16: Folding & Faulting

Question 1

What crustal forces happen as at convergent plate boundaries? What does this crustal force do to the surrounding rock?

Answer 1

Convergent plate boundaries -> compression

Compression:
- crustal rocks are pushed together, leading to horizontal shortening / vertical lengthening, causing folding & faulting

Question 2

What crustal forces happen as at divergent plate boundaries? What does this crustal force do to the surrounding rock?

Answer 2

Divergent plate boundaries -> Tension

Tension / extensional:
- crustal rocks are pulled apart, leading to horizontal lengthening / vertical shortening, causing faulting

Question 3

What crustal forces happen as at transform plate boundaries? What does this crustal force do to the surrounding rock?

Answer 3

Transform plate boundaries -> shear

shear:
- crustal rocks are pushed in opposite directions, leading to rocks breaking into sub-parallel blocks, causing strike slip faults

Question 4

What is Elastic deformation? What can this do / result in?

Answer 4

Elastic Deformation:
- Temporary change in size
- Recovery when stress is removed (reversible)
- stores energy

Question 5

What is ductile/plastic deformation? What can this do / result in?

Answer 5

Ductile/Plastic Deformation:
- Permanent change in shape or size
- in rocks, results in folding

Question 6

What is brittle deformation? What can this do / result in?

Answer 6

Brittle Deformation:
- loss of cohesion due to stress
- on rocks, results in faults or fractures

Question 7

How do you tell what kind of deformation rocks undergo? Elastic? Ductile/Plastic? Brittle?

Answer 7

Rock has an elastic limit (beyond which strain will not recover). All rocks are elastic to some point and experience elastic deformation.

After crossing the elastic limit (yield limit), rocks become plastic/ductile or brittle.

Question 8

How do you determine if a rock is ductile/plastic or brittle?

Answer 8

Depends on:
1. Kind of stress applied
2. Amount of stress
3. TEMPERATURE
4. Lithology of material being formed (strength)
5. Length of time rock is subjected to stress (strain rate)
6. CONFINING PRESSURE (DEPTH)

Question 9

How do you tell if a rock will behave plastically or brittle? Give examples:

Answer 9

Rocks display increasingly ductile behavior with in depth (increasing T/P)

Transition Zone (6-15km)
- depends on lithology, amount of fluid present, etc…

Past the lithosphere into the asthenosphere, we get ductile rock behavior

Few rock types exhibit brittle behavior below 30km depth

Question 10

What is folding? What are anticlines and synclines? Where is the youngest/oldest rock placed?

Answer 10

Folding:
- A type of deformation event that results from the horizontal compression of rock layers by internal forces of the earth along plate boundaries
- Plastic-ductile deformation
- no breaking; rock layers remain intact

Anticlines:
- a fold that resembles an arch, with the rock beds (or limbs) dipping away from the center.
- Oldest units in the middle (older rock is pushed down ON THE SIDES and younger rock cover it up).

Synclines:
- A fold that resembles a trough, where the rock layers are warped, dipping into the center of the structure
- Youngest units in the middle (older rock pushed further down IN THE MIDDLE and younger layers form above)

Question 11

What are recumbent folds and thrust faults? How do recumbent folds and thrust faults form?

Answer 11

Recumbent folds:
- They are a type of geological fold where the axial plane is nearly horizontal.
- Anticlines and synclines occur in pairs, with adjacent folds sharing a limb. Greater pressure results in inclined or asymmetrical folds, with enough pressure forming recumbent folds.

thrust fault:
- A break in the Earth’s crust where older rocks are pushed above younger rocks
- They form from recumbent folds
- They are also important in mountain building

Question 12

What is faulting? What causes fracturing? What’s it associated with?

Answer 12

Faulting:
- The process by which rocks break and move along a fracture
- elastic-brittle deformation
- fracturing: rock layers too rigid to bend/fold
- occurs in seconds, so highly associated with earthquakes

Question 13

What are the 3 types of faults? When do they form?

Answer 13

Normal:
- vertical motion along the fault plane so that the hanging wall is moving DOWN relative to the footwall
- form from tension

Reverse:
- vertical motion along the fault plane so that the hanging wall is moving UP relative to the footwall
- formed from compression

Strike-Slip:
- The principal movement here is horizontal and therefore parallel to the strike of the fault
- arises from shear stresses

Question 14

What is the hanging wall and the foot wall?

Answer 14

Hanging wall and foot wall can be seen as sides of a fault

hanging wall:
- the wall which “hangs” overtop of the footwall. Think of it as the wall which you would hang your lamp on

Foot wall:
- The wall which supports the hanging wall. Think of it as the wall which you would stand on

Question 15

What are horsts? What are Grabens? How are they formed?

Answer 15

Horst and Graben (valley and range) refers to a type of topography created when the earth’s crust is pulled apart. This process, called extension. As the crust is strained in this way, normal faults develop and blocks of the crust drop down to form grabens, or valleys.

Horsts:
- they are the up-lifted blocks (relative to the grabens). They form artificial mountains.

Grabens:
- They are the down-dropped blocks (relative to horsts). They form artificial valleys.

Question 16

How do you tell if a strike-slip fault is left or right lateral?

Answer 16

• Left lateral if the horizontal movement to an observer standing on either fault block is to the left
• If the movement is to the right, it’s a right laterla.

(Ex/ you’re standing on a fault block and the land Infront of you moves horizontally left. That’s a left lateral).

Question 17

What are fault zones? what do they represent? What can they provide?

Answer 17

A fault zone is an area where rocks deform due to faulting, which can change the lithology, pore pressure, and seismic velocity

Represent major zones of weakness

They can provide pathways for increased fluid flow (e.g. groundwater)

Question 18

What are joints? What’s the difference between joints and faults?

Answer 18

Joints are defined as fractures in rocks along which there has been little or no displacement.

Faults are fractures along which there has been displacement.

Question 19

How do joints generally occur and how many? What are the 3 types of joints?

Answer 19

Joints generally occur as joint sets which are families of parallel, evenly spaced joints. A rock mass may contain several of these sets, dipping in different directions.

3 types of joints:
Tectonic joints:
- tectonic stresses inducing tensile failure of the rock

unloading joints:
- uplift and erosion removes the overlying rocks, reducing compressive load and allowing the rock to expand laterally (side to side)

cooling joints:
- cooling of hot magma or lava introduces stresses that induce tensile failure of the rock