Page 110 - Global Tectonics

THE FRAMEWORK OF PLATE TECTONICS 97

contrast, subduction rates around the margins of the ies, using GPS (Global Positioning System) data
−1

Pacific are typically between 60 and 95 mm a . Thus the (Section 5.8). GPS measurements also make it possi-
oceanic plates of the Pacific are steadily reducing in size ble to determine the motion of these plates relative

as they are being consumed at subduction zones at a to adjacent plates, whereas this is not possible using
higher rate than they are being created at the East the techniques based on geologic and geophysical

Pacific Rise. By contrast, plates containing parts of the data described above. Most of the poorly defi ned
Atlantic and Indian oceans are increasing in size. A cor- zones of deformation surrounding these plates occur
ollary of this is that the Mid-Atlantic Ridge and Carls- within continental lithosphere, reflecting the profound

berg Ridge of the northwestern Indian Ocean must be difference between oceanic and continental litho-
moving apart. This has important implications for the sphere and the ways in which they deform (Sections
nature of the driving mechanism of plate tectonics dis- 2.10, 8.5.1).
cussed in Chapter 12. Not all ocean ridges spread in a
direction perpendicular to the strike of their magnetic
lineations. It may be significant that the major obliqui- 5.4 ABSOLUTE

ties of this type are found in the more slowly spreading
areas, in particular the North Atlantic, Gulf of Aden,
Red Sea, and southwestern Indian Ocean (Plate 4.1 PLATE MOTIONS
between pp. 244 and 245).
In contrast to accretionary plate margins, where the
spreading boundary is typically perpendicular to the The relative motion between the major plates, averaged
direction of relative motion, convergent margins are over the past few million years, can be determined with
not constrained in this way and the relative motion remarkable precision, as described in the preceding
vector typically makes an oblique angle with the plate section. It would be of considerable interest, particu-
boundary. Extreme examples, with very high obliquity, larly in relation to the driving mechanism for plate
occur at the western end of the Aleutian arc and the motions, if the motion of plates, and indeed plate
northern end of the Indonesian arc (Fig. 5.5). In subduc- boundaries, across the face of the Earth could also be
tion zones therefore, in addition to the component of determined. If the motion of any one plate or plate
motion perpendicular to the plate boundary, that pro- boundary across the surface of the Earth is known, then
duces underthrusting, there will be a component of the motion of all other plates and plate boundaries can
relative motion parallel to the plate boundary. This be determined because the relative motions are known.
“trench parallel” component often gives rise to strike- In general, within the framework of plate tectonics, all
slip faulting within the overriding plate immediately plates and plate boundaries must move across the face
landward of the forearc region. As a consequence, focal of the Earth. If one or more plates and/or plate bound-
mechanism solutions, for earthquakes occurring on the aries are stationary, then this is fortuitous. A particular
interface between the two plates beneath the forearc point on a plate, or, less likely, on a plate boundary, will
region, do not yield the true direction of motion be stationary if the Euler vector of the motion of that
between the plates. They tend to underestimate the plate or plate boundary passes through that point
trench parallel component of motion because part of (Fig. 5.6).
this is taken up by the strike-slip faulting (DeMets et al., The absolute motion of plates is much more diffi -

1990). Classic examples of such trench parallel strike- cult to define than the relative motion between plates
slip faults include the Philippine Fault, the Median Tec- at plate boundaries, not least because the whole solid
tonic Line of southwest Japan (Section 9.9), and the Earth is in a dynamic state. It is generally agreed that
Atacama Fault and the Liquiñe–Ofqui Fault (Section absolute plate motions should specify the motion of
10.2.3) in Chile. the lithosphere relative to the lower mantle as this
As indicated in Fig. 5.5, approximately 15% of the accounts for 70% of the mass of the solid Earth and
Earth’s surface is covered by regions of deforming deforms more slowly than the asthenosphere above
lithosphere; for example in the Alpine–Himalayan and the outer core below. In theory if the lithosphere
belt, southeast Asia, and western North America. and asthenosphere were everywhere of the same
Within these areas it is now possible to identify addi- thickness and effective viscosity, there would be no net
tional small plates, albeit often with diffuse boundar- torque on the plates and hence no net rotation of the