Molasse basin subsidence and paleodrainage evolution in the India-Asia collision zone: Insights from the Upper Oligocene-Lower Miocene Dazhuka Formation, Southeastern Tibet

The subsidence mechanisms of the inner molasse basins, specifically the Dazhuka Formation within the Yarlung Zangbo suture zone, remain a subject of debate, and the paleo-drainage evolution recorded in these strata has been insufficiently examined. In this study, we conduct a comprehensive analysis of the sedimentary characteristics, provenance, and uplift and exhumation history of the Dazhuka Formation in southeastern Tibet—a historically underexplored region—to elucidate the subsidence mechanisms of the inner molasse basin and the evolutionary processes of the paleo-Yarlung Zangbo River. Our findings indicate that deposition of the Dazhuka Formation in the Jiacha-Langxian region likely began in the late Oligocene, characterized primarily by proximal alluvial fan deposits with paleocurrents directed southward and detritus derived exclusively from the underlying Gangdese magmatic rocks to the north. Detrital apatite fission track (AFT) dating in the Remudang area yields a central age of ∼15 Ma, suggesting that uplift and exhumation occurred during this period. By integrating low-temperature thermochronological data, depositional age constraints, and provenance analysis from our study and regional data along the suture zone, we reveal that: (1) the depositional ages of the Dazhuka Formation become progressively younger from the syntaxis to Luobusha in an east-west direction, while exhumation ages remain nearly synchronous, aligning with regional slab breakoff and underthrusting processes and corroborating that subsidence was caused by slab rollback; (2) no axial paleodrainage system developed during the late Oligocene to early Miocene in the eastern part of the suture zone. However, by the middle Miocene, the paleo-Yarlung Zangbo River caused intense incision there, indicating that the evolution of the axial river system likely differed between areas east and west of Luobusha.