Rock Folding

Half-grabens are faults where a drop-down has occurred due to extension but there is no uplift on the other side. You can get series of half-Grabens where they look like dominoes in the process of falling down. These are more common than Horsts and Grabens.

ROCK FOLDING

Rock folding is different than a fault line. Faults are fractured areas, whereas folding happens to ductile rocks. Compression over time leads to folding. The strain rate needs to be low so you don't get fracturing. The two sides of any fold are called limbs. Figure 54 shows some geometry you need to know with regard to geological folding:

Figure 54.

Notice the axial plane running perpendicular to the hinge line and the two limbs on either side. A line drawn perpendicular to the axial plane is called the fold axis. The fold axis runs along the hinge line in a relatively straight way in any routine fold. The limbs will be the straightest part of the fold. The axial plane will divide the fold roughly in half as symmetrically as possible.

There are three known folding patterns you will see going on. These include the following:

• Monoclines – these are simple and involved the bending upwards of horizontal layers in an upward direction so that you basically see a downward hill with strata that are uniform and parallel to one another. If you draw an imaginary line perpendicular to the limb edges, it will not be vertical but will tilt downward. This imaginary line is called the axial plane.

• A syncline is a trough, where the limbs are on either side and horizontal, with a midline sinkage. The fold plane is one that divides two areas that are roughly symmetric on either side of the trough. Figure 55 shows you a syncline:

Figure 55.

• An anticline is an arch, with two limbs on either side and a fold axis that is still straight up. It is the opposite of a syncline. The fold plane will still separate roughly in equal halves of the arch.

Variations on these are domes and basins, where the layers are dipping in the center of a basin or rising in the middle of a dome. There is not just two limbs but a ring of rock that is different from the central portion. There is no real fold axis as the hinge just comes up to a point at the center of the dome or basin.

You can see where these things have happened only if there has been erosion of an area so you can see the layers. This is how you can see what's happened over time underneath the earth. The layers or strata in domes and basins will be concentric around a central point. Anticline folds get older as you get toward the central axis, while syncline folds get younger as the rock erodes toward the central plane of the fold.