Page 93 - Microtectonics

4.2 · Foliations 81
The morphology of crenulation cleavages may show a A foliation may change its morphology drastically
vast array of variation (Fig. 4.12, ×Video 4.12, ×Photo 4.12, within a single thin section (Fig. 4.15, ×Photo 4.15), or
Fig. 4.13, ×Photo 4.13; Figs. 4.19–4.21, 4.35, 4.37); impor- even disappear completely. This is generally related with
tant factors that influence the final morphology, apart the transition from one lithotype to another; foliation
from the lithotype, are temperature and deformation development is strongly dependent on lithotype. How-
intensity. Figures 4.18 and 4.19 (×Photo 4.19b1–7) show ever, local strain distribution around fold hinges has its
the inferred range of stages in crenulation cleavage de- influence on foliation development too, and may produce
velopment according to these two parameters (however, a remarkable variation in foliation morphology along a
see also Box 4.5). single layer.
A special type of spaced foliation is compositional lay- It is generally difficult to quantify the intensity or
ering, where microlithons and cleavage domains are wide strength of foliations. However, relative strength of foli-
and continuous enough to justify the use of the term lay- ations can be compared in samples with a continuous
ering. Normally, this term is applied if the layering is vis- foliation and similar grain size and mineral content us-
ible to the unaided eye in a hand specimen. ing X-ray texture goniometry (Sect. 10.3.5; van der Pluijm
Many transitional forms between foliation types as et al. 1994; Ho et al. 2001).
defined above occur in nature. In fact, the variation in
morphology is almost infinite and we should realise that 4.2.7
the proposed classification is meant as a way to facilitate Mechanisms of Foliation Development
communication between geologists and not as an objec-
tive in itself. For this reason, we have not tried to define 4.2.7.1
strict boundaries between categories, and we advocate the Introduction
use of a minimum of terminology. Where necessary, a
good photograph or detailed drawing can supplement a Secondary foliations develop in response to permanent
description. rock deformation. The main controlling factors on their

Fig. 4.16. Schematic diagram of some important mechanisms contributing to development of secondary foliations in rocks. a Fabrics at the
onset of deformation. b Fabric elements after deformation. 1 Elongate crystals (open rectangles) rotate in response to deformation in a way
similar to theoretical passive markers (solid lines) but there are differences; minerals may fold when normal to the shortening direction and
thus strengthen a preferred orientation, or rotate at slower rate than material lines when highly oblique to the shortening direction. 2 Min-
eral grains change shape by stress-induced solution transfer; grey is original material, white are overgrowths. 3 Mineral grains change
shape by crystalplastic deformation such as dislocation creep or solid-state volume diffusion. 4 Polymineralic aggregates develop foliations
by processes 1 + 2 when assisted by stress-induced solution transfer. 5 Grain growth of micas parallel to (001) during or after shortening
leads to an increase of foliation intensity because grains oriented in the direction of the foliation can grow to greater length than those in
oblique orientations. 6 Oriented nucleation and growth of a mineral in a stress field. 7 Mimetic growth of elongate grains due to restrictions
in growth direction imposed by an existing foliation. 8 Restricted growth parallel to platy minerals