Page 80 - Microtectonics

68 4 · Foliations, Lineations and Lattice Preferred Orientation
4.1 4.1
Introduction

Many microstructures in rocks are defined by a preferred
orientation of minerals or fabric elements. We distinguish
foliations, lineations and lattice-preferred orientation.
The word foliation (Fig. 4.1) is used here as a general
term to describe any planar feature that occurs penetra-
tively in a body of rock. It may refer to thin rhythmic bed-
ding in a sedimentary rock, to compositional layering in
igneous rocks or to cleavage, schistosity, or other planar
structures in metamorphic rocks (Sect. 4.5). Joints are
normally excluded for not being sufficiently penetrative.
We prefer this broad use of a descriptive term to genetic
terms since it is often difficult to decide what the origin
of a planar structure in a deformed rock is. Foliations may
be defined by a spatial variation in mineral composition
or grain size (Fig. 4.1a), by a preferred orientation of elon-
gate or platy grains or aggregates of grains (Fig. 4.1b–f),
by planar discontinuities such as microfractures (Fig. 4.1g),
or by any combination of these elements (Fig. 4.1h).
Stylolites are irregular surfaces, usually in metasedimen-
tary rocks that can define a crude foliation but may also
occur isolated or crosscutting. They are described in Box 4.3.
A lineation is defined as any linear feature that occurs
penetratively in a body of rock (Fig. 4.2). Fibres and stria-
tions on fault planes are not lineations since they only
occur on specific planar surfaces in the rock, not penetra-
tively. Two main types of lineations can be distinguished;
object lineations and trace lineations (Piazolo and
Passchier 2002a). Object lineations are defined by consti-
tuting elements that have a specific volume. Trace line-
ations are intersections of planes or microfolds on folia-
tion planes that lack a distinct volume (Sect. 4.3). Platelet
lineations are defined by planar minerals such as micas
that share a common axis. They have elements of object- Fig. 4.1. Diagrammatic presentation of various fabric elements that
may define a foliation. (After Fig. 5.1 in Hobbs et al. 1976). a Compo-
and trace lineations. sitional layering. b Preferred orientation of platy minerals (e.g. mica).
In three dimensions many foliations show an associ- c Preferred orientation of grain boundaries and shape of recrystal-
ated linear element, that is, the fabric elements defining lised grains (e.g. quartz, carbonate) in a grain shape preferred ori-
the foliation may appear stronger in some sections nor- entation. d Grain-size variation. e Preferred orientation of platy min-
mal to the foliation than in others. A complete transition erals in a matrix without preferred orientation (e.g. mica in mica-
ceous quartzite or gneiss). f Preferred orientation of lenticular min-
from pure S tectonites (only a foliation) to LS tectonites eral aggregates. g Preferred orientation of fractures or microfaults
(both a foliation and a lineation) to L tectonites (only a (e.g. in low-grade quartzites). h Combination of fabric elements a,
lineation) can be imagined. In practice, such transitions b and c; such combinations are common in metamorphic rocks
may actually exist within a single outcrop. Some lineations
may develop from or into foliations with time. An exam- tion from a rock sample (Sect. 12.5). Lineations are treated
ple of the latter is the progressive development of slaty cleav- in Sect. 4.3.
age at a high angle to bedding (Box 4.6, ×Video B.4.5). Many rocks have a lattice-preferred orientation (LPO),
It is important in the description of a foliation to give a non-random orientation of the crystallographic axes of
the relation with a lineation, if present. The linear ele- constituent minerals. Some foliations or lineations are
ments that are of the same age as foliations in a volume of defined by a LPO. However, we use the term here in a more
rock are important in tectonic studies because they may restricted sense for minerals with an equant shape like
furnish information on the direction of tectonic trans- quartz and calcite, the LPO of which cannot be seen in
port (Fig. 5.10). Such lineations must definitely been taken the field or in thin section without the aid of special tech-
into consideration when deciding how to cut a thin sec- niques. LPO is treated in Sect. 4.4.