Download Microscale - Structural Geology - Lecture Notes and more Study notes Geology in PDF only on Docsity!
Microscale Deformation
Ch. 9, p. 189-Ââ192; Ch. 10, p. 203-Ââ
1. Kinematic Indicators: Even if it is not possible to determine the true slip, the sense of motion can often be determined from kinematic indicators along the fault surface. Along an irregular fault surface, restraining bends may show stylolites (pressure solutions) whereas releasing bends show voids or mineral growth.
[Fig. 9.1. Mineral growth in releasing bends and pressure solution in restraining bends shows the kinematics of motion in the plane of the cross section]
2. Kinematic Indicators: On the fault surface, pressure solution stylolites may align along the slip direction, forming slickolites.
[Fig. 9.2. Slickolites indicate pure dip slip motion along this fault]
3. Kinematic Indicators: Frictionally polished fault surfaces (slickensides) may contain scratches called slickenlines or grooves that are parallel to the slip vector.
[Fig. 9.0. Slickenlines along a polished slickenside] [Figure. Slickenlines along a polished slickenside]
4. Kinematic Indicators: Hard parts (asperities) in the fault wall rocks may carve striations into the fault surface oriented parallel to the slip direction called gouge marks. These may form crescent shaped depressions at the start of the groove, and ridges in front of the asperity, creating ridge-Ââin-Ââgroove lineations.
[Fig. 9.4. Striations along a fault surface]
5. Kinematic Indicators: Motion along a fault may cause small fractures to develop in the adjacent wall rock called secondary fractures or subsidiary fractures. Based on their angles and motion sense, they form unique kinematic indicators that can be very useful for interpreting fault motions.
The fracture types include:
T-Ââfractures: oriented at 30°-Ââ50° to the fault plane (cf., pinnate fractures). Riedel shears: these include R (synthetic) and Râ (antithetic) conjugate Riedel fractures. P-Ââshears: antithetic shears that dip opposite to the fault at low angles.
[Fig. 9.3. Types of secondary fractures that provide evidence of fault kinematics]
6. Microdeformation: The type of deformation occurring in rock depends on the rheology, which in turn is influenced by temperature and stress conditions.
The deformation processes at work are sometimes best observed at the microscopic scale and are called microstructures.
Intercrystalline deformation (e.g., grain boundary sliding and fracturing) is common in the brittle regime.
Intracrystalline deformation occurs within grains and is common for plastic deformation. Atomic scale features may only be seen with electron microscopy.
[Fig. 10.0. Microscopic view of deformation in a quartzofeldspathic rock]
7. Brittle vs. Plastic Deformation Mechanisms: The transition from brittle to plastic microdeformation mechanisms depends on the rock type and PT conditions (pressure/temperature). Most rocks contain multiple minerals that may transition from brittle to plastic at different times (e.g., quartz and feldspar), resulting in both processes occurring in the same rock.
[Table 10.1. Microdeformation mechanisms in the brittle and plastic regimes]
8. Brittle Deformation Mechanisms: At shallow depths, brittle deformation mechanisms include granular flow and cataclastic flow.
Processes include grain boundary sliding and grain rotation, grain flaking, and transgranular fracturing (i.e., intergranular).
Cataclastic flow will reduce grain sizes and reduce porosity, producing breccia, cataclasite, or gouge.
[Fig. 10.1. Brittle microdeformation mechanisms]
9. Plastic Deformation: Mechanical Twinning: Stresses may result in mechanical kinking within the crystal lattice, producing mechanical twinning or deformation twins, particularly calcite and plagioclase. No fracturing occurs, just restructuring of the crystal lattice. In calcite, this may occur via twin gliding, where shearing occurs across a twin plane, producing shear strain at ~45° to Ď 1.
[Fig. 10.2. Mechanical twins in calcite] [Fig. 10.3. Simple shear in a calcite crystal produces mirror image twin planes]
10. Plastic Deformation: Diffusion Creep: Plastic deformation may occur through the migration of crystal defects such as point defects and planar defects called dislocations. The migration of point defects called vacancies forms the basis of diffusion (diffusion creep).
[Fig. 10.5. Types of point defects in a crystal lattice] [Fig. 10.6. The migration of vacancies through the crystal lattice constitutes diffusion]
11. Plastic Deformation: Diffusion Creep: Diffusion through a crystal is called volume diffusion or Nabarro-ÂâHerring creep. Vacancies move towards points of high stress in the crystal before disappearing at crystal boundaries, resulting in a permanently strained crystal shape. The rate is slow and temperature-Ââdependent, so is typical of lower crust and mantle conditions.
In the shallow crust, vacancies may migrate along grain boundaries (grain boundary diffusion or Coble creep).
Plastic deformation does not involve fracture at the scale of observation but occurs through microscopic processes like dislocation glide and diffusion.
The strain will increase without bounds for a constant applied yield stress for a perfectly plastic material (Saint Venant material).
[Fig. 6.2. Plasticity demonstrated as (g) a mechanical analog (a frictional block with some yield strength); (h) on a stress vs. strain rate graph; and (i) on a strain history curve]
19. Plastic Deformation: Although constitutive laws exist for plastic materials, the equations vary depending on the microdeformation mechanisms. The general flow law (relating stress and strain rate) is:
Ä = AĎdn^ exp (-ÂâQ/RT)
where A and n are constants, R is the gas constant, T is temperature, and Q is the activation energy. The process of dislocation creep is very dependent on the differential stress, Ďd , which provides the energy to allow dislocations to move. The power-Ââlaw creep constant n is ~3-Ââ5. At very high T (or for small grain size), n=1 (Newtonian viscous deformation).
[Fig. 6.9. Elastic-Ââplastic deformation histories. (a) Recoverable elastic strain. (b) Permanent strain after brittle fracture]
20. Microstructures Caused by Dislocation Creep: Although occurring at the atomic scale, dislocation creep creates microstructures visible in thin sections.
Misaligned crystal lattices may form the pile-Ââup locations of numerous dislocations, forming a dislocation wall.
[Fig. 10.10. Dislocation pile-Ââups visible through TEM imaging] [Fig. 10.13. Dislocation wall, where two sides of a crystal lattice are in different orientations]
21. Dislocation Walls in Thin Section: Dislocation walls result in undulose extinction because the variability in crystallographic orientation creates different extinction angles.
[Fig. 10.13. Dislocation wall, where two sides of a crystal lattice are in different orientations] [Fig. 10.14. Undulose extinction caused by dislocation walls within quartz crystals]
22. Recovery: As dislocations move, it may result in small polygonal patches of similar extinction angle in the grain, called subgrain formation. This process reduces the energy state of the grain and so is called recovery.
[Fig. 10.15. Formation of subgrains around a larger quartz grain surrounded by micas] [Fig. 10.16. Recovery by subgrain formation in quartz]
23. Recrystallization: The gradual removal of dislocations results in strain-Ââfree grains with no undulose extinction (i.e., recrystallization). If this occurs during deformation, it is dynamic recrystallization and causes preferred grain orientations. Static recrystallization occurs after deformation, producing equant grains with straight boundaries (also called annealing).
