Introduction to faults

Faults are shear fractures that are meters or more in length; when they are smaller

(centimetres) they are called shear fractures. Faults are very important structural

elements that affect the topography of the Earth. It is very common that large

faults actually consist of a network of smaller faults.

An inclined fault divides two separate blocks that are called the hanging wall and

the footwall (Fig. 1). The hanging wall is the bottom surface of the upper fault

block, and the footwall is the top surface of the lower fault block. Similarly, the

hanging wall block is the fault block above the fault; and the footwall block is

the fault block below the fault. These terms are of course not applicable to a vertical

fault. In that case geographic directions (north, south, etc.) are used.

High-angle faults are faults with a dip of more than 45 degrees; if it is less than

45 degrees, it is called a low-angle fault. Faults are furthermore divided in three

categories based on the orientation of the relative displacement (called slip): on

dip-slip faults, the slip is approximately parallel to the dip of the fault surface. This

goes for both normal and thrust faults; on strike-slip faults, the slip is more or

less horizontal and parallel to the strike of the fault surface; and on oblique-slip

faults, the slip is inclined obliquely on the fault surface (a combination of both dip-

slip and strike-slip faults). The dip-slip direction, which is a diagonal movement,

may be described in terms of the vertical displacement vector (throw) and the

horizontal displacement vector (heave).

Faults are also divided in terms of relative movement: on normal faults (Fig. 1),

the hanging wall block moves down relative to the footwall block; on thrust faults,

the hanging wall block moves up relative to the footwall block. High-angle thrust

faults are often called reverse faults; vertical faults cannot be described as normal

or thrust faults, so we simply describe if the left or right side has moved down;

strike-slip faults are right-lateral (dextral; Fig. 1) if the fault block across the fault

as seen from the observer’s point of view moved to the right; when the block

moved to the left, a strike-slip fault is called left-lateral or sinistral; oblique faults

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may be described according to the nature of the strike-slip and dip-slip components

(Fig. 1).

Fig. 1. Fault types.

Recognition of faults

Features intrinsic to faults

Faults can often be recognised by the characteristic textures and structures that

develop in rocks as a result of shearing. These textures are typically a function of

the depth of faulting, as temperature and pressure increase with depth (Fig. 2). Two

main rock types are formed: cataclastic rocks and mylonites. Cataclastic rocks

occur at depths less than 10 to 15 km. They occur in fault zones ranging from a few

mm to several km thicknesses. In general, the greater the thickness and the

smaller the grain size, the greater the amount of vertical displacement. Cataclastic

rocks are fractured or ground into powder and individual rock fragments are

generally angular and fractured. Cataclastic rocks usually lack an internal planar or

linear structure. Incohesive cataclasts (fragile, breakable by hand) are usually

formed by faulting at 1 to 4 km depth, and cohesive clasts at depths up to 15 km

(Fig. 2). Cataclastic rocks are divided into the breccia, gouge, cataclasites and

pseudotachylite series (Table 1). The breccia, gouge and cataclasites division is

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based on clast size and matrix content. Breccia is a rock consisting of rock

fragments in a finer matrix (Fig. 3). Gouge is a very fine type of breccia that looks

like a fine, white powder. Cataclasites are usually cohesive rocks. Pseudotachylite is

a dark-coloured, massive, very fine-grained or glassy rock that is often found in the

matrix of cataclastic breccia or injected as veins in the surrounding rock in the fault

zone (Fig. 4). It formed by melting of rock under relatively high temperatures; the

molten rock was then injected in small fractures due to the high pressure and

recrystallised into a dark, glass-like material.

Fig. 2.Faults and associated rock structures.

Mylonitic rocks are formed at depths

exceeding 10 to 15 km and generally

occur in shear zones of several mm to

several m in thickness. They are very

fine-grained rocks that formed under

ductile deformation. They show a strong

planar and linear internal structure

called foliation or lineation that trends

(sub)parallel to the fault zone (Fig. 5).

Mylonites form as the result of recrystallinisation of mineral grains during rapid

ductile deformation. They have polygonal grain boundaries and are in this sense

different from the angular breccia series from cataclastic rocks.

Fig. 3. Cataclastic breccia indicating a fault zone.

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Table 1. Subdivision of cataclastic and mylonitic rocks.

Fig. 4. Pseudotachylite.

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Fig. 5. Mylonite.

