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