Dilatation Sites: Fibrous Veins, Strain Shadows, Strain Fringes and Boudins

Abstract

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).