A resolved mantle anomaly as the cause of the Australian‐Antarctic Discordance

Abstract

We present evidence for the existence of an Australian‐Antarctic Mantle Anomaly (AAMA), which trends northwest‐southeast (NW‐SE) through the Australian‐Antarctic Discordance (AAD) on the Southeast Indian Ridge (SEIR), is confined to the upper 120 km of the mantle beneath the AAD, and dips shallowly to the west so that it extends to a depth of about 150 km west of the AAD. Average temperatures within the AAMA are depressed about 100°C relative to surrounding lithosphere and suggest very rapid cooling of newly formed lithosphere at the AAD to an effective thermal age between 20 and 30 Ma. A convective down welling beneath the AAD is not consistent with the confinement of the AAMA in the uppermost mantle. In substantial agreement with the model of Gurnis et al. [1998], we argue that the AAMA is the suspended remnant of a slab that subducted at the Gondwanaland‐Pacific convergent margin more than 100 Myr ago, foundered in the deeper mantle, and then ascended into the shallow mantle within the past 30 Myr, cutting any ties to deeper roots. The stability of the AAMA and its poor correlation with residual topography and gravity imply that it is approximately neutrally buoyant. The thermally induced density anomaly can be balanced by bulk iron depletion of less than 0.8%, consistent with the warmer conditions of formation for the Pacific than Indian lithosphere. We hypothesize that the low temperatures in the AAMA inhibit crustal formation and the AAD depth anomaly is formed at the intersection of the SEIR and the AAMA. The northward migration of the SEIR overriding the cold NW‐SE trending AAMA therefore presents a simple kinematic explanation for both the V‐shaped residual depth anomaly in the southeast Indian Ocean and the western migration of the AAD along the SEIR. Neither explanation requires the Pacific asthenospheric mantle to push westward and displace Indian asthenosphere. The AAMA may also act as a barrier to large‐scale flows in the shallow asthenosphere and may therefore define a boundary for mantle convection and between the Indian and Pacific isotopic provinces. The westward dip of the AAMA would also favor along‐axis flow from the Indian Ocean asthenosphere to the AAD that may contribute to the penetration of Indian Ocean mid‐ocean ridge basalts into the AAD.