Abstract The Gangdese belt of the southern Lhasa terrane (southern Tibet) records a Chileantype accretionary orogeny driven by subduction of Neotethyan oceanic lithosphere, prior to Indo-Asian collision and formation of the Tibetan Plateau. We present detailed structural analysis of outcrops and a drill core in the Jiama copper ore district along with 40Ar-39Ar cooling ages from white mica, plagioclase, and potassium feldspar and zircon U-Pb geochronology of granitoids and sandstone. These data add new constraints to the formation of a major angular unconformity, deformation along and within the footwall of the Gangdese décollement, and the coupling between deformation and magmatism. Structural analysis indicates that top- to- the- south motion along the décollement produced southvergent folding and thrusting of Upper Jurassic to Cretaceous strata in the Gangdese back-arc basin. A synthesis of new and compiled age data reveals that the décollement and associated south-vergent deformation occurred between ca. 90 and 65 Ma, contemporaneous with the formation of a major ca. 85–69 Ma angular unconformity between the overlying Paleocene–Eocene Linzizong Formation and the underlying Upper Cretaceous Shexing Formation. We posit that this deformation in the Gangdese belt resulted from flat-slab subduction of the Neotethyan oceanic slab beneath the southern margin of the Asian continent. A flat-slab subduction geometry is consistent with previously documented synchronous thrusting in the forearc and back-arc basins as well as the observed arc magmatic lull of the Gangdese belt between ca. 80 and 65 Ma.