Structural geology Flashcards
What are the two components of dip?
Direction of tilt using:
Compass direction / degrees of dip
And angle
What is strike?
Is the structural trend on the surface. It is the “grain” of the rocks. Strike is two opposite compass directions both at perpendicular to the dip direction (NE- SW) ETC
What is the preferred method for recording dip and strike data?
3 - digit compass bearing followed by 2 - digit dip angle, use the left hand rule
What is the difference between stress and strain?
Stress = Force applied
Strain = Resultant deformation of the stress
What is the difference between competent and incompetent?
Competent rocks are hard and brittle and tend to joint and fault
Incompetent rocks are weak and plastic and tend to fold or develop cleavage
What are 4 deformation types?
Plastic - changes shape when force is removed
Elastic - change of shape reverts back to what it originally was when force is removed
Ductile - Plastic
Brittle
What are 4 stresses?
Compressional - Inwards
Tensional - Outwards
Shear- Side by side
Confining- all directions
How do you calculate strain?
Strain = change in length/ original length
What is an antiform and a synform?
- Folded rock that bends upwards
- Synform is folded rock that bends downwards
What is a syncline and anticline?
Syncline = Younger rocks are in the core of the fold
Anticline= Older rocks are in the core of the fold
What are the four interlimb angles?
Gentle = 188-120 Degrees
Open = 120-70 degrees
Closed = 70-30 degrees
Tight = < 30 degrees
What is a fold limb?
The section between two “hinges” forming the sides of a fold.
Hinge = the line where there is a change in dip
What is the axial plain?
A plain that joins the hinges of all beds and bisects the fold
What are four fold orientations?
Upright horizontal
Horizontal overturned
Plunging
Recumbent
What are recumbent, nappes and isoclinal folds?
Recumbent = Folds that have its axial plain and fold limbs horizontal
Nappes = Recumbent folds that are broken along a fault
Isoclinal = Folds that have parallel limbs and are nearly vertical
What are domes and basins?
Dome - An anticline which dips from the centre in all directions
Basin - A syncline which dips towards the centre in the same direction but by different amounts
What is cleavage?
Describes how a mineral breaks along its weak points where the atoms are aligned.
What is slaty cleavage?
Where the slate minerals line up vertically , and are easily split parallel to the clay minerals.
What are tension joints?
Where the crest of a fold opens up.
What are two other types of jointing?
-Pressure release jointing
-Columnar jointing
What is pressure release jointing?
Found where the rock type is uniform and recent hills/valleys. Rocks removed by erosion unlock the underlying rocks, causing them to expand which creates joints parallel to the ground surface.
What is columnar jointing?
Form when lava cools and contracts steadily and slowly. Tensional forces are created during this and hexagons can tesselate across the surface which creates the joints.
Define:
Throw
Hanging wall
Foot wall
Throw = Vertical movement
Hanging wall = Wall that overhangs the fault
Foot wall = wall that is under the fault plane
What fault is caused by tensional forces?
Normal faults - creating a gap in the beds. Also known as a “dip slip fault”
What fault is created from compressional forces?
Reverse fault - Creates a repetition of beds and an overhanging wall
What fault is created from shear forces?
Tear faults = No throw, hanging wall or footwall. Also known as “strike fault”
Left movement = Sinistral
Right movement = Dextral
What is a thrust fault?
A low angle reverse fault
What is fault breccia?
Broken rock from either side of a fault creating a rubble zone
What is slickensides?
Small scratches where the rocks move past eachother.
What is an unconformity?
Represents a break in time where no sediment was depsoited
What is the difference between Graben and Horst
Grabren = Area that forms the downthrow between two faults
Horst = Area that forms the upthrow between two faults
G H
/-\______/——\
What are transform faults?
Faults created at conservative plate margins
What is fault gouge?
Fine particles of rock caused by the grinding of rock in faults. If the gouge’s heated up and pressurised it becomes hard and forms mylonite
How do angular unconformities form?
- Beds of young rock at top, old at bottom
- Tilt/folding of rock
- Erosion of tilted rock
- Deposition of new sediment
- Uplift and erosion expose the unconformity
