Fractures, faults and earthquakes - Introducing Tectonics, Rock Structures and Mountain Belts - page 38

At greater depth, fault rocks are
much harder and more cohesive. Here,
frictional heating caused by rapid move-
ment on the fault plane may cause local
melting of the crushed rock, forming
a material termed pseudotachylite,
which forms veins intruding into
the surrounding rock (Figure 5.9B).
Like the igneous basaltic rock, tachy-
lite , the matrix is a glass, in this case
formed from the melted crush rock.
Fault rocks at even greater depth may
become partly recrystallised to form
a metamorphic rock. A fine-grained
banded variety of such a rock is termed
mylonite ( see Figure 7.5C) and is found
along major deep-level thrusts such as
the Moine thrust of NW Scotland. At
mid-crustal levels and lower, faults and
fault rocks are replaced by shear zones ,
which are characterised by intense
deformation and recrystallisation, as
discussed in the following chapter.

A

compression

strike-slip
fault

extension

B

thrusting

normal
faulting

C

Figure 5.8 The San Andreas fault. A. Aerial view of a section of the San Andreas fault showing how
clearly the fault is reflected in the topography by a valley along the line of the fault and uplifted zones
on each side. Photo © Jim Wark. B, C. Bends in the course of a strike-slip fault create zones of
extension and compression resulting, respectively, in subsidiary normal faulting and thrusting along
those sectors of the fault that are oblique to the direction of movement. B, before movement; C, after
movement.

Physical conditions for fracturing
The process of fracturing, as discussed
in the previous chapter, is the result
of brittle behaviour where the stress,
and therefore the strain rate, is higher
than the level required to maintain
continuous viscous flow in the deform-
ing material ( see Figure 4.11). These
conditions normally apply only in the
upper 10-15 km of the crust, although
fracturing also occurs in subduc-
tion and collision zones where cooler
rocks have been transported to greater
depths. As explained in the previous
chapter, fracturing is critically depend-
ent on several variables: notably, the
magnitude of the applied stress, the
temperature, and the effective pressure
(lithostatic pressure minus pore-fluid
pressure), in addition to the strength

by a close association of uplifted
blocks that have been created in the
compressional zones and depressed
basins in the extensional zones.
Strike-slip faults forming part of
the plate boundary network, such as
the San Andreas fault, are known as
transform faults and are described
in Chapter 3. Active transform faults
in the oceans are very common
and are marked by periodic shallow
earthquakes. Underwater survey-
ing by side-scan sonar has identi-
fied linear ridges and troughs along
the course of some of these faults.

Fault rocks
Movement on a fault produces a zone
of broken and crushed rock frag-
ments of varying size; this process is
known as cataclasis . When formed
near the surface, cataclastic rocks may
be composed of large angular frag-
ments and are termed fault breccia
(similar in appearance to sedimen-
tary breccia) , as shown in Figure
5.9A; where the fragments are small,
a type of clay is formed, called fault
gouge . Such rocks are soft and easily
eroded, which explains the fact that
many faults are followed by valleys.