fault bend fold

8/16/2019 Fault Bend Fold

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GEOL411: Faults and Faulting

8.1 Introduction

I) Recall:

(i)

deformation can be brittle or ductile

(ii)

brittle deformation (cracks) can be tensile or shear cracks

(iii)

only mode I (tensile) cracks can propagate any distance

(iv)

large fractures with no shear displacement are called joints (topic

7)

(v) fractures with shear displacement are called faults, but these are

not simply large shear cracks

(vi) movement on faults is by frictional sliding

II) Terminology

Fault = discrete surface on which measurable slip has occurred by brittle

deformation processes

Slip = relative displacement between formerly adjacent points on opposite

sides of a fault, measured in the fault surface

Fault zone = Fault expressed as a zone of numerous small fractures. Small

fractures and faults branching off a larger fault are called splays

Shear zone = distributed zone of shear displacement, with macroscopically

ductile deformation. Includes microscopically ductile deformation

processes, and cataclasis.

Cataclasis is microscopically brittle but macroscopically ductile (zone of

fracturing and crushing, not a discrete surface)


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III) Why are faults important?

Control spatial arrangement of rock units (mapping)

Affect topography and landscape (geomorphology)

Affect permeability of rocks and sediments control fluid migration)

Affect distribution of natural resources (ores, fossil fuels, …)

Deformation: long-term, e.g. Wrangellia now spread along W coast of

US

short term, earthquakes!

Already covered a lot in topic 6; here we concentrate on mesoscopic

structures and processes. Will look at tectonics and faulting (macroscopic)

later on.


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8.2 Fault geometry and displacement

(I) More terminology…

Wall = rock adjacent to a fault surface

Fault block = body of rock that moved as a consequence of slip on the fault

For a non-vertical fault,

Hangingwall = block above the fault plane

Footwall = block below the fault plane

Movement on faults can be a combination of dip-slip and strike-slip.

Pure dip-slip faults can be normal (h/wall down) or reverse (h/wall up)

Pure strike-slip faults can be dextral (right-lateral) or sinistral (left-lateral)

Oblique-slip faults combine dip-slip and strike-slip components.

Net slip = displacement on a fault, measured in the plane of the fault.For oblique-slip faults we have to consider both the strike-lip and dip-slip

components.

The rake angle is the angle of the slip vector measured down from the

horizontal in the fault plane:


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In addition to normal/reverse and sinistral/dextral, we also classify a fault

according to its dip.

0 to 30̊ = low-angle, 30 to 60 ̊ = intermediate angle, 60 to 90̊ = high angle

Sometimes a fault will change displacement along its length (all faults do

this near their terminations)

Scissors fault = amount of slip changes along strike so that h/wall block

rotates around an axis perpendicular to the fault surface

(II) Map symbols

Note that the ornament is always in the hangingwall block!

Detachment = large low-angle fault system, (can be regionally extensional

or contractional, will often change apparent nature from outcrop to

outcrop)


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The hangingwall of a detachment is often referred to as

the allochthon (stuff which has moved) and the footwall as

the autochthon (stuff which is still in its place of origin)

Also note the interpretation of these symbols in cross-section.

e.g. where topography exposes the footwall of a large detachment, this is

called a window. Where topography has removed all but an isolated

remnant of hangingwall, this is called a klippe.

(III) Measuring slip

Net slip vector completely defined by:

(a)

distance

(b)

orientation (plunge and bearing of offset)

(c)

sense of slip

Note that separation refers to the offset between a particular marker

horizon from one side of the fault to another, measured along a specified

line.

This is usually not the same as net slip (imagine looking at a cross-section

of a fault – you can only see the dip-slip component of movement)

Animation from CD-ROM at this point?

