BATHYMETRIC ROUGHNESS OF THE SOUTHEAST INDIAN RIDGE - IMPLICATIONS FOR CRUSTAL ACCRETION AT INTERMEDIATE SPREADING RATE MIDOCEAN RIDGES

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 spreadi ng rate, mantle temperature, and lithospheric strength at midocean rid ges. In order to provide observational constraints on the nature of th e changes in axial morphology, we examined the regional-and segment-sc ale variations in axial and flank morphology at the intermediate sprea ding Southeast Indian Ridge (SEIR) using newly collected geophysical d ata. An empirical orthogonal function analysis was used to separate re gional and local components of the topography field and to estimate ba thymetric roughness. Three distinct types of axial morphology were ide ntified from the regional component of ridge topography in our area: a xial highs, shallow axial valleys, and ''Mid-Atlantic Ridge-type'' dee p axial valleys. Axial depth increases by similar to 2100 m from 88 de grees E and 118 degrees E, while off-axis depth only increases by simi lar to 500 m. In addition, except for one segment with a deep axial va lley, there is little change in off-axis depth within segments, in con trast to the large intrasegment variations in axial depth. These obser vations indicate that the overall and intrasegment variations in crust al thickness are much smaller than would be predicted from the variati ons in axial depth and that the major portion of the variations in rid ge axis depth are dynamically supported. There are step-like increases in bathymetric roughness as axial morphology changes from an axial hi gh 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 fro m one mode of axial morphology to another is accompanied by an abrupt change in the strength of the lithosphere. The abrupt changes in litho spheric strength may be due to the existence of a ''threshold'' mantle temperature or crustal thickness about which the lithospheric strengt h is very sensitive to small fluctuations. Systematic intrasegment var iations in roughness are also observed. Roughness shows V-shaped patte rns within segments with axial highs but no clear pattern within segme nts with axial valleys. The different patterns in roughness at axial h ighs and axial valleys on the SEIR may result from the presence or abs ence of a magma chamber. The presence of a magma chamber at a ridge se gment with an axial high implies weaker axial lithosphere and hence lo wer roughness near the center of segments relative to the segment ends .