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EART28202 Structural Geology > Extensional tectonic regimes > Flashcards

Extensional tectonic regimes Flashcards

1
Q

Give an example of a regional scale tectonic system

A

Rift

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2
Q

Describe rifting

A

Combination of active and passive rifting

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3
Q

Describe active rifting

A

Rifting driven by rising hot mantle material that causes crustal doming, generating tensile stress

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4
Q

Describe passive rifting

A

Rifting developed in respose to far-field stresses related to plate movement

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5
Q

What are the three stages of rifting?

A

Initial phase, main stretching phase, late subsidence phase

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6
Q

Describe the initial phase of lifting

A

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

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7
Q

Describe the main stretching phase of rifting

A

Formation of major faults/fault blocks that accommodate crustal thinning

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8
Q

Describe the late subsidence phase of rifting

A

Cooling and basin deepening
Fault movement is only caused by differential compaction of sedimentary succession

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9
Q

What does the post-rift sedimentary sequence depend on?

A

Geometry of the fault blocks and the extent of the thermal subsidence

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10
Q

What happens when the continental rift extends far enough?

A

Crust splits and is replaced by ocean crust
A passive margin is established on each side of the rift

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11
Q

Describe the fault movement on passive margins

A

Mostly gravity-driven, occuring on faults that merge down to a decollement horizon (salt/over-pressured mudstones)

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12
Q

Give examples of wehre extension can occur

A

Rifts and passive margines, contractional orogenic belts

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13
Q

Describe how extension occurs in contractional orogenic belts

A

Back-arc rifting and in the outer arc of the oceanic plate where it bends on entering the subduction zone

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14
Q

When can normal faults form in the orogenic wedge?

A

When it is overthickened by the incorporation of a basement slice

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15
Q

Describe channel flow

A

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

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16
Q

Describe gravitational orogenic collapse

A

The heating of thickened crust can lead to sufficient weaking for it to collapse under its own weight

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17
Q

What happens at the end of a contractional orogeny?

A

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

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18
Q

What can extension induced by delamination at the end of a contractional orogeny use?

A

May use thrusts formed previously in the collision (inversion) or form new normal faults

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19
Q

Describe the formation of metamorphic core complexes

A

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

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20
Q

Define a turtle back

A

The dome formed of the main normal fault, caued by isostatic uplift
Adjacent to overprinting of ductile fabrics by brittle structures

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21
Q

What can be found at slow spreading mid-ocean ridges?

A

Oceanic core complexes
They expose gabbros and peridotites

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22
Q

Describe megamullions in relation to oceanic core complexes

A

Well-developed lineations that are parallel to the extension direction on the domed surface of the footwall

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23
Q

Describe the process of footwall doming of oceanic core complexes

A

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

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24
Q

Describe what happens above structural domes

A

The uplifted and domed units are put into a state of circumferential extension
A set of radiating normal faults (displacement decreasing outwards) is formed

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25
Q

Describe membrane stresses around a loaded lithosphere

A

The curvature of the lithospheric shell counteracted by flexual subsidence when the load is wide compared to the planetary radius

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26
Q

What does flexural subsidence as a response to membraine stresses lead to?

A

Tensile hoops stresses around the load and a set of radial extensional faults

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27
Q

What faults form around loads that have a smaller diameter and flexural subsidence is still possible?

A

Circumferential extension faults

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28
Q

Describe ring faults

A

A set of concentric normal faults that form when a cavity forms at depth and the surface units collapse into it

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29
Q

What can global expansion lead to

A

Global distributions of grabens

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30
Q

Where are sets of polygonal normal faults commonly found?

A

Over vast areas with clay-rich lithologies

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31
Q

What are polygonal normal faults?

A

Layer-bound systems of small dip-slip normal faults arranged in 3D arrays with a polygonal map pattern

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32
Q

What are the two main hypotheses of the formation of polygonal normal faults?

A

Vertical compaction with limited lateral expansion, chemically-driven volume contraction during diagenesis

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33
Q

Where do contractional orogenic belts form?

A

In sequences that have a history of rifting and normal faulting on a passive margin

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34
Q

What happens to the pre-existing faults in contractional orogenic belts during contraction?

A

They can be reactivated as thrusts
The geometry of extensional faults systems has great influence on the geometry of subsequent thrust systems

35
Q

Define inversion

A

The reactivation of pre-existing faults but with the opposite sense of dip-slip displacement

36
Q

Describe shortcut faults

A

More gently-dipping faults near the surface, produced by contraction of normal faults (which tend to be steeper than thrusts)

37
Q

When is a fault extensional?

A

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

38
Q

When could certain reverse faults be classed as extensional?

