Strike-slip tectonic regimes

Question 1

Define strike-slip faults

Answer 1

Faults with a slip vector that is parallel to the strike of the fault and therefore parallel to the surface of the Earth
Curve locally but appear straight on maps

Question 2

Define tear faults

Answer 2

Relatively small scale strike-slip faults that accommodate/distribute slip on larger scale structures
Some tear faults are transfer faults

Question 3

Describe transform faults and transcurrent faults

Answer 3

Major regional strike-slip systems
Transform faults are transfer faults, transcurrent faults are not

Question 4

Describe transfer faults

Answer 4

Strike-slip faults with tips that terminate against other faults and serve to transfer displacement from one fault to another

Question 5

Describe transfer faults in contractional and regional terrains

Answer 5

Approximately parallel to the regional transport direction and connect adjacent reverse or normal faults within larger scale scontraction/extensional fault systems
May separate domains that have different geometry and displacement

Question 6

Describe the largest scale transfer faults

Answer 6

Transform faults
The active parts form plate boundaries

Question 7

What are the three types of oceanic transform faults?

Answer 7

Ridge-ridge, ridge-arc, arc-arc

Question 8

Describe ridge-ridge transform faults

Answer 8

The most common ocean transform faults, comprise of active parts between two ridge segments and inactive parts that were formally active and have passed by the adjacent ridge segment
Slip sense is opposite to the apparent offset of the ridges
Width and structural complexity increases with spreading rate

Question 9

Give two examples of continental transform faults

Answer 9

San Andreas Fault Zone, Alpine Fault (NZ)

Question 10

What must transform faults do?

Answer 10

Cut right through the lithosphere

Question 11

Define transcurrent faults

Answer 11

Faults with free tips that increase in length as the displacement accumulates

Question 12

Describe transcurrent faults that are thought to be able to cut right though the lithosphere

Answer 12

Subvertical, over 300km long, have an offet of over 30km

Question 13

Describe sinistral strike-slip faults on Enceladus

Answer 13

On the southern polar region, water vapour curtain geysers are continuously erupting from these faults

Question 14

Describe a stretching fault

Answer 14

Produced when the blocks on either side of a fault have different levels of deformability
There is slip parallel stretching of one side of the fault with respect to the other
Slip is zero at the fault tip and varies along the fault

Question 15

Describe sub-vertical strike-slip fault deforming in simple shear

Answer 15

The map is approx the XZ plane across a shear zone
Features seen in strike-slip fault zones can be considered by thinking about structures from shear zones

Question 16

Describe Riedel shears

Answer 16

Shears associated with strike-slip faults, they form oblique to the fault margins

Question 17

Give three other secondary features that are associated with strike-slip fractures

Answer 17

P-shears (synthetic to main fault), Y-shears (parallel to main fault), T-fractures (extension fractures that bisect angle between Riedel shears)

Question 18

What can develop in cover rocks in respose to strike-slip movement in the basement?

Answer 18

Folds
The hinges are normal to the incremental shortning direction and they rotate as shear strain increases

Question 19

Describe bends in strike-slip faults

Answer 19

Curved parts of a continuous fault trace connecting two parallel segments

Question 20

Describe stepovers in strike-slip faults

Answer 20

Overlapping en echelon fault segments

Question 21

When are bends and stepovers restraining and when are the releasing?

Answer 21

They are restraining if the rocks at them are compressed during slip on the fault and releasing if they are pulled apart

Question 22

Why can the displacement on the fault at bends and stepovers not be purely strike-slip?

Answer 22

Additional processes are required to accommodate the deformation

Question 23

Describe slip at releasing beds and stepovers

Answer 23

Leads to extensional fractures and normal faults (at larger scale) that may form an extensional duplex and generate pull-apart basins)

Question 24

What causes the flower structure of extensional duplexes in vertical section?

Answer 24

Extensional duplexes widen towards the surface
Negative flower structures are produced where the component faults tend to be listric and the central part is structurally lowered

Question 25

Describe the difference between releasing bends and simple shear strike-slip faults

Answer 25

Releasing bends result in wider pull-apart basins that are formed under overall transtension, they develop a set of oblqiue en echelon extension faults in the basin margin, and form multiple depocentres

Question 26

What does slip at restrain bends and stepovers lead to?

Answer 26

Contractional structures, including stylolites, cleavages, folds, and reverse faults (these generate topographic elevation), and sometimes contractional duplexes

Question 27

What structure do contractional duplexes form?

Answer 27

Positive flower structure, the component faults tend to be antilistric and the central part is structurally raised as a pop-up structure

Question 28

How do strike-slip faults terminate?

Answer 28

In an imbricate fan of either normal faults or reverse faults (dependent on location of deformation zone relative to the fault slip vector)
Or can terminate as a splay of small strike-slip faults (main fault displacement = sum of individual splay faults)

Question 29

What does strain in strike-slip fault zones determine?

Answer 29

The orientations of foliations and lineations in their fault rocks

Question 30

What planes and directions are foliations and stretching lineations in?

Answer 30

Foliation is in XY plane, stretching lineation in the X direction

Question 31

Describe what you can expect to see in vertical strike-slip faults?

Answer 31

No transgression or transtension, the foliation should be in XY plane to be vertical and oblique to the margins of the fault zone, stretching lineations should be towards X to be horizontal

Question 32

What are the shape and orientation of the strain ellipsoid influenced by?

Answer 32

The extent of transgression or transtension
Shape and orientation can be shown on a Flinn diagram

Question 33

What does the strain ellipsoid look like with strongly compressional transpression?

Answer 33

Vertical foliation and lineation

Question 34

What does the strain ellipsoid look like with strongly extensional transtension?

Answer 34

Horizontal foliation and lineation

Question 35

What does the strain ellipsoid look like with intermediate extensional transtension?

Answer 35

Vertical foliation and horizontal lineation

Question 36

Describe what happens when a fault or shear zone forms

Answer 36

The simple shear component of deformation is localised, leaving the surrounding volume of rock to accommodate the pure shear component by extension or contraction
This is shear partitioning

Question 37

What can be explain by shear partitioning?

Answer 37

Why fold traces along the San Andreas fault zone are sub-parallel with the fault and not oblique to it

Question 38

What can lead to additional complexities in fault zones?

Answer 38

Phyllosilicate-rich lithologies
e.g. Carboneras Fault Zone (complicated by back-arc extension)