Faults And Folds Flashcards
What is a fault?
A fracture in a rock along which there has been an observable amount of displacement
What is a fault plane?
A plane of fracture, along which the rocks have been displaced
What is an upthrow?
The side of the fault where the movement is upwards, in relation to the other side
What is the downthrow?
The side of the fault where the movement is downward, in relation to the other side
What is meant by ‘throw’?
The vertical displacement of rocks along the fault plane
What is the footwall?
The side of the fault that lies below the fault plane
What is the hanging wall?
The side of the fault that lies above the fault plane
How do we identify the hanging wall on a diagram/drawing?
Place a finger on the fault and move upwards
What is a joint?
A fracture in a rock with no observable displacement
What are the 4 dip-slip faults?
Normal, reverse, thrust & Graben and horst
What is a dip-slip fault?
Movement along fault plane is parallel to direction of dip along fault plane
What is a normal fault?
Results from tension (extension).
Hanging wall is on the downthrow of the fault.
Causes crustal extension
What is a reverse fault?
Caused by compressional forces.
Hanging wall on the upthrow.
Causes crustal shortening
What is a graben and horst fault?
Graben = Low point
Horst = high point.
Where 2 normal faults dip towards each other.
What is a thrust fault?
A reverse fault with a very shallow dip angle (10-40°).
Often associated with fold mountains.
Displacement can be km’s or 100s of km’s
What are slickenslides?
Striations (lines) and polishing found on a fault place, indicating the direction of relative movement
What is fault breccia?
Composed of fragments produced by rocks fracturing during faulting. It is found along the fault planes
What is fault gauge?
Composed of very finely ground particles, produced by grinding of rock during faulting. These are often fused together due to frictional heat and found along fault planes
What is mylonite?
A rock produced by dynamic recrystallisation (metamorphic) of minerals on a fault plane
What are the two strike-slip faults?
Transform and tear faults
What are strike-slip faults?
Usually shown as map view.
Occurs with shear forces (opposing forces).
What are transform faults (strike-slip)?
Always associated with divergent plate boundaries. A divergent boundary cannot be one continuous stretch, transform connect the divergence
What are tear faults?
Do not have to be associated with plate boundaries. The result of shear forces
What is an example of a transform fault?
San Andreas
What is an example of a tear fault?
Great Glen fault (Scotland)
What types of rock allow for faulting?
Cold, brittle, competent rocks
What types of rock allow for folding?
Incompetent rock
What forces cause folding?
Always compressional
What is an anticline fold?
An upright fold with the oldest rock at the core.
What is a syncline fold?
A downward pointing fold with the youngest rocks at the core
What is a symmetrical fold?
Can be synclines or anticlines.
Fold limbs dip at the same angle on both sides
What is an asymmetrical fold?
Can be synclines or anticlines.
Fold limbs dip at a different angle on both sides
What is a tight fold?
A fold with a <30° angle
What is a closed fold?
A fold with 30-70° angle
What is an open fold?
A fold with 70-120° angle
What is a gentle fold?
A fold with 120-180° angle
What is a fold limb?
The section of a fold between one hinge and the next, forming the side of the fold
What is a fold hinge?
The line along which there is a change in the amount and/or direction of dip forming the most sharply curved part of the fold
What is a fold crest?
The highest point of the folded bed
What is the fold trough?
The lowest point of the folded bed
What is the axial plane?
A plane that joins the hinges of all the beds. It bisects the fold.
What is the axial plane trace?
The outcrop of the axial plane at the Earth’s surface
What is plunge?
The angle of dip of the axial plane from the horizontal plane
What is a dome fold?
An anticline which dips away from the centre in all directions. With the oldest rock in the centre
What is a basin fold?
A syncline which dips towards the centre in all directions. With the youngest rocks in the centre
What is an overfold?
Folds which have both fold limbs dipping in the same direction but by different amounts
What is a recumbent fold?
