Fault Scrap

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Studies and its significance


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Faults Faults are fractures in rocks along which

appreciable displacement has taken place.

Sudden movements along faults are the cause ofmost earthquakes.

Classified by their relative movement which canbeHorizontal, vertical, or oblique


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Components of faults


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Types of Faults Dip Slip Faults.

Normal Faults.

Reverse Faults.Horst and Grabben.

Strike Slip Faults.

Oblique Slip Faults.


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Dip Slip Faults-

Movement is mainly parallel to the

dip of the fault surface

Normal Fault- Hanging wallblock moves down relative to

the footwall block.Accommodate lengthening or

extension of the crust


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Dip Slip Faults

Horst and Graben-

Horstand grabenrefer to regionsthat lie between normal faults andare either higher or lower than thearea beyond the faults. A horstrepresents a block pushed upwardby the faulting, and a graben is ablock that has dropped due to thefaulting.

Horst and Graben are formed whennormal fault of opposite dip occurin pair with parallel strike lines


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Dip Slip Faults Reverse Faults-Hanging wall

moves up relative to footwall.

Reverse faults have dips greaterthan 450 and thrust faults havedips less then 450

If the angle of the fault plane islow (generally less than 200degrees from the horizontal)and the displacement of theoverlying block is large (often

in the kilometer range) thefault is called an overthrust

Accommodate shortening ofthe crust


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Strike Slip Faults

Dominant displacement ishorizontal and parallel tothe strike of the fault.

Types of strike slip fault

Right lateral- as you face

the fault, the block on theopposite side of the faultmoves to the right.

Left lateral-as you facethe fault, the block on the

opposite side of the faultmoves to the left.


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Oblique Slip Faults

A fault which has a componentof dip-slip and a component ofstrike-slip is termedan oblique-slip fault

Nearly all faults will havesome component of bothdip-slip and strike-slip, sodefining a fault as oblique

requires both dip and strikecomponents to bemeasurable and significant.


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Fault Scarp-It is defined as a tectonic landform coincident or

roughly coincident with a fault plane that has dislocated the

ground surface.

Fault scarps can range from small ephemeral slopes createdby a single increment of displacement to a high bedrockescarpments that formed during repeated slip.

Fig-Simple scarps related to single increments of slip on a newly propagated fault. Modified from Hancock 1988


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Types of Normal-Faults Scarps

Piedmont Scarp-Productof single

increment of motion.Morphology-(After Wallace,1977)*Steep Free face (>500)*Moderately incline debris slope (300-400)*Gently incline wash slope (50-100)

Multiple Scarp-Related to formation

of fault splay during a single faultingevent.Producing multiple scarps.


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Composite Scarp-Related to Renewed slipon a fault coincident with an older,

degraded scarp.*Repeated displacements along the samefault.

Splintered Scarp(Cotton 1949)-Formed as a result of fault

displacement being distributed acrossoverlapping en echelon segments


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Scarps associated with Reverse Faults

When the near-surface material is strong,the reverse faulting occurs on a single slipplane to form a simple overhanging scarpthat collapse or erodes back.

When the surface material is weak,internal warping results in theformation of a topographic flexurecalled fold limb scarp


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Thrust-Front Scarps

*When escarpment rise above asharp break in slope coincident withthe emergent of a thrust-flat trace,the escarpment is not a true faultscarp but rather a margin of a thrust

sheet.

*The leading edge of the trust sheethas collapsed and been overridden.

*It is known as trust-front scarps in

order to distinguish it from thrust-fault scarps. (Ian S Stewart andPaul L Hancock, 1990)


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Strike-Slip Scarps

Faults associated with strike-slip

faults are mainly the result ofthe juxtapositon of formerlyseparate areas of different height

Fig- Selected geomorphic features associated with the active strike slipfaultAbbreviations-S-Sag pond; SR-Shutter Ridge; F-Fault Scarp; B-BeheadedChannel; O-Offset channel(Modified from Seih and Wallce (1987,fig 3)


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Effect of repeated increments of normal and reverse

faultsBedrock escarpments can form which are several hundred meters of height

in active fault zones.