Slickensides are smooth, polished surfaces on the fault plane that are the direct

result of shearing of the fault planes (Fig. 6). They are very important features in

determining the presence and direction of fault movement. Slickensides often show

linear features parallel to the fault called slickenlines or slickenside lineations,

or striations. These linear features may be recognised the presence of ridges and

grooves: the direction in which the grooves step down is the direction of fault

propagation (Fig. 7). Another important characteristic are mineral fibers (also called

slickenfibers). They are long, single-crystal mineral fibers that grow parallel to the

direction of fault displacement.

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Fig. 6. Slickensides in clay with striations.

Fig. 7. Slickensides with grooves showing fault propagation (arrows).

Faults that develop at shallow depths increase in volume due to the formation and

accumulation of fractures, which leads to water accumulation. This causes

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deposition of secondary minerals in veins in the fault zone such as calcite and silica-

bearing minerals such as quartz, opal or chalcedony.

Effects of faulting on geologic or stratigraphic units

Faulting usually results in an offset of the stratigraphic units in the two faulted

blocks (Fig. 8). Care should however been taken. A stratigraphic discontinuity can

also be the result of other factors. It may be the result of an intrusive contact, or an

unconformity as a result of erosional channels, other types of depositional contacts

or even local soil formation. The presence of a horse or fault slice along a

discontinuity is a clear indication of a fault. A horse is a volume of rock which is

surrounded on all sides by faults. It is a broken-off piece of rock from either the

hanging wall or the footwall and displaced by faults.

Fig. 8. Stratigraphic offset by faulting.

Repetition of certain strata in borehole logs may be indicative for the presence of a

fault (Fig. 9), but only if the undisturbed stratigraphy is known.

Sometimes a fault is not clearly visible in an outcrop, but local folding of the strata

may indicate the presence of a fault. Drag folds occur where sedimentary layers

trend at high angles to the fault plane (Fig. 10A and B). They are less likely to form

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when the fault trends at a low angle to the sedimentary layers (Fig. 10C). Drag

folds are often found in conjunction with thrust faults. Rollover anticlines are

often formed in conjunction with normal faults (Fig. 10D).

Fig. 9. Borehole logs indicating faulting and non-faulted sections.

Fig. 10. A) and B) drag folds; C) Low-angle fault without drag lines; D) Rollover anticline.

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Physiographic criteria for faulting

There are many criteria on landscape-scale that are indicative for fault activity.

Ongoing research in this field has led to a scientific subdiscipline called “tectonic

geomorphology”. Below a number of geomorphic elements are discussed:

Fault scarps occur where either normal or thrust faulting has generated a ridge in

the landscape (Fig. 11). Erosion of the ridge often leads to the deposition of alluvial

fans and facets or faceted spurs (Fig. 11). Fault scarps should not be confused

with marine terraces, which form due to a combination of sea level changes and

tectonic uplift, and should not be confused with river terraces, which occur in river

valleys and are the resultant of climate change and tectonic uplift.

Fig. 11. A) Fault scarp; B) alluvial fans and faceted spurs.

Unequal river terrace sequences on opposite sides of a river valley may be

indicative of an active fault underneath the current river bed causing unequal uplift

on both valley sides (Fig. 12). The formation of tectonic basins is a clear indication

for fault activity (Fig. 12). For instance, normal faulting may lead to the formation of

a horst and graben landscape, where the original surfaces on opposite sides of

the valley are the horsts, and the downthrown block forms the valley or the graben.

Normal and thrust faulting may lead to convex reaches (knickpoints) in river

profiles. Rivers tend to form smooth, concave profiles over time, but the presence of

a fault may lead to locally elevated reaches.

Strike-slip activity may lead to offset stream networks and local depressions

(sagponds) that fill up with water through the sheared and fractured rock area.

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Fig. 12. A) Unequal river terrace distribution on both river valley sides, indicating differential uplift on both valley sides; B) Tectonic basin and unequal river terraces on both valley sides, due to block subsidence and unequal uplift on both valley sides.

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Determination of fault displacement

Complete determination of the displacement on a fault requires knowledge of the

magnitude and direction of its displacement. The magnitude of fault displacement

is often difficult to establish in an absolute sense especially when the fault is not

located in sedimentary rocks that help to determine the amount of displacement by

their stratigraphy. In those cases intrusions, quartz veins, or even older fractures

that are found on both sides of the fault plane may be of use to establish the

absolute amount of displacement. When this is not possible often local reference

levels, such as sea level, or the relative fault motion between blocks is used to

establish the relative amount of displacement. In the case of normal and thrust

faults, it is possible that both blocks show displacement, but one block a bit more

than the other. It is possible to partially establish the fault displacement by a

number of small-scale structures that are found in the fault planes. The already

mentioned slickensides with their striations are a good tool.

