A resolved mantle anomaly as the cause of the 2 Australian - Antarctic Discordance

6 [1] We present evidence for the existence of an Australian-Antarctic Mantle Anomaly 7 (AAMA), which trends northwest-southeast (NW-SE) through the Australian-Antarctic 8 Discordance (AAD) on the Southeast Indian Ridge (SEIR), is confined to the upper 9 120 km of the mantle beneath the AAD, and dips shallowly to the west so that it extends to 10 a depth of about 150 km west of the AAD. Average temperatures within the AAMA are 11 depressed about 100 C relative to surrounding lithosphere and suggest very rapid cooling 12 of newly formed lithosphere at the AAD to an effective thermal age between 20 and 13 30 Ma. A convective down welling beneath the AAD is not consistent with the 14 confinement of the AAMA in the uppermost mantle. In substantial agreement with the 15 model of Gurnis et al. [1998], we argue that the AAMA is the suspended remnant of a slab 16 that subducted at the Gondwanaland-Pacific convergent margin more than 100 Myr ago, 17 foundered in the deeper mantle, and then ascended into the shallow mantle within the past 18 30 Myr, cutting any ties to deeper roots. The stability of the AAMA and its poor 19 correlation with residual topography and gravity imply that it is approximately neutrally 20 buoyant. The thermally induced density anomaly can be balanced by bulk iron depletion 21 of less than 0.8%, consistent with the warmer conditions of formation for the Pacific 22 than Indian lithosphere. We hypothesize that the low temperatures in the AAMA inhibit 23 crustal formation and the AAD depth anomaly is formed at the intersection of the SEIR 24 and the AAMA. The northward migration of the SEIR overriding the cold NW-SE 25 trending AAMA therefore presents a simple kinematic explanation for both the V-shaped 26 residual depth anomaly in the southeast Indian Ocean and the western migration of the 27 AAD along the SEIR. Neither explanation requires the Pacific asthenospheric mantle to 28 push westward and displace Indian asthenosphere. The AAMA may also act as a barrier 29 to large-scale flows in the shallow asthenosphere and may therefore define a boundary 30 for mantle convection and between the Indian and Pacific isotopic provinces. The 31 westward dip of the AAMA would also favor along-axis flow from the Indian Ocean 32 asthenosphere to the AAD that may contribute to the penetration of Indian Ocean 33 mid-ocean ridge basalts into the AAD.