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.
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 ~30 mm/yr (e.g., Lin and Parmentier, 1989) predict
that the depth to the 700¡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.
Mitchell, N. C., "Distributed extension at the Indian Ocean Triple Junction", J.
Geophys. Res., 96, 8019-8043, 1991.