Linkage between crustal structure, mantle temperature and axial morphology: A study of the intermediate spreading Southeast Indian ridge

There is a systematic variation in axial morphology and axial depth along the Southeast Indian ridge (SEIR) with distance away from the Australian Antarctic Discordance, an area of cold uppermost mantle. Since spreading rate (72-76 mm/yr) and mantle geochemistry appear constant along this portion of the SEIR, the observed variations in axial morphology and axial depth are attributed to a gradient in mantle temperature. In this study, we report results from a multichannel seismic investigation of on-axis and off-axis crustal structure along this portion of SEIR. Axial highs have a shallow magma lens and a thin layer 2A along the ridge crest that doubles in thickness off-axis. Rifted axial highs have a deeper magma lens and thicker layer 2A on-axis that thickens by 50% off-axis. Beneath shallow axial valleys, no magma lens is imaged, and layer 2A is thick with no pattern of thickening off-axis. Near the ridge axis hydrothermal circulation has a large role in the observed seismic layer 2A velocity increase and thickening off-axis. Near-axis, in the vicinity of a magma lens, active hydrothermal circulation occurs, infilling pore spaces, as well as resulting downward cracking. Where there is no magma lens imaged, little active hydrothermal circulation occurs, thus resulting in a small velocity increase. With an observed magma lens, eruption of less viscous, far-traveling lobate laves can extend far off-axis, whereas in areas with no observed magma lens, lava may be emplaced through eruption of cooler more viscous lavas that form constructional pillows. The velocity of layer 2B increases through cooling and healing of fractures through infilling due to hydrothermal circulation. We conclude that a small change in melt production can cause a large change in axial morphology and shallow crustal structure. This implies a threshold type behavior where a small change in melt production results in a transition between two very different modes of crustal generation, resulting in very different shallow crustal structure and axial morphology.