Geoscience Reference
In-Depth Information
Figure 2.6.
(a) A
three-plate model on a flat
planet. Plate A is
unshaded. The western
boundary of plate B is a
ridge spreading at a
half-rate of 2 cm yr
−1
. The
boundary between plates
A and C is a subduction
zone with plate C
overriding plate A at
6cmyr
−1
. (b) Relative
velocity vectors for the
plates shown in (a). (c)
The solution to the model
in (a): the northern and
southern boundaries of
plate B are transform
faults, and the eastern
boundary is a subduction
zone with plate C
overriding plate B at
10 cm yr
−1
. (d) Vector
addition to determine the
velocity of plate B with
respect to plate C,
C
v
B
.
shape of plate B, we can see that its northern and southern boundaries must be
transform faults. The northern boundary is
sinistral
,orleft-handed; rocks are
offset to the left as you cross the fault. The southern boundary is
dextral
,orright-
handed; rocks are offset to the right as you cross the fault. The eastern boundary
is ambiguous:
A
v
B
indicates that plate B is approaching plate A at 4 cm y
−
1
along
this boundary, which means that a subduction zone is operating there; but there is
no indication as to which plate is being subducted. The two possible solutions for
this model are shown in Figs. 2.5(c) and (d).Figure 2.5(c) shows plate A being
subducted beneath plate B at 4 cm yr
−
1
. This means that plate B is increasing in
width by 2 cm yr
−
1
, this being the rate at which new plate is formed at the ridge
axis. Figure 2.5(d) shows plate B being subducted beneath plate A at 4 cm yr
−
1
,
faster than new plate is being created at its western boundary (2 cm yr
−
1
); so
eventually plate B will cease to exist on the surface of the planet.
If we introduce a third plate into the model, the motions become more complex
still (Fig. 2.6(a)). In this example, plates A and B are spreading away from the
ridge at a half-rate of 2 cm yr
−
1
, just as in Fig. 2.5(a). The eastern boundary of
plates A and B is a subduction zone, with plate A being subducted beneath plate
