2.20 Faults

Question 1

What is a fault?

Answer 1

a fracture in a rock along which there has been an observable amount of displacement

Question 2

What is a fault plane?

Answer 2

a plane of fracture along which the rocks have been displaced
- the are not always simple fractures, there are often a relatively narrow zone of shattering along the fault
- the fault may be brecciated or may contain finely ground rock, called fault gouge
- the area of the fault zone may become a route for mineralising fluids or water

Question 3

What is meant by the term throw in relation to a fault?

Answer 3

the vertical distance between the top of the same bed seen on both sides of the fault

Question 4

What is meant by the upthrow side of a fault?

Answer 4

the side of the fault where the movement is upwards in relation to the other side

Question 5

What is meant by the downthrow side of a fault?

Answer 5

the side of the fault where the movement is downward, in relation to the other side

Question 6

What is meant by the footwall of the fault?

Answer 6

the side of the fault that lies below the fault plane, if the fault is not vertical

Question 7

What is meant by the hanging wall of the fault?

Answer 7

the side of the fault that lies above the fault plane, if the fault is not vertical

Question 8

What is a normal fault?

Answer 8

• a fault where the hanging wall is the downthrow side
• a fault caused by extension of the crust (tensional forces)
• there is a gap created in the formerly continuous beds or rock as the downthrow side of the fault sinks under the influence of gravity
• the principal stress direction is vertical due to the weight of overlying rocks, with the minimum stress being horizontal tensional forces

Question 9

What is a graben?

Answer 9

where two normal faults face each other (dip towards each other) a graben or rift valley is formed

Question 10

What is a horst?

Answer 10

where two normal faults face away from each other (dip away from each other) an elevated block forms called a horst where the areas either side have dropped downwards due to crustal extension

Question 11

What is a reverse fault?

Answer 11

• a fault where the hanging wall is the upthrow side
• a fault caused by shortening of the crust (compressional forces)
• there is an overlap of the strata
• the principal stress direction is horizontal with the minimum stress being vertical, which suggests that these faults form relatively close to the Earth’s surface where there is less overlying rock

Question 12

What are thrust faults?

Answer 12

• thrust faults are a type of reverse fault, where the fault dip is less than 45 degrees but typically of a low angle between 10 and 40 degrees
• they are formed by compressional forces
• they are often associated with major fold mountain systems and have displacements which can be measured in kilometres

Question 13

What is a strike-slip fault?

Answer 13

a fault where the fault plane is vertical or near vertical and the movement along the fault is horizontal (parallel to the strike of the fault plane

Question 14

What is a strike-slip fault?

Answer 14

a fault where the fault plane is vertical or near vertical and the movement along the fault is horizontal (parallel to the strike of the fault plane

Question 15

What is a tear fault?

Answer 15

• the result of shearing forces applied to rocks
• ## geological features such as batholiths are displaced horizontally

Question 16

What is the name for a fault where the displacement of features is to the right?

Answer 16

dextral tear fault

Question 17

What is the name for a fault where the displacement of features is to the left?

Answer 17

sinistral tear fault

Question 18

What is a transform fault?

Answer 18

• associated with plate margins and may be described as a conservative plate margin
• they are common at right angles to a mid-ocean ridge
• they can be the result of different rates of movement within a plate and allow the rigid plates to adjust for these differences in the rate of movement

Question 19

What is a transform fault?

Answer 19

• associated with plate margins and may be described as a conservative plate margin
• they are common at right angles to a mid-ocean ridge
• they can be the result of different rates of movement within a plate and allow the rigid plates to adjust for these differences in the rate of movement

Question 20

What is a slickenside?

Answer 20

scratches found on fault planes - they are left by the grinding of pieces of rock along the fault plane as the fault moves
- they are best seen where the fault plane is coated in a mineral precipitated from a fluid that moved along the fault plane
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Question 21

What is a slickenside?

Answer 21

scratches found on fault planes - they are left by the grinding of pieces of rock along the fault plane as the fault moves
- they are best seen where the fault plane is coated in a mineral precipitated from a fluid that moved along the fault plane
-

Question 22

What is fault breccia?

Answer 22

• they are found along fault planes and consist of fragments broken from the rocks on either side of the fault plane
• the fragments are large and angular and made of hard, competent rock
• they may be cemented by minerals precipitated in the fault zone at a later stage

Question 23

What is fault gouge?

Answer 23

in some faults, the grinding of rock along the fault plane can be extreme, producing fault gouge consisting of very fine particles of rock

Question 24

What is mylonite?

Answer 24

a rock produced by dynamic recrystallisation of minerals (fault gouge) on a fault plane

Question 25

What is stress transfer?

Answer 25

• when a section of crust moves during an earthquake, stress is released
• for large earthquakes, the stress may not be totally used up in the displacement of the crust
• the remaining stress may be transferred to another section of the fault triggering other earthquakes

Question 26

What is seismic gap theory?

Answer 26

‘locked’ areas of a fault where no earthquakes have occurred to release the stress can monitored and the probability of an earthquake assessed