The formation and expansion of the eastern Proto-Tibetan Plateau: Insights from low-temperature thermochronology

We report new apatite fission track (AFT) ages and thermal modeling results from the central Songpan-Ganzi, central and easternmost Lhasa, and eastern Qiangtang blocks to provide robust constraints on the extent and expansion of the Proto-Tibetan Plateau. Our results reveal two main stages of rapid cooling events in different geological units. The central Songpan-Ganzi, central Lhasa and eastern Qiangtang blocks underwent a phase of rapid cooling event during the Paleocene-Early Eocene (similar to 62-40 Ma) followed by extremely slow cooling since then. Such a sharp transition from relatively rapid to very slow cooling in approximately 40 Ma in a large area, suggest the eastern part of the Proto-Tibetan Plateau had formed since 40 Ma. The easternmost Lhasa block experienced a stage of rapid cooling during the Late Eocene-Oligocene (similar to 40-24 Ma). This stage of rapid cooling event was coeval with the first stage of rapid cooling event in the Longmen Shan thrust belt, suggestive of the eastward expansion of the eastern part of the Proto-Tibetan Plateau. Our Paleogene AFT ages and their corresponding rapid exhumation, synthesized with other thermochronological and paleoaltimetry data, are not consistent with those of the predictions from the channel flow model. Instead, these observations favor the outward extrusion model. The eastward expansion of the Proto-Tibetan Plateau improved the previous northward and southward growth model.