Fossen Brittle deformation Flashcards
Name and briefly describe the two mechanisms of brittle deformation
- Granular flow - intergranular deformation where minerals slide against one another without being crushed. Common in unconsolidated sediment.
- Cataclasis - intragranular deformation where friction between grains stops grain sliding causing them to break. Typical in consolidated sediment
Where do tensile fractures develop?
Perpendicular to the least principle stress axis and align with the plane defined by the max and intermediate principal stress axes.
Under what conditions are tensile fracture nucleations common?
Low confining pressure and low differential stress
How many degrees off of the maximum principal stress do shear fractures form?
20-30 degrees
Why are rocks generally more susceptible to shear failure under lower confining pressures than higher?
The higher the confining pressure the larger the differential stress the rock can sustain before it yields
What is the difference between mode 2 and mode 3 fractures?
- Mode 2: Form by oppositely directed translation of the side of the walls parallel to the fracture surface
- Mode 3: involve both translation and rotation of the slide in a scissor like fashion
What determines the effective stress on a rock body?
The difference between total stress and fluid pressure
Why does fluid pressure reduce the total stress?
Fluids tend to fill pores which reduces the load at grain contact surfaces
What does Griffith’s failure criteria assume?
Both shear fractures and joints initiate from growth and linkage of micro-defects in the rock.
What shape of micro-defect is most likely to cause rock failure?
Ellipsoidal due to high concentration of stress around the tip.
How do tensile fractures (joints) grow?
Incrementally. The growth process involves concentric growth from a nucleation point to produce elliptical or circular joint surfaces
How do shear fractures form?
Growth and linkage of micro-defects such as cracks
What is the precursory stage to the fracture surface being established?
Local cataclasis and gouge formation
What are the three kinematic based deformation bands?
- Compaction bands
- Dilation bands
- Shear bands
What are the three mechanism based deformation bands?
- DIsaggregation bands
- Phyllosilicate bands
- Cataclastic bands
Under what circumstances do each of the different types of kinematic based deformation bands form?
Simple shear bands - displacement parallel to band
Compactional and dilation bands - displacement perpendicular to band
How are compaction bands formed?
Sliding, rotation and crushing of grains during shear.
Why can compaction bands be useful?
- Form perpendicular to shortening direction so useful strain marker.
- If not rotated can also indicate maximum compressional stress direction
How do disaggregation bands form and how do they appear?
Sliding and rotation of mineral grains with little cataclasis. Presence seen by offset of sedimentary laminations.
What two conditions favour disaggregation bands?
Poorly consolidated host rock and shallow burial depth with excess fluid pressure
What do phyllosilicate bands do to the rock?
Decrease permeability as grains rotated to form barriers to fluid flow
When does catalclastic deformation occur?
When intragranular strength is greater than the strength of the grains.
What are the three main controls of the concentration of stress within the grains?
- Confining pressure
- Imposed shear stress
- Grain contact area
What textural characteristics favour low grain contact area and high intragranular stress so that grains may fail?
- Good sorting
- Well rounded grains
- Coarse grain size
How does cementation and dissolution restrict cataclastic deformation?
Increase contact area between the grains reducing grain contact stress.
