Figure 3. Mantle boundary configurations allowed by pre-Leg 187 geochemical data from the dredge sites shown. Indian and Pacific populations are the same as those in Figure 2. Migration across Segment B5 is confirmed, with the boundary constrained to the vertically ruled area labeled "Migrating boundary in B5." East of the Australian Antarctic Discordance (AAD), a hypothetical boundary produced by long-term westward migration is constrained to the medium-gray shaded region in the upper right quadrant. Alternate boundary configurations are associated with the curving trace of the depth anomaly or oscillation between the easternmost AAD transforms. The more southerly Leg 187 drill sites are shown as dark gray solid circles. The remaining sites lie to the north and east of this map. Magnetic anomalies are numbered 2-5. SEIR = Southeast Indian Ridge.