General Characteristics of Migmatites and Migmatite Terranes

Abstract

Structures and structural relationships resulting from superimposed deformation have long been recognized by geologists, and references to this perception can be found in the literature at least as far back as the late nineteenth century (Clough, The Geology of Cowal (Gunn et al, 1897, pp. 23, 24), and here, under ‘Fold (Structure) sets’, section 5.3), and even earlier than this, in the eighteenth century, James Hutton in his Theory of the Earth, Chapter IV, p. 19 (1795), described overprinting relationships between granite, ‘schistus’ and a later transecting granite vein observed on a boulder in Glen Tilt, Scotland (see also section 8.1.2). The systematic study and geometrical analysis of superimposed folds, i.e. the effects of overprinting (Überprägung), were formalized in the early 1900s by the work of the Austrian geologists Sander (1930, 1970) and Schmidt (1932). The publication of Sander’s work, particularly that in English in the 1930s (Knopf, 1933) greatly stimulated the study of structural relationships in folded rocks, a study which increasingly employed stereographic projection as an analytical tool. This in turn led to an increase in studies tending to concentrate on two major aspects of structural geology; one concerned with the geometrical relationships and behaviour of structures during refolding by different mechanisms, and the other involving statistical analysis of refold relationships with Schmidt nets, using Lambert equal area projection, of data concerning refold relationships collected in the field (see Turner and Weiss 1963; Phillips 1971; and Figures 2.10, 2.11,2.12).