(IV) Fault bends


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Fault bend = sudden change in dip and/or strike of a fault

e.g. Listric faults have concave-up shapes (shallow at depth, steep near the

surface):

e.g. Thrust faults frequently display “ramp and flat” geometry. Note

distinction between footwall flat and hangingwall flat.

e.g. Strike-slip faults often contain fault bends, which are classified as:

restraining bends

(transpression = combination of strike-slip & compression)

releasing bends

(transtension = combination of strike-slip & extensional movement)


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(V) Fault terminations

All faults have to end somewhere

An emergent fault ends at the Earth’s surface

A blind fault terminates inside the Earth and is not seen at the surface

(Blind faults can be later exposed by erosion – not all faults you see in

outcrop necessarily reached the surface when they were active)

The edge of a fault is called the tip line (separates slipped & unslipped

regions):

Faults can die out along their length in a number of ways, e.g.:

(a)

as many smaller fault splays, forming a horsetail

(b)

in a zone of ductile deformation, becoming more diffuse

away from the fault tip

OH: Fault displacement is related to fault length – longer faults usually

have greater displacement. (Fig. 8.14 , b)

8.3 Fault rocks

I) Classification


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Fault rocks are classified according to the size of the fragments, and

whether they are cohesive or not.

Fault gouge: Fine-grained (< 1mm) noncohesive fault rock, grain size

reduced by pulverisation, gouge may be sheared to form foliation.

Often altered to clay minerals (in which case gouge or fluid flow?).

If cemented by minerals precipitated from circulating groundwater it

is an indurated gouge.

Fault breccia: Coarse noncohesive fault rock, angular rock fragments >

1mm (can be several m). If breccia blocks are cemented by vein

material, it is a vein-filled breccia (or indurated breccia).

Cataclasite: Cohesive fault rocks composed of broken, crushed and

rolled grains. Does not disintegrate when hammered (c.f. gouge,

breccia).

Pseudotachylyte: Glassy or microcrystalline material formed by melting

due to frictional heating during slip on a fault. Flows between breccia

fragments or nto cracks in fault wall. Usually mm to cm lenses or

sheets. [Can be several m thick at impact sites, e.g. Vredefort dome.]

II) Slickensides and Fibers

Slickensides are fault surfaces polished by frictional sliding, often

containing groove lineations (striations) caused by asperities ploughing

into the opposite wall.

When fault movement was by the crack-seal mechanism (remember topic

6), any space between the two fault planes is filled with fluid. On slipping,

the pressure decreases and minerals may be precipitated. If slip occurs in

many small amounts, these minerals will grow as elongate fibres parallel to

the slip direction. This can often give a sense of slip on the fault:


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Remember that multiple slip episodes may have occurred in multiple

orientations. The latest slip may have erased earlier grooves or fibres which

grew in a different orientation.

III) Slip indicators

Grooves and/or fibres can tell us the orientation of slip, but we also want to

know the sense of slip (e.g. normal or reverse for dip-slip, and dextral or

sinistral for strike-slip).

There are several sense of slip indicators for brittle faults and fault zones:

(i)

offset markers (the easiest, but beware apparent offset!)

(ii)

en echelon veins / sigmoidal en echelon veins (topic 7)(iii)

fault-related folds (asymmetry can tell you the shear sense)

(iv) fiber-sheet imbrication

(v) carrot-shaped grooves on slickensides (opposite wall moved from

deep wide end of groove towards narrow shallow end)

(vi)

steps on slickensides (very small steps – usually feel with hand)

(vii)

pinnate fractures (near a fault tip)

IV) Change in character with depth

Physical appearance of a fault depends on magnitude of displacement,

whether slip is on a pre-existing surface or reactivates an older surface, and

the pressure-temperature (PT) conditions of faulting (which obviously vary

with depth.)

Note that the behaviour of rocks is controlled by temperature, composition,

strain rate, confining pressure, fluid pressure, previous deformation, etc.


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Because of the composition dependence, a major strike-slip fault may look

different at the same depth, if different rock types are involved.

(i) At the surface, faults may be characterized by at least three types of

topographic feature:

Fault scarp = topographic offset caused by dip-slip motion on the fault

plane. The scarp is the fault plane.

Fault-line scarp = topographic offset caused by differential erosion on

different sides of the fault. The fault plane may not be exposed.

In addition, notches may result from preferential erosion of weak

fault gouge or breccia. Streams often follow the traces of faults (good

for mapping). Rarely, indurated breccia (with lots of mineral

precipitation) may be stronger than rocks on either side, in which

case the fault trace will be expressed as a ridge.

(ii) At shallow depths (≤ about5 km), mesoscopic faults can reactivate

bedding planes or joint surfaces (will usually result in slickensides)

or fracture previously intact rock (will result in breccia and/or

gouge). Bigger faults have wider brecciated zones and may have

small fault splays.