Another excellent indicator is secondary fractures that form perpendicular to the

shear plain (see Fig. 1.2 Fractures Chapter). Those fractures may either be

extension or shear fractures (see Fig. 1 Fractures Chapter for extension and shear

fractures). Secondary extension fractures are usually not striated and may be filled

with secondary minerals. Secondary shear fractures are usually striated. Secondary

fractures may both form steps that are comparable to the ridges on the slickensides

that are used to determine the direction, but contrary to the slickensides, they

cannot be used to determine the direction of fault propagation because they may

form in both the downstream direction (congruous steps) and upstream direction

(incongruous steps) on the fault plane. There are however other ways to use

secondary fractures to determine the direction of fault displacement:

a) extension fractures cut in the fault surface at an angle of 30 to 50 degrees in the

direction of movement (Fig. 13A);

b) sometimes those extension fractures are crescent-shaped. The hollow part of the

fracture is in the direction of fault propagation (slip, see Fig. 13B);

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c) striated secondary shear fractures may cut into the wall of the main fault plane

and are oriented towards the direction of fault movement (Fig. 13C). Sometimes

incongruent steps are formed that are either linear-shaped (Fig. 13C) or lunar-

shaped (Fig. 13D);

d) sometimes the secondary striated fractures may occur together with the

extension fractures, leading to a combination of incongruous steps and extension

fractures that cut deep into the fault wall indicating direction of fault displacement

(Fig. 13E). Or there may be very little evidence of fracturing at all (Fig. 13F).

Fig. 13. Secondary fracturing

Gouging of the fault surface is another way to deduct the movement of brittle fault

displacement (Fig.14). Gouging occurs by fragments of hard rock or minerals on the

wall of the relatively downward moving block that cut into the fault plane of the

other block. Depending on how the rock fragment cuts into the fault plane of the

opposite block, and whether the rock fragment breaks of, various patterns are

formed (Fig. 14). These patterns can only be used if the point on the fault plane

where the gouging began (the pluck face) can be located (Fig. 14).

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Fig. 14. Gouging

Ductile shear zones may contain a number of small-scale structures that indicate

the shear sense. Platy minerals may become aligned to form a foliation. The

alignment is about at 45 degrees to the fault plane close to the plane, and less

inclined in the centre of deformation (Fig. 15A). Another structure is formed by

sheat folds that form parallel to the direction of slip on the ductile fault (Fig. 15B);

sometimes porphyroclasts are formed. Those are relict crystals that survived the

shearing and reduction in grain size from the original rock. They are very common

in mylonites. They grow “tails” of very fine mineral grains during ductile shearing

(Fig. 5), indicating the shear direction (Fig. 15CD). Porphyroblasts are mineral

grains such as garnet and staurolite that grow to relatively large size in a rock

during metamorphism. During ductile shear they do not deform, but they rotate like

a wheel and during this process they enclose adjacent minerals. In doing so, the

enclosed minerals attain a spiral-like form, which can be used to indicate the sense

of shear in a rock (Fig. 15EF).

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Fig. 15. Shear sense criteria in ductile shear zones

Partial determination of displacement from large-scale structures.

Sometimes large-scale landscape features can be used to determine the amount of

fault displacement. The most well-known method is measuring the amount of

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horizontal displacement due to strike-slip faulting. Off-set river systems are often

used, but also the occurrence of certain types of sediment or rocks can be used. See

for instance the displacement along the San Andreas fault in California, which was

determined on basis of off-set marine terrace sediments (Fig. 16).

Fig. 16. San Andreas fault displaced marine sediments

Nonunique constraints on fault displacement

Frequently, the principal evidence for a fault consists of the offset of a planar

structure, typically sedimentary bedding. This offset alone can never define the

displacement on the fault, regardless of the appearance of the outcrop pattern. Fig.

17 explains why. The true displacement vector (D) defines the actual movement

on the fault plane and incorporates both a vertical displacement vector n with

respect to the fault, and a horizontal vector p (strike-slip component). In an outcrop

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–assuming that it is oriented exactly parallel to the strike of the fault– often the

vertical component can be observed, but it is impossible to tell if there has been any

horizontal movement towards or away from us. Fig. 17. clearly shows that for a

given vertical offset as defined by the vector n, different horizontal offsets p still

lead to the same observed offset of the footwall and the hanging wall blocks. Thus,

we cannot talk about the slip or displacement on the fault because we cannot

determine it. We speak instead of the separation, which is the distance measured

in a specified direction between the same planar feature on opposite sides of the

fault. The separation enables us to determine only the component of displacement

normal (vector n) to the cutoff line.

