Structural Geology

Question 1

structural geology

Answer 1

study of faults, folds, and other tectonically produced features

Question 2

stress (and its 3 types)

Answer 2

• force applied per unit of area (N/m^2) aka Pascals (Pa)
  1) Compressive stress: forces act towards each other -> Results in shortening or flattening
  2) Tensile stress: forces act away from each other -> Results in stretching and/or breaking
  3) Shear stress: forces are parallel but act on opposite sides of a plane (think pushing a deck of cards so it ends up diagonal)

Question 3

strain

Answer 3

change in shape or size of a body because of application of stress: deformation

Question 4

for rocks and soils, deformation will be a consequence of 3 things:

Answer 4

1. type of stress applied
2. temperature
3. material properties

Question 5

3 types of deformation

Answer 5

1. elastic
2. plastic
3. brittle

Question 6

elastic deformation

Answer 6

• Temporary change in shape or size
• Recovers when stress is removed (reversible)
• Stores energy
• Think: elastic band

Question 7

plastic deformation

Answer 7

• Permanent change in shape or size
• Results in folding of rocks
• Think: piece of gum
• Generally compressive stress, warm

Question 8

brittle deformation

Answer 8

• Stress beyond the elastic limit
• Material fractures, result in faults
• Generally occurs at cold rocks that have relatively low - pressure
• Think: broken vase

Question 9

folds and their types

Answer 9

• folds are plastic deformation
• anticline
• syncline
• upright folds
• overturned folds
• recumbent folds

Question 10

anticline vs. syncline

Answer 10

• Anticline: upturned folds, limbs dip away from each other

- Synclines: downturned folds, limbs dip towards each other (Synclines smile)

Question 11

upright folds

Answer 11

can be vertical (symmetrical) or inclined (asymmetrical)

Question 12

overturned folds

Answer 12

parallel axial planes (inclined, limbs dipping in same direction)

Question 13

recumbent folds

Answer 13

axial plane is horizontal or near horizontal

Question 14

faults/fractures

Answer 14

• type of brittle deformation
• Loss of cohesion due to stress
• Material fractures -> faults
• Stress > strength of rock (shallow crustal levels)
• Typically preceded by a bit of elastic deformation (think: bending dry spaghetti)
• Scale of movement can range from mm to km (Mm-cm: micro-faults or factures; M-km: faults)

Question 15

fault types

Answer 15

• normal faults
• reverse faults
• thrust faults
• strike-slip faults

Question 16

normal faults

Answer 16

• Caused by tension
• Blocks move away
• Hanging wall block moves down relative to footwall
• Can produce horst and graben landscapes -> landscapes under tension

Question 17

hanging wall vs. foot wall

Answer 17

• Hanging wall lies above the fault plane

- Footwall block lies below the fault plane

Question 18

horst vs. graben landscapes

Answer 18

• both are normal faults
• Graben: down-thrown block
• Horst: up-thrown block

Question 19

reverse faults

Answer 19

• Caused by compression
• Blocks move towards each other
• Hanging wall block moves up relative to footwall

Question 20

thrust faults

Answer 20

• Very low angled

- Can result in overthrusting: older rocks nearly horizontal above younger rocks

Question 21

strike-slip faults

Answer 21

• Caused by shear stress
• Motion is lateral (side to side)
• Fault is vertical (no hanging or footwall blocks)
• Described as either left lateral or right lateral
• To figure out, imagine yourself standing on one side of the block and asking which way the fault moved

Question 22

strike vs. drip

Answer 22

• both common features on geological maps
• Strike is the orientation of a geological feature in space and its dip is how it is angled with respect to the horizontal

Question 23

dip symbols on a geological map

Answer 23

• opposite directions in anticlines
• towards each in synclines
• If dip angles are different, fold is not symmetrical (limbs dip at different angles)

Question 24

oldest and youngest rocks: anticlines vs. synclines

Answer 24

• Anticlines: oldest rocks in centre

- Synclines: youngest rocks in centre

Question 25

plunging folds

Answer 25

• fold axis is inclined/dipping
• You’ll see a different outcrop pattern -> “v” pattern (non-plunging folds are just parallel strata
• Plunging syncline: V’s point opposite to direction of plunge
• Plunging anticline: V’s point towards direction of plunge
• On a geologic map, a plunging fold can be indicated by an arrow that runs along the axis and points in the direction of the plunge (with the youngest exposed rocks in the direction the arrows are pointing)

Question 26

domes vs. basins

Answer 26

• Domes: anticlinal: beds dip away from center, oldest rocks exposed at the center surface; Great traps for oil and gas to pool
• Basin: synclinal beds dip towards the center, youngest rocks exposed at center (bowl-shaped)

Question 27

depiction of faults on geological maps

Answer 27

• Depicted in various ways on maps

- Ex. “teeth” = hanging wall thrust fault, arrows on strike-slip fault