Recent multibeam bathymetric and geophysical data from the Southeast Indian Ridge (SEIR) reveal eight propagating ridges (PRs) moving down the regional bathymetry gradient toward the Australian-Antarctic Discordance (AAD). These PRs are propagating at a nearly uniform rate of 40- mm yr-1 despite dramatic variations in axial morphology and segment length, and local reversals of the regional bathymetric gradient. Because existing dynamic rift propagation models do not explain the continued, uniform propagation of the diverse SEIR PRs, we have developed a new model that includes variable crustal thickness and thermally-driven, along-axis asthenospheric flow, in addition to topographic gradients, as positive contributions to the forces driving rift propagation. Along-axis asthenospheric flow and variations in crustal thickness along the SEIR appear to provide the first-order driving force required for ridge propagation, even in the highly segmented, magmatically starved region in and near the AAD where previous models for ridge propagation fail. Further, our model development requires that the rift valley at the propagating tip is a consequence of rifting in the cooler thermal regime of preexisting lithosphere, rather than a consequence of viscous resistance to flow. This explanation of PR tip topography is consistent with gravity data from the SEIR that suggest that the rift tip and associated pseudofaults are at least partially compensated by crustal thinning. ¿ 1999 American Geophysical Union