Figure Spanish Fork Peak area of the Wasatch

front, Utah, USA, highlighting the mainmorphological characteristics of tectonicallyactive range fronts in the Basin and Range

provinceAbbrevations- transverse spurs (S) andnarrow, V-shaped valleys (V), triangular facets(T), piedmont (P). ( from Anderson 1977,fig.17)


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Effect of repeated increments of normal and reverse

faults

Aegean-type range front showingboth step like (S) and ramplike (R)morphologies. Taken from Stewart

andHancock (1991, fig. 10)

Product of distributed normal splayfaulting.


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Geomorphic modification of fault scarpA-Creation of fresh piedmontscarp.B-Debris falling from a free face

which results in the rapid retreatof the scarp face and progressivebuild up of debris slope.C-Free face is completely buriedby the by the debris slope, thescarp becomes rounded.(101-102

yrs)D-The scarp decline results inthe scarp profile beingdominated initially by debrisslope.E-In the later stage it is being

dominated by wash slope

*Thus. the change from a fault scarp to a residual fault scarp (scarps from which thelast remnants of the original tectonic surface has been removed) in a piedmontsetting occurs when the free face becomes completely buried* In bedrock fault scarps. the removal by denudation of tectonic lineations on the

fault-scarp surface would be diagnostic of a residual morphology


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Fault Scarps degradation

Factors- Lithology Topography- degradation. as scarps crossing ridges generally are

better preserved than scarps on midslopes or along river channels(Yuming. 1989)

The dip of the ground surface before faulting will greatly affect the

preservation potential of a scarp, because scarps that slope in thedirection of the ground surface before faulting are generally moredegraded than scarps that face against the preexisting slope.

Microclimatic differences, such as variations in slope aspect, also havea large effect on degradation rates. For example, Pierce and Colman1986) demonstrated that south-facing scarps in central Idaho, USA.

degrade three times as fast as north-facing scarp. Disparity in degradation rates became intensified as scarp height

increase


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Faults Scarp Degradation

Fault scarps in consolidated materials

are less susceptible to denudation thanthose in poorly consolidatedsediments, their preservationpotential is greater. Thus, they arecapable of describing longer, if moreapproximate, records of tectonicactivity (Mayer, 1986)

Wallace (1977, p. 1272) contented thatbedrock scarps are characterized byslower rates of change

However degradation of fault scarpsfollows a similar sequence ofdegradation despite the variation inlithology as shown in the fig. viz-

gravity controlled-debris controlled-wash-controlled with decreasing slopeangle with time.

Fig-Limits of maximum slope angle versus ageof fault scarp. Fractured bed rock are moremodified slowly than in fanglomerate. They

follow a similar sequence (Modified from

Wallace 1977,fig 12)


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Complications in Fault Scarp Degradation

It is mainly controlled by structural heterogenity

They do not progress through a systematic and predictablesequence of morphologic change.

For eg- Pattern of degradation exhibited by normal faultsscarps in carbonate bedrock in the Aegean region.(Stewartand Hancock 1988)

A newly emergent and corrugatedfault plane cut by tectonic cavities,such as pluck holes (P), and fracturesand underlain by alternating layers of

compact breccia (CB) and incohesivebreccia (IB)


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Localized breaching of the armored compact-breccia carapace occurs at sites of enhancedweathering

Uneven degradation creates a cavitated scarp.

A residual ragged and embayed fault scarpforms following the removal of the uppermostcompact breccia sheet.


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Conclusions Fault scarps includes diverse range of tectonic

landforms that need to be distinguished from oneanother.

Contrasting tectonic styles and degradationalprocesses can lead to scarps of similar appearance, andhence, careful tectonic and geomorphic investigationis required before fault-generated landforms can beused to date faulting events.


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Fault Scrap

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