The Australian Antarctic Discordance (AAD) (Fig. F1) is a long-lived regional tectonic feature that encompasses the deepest (4–5 km) regions of the Southeast Indian Ridge (SEIR) and, indeed, the entire global mid-oceanic spreading system. Although the AAD has become well known for its anomalous depth, its defining characteristic is the unusual crenellated geometry formed by short (~100 km), deep axial valleys that are offset by long (2–300 km) transforms with alternating offset directions. This geometry has only developed during the past 25–30 m.y. (Vogt et al., 1983; Marks et al., 1999).
The present-day anomalously deep bathymetry of the AAD is the manifestation of the Australian Antarctic Depth Anomaly (AADA), a long-lived residual depth anomaly that extends from the Australian to the Antarctic continental margins and most likely predates the onset of continental rifting at ~100 Ma (Veevers, 1982; Mutter et al., 1985; Gurnis and Müller, 2003). North and south of the SEIR, the axis of the AADA describes a shallow arc that is convex to the west and cuts across the major eastern AAD fracture zones (Figs. F1, F2). The shape of this arc (Fig. F1) indicates a slow (~15 mm/yr) long-term eastward motion of the SEIR spreading system relative to a fixed north-south elongate mantle thermal anomaly (Marks et al., 1990, 1999; Gurnis et al., 1998; Ritzwoller et al., 2003). Initially, the AAD and the AADA were geographically distinct entities. The northeastward absolute motion of the plate boundary has systematically decreased their spatial separation through time, and they have come into conjunction only since ~12 Ma. These and other abbreviated terms are defined in Table T1.
The anomalous depth and segmentation of the AAD region are associated with unusually low magma production and with relatively cool upper mantle temperatures (e.g., Forsyth et al., 1987; Ritzwoller et al., 2003). East of the AAD, the SEIR in Zone A (Fig. F1) is characterized by a pronounced axial-ridge morphology with smooth off-axis topography, features that are typically associated with fast-spreading centers (>90 mm/yr). Within the AAD (Zone B), the SEIR is characterized by deep axial valleys with rough off-axis topography, features that are typically associated with slow-spreading centers (<50 mm/yr) (Palmer et al., 1993; Sempéré et al., 1991, 1996; West et al., 1994, 1997). In parts of the AAD, unusually large areas are dominated by chaotic seafloor terrain formed by listric faulting in a magma-poor, predominantly tectonic extensional environment (Christie et al., 1998). The contrasts in seafloor spreading between Zone A and the AAD are accompanied by distinct contrasts in the compositions and compositional diversity of basaltic lavas. These differences imply that a distinct change in fundamental seafloor accretionary processes occurs over a remarkably short distance along axis. Because it occurs at a uniform intermediate spreading rate of 74 mm/yr (DeMets et al., 1990), this change must reflect a fundamental change in magma supply (Palmer et al., 1993; Sempéré et al., 1991, 1996).
The AADA is a unique bathymetric feature that spans the Southern Ocean Basin from the Australian to the Antarctic continental margins. It is defined by seafloor depths that are as much as 1000 m deeper than surrounding (normal) seafloor of comparable age. The geodynamic process(es) that created and maintain the AADA have been active at least since seafloor spreading began at ~100 Ma and possibly for as long as 300 m.y. (Veevers, 1982; Mutter et al., 1985; Gurnis and Müller, 2003).
Gurnis et al. (1998) and Gurnis and Müller (2003) used plate reconstruction models to demonstrate that the inferred cold north-south elongate source of the depth anomaly coincides with the location, in a fixed-mantle reference frame, of a long-lived, pre-100-Ma western Pacific subduction zone. They argued that cold mantle beneath the AADA is associated with refractory subducted material from this subduction zone that accumulated at the 660-km mantle discontinuity. Mantle flow models based on this conclusion suggest that some of this refractory material with its associated mantle wedge material has recently been entrained into the upwelling upper mantle beneath the AAD.
More recently, Ritzwoller et al. (2003) identified the Australian Antarctic Mantle Anomaly (AAMA), a northwest-southeast–trending linear band of high upper mantle seismic velocities that passes beneath the AAD. Northward migration of the SEIR over this oblique mantle anomaly would account for westward migration of the depth anomaly at a rate consistent with the curvature of the depth anomaly. They suggested that the AAMA consists of residual cold material that has risen to a level of neutral buoyancy in the upper mantle. The northwest-southeast orientation of the high-velocity AAMA appears, however, to be inconsistent with Gurnis's tectonic reconstruction. No mechanism has yet been proposed to account for either the persistence of cold, dense material at shallow mantle depths or the apparent 45° rotation of an original north-south–trending western Pacific paleosubduction zone.
