structural geology faults & faulting reporters: babala, arjaylyn b. putian, janice d. samaniego,...

Structural Geology Faults & faulting

Reporters:Babala, Arjaylyn B.

Putian, Janice D.Samaniego, Irvin B

Definitions and Characteristics

A fault consists of a zone along which slip (shear displacement) has occurred. The zone is separated from in-tact rock by two surfaces. The zone can vary in width from less than a mm to more than a km.


Fault Rock is the material in a fault zone it consists of:gouge Breccia CataclasitePseudotachylite


Gouge is mostly clay sized, poorly consolidated material pulverized by fault slip.

Breccia consists of angular, poorly sorted clasts up to boulder size that have been broken up by fault slip.


Cataclasite is a fine grained poorly sorted breccia that is usually well lithified.


Pseudotachylite is also glass, but formed by melting rock due the frictional heat along a fault .

Normally forms at depth where rock is already heated.

Classification of Faults

Slip and Separation: Slip is a displacement vector that connects two points on

either side of the fault zone that were connected before faulting. A bedding surface alone can never be used to determine slip.

Separation is an apparent displacement parallel to the strike and/or parallel to the dip. It is not the slip, but may be a component of the slip.

Strike separation is not the same as strike slip (Next Slide)


Examples of Slip and Separation of bedding surfaces:


Faults are classified based on their orientation relative to the surface (strike and dip) and sense of slip – i.e. relative displacement of the fault blocks.

There are four general categories of faults:Strike-slipDip-slipOblique-slipRotational-slip


Strike-slip Faults

Sense of slip: Left-Lateral Right-Lateral


High angle Dip-slip Faults (>50o dip)

Hanging Wall

Foot Wall

Sense of slip: Normal (extension) Reverse (shortening)


Low angle Dip-slip Faults (<50o dip)

Hanging Wall

Foot Wall

Sense of slip: Detachment

(Extension) Thrust (shortening)


Oblique slip

Sense of slip: Normal/left slip Normal/right slip

(Not shown) Reverse/left slip Reverse/right slip

(not shown)


Rotational slip

Looking across fault: Clockwise Counter clockwise

Slip DirectionDetermination of slip

Slip direction can frequently be determined from slickenlines and fiber lineations.

Slip direction

Slickenlines give only the direction of slip. Sometime the sense of slip can be determined as well. For example, the clast causing the groove is present indicating the overlying fault block moved upward.

Slip Direction

Fibrous mineral growths may also give the sense of slip. The fibers tend to be stepped because they grow from irregularities on the fault surface. The steps indicate sense of motion of the block above them.


What is the sense of slip of the fault blocks in the picture

Fault BendsWhen ever a fault has a bend it will commonly produce folds

in layered sediment.

Dip slip fault bends produce fault bend folds

Strike slip bends produce pop-up folds or sags

Thrust Fault Bends

Thrust faults usually follow bedding plane surfaces and then bend up (ramp) to another bedding plane producing a stair-step geometry.

A ramp generates a fault-bend fold which may develop a flat top if there is enough displacement.

Thrust Fault Bends

Because of the ramp-flat geometry, thrust belts can be very complicated.

A ramp cuts off layering producing a hanging wall cutoff and a footwall cutoff.

Likewise there are both hanging wall flats and footwall flats.

Normal Fault Bends

Listric normal faults also produce fault bend folds. These have been called a variety of names:

Rollover folds

Reverse Drag Folds

Strike slip Fault Bends

Strike slip fault bends have two types:

Releasing bends produce extensional structures: sags (basins) and normal faults

Restraining bends produce shortening structures: uplifts, folds and thrust faults

Strike slip Fault Bends

The strike slip fault at left has a bend. What type of bend is it? That is the sense of displacement on the fault?

Fault Terminations

When faults terminate, folding commonly occurs to accommodate the change in displacement.

Both fault bends and fault terminations can generate folds

Recognition of Faults

A fault scarp is an offset of the topographic surface that is produced by recent movement on a fault.


A fault line scarp is also a step in the topography, but represents differential erosion along an old fault that has rock on one side that erodes faster than that on the other side.

In the photo the fault block on which the person is standing has actually moved up. It appears the opposite because of differential erosion.

Problems with Faulting

There are two problems associated with the origin of faults:

Strength of rock increases with pressure (depth) so that the rocks at depth are too strong for the available stresses in the Earth to break them.

Problems with Faulting

The other problem is Large, nearly

horizontal thrust faults cannot move because the horizontal stress on the hanging wall block would crush the end before it moved the block.

Fault Arrays

Thin Skinned – Fault arrays that are confined to the sedimentary sequence and do not penetrate basement

Thick skinned – Fault arrays that do penetrate basement

Master faults – large regionally significant faults Synthetic faults – smaller faults that parallel the master

fault Antithetic faults – smaller faults that are conjugate to

the master fault.

Fault Arrays

Terms describing fault arrays in map view.

Parallel array Anastomosing En echelon Relay Conjugate Nonsystematic – usually from

reactivation of older faults.

Fault Arrays

Terms describing extensional fault arrays in profile. Listric faults are usually thin skinned (but not always) Horsts & graben Half graben Rift – large regional feature that contains horsts & graben

and/or half graben.

Fault ArraysTerms describing shortening fault arrays in

profile. Listric faults are usually thin skinned

(but not always) Structures occur in fold-thrust belts Flats & ramps Imbricate fans

Fault ArraysMore terms describing shortening fault

arrays in profile.

Duplex & horses (multiple, closely spaced ramps)

Roof Thrust & floor thrust

Fault ArraysStrike slip systems in profile Flower structures – master fault

branches upward forming a stem with petals

Negative & positive flowers.

Faults, Resources & Earthquakes

Faults and earthquakes are usually associated, but aseismic fault creep does occur.

Displacement on large faults is accumulated from smaller, sudden displacements producing earthquakes

An earthquake displacement event does not occur over the whole length of a large fault – so displacement is not only accumulated over time, but also over space – only a small area of a fault will displace at one time with another area displacing at another time.


An Earthquake and fault displacement at one particular zone along a large fault may occur at regular intervals of time – the recurrence interval

The recurrence interval has a large standard deviation – it’s not Old Faithful.

Predicting regions with a high probability of a major earthquake is critical for land use planning, especially for critical installations like nuclear power plants and dams.


Faults are also economically important for resource recovery:

They can form barriers or channels for fluid flow, whether groundwater or petroleum

They are sources and zones for mineralization and ores

They may offset economically valuable strata (coal, petroleum reservoirs) leading to recovery complications.

structural geology faults & faulting reporters: babala, arjaylyn b. putian, janice d. samaniego,...

Documents