Environmental Engineering Reference
In-Depth Information
folds, (c) types of fold, showing increasing deformation to the
right.
The principal forms of plastic deformation are
foliations
(including cleavage) and
folds
,
which produce banded and wave-like structures respectively. Foliation tends to occur at
small to intermediate scales, seen typically in microscope to hand specimens. Folds occur
at all scales, with wavelengths up to 10
1-2
km, and in a variety of wave forms (Figure
12.21b, c). Material between each foliation or fold plane is stretched or compressed and
particles are realigned parallel to the deformation. This creates strength anisotropy, which
is exploited by denudation. Foliation is a form of low-grade metamorphism. Folding, by
comparison, represents a shortening of rock mass in the direction of compression and
corresponding extension at right angles (Figure 12.20). The degree of folding reflects
stress intensity and rock deformability (see Colour Plate 11 between pp. 272 and 273).
Nappes
bridge the difference between folds and faults. Rocks, well folded past the
vertical, assume a recumbent form, but shortening does not stop there. The upper limb
continues to deform by faulting in the thrust direction against the lower limb. In
continental collision, slivers of cratonic basement may be thrust forward in this way.
However, displacement may occur along a
décollement
or detachment surface instead,
requiring - or creating - a zone of significant change in material properties, rather than a
fault
sensu stricto
. Such low-angled surfaces may also generate tectonic-scale gravity
sliding in the opposite direction. The Alpine thrust zone (see Figure 25.4), Canadian
Rockies and Appalachians show nappe/
décollement
structures and the basin-range system
of the southwestern United States exemplifies gravity sliding (see Figure 25.2).
Deformation is also associated with
diapirism
, forcing overlying rock into a dome
intruded by less dense magma (hot) or salt and mud (cold) diapirs.
Brittle failure fractures rock without disturbing the intervening rock mass, at a strain
rate and magnitude which exceed its intact strength.
Joints
are incipient faults, generally
restricted to individual facies, and occur initially in response to contraction brought about
by cooling (igneous) and dewatering (sedimentary) processes.
Faults
are joints with
differential movement on opposite sides and are identified by the principal displacement
direction (Figure 12.22). Faults and joints are organized in geometric (usually
orthogonal) patterns and each plane is defined by a
dip
and
strike
. They differ in scale,
with joint spacing at 10
1-3
cm and faults two or three orders of magnitude larger.
Fractures may be 'clean breaks' or occasionally smoothed, where movement has abraded
opposing faces to form
slickensides
. However, both forms may be lined with a 'fill'
composed of coarse rock fragments (
fault breccia
), fine debris (
gouge
) or cement -
which may be weaker or stronger than the fractured rock mass itself.
Deformation accompanies both small-scale rock-forming processes, when they are
lithological in nature, and the creation of large-scale tectonic structures. Subsequent
applied stresses are likely to be accommodated along existing structures first and, in that
way, Earth's principal mobile belts often drive younger plate motions and orogens. In
addition to their primary function, they are of vital importance to denudation (Plate 12.4).
