To determine recency of fault activity, which dating method would be best for dating undisturbed soils on top of the fault trace?

• A. Carbon-14
• B. Uranium-Lead
• C. Potassium-Argon
• D. Luminescence dating

Answer: (A)

The key idea is using a dating method that directly ages the material recording recent surface conditions. Undisturbed soils that have formed on top of a fault trace hold organic matter—such as plant residues, roots, and soil carbon—that can be dated. Radiocarbon dating works on this organic material and is reliable for ages up to around 50,000 years, which covers the timescales typically relevant for recent fault activity. By dating the organic components in the top soils, you obtain the time when those soils formed and the surface remained stable after faulting, giving a direct constraint on when fault movement last affected the surface.

Uranium–Lead and Potassium–Argon dating are designed for igneous or metamorphic rocks and mineral phases, not for soils, so they aren’t appropriate here. Luminescence dating can date when sediment grains last saw light, but in soils this method can be more complex and less straightforward to interpret for the specific goal of constraining recent fault movement. Radiocarbon dating of soil organic matter provides a direct, applicable age for soil formation on the fault surface, making it the best choice in this scenario.