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The Cenozoic evolution of SE Asia is marked by thousands of kilometres extension on the continental margin, including normal faults, half‐grabens, and volcanic rocks, but dating of brittle faults in the shallow crust is difficult because of a lack of synkinematic minerals. The Cenozoic extension in the Youjiang fold‐thrust belt in southwest China is marked by brittle normal faults and half‐grabens filled with Paleogene sediments in the region. Here, we identified a strongly weathered mafic sill that was truncated by normal faults in the fold‐thrust belt and present the age and chemistry of zircons from a weathered sill. The zircons from the sill yielded a mean age of ~35 Ma. Integrating cross‐cutting relationship and zircon U–Pb dating suggest that the Cenozoic extension was initiated later than ~35 Ma in Youjiang fold‐thrust belt in southwest China. Combined with structural analysis and previous fission track data in the region, we suggest that the normal faults in this study provide evidence of surface fault activity of Cenozoic extension in SE Asia caused by retreat of a trench system. Furthermore, this study provides a case study to discuss how to decipher brittle normal faulting using zircon geochronology and structural and textural analysis from the mesoscale to the microscale. [ABSTRACT FROM AUTHOR]