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How do thrust faults place older rocks above younger rocks?

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Grand Canyon rock layering — geochronology The steep walls of the Grand Canyon contain a number of layers of sedimentary rock laid down over millions of years. The lower formations belong to the early Precambrian age, while the upper layers are of the Paleozoic age. The line between the two sets of formations is called the Great Unconformity.

In the context of geochronology and Earth science, the principle of superposition generally dictates that in an undisturbed sequence of sedimentary layers, the oldest rocks are at the bottom and the youngest are at the top.1

However, thrust faults—which are reverse faults that dip at an angle of less than 45°—can disrupt this sequence. These faults occur in zones of horizontal compression where the Earth's crust is shortened. During this process, the hanging wall is pushed up and over the footwall.2

When this movement occurs, a block of older, deeper crustal material can be displaced and thrust horizontally over younger sedimentary units. Because the fault plane may follow bedding planes before cutting across strata at a steep angle, it effectively stacks older rock units on top of younger ones, creating a sequence that appears to contradict the standard law of superposition.31