Extensional tectonic regimes Flashcards
Give an example of a regional scale tectonic system
Rift
Describe rifting
Combination of active and passive rifting
Describe active rifting
Rifting driven by rising hot mantle material that causes crustal doming, generating tensile stress
Describe passive rifting
Rifting developed in respose to far-field stresses related to plate movement
What are the three stages of rifting?
Initial phase, main stretching phase, late subsidence phase
Describe the initial phase of lifting
Rifting as a result of large scale crustal doming
A steep fracture forms and it is possible for the intrusion of magma generated at depth
Describe the main stretching phase of rifting
Formation of major faults/fault blocks that accommodate crustal thinning
Describe the late subsidence phase of rifting
Cooling and basin deepening
Fault movement is only caused by differential compaction of sedimentary succession
What does the post-rift sedimentary sequence depend on?
Geometry of the fault blocks and the extent of the thermal subsidence
What happens when the continental rift extends far enough?
Crust splits and is replaced by ocean crust
A passive margin is established on each side of the rift
Describe the fault movement on passive margins
Mostly gravity-driven, occuring on faults that merge down to a decollement horizon (salt/over-pressured mudstones)
Give examples of wehre extension can occur
Rifts and passive margines, contractional orogenic belts
Describe how extension occurs in contractional orogenic belts
Back-arc rifting and in the outer arc of the oceanic plate where it bends on entering the subduction zone
When can normal faults form in the orogenic wedge?
When it is overthickened by the incorporation of a basement slice
Describe channel flow
Where a detached and heated basement slice can be buoyant enough to ascend
It has a thrust on the lower side and a normal fault on its upper side
Describe gravitational orogenic collapse
The heating of thickened crust can lead to sufficient weaking for it to collapse under its own weight
What happens at the end of a contractional orogeny?
Cool lithosphere mantle root under the orogenic belt and phase transitions can make the root sufficiently dense for it to delaminate and sink into the deeper mantle
Replacement by hot material leads to uplift and gravitational collapse
What can extension induced by delamination at the end of a contractional orogeny use?
May use thrusts formed previously in the collision (inversion) or form new normal faults
Describe the formation of metamorphic core complexes
The thinning and isostatic compensation during extreme extension that elevates deeper levels of the crust
The high grade footwall rocks can be exposed at the surface
Define a turtle back
The dome formed of the main normal fault, caued by isostatic uplift
Adjacent to overprinting of ductile fabrics by brittle structures
What can be found at slow spreading mid-ocean ridges?
Oceanic core complexes
They expose gabbros and peridotites
Describe megamullions in relation to oceanic core complexes
Well-developed lineations that are parallel to the extension direction on the domed surface of the footwall
Describe the process of footwall doming of oceanic core complexes
Slip is initiated on high angle normal faults, most cease at small distances but some allow runaway slip (leading to doming) by the formation of talc during periods of waning magmatism
Describe what happens above structural domes
The uplifted and domed units are put into a state of circumferential extension
A set of radiating normal faults (displacement decreasing outwards) is formed
Describe membrane stresses around a loaded lithosphere
The curvature of the lithospheric shell counteracted by flexual subsidence when the load is wide compared to the planetary radius
What does flexural subsidence as a response to membraine stresses lead to?
Tensile hoops stresses around the load and a set of radial extensional faults
What faults form around loads that have a smaller diameter and flexural subsidence is still possible?
Circumferential extension faults
Describe ring faults
A set of concentric normal faults that form when a cavity forms at depth and the surface units collapse into it
What can global expansion lead to
Global distributions of grabens
Where are sets of polygonal normal faults commonly found?
Over vast areas with clay-rich lithologies
What are polygonal normal faults?
Layer-bound systems of small dip-slip normal faults arranged in 3D arrays with a polygonal map pattern
What are the two main hypotheses of the formation of polygonal normal faults?
Vertical compaction with limited lateral expansion, chemically-driven volume contraction during diagenesis
Where do contractional orogenic belts form?
In sequences that have a history of rifting and normal faulting on a passive margin
What happens to the pre-existing faults in contractional orogenic belts during contraction?
They can be reactivated as thrusts
The geometry of extensional faults systems has great influence on the geometry of subsequent thrust systems
Define inversion
The reactivation of pre-existing faults but with the opposite sense of dip-slip displacement
Describe shortcut faults
More gently-dipping faults near the surface, produced by contraction of normal faults (which tend to be steeper than thrusts)
When is a fault extensional?
If it leads to elongation perpendicular to the strike of the fault
Includes faults with a normal component of dip-slip direction
Uses the horizontal surface of the earth for reference
When could certain reverse faults be classed as extensional?
When an extensional fault is defined as one that lengthens a given layer
Describe detachments
Extensional faults with large displacements and are gently dipping
How are extensional detachments different from thrusts
Thy bring younger rocks over older ones or rocks of low metamorphic grade over those of a higher grade
Describe the fault zone on detachments
A thick region of distributed cataclasis
Reflects the late stages of movement under transtensional conditions, often in the presence of a large pore fluid pressure
Give the three types of extensional fault surfaces
Planar, listric, antilistric
What type of geometry do large displacement extensional faults have?
ramp-flat-ramp
When do extensional faults develop ramp-flat-ramp geometry?
