A fault is a line along which pressures or movement in surrounding rocks have caused severe cracking. The angle of the pressures will largely determine the type of fault which is produced.
In the case of a thrust fault, the angle is very low and the upper beds of rock are pushed forward and over the beds beneath them.
Some rock strata tend to fold under pressure, while others tend to fault. In order to consider the difference think of two rulers, one plastic and one wooden. If the same amount of pressure is applied when bending these rulers, the plastic ruler will bend almost double before breaking. However the wooden ruler will seem to withstand the pressure until it suddenly gives and snaps. In this example the plastic ruler is behaving like rocks which fold, while the wooden ruler represents rocks which fault.
Although it initially folds, if sufficient pressure is exerted on the plastic ruler, it will eventually snap. So it is with rocks.
Many rocks are distorted by folding under the pressures of earth movements. For example, if a series of horizontal rock strata are pushed inwards from either side, the strata will tend to fold up and down into a series of wave like formations. This can be illustrated by holding several sheets of paper horizontally and pushing in on them. These folds in rocks can cover enormous distances, although in other cases they can be quite small.
If such a series of folds occurs, parts of the fold will go up and parts will go down, as in the case of the sheets of paper. An upfold is called an anticline and a downfold is called a syncline.
If the pressures which formed an anticline continue, it is possible that the rock strata will reach a stage at which they can no longer give gradually and fold, and there will be a sudden break, as in the case of the plastic ruler. Such a break will occur along a line of relative weakness in the strata. This line will form a low angle to the horizontal and will cause the upper beds of rock to slide across and over the other beds beneath them.
This is the fundamental cause of a thrust fault. The thrust fault itself is the line along which the cracking and breaking has taken place and the upper rocks have moved.
Evidence for a thrust fault in any landscape can be found from a discontinuity of rock strata. When rock strata are folded, although they distort, the sequence of rocks remains unchanged. However when they break into a thrust fault, the rocks on either side of a fault will not necessarily be the same, due to the sliding process.