Development Of Crystallographic Preferred Orientation Research Articles

A Review of Deformation Microstructures

Microstructural geology is playing an ever more important role in structural geology, yet it is only slowly being incorporated into structural geology courses because of the difficulties in demonstrating the great variety of deformation processes. We have found that the combination of a video program showing real-time formation of microscopic deformation features in rock analogues and lectures on deformation is a good way of introducing this material. Our method introduces students to an atomistic view of dislocations and related deformation mechanisms such as twinning, kinking, and glide. Development of these features is shown to be related to the development of crystallographic preferred orientations. On the microscopic level, dislocation motion manifests itself as subgrain formation and recrystallization. The video shows how S-C structures can be used to determine the sense of movement along shear zones in metamorphic rocks. Furthermore, students are introduced to the concept of grain-boundary sliding ...

Flow mechanisms in sulphide minerals

The sulphide minerals as a group display a diverse range in strength and mechanical behaviour in crustal environments, deforming by a range of different flow mechanisms which can lead to the development of characteristic microfabrics. The dependence of creep rate on imposed parameters, such as stress difference, temperature, grainsize, effective confining pressure, and the activities of various components, varies for each type of flow mechanism. For particular phases, changes in the dominant operative flow mechanism may occur in response to changes in the imposed environmental variables. At high applied stress differences, low effective confining pressures, and low temperatures, brittle failure and cataclastic flow are significant deformation mechanisms in some sulphide minerals. At elevated confining pressures, brittle failure can be suppressed and low-temperature plasticity processes involving dislocation glide and/or deformation twinning may operate. With decreasing applied stress difference, and at elevated temperatures, even though strain may be accommodated largely by dislocation glide, deformation may be rate-controlled by recovery processes, and dynamic recrystallization may be significant in influencing microfabric development. The operation of dislocation glide processes leads to the development of crystallographic preferred orientations. At very low applied stress difference, various grainsize-dependent atomic-transfer deformation mechanisms may be important. In many fluid-present, low- to medium-grade metamorphic environments, solution-precipitation creep processes are active in the deformation of some sulphide phases; in fluid-absent situations, grain-boundary diffusion creep may be significant. At higher temperatures, where intragranular diffusion is relatively rapid, lattice-diffusion creep processes are expected to be a major deformation mechanism. In fine-grained sulphide assemblages at elevated temperatures, grain-boundary sliding may be an important deformation mechanism.

Microstructure and fabric transitions in calcite tectonites from the Sierra Alhamilla (Spain)

A suite of marble specimens from the Sierra Alhamilla (Spain), deformed to large strains under natural conditions at about 300° C shows distinct variations in microstructure and fabrics. It can be demonstrated that the development of crystallographic preferred orientations and grain shape fabrics are strongly dependent on recrystallized grain size. This is interpreted to reflect the relative importance of various deformational mechanisms. Superplasticity seems to have a sharp upper grain size limit at 10–15 /smm. Within the power law creep regime, deformation in fine grained aggregates is probably dominated by diffusional, in coarser ones by dislocation mechanisms. The transition is a gradual one, and may span a grain size range of 30 /sm.

The simulation of fabric development during plastic deformation and its application to quartzite: the influence of deformation history

The effect of deformation history on the development of crystallographic preferred orientation in quartzities has been simulated using a computer program based on the Taylor-Bishop-Hill analysis. Model quartzities with different combinations of glide systems have been subjected to various coaxial and non-coaxial deformation histories. It is possible to obtain information from the fabrics that develop during simple histories; for example, the location of the axis of extension is generally associated with a pole free area on a c-axis plot, and progressive axial shortening, plane strain and axial shortening produce characteristic fabrics. In progressive simple shear the fabric skeleton becomes asymmetric relative to the sense of shear and a-axes preferentially align in the flow plane parallel to the flow direction. However, this example illustrates that the fabric orientation and characteristics are controlled by the kinematic framework and bear only an indirect relationship to the finite strain accumulated to that point in the history. The imprint of the closing stages of deformation limits to some degree the use of crystallographic fabrics as a tool for structural geologists, but in favourable circumstances data can be obtained concerning characteristics of the deformation history, on the scale of the hand-specimen, for the last part of this history.