Physical Processes in Earth and Environmental Sciences Phần 6 pot

156 Chapter 4

to a lower level, while maintaining the differential

stress(Fig. 4.91). With low differential stresses, even when

the applied stress may be compressive, and fully located in

the field of stress stability, fluid pore pressure can reduce

the effective stress displacing the circle to the tensile field

and producing joints if the condition is satisfied. E3 T0 ˜

4.15 Faults

Εs1

Εs3

Effective

stress Applied

stress

t

sn Εsn

sn

s s1 3

Pf

2uf

= 120º

2uf

= –120º

(a) (b)

Εs1

Εs3 = T0

Effective

stress

Applied

stress

t

sn s3 s1

Pf

2u = 180º

Failure envelope

Stable field

Fig. 4.91 Effect of pore fluid pressure in fracture formation. (a) With high differential stresses Coulomb fractures can be produced when the

Mohr circle moves to the left by pore fluid pressure. (b) With low differential stresses, even when the applied stress may be compressive, and

fully located in the field of stress stability, fluid pore pressure can reduce the effective stress displacing the circle to the tensile field and

producing joints if the condition E3 T0 is satisfied.

Faults are fracture surfaces or zones where several adjacent

fractures form a narrow band along which a significant

shear displacement has taken place (Fig. 4.92a, b).

Although faults are often described as signifying brittle

deformation there is a transition to ductile behavior where

shear zones develop instead. As described in Section 4.14,

shear zones show intense deformation along a narrow band

where cohesive loss takes place on limited, discontinuous

surfaces (Fig. 4.92c). Faults are commonly regarded as large

shear fractures, though the boundary between features

properly regarded as shear fractures or joints is not sharply

established. In any case, although millimeter-scale shear

fractures are called microfaults, faults may range in length of

order several decimeter to hundreds of kilometers: they can

be localized features or of lithospheric scale defining plate

boundaries (Section 5.2). Displacements are generally con￾spicuous (Fig. 4.93), and can vary from 103 m in hand

specimens or outcrop scale to 105 m at regional or global

scales. Faults can be recognized in several ways indicating

shear displacement, either by the presence of scarps in recent

faults (Fig. 4.93a and b), offsets, displacements, gaps, or

overlaps of rock masses with identifiable aspects on them

such as bedding, layering, etc. (Fig. 4.93c).

4.15.1 Nomenclature and orientation

Fault nomenclature is often unclear, coming from widely

different sources. For example, quite a lot of the terms

used to describe faults comes from old mining usage, even

the term fault itself, and the terms are not always well con￾strained. Fault surfaces can be inclined at different angles

and their orientation is given, as any other geological sur￾face, by the strike and dip (Fig. 4.94a). A first division is

made according to the fault dip angle; high-angle faults are

those dipping more than 45 and low-angle faults are those

dipping less than 45. Faults divide rocks in two offset

blocks at either side of the fracture surface. If the fault is

inclined, the block which is resting over the fault surface is

named the hanging wall block (HWB, Fig. 4.95) and its

corresponding surface the hanging wall (HW, Fig. 4.96);

and the underlying block which supports the weight of the

hanging wall is called the footwall block (FWB, Fig. 4.95);

the corresponding fault surface is called the footwall (FW,

Fig. 4.96). If homologous points previous to fracturing at

each side of the fault can be recognized, the reconstruc￾tion of the relative displacement vector or slip can be

reconstructed over the fault surface, both in magnitude

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Flow, deformation, and transport 157

and direction. The relative movement can be either paral￾lel to the fault dip direction (dip-slip faults) or to the fault

strike (strike-slip faults). Dip-slip faults show vertical dis￾placements of blocks whereas in strike-slip faults the

displacement is hori-zontal. In a composite case, the

movement of blocks can be oblique; in these oblique-slip

faults blocks move diagonally along the fault surface,

allowing the separation of a dip-slip component and a

strike-slip component (Fig. 4.94a). The dip–slip component

can be separated into a horizontal part which is called

heave and a vertical part known as throw (Fig. 4.94b).

