Brittle Faults and Mylonites

Question 1

How do brittle fault rocks form?

Answer 1

Faults propagation through intact rock commonly along older plane of weakness

Question 2

How does fluid infiltration effect the mechanical behaviours of brittle fault rocks?

Answer 2

• An increase in fluid pressure decreases the strength of the fault by decreasing the effective normal stress over the fault.
• Fluids may also cause weakening by stress corrosion or weakening by reaction of of stronger phases to weaker minerals
• Fluids can cause fault rock strengthening by precipitation of vein material such as quart cementing the rock (Wintch, 1998)

Question 3

Under what conditions are incohesive brittle fault rocks formed?

Answer 3

Usually found in faults which have been active at shallow crustal levels.

Question 4

What three forms do incohesive brittle fault rocks take and how are they distinguished between?

Answer 4

Fault breccia - Over 30% angular rock fragments
Fault cataclasite - Less than 30% rock fragments
Fault gouge - Few large fragments isolated in matrix.

Question 5

What are deformation bands?

Answer 5

mm-wide planar shear zones in undeformed, porous quartz rich, clay poor sedimentary rocks

Question 6

Under what four conditions do deformation bands form?

Answer 6

Either in:
- High porosity rocks
OR
- High differential stress
- High mean stress 
- High strain

Question 7

Why are deformation bands associated with a change in porosity economically important?

Answer 7

Influence rock permeability and the shape of water and hydrocarbon reservoirs in rocks

Question 8

What are the three sub-divisions of cohesive fault rocks?

Answer 8

• Cohesive breccia
• Cohesive cataclasite
• Pseudotachylyte

Question 9

What causes the cohesive nature of the cohesive fault rocks?

Answer 9

Precipitation crystallisation of minerals such as quartz, calcite and epidote from a fluid

Question 10

Why is it easier to identify incohesive cataclasite than cohesive cataclasite in quartzite?

Answer 10

Incohesive cataclasite weathers very contrastingly to quartzite. Cohesive quartzite may only be differentiated from the quartzite by a slightly darker colour

Question 11

Where do cohesive cataclasite and breccia form in relation to the incohesive form?

Answer 11

Greater crustal depth

Question 12

Name three common deformation mechanisms in cohesive cataclasite

Answer 12

• Cataclastic flow in and between grains
• Grain boundary sliding
• Pressure solution.

Question 13

What is a pseudotachylyte?

Answer 13

A cohesive glassy very fine grained fault rock with a very distinct fabric

Question 14

How do pseudotachylytes form?

Answer 14

Melt veins that develop in brittle faults during fast fault slip. Locally temperature is high enough to melt the rock.

Question 15

What are psudotachyltyes thought to indicate?

Answer 15

Past earthquakes

Question 16

In what kind of rock do pseudotacylytes form?

Answer 16

Dry and competent rocks

Question 17

What is a mylonite?

Answer 17

A foliated rock that shows evidence for strong ductile deformation. It is strictly a structural term that refers only to the fabric of the rock.

Question 18

Is the fabric change between the mylonite zone and the unaffected rock sharp or gradual?

Answer 18

Gradual

Question 19

The displacement on brittle faults can be measured from slicken lines and slicken fibres. How do each form?

Answer 19

• Slicken lines: the surface is scratched which leads to striations parallel to fault movement
• Slicken fibres: new crystals grow in the direction of movement

Question 20

What are slickolites and how can you identify movement direction from them? How does it vary from slicken fibres?

Answer 20

• They form in a restraining bend in carbonates and develop a rough surface. The rough teeth show the direction of movement.
• In slicken fibres a releasing bend forms which fills with vein material. Movement direction is the opposite of the slickolites.

Question 21

What are the common structures that can be found in brittle rocks and what are they?

Answer 21

Y-shears - parallel to the main shear direction
S - is a foliation that develops in the fault rock
R - Riedel shear with small angle to main fault plane with the same shear sense
R’ - Riedel shear orientated at 90 degrees to main plane, small but opposite shear sense than the main fault
P surfaces - small shears that are aligned parallel to the foliation
T - small fractures opening as tensile cracks, these may be filled with minerals and become veins

Question 22

What steps do you need to take to determine the shear sense of a shear zone?

Answer 22

1. Find the foliation - the main foliation is parallel to the shear zone boundary
2. Find a stretching lineation - stretched crystals, veins etc. The stretching lineation will lie in the foliation plane
3. Look at sections perpendicular to foliation and parallel to stretching lineation. These will show the right shear sense.

Question 23

What are C-type shear bands?

Answer 23

Shear band parallel to the main shear direction of the shear direction of the shear zone and the foliation developed during shearing. Formed in the early phase deformation.

Question 24

What are C’-type shear bands?

Answer 24

Form later. The foliation is rotated and is more or less parallel to shear zone boundary. C’ shear bands form with an angle to the shear zone boundary and are extensional.