Geology Reference
In-Depth Information
4.7 Mechanistic Approach
In the following chapters, we turn from the macroscopic or continuum view of
deformation to the microscopic or discontinuum view. We shall now consider the
elementary processes whereby the deformation takes place physically, that is, the
deformation mechanisms. The study of mechanisms is involved with the real
structure of the material, that is, with the nature of the building blocks or ele-
mentary structural units of which the material is made, the way in which these
units are assembled and interact, and the way in which the assemblage is affected
by the deformation.
In practice, there is a hierarchy of structural units at different scales to be
considered but for a particular deformation mechanism we can usually distinguish
structural units at a particular scale that can be regarded as the fundamental flow
units for that mechanism. Depending on the scale, there are three general cate-
gories of flow units (Paterson
1979
):
1. Atomic: individual atoms or molecules.
2. Intragranular: especially the intracrystalline blocks or glide packets that slide
over each other in crystallographic slip.
3. Granular: individual grains, clastic fragments or particles.
The identification of the principal flow units and their pattern or relative
movement defines the microgeometry of the deformation.
The microdynamics of the deformation involves the dynamical interactions
between the flow units. These interactions can generally be analyzed in terms of
processes in the zones of contact between the flow units, for which it is sometimes
useful to introduce finer-scale structural entities, such as dislocations or other
crystal defects. Thus, the microdynamical treatment may variously concentrate on
such effects as friction at particle contacts, the dynamics of dislocation multipli-
cation and propagation, the accommodation of geometrical impediment between
flow units, or the kinetics of diffusion or solution and redeposition, and it will
entail establishing which is the rate controlling process in a particular situation.
The variety in types of flow units and of interaction between them naturally
leads to there being a wide variety of possible deformation mechanisms. However,
in dealing with rocks and minerals, this range can be usefully divided into three
categories corresponding to the categories of flow units listed above:
1. Atomic transfer flow: This may involve solid-state diffusional flow as in change
of shape by material transfer by inter- or intra-crystalline diffusion, or viscous
flow in amorphous material; point defects may be important for the diffusion
process. Alternatively, it may involve solution-transfer processes, involving
change of shape by material transfer via a fluid phase, at high or at low
temperature.
2. Crystal plasticity: Intragranular deformation by slip and twinning, much studied
in metals and other ductile materials over a wide range of temperatures;
involves crystal dislocations in a variety of ways.
