How can a thrust fault place older rocks above younger rocks?

A thrust fault can place older rocks above younger rocks because it is a type of reverse fault driven by horizontal compressional forces that cause the Earth's crust to shorten. In this process, the hanging wall—the block of rock situated above the fault plane—is pushed upward and over the footwall.1

When this movement occurs, a large body or sheet of rock, known as a nappe, can be displaced from its original position and thrust onto the surface of younger, underlying rock. Because these faults often follow bedding planes before cutting across strata at an angle, they effectively override the existing sequence. This mechanism disrupts the principle of superposition, which normally dictates that in an undisturbed sedimentary sequence, the oldest layers are at the bottom and the youngest are at the top. Consequently, the older rock units are transported and stacked on top of younger ones, creating a geological configuration where the standard age order is inverted.23