Geology Reference
In-Depth Information
minimum principal strain is appreci-
ably smaller than the other two strain
axes, which again are approximately
equal. In plane strain, all three strain
axes are different and the intermediate
strain axis is unchanged. This type of
strain is characteristic of
shear zones
,
which we shall discuss in Chapter 6.
Shear strain, on the other hand,
is rota-
tional
, as the strain axes progressively
rotate as the deformation continues
(Figure 4.5E). This type of strain is also
known as
non-co-axial strain
or
simple
shear
but the term 'rotational strain' is
preferred as being more descriptive.
Since we can only observe the final
strain geometry (the so-called
finite
strain
) it may not be obvious whether
the strain was co-axial or rotational.
However, if the unstrained rock con-
tained planar features with varying
orientations, the finite strain will dif-
ferentiate between them as explained
below.
Geometrical features of progressive
strain
Rocks that contain a variety of planes
or lines of varying orientation in their
unstrained state, such as many meta-
morphic rocks, will show progressive
geometrical changes as the strain
increases in intensity, as shown in
Figure 4.6. Depending on their initial
orientation, certain planes or lines
will extend and others will contract.
Moreover, the fields of extension and
contraction in the two types of strain
differ, depending on whether the strain
was co-axial (Figure 4.6A) or rotational
(Figure 4.6B). This property is particu-
larly useful in determining the sense
Co-axial and rotational strain
Distortional strain where the strain axes
maintain their orientation throughout
the course of the deformation is termed
co-axial strain
, as in Figures 4.5B-D.
The alternative term for this type of
deformation,
pure shear
, is potentially
confusing and is not recommended.
Figure 4.6
Progressive co-axial and
rotational strain (in two dimensions).
A.
The effect of co-axial strain on a set of
lines 1-9 at 20° intervals: at a strain of
X:Z = 4:1 the lines begin to concentrate
around the X direction; black lines 1-3
and 7-9 are elongated, green lines 5
and 6 are shortened, and orange line 4
remains the same length; at a strain of
X:Z = 16:1 all the black lines 1-4 and
6-9 are elongated and are concentrated
within an angle of 6° of the X axis; only
green line 5 is shortened.
B.
In rotational
(shear) strain, the strain axes (in red)
rotate during progressive shear strain:
at a shear strain of 0.8 (X:Z = 2:1), black
lines 5-9 are elongated, green lines 1-3
are shortened, and orange line 4 remains
the same length; at a shear strain of
1.8 (X:Z = 4:1), all of the black lines
3-9 are elongated while
green lines 1 and 2 are
shortened; line 3, which
had shortened at the
smaller shear strain, has
now elongated.
4
5
6
7
Z
3
8
9
5
6
4
7
3
2
8
2
9
1
1
X
X
X:Z = 4:1
Z
unstrained
Z
A
co-axial strain
6
4
5
X
X
X:Z = 16:1
Z
X
X
3
6
7
3
4
5
6
7
4
5
3
4
5
6
7
Z
Z
8
8
8
2
2
2
9
9
1
9
1
1
Z
Z
X:Z = 2:1
X:Z = 4:1
X
X
B
rotational strain
shear strain = 0.8
shear strain = 1.8
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