Within Segment B5, the easternmost AAD segment, a distinct discontinuity in the Sr, Nd, and Pb isotopic signatures of axial lavas marks the boundary between Indian Ocean and Pacific Ocean mantle provinces (Klein et al., 1988; Pyle et al., 1990, 1992). The boundary is remarkably sharp, although lavas with "transitional" characteristics occur within 50-100 km of the boundary (Fig. 2). Along the axis of the SEIR, the boundary is located within 20-30 km of the ~126°E transform, the western boundary of Segment B5. The boundary has migrated westward across Segment B5 during the last 3-4 m.y. (Pyle et al., 1990, 1992; Lanyon et al., 1995; Christie et al., 1998).
Although the recent history of this uniquely sharp boundary between
ocean basin-scale upper mantle isotopic domains has been reasonably well
defined by mapping and conventional dredge sampling, its long-term
relationship to the remarkable geophysical, morphological, and
petrological features of the AAD had not been determined prior to Leg 187.
The AAD is a long-lived major tectonic feature. Its defining characteristic
is its unusually deep bathymetry, which stretches across the ocean floor
from the Australian to the Antarctic continental margins and which may
have existed well before continental rifting began ~100 m.y. ago (Veevers,
1982; Mutter et al., 1985). The trend of this depth anomaly forms a
shallow west-pointing V-shape, cutting across the major fracture zones
that currently define the eastern AAD segments (Fig. 1, Fig. 3). This V-shape
implies that the depth anomaly has migrated westward at a long-term
rate of ~15 mm/yr (Marks et al., 1991), which is much slower than either
the recent migration rate of the isotopic boundary or the majority of the
known propagating rifts along the SEIR. Further, the relatively rapid
northward absolute motion of the SEIR requires that the mantle "source"
of the depth anomaly be linear and oriented approximately north-south.
Recently, Gurnis et al. (1998) have suggested that the source of this cold
linear anomaly lies in a band of subducted material that accumulated
before ~100 Ma at the 660-km mantle discontinuity beneath a long-lived
western Pacific subduction zone.
History of the Isotopic Boundary
Prior to Leg 187, the locus and history of the isotopic boundary before ~5 Ma were almost completely unknown. Possible long-term relationships between the isotopic boundary and the morphologically defined AAD could be divided into two distinct classes (schematically illustrated in Fig. 3). Either the recent (0-4 Ma) isotopic boundary migration simply reflects a localized (~100 km) perturbation of a geochemical feature that has been associated with the eastern boundary of the AAD since the basin opened, or the migration is a long-lived phenomenon that only recently brought Pacific mantle beneath the AAD. In the first case, the boundary could be related either to the depth anomaly or to the eastern bounding transform but not, in the long term, to both. The second possibility, that the isotopic boundary only recently arrived beneath the AAD, was first proposed by Alvarez (1982, 1990), who suggested that Pacific mantle began migrating westward when the South Tasman Rise first separated from Antarctica 40-50 m.y. ago. Limited geochemical support for this hypothesis came from the Indian and transitional isotopic signatures of altered ~38- and ~45-Ma basalts dredged to the north and east of the AAD by Lanyon et al. (1995) and from 60- to 69-Ma Deep Sea Drilling Project (DSDP) basalts that were drilled close to Tasmania (Pyle et al., 1992). Unfortunately, neither sample set is definitive. The dredged samples are from sites within the residual depth anomaly and therefore support two of the three possible configurations. The DSDP samples lie far to the east of the depth anomaly but very close to the continental margin. Their apparent Indian affinity is suspect because of the possibility that their mantle source has been contaminated by nearby subcontinental lithosphere. Finally, the fact that the oldest (~7 Ma) off-axis dredge sample from Zone A is of Pacific type (Christie et al., 1998) constrains possible loci of the Indian-Pacific boundary to intersect the eastern AAD transform north of approximately 47°45'S.
Introduction (continued) | Table of Contents