1.2.7 Triple junctions

It is common, although imprecise, to refer to a plate margin by its dominant topographic feature, rather than by the nature of the margin. A ridge (R) represents a con­structive margin or spreading center, a trench (T) refers to a destructive margin or subduction zone, and a transform fault (F) stands for a conservative margin. Each margin is a location where two tectonic plates adjoin. Inspection of Fig. 1.11 shows that there are several places where three plates come together, but none where four or more plates meet. The meeting points of three plate boundaries are called triple junctions. They are important in plate tecton­ics because the relative motions between the plates that form a triple junction are not independent. This may be appreciated by considering the plate motions in a small plane surrounding the junction.

Consider the plate velocities at an RTF junction formed by all three types of boundary (Fig. 1.20a). If the plates are rigid, their relative motions take place entirely at their margins. Let _AV_B denote the velocity of plate B rela­tive to plate A, _fiV_c the velocity of plate C relative to plate B, and _CV_A the velocity of plate A relative to plate C. Note that these quantities are vectors; their directions are as important as their magnitudes. They can be represented on a vector diagram by straight lines with directions paral­lel to and lengths proportional to the velocities. In a circuit about the triple junction an observer must return to the starting point. Thus, a vector diagram of the interplate velocities is a closed triangle (Fig. 1.20b). The velocities are related by

Fig. 1.20 (a) Triple junction formed by a ridge, trench and transform fault, and (b) vector diagram of the relative velocities at the three boundaries (after McKenzie and Parker, 1967).

This planar model is a "flat Earth" representation. As discussed in Section 1.2.9, displacements on the surface of a sphere are rotations about Euler poles of relative motion. This can be taken into account by replacing each linear velocity V in Eq. (1.7) by the rotational velocity w about the appropriate Euler pole.

1.2.7.1 Stability of triple junctions

The different combinations of three plate margins define ten possible types of triple junction. The combinations correspond to all three margins being of one type (RRR, TTT, FFF), two of the same type and one of the other (RRT, RRF, FFT, FFR, TTR, TTF), and all different (RTF). Different combinations of the sense of subduc­tion at a trench increase the number of possible junctions to sixteen. Not all of these junctions are stable in time. For a junction to preserve its geometry, the orientations of the three plate boundaries must fulfil conditions which allow the relative velocities to satisfy Eq. (1.7). If they do so, the junction is stable and can maintain its shape. Otherwise, the junction is unstable and must evolve in time to a stable configuration.

The stability of a triple junction is assessed by consid­ering how it can move along any of the plate boundaries

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