Faults and Joints

Step 3 is to determine the dip. This is a perpendicular line down the face of the incline toward the water in the example. You might say it is 35 degrees West of North.

Step 4 is the measure the dip angle. The dip angle is the angle of the incline compared to the horizontal. You might say it is 65 degrees angulated with respect to the horizontal.

FAULTS AND JOINTS

A joint is any separation of a block of wall where there has been no movement on either side of the crack. It takes pressure from gravity or from forces behind or beneath onehalf of the joint to create a fault.

A fault is a good example of what happens when brittle rock fractures or breaks. You will see an offset at this point because the two halves didn't just break and stay in place. They broke and then moved in some direction away from each other. The reference point is the inclined plane. The block below the inclined plane is called the footwall, while the one above the inclined plane is the hanging wall. The hanging wall will either fall due to gravity or rise when a force thrusts it up the plane.

A geologist's goal is to describe what has happened to the two halves. Note the inclined plane aspect of the images you look at in figure 52 as you think about which fault is which. Figure 52 shows you the different fault types in better detail:

Figure 52.

Here is how you describe the different faults and their respective movement:

• Dip-slip faults – this is when two segments rise or dip compared to another segment. There are two types of these. In a "normal" dip-slip fault, a segment dips downward, just as you'd expect would happen because of gravity. In a reverse dip-slip or "thrust" dip-slip, a force opposite to gravity has force a section of rock to move upward in relation to another one.

• Strike-slip faults – this is when two halves of a fault slip past one another horizontally. You need to think about these faults in terms of their relative directions. A right lateral strike-slip is when the right side slips downward as the left slips upward. If you think of a line or fault as being on a clock face, a right lateral slip has the right side facing the 6 o-clock position, while the left lateral strike slip has the right side facing the 12 o-clock position. Transform boundaries along tectonic plates are just giant strike-slip faults.

There are different patterns of faults you can encounter. Some are related to extension along two sections of rock, such as along a rift zone.

A Horsts and Grabens situation is where there is tensional stress that causes faults to dip in opposite directions. There are down-dropped sections called grabens along with uplifted blocks called horsts. A rift valley is a Horsts and Grabens situation. You might see these in places like Idaho, Utah, and Nevada, where newer crustal areas have gotten extended in places, creating new valleys. Figure 53 is a Horsts and Grabens situation: