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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]

The southwestern part of the South China Block (SCB) records Triassic and subsequent deformations and is a key region that provides evidence of the post-amalgamation tectonic evolution of Southeast Asia. Here, we outline the tectonothermal evolution of early Mesozoic orogenesis in this region using structural analysis and fission track thermochronology. This region is divided into three tectonic units (south to north): the Youjiang Fold-and-Thrust Belt (YFTB), the Qianzhong Massif (QZM), and the Thin-skinned Thrust Belt (TTB) of the southeastern Sichuan Basin, all three of which record three deformation events, here termed D 1 , D 2 , and D 3 . D 1 deformation is represented by Triassic top-to-the-north thrusts (F 1 ) and fault-related folds (f 1 ) that are indicative of N–S shortening. The intensity of deformation decreases toward the north. The subsequent D 2 and D 3 deformations are marked by Cretaceous top-to-the-NW thrusting and fault-propagation folding, and Cenozoic NE–SW trending normal faults (F 3 ), respectively. The temperature–time ( t – T ) path obtained by apatite fission-track modeling of samples from the YFTB, QZM, and TTB areas provides evidence of uplift and denudation. The D 1 deformation at ∼230–210 Ma is characterized by buried thrusts within the relatively stable YFTB and QZM, and an increase in depth of the TTB. This was coeval with the Late Triassic northward migration of uplift and denudation that is evidenced by a Late Triassic stratigraphic gap within the YFTB, a thin layer of coarse-grained continental clastics within the QZM, and a thick layer of clastic rocks within the TTB. The study area records a Triassic thrust system (D 1 ) that progressively migrated northwards and represents a foreland fold-and-thrust belt that formed during the Indochina–SCB collision. This initial deformation was subsequently overprinted by Late Jurassic to Cretaceous NW–SE thrusting (D 2 ) and the development of Cenozoic NW–SE extensional structures (D 3 ). [ABSTRACT FROM AUTHOR]