Lecture index: Lattice imperfections. / Dislocation glide. / Diffusion. / Recrystallization and recovery. /
Reading: Chapter 9: Ductile Deformation Processes and Microstructures in van der Pluijm B. A. & Marshak, S., Earth Structure - An Introduction to Structural Geology and Tectonics, McGraw-Hill, p. 179-206. This is a very nice summary with some depth and very nice illustrations. Read it carefully.
From introductory courses in geology one might be forgiven of thinking of crystal lattices as being a model of perfection, all the atoms lined up and in their place. Real world crystals are full of imperfections, and those imperfections are crucial to understanding how intracrystalline deformation takes place.
The most fundamental mechanism of intracrystalline deformation is the propagation of line dislocations through the lattice structure.
This is a thin section of a deformed quartz grain in a pegmatite. It is reasonable to assume this entire view started out as one optically continuous grain and that the microstructures in it are due to deformation. Within the grain deformation bands are evident (double arrow) and at grain margins new subgrains occur (larger arrow). In addition the grains now have a ribbon texture. This suggests a prolonged deformation with a lot of dislocation glide and limited diffusion and recovery.
Photomicrograph of folded and colorful muscovite flake under crossed nicols. Note how in the hinge zones the mica grain has gone to parallel extinction, and towards the limb the birefrigence colors get stronger. This is an example of undulose extinction due to lattice reorientation. Dislocation glide along the basal cleavage of mica is particularly easy because of the low bond strengths along this lattice plane. This allows mica to deform easily and develop microfolds, and mica-rich rocks are known for being incompetent, ductilely weak. Much of the low birefrigent material in here (white to gray to black) is quartz. Note how some of the larger grains show bands and patches of slightly different shades of gray in the same crystal. This is undulose extinction as manifest in quartz, and is due to lattice misorientation from accumulated line dislocations, and is good evidence of the operation of dislocation glide. This is a deformed gneiss from the Piedmont of Georgia.
Diffusion occurs when there is ability of ions to migrate through the lattice structure. This does open a whole new series of questions.
Superplastic creep: enhanced ground boundary sliding. May occur in mantle. Aided by small grain sizes.
Many deformed rocks show evidence of recrystallization. Consider marbles with their 'sugary' texture.
Thin section photomicrograph of deformed Piedmont gneiss under crossed nicols and with gypsum plate in (producing the unusual colors). Quartz, muscovite and some feldspar occur. Here the slide is dominated by quartz. Note how many of the quartz grains show local straight ground boundaries, triple junctions, and a polygonal character (although the grains are not equant, but do show a long axis preferred orientation. This is a texture characteristic of dynamic recrystallization where both dislocation glide and climb where occurring. The larger quartz grain aggregate in the middle shows some undulose extinction, and smaller grains concentrated at its edges. This is a quartz grain that suffered less deformation and recrystallization because of an original favorable lattice orientation, or protection by the adjacent micas, or an originally larger grain size. Note as you move along the foliation from this porphyroclast of quartz the recrystallization is better and better developed, with greater ground boundary distinction and smaller grain size. One interpretation is that these are tails pulled of the original quartz grain taffy-like.
This is a thin section under crossed nicols of a feldspar 'augen' in a deformed amphibolite from the Piedmont of Georgia. The twinning indicates it is plagioclase. The surrounding material is hornblende. Note the small grains on the periphery of the larger twinned center, some of which have twins identifying them as plagioclase grains. Note also, how the twins disappear along their length. This is likely due to lattice misorientation and line dislocations and lattice glide in the feldspar grain. An explanation for this texture is that plagioclase is a relatively strong mineral, and deformation is being concentrated at its margins producing the new smaller grains by recrystallization. relatively high temperatures are required for plagioclase to deform like this.
Grain size controlled by competition between dynamic and static recrystallization, with the one decreasing and the other increasing grain size. It can be cast as a competition between strain rate and static recovery aided by temperature.
Deformation maps: These are maps indicating for a given material the type of deformation mechanisms that operate at different conditions. Typically the space looked at is temperature and deviatoric stress, with lines of constant strain rate superimposed. An important implication is that rock systems can behave discontinuously in terms of strain. At a certain temperature for example, a new deformation mechanism may contribute and the rock could soften considerably.
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