Strike-slip tectonic regimes Flashcards
Define strike-slip faults
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
Define tear faults
Relatively small scale strike-slip faults that accommodate/distribute slip on larger scale structures
Some tear faults are transfer faults
Describe transform faults and transcurrent faults
Major regional strike-slip systems
Transform faults are transfer faults, transcurrent faults are not
Describe transfer faults
Strike-slip faults with tips that terminate against other faults and serve to transfer displacement from one fault to another
Describe transfer faults in contractional and regional terrains
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
Describe the largest scale transfer faults
Transform faults
The active parts form plate boundaries
What are the three types of oceanic transform faults?
Ridge-ridge, ridge-arc, arc-arc
Describe ridge-ridge transform faults
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
Give two examples of continental transform faults
San Andreas Fault Zone, Alpine Fault (NZ)
What must transform faults do?
Cut right through the lithosphere
Define transcurrent faults
Faults with free tips that increase in length as the displacement accumulates
Describe transcurrent faults that are thought to be able to cut right though the lithosphere
Subvertical, over 300km long, have an offet of over 30km
Describe sinistral strike-slip faults on Enceladus
On the southern polar region, water vapour curtain geysers are continuously erupting from these faults
Describe a stretching fault
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
Describe sub-vertical strike-slip fault deforming in simple shear
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
Describe Riedel shears
Shears associated with strike-slip faults, they form oblique to the fault margins
Give three other secondary features that are associated with strike-slip fractures
P-shears (synthetic to main fault), Y-shears (parallel to main fault), T-fractures (extension fractures that bisect angle between Riedel shears)
What can develop in cover rocks in respose to strike-slip movement in the basement?
Folds
The hinges are normal to the incremental shortning direction and they rotate as shear strain increases
Describe bends in strike-slip faults
Curved parts of a continuous fault trace connecting two parallel segments
Describe stepovers in strike-slip faults
Overlapping en echelon fault segments
When are bends and stepovers restraining and when are the releasing?
They are restraining if the rocks at them are compressed during slip on the fault and releasing if they are pulled apart
Why can the displacement on the fault at bends and stepovers not be purely strike-slip?
Additional processes are required to accommodate the deformation
Describe slip at releasing beds and stepovers
Leads to extensional fractures and normal faults (at larger scale) that may form an extensional duplex and generate pull-apart basins)
What causes the flower structure of extensional duplexes in vertical section?
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
Describe the difference between releasing bends and simple shear strike-slip faults
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
What does slip at restrain bends and stepovers lead to?
Contractional structures, including stylolites, cleavages, folds, and reverse faults (these generate topographic elevation), and sometimes contractional duplexes
What structure do contractional duplexes form?
Positive flower structure, the component faults tend to be antilistric and the central part is structurally raised as a pop-up structure
How do strike-slip faults terminate?
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)
What does strain in strike-slip fault zones determine?
The orientations of foliations and lineations in their fault rocks
What planes and directions are foliations and stretching lineations in?
Foliation is in XY plane, stretching lineation in the X direction
Describe what you can expect to see in vertical strike-slip faults?
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
What are the shape and orientation of the strain ellipsoid influenced by?
The extent of transgression or transtension
Shape and orientation can be shown on a Flinn diagram
What does the strain ellipsoid look like with strongly compressional transpression?
Vertical foliation and lineation
What does the strain ellipsoid look like with strongly extensional transtension?
Horizontal foliation and lineation
What does the strain ellipsoid look like with intermediate extensional transtension?
Vertical foliation and horizontal lineation
Describe what happens when a fault or shear zone forms
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
What can be explain by shear partitioning?
Why fold traces along the San Andreas fault zone are sub-parallel with the fault and not oblique to it
What can lead to additional complexities in fault zones?
Phyllosilicate-rich lithologies
e.g. Carboneras Fault Zone (complicated by back-arc extension)
