Initiation of the Gangdese Retroarc Fold‐and‐Thrust Belt in Southern Tibet: Evidence From New Geochronologic Data of the Late Cretaceous Shexing Formation

The initial exhumation of the fold‐and‐thrust belt in the Gangdese retroarc provides key insights into the development of an Andean‐type continental margin in southern Tibet. Here, we present new zircon U‐Pb geochronology, low‐temperature thermochronology and balanced cross section from the Late Cretaceous retroarc and forearc basins to reconstruct the pre‐collisional exhumation history of the Gangdese fold‐and‐thrust belt. Interbedded andesite and tuffite layers from the upper Shexing Formation in the retroarc basin yield weighted mean U‐Pb ages of 85.5 ± 4.4 and 84.6 ± 0.9 Ma, providing refined constraints on the depositional age. Thermochronologic data from the retroarc and forearc basins record Late Cretaceous‐Cenozoic cooling, with inverse thermal modeling indicating the onset of rapid retroarc exhumation at ∼78–70 Ma, shortly after deposition. This timing likely corresponds to the onset of crustal shortening in the Gangdese retroarc, with a minimum estimate of ∼67% as constrained by balanced cross section restoration. This earliest exhumation signal is also recognized in the forearc and north‐central Lhasa terrane, coinciding with a magmatic lull in the Gangdese arc and northward migration of magmatism. Collectively, we interpret that shallow or flat subduction of the Neo‐Tethyan oceanic lithosphere may have provided the primary geodynamic forcing, whereas surface erosion modulated the mechanical state of the orogenic wedge, jointly promoting Late Cretaceous crustal shortening.