Page 126 - Global Tectonics

THE FRAMEWORK OF PLATE TECTONICS 113

necessary information, from oceanic magnetic anoma- ment of the Alpine Fault of New Zealand (Fig. 5.19c),
lies and fracture zones, is available. which is a dextral transform fault linking the Tonga-
Kermadec Trench, beneath which Pacifi c lithosphere is
underthrusting in a southwesterly direction, to a trench
5.10 STABILITY OF to the south of New Zealand where the Tasman Sea is
being consumed in a northeasterly direction (McKenzie
TRIPLE JUNCTIONS & Morgan, 1969).
A more complex and potentially unstable situation
arises when three plates come into contact at a triple
junction. Quadruple junctions are always unstable, and
The stability of the boundaries between plates is depen- immediately devolve into a pair of stable triple junctions,
dent upon their relative velocity vectors. If a boundary as will be shown later.
is unstable it will exist only instantaneously and will The Earth’s surface is covered by more than two
immediately devolve into a stable confi guration. plates, therefore there must be points at which three
Figure 5.19a shows an unstable boundary between plates come together to form triple junctions. In a
two plates where plate X is underthrusting plate Y at bc similar fashion to a boundary between two plates, the
in a northeasterly direction and plate Y is underthrust- stability of triple junctions depends upon the relative
ing plate X at ab in a southwesterly direction. The directions of the velocity vectors of the plates in
boundary is unstable because a trench can only consume contact. Figure 5.20 shows a triple junction between a
in one direction, so to accommodate these movements ridge (R), trench (T), and transform fault (F). From this

a dextral transform fault develops at b (Fig. 5.19b). This figure it can be appreciated that, in order to be stable,
sequence of events may have occurred in the develop- the triple junction must be capable of migrating up or

Figure 5.19 (a,b) Evolution of a trench. (c) Alpine Fault of New Zealand (redrawn from McKenzie & Morgan, 1969, with
permission from Nature 224, 125–33. Copyright © 1969 Macmillan Publishers Ltd).