The nature of the transition from axial highs to axial valleys at mid-ocean ridges and the physical processes involved in the transition are important for understanding how axial morphology changes with spreading rate, mantle temperature, and lithospheric strength at midocean ridges. In order to provide observational constraints on the nature of the changes in axial morphology, we examined the regional-and segment-scale variations in axial and flank morphology at the intermediate spreading Southeast Indian Ridge (SEIR) using newly collected geophysical data. An empirical orthogonal function analysis was used to separate regional and local components of the topography field and to estimate bathymetric roughness. Three distinct types of axial morphology were identified from the regional component of ridge topography in our area: axial highs, shallow axial valleys, and ''Mid-Atlantic Ridge-type'' deep axial valleys. Axial depth increases by similar to 2100 m from 88 degrees E and 118 degrees E, while off-axis depth only increases by similar to 500 m. In addition, except for one segment with a deep axial valley, there is little change in off-axis depth within segments, in contrast to the large intrasegment variations in axial depth. These observations indicate that the overall and intrasegment variations in crustal thickness are much smaller than would be predicted from the variations in axial depth and that the major portion of the variations in ridge axis depth are dynamically supported. There are step-like increases in bathymetric roughness as axial morphology changes from an axial high to a shallow axial valley and from a shallow axial valley to a deep axial valley. The step changes in roughness imply that the change from one mode of axial morphology to another is accompanied by an abrupt change in the strength of the lithosphere. The abrupt changes in lithospheric strength may be due to the existence of a ''threshold'' mantle temperature or crustal thickness about which the lithospheric strength is very sensitive to small fluctuations. Systematic intrasegment variations in roughness are also observed. Roughness shows V-shaped patterns within segments with axial highs but no clear pattern within segments with axial valleys. The different patterns in roughness at axial highs and axial valleys on the SEIR may result from the presence or absence of a magma chamber. The presence of a magma chamber at a ridge segment with an axial high implies weaker axial lithosphere and hence lower roughness near the center of segments relative to the segment ends.
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