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(a)
(b)
(c)
Plate A
Plate B
ab
B
v
A
C
C
A
v
B
bc
A
v
v
v
CB
CB
C
v
B
A
A
B
v
A
B
B
ac
A
v
PlateC
Figure 2.17.
Determination of stabilty for a triple junction involving three subduction
zones, TTT(a) of Fig. 2.16. (a) The geometry of the triple junction and relative
velocities; this is the same example as Fig. 2.15(a). (b) Relative-velocity triangles
(Fig. 2.15(b)). Sides BA, CB and AC represent
B
v
A
,
C
v
B
and
A
v
C
, respectively. The
corner C represents the velocity of plate C. Thus for example, relative to plate A, the
velocity of plate C,
A
v
C
,isrepresented by the line from point A to point C, and the
line from point B to point A represents
B
v
A
, the velocity of plate A relative to plate B.
(c) The dashed lines ab, ac, and bc drawn onto the velocity triangle ABC represent
possible velocities of the boundary between plates A and B, plates A and C and
plates B and C, respectively, which leave the geometry of those boundaries
unchanged. The triple junction is stable if these three dashed lines intersect at a
point. In this example, that would occur if ab were parallel to ac or if the velocity
A
v
C
were parallel to bc. If the geometry of the plate boundaries and the relative velocities
at the triple junction do not satisfy either of these conditions, then the triple junction
is unstable. If it is unstable, the geometry of the plate boundaries will change; this
particular geometry and triple junction can exist only momentarily in geological
time.
TTT(a) in Fig. 2.16,isshown in Fig. 2.17. The subduction zone between plates
A and B does not move relative to plate A because plate A is overriding plate B.
However, because all parts of the subduction zone look alike, any motion of the
subduction zone parallel to itself would also satisfy this condition. Therefore, we
can draw onto the velocity triangle a dashed line ab, which passes through point
A and has the strike of the boundary between plates A and B (Fig. 2.17(c)). This
line represents the possible velocities of the boundary between plates A and B
which leave the geometry of these two plates unchanged. Similarly, we can draw
a line bc that has the strike of the boundary between plates B and C and passes
through point C (since the subduction zone is fixed on plate C) and a line ac
that passes through point A and has the strike of the boundary between plates A
and C. The point at which the three dashed lines ab, bc and ac meet represents
the velocity of a stable triple junction. Clearly, in Fig. 2.17(c), these three lines
do not meet at a point; therefore, this particular plate-boundary configuration is
unstable. However, the three dashed lines would meet at a point (and the triple