When faults show a dip-slip movement the block which is

displaced relatively downward is called down-thrown block

(DTB, Fig. 4.94) and the one displaced relatively upward

up-thrown block (UTB, Fig. 4.95). Blocks in strike–slip

faults are generally referred to according to their orienta￾tion (for instance: north block and south block, etc.). In

most cases accurate deduction of movement vectors is not

(a) (b) (c)

Fig. 4.92 (a) Fault, (b) fault zone, and (c) ductile shear zone. Faults are well-defined surfaces produced by brittle deformation. Weak rocks

can be deformed by brittle deformation giving rise to a fault zone with multiple, closely spaced, sometimes interconnected surfaces. Shear

bands develop in the ductile field.

(b) (a)

(c)

Fig. 4.93 Faulting is marked by conspicuous shear displacements, forming distinctive features on fault surfaces like (a) bends and grooves

(b) slickenlines. In (c), originally continuous bedding traces seen in vertical section show up fault displacement (all photos taken in central

Greece.

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158 Chapter 4

possible, and the displacement has to be guessed by the

observation of offset layers. In this case the separation can

be defined as the distance between two homologous

planes or features at either side of the fault, that can be

measured in some specific direction (like the strike and dip

directions of the layer).

Faults initially form to a limited extent and progressively

expand laterally; the offset between blocks increasing with

time. The limit of the fault or fault termination, where

there is no appreciable displacement of blocks is called tip

line (Fig. 4.96). In the case of faults that reach the Earth’s

surface, the intersection line between the fault plane and

the topographic surface is called the fault trace and the point

where the fault trace ends is called the tip point or tip. Blind

faults are those which terminate before reaching the

Earth’s surface and although they can cause surface defor￾mation, like monocline folds, there is no corresponding

surface fault trace (Fig. 4.96) to the fault bounded at the

front and upper ends by termination or tip lines.

Fault planes can have different forms. At the surface

most faults appear as fairly flat surfaces (Fig. 4.97a) but at

depth they can show changes in inclination. Some faults

show several steps: in high angle faults, stepped segments

showing a decrease in dip are called flats (Fig. 4.97b),

whereas in low angle faults, segments showing a sudden

increase in dip are called ramps (Fig. 4.97c). Flats and

ramps give way to characteristic deformation at the topo￾graphic surface; in normal faulting, for instance, bending

of rocks in the part of the hanging wall block located over

a ramp results in a synclinal fold, whereas the resulting

deformation over a flat is an anticlinal fold. Ramps can be

also present in faults with vertical surfaces as in strike–slip

faults, which are called bends, or orientated normal (side￾wall ramp) or oblique (oblique ramps) to the fault strike.

Listric faults are those having a cylindrical or rounded sur￾face, showing a steady dip decrease with depth and ending

in a low-angle or horizontal detachment (Fig. 4.97c).

Detachment faults can be described as low-angle faults,

generally joining a listric fault in the surface that separates

a faulted hanging wall (with a set of imbricate listric or flat￾surface faults) from a nondeformed footwall. Detachments

form at mechanical or lithological contacts where rocks

show different mechanical properties, a decrease in friction

H

T

Fault Surface

DV

N

b d

DV

dc sc

r

(a)

(b)

Fig. 4.94 (a) Total displacement vector (DV) in a fault (general

case). If the movement is oblique, a dip component (dc) and a slip

component (sc) can be defined. DV can be orientated by the rake (r)

over the fault surface, whose orientation is given by the strike ()

and slip () angles. (b) Other components can be separated from

DV: the vertical offset or throw (T) and the horizontal offset or

heave (H).

HWB HWB

FWB

FWB DTB

DTB

UTB

UTB

(a) (b)

Fig. 4.95 Relative position of blocks in a fault: hanging wall block

(HWB); footwall block (FWB); upthrow block (UTB); and

downthrow block (DTB) in (a) a reverse fault and (b) a normal

fault.

tip

A

TL tip

TL

TL

B FW

HW

HW

FW

F. trace

Fig. 4.96 (A) Faults have a limited extent and can cut through the

surface (A) or not (B), in which case they are regarded as blind

faults. Fault terminations (tip and tip lines: TL) are marked in both

cases. FW marks the footwall and HW the hanging wall of the fault

surfaces.

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