THE SOUTHEAST INDIAN RIDGE BETWEEN 88-DEGREES-E AND 118-DEGREES-E - VARIATIONS IN CRUSTAL ACCRETION AT CONSTANT SPREADING RATE

The temperature of the mantle and the rate of melt production are para meters which play important roles in controlling the style of crustal accretion along mid-ocean ridges. To investigate the variability in cr ustal accretion that develops in response to variations in mantle temp erature, we have conducted a geophysical investigation of the Southeas t Indian Ridge (SEIR) between the Amsterdam hotspot and the Australian -Antarctic Discordance (88 degrees E-118 degrees E). The spreading cen ter deepens by 2100 m from west to east within the study area. Despite a uniform, intermediate spreading rate (69-75 mm yr-l), the SEIR exhi bits the range in axial morphology displayed by the East Pacific Rise and the Mid-Atlantic Ridge (MAR) and usually associated with variation s in spreading rate. The spreading center is characterized by an axial high west of 102 degrees 45'E, whereas an axial valley is prevalent e ast of this longitude. Both the deepening of the ridge axis and the ge neral evolution of axial morphology from an axial high to a rift valle y are not uniform. A region of intermediate morphology separates axial highs and MAR-like rift valleys. Local transitions in axial morpholog y occur in three areas along the ridge axis. The increase in axial dep th toward the Australian-Antarctic Discordance may be explained by the thinning of the oceanic crust by similar to 4 km and the change in ax ial topography. The long-wavelength changes observed along the SEIR ca n be attributed to a gradient in mantle temperature between regions in fluenced by the Amsterdam and Kerguelen hot spots and the Australian-A ntarctic Discordance. However, local processes, perhaps associated wit h an heterogeneous mantle or along-axis asthenospheric flow, may give rise to local transitions in axial topography and depth anomalies.