Triple junction stability

Triple junction stability is a term used in geology. A triple junction is the point at which three plate boundaries meet. These boundaries can either be ridges (R), trenches (T) or transform faults (F). Triple junctions were once defined simply as the meeting point of three spreading ridges. This is because this sort of triple junction is very common, usually with one of the ridges failing (forming an aulacogen) and the other two continuing to open a new ocean. The meeting of 4 or more plates is also theoretically possible but junctions will only exist instantaneously.

Triple junctions may be described and their stability assessed without use of the geological details but simply by defining the properties of the ridges, trenches and transform faults involved, making some simplifying assumptions and applying simple velocity calculations. This assessment can generalise to most actual triple junction settings provided the assumptions and definitions broadly apply to the real earth situation.

A stable junction is one at which the geometry of the junction is retained with time as the plates involved move. This places restrictions on relative velocities and plate boundary orientation. An unstable triple junction will change with time, either to become another form of triple junction (RRF junctions easily evolve to FFR junctions), will change geometry or are simply not feasible (as in the case of FFF junctions).

By assuming that plates are rigid and that the earth is spherical, Leonhard Euler’s theorem of motion on a sphere can be used to reduce the stability assessment to determining boundaries and relative motions of the interacting plates. The rigid assumption holds very well in the case of oceanic crust, and the radius of the earth at the equator and poles only varies by a factor of roughly one part in 300 so the earth approximates very well to a sphere.

McKenzie and Morgan first analysed the stability of triple junctions using these assumptions with the additional assumption that the Euler poles describing the motions of the plates were such that they approximated to straight line motion on a flat surface. This simplification applies when the Euler poles are distant from the triple junction concerned. The definitions they used for R, T and F are as follows:

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