Deformation, stress and strain - Introducing Tectonics, Rock Structures and Mountain Belts - page 26

strains a rough approximation to the
strain can be given by measuring the
spread of orientations about the X axis.

Figure 4.8 Contraction and
extension in layered rocks.
Contraction of a strong layer
within weaker material will
produce folds ( A ), whereas
extension may produce
boudinage ( B ). C. Extension
has resulted in the stronger,
sandstone layer being pulled
apart forming boudins separated
by narrow 'necks'; the weaker
shale layers have flowed
into the areas between the
boudins; see car key for scale.

stronger

weaker
stronger

weaker

boudinage

Structures produced by compression
and extension
A body of rock subjected to compression
or extension may deform homogene-
ously if it is composed of material with
approximately the same physical prop-
erties throughout, as in Figure 4.7, for
example. However, if the rock contains
layers of stronger material enclosed
within weaker, the layers, when com-
pressed, may contract, forming folds
(Figure 4.8A). When subjected to exten-
sional strain, a stronger layer may divide
into blocks separated by thin necks,
or become completely isolated by the
enclosing weak material, as illustrated
by Figure 4.8B and C. The latter process
is termed boudinage and the separate
blocks, which may be either sausage-
shaped or brick-like, are boudins (from
a French word for a type of sausage).

B extension

A shortening

C

X

extension
direction

planar
fabric

Figure 4.9 Strain and fabric.
Undeformed block ( A ) containing
spherical objects is subjected
to co-axial strain ( B ): planes
parallel to the YZ strain axes are
shortened (folded) while planes
parallel to the XY strain axis are
lengthened and flattened (to
give a planar fabric, or foliation).
The spheres are extended in
the X direction and flattened in
the XY plane. A linear fabric,
in the form of an elongation
lineation is parallel to X.

Y

linear
fabric

Z

Transpression and transtension
A block or layer of material may be
subjected to compressional stress
and shear stress simultaneously, in
which case the stress state is termed
transpression . Shear stress added to
extension, is termed transtension .

Z

compression
direction

Y

A

B

X

Strain and fabrics
The effect of large strains, mainly on
metamorphic rocks but also on some
unmetamorphosed rocks, is to produce
a set of new microstructural elements
that are collectively known as the fabric .
The subject of rock fabric is discussed
in Chapter 7. Fabrics include sets of
planar and linear structures (termed
respectively foliations and lineations )

that penetrate through the whole rock
on the scale of the outcrop or the hand
specimen, and these fabrics can be used
to indicate both the strain axes and, in
favourable circumstances, the amount
of strain. Thus a foliation that represents
a plane of flattening, as in Figure 4.9B,
will contain the principal strain axes X

and Y, and will be perpendicular to Z. A
lineation that represents the direction
of elongation within this XY plane will
be the principal strain axis X. Under
these conditions, therefore, measure-
ments of the strain ratio of flattened or
elongated objects in these planes can be
used to estimate the strain ratios X:Y:Z.