Geology Reference
In-Depth Information
the shear zones become faults, and the material is better described as a brittle solid
rather than as a fluid.
The general term for the mechanical response of materials to forces and stresses
is
rheology
, and the rheology of rocks is quite complicated, depending not only
on temperature but also on such things as the confining pressure, the shear stress
causing deformation, the particular minerals present, their grain size and some
details of their chemical composition, such as a few hundred parts per million of
dissolved water. It is thus hard to characterise the rheology of mantle rocks, and
they are not all that well known in the intermediate ranges of temperature and stress
that apply in the Earth's lithosphere. Nevertheless a couple of general points can
be made fairly confidently.
At the Earth's surface, rocks are brittle. They will break along fractures, and
sliding can occur along fractures, which are then known as faults. At increasing
depths, the stress required to cause sliding increases because friction increases with
increasing confining pressure. On the other hand, at a sufficiently high temperature
the rock will deform like a fluid. If the fluid-like deformation is fast enough,
it may relieve the stress before any more fracturing or sliding occurs, and fluid
deformation will then be the dominant mode of deformation. At greater depths and
temperatures, the rocks will deform more easily, so their resistance to deformation
will decrease again.
This general pattern is illustrated in Figure 6.1 for oceanic lithosphere and
for continental lithosphere. It shows a plot of 'strength', loosely defined, versus
depth. For oceanic lithosphere the strength increases with depth, starting from the
surface. Temperature also increases with depth through oceanic lithosphere, as
depicted schematically in Figure 5.5. The increasing temperature promotes fluid-
like deformation, so that at depths greater than about 40 km the strength decreases
with increasing depth as the fluid deformation becomes dominant. In this example
there is an intermediate regime called semi-brittle deformation that limits the
strength between about 10 km and 40 km depth. The three deformation regimes
(brittle, semi-brittle and fluid/ductile) define a
strength envelope
, which indicates
roughly what level of shear stress the rocks can resist at each depth.
The strength envelope for continents is rather different, because crustal rocks are
generally not as strong as mantle rocks. Thus the fluid-like deformation becomes
dominant at about 15 km depth in this example, and the lower crust is relatively
deformable. In this example the crust is taken to be 35 km thick, and below this
depth the strength jumps up again because of the greater strength of mantle rocks.
Thus the continental lithosphere may be stronger near the top and bottom than in
the middle, within the lower crust.
Because its middle part is relatively weak, the continental lithosphere is also
weaker overall than oceanic lithosphere. This inference, based on laboratory
