Gravity anomalies, flexure of axial lithosphere, and along-axis asthenospheric flow beneath the Southeast Indian Ridge

Mantle Bouguer and residual gravity anomaly patterns consistent with two- and three-dimensional mantle flow and/or crustal thickness variability, are observed juxtaposed within the five first-order segments of the Southeast Indian Ridge (SEIR) surveyed between 98° and 112°E. Within each segment ‘bullʹs-eye’ or ‘triangular’ patterns of mantle Bouguer and residual gravity anomalies, consistent with focused mantle upwelling and/or three-dimensional crustal thickness patterns, occur systematically toward the western ends of segments which adjoin significant left-stepping offsets in the ridge axis. The eastern ends of these segments and those that abut smaller, right-stepping transforms display a more two-dimensional anomaly character. Axial morphology also varies along strike; axial highs coincide with focused gravity anomalies, while rifted highs occur elsewhere. These observations imply that more magmatically robust extension, more vigorous mantle upwelling, and/or thicker crust characterize the western parts of those segments adjacent to significant ridge offsets. These trends are mirrored east of the deepest portions of the Australian-Antarctic Discordance (AAD), where more vigorous upwelling is located toward the eastern end of segment B5. In addition to the usual interpretations in terms of crustal thickness and/or mantle temperature, we suggest that the systematics of topographic style and gravity anomaly character provide further evidence for along-axis asthenospheric flow beneath the SEIR. In western portions of segments between 98° and 112°E, sub-axial flow toward the AAD causes asthenosphere to be drawn from beneath older lithosphere to the west, thereby increasing the region of mantle upwelling and melting supplying the ‘upstream’ portion of the ridge axis.