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Topography - Structural Geology - Lecture Notes, Study notes of Geology

West Bengal State UniversityGeology

In these Lecture notes, Professor has tried to illustrate the following points : Topography, Slip, Regimes, Vertical, Slip, Vectors, Motions, Topography, Vectors, Parallel

Typology: Study notes

2012/2013

Uploaded on 07/22/2013

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% % Lecture%17%
!
1!
Strike:Slip%Regimes%
%%%
Ch.%18,%p.%355+363%
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1.!Strike*Slip!Faults:%Strike+slip%faults%are%usually%vertical%and%have%slip%vectors%parallel%to%the%Earth’s%surface.%They%
form%straight%fault%traces,%unaffected%by%topography.%Motions%are%either%right+lateral%(dextral)%or%left+lateral%
(sinistral).%
%%%%%%%%%%%%%%%%%%%%%%%%%
[Fig.&18.1.&Strike.slip&faults&are&vertical&and&have&slip&vectors&parallel&to&the&Earth’s&surface]&&
&
&
2.!Transfer!Faults:%Strike+slip%faults%can%act%as%transfer%faults%that%connect%other%structures%(normal%faults,%thrust%
faults,%dikes,%veins,%rift%segments,%etc.).%
%%%
[Fig.&18.2.4.&Strike.slip&faults&behaving&as&transfer&fault]&&
&
&
3.!Transform!Faults:%One%type%of%transfer%fault%is%found%along%a%plate%boundary%and%is%called%a%transform%fault.%
These%faults%connect%adjacent%spreading%ridge%segments%or%subduction%zones.%
%%%%
[Fig.&18.5.&Transform&faults&can&accommodate&offsets&between&spreading&ridges]&[Fig.&18.6.&Varieties&of&transform&
faults]&&
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4.!Transform!Faults:%Transform%faults%can%be%>1000%km%long%and%may%present%significant%earthquake%hazards%
where%they%come%on+land.%
%
e.g.%San%Andreas%fault,%California;%North%Anatolian%fault,%Turkey%
%
[Fig.&18.0.&The&San&Andreas&is&a&transform&fault]&&
&
&
5.!Transcurrent!Faults:%Major%strike+slip%faults%entirely%contained%within%continental%lithosphere%are%transcurrent%
faults%or%wrench%faults.%They%are%unconstrained%at%their%ends.%
%%
[Fig.&18.7. Transcurrent&fault&that&soles&into&a&subduction&zone]&[Figure.&Examples&of&(a)&a&transform&fault&and&(b)&a&
transcurrent&fault.&(Twiss&&&Moores,&2007)]&
&
&
6.!Kinematics!of!Strike*Slip!Faults:%Strike+slip%faults%may%have%complex%surface%geometries%comprised%of%multiple%
fractures.%These%commonly%form%an%en%echelon%pattern%with%the%step%sense%dependent%on%the%slip%sense.%
%%
&[Fig.&18.8.&En&echelon&shear&fractures&along&a&strike.slip&fault&trace&(from&Riedel’s&clay&experiments)]&[Figure.&En&
echelon&veins&(tension&fractures)&produced&by&shearing]%
&
&
7.!Kinematics!of!Strike*Slip!Faults:%As%described%earlier,%shear+related%fractures%have%specific%orientations%relative%
to%the%shear%direction:%
%
R+shears:%these%are%Riedel%shears%at%~20°%to%the%fault%with%the%same%shear%sense.%
%
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Strike-­‐Slip Regimes

Ch. 18, p. 355-­‐

  1. Strike-­‐Slip Faults: Strike-­‐slip faults are usually vertical and have slip vectors parallel to the Earth’s surface. They form straight fault traces, unaffected by topography. Motions are either right-­‐lateral (dextral) or left-­‐lateral (sinistral).

