Evolution of the Australian-Antarctic discordance since Miocene time

In this study we chronicle the development of the Australian-Antarctic disc ordance (AAD), the crenelated portion of the Southeast Indian Ridge between similar to 120 degrees and 128 degrees E, since anomaly 6y time (19 Ma). W e reconstruct satellite-derived marine gravity fields and depth anomalies a t selected times by first removing anomalies overlying seafloor younger tha n the selected age, and then rotating the remaining anomalies through impro ved finite rotations based on a very detailed set of magnetic anomaly ident ifications. Our gravity field reconstructions reveal that the overall lengt h of the Australian-Antarctic plate boundary within the AAD has been increa sing since 19 Ma. Concomitantly, the number of propagating rifts and fractu re zones in the vicinity of the discordance has increased dramatically in r ecent times, effectively dividing it into its present-day configuration of five distinct spreading corridors (B1-B5) that are offset alternately to th e north and south and exhibit varying degrees of asymmetric spreading. Our bathymetric reconstructions show that the regional, arcuate-shaped, negativ e depth anomaly (deeper than predicted by normal lithospheric cooling model s) presently centered on the discordance began migrating westward before an omaly 5ad time (similar to 14.4 Ma), and that a localized depth anomaly low , which at time 5ad lay on the ridge axis in spreading corridor B5, has bee n split apart by subsequent seafloor spreading. The magnetic anomaly patter ns suggest that the depth anomaly is not always associated with a particula rly contorted plate boundary geometry. Although the plate boundary within t he AAD has been getting progressively more crenelated with time, this effec t shows little to no migration along the ridge axis since 19 Ma. Thus any g eodynamic models of the evolution of the discordance must account for the f ollowing observations: (1) the crenelation of the plate boundary within the AAD has increased with time, (2) the center of the crenelated zone does no t appear to have migrated along the ridge crest, and (3) both the depth ano maly and the isotopic boundary between Pacific and Indian mantle have been migrating westward along the ridge axis but at apparently different rates. We suggest that both along-axis migration of the depth anomaly and isotopic boundary, as well as temporal variation in the upwelling mantle material b eneath the AAD, and local tectonic effects are required in order to explain these observations.