A

When an extensional fault is defined as one that lengthens a given layer

39
Q

Describe detachments

A

Extensional faults with large displacements and are gently dipping

40
Q

How are extensional detachments different from thrusts

A

Thy bring younger rocks over older ones or rocks of low metamorphic grade over those of a higher grade

41
Q

Describe the fault zone on detachments

A

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

42
Q

Give the three types of extensional fault surfaces

A

Planar, listric, antilistric

43
Q

What type of geometry do large displacement extensional faults have?

A

ramp-flat-ramp

44
Q

When do extensional faults develop ramp-flat-ramp geometry?

A

When two steep fault segments link up long a weak horizon

45
Q

Describe extensional imbricate fans

A

Sets of listric extensional faults that merge down into a detachment

46
Q

Describe an extensional duplex

A

When an extensional imbricate fan merges upwards into an extensional roof

47
Q

Define rider

A

Fault blocks in an imbricate fan

48
Q

Where can smaller scale faults be found in extensional fault systems

A

Within the hanging wall of the main fault when significant displacement has occurred

49
Q

Describe the two types of smaller faults that can be found in extensional fault systems

A

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

50
Q

Define a graben

A

A downthrown block that is bounded on each side by conjugate normal faults that dip towards the downthrown block

51
Q

Define a horst

A

A relatively uplifted block that is bound by two conjugate normal faults that each dip away from the uplifted block

52
Q

Define a half-graben

A

A downthrown and tilited block that is bound on only one side by a major normal fault

53
Q

Describe half-grabens

A

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

54
Q

Describe a classical normal fault

A

Planar and involved translation without rotational slip
Should dip at ~60deg

55
Q

Describe domino mode

A

A series of rotated fault blocks that are bound by planar faults
Seen in extended parts of the upper crust

56
Q

What happens when dominoes are rotated?

A

Voids open up during deformation

57
Q

How are voids avoided when dominoes are rotated?

A

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)

58
Q

What has to happen during extension on listric faults?

A

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

59
Q

How is a rollover antiform formed from listric rotational faults?

A

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

60
Q

How is the space problem from rollover antiforms alleviated?

A

Allowign layer-parallel extension through the formation of a set of antithetic faults

61
Q

How are the space problems created by slip on a listric surface resolved?

A

A combination of rollover antiform and synthetic/antithetic fault forming processes

62
Q

Define hanging wall collapse

A

The general process of hanging wall deformation and subsidence in extensional systems
More complex on faults with abrupt changes in dip

63
Q

Describe hanging wall deformation in a flat-ramp-flat geometry

A

Hanging wall forced to deform into a ramp synform

64
Q

Describe hanging wall deformation in a ramp-flat-ramp geometry

A

Fault-bend antiform produced in the hanging wall

65
Q

What does mapping the axial traces of folds from hanging wall deformation tell us?

A

The footwall ramp geometry because the traces run parallel to the associated ramp

66
Q

What can be inferred from a rollover antiform?

A

The underlying fault must be listric

67
Q

What is the relationship between rollover antiforms and drag folding

A

The bending of the layering is on the opposite sense

68
Q

Where can reverse drag be produced?

A

On a planar fault that dies out at the tip

69
Q

Describe footwall collapse

A

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

70
Q

How can the resulting geometry of footwall collapse be complicated?

A

Out-of-sequence faulting and/or increased faulting in hanging wall

71
Q

How can the sequence of events in extensional systems be deduced?

A

Establishing patterns of subsidence from syntectonic sediments

72
Q

Give the three stages in the development of an extensional fault system

A

Distributed faulting, formation of half-grabens, localisation of almost all slip

73
Q

Describe distributed faulting

A

Faults dipping in both cojugate orientation and slow overall extension

74
Q

Describe the formation of half grabens

A

The outward dipping faults die and the inward dipping faults extend, link and increase their rate of slip (inner faults by more)

75
Q

Describe the location of almost all slip

A

Almost all slip is localised on to one through-going fault with the other faults becoming inactive

76
Q

How do faults die out?

A

Along strike

77
Q

What happens when a fault dies out?

A

Regional extension is taken up by adjacent faults
Usually a transfer zone between these faults

78
Q

Describe the transfer zone between the end of a fault and adjacent faults

A

Deformation is accommodated by folding and faulting, they may be strike-slip transfer faults

79
Q

Where can extensions in rifts be concentrated?

A

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

80
Q

What are the two end-member models of extension at the lithospheric scale?

A

Pure shear and simple shear

81
Q

Define pure shear

A

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

82
Q

Define simple shear

A

Extension is localised on to a dipping shear zone that transects the lithosphere

83
Q

Where are the highest geothermal gradients in the pure shear model and the simple shear model?

A

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

EART28202 Structural Geology (10 decks)

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