Lecture 2- Joints, Cleavage, Veins, Faults, Earthquakes

Question 1

What are joints?

Answer 1

fractures with minimum opening (mm to cm) with no displacement

Question 2

What do joints form in response to?

Answer 2

extensions and perpendicular to stretching direction

Question 3

How do joints form from lavas?

Answer 3

By contraction vertically due to thermal gradient cool at tip and base

Question 4

What are plumouse structures?

Answer 4

feather like on top of joints and develop when the crack is propagating laterally

Question 5

What can plumouse structures allow you to distinguish?

Answer 5

Joints from bedding planes

Question 6

Why are joints imporant?

Answer 6

Paleostress analysis
Oil exploration
Volcanic hazards
Seismic hazard
Landslide Hazard

Question 7

How are joints arranged in simple shear?

Answer 7

‘En echelon’ in shear zone (will step down) and dip in shear direction

Question 8

What are joints like for pure shear?

Answer 8

Parallel and overlap

Question 9

What are veins?

Answer 9

joints filled with minerals that precipitate from fluids circulating through fractures

Question 10

How do veins form from pure shear?

Answer 10

straight veins (lens shape)

Question 11

How will simple shear veins form?

Answer 11

Sigmoidal veins (S shape) as rotation occurs the core and growth rotates

Question 12

What can veins form?

Answer 12

large ore deposits (gold, copper, lead, zinc, silver)

Question 13

What is a fault?

Answer 13

planar surface or zone which one side is displaced relative to the other

Question 14

What is the range for displacement?

Answer 14

mm to Km

Question 15

What are the 2 main forms of deformation?

Answer 15

Brittle
Ductile

Question 16

What is brittle deformation?

Answer 16

rock break into rigid blocks each portion separated by discontinuities

Question 17

What is ductile deformation?

Answer 17

apply stress, no discontinuity but will deform as single piece

Question 18

What are the conditions of brittle deformation?

Answer 18

<350*c
or
High strain rates

Question 19

What are the conditions of ductile deformation?

Answer 19

> 350*c
or
Low strain rates

Question 20

What deformation leads to faults?

Answer 20

brittle deformation

Question 21

What are the typical conditions for fault formation if by brittle deformation?

Answer 21

Shallow depth (less than approx ~15km)
where temp is below 350*c

Question 22

How do faults form?

Answer 22

when the internal friction of rocks is overcome due to stress

Question 23

What happens when stress approaches internal friction?

Answer 23

micro-cracks form a fault form when these microcracks merge together

Question 24

What plays a key role in fault formation?

Answer 24

pre-existing weaknesses

Question 25

What are the 2 blocks in a fault called?

Answer 25

Hanging wall (above the fault plane) Footwall (below the fault plane)

Question 26

What is a normal fault?

Answer 26

the hanging wall moves down relative to the footwall

Question 27

What is a reverse (thrust) fault?

Answer 27

the hanging wall moves up relative to the footwall

Question 28

What are strike slip faults?

Answer 28

the two blocks are displaced laterally (i.e. in the horizontal plane). There is no hanging wall or footwall

Question 29

What is Dextral and Sinistral with strike-slip faults?

Answer 29

Dextral- right lateral Sinistral- Left lateral

Question 30

What are thrust faults?

Answer 30

low angle <30* reverse faults

Question 31

What are detachment faults?

Answer 31

low angle ~30* normal faults (normal fault typically 60*)

Question 32

What does thrust play a key role in?

Answer 32

Orogenesis (mountain building)

Question 33

What does detachment fault allow?

Answer 33

deep rocks to be exhumed at the surface

Question 34

What is heave?

Answer 34

horizontal displacement between 2 points which were adjacent

Question 35

What is throw?

Answer 35

vertical displacements between 2 points that were once adjacent

Question 36

What is total slip?

Answer 36

displacement along the fault plane between two points that were once adjacent

Question 37

How do faults terminte?