Mid-ocean-ridge basalt (MORB) 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 (Subbarao and Hedge, 1973; Dupré and Allégre, 1983; Hart, 1984; Hamelin and Allégre, 1985; Hamelin et al., 1986; Price et al., 1986; Dosso et al., 1988; Mahoney et al., 1989, 1992). Along the SEIR, the Indian Ocean MORB mantle (IMM) isotopic province abuts the Pacific Ocean MORB mantle (PMM) province at a uniquely sharp boundary within the AAD. This was first recognized by Klein et al. (1988). Subsequent along-axis dredging (Moana Wave cruise MW88; Pyle et al., 1992) defined a very narrow (~50 km) zone within which lavas have transitional MORB mantle (TMM) isotopic signatures. This transition zone characterizes the present-day PMM/IMM boundary that exists beneath the western end of Segment B5 within the eastern AAD. Pyle et al. (1992) also recovered IMM lavas from two off-axis (3–4 Ma) sites located on flow lines directly south of sections of the B5 axis that presently erupt TMM and PMM lavas. The simplest explanation for this change in mantle source beneath Segment B5 is a recent westward migration of PMM during the last 4 m.y. at ~25 mm/yr (Pyle et al., 1992).
The apparent migration rate of the isotope boundary across Segment B5 (~25 mm/yr) is intermediate between the rate of westward migration of the AADA (~15 mm/yr) and rates of rift propagation toward the AAD from the east (30–40 mm/yr). These faster rates are consistent with hypotheses of Alvarez (1982, 1990) that invoke large-scale upper mantle flow from the shrinking Pacific to the expanding Indian Ocean Basin as Australia separated from Antarctica. If such a flow was initiated 30–40 m.y. ago upon final continental separation of the South Tasman Rise from Antarctica (Hinz et al., 1990), migration velocities of ~40 mm/yr would be required to account for the recent arrival of PMM beneath the eastern margin of the AAD. The IMM/PMM boundary configuration produced by this type of migration is indicated by the heavy black lines in Figure F1.
Pyle et al. (1995) set out to evaluate the Alvarez rapid migration hypothesis through a regional geochemical study based on basalts from Deep Sea Drilling Project (DSDP) Legs 28 and 29. At the same time Lanyon et al. (1995) studied altered basalts from four dredge sites close to the continent/ocean boundary north of the AAD. Both studies showed that IMM was present east of the AAD prior to 30 Ma, apparently consistent with large-scale upper mantle flow from the east. However, because of the sparse and nonideal distribution of available samples, it was not possible to conclusively distinguish between possible off-axis boundary configurations consistent with rapid migration and a boundary that was permanently associated with the AADA.
To better locate the Indian/Pacific mantle boundary off axis, the Boomerang 05 (BMRG05) and Sojourn 05 (SOJN05) expeditions of the Melville mapped and sampled 3- to 7-Ma seafloor in Segment B5 and the adjacent Segments A1 and B4. Christie et al. (1998) showed that the eastern limit of the IMM domain coincides with a west-pointing, V-shaped topographic boundary that separates chaotic terrain to the west from normal seafloor. The geometry of this remarkable boundary is consistent with the ~25 mm/yr westward migration of the PMM/IMM boundary across Segment B5 during the last ~4 m.y. (Fig. F2). Off-axis dredging east of the AAD recovered PMM lavas from seafloor as old as ~7 Ma. This inability to locate the PMM/IMM boundary in Zone A left the two principal mantle migration hypotheses unresolved. The question of whether the topographic and isotopic boundary within Segment B5 represented the culmination of long-term, rapid migration or a localized perturbation in a boundary associated in the long term with the AADA remained unresolved.
The principal objective of Ocean Drilling Program (ODP) Leg 187 was to evaluate the long-term (10–30 Ma) history of this mantle domain boundary, focusing on the two alternate hypotheses outlined above: (1) that the recent migration is the culmination of long-term, rapid migration of PMM from the east or (2) that the IMM/PMM boundary is an intrinsic feature of the AADA. To be consistent with the second hypothesis, the 0- to 4-Ma migration must represent a localized short-term perturbation of a stable long-term boundary location (Pyle et al., 1992; Christie et al., 1998; Christie, Pedersen, Miller, et al., 2001).