Early Cenozoic Reactivation of a Pre‐Existing Crustal Boundary Along the Kuantan Shan‐Hei Shan Area North of the Jiuxi Basin, Northeastern Tibetan Plateau

The Cenozoic India‐Eurasia collision and continued convergence resulted in the formation of the Tibetan Plateau and the significant reactivation of intracontinental structures located over 1,500 km from the collision zone. However, the timing and mechanisms driving this reactivation remain debated, resulting in competing models about the Cenozoic evolution of the plateau margins. This study aims to address this issue by focusing on the northeastern Tibetan Plateau and investigating the depositional sequences, source‐to‐sink relationships, and deformation structures along the northeastern margin. Stratigraphic analysis reveals an upward‐thinning sequence beginning with high‐energy conglomerates in the Eocene, followed by an upward‐coarsening sequence. Detrital zircon U‐Pb dating analysis indicates a reorganization of sediment dispersal during the middle Miocene, shifting from northern provenances to southern provenances. Structural analysis of a seismic profile traversing the present deforming front of the northeastern Tibetan Plateau shows steep boundary faults and positive flower structures, accommodating ∼2.2 km of Cenozoic shortening (∼7.5% strain). These findings suggest that pre‐existing weak zones within the Eurasian plate were reactivated immediately following the initial India‐Eurasia collision. Combined with previous geophysical studies, a pre‐existing crustal boundary is also imaged along the deforming front of the northeastern Tibetan Plateau, acting as a fixed convergence zone between two terranes throughout the Cenozoic. This zone is characterized by steeply dipping faults, positive flower structures, and minimal shortening, exemplifying intracontinental convergence between terranes of comparable rheological strength.