Faults are usually younger than the rocks that they cut. The San Andreas Fault is a good example. This fault traverses the coastal rocks of California. First the rocks were deposited most likely including some sedimentary rocks. Then more recently subsequently the rocks were subjected to stresses from plate tectonics and this fault was formed. Another example is the Lornex Fault at the Highland Valley Copper MIne in British Columbia in Canada. This fault obviously cuts one phase of the Guichon Ck Batholith and juxtaposes it with another phase in a sharp contact whereas if it was just a lithological contact it would be gradational.