Page 113 - Microtectonics

4.3 · Lineations 101

Box 4.9 Transposition 4.3 4.3
Lineations
In many metamorphic terrains it is difficult or impossible to
use bedding as a reference plane in outcrop. The distribution
of lithotypes on the map may allow establishment of approxi- 4.3.1
mate contacts between stratigraphic units, but these may not Terminology of Lineations
coincide with lithologic contacts in outcrop. The latter contacts
are usually parallel to “the main foliation” and may be difficult Terminology of lineations was redefined by Piazolo and
to follow along strike. In such areas, transposition of one or Passchier (2002a), as follows: Object lineations can be sub-
more foliations (including bedding) has occurred. Transposi-
tion is usually defined as the progressive erasure of a reference divided into grain lineations and aggregate lineations
surface (S 0 , S 1 , S n , etc.) due to tight folding accompanied by some (Fig. 4.2). Grain lineations are defined by parallel oriented
differentiation process. However, it can also be used in a more elongate single crystals. These can be deformed single
general sense for erasure of an older structure by strong crystals of normally equidimensional shape such as quartz
younger deformation. Turner and Weiss (1963; see also Davis or calcite (grain shape preferred orientation – Box 4.2),
1984) have given some good examples of bedding transposi- or of euhedral or subhedral mineral grains with an elon-
tion on the outcrop scale. The concept is clearly scale-depend-
ent; a number of en-echelon disrupted bedding lenses may be gate shape such as amphibole, tourmaline or sillimanite.
mistaken for real bedding if seen in an outcrop smaller than An aggregate lineation is a type of shape preferred orien-
these lenses. In a large outcrop where a number of lenses are tation defined by elongate aggregates of equidimensional
visible, the oblique position of the enveloping surface of bed- or slightly elongate grains (Box 4.2). Common examples
ding may still be recognisable. are aggregates of dynamically recrystallised grains replac-
Transposition may also occur on the scale of a thin section.
The sequence of crenulation cleavage development (Figs. 4.18, ing a large deformed older crystal. It is possible and even
4.19) is a good example of transposition of S 1 by S 2 . Figure 4.38 common that only part of the rock volume defines an
shows a natural example of a D fold where S is clearly distinct object lineation (Fig. 4.2). Trace lineations include crenu-
2
2
from S 0 /S 1 in the fold hinges, whereas in the limbs transposi- lation lineations and intersection lineations (Fig. 4.2). In-
tion has occurred and all three planes, S 0 , S 1 and S 2 , have be- tersection lineations are formed by intersecting foli-
come parallel. The parallelism of S and S probably indicates
0
1
that a similar process occurred during D 1 . ations (Fig. 4.2) while crenulation lineations are defined
by hinge lines of microfolds in a foliation surface (Fig. 4.2).
Several other terms are used in the geological litera-
ture for lineations. The word stretching lineation is com-
monly used as a general term for aggregate lineation and
grain lineation if the constituting grains are defined by
deformed aggregates or single crystals. However, the term
stretching lineation has genetic implications and can be
misleading. Elongate crystals or aggregates can form by
stretching, but also by boudinage into thin strips normal
to the stretching direction, or by vein formation. Therefore,
the term stretching lineation should only be used if it is
clear that aggregates lie in the direction of the X-axis of the
finite strain ellipsoid. Grain lineations made up of deformed
large crystals can also be classified as stretching lineations
for this reason (Fig. 4.2). The term mineral lineation has
been used for the preferred orientation of non- or little de-
formed euhedral or subhedral mineral grains with an elon-
gate shape such as amphibole, tourmaline or sillimanite.
Mineral lineation is a special type of grain lineation. Cata-
clastic lineation (Tanaka 1992) consists of elongate frag-
ments and aggregates of fragments in the more fine-grained
matrix of a cataclasite (Sect. 5.2).
Since lineations are defined as linear structures that
occur penetratively in a volume of rock, they do not in-
clude linear features that only occur on certain surfaces
in the rock. For example, slickenside striations and simi-
lar structures that occur restrictedly on slickensides (e.g.
Means 1987) or other fault planes are not considered to
Fig. 4.38. Isoclinal D fold showing the parallel orientation of S , S 1 be lineations and are not treated here, since they can rarely
2
0
and S in fold limbs and S oblique to S /S in the hinges be studied in thin section. Another common mistake is to
2
2
1
0