Do ridge^ridge^fault triple junctions exist on Earth? Evidence from the Aden^Owen^Carlsberg junction in the NW Indian Ocean

The triple junctions predicted to be ridge^ridge^fault (RRF) types on the basis of large-scale plate motions are the Azores triple junction between theGloria Fault and theMid-Atlantic Ridge, the Juan Fernandez triple junction between the ChileTransform and the East Paci¢c Rise and the Aden^ Owen^Carlsberg (AOC) triple junction between theOwen fracture zone (OFZ) and theCarlsberg and Sheba ridges. In the ¢rst two cases, the expected RRF triple junction does not exist because the transform fault arm of the triple junction has evolved into a divergent boundary before connecting to the ridges. Here, we report the results of a marine geophysical survey of the AOC triple junction, which took place in 2006 aboard theR/VBeautemps-Beaupre ¤.We show that a rift basin currently forms at the southern end of the OFZ, indicating that a divergent plate boundary between Arabia and India is developing at the triple junction.The connection of this boundary with the Carlsberg and Sheba ridges is not clearly delineated and the triple junction presently corresponds to a widespread zone of distributed deformation.The AOC triple junction appears to be in a transient stage between a former triple junction of the ridge^fault^fault type and a future triple junction of the ridge^ridge^ridge (RRR) type. Consequently, the known three examples of potential RRF triple junctions are actually of the RRR type, and RRF triple junctions do not presently exist on Earth