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EART28202 Structural Geology > Faulting > Flashcards

Faulting Flashcards

Choosing this module is my biggest fault

1
Q

Define fault

A

A surface or narrow zone along which there is measurable surface-parallel displacement

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2
Q

Even though faults are a surface, what do they have?

A

Thickness because they are zones of deformation

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3
Q

What can we consider a fault to be because it has thickness?

A

A tabular volume of rock with a central core (formed by intense shearing) with a surrounding damage zone (less intensely affected by brittle deformation)

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4
Q

What do non-vertical faults have?

A

Footwall and hanging wall

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5
Q

What is the difference between the footwall and the hanging wall?

A

The footwall is the block below the fault surface and the hanging wall is the block above the fault surface

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6
Q

What is used to describe faults?

A

Dip
Low-angle faults dip less than 30deg, high-angle faults dip more than 60deg

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7
Q

Define listric and anti-listric faults

A

Listric faults flatten downwards (common), anti-listric faults steepen downwards (unusual)

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8
Q

Describe the geometry of staircase faults

A

Ramps are the parts that are at a relatively high angle to the laying
Flat are the parts that are sub-parallel to the layering

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9
Q

Define slip vector

A

The displacement vector joing two points that were connected before faulting

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10
Q

What compenents can the slip vector be separated into?

A

Strike-slip component and dip-slip component

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11
Q

Define the strike-slip component of the slip vector

A

It is parallel to the strike of the fault and has dextral or sinistral sense

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12
Q

Define the dip-slip component of the slip vector

A

It is parallel to the true dip of the fault and has normal or reverse sense

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13
Q

What are the three types of fault based on slip vector?

A

Pure dip-slip, pure strike-slip, oblique

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14
Q

Describe a rotational fault

A

Where the slip vector has changed orientation along the strike

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15
Q

What can be used to determine the orientation and magnitude of the slip vector?

A

The strike/dip of the fault
AND EITHER
The direciton of slip on the fault plane (usually from slickensides) and one apparent offset of a surface with known strike/dip
OR
Two aparent offsets of surface with known (and different) strike/dips

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16
Q

What do slickensides indicate?

A

Direction of slip on the fault plane

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17
Q

Define separation

A

When we only have strike/dip of the fault and the offset of one planar feature (e.g. a lithological contact)

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18
Q

What can drag folds indicate?

A

Slip vector movement sense and orientation

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19
Q

Describe drag folds

A

The bending of the layering cut by faults

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20
Q

When are drag folds most easily formed

A

When the cut-off lines are at a high angle to the slip vector

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21
Q

Where are drag folds most commonly seen in sub-horizontal rocks?

A

In high-angle normal and reverse faults

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22
Q

Describe breached fault propagation folds

A

Formed when drag folds are produced by the bending of layers ahead of propagating fault tips
Folding preceeds the passage of the fault

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23
Q

When can a hanging wall antiform-footwall synform pair be formed?

A

When a thrust fault breaches the fault propagation fold

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24
Q

Give another example of how drag folds can develop

A

In the zone of interaction between two overlapping en echelon faults

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25
Q

When else can folding occur?

A

Where faults change orientation
Either in map view (fault strike) or section (fault dip)

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26
Q

What does change in fault dip require?

A

Internal deformation within the hanging wall to avoid space problems
In normal faults, if the fault steepens downwards a hanging wall synform is formed, if it shallows, a hanging wall antiform is formed

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27
Q

When is a roll-over antiform formed within the hanging wall?

A

When the dip is downwardly decreasing on a listric normal fault

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28
Q

Describe slickensides

A

Fault surfaces becoming smooth and polished in reponse to movement

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29
Q

What are the three types of strongly oriented lineations that develop in fault surfaces (e.g. slickensides)?

A

Ridges and grooves, slickenfibres, mineral streaks

30
Q

Describe ridges and grooves

A

Formed by gouging of the fault surface and by accumulation of debris behind asperities

31
Q

Describe slickenfibres

A

Formed by growth of minerals parallel to the fault slip vector during slow shearing

32
Q

Describe mineral streaks

A

formed from pulverised and streaked out mineral debris

33
Q

What does shearing on non-planar surfaces lead to?

A

Regions of compression (where slickolites can develop) and dilatancy
Can beused to indicate slip sense, the lineation can provide line of slip (where slickenfibres can precipitate)

34
Q

When do slickenfibres grow?

