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Crustal Deformation
Structural Geology
Crustal Deformation
Structural Geology
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
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
Stress - Force applied to a given area. Determines the concentration of force.
3 major types of differential stress Compressional stress Tensional stress Shear stress
Stress
“Push-together” stress. Shortens and thickens crust. Associated with mountain building
“Push-together” stress. Shortens and thickens crust. Associated with mountain building
Compressional Stress
“Pull-apart” stress. Thins and stretches crust. Associated with rifting.
Tensional Stress
Slippage of one rock mass past another. Slippage of one rock mass past another.
Shear Stress
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
Factors controlling rock strength and deformation style. Temperature and confining pressure
Low T and P = faulting deformationHigh T and P = folding deformation
Rock type – Mineral composition controls strength.
Time – Stress applied for a long time generates change.
How Rocks Deform
The Formation A mappable rock unit.
The Formation A mappable rock unit.
Folds Rocks are bent by crustal deformation into a
series of wave-like undulations called folds. Most folds result from compressional stresses
which 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 stresses which shorten and thicken the crust.
Stephen Marshak
Anticline – Upfolded or arched rock layers. Syncline – Downfolds or rock troughs. (Think
“sink”)
Common Types of Folds
AnticlineAnticline
SynclineSyncline
Anticlines and Synclines are common in fold and thrust belts related to mountain belts.
Monoclines – Large, step-like folds in otherwise horizontal sedimentary strata.
Domes -Upwarped circular or slightly elongated structure. Oldest rocks in center, younger rocks outside.
Basins – Downwarped circular or slightly elongated structure. Youngest rocks are found near the center, oldest rocks on the flanks.
Common Types of Folds
FaultsFaults
Breaks in rock that exhibit offset. Exist at a variety of scales. Sudden movements along faults are the cause
of most earthquakes. Classified by movement…
HorizontalVertical
Breaks in rock that exhibit offset. Exist at a variety of scales. Sudden movements along faults are the cause
of most earthquakes. Classified by movement…
HorizontalVertical
Faults
Dip-slip faults – Motion is up & down Strike-slip faults – Motion is left or right
Fault Types
May produce long, low cliffs called fault scarps.
Dip Slip Faults
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
Two dominant types Normal fault Reverse Fault
Types of Dip-Slip Faults
Normal fault Hanging wall moves down relative to the
footwall. Accommodate lengthening or extension of the
crust.
Types of Dip-Slip Faults
Larger scale normal faults are associated with fault-block mountains (Basin and Range of Nevada).
Normal Faults
Fig. 11.17b
W. W. Norton
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
Types of Dip-Slip Faults
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
Types of Dip-Slip Faults
U.S. Geological Survey
Displacement is horizontal 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 on the opposite side of the fault moves to the left
Strike-Slip Faults
Strike-slip faultTransform fault– Large strike-slip fault that cuts through the
lithosphere
– Accommodates motion between two large crustal plates
Strike-Slip Faults
Joints are a very common
rock structure. They are fractures with no
offset. Occur in parallel groups.
Joints
The End