The AAD is a segment of anomalous "chaotic" bathymetry and gravity along the SEIR between 120° and 128°E. Weissel and Hayes (1971) subdivided the SEIR south of Australia into three zones, Zone A (east of the AAD), Zone B (the AAD itself), and Zone C (west of the AAD), based on striking contrasts in seafloor morphology and geophysical data. In spite of a regionally uniform spreading rate (~74 mm/yr full rate), axial morphology between Zone A and Zone B (AAD) is quite different; Zone A exhibits a bathymetry similar to the fast-spreading East Pacific Rise-type spreading axis, whereas Zone B (AAD) has a deeper axial ridge similar to the slow-spreading Mid-Atlantic Ridge (e.g., Palmer et al., 1993; Shipboard Scientific Party, 2001).
Within the AAD (Zone B), five short but distinct and well-defined segments were denoted as B1-B5 from west to east (Vogt et al., 1983). The isotopic boundary between the Indian Ocean and Pacific Ocean mantle isotopic provinces for zero-age MORB lies within Segment B5 of the AAD (Pyle et al., 1992; Christie et al., 1998). The locality where Pacific-type MORB was recovered in Segment B5 is situated in "normal" seafloor terrain within a west-pointing V-shaped region (Christie et al., 1998). The bathymetric depth anomaly forms a shallow, west-pointing V or U shape, which stretches across the ocean floor from Antarctica to Australia (Marks et al., 1990) (Fig. F1A).
In order to determine the location of the isotopic boundary, 13 sites were drilled during Leg 187: four sites in Segments B4 and B5, six sites in the western part of Zone A, and two sites in central Zone A were chosen for drilling (Fig. F1B). Isotopic domains (Pacific or Indian) used in this study refer to Christie et al. (this volume) rather than shipboard discrimination (Shipboard Scientific Party, 2001). Based on postcruise isotope analysis, samples from Leg 187 are divided into two mantle domains: Pacific-type MORB and Indian-type MORB (Christie et al., this volume).