MORPHOLOGY OF THE TRANSITION FROM AN AXIAL HIGH TO A RIFT-VALLEY AT THE SOUTHEAST INDIAN RIDGE AND THE RELATION TO VARIATIONS IN MANTLE TEMPERATURE

The Southeast Indian Ridge exhibits a transition in axial morphology f rom an East Pacific Rise-like axial high near 100 degrees E to a Mid-A tlantic Ridge-like rift valley near 116 degrees E but spreads at a nea rly constant rate of 74-76 mm/yr. Assuming that the source of this tra nsition lies in variations in mantle temperature, we use shipboard gra vity-derived crustal thickness and ridge flank depth to estimate the v ariations in temperature associated with the changes in morphological style. Within the transitional region, SeaBeam 2000 bathymetry shows s cattered instances of highs, valleys, and split volcanic ridges at the axis. A comparison of axial morphology to abyssal hill shapes and sym metry properties suggests that this unorganized distribution is due to the ridge axis episodically alternating between an axial valley and a volcanic ridge. Axial morphology can then be divided into three class es, with distinct geographic borders: axial highs and rifted highs are observed west of a transform fault at 102 degrees 45'E; rift valleys are observed east of a transform fault at 114 degrees E; and an interm ediate-style morphology which alternates between a volcanic ridge and a shallow axial valley is observed between the two. One segment, betwe en 107 degrees and 108 degrees 30'E, forms an exception to the geograp hical boundaries. Gravity-derived crustal thickness and flank depth ge nerally vary monotonically over the region, with the exception of the segment between 107 degrees E and 108 degrees 30'E. The long-wavelengt h variations in these properties correlate with the above morphologica l classification. Gravity-derived crustal thickness varies on average similar to 2 lan between the axial high and rift valley regions. The a pplication of previous models relating crustal thickness and mantle te mperature places the corresponding temperature variation at 25 degrees C-50 degrees C, depending on the model used. The average depth of rid ge flanks varies by similar to 550 m over the study area. For a variat ion of 25 degrees-50 degrees C, thermal models of the mantle predict d epth variations of 75-150 m. These values are consistent with observat ions when the combined contributions of crustal thickness and mantle d ensity to ridge flank depth are considered, assuming Airy isostasy. Cr ustal thickness variations differ at the two transitions described abo ve: A difference of 750 m in crustal thickness is observed at the rift valley/intermediate-style transition, suggesting small variations in crustal thickness and mantle temperature drive this transition. At the axial high-rifted high/intermediate-style transition, crustal thickne ss variations are not resolvable, suggesting that this transition is c ontrolled by threshold values of crustal thickness and mantle temperat ure, and is perhaps related to the presence of a steady state magma ch amber.