structural geology faults & faulting reporters: babala, arjaylyn b. putian, janice d. samaniego,...
TRANSCRIPT
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Structural Geology Faults & faulting
Reporters:Babala, Arjaylyn B.
Putian, Janice D.Samaniego, Irvin B
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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.
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Definitions and Characteristics
Fault Rock is the material in a fault zone it consists of:gouge Breccia CataclasitePseudotachylite
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Definitions and Characteristics
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.
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Definitions and Characteristics
Cataclasite is a fine grained poorly sorted breccia that is usually well lithified.
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Definitions and Characteristics
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.
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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)
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Classification of Faults
Examples of Slip and Separation of bedding surfaces:
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Classification of Faults
Examples of Slip and Separation of bedding surfaces:
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Classification of Faults
Examples of Slip and Separation of bedding surfaces:
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Classification of Faults
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
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Classification of Faults
Strike-slip Faults
Sense of slip: Left-Lateral Right-Lateral
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Classification of Faults
High angle Dip-slip Faults (>50o dip)
Hanging Wall
Foot Wall
Sense of slip: Normal (extension) Reverse (shortening)
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Classification of Faults
Low angle Dip-slip Faults (<50o dip)
Hanging Wall
Foot Wall
Sense of slip: Detachment
(Extension) Thrust (shortening)
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Classification of Faults
Oblique slip
Sense of slip: Normal/left slip Normal/right slip
(Not shown) Reverse/left slip Reverse/right slip
(not shown)
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Classification of Faults
Rotational slip
Looking across fault: Clockwise Counter clockwise
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Slip DirectionDetermination of slip
Slip direction can frequently be determined from slickenlines and fiber lineations.
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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.
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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.
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Definitions and Characteristics
What is the sense of slip of the fault blocks in the picture
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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
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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.
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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.
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Normal Fault Bends
Listric normal faults also produce fault bend folds. These have been called a variety of names:
Rollover folds
Reverse Drag Folds
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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
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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?
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Fault Terminations
When faults terminate, folding commonly occurs to accommodate the change in displacement.
Both fault bends and fault terminations can generate folds
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Recognition of Faults
A fault scarp is an offset of the topographic surface that is produced by recent movement on a fault.
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Definitions and Characteristics
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.
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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.
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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.
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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.
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Fault Arrays
Terms describing fault arrays in map view.
Parallel array Anastomosing En echelon Relay Conjugate Nonsystematic – usually from
reactivation of older faults.
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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.
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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
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Fault ArraysMore terms describing shortening fault
arrays in profile.
Duplex & horses (multiple, closely spaced ramps)
Roof Thrust & floor thrust
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Fault ArraysStrike slip systems in profile Flower structures – master fault
branches upward forming a stem with petals
Negative & positive flowers.
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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.
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Faults, Resources & Earthquakes
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.
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Faults, Resources & Earthquakes
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.