Page 128 - Global Tectonics

THE FRAMEWORK OF PLATE TECTONICS 115

Figure 5.22 (a) Transform fault (F) between plates B and C; (b) its representation in velocity space with the velocity
line bc corresponding to its related triple junction.

Figure 5.23 (a) Ridge (R) between plates A and C; (b) its representation in velocity space with the velocity line ac
corresponding to its related triple junction.

of velocity vector CA if the accretion process is sym- of an RTF triple junction, which is unstable, is shown
metric with plates A and C each moving at half the rate in Fig. 5.25.
of accretion. Figure 5.26 illustrates how an unstable triple junc-
By combining the velocity space representations tion can evolve into a stable system, and how this evo-
(Fig. 5.24), the stability of the triple junction can be lution can produce a change in direction of motion.
determined from the relative positions of the velocity The TTT triple junction shown in Fig. 5.26a is unstable,
lines representing the boundaries. If they intersect at as the velocity lines representing the trenches do not
one point, it implies that a stable triple junction exists intersect at a single point (Fig. 5.26b). In time the
because that point has the property of being able to system evolves into a stable confi guration (Fig. 5.26c)
travel up and down all three plate margins. In the case in which the new triple junction moves northwards
of the RTF triple junction, it can be appreciated that a along trench AB. The dashed lines show where plates
stable triple junction exists only if velocity line ac passes B and C would have been if they had not been sub-
through B, or if ab is the same as bc, that is, the trench ducted. The point X (Fig. 5.26a,c) undergoes an abrupt
and transform fault have the same trend, as shown here. change in relative motion as the triple junction passes.
If the velocity lines do not all intersect at a single point This apparent change in underthrusting direction can
the triple junction is unstable. The more general case be distinguished from a global change as it occurs at