Page 129 - Global Tectonics

116 CHAPTER 5

Figure 5.24 Velocity space representation of the plate system shown in Fig. 5.20. Velocity lines ab, bc, and ac intersect
at the single point J, which thus represents a stable triple junction.

Figure 5.25 (a) Ridge (R)–trench (T)–transform fault (F) triple junction between plates A, B, and C. (b) Its
representation in velocity space. As the velocity lines ab, bc, and ac do not intersect at a single point, the triple junction
must be unstable.

different times and locations along the plate boundary. tions of trench, ridge, and transform fault (Fig. 5.27),

In order to be stable, the plate configuration shown in taking into account the two possible polarities of
Fig. 5.26a must be as in Fig. 5.26d. When plotted in trenches, but not transform faults. Of these, only the
velocity space (Fig. 5.26e) the velocity lines then inter- RRR triple junction is stable for any orientation of the
sect at a single point. ridges. This comes about because the associated veloc-
McKenzie & Morgan (1969) have determined the ity lines are the perpendicular bisectors of the triangle
geometry and stability of the 16 possible combina- of velocity vectors, and these always intersect at a