Distributed extension at the Indian Ocean triple junction

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 ~ 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. The extension may be amagmatic near the triple junction. The high relief of the SWIR rift flanks may be an isostatic response of the lithosphere to the deep valley produced by this rifting. These observations, which suggest that progressive development of a propagating SWIR rift by the extension of preexisting seafloor, may have general implications for the dynamics of ocean spreading centers. -from Author