Macroscopic imprints of ductile deformation

Abstract

The fundamentals of structural geology are presented, namely, folds, planar structures (cleavage or schistosity, foliation) and linear ones (lineations), regarded as emblematic for geologists. Ductile imprints of folds, affecting stratified formations, combined with brittle imprints, often remain modest in terms of strain intensity. Folding is essentially inhomogeneous and often results from the buckling (bending) of the layers (or stratification) as a consequence of layer parallel compression. Folded structures are frequently accompanied by fractures. Hence they may be classified as brittle–ductile. They are mostly encountered at low depths and constitute the upper structural level of the Earth’s crust. Ductile deformation sensu stricto appears at the lower structural level. The macroscopic aspects of ductile deformations and their implications will be examined. The principal operating mechanism, crystalline plasticity, represents the mechanical aspect of deformation, sometime assisted by chemical aspects (pressure-solution). While homogeneous deformation constitutes our principal concern, heterogeneous deformation is often present, particularly when examined at fine scales. At low shear strain (γ‎ < 0.7, or θ‎ ~35°, equivalent to ~30% shortening), plastic deformation generally leads to a planar and a linear anisotropy strengthening with increasing deformation. At higher shear strain, any pre-existing planar structure becomes so stretched that it cannot be recognized. The new structure may be purely planar, purely linear or plano-linear. Lattice fabrics, appearing in rocks subjected to plastic deformation and resulting from deformation mechanisms at the grain-scale, are examined in detail in Chapter 6.