Folds have axial planes and fold limbs which are close to horizontal and always less than 30°
What is a nappe?
Recumbent folds that are broken along thrust planes.
What is an isoclinal fold?
Folds have parallel limbs that are nearly vertical and in very tight folds
If given a 6 marker on folds, what should you write about?
Anticlines & synclines - both symmetrical and asymmetrical.
Domes & basins.
Isoclinal & recumbent (&nappes)
Perhaps overfolds
What is a plunging fold?
A fold where the hinge is dipped.
How can plunge be measured?
The dip angle from the horizontal
How can a plunging fold be identified on a map?
If the beds loop/join.
On a plunging anticline, which direction is it plunging relative to the map?
Towards the ‘loop’
On a plunging syncline, which direction is it plunging relative to the map?
Away from the ‘loop’
What is stress?
The force applied to rocks
What is strain?
A change in shape or volume of a body as a result of applied stress
What is competent rock?
Strong and brittle and tend to joint and fault
What is incompetent rock?
Weak and plastic and tend to fold and develop cleavage
What is tension?
The force trying to pull rocks apart. Causes crustal extension
What is compression?
The force trying to push rocks together. Causes crustal shortening
What is shear?
Forces which act along a plane in the rock and promote sliding along the plane
What are the three main factors affecting stress and strain in rocks?
Temperature, strength and time
How does temperature affect stress and strain in rocks?
The higher the temperature, the more plastic the rocks will be.
At high temps, rocks will fold, not fault. Cold rocks fault
How does the rock strength affect stress and strain?
Strength increases with confining pressure at depth. Therefore, rocks at depth are more difficult to deform
How does the time affect stress and strain?
If pressure is applied for short period of time, rocks may fault more.
If pressure is applied over longer periods of time, more likely to act as plastic deformation (fold)
How can deformed fossils be used to measure strain?
Strain = difference of line length / original length of line.
Line is the bilat symmetry
As well as fossils, what else can be used to measure strain from deformation?
Deformation of ooliths
How can oolites be used to measure strain?
They are spherical, if deformed, they are not spherical.
More deformation = more strain
How do oolites form?
A small grain or fragment which rolls in calcium carbonate mud on the ocean floor
What are 4 types of joints?
Tension, cross, cooling, unloading
What are the two types of tectonic joints?
Tension and cross joints
What are tension joints?
Extensional forces over the crest or trough of a fold. (stretches so cracks).
Parallel to the axial plane trace of the fold
What are cooling joints?
Columnar jointing.
As it cools, it contracts.
Joints perpendicular to cooling surfaces (often vertical)
What are unloading joints?
When pressure/mass is released (e.g. glacial retreat), rock uplifts (isostacy). This causes stretching in direction of release.
Roughly parallel to the Earth’s surface
What is cleavage?
Planes of weakness in incompetent sedimentary rocks, e.g. shale. an also be sometimes in low grade met rocks e.g. slate.
Planes in which rocks split or fracture. Form perpendicular to the direction of pressure
How do cleavages form?
Compressive strength applied to rocks. Flat platy minerals (clay) align at right angles to direction of pressure.
With metamorphism, clay recrystallises into micas.
How should cleavage be drawn in fold diagrams?
Vertical along the whole fold. Not parallel to the fold.
What is bedding?
A unit of sedimentation which can vary in thickness
How do we recognise bedding?
Differences in colour or composition between beds.
Differences in grain size between beds.
Structures within beds and along bedding planes
What rock types can bedding be seen in?
Sedimentary, igneous and metamorphic
How do we recognise jointing?
Fractures along which there is no displacement.
Often perpendicular to bedding.
Possible mineralisation along the joints
What rocks can jointing be seen in?
Only competent rocks such as sandstones or limestones
How do we recognise cleavage?
Rocks split easily along parallel planes.
Planes very close together
What rocks can cleavage be seen in?
Only develops in incompetent rocks such as shales, which contain platy clay minerals and micas