[Fig. 18.1. Strike-­‐slip faults are vertical and have slip vectors parallel to the Earth’s surface]

  1. Transfer Faults: Strike-­‐slip faults can act as transfer faults that connect other structures (normal faults, thrust faults, dikes, veins, rift segments, etc.).

[Fig. 18.2-­‐4. Strike-­‐slip faults behaving as transfer fault]

  1. Transform Faults: One type of transfer fault is found along a plate boundary and is called a transform fault. These faults connect adjacent spreading ridge segments or subduction zones.

[Fig. 18.5. Transform faults can accommodate offsets between spreading ridges] [Fig. 18.6. Varieties of transform faults]

  1. Transform Faults: Transform faults can be >1000 km long and may present significant earthquake hazards where they come on-­‐land.

e.g. San Andreas fault, California; North Anatolian fault, Turkey

[Fig. 18.0. The San Andreas is a transform fault]

  1. Transcurrent Faults: Major strike-­‐slip faults entirely contained within continental lithosphere are transcurrent faults or wrench faults. They are unconstrained at their ends.

[Fig. 18.7. Transcurrent fault that soles into a subduction zone] [Figure. Examples of (a) a transform fault and (b) a transcurrent fault. (Twiss & Moores, 2007)]

  1. Kinematics of Strike-­‐Slip Faults: Strike-­‐slip faults may have complex surface geometries comprised of multiple fractures. These commonly form an en echelon pattern with the step sense dependent on the slip sense.

[Fig. 18.8. En echelon shear fractures along a strike-­‐slip fault trace (from Riedel’s clay experiments)] [Figure. En echelon veins (tension fractures) produced by shearing]

  1. Kinematics of Strike-­‐Slip Faults: As described earlier, shear-­‐related fractures have specific orientations relative to the shear direction:

R-­‐shears: these are Riedel shears at ~20° to the fault with the same shear sense.

R’-­‐shears: conjugate Riedel shears at ~80° to the fault; opposite slip sense.

P-­‐shears: opposite rotation sense to R-­‐shears; ~10° to fault; same slip sense.

T-­‐fractures: pinnate tension fractures at ~45° to fault. Also get normal faults.

Folds and stylolites: form perpendicular to contraction direction.

[Fig. 18.9. A range of Riedel shear fractures, extension features, and contractional features can indicate the shear sense along a strike-­‐slip fault zone]

  1. Kinematics of Strike-­‐Slip Faults: A mirror-­‐image pattern is produced by the opposite sense of slip.

[Figure. Riedel shear arrays]

  1. Strike-­‐Slip Fault Geometries: Similar to normal and thrust faults, strike-­‐slip faults can be segmented at the surface, resulting in small steps along the fault trace.

[Fig. 18.0. Segmented trace of the San Andreas fault]

  1. Strike-­‐Slip Fault Geometries: Depending on the sense of step and sense of slip, the steps may be extensional (releasing) or contractional (restraining).

[Figure. Releasing steps occur where the sense of step mirrors the sense of slip. If they are opposite, a restraining step occurs. Twiss & Moores (2007)]

  1. Strike-­‐Slip Fault Geometries: Restraining steps result in contractional strike-­‐slip duplexes (thrust faults and folds); releasing steps produce extensional strike-­‐slip duplexes (normal faults).

[Fig. 18.13. Extensional and contractional strike-­‐slip duplexes]

  1. Strike-­‐Slip Fault Geometries: Strike-­‐slip duplexes are broad zones of deformation that fan upwards towards the surface to form flower structures.

[Fig. 18.15. Flower structures in strike-­‐slip duplexes]

  1. Strike-­‐Slip Fault Geometries: An example of extension in a releasing step is given by Death Valley. These low-­‐ lying areas are also called pull-­‐apart basins.

[Fig. 18.14. Death Valley formed in a releasing step]

  1. Pull-­‐Apart Basins: Pull-­‐apart basins can form sag ponds or lakes at a small scale, or large rhomb-­‐shaped sedimentary basins at the crustal scale.

[Figure. Pull-­‐apart basins]