Plate Motions - Quantitative Plate Tectonics - page 47

Table 2.1 Order of modern tectonic plates
Plate

N Plate N Plate N Plate N

Pacific 13 Nazca 7 Philippine 5 Scotia 3

N. America 10 Australia 5 Arabia 5 Anatolia 3

Eurasia 9 India 5 Sundaland 5 Amurian 3

Antarctica 9 Somalia 5 Caribbean 4 J. de Fuca 2

S. America 8 Cocos 5 Rivera 3 Easter 2

Africa 7 Okhotsk 5 S. Sandwich 3

existing boundaries determines further increase
of the total number of plate boundaries by two
units. Therefore, there are three additional bound-
aries for each new plate. This proves Eq. ( 2.34 ).
The present day configuration illustrated in
Fig. 2.16 includes 23 plates. Thus, Eq. ( 2.34 )
requires that j D 42 and b D 63. The order N of
these plates is listed in Table 2.1 . The order of a
tectonic plate measures the degree of interaction
with the global system, because it coincides with
the number of neighbor plates. For example, in
the modern Earth's configuration the dynamics
and kinematics of the Pacific and N. American
plates have the largest impact on the global plate
system, because they are interacting with 14 of
the remaining 21 tectonic plates.
The classification and the kinematics of triple
junctions has been the subject of several studies
since the 1960s (McKenzie and Morgan 1969 ;
Patriat and Courtillot 1984 ; Kleinrock and Phipps
Morgan 1988 ). The basic principle describing the
instantaneous kinematics of these important tec-
tonic features is represented by the closure rule .
In general, if ¨ AB , ¨ BC ,and¨ CA are respectively
the Euler vectors of a plate A with respect to
another plate B ,of B with respect to a third plate
C , and of C relative to A , then the closure rule
simply states that:

The velocity triangle associated with Eq.
( 2.36 ) can be used to predict the kinematics
of triple junctions. The method is illustrated
in Fig. 2.17 through four significant examples.
It is useful to assume a reference frame fixed
to one of the three plates (for example, A ).
Strike-slip boundaries and trenches must be
moved according to the magnitude of the relative
velocity vectors. However, trenches are always
displaced with the upper (overriding) plate ,
thereby they remain at rest when this coincides
with the reference plate. An important geological
consequence of this behaviour is represented
by the development of strike-slip boundaries at
triple junctions where a subduction flip occurs
(Fig. 2.17 bottom right). This is a general result,
which in principle may be observed along any
composite flipping convergent boundary between
two plates, as illustrated in Fig. 2.18 .
Differently from the other plate boundaries,
ridges move at half of the relative velocity v
between two conjugate plates (Fig. 2.17 top left).
In the case of an RRR junction, an extra space
of triangular shape is created during the dis-
placement of the three spreading segments, with
edges given by: v AB t , v BC t ,and v CA t .The
new triple junction will be placed within this
triangle, but the link to the original segments may
be somewhat complicated. It may involve either
a simple propagation of the spreading segments
toward the new location of the triple junction,
or the formation of new spreading segments and
even of a small microplate, as it is observed in the
East Pacific region (Juan Fernandez and Galapa-
gos microplates). The fact that a ridge moves at
half velocity with respect to the reference plate
clearly implies that any set of points located near
a spreading segment at time t will be displaced

¨ AB C ¨ BC C ¨ CA D 0 (2.35)

If this three-plates system is connected
through a triple junction J , then this point
belongs simultaneously to A , B ,and C . Therefore,
applying Eq. ( 2.17 ) we have that in this case the
closure rule can be expressed in terms of linear
velocities at the triple junction:

v AB C v BC C v CA D 0 (2.36)