(iii) Between about 5 and 10-15 km, rocks become more ductile. Cataclastic

shear zones form here (macro ductile, micro brittle). The brittle-

plastic transition is at about 10-15 km depth (c. 250 to 350 ̊C). Below

this, ductile crystal-plastic deformation mechanisms dominate, and

mylonite forms (very fine-grained strongly foliated rock). Will cover

this in later topics.


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8.4 Faults and folds

We haven’t covered folding in detail yet, but important to note folds and

faults often associated. Some examples:

(I) Fault-inception fold – deformation by folding is overprinted by faulting

(II) Fault-propagation fold – e.g. folding above and beyond a thrust fault tip

line

(III) Fault-bend fold – forms passively as gravity prohibits void formation

Can be related to thrust or normal fault, called a “drape fold” when

sedimentary cover passively drapes over a basement-offsetting fault

8.5 Anderson’s theory of faulting


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Recall that faulting occurs in response to shear stress

(presence of a shear stress requires that differential stress,σd =σ1 –σ3 ≠ 0).

Now recall that the ratio of shear stress to normal stress is a maximum on

planes oriented at about 30̊ toσ1 and containingσ2 (which is why Coulomb

shear fractures initiate at about this angle).

Now recall that the Earth’s surface is a free surface (cannot transmit shear

stress), therefore it must also be one of the three principal planes of stress.

One principal plane of stress usually remains (sub-) parallel to the Earth’s

surface at depth, because gravity is a major contributor to stress state.

Andersons’s theory of faulting uses these relations to predict basic fault

geometries:

(a) normal faulting (b) thrust faulting (c) strike-slip

faulting

σ1 vertical σ1 horizontal σ1 horizontal

σ2 horizontal σ2 horizontal σ2 vertical

σ3 horizontal σ3 vertical σ3 horizontal

Note this does not always work:

Frictional sliding on pre-existing surface is often easier than initiating

new fractures (pre-existing surface may become oblique-slip

fault)

Fault surface may be rotated by deformation to a different orientation

Anderson’s theory is for isotropic homogeneous crust & stress field


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As an example of fault geometry controlled by local stress field, we now

look at:

Listric faults – concave-up faults, steep near the surface but shallow at

depth.

In part due to curving stress field, especially curving trajectories

ofσ1 andσ3. (Recall the sliding block from Topic 3).O/H

8.6 Fluids and faulting

Recall indurated gouge, vein-filled breccia, gouge often altered to clay

minerals. Fluids are often associated with fault zones.

Seismic pumping - occurs when movement on a fault creates space (of

course it doesn’t really create space, but the pressure within the fault zone

is reduced temporarily). This pressure gradient drives groundwater into

the fault zone.

Fluids affect the shear stress at which faulting occurs in three ways:

(i) Alteration - clay minerals are weak, with low shear strength

(ii) Hydrolytic weakening of silicate minerals (without

transformation to clays)

(iii)

Pore pressure (Pfluid) decreases effective normal stress, and

decreases the shear stress at which brittle failure occurs (topic

6).

This last point is very important as it explains how large thrust sheets can

move intact – to overcome the friction, large horizontal stresses are required

(large enough that one would expect the thrust sheet to break before it

slides).

[Analogy: try pushing a large piece of carpet at one end and see how it just

wrinkles next to where you push, instead of sliding the whole carpet]


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The Hubbert-and-Rubey hypothesis is that if Pfluid in the detachment one is

near to lithostatic pressure, effective normal stress across the contact

approaches zero, and the shear stress required for sliding becomes smaller

than that required for internal deformation of the sheet.

So, what magnitude of stress is required for faulting to occur?

Recall effects of confining pressure, fluid pressure, lithology, strain rate,

temperature, pre-existing fractures…

Also depends on type of fault (largerσd required for thrust than normal

fault)

Estimates from shear heating (σS.u = Ee + Es + Q) where u is amount of

slip, Ee is earthquake energy, Es is energy used making new fractures, and

Q is heat generated. But San Andreas Fault shows little or no heat flow

anomaly…

Estimates from stress drop (minimum estimate forσS range from 0.1 to 150

MPa (average c. 3 MPa).

Estimates from laboratory studies are around 20 to 100 MPa.


