Microtectonics

From their first use in the last century, thin sections of rocks have been an important source of information for geologists. Many of the older textbooks on structural geology, however, did not treat microscopic aspects of structures, while petrologists would describe microscopic structures as, for example, lepidoblastic or nematoblastic without paying much attention to kinematic and dynamic implications. During the last decades, however, structural geologists learned to profit from the wealth of data that can be obtained from the geometry of structures studied in thin section, and metamorphic petrologists have appreciated the relation of structural evolution on the thin section scale and metamorphic processes.

Many microstructures in rocks are defined by a preferred orientation of minerals or fabric elements. We distinguish foliations, lineations and lattice-preferred orientation.

Many deformed rocks contain sites with a deviant mineralogy and fabric, interpreted as an effect of rearrangement of material by local dilatation and precipitation during deformation. Such ‘dilatation sites’ can be isolated and elongate (veins), flanking rigid objects (strain shadows) or occur in the neck of boudinaged layers or elongate crystals (Fig. 6.1). Strain shadows are also referred to in the literature as pressure shadows. Most veins and many strain shadows and boudin necks have sharp contacts with the wall rock and may form by precipitation of material from an aqueous solution in a fracture, as outlined below. Such sites are usually filled with polycrystalline material which may be massive, but commonly consists of rod-shaped crystals known as fibres (Figs. 6.1–6.3). Fibrous veins and strain shadows (the latter also known as strain fringes; Fig. 6.3) are some of the most complex microstructures to be found in rocks, and contain much information about deformation and deformation history (Figs. 6.1–6.3; Zwart and Oele 1966; Choukroune 1971; Durney and Ramsay 1973; Beutner and Diegel 1985; Etchecopar and Malavieille 1987). Strain fringes are also known as pressure fringes. Since the shape of the aggregates gives primarily information on strain distribution around an object, and not on forces, we advocate the use of the term strain fringe. The same applies to pressure shadow where we prefer the use of strain shadow. Some veins and strain shadows have fuzzy boundaries (Fig. 6.1). They may form by local alteration of the wall rock along a fracture (replacement veins) or rigid object, or by deformation and recrystallisation of veins with sharp boundaries (Chap. 6.5, 7.5 and 7.6.8).

A volume of rock involved in deformation and metamorphism will continuously undergo changes in structure and mineral content. This chapter treats mineral growth and replacement structures and the way in which their geometry can be used to reconstruct tectonic history. Two types of informative structures are treated: porphyroblasts and reaction rims.

A sound analysis of microstructures relies on correct sampling and on the right choice of the direction in which thin sections are cut from samples. This chapter discusses the steps of sample collection; choice of sectioning plane, and problems involved in the interpretation of three-dimensional structures from two-dimensional sections.

This chapter presents photographs of microstructures similar to those discussed in the previous chapters. The photographs are meant as exercises, to be studied and interpreted by the reader. We have given only the essential information in the figure captions, and our interpretations of the structures are given at the end of the section. The subjects of the problems follow the sequence of treatment in the book. The reader may not always agree with our interpretations, but the main purpose of this chapter is to provide some examples of how information can be obtained from thin sections and which arguments can be used for their interpretation.