When two steep fault segments link up long a weak horizon
Describe extensional imbricate fans
Sets of listric extensional faults that merge down into a detachment
Describe an extensional duplex
When an extensional imbricate fan merges upwards into an extensional roof
Define rider
Fault blocks in an imbricate fan
Where can smaller scale faults be found in extensional fault systems
Within the hanging wall of the main fault when significant displacement has occurred
Describe the two types of smaller faults that can be found in extensional fault systems
Synthetic faults are parallel to the main fault with the same sense of shear and antithetic faults are are conjugate orientation with the main fault
Define a graben
A downthrown block that is bounded on each side by conjugate normal faults that dip towards the downthrown block
Define a horst
A relatively uplifted block that is bound by two conjugate normal faults that each dip away from the uplifted block
Define a half-graben
A downthrown and tilited block that is bound on only one side by a major normal fault
Describe half-grabens
The downthrown block is back-tilted, the angle depending on the dip of the fault surface andhow the displacement is accommodated in the hanging wall
Describe a classical normal fault
Planar and involved translation without rotational slip
Should dip at ~60deg
Describe domino mode
A series of rotated fault blocks that are bound by planar faults
Seen in extended parts of the upper crust
What happens when dominoes are rotated?
Voids open up during deformation
How are voids avoided when dominoes are rotated?
Listric fault is introduced at the end of the system and the voids between base of blocks and the substrate are removed by introducing a mobile underlying medium and/or internal deformation (brecciation/ductile flow)
What has to happen during extension on listric faults?
Must be accompanied by significat hanging wall deformation to resolve space problems
Rigid displacement of a hanging wall of a listric fault opens a large gap
How is a rollover antiform formed from listric rotational faults?
When the bottom edge of the hanging wall is forced to conform to the shape of the fault and the layer length remains constant
Generates another space problem at the end of the section
How is the space problem from rollover antiforms alleviated?
Allowign layer-parallel extension through the formation of a set of antithetic faults
How are the space problems created by slip on a listric surface resolved?
A combination of rollover antiform and synthetic/antithetic fault forming processes
Define hanging wall collapse
The general process of hanging wall deformation and subsidence in extensional systems
More complex on faults with abrupt changes in dip
Describe hanging wall deformation in a flat-ramp-flat geometry
Hanging wall forced to deform into a ramp synform
Describe hanging wall deformation in a ramp-flat-ramp geometry
Fault-bend antiform produced in the hanging wall
What does mapping the axial traces of folds from hanging wall deformation tell us?
The footwall ramp geometry because the traces run parallel to the associated ramp
What can be inferred from a rollover antiform?
The underlying fault must be listric
What is the relationship between rollover antiforms and drag folding
The bending of the layering is on the opposite sense
Where can reverse drag be produced?
On a planar fault that dies out at the tip
Describe footwall collapse
Extension->unloading causes cutting into footwall->extensional duplex formed if there is a ramp and flat->antithetic faults form in conjugate to accom deformation in hanging wall
How can the resulting geometry of footwall collapse be complicated?
Out-of-sequence faulting and/or increased faulting in hanging wall
How can the sequence of events in extensional systems be deduced?
Establishing patterns of subsidence from syntectonic sediments
Give the three stages in the development of an extensional fault system
Distributed faulting, formation of half-grabens, localisation of almost all slip
Describe distributed faulting
Faults dipping in both cojugate orientation and slow overall extension
Describe the formation of half grabens
The outward dipping faults die and the inward dipping faults extend, link and increase their rate of slip (inner faults by more)
Describe the location of almost all slip
Almost all slip is localised on to one through-going fault with the other faults becoming inactive
How do faults die out?
Along strike
What happens when a fault dies out?
Regional extension is taken up by adjacent faults
Usually a transfer zone between these faults
Describe the transfer zone between the end of a fault and adjacent faults
Deformation is accommodated by folding and faulting, they may be strike-slip transfer faults
Where can extensions in rifts be concentrated?
Slip on a master fault on one of the flanks
There are a variety of transfer zone structures;
antithetic interbasin ridge
antithetic interference zone
transfer fault
synthetic relay ramp
What are the two end-member models of extension at the lithospheric scale?
Pure shear and simple shear
Define pure shear
Overall deformation across the region of extension, creasted symmetric thinning of the lithosphere
Accommodated by brittle faulting in the upper curst and plastic deformation at greater depth
Define simple shear
Extension is localised on to a dipping shear zone that transects the lithosphere
Where are the highest geothermal gradients in the pure shear model and the simple shear model?
Pure shear: under the centre of the region of extension
Simple shear: offset to one side
Has important consequences for uplift and subsidence patterns and therefore for basin development
