STRUCTUAL GEOLOGY Flashcards
1
Q
stress and strain
A
- stress - force applied
- strain - resultant deformation due to stress
2
Q
Competent vs incompetent
A
competent
- brittle
- breaks/faulting
- cold rocks near surface
- e.g granite, sandstone, limestone
incompetent
- soft
- changes shape if force is applied
- folding
- warm rocks at depth
- e.g clay
- plastic incompetence - deformation is permanent
- elastic incompetence - material returns to original state once force is removed
3
Q
Factors which affect competence
A
time
- if force is applied slowly, rocks will act incompetent
- if force applied quickly, rocks will act competent
confining pressure
- burial at great depth/pressure cause rocks to act incompetent
temperature
- higher temperature cause incompetence
- e.g chocolate is competent and brittle, but at high temperature will undergo plastic deformation
4
Q
Measuring strain
A
- strain = change in length of line/ original length of line
- measured using: fossils, pebbles in conglomerate, etc
5
Q
Stress - strain graphs
A
diagram in notes 4
competent
- as stress increases, strain increases until rock breaks
Incompetent
- stress and strain increases until plastic deformation occurs
- hot Incompetent rock deforms at lower stress than cold Incompetent rock
6
Q
Types of force/stress
A
compressional –> <–
- makes folds and faults
tensional <– –>
- makes faults
shear ⬆️⬇️
- makes folds and faults
7
Q
Dip and strike
A
Strike
- structural trend of beds on the surface
- perpendicular to dip
Dip
- direction of tilt (compass)
- angle of dip (measured from horizontal)
- true dip - steepest angle on the planar surface
- apparent dip - result of of a rock face not being at 90° to dip. Always less than true dip
8
Q
Measuring the strike
A
- set compass-clinometer into clinometer mode
- move clinometer around the bedding plane until there’s 0 dip
- Mark the strike on the bed
- hold compass-clinometer flat and align the side with the mark
- move the dial so red arrow and needle line up and record value indicated at top
- use left hand rule to decide direction - fingers parallel to strike, thumb down dip
- strike measured from north as a 3 figure bearing
9
Q
Measuring the dip
A
- set compass-clinometer into clinometer mode
- measure true dip at right angle to the strike
- dip measured from the horizontal in degrees as two figures
10
Q
Describing a fold
A
- antiform - upfold ^
- synform - downfold v
- anticline - upfold with oldest rocks in core
- syncline - downfold with youngest rocks in core
- interlimb angle - gentle,open,closed,tight
- symmetrical - vertical axial plane, limbs have equal dips
11
Q
Extreme folding
A
- caused by strong compressional forces
Isoclinal
- vertical, parallel axial planes
- parallel fold limbs
Recumbent
- horizontal, parallel axial planes
- parallel fold limbs
Overfold
- 2 fold limbs dip in the same direction but by different amounts
12
Q
Plunge folds
A
- the angle of dip of the axial plane from the horizontal l
- characteristic M or W outcrop
13
Q
Fault terminology
A
- fault - a break in a rock where movement occurs
- throw - vertical movement
- upthrown / downthrown block
- hanging wall / foot wall
- slickenside - striations on the fault plane
- fault breccia - rubble produced by large fault movement
14
Q
Normal fault
A
- caused by tension <– –>
- hanging wall on downthrown block
- steep fault plane dip (>45°)
- “dip slip fault”
- causes crustal stretching
- e.g horst and graben
15
Q
Reverse faults
A
- caused by compression –> <–
- hanging wall on upthrown block
- gentle fault plane dip (<30°)
- “thrust fault”
- causes crustal shortening
