Folding and Faulting Flashcards
Fault
is a fracture in a rock along which there has been an observable amount of displacement
Throw
is the amount of vertical displacement measured vertically between the top of the same bed seen on both sides of the fault
Hanging wall
is the side that lies above the fault plane
Footwall
is the side that lies below the fault plane
Downthrow
is the side of the fault that has moved down
Upthrow
is the side of the fault that has moved up
Fault types
Dip-slip, strike-slip
Dip-slip fault
Where the movement along the fault plane is parallel to the dip of the fault plane
Normal
Hanging wall on the downthrow side
Dip-slip movement
Reverse
Hanging wall on the upthrow side
Dip-slip movement
Strike-slip fault
Where the fault plane is vertical and the movement along the fault is horizontal - parallel to the strike of the fault plane
Sinistral
Left handed movement
Dextral
Right handed movement
Features formed by faults
Graben and horsts, Slickensides, Fault breccia
Slickensides
is the polishing and striations found on a fault plane indicating the direction of relative movement
Fold
is a flexure in rocks, where there is a change in the amount of dip of a bed
Axial plane
is a plane that joins the hinges of all the beds. It bisects the fold
Plunge
is the angle of dip of the fold axis from the horizontal
Antiform
is a upward closing fold
Synform
is a downward closing fold
Symmetrical folds
Formed by equal pressure from both sides
The length of fold limbs is the same
Asymmetrical folds
Formed by more pressure on one limb than the other limb
Limb length will be different on either side of the axial plane
Overfolds
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
Recumbent folds
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
Isoclinal
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
Monoclines
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
Nappes
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
Domes
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
Basins
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
Plunging
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
