Geoscience Reference
In-Depth Information
These local changes in the plate boundary are a geometric, consequence of
the motions of the three rigid plates rather than being caused by any disturbing
outside event. A complete study of all possible interactions of three plates is made
in the next section. Such a study is very important because it enables us to apply
the theory of rigid geometric plates to the Earth and deduce past plate motions
from evidence in the local geological record. We can also predict details of future
plate interactions.
2.6 Triple junctions
2.6.1 Stable and unstable triple junctions
A triple junction is the name given to a point at which three plates meet, such as
the points T in Fig. 2.14.Atriple junction is said to be 'stable' when the relative
motions of the three plates and the azimuth of their boundaries are such that
the configuration of the junction does not change with time. The two examples
shown in Fig. 2.14 are thus stable. In both cases the triple junction moves along
the boundary of plate C, locally changing this boundary. The relative motions of
the plates and triple junction and the azimuths and types of plate boundaries
of the whole system do not change with time. An 'unstable' triple junction exists
only momentarily before evolving to a different geometry. If four or more plates
meet at one point, the configuration is always unstable, and the system will evolve
into two or more triple junctions.
As a further example, consider a triple junction where three subduction zones
meet (Fig. 2.15): plate A is overriding plates B and C, and plate C is overriding
plate B. The relative-velocity triangle for the three plates at the triple junction
is shown in Fig. 2.15(b).Now consider how this triple junction evolves with
time. Assume that plate A is fixed; then the positions of the plates at some later
time are as shown in Fig. 2.15(c). The dashed boundaries show the extent of
the subducted parts of plates B and C. The subduction zone between plates B
and C has moved north along the north-south edge of plate A. Thus, the origi-
nal triple junction (Fig. 2.15(a))was unstable; however, the new triple junction
(Fig. 2.15(c))isstable (meaning that its geometry and the relative velocities of
the plates are unchanging), though the triple junction itself continues to move
northwards along the north-south edge of plate A. The point X is originally on
the boundary of plates A and B. As the triple junction passes X, an observer there
will see a sudden change in subduction rate and direction. Finally, X is a point
on the boundary of plates A and C.
In a real situation, the history of the northward passage of the triple junction
along the boundary of plates A and C could be determined by estimating the
time at which the relative motion between the plates changed at a number of
locations along the boundary. If such time estimates increase regularly with posi-
tion along the plate boundary, it is probable that a triple junction migrated along
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