The Southeast Indian Ridge between 88°E and 118°E: Variations in crustal accretion at constant spreading rate

Abstract

The temperature of the mantle and the rate of melt production are parameters which play important roles in controlling the style of crustal accretion along mid‐ocean ridges. To investigate the variability in crustal accretion that develops in response to variations in mantle temperature, we have conducted a geophysical investigation of the Southeast Indian Ridge (SEIR) between the Amsterdam hotspot and the Australian‐Antarctic Discordance (88°E–118°E). The spreading center deepens by 2100 m from west to east within the study area. Despite a uniform, intermediate spreading rate (69–75 mm yr −1 ), the SEIR exhibits the range in axial morphology displayed by the East Pacific Rise and the Mid‐Atlantic Ridge (MAR) and usually associated with variations in spreading rate. The spreading center is characterized by an axial high west of 102°45′E, whereas an axial valley is prevalent east of this longitude. Both the deepening of the ridge axis and the general evolution of axial morphology from an axial high to a rift valley are not uniform. A region of intermediate morphology separates axial highs and MAR‐like rift valleys. Local transitions in axial morphology occur in three areas along the ridge axis. The increase in axial depth toward the Australian‐Antarctic Discordance may be explained by the thinning of the oceanic crust by ∼4 km and the change in axial topography. The long‐wavelength changes observed along the SEIR can be attributed to a gradient in mantle temperature between regions influenced by the Amsterdam and Kerguelen hot spots and the Australian‐Antarctic Discordance. However, local processes, perhaps associated with an heterogeneous mantle or along‐axis asthenospheric flow, may give rise to local transitions in axial topography and depth anomalies.