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8.7 Fault systems

Faults are manifestation of fundamental way that stress causes deformation

in the upper crust. They do not usually occur in isolation, but rather in faultsystems, also called fault arrays.

Fault systems can be classified according to geometric and tectonic features.

I) Geometric classification:

Groups of faults may be a parallel array, anastomosing array, en echelon

array, relay array, conjugate array, or random array:

Usually most faults in a parallel array will dip in the same direction.

Subsidiary faults parallel to major faults are called synthetic. Sometimes a

fault dips in the opposite direction, in which case it is anantithetic fault:


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II) Tectonic classification:

(i) Normal fault system: these often form in rifts, where the

lithosphere is extending, along passive margins, and along mid-

ocean ridges. Usually relay or parallel arrays of listric or planar

faults. Movement on either usually results in rotation / tilting of

overlying fault blocks. Two different styles, (a) horst-and-graben

(planar faults) and (b) half-graben systems (listric faults):

(ii) Thrust fault system: accommodate regional shortening, e.g.

margins of convergent plate boundaries and collisional orogens.

Usually relay or parallel arrays, often combined with folding,

resulting infold-thrust belts. Flats follow weak horizons,

and ramps cut across rigid beds. Imbricate

fan or duplex structure:

(iii) Strike-slip fault system: occur at transform plate boundaries,

within plates, and as components of collisional or convergent

orogens. Typically splay into many separate faults near the surface

(flower structure). In a transpressional or transtensional setting,

these are either “positive” or “negative” flower structures.


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Reactivated fault systems: We know that often a pre-existing fault

surface will be reactivated in preference to initiating a new fracture

plane. There is no reason why an old fault plane should be reactivated

with the same sense of slip as when it first formed, since there may be

hundreds of millions of years in between for the regional stress field to

change.

Fault inversion is the reversal of displacement on a fault during

reactivation

e.g. normal faults formed during rifting of a continental margin may be

reactivated as a thrust fault if that margin is later caught in a continental

collision.

e.g. border faults of a small half-graben basin may be reactivated as

reverse or oblique-slip faults if the region is later subjected to

compression.

III) A final distinction that is often made is between thin-skinned and thick-

skinned fault systems (but the two are often laterally related):

Thin-skinned fault system: faults occur only at shallow depths in the

sedimentary cover, separated from unfaulted deeper sediments and

basement by a detachment

Thick-skinned fault system: faults involve basement as well as cover.

8.8 Review / summary

(i) Fault = shear displacement (some movement parallel to plane of

contact)

(ii) Normal, reverse, strike-slip, oblique

(iii)

Rock types: gouge and breccia, cataclasite, mylonite (plastic)

(iv)

Slip indicators, slickensides, fibres, etc.


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(v) Fault-related folds

(vi)

Anderson’s theory of faulting (σ1 andσ3 orientation)

(vii) Fluids and faulting (seismic pumping; how do large thrust sheets

move?)

(viii)

Fault systems and fault reactivation

http://ijolite.geology.uiuc.edu/07fallclass/geo411/Faults/T8faults.htm

Free and forced folding

Fault-bend fold and fault-propagation fold are two important styles of ‘forced folding’ in which the

fold shape is controlled by thrust fault (after John Suppe, Principles of Structural Geology, !"#$%

In free folding, rock layers are free to exert their mechanical properties on the

development and shape of the folded stack and thus layer-parallel strain dominantly

takes place. Buckling discussed above typically produces free folds.

In forced folding, the shape and geometric features of the folded stack are ‘forced on’

the layers usually by a fault that is the primary structure. In this case, to uote


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!merican geologist "eorge #avis in his textbook $tructural "eology %&''(), the rock

layers *+ust go along for a ride. otable examples of forced folding include drape

fold %folding of sediments overlying a high-angle basement fault), faultbend fold

%bending and slip of an anticlinal fold as a thrust block overrides the footall block

along a ramp), and fault propagation fold %asymmetric bending of rock strata along a

thrust ramp). In these examples, folding depends on faults, and bending is the main

process of folding.

http://www.geoexpro.com/articles/2015/01/folds-and-folding-part-ii
http://www.geoexpro.com/articles/2015/01/folds-and-folding-part-iihttp://www.geoexpro.com/articles/2015/01/folds-and-folding-part-ii

fault bend fold

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