Lecture 3 - Faulting

Question 1

What is the difference between extension and shear faults?

Answer 1

• . extension fractures - joints that open but have no displacement along plane
• . shear fractures - faults that have a displacement along the fracture plane

Question 2

Hexagonal symmetry is often seen in which kind of cracks?

Answer 2

Shrinkage cracks

Question 3

What can cause shrinkage cracks to occur?

Answer 3

The shrinkage process may be induced by the loss of water through evaporation or by a shrinkage process due to thermal contraction during cooling

Question 4

How can curved joints be formed?

Answer 4

Curved joints can form when a stressed rock is brought to the Earth‘s surface. They may be oriented parallel to the topography, typical examples are large granites.

Question 5

How do joints form in relation to bedding?

Answer 5

Perpindicular

Question 6

How do joint sets form?

Answer 6

Assume that 1 is vertical and that 2 and 3 are relatively small. High fluid pressure may result in a negative effective stress in the 3 direction - first joint set forms. Once joints are present the stress in the 3 direction becomes zero. This can now lead to a switch of 3 and 2, and new joints form perpendicular to the new 3 direction.

Question 7

The Anderson Principle explains the three main fault types (normal, strike-slip and reverse). What is this principle?

Answer 7

The surface of the Earth is “open“, it cannot sustain shear stresses. Therefore one of the three principle stresses has to be perpendicular to the Earth‘s surface. The three possible stress orientations lead to the three main fault types. -> normal faults have steep angles, reverse faults flat angles and strike slip faults are vertical.

Question 8

What scenario of faulting would bring about high grade rocks on low grade rocks?

Answer 8

Large displacement

Question 9

Describe the role of the piercing point on a linear structure

Answer 9

Allows the real displacement point of a fault

Question 10

What is the difference between a releasing bend and a restraining bend?

Answer 10

a releasing bend (quite common)
fills with vein material - slicken fibres
a restraining bend (in carbonates)
develops a rough surface - slickolites

Question 11

What are slickolites?

Answer 11

Slickolites are like stylolites, the rock dissolves locally and is roughening. The rough “teeth” show the movement direction.

Question 12

What is a pseudotachylite?

Answer 12

Pseudotachylites are melt veins that develop in brittle faults during fast fault slip - locally the temperature is high enough to melt the rock