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 t wo-and three-dimensional mantle flow and/or crustal thickness variabil ity, are observed juxtaposed within the five first-order segments of t he Southeast Indian Ridge (SEIR) surveyed between 98 degrees and 112 d egrees E. Within each segment 'bull's-eye' or 'triangular' patterns of mantle Bouguer and residual gravity anomalies, consistent with focuse d mantle upwelling and/or three-dimensional crustal thickness patterns , occur systematically toward the western ends of segments which adjoi n significant left-stepping offsets in the ridge axis. The eastern end s of these segments and those that abut smaller, right-stepping transf orms display a more two-dimensional anomaly character. Axial morpholog y also varies along strike; axial highs coincide with focused gravity anomalies, while rifted highs occur elsewhere. These observations impl y that more magmatically robust extension, more vigorous mantle upwell ing, and/or thicker crust characterize the western parts of those segm ents 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 cr ustal thickness and/or mantle temperature, we suggest that the systema tics of topographic style and gravity anomaly character provide furthe r evidence for along-axis asthenospheric flow beneath the SEIR. In wes tern portions of segments between 98 degrees and 112 degrees E, sub-ax ial 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 a xis. (C) 1998 Elsevier Science B.V. All rights reserved.