Geology Reference
In-Depth Information
0
Continent
20
40
Ocean
60
0
1000
Differential stress (MPa)
Figure 6.1. Strength envelopes estimated for representative oceanic and continen-
tal geotherms. Such estimates depend greatly on details assumed (see text). Each
case comprises three regimes: brittle (straight lines), semi-brittle (dashed lines)
and fluid/ductile (curves). In the continental case, the crustal ductile response
changes to the mantle ductile response at 35 km depth. In these examples the
deformation rate is assumed to be 10
−
15
s
−
1
. After Kohlstedt
et al.
[39]. Copyright
by the American Geophysical Union.
measurements of rock deformation, is borne out by the fact that the continen-
tal parts of tectonic plates have more internal deformation, as exemplified by the
broad deformation zones through China and the western USA. In contrast, oceanic
lithosphere undergoes little deformation away from plate boundaries.
As we saw in Chapter 3, Tuzo Wilson conceived of plate tectonics by looking
at the surface tectonic features of the Earth. What he saw was a network of mobile
belts dividing the Earth's surface into large pieces, like a jigsaw puzzle. He named
the pieces
plates
, and described them as essentially rigid, though moving. The fact
that the Earth's surface is not deforming in most places, and that deformation is
usually confined to narrow zones, implies that the lithosphere behaves like a brittle
solid, to a good approximation. This is why Wilson was able to take the notion of
a mobile belt to its geometric limit and treat it as a narrow fault. He also took the
behaviour of the intervening plates to its limit, and described them as rigid, even
though there is some relatively small deformation in some parts of some plates.
The implication is that the behaviour of the lithosphere is dominated by its upper,
brittle part. Its lower parts evidently adjust to the pattern of deformation imposed
by the upper brittle part, even though they may be semi-brittle or 'ductile', in the
sense of forming ductile shear zones. (Strictly speaking, ductile refers to the ability
