Geoscience Reference
In-Depth Information
(a)
(c)
(b)
Plate A
Plate B
Plate A
Plate B
A
v
A
v
B
v
CB
X
X
C
v
B
B
v
A
A
v
Plate C
Plate C
Figure 2.15.
(a) A triple junction where three subduction zones intersect. Plate A
overrides plates B and C, while plate C overrides plate B.
A
v
B
,
C
v
B
and
A
v
C
are the
relative velocities of the three plates in the immediate vicinity of the triple junction.
(b) The relative-velocity triangle for (a). (c) The geometry of the three subduction
zones at some time later than in (a). The dashed lines show where plates B and C
would have been had they not been subducted. The point X in (a) was originally on
the boundary between plates A and B; now it is on the boundary between plates A
and C. The original triple junction has changed its form. (After McKenzie and
Morgan (1969).)
the boundary. The alternative, a change in relative motion between the plates,
would occur at one time. It can be seen that, although the original triple junction
shown in Fig. 2.15(a) is not stable, it would be stable if
A
v
C
were parallel to the
boundary between plates B and C. Then the boundary between B and C would
not move in a north-south direction relative to A, so the geometry of the triple
junction would be unchanging with time. The other configuration in which the
triple junction would be stable occurs when the edge of the plate either side of the
triple junction is straight. This is, of course, the final configuration illustrated in
Fig. 2.15(c).
Altogether there are sixteen possible types of triple junction, all shown in
Fig. 2.16.Ofthese sixteen triple junctions, one is always stable (the ridge-ridge-
ridge junction) if oblique spreading is not allowed, and two are always unstable
(the fault-fault-fault and fault-ridge-ridge junctions). The other thirteen junc-
tions are stable under certain conditions. In the notation used to classify the types
of triple junction, a ridge is written as R, a transform fault as F and a subduction
zone (or trench) as T. Thus, a ridge-ridge-ridge junction is RRR, a fault-fault-
ridge junction is FFR, and so on.
To examine the stability of any particular triple junction, it is easiest to draw
the azimuths of the plate boundaries onto the relative velocity triangle. In Fig.
2.16 the lengths of the lines AB
BC and AC are proportional and parallel to
the relative velocities
A
v
B
,
B
v
C
and
A
v
C
. Thus, the triangles are merely velocity
triangles such as that shown in Fig. 2.15(b). The triple junction of Fig. 2.15, type
,
