A 2¡ X 2¡ map of spreading centres and fracture zones surrounding the
Indian Ocean RRR triple junction, at 25.5¡S, 70¡E, is described from a data
set of GLORIA side-scan sonar images, bathymetry, magnetic and gravity
anomalies. The GLORIA images show a pervasive fabric due to linear
abyssal hills oriented parallel to the two medium-spreading ridges (the
Central Indian Ridge (CIR) and Southeast Indian Ridge (SEIR)). A
curvature of the fabric occurs along fracture zones, which are also located by
lows in the bathymetry and gravity data and by offsets between magnetic
anomalies. The magnetic anomalies also record periods of asymmetric
spreading marking the development of the fracture zones, including the
birth, at anomaly 2A, of a short fracture zone 50 km north of the triple
junction on the CIR, and its death near the time of the Jaramillo anomaly. In
some localities, a fine-scale fabric corresponds to a coarser fabric on the
opposite flank of the CIR, possibly indicating a persistent asymmetry in the
faulting at the median valley walls if the fabric has a tectonic and not a
volcanic origin. A plate velocity analysis of the triple junction shows that
both the CIR and Southwest Indian Ridge (SWIR) are propagating
obliquely; the CIR appears to form an oblique trend by segmenting into a
series of almost normally-oriented segments separated by short-offset
fracture zones. For the last 4 m.y., the abyssal hill lineations indicate that
the CIR segment immediately north of the triple junction has been spreading
with an average 10¡ obliquity. The present small 5 km offset of the centres
of the CIR and SEIR median valleys (Munschy and Schlich, 1989) is shown
to be the result of this obliquity and a 30% spreading asymmetry between
anomaly 2 and the Jaramillo on the CIR segment immediately north of the
triple junction.
Mitchell, N. C., "An evolving ridge system around the Indian Ocean Triple
Junction", Marine Geophysical Researches, 13, 173-201, 1991.