The structure and segmentation of the Southeast Indian Ridge

摘要

The Southeast Indian Ridge (SEIR) spreads at arelatively narrow range of intermediate rates (59-75 km/Ma) butexhibits the full range of slow to fast spreading morphology andsegmentation. Satellite gravity data reveal transitions in thestructure of the spreading center where it is influenced by theAmsterdam and Kerguelen hotspots and at the Australian-AntarcticDiscordance (AAD). Although the spreading rate between thehotspots and the AAD is nearly constant, the ridge exhibits avariety of distinct styles of morphology and segmentation notobserved at fast or slow spreading centers. Recently, collectedmultibeam bathymetry data reveal a transition from East PacificRise style overlapping axial highs near 92°E to Mid-AtlanticRidge style axial valleys with non-transform offsets near 116°E.The intervening segmentation is characterized by propagatingoffsets coexisting with stationary transforms which exhibitdifferent degrees of temporal stability. Currently, there are 10transform offsets between the hotspots and the AAD but only fiveof these have persisted since seafloor spreading stabilized at 35Ma.The other fove appear to have formed since 35 Ma and severalmore have disappeared by transform shortening or coalesced byalong-axis propagation. There is a transition from monotonicoffset propagation near the hotspots to oscillatory propagationapproachin the AAD. This change in offset stability corresponds totransitions in depth, axial morphology and offset structure.Through much of the transitional region higher order segmentationis characterized by en-echelon offsets of a diffuse spreading axisthat generally lacks a well defined neovolcanic zone. Since thespreading rate is nearly constant, the regional variation in axialmorphology and segmentation appears to be controlled by an uppermantle thermal gradient-possibly a result of flux of asthenospherefrom the hotspots to the AAD. This is consistent with the gradualincrease in average ridge flank depths along this part of the plateboundary but segment scale changes in axial depth reveal spatio-timporal variability in the dynamic topography that are notpreserved on older lithosphere. Intrasegment transitions in axialmorphology and en-echelon offsets within first order segmentssuggest that local variations in mantle thermal structure introduceshort-lived instabilities in higher order segmentation and dominatethe short therm evolution of the plate boundary.