Faulting and folding Flashcards
What are the four main types of fault?
Normal
Reverse
Strike-slip
Thrust
Movement of hanging wall and footwall on normal faults?
Hanging wall down
Footwall up
Where are the rocks on the normal fault in relation to the fault plane?
Footwall= rocks underneath fault plane
Hanging wall= rocks above the fault plane
Is a normal fault dip-slip?
Yes
Normal fault, force?
Tensional (crust extended)
Normal fault degree/angle of dip?
Steep, 60-65 degrees
Movement of hanging wall and footwall on reverse faults?
Hanging wall up
Footwall down
Where are the rocks on the normal fault in relation to the fault plane?
Footwall= rocks above fault plane
Hanging wall= rocks underneath the fault plane
Is a reverse fault dip-slip?
Yes
Can reverse faults be normal faults reactivated?
Yes
In a reverse fault, how can the beds of rock be repeated?
In a borehole
Reverse fault, forces?
Compressional (push rocks together)
Angle of dip/degree of reverse faults?
Steep 60-65 degrees
Thrust fault, which is the upthrown side and which is the downthrown side?
Upthrown side= hanging wall
Downthrown side= footwall
Thrust fault forces?
Formed via compression
What is the angle of dip/degree of a thrust fault?
<45 degrees
Thrust fault, dip-slip (direction of movement is same as the fault)?
Yes
Thrust fault, which side is above the fault and which side is below the fault?
Side above fault= hanging wall
Side below fault= footwall
Is a strike-slip fault compressional?
No, it is neither compressional or tensional, it is shear
Degree/ angle of dip of strike-slip fault?
Vertical
What does strike-slip mean?
Same movement as surface direction of the fault
Strike-slip fault, hanging and footwall directions?
There is not a hanging wall or a footwall in relation to strike-slip faults
Immediately after faulting, what are the relative positions of the upthrown and downthrown blocks?
Upward block will be above the downthrown block
What happens to the upthrown block and what does this indicate?
The upthrown block is eroded to the level of the downward block and this indicates that the rocks exposed at the surface on the downthrown block will be younger than those on the surface of the upthrown block
What are marker beds used for?
To establish the relative displacement of rocks across a fault
What is fault throw?
The amount of vertical displacement of beds due to fault movement
What is a right lateral/dextral strike-slip fault?
Where relative movement is to the right due to shear forces
What is a left lateral/sinistral strike-slip fault?
Where relative movement is to the left due to shear forces
What are the two main types of fold?
Antiforms and synforms
What is folding caused by?
Compressional forces
What is an antiform?
Arch shaped rock structures , which close upwards. There is no indication of the relative age of the beds involved
What is an anticline?
Arch shaped folds, where the oldest bed is in the centre (core)
What is a synform?
Trough shaped rock structures, which close downwards. There is no indication of the relative age of the beds involved
What is a syncline?
Trough shaped folds, where the youngest bed is in the centre (core)
What is important to remember about anitclines?
All anticlines are antiforms but not all antiforms are anticlines. The antiform could be an upside-down or overturned syncline
What is important to remember about synclines?
All synclines are synforms but not all synforms are synclines. The synform could be an upside-down or overturned anticline
What position are the axial planes if the limbs dip away from the fold axis at the same angle?
Vertical
What are some key characteristics of anticlines with limbs of unequal dip?
-Gentle angle of dip=wider outcrop
-Axial planes are inclined if the limbs dip away from the fold axis at different angles
-Steeper dip=narrower outcrop
-Core is the oldest
-Axial plane dips towards more gently dipping side
-Beds get younger away from core in both directions
What are symmetrical folds?
Folds with limbs of equal length
What are asymmetrical folds?
Folds with limbs of differing lengths
What is the fold crest?
The top of the fold
What is the fold trough?
The bottom of the fold
What is wavelength?
The horizontal distance between adjacent fold troughs or fold crests
What is amplitude?
The vertical difference between the fold crest and the fold trough in a fold divided by 2
What are the interlimb angles of gentle folds?
120°-180°
What are the interlimb angles of open folds?
70°-120°
What are the interlimb angles of closed folds?
30°-70°
What are the interlimb angles of tight folds?
<30°
What is special about the limbs in isoclinal folds?
The limbs are parallel to each other
What is a monocline?
A fold with only one limb
What are characteristics of overturned folds/overfolds?
-Upside down sedimentary structures
-Youngest bed at top and oldest bed at bottom
-Axial planes dips at 45° and in the same direction as the limbs
-Overturned strata present here
What are some characteristics of recumbent folds?
-Youngest bed at top and oldest bed at bottom
-Sedimentary structures upside down or inverted
-Axial plane is between horizontal and 10°
What is the axial plane?
The plane/surface that divides the fold as symmetrically as possible
What is the axial plane trace?
Outcrop of the axial plane at the Earths surface
What is the hinge?
Line or point where the curve of the fold is greatest
What is the fold limb?
Sides of the fold structure
What is the fold axis?
The imaginary line that runs along the crest (anticline) or trough (syncline)
What is plunge?
The vertical angle between the horizontal plane and the axis or line of maximum elongation of a feature.
