Constraints on the mantle temperature gradient along the Southeast Indian Ridge from crustal structure and isostasy: implications for the transition from an axial high to an axial valley

Axial and ridge flank depths on the Southeast Indian Ridge (SEIR) systematically increase for 2500 km from 90°E to 120°E approaching the Australian-Antarctic Discordance. The SEIR also experiences an abrupt change in ridge axis morphology near 103°30′E with axial highs found to the west and axial valleys to the east. Since the spreading rate is constant throughout this region, these variations have been ascribed to an along-axis gradient in mantle temperature. A seismic refraction experiment provides information on the crustal thickness and seismic velocity structure of two segments with differing axial morphology. Segment P2, centred near 102°E with an axial high has a mean crustal thickness of 5.9 ± 0.2 km, while the mean crustal thickness is 5.3 ± 0.3 km at Segment S1 with an axial valley and centred near 109°45′E. Isostatic compensation of the difference in crustal thickness and density structure between the two segments only accounts for 33 m of the 198 m difference in average ridge flank depth between the two segments. The remaining depth difference must result from a difference in mantle density. Melt production models imply a mantle temperature difference of 11–13.5 °C to produce the observed difference in crustal thickness. Isostatic compensation of the two segments requires that the resulting density difference must extend to about 300 km in the mantle. The transition in axial morphology along the SEIR is very abrupt occurring over a narrow zone within a single segment in which the transition is complete. If a linear mantle temperature gradient is assumed, the temperature difference across the transition segment is only 2.4 °C. The change in axial morphology is accompanied by abrupt changes in other parameters including abyssal hill height, magnetic anomaly amplitude, layer 2a thickness and the presence or absence of an axial magma lens. The abrupt, coincident change in a number of parameters with a very small change in mantle temperature strongly suggests a threshold change between two distinctly different modes of crustal accretion. The trigger for the transition appears to be whether a steady-state crustal magma lens can be maintained.