Answer 37

Faults may terminate laterally by decreasing progressively their displacement.

Question 38

What is displacement at tip points?

Answer 38

zero

Question 39

What is a transfer fault?

Answer 39

when faults terminate against another fault

Question 40

What are relay ramps?

Answer 40

when a fault terminates by overlapping eachother

Question 41

How do normal faults affect stratigraphy?

Answer 41

OMIT and thin the crust (erase)

Question 42

How do reverse faults affect stratigraphy?

Answer 42

Repeat stratigraphy so thicken crust

Question 43

What are the 2 divisions of fault rock?

Answer 43

Incoesive Coesive

Question 44

Where do incoesive rocks faults form?

Answer 44

shallow part of crust

Question 45

Where do coesive fault rocks form?

Answer 45

deep part of crust

Question 46

What are the 2 types of incoesive rock?

Answer 46

Fault breccia (>30% visible grains) Fault gouge (<30% visible grains)

Question 47

What is the reason for the difference between fault breccia and fault gouge?

Answer 47

Fault gouge has experienced more deformation and friction

Question 48

What are the coesive fault rocks?

Answer 48

Crush breccia - Matrix <10% Cataclasite - matrix predominate

Question 49

What are mylonites?

Answer 49

Fine grained rocks formed by dynamic recrystallization of new minerals during strain at high temperature

Question 50

What depth do mylonites form at?

Answer 50

>15km shear zones where deformation is ductile

Question 51

What is the stick phase of the stick-slip mechanism?

Answer 51

stress increases and elastic strain builds up over years

Question 52

What is the slip phase?

Answer 52

when the elastic limit of rocks is reached and a fault forms

Question 53

What is the speed of elastic recovery?

Answer 53

recovered in seconds

Question 54

What is generated when elastic strain is recovered?

Answer 54

seismic waves

Question 55

What are the types of seismic waves?

Answer 55

Compressional P waves Shear S waves

Question 56

What are the characteristics of compressional P waves?

Answer 56

Faster, arrive first to seismograms (aka “primary”) Travel as a caterpillar Travel in both solids and liquids

Question 57

What are the characteristics of Shear S waves?

Answer 57

Slower, arrive after P waves (aka “secondary”) Travel as a snake Can only travel in solids

Question 58

What is the seismic cycle?

Answer 58

Interseismic (strain build up) and Coseismic (strain recovered)

Question 59

What is the primary cause of earth quakes?

Answer 59

Faults

Question 60

How do faults grow?

Answer 60

repeated slip; so, large-offset faults are the products of hundreds/thousands of earthquakes

Question 61

Why is repeated slip likely to occur on faults?

Answer 61

more likely to be reactivated due to the presence of the weakness

Question 62

What increases at a fault with continued slip?

Answer 62

displacement and length

Question 63

What is the ratio of length to displacement?

Answer 63

Length 10,000 times displacement (10km length = 1m displacement)

Question 64

What is aseismic slip?

Answer 64

If the frictional resistance on the fault is low elastic energy cannot be stored, and displacement is accumulated at a constant rate without causing any earthquake

Question 65

Where does aseismic slip commonly occur?

Answer 65

uppermost part of the crust (3-4km) where normal stress is lower (not enough friction)

Question 66

What is the depth range for stick-slip?

Answer 66

~4 to ~15 km

Question 67

Why does aseismic slip occur after 15km?

Answer 67

as rocks become ductile due to high temperature (>350*c)

Question 68

How can earthquakes occur below 15km?

Answer 68

at subduction zones in the subducting plate as the temperature at the plate interface is <350*c

Question 69

What is the thermal gradient at a subduction margin compared to normal thermal gradient?

Answer 69

subduction - 10*c per km Normal - 30*c per km

Question 70

What are the depth of shallow, intermediate and deep earthquakes?

Answer 70

Shallow <99km Intermediate 100-300km Deep 300~700 km