg 192, suggesting that it may be used to illustrate the variability present in many other pillows.
A directional analysis of the anisotropy of magnetic susceptibility axes was also carried out for samples from each site in order to determine whether a preferred azimuth attributable to flow is present and also to examine differences in this azimuth between individual sites that might reflect the local rather than regional nature of the flow.