1

Structure and Deformation:

Folds and Faults

Deformation Terms• Stress• Strain• Compression stress• Tension stress• Shear stress• Elastic strain• Plastic strain• Ductile strain• Brittle strain• Anticline• Syncline• Monocline

• Strike• Dip• Joint• Fault• Dip-slip fault• Strike-slip fault• Oblique-slip fault• Normal fault• Reverse fault• Thrust fault• Left-lateral• Right-lateral

Structure and Deformation:

• The driving force for geologic deformation is Earth’s internal heat engine:

• Radioactive decay of U, Th, K.

• The rocks get deformed because the continents move.

Stress and Strain:

•Stress is an applied force.•Strain is the resultant deformation.

Stress is an applied force

•Compressional (pushed together)

•Tensional (pulled apart)•Shear (tearing)•Isostatic (hydrostatic)

Strain is deformation• Elastic deformation is temporary.

– Like an elastic band• Plastic deformation is permanent.

– Materials that undergo continuous plastic deformation are said to be ductile.

– Materials that fracture are said to be brittle.

2

Folds and Faults• Stress applied to rocks can

result in ductile behavior (folding) or brittle behavior (faulting).

• Even though rocks are usually brittle, small stresses applied over long times can result in ductile behavior.

Folds• An anticline is a concave-

downward fold. (arch)• A syncline is a concave-

upward fold. (smile)• A monocline is a flexure.

Folds in SedimentsAnticline, Split Mountain

Syncline

Monocline

3

Monocline Recumbent Fold, Yukon

Strike and Dip• Strike and dip are used to

describe the orientation of any planar feature such as bedding in sedimentary rocks.– Strike is the compass direction of the

intersection of the plane with the horizontal.

– Dip is the measured angle between the plane and the horizontal.

Strike and Dip

Rock Fractures• A joint is a fracture in rock on which

there has been no movement.• A fault is a fracture in rock on which

there has been movement.– Dip-slip is movement in the direction of

dip.– Strike-slip is movement in the direction

of strike.– Oblique-slip is movement in both

directions.

Joints in basalt: Giant’s Causeway, Co. Antrim, Ireland

4

Joints in basalt: Grand Canyon Folds and Faults

Dip-Slip• Dip-slip is movement of one

block up or down the plane of the fault.– Normal fault: upper block moves down.– Reverse fault: upper block moves up.– Thrust fault: low angle reverse fault.

Faults

Stress and Fault Movement

•Normal faults are tensional.•Reverse faults and thrustfaults are compressional.

Strike-Slip Faults• A strike-slip fault is the

horizontal movement of the blocks relative to each other.–Left-lateral: the opposite block has moved to the left.

–Right-lateral: The opposite block has moved to the right.•(It doesn’t matter which block you’re on.)

5

San Andreas

Fault(Right-

Lateral)

Unconformities• An unconformity is any break in

the sedimentary geologic record.–A disconformity is a break with no deformation.

–An angular unconformity is a break in which the lower beds have been tilted and eroded before sedimentation resumed.

Angular Unconformity, GCNP

Deformation Terms• Stress• Strain• Compression stress• Tension stress• Shear stress• Elastic strain• Plastic strain• Ductile strain• Brittle strain• Anticline• Syncline• Monocline

• Strike• Dip• Joint• Fault• Dip-slip fault• Strike-slip fault• Oblique-slip fault• Normal fault• Reverse fault• Thrust fault• Left-lateral• Right-lateral

Clicker

• A. Normal Fault• B. Reverse Fault• C. Thrust Fault• D. Strike-slip fault

6

Clicker

• A. Normal Fault• B. Reverse Fault• C. Thrust Fault• D. Strike-slip fault

1

Clicker

• A. Normal Fault• B. Reverse Fault• C. Thrust Fault• D. Strike-slip fault

2

Clicker

• A. Normal Fault• B. Reverse Fault• C. Thrust Fault• D. Strike-slip fault

3

• A. Normal Fault• B. Reverse Fault• C. Thrust Fault• D. Right-lateral Strike-slip Fault• E. Left-lateral Strike-slip Fault

Clicker

• A. Anticline• B. Syncline• C. Monocline• D. Dome

• A. Angular unconfomity• B. Disconformity• C. Thrust Fault• D. Right-lateral Strike-slip Fault• E. Left-lateral Strike-slip Fault

7

Clicker

• A. Anticline• B. Syncline• C. Monocline• D. Reverse fault

1

Clicker

• A. Anticline• B. Syncline• C. Monocline• D. Reverse fault

2

Clicker

• A. Anticline• B. Syncline• C. Monocline• D. Reverse fault

3