An Introduction to Structural Geology and Tectonics

9. Ductile Deformation Processes and Microstructures

TEM image of authigenic mica; scale bar is 100 nmWhy is it that a layer of rock can be permanently bent into a fold, or that a material such as ice can be permanently contorted (or flow) over a period of time while it remains a solid? Ice is perhaps a particularly instructive example of flow in crystalline solids because it moves on human time scales. At first you might think that this is accomplished by bending and stretching of atomic bonds in the crystal lattice, but bending and stretching is elastic deformation, and, as described in Chapter 5, elastic deformation is recoverable (i.e., non-permanent). The movement of glaciers, however, is a permanent feature and they only retreat by melting. Such permanent bends and contortions represent ductile deformation and it is not recoverable. If we would be able to remove a folded layer from an outcrop or a deformed mineral from a hand specimen they would not jump back to their original forms. So the distortions that have occurred must also be a result of permanent changes in the material. Understanding some of the principles that underlie this ability of rocks to accumulate permanent strain is contained in a vast and ever growing body of materials science literature.
Structural geologists have applied many of the concepts from materials science to geologic conditions; use of the associated terminology, however, is not always consistent between these fields. In trying to keep the terms and concepts to a minimal, we invariably have to make some choices in this chapter; otherwise we will loose site of our ultimate goal: understanding the way rocks deform in the ductile regime. In Chapter 5 we first introduced the concept of flow. At that point we described this topic merely in terms of stress and strain rate. We contrasted linear viscous (Newtonian) and non-linear viscous (non-Newtonian) behavior using analogs and simple mechanical models. In this chapter we will examine the physical processes that underlie this ability of materials to undergo appreciable, permanent distortions that are distributed through a volume of rock. Just to refresh your memory, strain that is distributed over the body is what distinguishes ductile behavior from brittle behavior. But strain that appears homogeneous on one scale may represent heterogeneity on another, so we again have to include the scale of our observation. The scales of observation in structural geology range from nanometers (10^-9 m; Figure) to kilometers, and single minerals to mountain ranges. For practical reasons we therefore define ductile behavior as uniform flow down to the scale of the hand specimen. We distinguish three fundamental mechanisms that produce ductile behavior in rocks and minerals: (1) cataclastic flow, (2) diffusional mass transfer and (3) crystal plasticity. Which of these processes actually occurs at a given time in a rock is primarily a function of temperature, stress, grain size, rock composition and fluid presence. At the onset we mention that temperature is a particularly important parameter, but different minerals behave ductily at different temperatures. What is considered high temperature behavior for one mineral may be low temperature behavior for another mineral. Thus, when talking about the relationship between temperature and deformation, we need to use a normalized parameter that is called the homologous temperature, Tm. The homologous temperature is a dimensionless parameter that is defined as the absolute temperature divided by the absolute melting temperature of the material: Th = T/Tm,  with T for temperature and Tm for the melting temperature of the material, both in K. Now, we will looely define 'low temperature conditions' as 0>Th>0.3, 'medium temperature conditions' as 0.3>Th>0.7, and 'high temperature conditions' as 0.7>Th>1. After discussing the fundamental mechanisms and the associated microstructures (i.e., mineral geometries on the microscopic scale), we will close this chapter by examining the interrelationship between the various rheologic parameters (such as stress and strain rate; see Chapter 5), and we introduce the powerful concept of deformation mechanism maps. Now let us first turn to the three mechanisms of ductile behavior.

Table of Deformation Mechanism by Jessell and Bons (Monash Univ.)

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