Introduction (continued) | Table of Contents


The Indian and Pacific Mantle Isotopic Provinces
Lavas erupted at Indian Ocean spreading centers are isotopically distinct from those of the Pacific Ocean, reflecting a fundamental difference in the composition of the underlying upper mantle. Along the Southeast Indian Ridge (SEIR), the Indian Ocean and Pacific Ocean mantle isotopic provinces are separated by a uniquely sharp boundary. This boundary has been located to within 25 km along the spreading axis of the SEIR within the Australian Antarctic Discordance (AAD) (Klein et al., 1988; Pyle et al., 1992; Christie et al., 1998), and subsequent off-axis dredge sampling has shown that the Pacific mantle has migrated rapidly westward during at least the last 4 m.y. The principal objective of Leg 187 was to delineate this boundary farther off axis, allowing us to infer its history over the last 30 m.y.

The Australian Antarctic Discordance
The AAD (Fig. 1) is a unique region, encompassing one of the deepest (4-5 km) regions of the global mid-oceanic spreading system. Its anomalous depth reflects the presence of unusually cold underlying mantle and, consequently, of thin crust. Despite a uniform, intermediate spreading rate, the SEIR undergoes an abrupt morphologic change across the eastern boundary of the AAD. The region east of the AAD, known as Zone A, is characterized by an axial ridge with smooth off-axis topography (characteristics usually associated with fast-spreading centers), whereas the AAD, also known as Zone B, is characterized by deep axial valleys with rough off-axis topography (characteristics usually associated with slow spreading centers). Other anomalous characteristics of the AAD include a pattern of relatively short axial segments separated by long transforms with alternating offset directions, unusually thin oceanic crust, chaotic seafloor terrain dominated by listric extensional faulting rather than magmatism, high upper mantle seismic wave velocities, and an intermittent asymmetric spreading history (Weissel and Hayes, 1971, 1974; Forsyth et al., 1987; Marks et al., 1990; Sempéré et al., 1991; Palmer et al., 1993; West et al., 1994; West, 1997; Christie et al., 1998). The morphological and geophysical contrasts across the eastern boundary of the AAD are paralleled by distinct contrasts in the nature and variability of basaltic lava compositions, reflecting fundamental differences in magma supply because of strong contrasts in the thermal regime of the spreading center.

Mantle Flow and the Isotopic Boundary
The AAD appears to be the locus of converging asthenospheric mantle flows. This is suggested by multiple episodes of ridge propagation from both east and west toward the AAD (Vogt et al., 1984; Cochran et al., 1997; Sempere et al., 1997; Sylvander, 1998; West et al., 1999) and by recent numerical model studies suggesting that significant, convergent, subaxial mantle flow is an inevitable consequence of gradients in axial depth and upper mantle temperature around the AAD (West and Christie, 1997).

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