Fault geometry

Smaller faults are much more

common than larger faults. In fact, it

has been shown that the relation

between fault length of a fault system

and the number of faults can be

scaled using a power-law function.

When observing a large dataset of

faults, one can therefore expect to

find a small number of large faults,

but a large number of small faults.

Similarly, the measured total

displacement along a fault is often an

accumulation of many small

displacements over time.

Fault displacement is largest along the

central part of the fault, and

approaches zero displacement at its

tips. For that reason there is no 1:1 scaling relationship between fault length and

fault displacement. Typically a fault of several meters will only show several

Fig. 17. Nonunique indicators of fault displacement

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centimeters of displacement; a fault of several kilometers will shows several

hundreds of meters of displacement. A rule of thumb indicates that fault

displacement is about 3% of the total fault length. The termination line end of a

fault is a tip line, where the fault displacement has decreased to the extent that it

can be accommodated by coherent deformation distributed through the solid rock.

It should be realised that faults and their tip lines may look like lines in an outcrop.

In reality however, they are three-dimensional, often elliptical features that form a

3-D surface (the fault plain; see Fig. 17).

If one fault terminates against another of the same age, the intersection line of the

faults must be parallel to the displacement direction on both faults (Fig. 18A). If a

younger fault terminates against an older fault, the displacement vector on the

younger fault must parallel the termination line (Fig. 18B). If an older fault is cut

and offset by a younger fault the termination line of the older fault against the

younger has no relationship to the slip direction on either fault (Fig. 18C).

Fig. 18. Fault termination lines and fault traces

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A fault in the subsurface that reaches the Earth surface is called a fault trace; one

that does not reach the Earth’s surface is called a blind fault. Sometimes a fault

splits into two fault surfaces when it intersects with another fault surface. This is

called a branch line. In Fig. 19 the horse is bounded on all edges by branch lines.

Faults of all types commonly die out in a set of splay faults, which are smaller

subsidiary faults that branch off from the main fault. Where splay faults branch off

from the main fault at fairly regular intervals and have comparable geometries, they

form an imbricate fan, which can be either extensional or contractional (reverse).

Fig. 19. Splay faults and a horse

Fault surfaces are not necessarily planar. It is quite common for the attitude of the

fault to change down dip or along strike. Sometimes the dip of fault decreases with

depth, so that the fault overall has a concave-up shave. These types of faults are

called listric faults. If the dip and/or strike of a fault abruptly changes, the location

of the change is called a fault bend. Some faults run parallel to bedding, called

flats, and some cut across bedding, called ramps. If the fault has not been folded

subsequent to its formation, flats are (sub)horizontal, whereas ramps have dips of

about 30 to 45 degrees.

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Fault bends (or steps) along strike-slip faults cause changes in the strike of the

fault. Locations where the bend is oriented such that blocks on opposite sides of the

fault are squeezed together are restraining bends (Fig. 20), whereas locations

where the bend is oriented such that blocks on opposite sides of the fault pull away

from each other are releasing bends (Fig. 20).

Fig. 20. Restraining and releasing bends

Where movement across a segment of a strike-slip fault results in some

compression, transpression is occurring. Where movement results in extension,

transtension is occurring.

If a fault is segmented but the segments are not connected by a distinct ramp

structure, the structure is a step-over.

Because individual faults die out along the fault system, the total displacement

across the system is maintained by the transfer of slip from one end of a fault to the

adjacent parallel fault. Such zone is called a transfer zone or relay zone. If this

zone consists of one fault, it is called a transfer fault or relay fault.

Where faulting occurs, a damage zone is created as well. This is an area

surrounding the fault, or extending from the fault tips, where fracturing occurs. This

area can be of variable extent, but in some cases it can be considerably wider than

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the fault zone itself. Characteristics of the secondary fracturing at a fault tip depend

in part on the orientation of the displacement vector relative to the tip line. Damage

zones that develop at mode II (shearing; strike-slip) tip lines may develop

extension fractures,

splay faults or branch

faults. Examples of such

secondary fracture

zones include wing

cracks, horsetail

splays, synthetic

branch faults and

antithetic faults (Fig.

21). Damage zones also

develop in step-overs,

where one strike-slip

fault ends and an

adjacent parallel strike-

slip fault begins.

Depending on the local

shear sense, either

extenstional steps (e.g.

pull-apart basins) or

contractional steps (e.g.

rotated blocks) form

(Fig. 21).

Fig. 21. Secondary fracture zones

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