Page 38 - Microtectonics

26 3 · Deformation Mechanisms
3.1 3.1
Introduction

Deformation in rocks is achieved by a large number of proc-
esses on the scale of individual grains. The actual processes
involved depend on factors such as mineralogy, composi-
tion of the intergranular fluid, grain size, lattice-preferred
orientation, porosity and permeability; and on external
controls such as temperature, lithostatic pressure, differen-
tial stress, fluid pressure and externally imposed strain rate.
In this chapter, we will briefly introduce the most impor-
tant rock deformation processes in a sequence from low
temperature-high strain rate to high temperature-low stain
rate. Grain-scale microstructures that are thought to be
formed in response to these processes are highlighted, and Fig. 3.1. Microcrack propagating in extension a and shear b. When
it is shown how such microstructures can be used to iden- the crack opens, the tips propagate in extension mode (e), sliding
tify deformation processes that have been operating. mode (s) or tearing mode (t)
Grains are volumes of crystalline material separated
from other grains of the same or different minerals by a (Fig. 5.1; Hallbauer et al. 1973; Blenkinsop and Rutter
grain boundary. If a grain boundary separates grains of 1986; Lloyd and Knipe 1992; Moore and Locker 1995).
the same mineral, they must have a significantly different Motion on the fault then gradually separates grain seg-
lattice orientation. Some authors restrict the use of the ments and a volume of brittle fault rock is produced along
term grain boundary for surfaces separating grains of like the active fault (Fig. 5.1).
minerals, and use the term interphase boundary for sur- Microcracks are planar discontinuities in rocks on the
faces separating different minerals (Fliervoet et al. 1997). grain scale or smaller, commonly with some dilation but
In practice, it is difficult to maintain this distinction when with negligible displacement. They may nucleate on mi-
describing aggregates composed of many grains, and we nor flaws in the crystal lattice, fluid or solid inclusions in
therefore use grain boundary for both types of surfaces. crystals, or on grain boundaries (Tapponier and Brace
Structures visible within grains are known as intracrys- 1976). Microcracks propagate laterally by movement of
talline deformation structures. their tips into intact surrounding material. When the crack
Although we treat deformation processes and micro- opens the walls can be displaced in a tensional regime, in
structures one by one, this does not mean that they occur a shear regime or in a combination of both. If a shear
isolated in deformed rocks. Most deformed rocks have a component is present the structure is better referred to
long and complicated history of burial, deformation, meta- as a microfracture, and motion can be towards a tip line,
morphism and uplift, and several stages of this process or parallel to it (Fig. 3.1b). In all cases, elastic displace-
may have contributed to the final fabric. Since peak meta- ment creates a differential stress increase at the tip of the
morphic conditions tend to erase earlier structures, most fracture that depends on fracture length, applied bulk
overprinting structures tend to be higher temperature stress, elastic properties of the material and resistance to
features which are overprinted by lower temperature ones. breaking atomic bonds at the crack tip, known as frac-
ture toughness. Displacement on a microfracture can lead
3.2 3.2 to fracture propagation if a certain critical differential
Brittle Fracturing – Cataclasis stress is reached, in extension, sliding or tearing mode
(Fig. 3.1). This displacement is usually in the plane of the
At low temperature or high strain rate, rocks change shape microfracture if it lies isolated in a homogeneous isotropic
by brittle deformation, i.e. by fracture formation and material such as glass (Fig. 3.1). However, microfractures
propagation associated with movement along faults. In may also obtain a curved shape if the stress field at the
the terminology of brittle deformation a fracture is a pla- tip interferes with that of a neighbouring fracture or an-
nar discontinuity usually with some dilation, including other inhomogeneity such as an inclusion (Fig. 3.2). In
cracks, joints (large cracks) and faults. A crack or joint rocks, most minerals are mechanically anisotropic and
opens at right angles to the plane of the fracture and has microfractures commonly form along certain crystallo-
no displacement (Fig. 3.1a); a fault has lateral displace- graphic directions such as the cleavage direction in mi-
ment (Fig. 3.1b). A propagating fault has a progress zone cas (Wong and Biegel 1985), feldspars, amphiboles, py-
at its tip (Fig. 10.9) where isolated microcracks form and roxenes (Williams et al. 1979; Brown and Macaudiere 1984;
propagate, microcrack density gradually increases, and Tullis and Yund 1992) and calcite; even quartz is slightly
finally microcracks link to form a through-going fault anisotropic for fracturing (Vollbrecht et al. 1991). If there