Geology Reference
In-Depth Information
Figure 3.22 Fused
volcanic rock and
granitic intrusion,
Anderson Road
Quarry, Hong
Kong.
mm movements, to breaks in the Earth
s crust extending many
kilometres laterally and vertically and with cumulative displacements
over many years, also of many kilometres. Faults can form in com-
pressive regions (reverse faults) and in extensional zones (normal
faults). The term
'
originates from coal mining in the UK,
because most fault blocks dropped away, down the dip of the fault,
and the miners knew in which direction the productive coal seam was
likely to be found. Faults in the upper 10 km or so of the Earth
normal
'
'
'
s crust
break in a brittle manner, producing fractured, brecciated rock. At
greater depth, where the temperature and stress is much higher, faults
occur more plastically. Typical features of brittle and plastic fault
zones, now exposed at
s surface, are illustrated
in
Figure 3.23.
Fault zones can be very extensive, with metres of
broken rock and gouge between the walls of the fault (
Figure 3.24)
but can otherwise be represented by a single surface with very little
gouge (
Figure 3.25).
One example presented in
Chapter 7
(TBM tunnel
collapse) serves as an illustration of how important it is to know the
nature of any fault zone. Sometimes the fault movement results in a
the Earth
'
Figure 3.23
Schematic
representation of
ductile and brittle
fault zones now
exposed at the
Earth
mylonite
quartz veins
folded
metasediments
s-c fabrics
s surface
(after Fletcher,
2004).
'
granite
quartz veins
fault gouge
fault breccia