A

During slow aseismic slip as dilant surfaces separate
They become exposed by the removal of part of one of the fault blocks

35
Q

Give two examples of ridges and grooves

A

Tool marks and pluck holes

36
Q

Describe tool marks

A

Created by asperities (hard mineral grains)

37
Q

What secondary fractures can indicate shear sense?

A

Tension gashes, Riedel shears, pinnate fractures

38
Q

When does the brittle regime in rocks get stronger?

A

With increasing confining pressure

39
Q

What is α on a Mohr diagram?

A

The angle between the fault plane and σ1
Typically ~30deg

40
Q

What is the result α ~30deg and σ1 is vertical?

A

There is a normal fault dipping at ~60deg

41
Q

What is the result α ~30deg and σ2 is vertical?

A

There is a vertical strike-slip fault

42
Q

What is the result α ~30deg and σ3 is vertical?

A

There is a reverse fault dipping at ~30deg

43
Q

σv=

A

ρrock g z

44
Q

What does faulting require and why?

A

Attaining the stresses required for frictional sliding because most faulting occurs in rocks with pre-existing fractures, major faulting occurs by reactivating old faults

45
Q

What is the difference between the failure envelope for frictional sliding between intact and fractured rocks?

A

Similar but the cohesive strength is smaller for fractured rocks

46
Q

Define Byerlee’s Law

A

Almost all lithologies have a similar coefficient of frictional sliding

47
Q

Give a factor that can promote brittle fracture

A

Pore fluid pressure

48
Q

How does brittle failure occur?

A

Through the formation and coalescence of microcracks that have developed around flaws in the material

49
Q

What happens when a new crack is formed?

A

Local stresses are relieved so the next crack forms elsewhere

50
Q

Where are stresses concentrated and what does this lead to?

A

Around crack tips
Regions with high crack density become more stressed, microfracturing then focuses in these regions, these coalesce and defromtion is localised on a through-going fracture

51
Q

Where does microcracking focus when a macroscopic fracture is already present?

A

In the region just ahead of the crack tip where the stresses are the greatest
This region of deformation is the process zone

52
Q

What does the concentration of microcracking activity result in?

A

Faults having a well defined core of intense deformation and a surround damage zone of less intense fracturing

53
Q

Describe what happens when fracture propagation is fast

A

Damage around the fault si linked to transient stress conditions at the fracture tip
This is dynamic fracturing

54
Q

Where does pervasive fracturing occur during fast fracture propagation?

A

On the side of the fault where the transient stresses are tensile

55
Q

When is fault zone thickness greater than in fault core damage scenarios?

A

At given fault displacement

56
Q

Where can incipient faults be found?

A

Developing though the linking components of an en echelon array or extension fractures

57
Q

What is found when faults and associated fractures are mapped over a wide range of scales?

A

Plots of cumulative frequence vs fault trace length show a power law relationship
This relationship is overlooked at very small or long lengths

58
Q

When does the slope of the power law decrease?

A

When deformation increases (more long faults)

59
Q

Why is the displacement not constant over the whole fault surface?

A

Because it must decrease to zero at the tip of the faulr

60
Q

What are the axes on displacement profiles and what shapes do they produce?

A

Displacement vs distance
Peak, bell, plataeu types

61
Q

What can be inferred from displacement profiles?

A

Information about the growht history of the fault and the nature of interactions with other faults

62
Q

Describe the maximum displacement (Dmax) on a fault

A

Often has a power law relationship with fault length

63
Q

When do relay ramps form?

A

When two faults join together for form a longer fault

63
Q

Define a transfer zone

A

The structure that form in the zone overlapping between two faults

64
Q

Describe the two types of transfer zones

A

Releasing overlap zones (where the deformation is extensional) and restraining overlap zones (where the deformation is contractional)

65
Q

What causes bends and jogs?

A

Fault segments that have linked via the creation and breaching of relay ramps

66
Q

How can bends and jogs result in dilation?

A

When similar bends and jogs change in fault dip with depth
If there are pore fluids, there is decompression (can be explosive, leading to brecciation, if fault slip is fast) and precipitation

67
Q

What must the tip-line of a fault do?

A

Must form a closed loop
It connects to the end of the trace of the fault that loops around below the surface

68
Q

Describe a branch-line

A

The line of intersection between two faults that allows the tip-line to not need to form a closed loop

69
Q

Define a horse

A

Where a fault block is completely surrounded by faults

70
Q

Describe an imbricate fan

A

A set of splay faults, this is commonly how faults die out
Can be extensional or contractional

EART28202 Structural Geology (10 decks)

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