The Indian Ocean triple junction is a ridge-ridge-ridge type joining two medium-spreading ridges with one slow-spreading ridge. GLORIA long-range side scan sonar images show that, while the axial valleys of the two medium-spreading ridges are almost colinear, apart from a small approximately 5 km offset, the valley of the slow-spreading third axis does not meet the other two in a simple fashion. The axis of this slow-spreading Southwest Indian Ridge (SWIR), beyond the rift valley walls of the other two ridges, steadily deepens by over 1000 m away from the triple junction to reach 5000 m at 35 km. The GLORIA images show large normal faults around the deep SWIR rift valley, which increase in heave away from the triple junction and crosscut the abyssal hills formed at the faster-spreading Central Indian Ridge, indicating that extension across the SWIR is distributed over a zone 10 km or more wide. This zone also shows no evidence for the formation of new oceanic crust, suggesting that the extension may be amagmatic near the triple junction. The high relief of the SWIR rift flanks, containing tilted seafloor of the other two ridges, may be an isostatic response of the lithosphere to the deep valley produced by this rifting. These observations, which suggest the progressive development of a propagating SWIR rift by the extension of preexisting seafloor, may have general implications for the dynamics of oceanic spreading centers. In particular, the deepening and widening of the valley away from the triple junction is attributed to the competing effects of tectonic thinning and lithospheric cooling. Thermal models of mid-ocean ridges spreading at approximately 30 mm/yr (e.g., Lin and Parmentier, 1989) predict that the depth to the 700-degrees-C isotherm increases with distance from the spreading axis by a proportion similar to the observed doubling in the total width of the deformation with distance from 10 to 35 km away from the triple junction. This rate of widening of the deformation zone may therefore support lithospheric necking models for oceanic rift valleys, if the width and depth of the deforming lithosphere are related in a simple manner.
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