Fossen Brittle deformation

Question 1

Name and briefly describe the two mechanisms of brittle deformation

Answer 1

1. Granular flow - intergranular deformation where minerals slide against one another without being crushed. Common in unconsolidated sediment.
2. Cataclasis - intragranular deformation where friction between grains stops grain sliding causing them to break. Typical in consolidated sediment

Question 2

Where do tensile fractures develop?

Answer 2

Perpendicular to the least principle stress axis and align with the plane defined by the max and intermediate principal stress axes.

Question 3

Under what conditions are tensile fracture nucleations common?

Answer 3

Low confining pressure and low differential stress

Question 4

How many degrees off of the maximum principal stress do shear fractures form?

Answer 4

20-30 degrees

Question 5

Why are rocks generally more susceptible to shear failure under lower confining pressures than higher?

Answer 5

The higher the confining pressure the larger the differential stress the rock can sustain before it yields

Question 6

What is the difference between mode 2 and mode 3 fractures?

Answer 6

• 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

Question 7

What determines the effective stress on a rock body?

Answer 7

The difference between total stress and fluid pressure

Question 8

Why does fluid pressure reduce the total stress?

Answer 8

Fluids tend to fill pores which reduces the load at grain contact surfaces

Question 9

What does Griffith’s failure criteria assume?

Answer 9

Both shear fractures and joints initiate from growth and linkage of micro-defects in the rock.

Question 10

What shape of micro-defect is most likely to cause rock failure?

Answer 10

Ellipsoidal due to high concentration of stress around the tip.

Question 11

How do tensile fractures (joints) grow?

Answer 11

Incrementally. The growth process involves concentric growth from a nucleation point to produce elliptical or circular joint surfaces

Question 12

How do shear fractures form?

Answer 12

Growth and linkage of micro-defects such as cracks

Question 13

What is the precursory stage to the fracture surface being established?

Answer 13

Local cataclasis and gouge formation

Question 14

What are the three kinematic based deformation bands?

Answer 14

• Compaction bands
• Dilation bands
• Shear bands

Question 15

What are the three mechanism based deformation bands?

Answer 15

• DIsaggregation bands
• Phyllosilicate bands
• Cataclastic bands

Question 16

Under what circumstances do each of the different types of kinematic based deformation bands form?

Answer 16

Simple shear bands - displacement parallel to band

Compactional and dilation bands - displacement perpendicular to band

Question 17

How are compaction bands formed?

Answer 17

Sliding, rotation and crushing of grains during shear.

Question 18

Why can compaction bands be useful?

Answer 18

• Form perpendicular to shortening direction so useful strain marker.
• If not rotated can also indicate maximum compressional stress direction

Question 19

How do disaggregation bands form and how do they appear?

Answer 19

Sliding and rotation of mineral grains with little cataclasis. Presence seen by offset of sedimentary laminations.

Question 20

What two conditions favour disaggregation bands?

Answer 20

Poorly consolidated host rock and shallow burial depth with excess fluid pressure

Question 21

What do phyllosilicate bands do to the rock?

Answer 21

Decrease permeability as grains rotated to form barriers to fluid flow

Question 22

When does catalclastic deformation occur?

Answer 22

When intragranular strength is greater than the strength of the grains.

Question 23

What are the three main controls of the concentration of stress within the grains?

Answer 23

• Confining pressure
• Imposed shear stress
• Grain contact area

Question 24

What textural characteristics favour low grain contact area and high intragranular stress so that grains may fail?

Answer 24

• Good sorting
• Well rounded grains
• Coarse grain size

Question 25

How does cementation and dissolution restrict cataclastic deformation?

Answer 25

Increase contact area between the grains reducing grain contact stress.