Faults

In the previous post, I discussed folds, which precede faults. That is to say, if the forces that create a fold becomes too strong, the rock layers (beds) eventually fracture, rather than just bend. The dictum is: FOLDING PRECEDES FAULTING. 

Horizontal layers prior to being deformed. The geologically youngest layer is at the top.

Children's clay makes useful fault models, as shown here. The blue layer (bed) is the oldest, and the green layer is the youngest.

In the following images, I shall discuss the different kinds of faults. Afterward, I shall show examples of some of the faults common to southern California.

DIFFERENT KINDS OF FAULTS:

A reverse fault forms when compression causes localized bending and subsequent breakage. The compression causes the "hanging-wall" block on the right side to move up relative to the "foot-wall" block on the left side. The compression causes the layers (beds) to overlap. The fault in this scenario is a high-angle reverse fault. The terms "foot-wall" and "hanging wall" stem from the early days when a miner would dig a mine shaft down along a fault plane (minerals commonly form in this zone of breakage because that is where fluids would easily flow). The miner's feet would be on the "footwall," and the "hanging wall" would be above his head.

If, however, the angle of the reverse fault is low, then the fault is referred to as a thrust fault. The relative motions of the hanging and footwalls are the same as for a high-angle reverse fault.

A normal fault forms when extension (= the opposite of compression) caused the "hanging-wall" block on the right side to move down relative to the "foot-wall" block on the right side. The layers do not overlap themselves. The fault in this scenario is a high-angle normal fault. These kind of faults are uncommon in southern California because this area has mostly undergone (and still is undergoing) compression, rather than extension.

A strike-slip fault has mostly horizontal displacement (shown by the arrows in the diagram above). The sense of displacement is that one side moves in one direction, and the other moves in the opposite direction or is stationary. The San Andreas Fault is an excellent example of a strike-slip fault.

In southern California, all faults, except normal faults, are  common because the area has been and is still undergoing compression. 

EXAMPLES OF FAULTS:

In this reverse fault (the red line), the "OLDER FORMATION (A)," consisting of layers (beds) has been displaced, and is now situated above the "YOUNGER FORMATION (B)." This is not the way things were originally, because, in unfaulted situations, older beds are below younger beds (as is the case on the far left side of the image).

In this thrust fault (red line), older brown sandstone and gray mudstone rocks, in the "shadowy" upper right-hand side of the image, have been moved sideways at a low angle (nearly horizontal) and now overlie younger conglomerate (reddish sandstone and small boulder) beds. Prior to the faulting, the brownish and gray beds had a gradational contact with the underlying reddish beds. Now, they have a fault contact with the underlying reddish beds. The two rock units belong to different formations of considerably different geologic age and much different ancient environments of deposition. Low-angle thrust faults like this one can be very difficult to detect unless the geologist knows full well the lithologic (rock types) and their vertical changes within a formation or a sequence of formations.

The vertical cliff in the image shown above is at least 50 feet high. This fault (indicated by the red arrow) is along the sharp line of color difference between the white rock, which is an igneous rock that formed several hundred million years ago, and the dark gray rock, which formed about 2 billion years ago. This fault is a strike-slip fault and originally involved sideways motion of one rock mass sliding pass the other. 

This is a closeup along the fault shown in the previous image. One can truly put a finger on the fault.

This is a vertical strike-slip fault.  The grayish rocks left of the red-and-white measuring stick (1.5 m in length) are geologically old "basement" metamorphic rocks, and the red rocks on the right of the staff are much younger sedimentary rocks consisting of sandstone and siltstone.