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Faults

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Why is this called a fault?

The word "fault" refers to a defect. There may be no greater defect than the scar of the San Andreas Fault across California. Rocks on either side of the fault are estimated to have originated in locations about 350 miles apart! We're still in the arid western United States, but now our searching for geological features is more dangerous!

Fractures

A rock under enough stress will fracture. There may or may not be movement along the fracture.

Joints

If there is no movement on either side of a fracture, the fracture is called a joint . These rocks in Spain show horizontal and vertical jointing. These joints formed when the confining stress was removed from the rocks as shown in ( Figure below ).

Joints in rocks at Joshua Tree National Park, in California.

Faults

If the blocks of rock on one or both sides of a fracture move, the fracture is called a fault ( Figure below ). Sudden motions along faults cause rocks to break and move suddenly. The energy released is an earthquake.

Faults are easy to recognize as they cut across bedded rocks.

How do you know there's a fault in this rock? Try to line up the same type of rock on either side of the lines that cut across them. One side moved relative to the other side, so you know the lines are a fault.

Slip is the distance rocks move along a fault. Slip can be up or down the fault plane. Slip is relative, because there is usually no way to know whether both sides moved or only one. Faults lie at an angle to the horizontal surface of the Earth. That angle is called the fault’s dip. The dip defines which of two basic types a fault is. If the fault’s dip is inclined relative to the horizontal, the fault is a dip-slip fault ( Figure below ).

Dip-Slip Faults

There are two types of dip-slip faults. In a normal fault , the hanging wall drops down relative to the footwall. In a reverse fault , the footwall drops down relative to the hanging wall.

This diagram illustrates the two types of dip-slip faults: normal faults and reverse faults. Imagine miners extracting a resource along a fault. The hanging wall is where miners would have hung their lanterns. The footwall is where they would have walked.

An animation of a normal fault is seen here: http://earthquake.usgs.gov/learn/animations/animation.php?flash_title=Normal+Fault&flash_file=normalfault&flash_width=220&flash_height=320 .

A thrust fault is a type of reverse fault in which the fault plane angle is nearly horizontal. Rocks can slip many miles along thrust faults ( Figure below ).

An animation of a thrust fault is seen here: http://earthquake.usgs.gov/learn/animations/animation.php?flash_title=Thrust+Fault&flash_file=thrustfault&flash_width=220&flash_height=320 .

At Chief Mountain in Montana, the upper rocks at the Lewis Overthrust are more than 1 billion years older than the lower rocks. How could this happen?

Normal faults can be huge. They are responsible for uplifting mountain ranges in regions experiencing tensional stress.

Strike-Slip Faults

A strike-slip fault is a dip-slip fault in which the dip of the fault plane is vertical. Strike-slip faults result from shear stresses. Imagine placing one foot on either side of a strike-slip fault. One block moves toward you. If that block moves toward your right foot, the fault is a right-lateral strike-slip fault; if that block moves toward your left foot, the fault is a left-lateral strike-slip fault ( Figure below ).

Strike-slip faults.

California’s San Andreas Fault is the world’s most famous strike-slip fault. It is a right-lateral strike slip fault (See opening image).

A strike-slip fault animation: http://earthquake.usgs.gov/learn/animations/animation.php?flash_title=Strike-Slip+Fault&flash_file=strikeslip&flash_width=240&flash_height=310 .

People sometimes say that California will fall into the ocean someday, which is not true. This animation shows movement on the San Andreas into the future: http://visearth.ucsd.edu/VisE_Int/aralsea/bigone.html .

Summary

  • A fracture with no movement on either side is a joint.
  • Dip-slip faults show vertical movement. In a normal fault, the hanging wall drops down relative to the footwall. The reverse is true of a reverse fault.
  • Strike-slip faults have horizontal motions due to shear stress.

Practice

Use this resource to answer the questions that follow.

http://www.iris.edu/gifs/animations/faults.htm

1. What causes normal fault motion?

2. What type of motion results from a normal fault?

3. Explain a reverse fault. What type of motion results from this fault?

4. Describe a strike-slip fault.

5. What causes an oblique-slip fault?

Review

1. Imagine you're looking at an outcrop. What features would you see to indicate a fault?

2. If the San Andreas Fault has had 350 miles of displacement, where did the rocks in San Francisco (on the west side of the fault) originate? How do scientists know?

3. How do you imagine the Grand Teton mountain range rose? In one earthquake? Along one fault? Or is there a more complex geological history?

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