Melt Production and Morphology Variations Along the Southeast Indian Ridge

The Southeast Indian Ridge (SEIR) is an intermediate spreading rate ridge with an along-axis transition in axial ridge morphology from an axial high to an axial valley, which occurs independently of geochemistry and spreading rate. We investigate possible mechanisms for this transition by considering two segments separated by 600 km, one characterized by an axial high and the other by a shallow axial valley. The crustal structure of these two segments is well constrained through seismic reflection and refraction studies. We find that crustal structure differences account for only 70 m difference in off-axis (isostatic) depth, compared with an observed 237 m depth difference. We explore two models to account for the variation in morphology, crustal structure and depth by varying mantle properties. First, we consider a model that assumes a decrease in mantle temperature approaching the Australian-Antarctic Discordance. Mantle melt production models imply a 12°- 15°C mantle temperature difference between the two ridge segments based on the difference in crustal thickness. This result is compatible with the observed difference in mantle Bouguer anomalies between the two segments. Since the transition from an axial high to an axial valley occurs within a single segment located between the two segments for which we have seismic data, the actual temperature difference over which the change in the mode of crustal accretion occurs must be significantly less than 15°C. In the second model, we consider a constant temperature asthenosphere that thins toward the AAD. If the base of the asthenosphere lies above or cuts across the base of the potential melting zone, melt production would decrease as the AAD is approached. If a 100°C difference in temperature between the asthenosphere and mesosphere is assumed, then an ~3° slope (33 km depth difference over 600 km) on the base of the asthenosphere is required to match the gravity data. Both models predict that axial morphology and shallow crustal structure are highly sensitive to small changes in melt production. This is consistent with a threshold type relationship between melt production and the mode of crustal generation that can result in an abrupt along-axis transition in crustal structure and axial morphology driven by small-scale changes in mantle properties.