Page 37 - Microtectonics

3

Deformation Mechanisms

3.1 hapter 3 deals with deformation structures on the scale of individual grains.
Introduction CGrain scale brittle deformation and cataclastic flow occur in the upper crust
3.2 or at high strain rate. At deeper crustal levels, rocks deform by ductile flow through
Brittle Fracturing – a range of mechanisms of ductile grain scale deformation such as dissolution-
Cataclasis precipitation, intracrystalline deformation by dislocation glide and creep, diffu-
sion creep, twinning and kinking. Ductile deformation in rocks could not lead to
3.3 high strain if it was not accompanied by mechanisms that reduce the damage
Dissolution-Precipitation
imposed during the deformation process. There are two main groups of such
3.4 mechanisms; recovery, which removes dislocations inside the crystal lattice, and
Intracrystalline recrystallisation that operates by migration of grain boundaries. Three main types
Deformation of dynamic recrystallisation are treated; subgrain rotation, bulging and high tem-
3.5 perature grain boundary migration. After deformation slows down or stops, grain
Twinning and Kinking boundary migration can continue by grain boundary area reduction, and so-called
foam textures can develop. In absence of deformation this process is know as
3.6 static recrystallisation.
Recovery
The second part of this chapter discusses grain scale deformation processes
3.7 for a number of rock-forming minerals. This is necessarily a short description of
Recrystallisation what is presently known, with a large number of references for further reading.
3.8 Treated are quartz, calcite, dolomite, feldspars, micas, olivine, pyroxenes, garnet
Solid-State Diffusion Creep, and amphibole. Finally, a short outline is given of the deformation of polyminer-
Granular Flow and alic rocks with quartz-feldspar aggregates as an example. The final section of this
Superplasticity chapter treats flow laws and deformation mechanism maps.
3.9
Competing Processes
During Deformation
3.10
Grain Boundary Area
Reduction (GBAR)
3.11
Static Recrystallisation
3.12
Deformation of Some
Rock-Forming Minerals
3.13
Deformation of
Polymineralic Rocks
3.14
Flow Laws and Deformation
Mechanism Maps