An Evolving Ridge System around the Indian-Ocean Triple Junction

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Marine Geophysical Researches
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A 2-degrees x 2-degrees map of spreading centres and fracture zones surrounding the Indian Ocean RRR triple junction, at 25.5-degrees-S, 70-degrees-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 cuvature 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-degrees 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.


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