Lecture index: Lattice imperfections. / Dislocation glide. / Diffusion. / Recrystallization and recovery. /
Image to right is of a ductiley deformed quartz-mica mylonite from the Kjeiserhjelmen core complex detachment in the NW part of Spitsbergen, Norway. To finish off the discussion on ductile deformation and deformational fabric development it will be useful to consider further deformation mechanisms at smaller scales.
Cataclastic flow occurs within granular material such as sand. This is obviously not deformation at a lattice scale, but at the thin section and even larger scale. However, it provides a transition to thinking about mechanisms at an even smaller scale, and also includes an important suite of 'ductile' or distributed slip deformation mechanisms.
Examples of where it can occur:
Soft sediment faults in Triassic strata of Edgeøya. Note how they 'disappear' into the massive sand.
Photomicrograph mosaic under plain light of small soft sediment faults in fine sands with shale laminations from the sample above. Instead of a distinct slip surface or brecciation the fault is defined by a zone of cataclastic flow with grain sliding and reorientation.
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 evident here are due to deformation. Within the grain deformation bands (distinctly different type that deformation bands seen in porous rocks such as sandstone at a thin section and hand specimen scale) 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. The preservation of undulose extinction in the quartz suggests that the last bit of ductile deformation occurred at lower temperatures, where dynamic recrystallization (see below for further explanation) did not take place.
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.
Many deformed rocks show evidence of recrystallization. Consider marbles with their 'sugary' texture.
Photomicrograph mosaic of dunite from North Carolina showing the fairly well developed polygonal texture due to recrystallization with strain free grain interiors, a texture typical of where recovery processes are efficient. Note that there is an elongation of the grains running diagonally (from upper left to lower right), suggesting that this was dynamic recovery occurring during deformation.
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, temperatures consistent with amphibolite facies metamorphism.
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 (introducing new dislocations) and recovery that collects the dislocations into distinct grain boundaries. For constant conditions a resulting equilibrium grain size is a function of the strain rate, which in turn is driven by the deviatoric stress (as captured in flow laws). Thus in some situations grain size can be used to estimate the driving stress (Stipp, M and Tullis, J., 2003, The recrystallized grain size piezometer for quartz; Journal of Geophysical Research 30: doi:10:1029/2003GL018444).
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.
Examples of deformation maps:
More information: 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.
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