Faulting Flashcards
Choosing this module is my biggest fault
Define fault
A surface or narrow zone along which there is measurable surface-parallel displacement
Even though faults are a surface, what do they have?
Thickness because they are zones of deformation
What can we consider a fault to be because it has thickness?
A tabular volume of rock with a central core (formed by intense shearing) with a surrounding damage zone (less intensely affected by brittle deformation)
What do non-vertical faults have?
Footwall and hanging wall
What is the difference between the footwall and the hanging wall?
The footwall is the block below the fault surface and the hanging wall is the block above the fault surface
What is used to describe faults?
Dip
Low-angle faults dip less than 30deg, high-angle faults dip more than 60deg
Define listric and anti-listric faults
Listric faults flatten downwards (common), anti-listric faults steepen downwards (unusual)
Describe the geometry of staircase faults
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
Define slip vector
The displacement vector joing two points that were connected before faulting
What compenents can the slip vector be separated into?
Strike-slip component and dip-slip component
Define the strike-slip component of the slip vector
It is parallel to the strike of the fault and has dextral or sinistral sense
Define the dip-slip component of the slip vector
It is parallel to the true dip of the fault and has normal or reverse sense
What are the three types of fault based on slip vector?
Pure dip-slip, pure strike-slip, oblique
Describe a rotational fault
Where the slip vector has changed orientation along the strike
What can be used to determine the orientation and magnitude of the slip vector?
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
What do slickensides indicate?
Direction of slip on the fault plane
Define separation
When we only have strike/dip of the fault and the offset of one planar feature (e.g. a lithological contact)
What can drag folds indicate?
Slip vector movement sense and orientation
Describe drag folds
The bending of the layering cut by faults
When are drag folds most easily formed
When the cut-off lines are at a high angle to the slip vector
Where are drag folds most commonly seen in sub-horizontal rocks?
In high-angle normal and reverse faults
Describe breached fault propagation folds
Formed when drag folds are produced by the bending of layers ahead of propagating fault tips
Folding preceeds the passage of the fault
When can a hanging wall antiform-footwall synform pair be formed?
When a thrust fault breaches the fault propagation fold
Give another example of how drag folds can develop
In the zone of interaction between two overlapping en echelon faults
When else can folding occur?
Where faults change orientation
Either in map view (fault strike) or section (fault dip)
What does change in fault dip require?
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
When is a roll-over antiform formed within the hanging wall?
When the dip is downwardly decreasing on a listric normal fault
Describe slickensides
Fault surfaces becoming smooth and polished in reponse to movement
What are the three types of strongly oriented lineations that develop in fault surfaces (e.g. slickensides)?
Ridges and grooves, slickenfibres, mineral streaks
Describe ridges and grooves
Formed by gouging of the fault surface and by accumulation of debris behind asperities
Describe slickenfibres
Formed by growth of minerals parallel to the fault slip vector during slow shearing
Describe mineral streaks
formed from pulverised and streaked out mineral debris
What does shearing on non-planar surfaces lead to?
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)
When do slickenfibres grow?
During slow aseismic slip as dilant surfaces separate
They become exposed by the removal of part of one of the fault blocks
Give two examples of ridges and grooves
Tool marks and pluck holes
Describe tool marks
Created by asperities (hard mineral grains)
What secondary fractures can indicate shear sense?
Tension gashes, Riedel shears, pinnate fractures
When does the brittle regime in rocks get stronger?
With increasing confining pressure
What is α on a Mohr diagram?
The angle between the fault plane and σ1
Typically ~30deg
What is the result α ~30deg and σ1 is vertical?
There is a normal fault dipping at ~60deg
What is the result α ~30deg and σ2 is vertical?
There is a vertical strike-slip fault
What is the result α ~30deg and σ3 is vertical?
There is a reverse fault dipping at ~30deg
σv=
ρrock g z
What does faulting require and why?
Attaining the stresses required for frictional sliding because most faulting occurs in rocks with pre-existing fractures, major faulting occurs by reactivating old faults
What is the difference between the failure envelope for frictional sliding between intact and fractured rocks?
Similar but the cohesive strength is smaller for fractured rocks
Define Byerlee’s Law
Almost all lithologies have a similar coefficient of frictional sliding
Give a factor that can promote brittle fracture
Pore fluid pressure
How does brittle failure occur?
Through the formation and coalescence of microcracks that have developed around flaws in the material
What happens when a new crack is formed?
Local stresses are relieved so the next crack forms elsewhere
Where are stresses concentrated and what does this lead to?
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
Where does microcracking focus when a macroscopic fracture is already present?
In the region just ahead of the crack tip where the stresses are the greatest
This region of deformation is the process zone
What does the concentration of microcracking activity result in?
Faults having a well defined core of intense deformation and a surround damage zone of less intense fracturing
Describe what happens when fracture propagation is fast
Damage around the fault si linked to transient stress conditions at the fracture tip
This is dynamic fracturing
Where does pervasive fracturing occur during fast fracture propagation?
On the side of the fault where the transient stresses are tensile
When is fault zone thickness greater than in fault core damage scenarios?
At given fault displacement
Where can incipient faults be found?
Developing though the linking components of an en echelon array or extension fractures
What is found when faults and associated fractures are mapped over a wide range of scales?
Plots of cumulative frequence vs fault trace length show a power law relationship
This relationship is overlooked at very small or long lengths
When does the slope of the power law decrease?
When deformation increases (more long faults)
Why is the displacement not constant over the whole fault surface?
Because it must decrease to zero at the tip of the faulr
What are the axes on displacement profiles and what shapes do they produce?
Displacement vs distance
Peak, bell, plataeu types
What can be inferred from displacement profiles?
Information about the growht history of the fault and the nature of interactions with other faults
Describe the maximum displacement (Dmax) on a fault
Often has a power law relationship with fault length
When do relay ramps form?
When two faults join together for form a longer fault
Define a transfer zone
The structure that form in the zone overlapping between two faults
Describe the two types of transfer zones
Releasing overlap zones (where the deformation is extensional) and restraining overlap zones (where the deformation is contractional)
What causes bends and jogs?
Fault segments that have linked via the creation and breaching of relay ramps
How can bends and jogs result in dilation?
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
What must the tip-line of a fault do?
Must form a closed loop
It connects to the end of the trace of the fault that loops around below the surface
Describe a branch-line
The line of intersection between two faults that allows the tip-line to not need to form a closed loop
Define a horse
Where a fault block is completely surrounded by faults
Describe an imbricate fan
A set of splay faults, this is commonly how faults die out
Can be extensional or contractional
