Structural Geology Flashcards
structural geology
study of faults, folds, and other tectonically produced features
stress (and its 3 types)
- 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)
strain
change in shape or size of a body because of application of stress: deformation
for rocks and soils, deformation will be a consequence of 3 things:
- type of stress applied
- temperature
- material properties
3 types of deformation
- elastic
- plastic
- brittle
elastic deformation
- Temporary change in shape or size
- Recovers when stress is removed (reversible)
- Stores energy
- Think: elastic band
plastic deformation
- Permanent change in shape or size
- Results in folding of rocks
- Think: piece of gum
- Generally compressive stress, warm
brittle deformation
- Stress beyond the elastic limit
- Material fractures, result in faults
- Generally occurs at cold rocks that have relatively low - pressure
- Think: broken vase
folds and their types
- folds are plastic deformation
- anticline
- syncline
- upright folds
- overturned folds
- recumbent folds
anticline vs. syncline
- Anticline: upturned folds, limbs dip away from each other
- Synclines: downturned folds, limbs dip towards each other (Synclines smile)
upright folds
can be vertical (symmetrical) or inclined (asymmetrical)
overturned folds
parallel axial planes (inclined, limbs dipping in same direction)
recumbent folds
axial plane is horizontal or near horizontal
faults/fractures
- 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)
fault types
- normal faults
- reverse faults
- thrust faults
- strike-slip faults
normal faults
- Caused by tension
- Blocks move away
- Hanging wall block moves down relative to footwall
- Can produce horst and graben landscapes -> landscapes under tension
hanging wall vs. foot wall
- Hanging wall lies above the fault plane
- Footwall block lies below the fault plane
horst vs. graben landscapes
- both are normal faults
- Graben: down-thrown block
- Horst: up-thrown block
reverse faults
- Caused by compression
- Blocks move towards each other
- Hanging wall block moves up relative to footwall
thrust faults
- Very low angled
- Can result in overthrusting: older rocks nearly horizontal above younger rocks
strike-slip faults
- 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
strike vs. drip
- 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
dip symbols on a geological map
- opposite directions in anticlines
- towards each in synclines
- If dip angles are different, fold is not symmetrical (limbs dip at different angles)
oldest and youngest rocks: anticlines vs. synclines
- Anticlines: oldest rocks in centre
- Synclines: youngest rocks in centre
plunging folds
- 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)
domes vs. basins
- 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)
depiction of faults on geological maps
- Depicted in various ways on maps
- Ex. “teeth” = hanging wall thrust fault, arrows on strike-slip fault
