Page 16 - Microtectonics

2 1 · A Framework of Microtectonic Studies
1.1 1.1 Box 1.1 Fabric, texture, microstructure
Introduction
In this book we mainly deal with fabrics. A fabric “includes the
complete spatial and geometrical configuration of all those
th
From their first use in the 19 century, thin sections of
components that make up a rock. It covers concepts such as
rocks have been an important source of information for texture, structure and crystallographic preferred orientation“
geologists. Many of the older textbooks on structural ge- (Hobbs et al. 1976). The parts that make up a fabric, also known
ology, however, did not treat microscopic aspects of struc- as fabric elements, should be penetratively and repeatedly de-
tures, while petrologists would describe microscopic veloped throughout a volume of rock; a single fault in a vol-
ume of rock is not considered to be part of the fabric, but a
structures as, for example, lepidoblastic or nematoblastic
large number of parallel foliation planes are. Fabric elements
without paying much attention to kinematic and dynamic are therefore dependent on scale (cf. Fig. 2.4). A volume of rock
implications. During the last decades, however, structural may have a random fabric, i.e. a random distribution and ori-
geologists learned to profit from the wealth of data that entation of its elements or, more commonly, a non-random fab-
can be obtained from the geometry of structures studied ric, including foliations and lineations.
in thin section, and metamorphic petrologists have ap- In this book, we mainly deal with fabrics on microscopic
scale, or microfabrics. Microfabric elements may include grain
preciated the relation of structural evolution on the thin shape, grain boundaries, deformation lamellae, aggregates of
section scale and metamorphic processes. grains with similar shape, and lattice preferred orientation.
Deformed rocks are one of the few direct sources of In the non-geological literature about metals and ceramics,
information available for the reconstruction of tectonic the term texture is generally used for lattice-preferred orienta-
evolution. Nevertheless, observations on the geometry of tion. On the other hand, most of the older textbooks on meta-
morphic petrology (e.g. Turner 1968; Miyashiro 1973; Best 1982;
structures in deformed rocks should be used with care; Williams et al. 1982; Bucher and Frey 1994) make a distinction
they are the end product of an often complex evolution between the texture and the structure of a metamorphic rock.
and we can only hope to reconstruct this evolution if we In these texts, texture refers to the geometrical aspects of the
correctly interpret the end stage. Simple geometries such component particles of a rock including size, shape and ar-
as folds can be formed in many ways and it may seem rangement, whereas structure usually refers to the presence of
compositional layering, folds, foliation, lineation, etc. In fact
hopeless to try and reconstruct a complex evolutionary
there is no clear difference between the two concepts and the
sequence from geometrical information only. However, subcommission on the systematics of metamorphic rocks of
despite the simple geometry of our face, we can individu- the IUGS recommends substituting the term texture by micro-
ally recognise most of the six billion people on our planet. structure. In this book, we use the terms microstructure and
It is likely that structures in rocks also contain a large amount microfabric (see also Sect. 1.1) as synonyms.
of detail, which we cannot (yet) recognise and interpret
because we are not trained to do so, and partly because we
do not know what to look for. It is interesting to page through chapter can be read before, but also in conjunction with
old publications on microstructures, e.g. on inclusions in the other chapters.
garnet or on porphyroclasts, to see how drawings evolved In theory, one could expect that a sedimentary rock,
from simple to complex while understanding of the proc- which is buried, deformed, metamorphosed and brought
esses related to the development of these structures in- back to the surface, should have the same mineral compo-
creased. At any time, some degree of misinterpretation of sition as the original sediment if perfect equilibrium con-
structural evolution is unavoidable and part of the nor- ditions were to be attained at each stage. A simple fabric
mal process of increasing our understanding of the sub- should be developed in such a case in response to gradual
ject. This book is therefore a-state-of-the-art description changes in the stress field and in metamorphic conditions.
of microstructures and their interpretation. Fortunately for the geologist, who relies on structures and
Observations on the microstructure or fabric of a rock mineral assemblages in deformed rocks as a source of in-
(Box 1.1), specifically in thin section, can be used in two formation, this is almost never the case. In most deformed
major fields. They can be applied to thematic studies, to rocks, structures with different style and orientation and
understand mechanisms of rock deformation and meta- minerals, which represent different metamorphic grades,
morphism; or they can be used to reconstruct the struc- overprint each other. This means that equilibrium is gener-
tural and metamorphic history of a volume of rock. Thin ally not attained at each stage: mineral assemblages rep-
section studies are mostly in the latter field. Because such resentative of different metamorphic conditions may be
thin section studies can serve to reconstruct tectonic evo- ‘frozen in’ at different stages during burial and uplift. With
lution, we use the term microtectonics. overprint we mean that structures or mineral assemblages
This chapter is not only meant as a general introduc- are superposed on each other and must therefore differ
tion to the subject of microtectonics, but also serves as a in age; this may be visible through crosscutting relations,
definition of the framework within which we see studies overgrowth, or even differences in deformation intensity.
in microtectonics. As such, it contains terminology that is In practice, however, overprinting relations can be diffi-
explained only later in the text and in the glossary. This cult to establish. This book mainly serves to illustrate the