Geoscience Reference
In-Depth Information
(a)
Figure 2.14.
Two examples of a plate
boundary that locally changes with time. (a)
A three-plate model. Point T, where plates A,
B and C meet, is the triple junction. The
western boundary of plate C consists of
transform faults. Plate B is overriding plate A
at 4 cm yr
−1
. The circled part of the boundary
of plate C changes with time. (b)
Relative-velocity vectors for the plates in (a).
(c) A three-plate model. Point T, where plates
A, B and C meet, is the triple junction. The
boundary between plates A and B is a ridge,
that between plates A and C is a transform
fault and that between plates B and C is a
subduction zone. The circled part of the
boundary changes with time. (d)
Relative-velocity vectors for the plates in (c).
In these examples, velocity vectors have
been used rather than angular-velocity
vectors. This is justified, even for a spherical
Earth, because these examples are
concerned only with small regions in the
immediate vicinity of the triple junctions,
over which the relative velocities are
constant.
Plate A
2
(b)
4
T
Plate
4
B
v
A
v
6
=
+
BC
C
2
A
v
6
Plate B
(c)
(d)
Plate C
T
B
v
A
C
v
A
C
v
B
Plate
A
Plate B
and indicate that the Pacific-Farallon pole position changed slightly a number of
times during the Tertiary.
Partsofplate boundaries can change locally, however, without any major
'plate' or 'pole' event occurring. Consider three plates A, B and C. Let there be
a convergent boundary between plates A and B, and let there be strike-slip faults
between plates A and C and plates B and C, as illustrated in Figs. 2.14(a) and
(b).From the point of view of an observer on plate C, part of the boundary of
C (circled) will change with time because the plate to which it is adjacent will
change from plate A to plate B. The boundary will remain a dextral (right-handed)
fault, but the slip rate will change from 2 cm yr
−
1
to 6 cmyr
−
1
. Relative to plate
C, the subduction zone is moving northwards at 6 cm yr
−
1
. Another example of
this type of plate-boundary change is illustrated in Figs. 2.14(c) and (d).Inthis
case, the relative velocities are such that the boundary between plates A and C
is a strike-slip fault, that between plates A and B is a ridge and that between
plates B and C is a subduction zone. The motions are such that the ridge migrates
slowly to the south relative to plate C, so the circled portion of plate boundary
will change with time from subduction zone to transform fault.
