On the stability of triple junctions and its relation to episodicity in spreading

A convenient representation of triple junctions that involve only ridges (R) and transform faults (F) is proposed: this representation combines in a simple way information from geographic and velocity spaces. The velocity triangle provides the budget of lithospheric surface change which directly results from interactions of the three plates. A discussion of the relative positions of the triple junction with respect to the velocity triangle demonstrates that, in general, there are several triple junction configurations that are compatible with a given triangle. This discussion stresses the importance of oblique and asymmetric spreading. Pairs of triple junction configurations that are of particular interest are RRR‐RRF and RRR‐FFR. When the triple junction lies outside of the triangle, at least one of the ridges will shorten, leading to yet another type of potentially unstable configuration. The present configurations of the Bouvet, Galapagos, and Indian Ocean triple junctions are reviewed. A method is proposed to reconstruct past configurations of the Indian Ocean junction at the time of anomalies 23 and 28. The main parameters that influence triple junction evolution are the lengths of transform faults, the availability of magma and related connectivity of magma chambers and the (also related) spreading velocities. This study confirms an earlier suggestion that activity at constructive plate boundaries occurs in two preferred modes: the effusive and tectonic modes, corresponding here to RRR and to RRF‐RFF configurations respectively. These modes apparently alternate in episodes of typically 1 Ma duration. The significance of this time constant and consequences for lithospheric mechanics are briefly discussed