Microstructures due to intracrystalline deformation
- Typical of a deformed sandstone at « low T » : around 450°C … the grains are not recrystallized
- Typical of a deformed quartzite at High T » : more than 500°C … pervasive recrystallization
II. Diffusional processes
1. Diffusion along surfaces : pressure solution
Pressure solution is the second most important mechanism of deformation in rocks: it looks like simple but its analysis is complex (Figure). It is the main mechanism responsible for the formation of schistosity in rocks from the upper crust (where water is abundant).
If a fluid is in contact with the grains of a rock, even under lithostatic (macroscopic) pressure, the (local) differential stress acting on the grains with be responsible for some dissolution of the grains (if their solubility in non zero). If the solution is mobile (needs a fluid pressure gradient, hence some permeability) it will be transported in another place. Eventually the soluble fraction will precipitate at places where the solid fraction is no more dissolved (due to a pressure drop, generally, but other factors may be active). This pressure solution mechanism is also called : dissolution-precipitation or dissolution-recrys-tallization.
Displacement of atoms from one place to another is a deformation. Such a mechanism is composed of several sub-mechanisms (dissolution, transport and precipitation), themselves dependent on several factors (pressure, temperature, composition of the solid fraction, composition of the fluid, solubility degree, itself related to the fluid flux, distance of transportation, in relation with grain size etc…). Therefore, it cannot be analysed with the same simplicity as for intracrystalline plasticity. The flow law has to integrate all these parameters:
e’ = k f (water/rock ratio, solubility, concentration, stress, grain size ) x (distance of transportation - from grain-size distance to karstic conduits-) …
Discuss some of these parameters : grain-size, concentration of the solution, stress magnitude, temperature …
In rocks, such a mechanism is a rather low-T mechanism, a very slow mechanism, and needs the presence of a fluid (water in general). In metallurgy this mechanism is called Cobble-creep.
Geological examples of structures due to pressure solution : schistosity, styloliths, fibers along shadow zones, tension gashes …
Figure. Dissolution of an organism in a limestone. Maximum dissolution zones are underlined by insoluble (opaque) minerals ; mineral precipitation zones are lighter in colour (calcite principally). Total reduction in height : about 25%.
Figure. Striated surface of a fault-plane attesting to a mechanism of pressure solution that took place during the asismic displacement along the plane. What is the sense of movement along the fault-plane ?
2. Diffusion in volume: Nabarro-Herring creep
This deformation mechanism is effective close to the melting temperature of materials. It is a low-stress- very HT mechanism.
II. (T, s, e’) deformation maps : for olivine
Figure. Deformation map (T, s) of olivine. Inside : strain-rate curves labelled in s-1. Tf : melting temperature. m : shear modulus.
for quartz (pressure solution domain : T < 650°C)
and calcite (note the differences in temperature with respect to quartz and olivine : pressure solution domain : T< 450°C)