The Southeast Indian Ridge (SEIR) spreads at a relatively narrow range of intermediate rates (59-75 km/Ma) but exhibits the full range of slow to fast spreading morphology and segmentation. Satellite gravity data reveal transitions in the structure of the spreading center where it is influenced by the Amsterdam and Kerguelen hotspots and at the Australian-Antarctic Discordance (AAD). Although the spreading rate between the hotspots and the AAD is nearly constant, the ridge exhibits a variety of distinct styles of morphology and segmentation not observed at fast or slow spreading centers. Recently, collected multibeam bathymetry data reveal a transition from East Pacific Rise style overlapping axial highs near 92 degrees E to Mid-Atlantic Ridge style axial valleys with non-transform offsets near 116 degrees E. The intervening segmentation is characterized by propagating offsets coexisting with stationary transforms which exhibit different degrees of temporal stability. Currently, there are 10 transform offsets between the hotspots and the AAD but only five of these have persisted since seafloor spreading stabilized at 35 Ma. The other five appear to have formed since 35 Ma and several more have disappeared by transform shortening or coalesced by along-axis propagation. There is a transition from monotonic offset propagation near the hotspots to oscillatory propagation approaching the AAD. This change in offset stability corresponds to transitions in depth, axial morphology and offset structure. Through much of the transitional region, higher order segmentation is characterized by en-echelon offsets of a diffuse spreading axis that generally lacks a well defined neovolcanic zone. Since the spreading rate is nearly constant, the regional variation in axial morphology and segmentation appears to be controlled by an upper mantle thermal gradient - possibly a result of flux of asthenosphere from the hotspots to the AAD. This is consistent with the gradual increase in average ridge flank depths along this part of the plate boundary but segment scale changes in axial depth reveal spatio-temporal variability in the dynamic topography that are not preserved on older lithosphere. Intrasegment transitions in axial morphology and en-echelon offsets within first order segments suggest that local variations in mantle thermal structure introduce short-lived instabilities in higher order segmentation and dominate the short term evolution of the plate boundary. (C) 1999 Elsevier Science B.V. All rights reserved.
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