structure geology[1]

8/9/2019 Structure Geology[1]

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Structural GeologyStructural Geology


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Structural Geology Tectonic collision deforms crustal rocks

producing geologic structures.

Folds

Faults

Joints and Fractures

Tectonic collision deforms crustal rocks

producing geologic structures.

Folds

Faults

Joints and Fractures


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Deformation

All changes in the original location,

orientation or form of a crustal rock body.

Deformation common

at plate margins.

Deformation concepts

Force

Stress

Strain


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Force Force Mass x acceleration (F = ma)

The action that puts stationary objects in

motion or

Changes the motion of moving objects.


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Stress - Force applied to a given area.

Determines the concentration of force.

Differential Stress Unequal in different

directions.

3 major types of differential stress

Compressional stress

Tensional stress

Shear stress

Stress


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Push-together stress.

Shortens and thickens crust.

Associated with orogenesis (mtn. building).

Push-together stress.

Shortens and thickens crust.

Associated with orogenesis (mtn. building).

Compressional Stress


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Pull-apart stress.

Thins and stretches crust.

Associated with rifting.

Tensional Stress

Stephen Marshak


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Slippage of one rock mass past another.

In shallow crust, shear is often

accommodated by bedding planes.

Slippage of one rock mass past another.

In shallow crust, shear is often

accommodated by bedding planes.

Shear Stress


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Changes in the shape or size of a rock body

caused by stress.

Strain occurs when stresses exceed rock

strength.

Strained rocks deform by folding, flowing, or

fracturing.

Strain


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Factors controlling rock strength anddeformation style.

Temperature and confining pressure

Low T and P = brittle deformationHigh T and P = ductile deformation

Rock type Mineral composition controlsstrength.

Time Stress applied for a long time generateschange.

How Rocks Deform


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Mapping Geologic Structures Geologists describe and interpret rock structures.

Structure usually determined from a limited number of

outcrops. Mapping is aided by advances in aerial photography,

satellite imagery and Global Positioning Systems (GPS).

The most common and useful technique for geological

mapping remains.

FIELD WORK !!


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The FormationA mappable rock unit.

The FormationA mappable rock unit.


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Describing and mapping the orientation of a

geologic structure or fault surface involves

determining

Strike (trend)

Dip (inclination)

Mapping Geologic Structures


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Strike (trend)

The compass direction of the line produced by the

intersection of an inclined rock layer or fault witha horizontal plane.

Generally expressed an an angle relative to north.

N37E

N12W



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Dip (inclination)

The angle of inclination of the surface of a rockunit or fault measured from a horizontal plane.

Includes both an angle of inclination and a

direction toward which the rock is inclined.

82SE17SW



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Folds Rocks are bent by crustal deformation into a

series of wave-like undulations called folds.

Most folds result from compressional stresseswhich shorten and thicken the crust.

Rocks are bent by crustal deformation into a

series of wave-like undulations called folds.

Most folds result from compressional stresseswhich shorten and thicken the crust.

Stephen Marshak


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Parts of a fold

Limbs The two sides of a fold.

Fold axis or hinge line A line connecting points

of maximum curvature along a fold.

Axial plane An imaginary surface that divides a

fold symmetrically.

Parts of a fold

Limbs The two sides of a fold.

Fold axis or hinge line A line connecting points

of maximum curvature along a fold.

Axial plane An imaginary surface that divides a

fold symmetrically.

Characteristics of Folds


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Anticline Upfolded or arched rock layers.

SynclineDownfolds or rock troughs. (Thinksink)

Depending on their orientation, anticlines andsynclines can be described as

Symmetrical

Asymmetrical

Recumbent (an overturned fold)

Plunging

Common Types of Folds


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AnticlineAnticline


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SynclineSyncline


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Anticlines and Synclines are common in fold

and thrust belts related to mountain belts.


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MonoclinesLarge, step-like folds in otherwise

horizontal sedimentary strata.

Domes -Upwarped circular or slightly elongated

structure. Oldest rocks in center, younger rocks

outside.

BasinsDownwarped circular or slightly elongatedstructure. Youngest rocks are found near the

center, oldest rocks on the flanks.

Common Types of Folds


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FaultsFaults


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Breaks in rock that exhibit offset.

Exist at a variety of scales.

Sudden movements along faults are the causeof most earthquakes.

Classified by movement

HorizontalVerticalOblique

Breaks in rock that exhibit offset.

Exist at a variety of scales.

Sudden movements along faults are the causeof most earthquakes.

Classified by movement

HorizontalVerticalOblique

Faults


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Faults grind rocks to create fault gouge.

Walls of a fault bear evidence of this grinding

as slickensides.

Slicks reveal

fault direction.

Faults grind rocks to create fault gouge.

Walls of a fault bear evidence of this grinding

as slickensides.

Slicks reveal

fault direction.

Faults


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Dip-slip faults Motion is parallel to fault

dip.

Strike-slip faults Motion is parallel to fault

strike.

Fault Types


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Footwall (rock mass

below the fault)

Footwall (rock mass

below the fault)

Dip Slip Faults

Hanging wall

(rock mass

above the fault)

Hanging wall

(rock mass

above the fault)

Fault blocks classified as Fault blocks classified as


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Two dominant types

Normal fault

Reverse Fault

Thrust (a low angle reverse fault)

Types ofDip-Slip Faults


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Normal fault

Hanging wall moves down relative to the

footwall. Accommodate lengthening or extension of the

crust.

Exhibit a variety of scales.



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Larger scale normal faults are associated

with fault-block mountains (Basin and Range

ofNevada).

Normal fault bounded valleys are called

grabens (Rhine graben).

Normal fault bounded ridges are calledhorsts.

Normal Faults


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Fig. 11.17b

W. W. Norton


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Reverse faults

Hanging wall block moves up relative to the

footwall block Reverse faults have dips greater than 45o and

thrust faults have dips less then 45o

Accommodate shortening of the crust

Strong compressional forces



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Thrust faults - A special case of reverse fault.

Hanging wall block moves up relative to the

footwall block Thrust faults are characterized by a low dip angle

(less then 45o).

Accommodate shortening of the crust

Strong compressional forces



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Fig. 11.17a

W. W. Norton


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U.S. Geological Survey


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Dominant displacement is horizontal and

parallel to the strike of the fault

Types of strike-slip faults

Right-lateral as you face the fault, the block on

the opposite side of the fault moves to the right

Left-lateral as you face the fault, the block onthe opposite side of the fault moves to the left

Strike-Slip Faults


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Strike-slip fault

Transform fault

Large strike-slip fault that cuts through the

lithosphere

Accommodates motion between two large

crustal plates

Strike-Slip Faults


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Joints are a very common

rock structure.

They are fractures with no

offset.

Result from tectonic

stresses on rock mass.

Occur in parallel groups.

Joints


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Chemical weathering tends to be

concentrated along joints

Many important mineral deposits are

emplaced along joint systems

Highly jointed rocks often represent a risk to

construction projects

Chemical weathering tends to be

concentrated along joints

Many important mineral deposits are

emplaced along joint systems

Highly jointed rocks often represent a risk to

construction projects

Significance of Joints


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structure geology[1]

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