Folding and Faulting

Question 1

Fault

Answer 1

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

Question 2

Throw

Answer 2

is the amount of vertical displacement measured vertically between the top of the same bed seen on both sides of the fault

Question 3

Hanging wall

Answer 3

is the side that lies above the fault plane

Question 4

Footwall

Answer 4

is the side that lies below the fault plane

Question 5

Downthrow

Answer 5

is the side of the fault that has moved down

Question 6

Upthrow

Answer 6

is the side of the fault that has moved up

Question 7

Fault types

Answer 7

Dip-slip, strike-slip

Question 8

Dip-slip fault

Answer 8

Where the movement along the fault plane is parallel to the dip of the fault plane

Question 9

Normal

Answer 9

Hanging wall on the downthrow side
Dip-slip movement

Question 10

Reverse

Answer 10

Hanging wall on the upthrow side
Dip-slip movement

Question 11

Strike-slip fault

Answer 11

Where the fault plane is vertical and the movement along the fault is horizontal - parallel to the strike of the fault plane

Question 12

Sinistral

Answer 12

Left handed movement

Question 13

Dextral

Answer 13

Right handed movement

Question 14

Features formed by faults

Answer 14

Graben and horsts, Slickensides, Fault breccia

Question 15

Slickensides

Answer 15

is the polishing and striations found on a fault plane indicating the direction of relative movement

Question 16

Fold

Answer 16

is a flexure in rocks, where there is a change in the amount of dip of a bed

Question 17

Axial plane

Answer 17

is a plane that joins the hinges of all the beds. It bisects the fold

Question 18

Plunge

Answer 18

is the angle of dip of the fold axis from the horizontal

Question 19

Antiform

Answer 19

is a upward closing fold

Question 20

Synform

Answer 20

is a downward closing fold

Question 21

Symmetrical folds

Answer 21

Formed by equal pressure from both sides
The length of fold limbs is the same

Question 22

Asymmetrical folds

Answer 22

Formed by more pressure on one limb than the other limb
Limb length will be different on either side of the axial plane

Question 23

Overfolds

Answer 23

Formed by a lot more pressure one one side than the other side
Both limbs dip in the same direction but at different angles
The axial plane is inclined from the vertical
One limb of the fold is inverted

Question 24

Recumbent folds

Answer 24

Formed by very high pressures from one side
The axial plane must be sub horizontal less than 30 degrees
Both the limbs and the axial plane will be at very low angles
One limb of the recumbent fold is inverted
The rocks are usually incompetent or plastic

Question 25

Isoclinal

Answer 25

Formed by large amounts of pressure equally from both sides
They can be recognised by parallel limbs at angles of over 70 degrees forming closed and tight folds
Axial plane traces are usually vertical
They can be recognised on maps by outcrops of repeated and parallel beds

Question 26

Monoclines

Answer 26

Formed by pressure from only one side
They are folds with only one limb so they are step shaped folds
They have two areas of nearly horizontal strata separating an area of very steeply dipping strata

Question 27

Nappes

Answer 27

These are huge recumbent folds that have broken along thrust planes
Movement is horizontal along thrust planes which are the lines along which a recumbent fold breaks

Question 28

Domes

Answer 28

A dome is an anticlinal fold shaped like an upturned bowl
The beds dip outwards all directions
The oldest rocks are found in the centre
Domes have roughly circular outcrop patterns

Question 29

Basins

Answer 29

A basin is a synclinal fold shaped like a saucer
The beds dip inwards in all directions
The youngest rocks are found in the centre
Basins have roughly circular outcrop patterns

Question 30

Plunging

Answer 30

Where the axes of the folds are tilted so they dip away from the horizontal or plunge
The angle of plunge is the angle between the axis and the horizontal plane
A plunging fold will have a V shaped outcrop pattern
It will have three angles of dip