Mesozoic Evolution of the Southeastern Tibetan Plateau From Sediment Provenance of the Qamdo Basin: Implications for Proto‐Plateau Development.

Provenance analysis of the Qamdo basin reveals convergent tectonic evolution of the southeastern Tibetan plateau region from initiation as a Late Triassic foreland basin to a final phase as a Cretaceous intermontane basin. Sandstone composition and detrital zircon ages consistently reflect recycled sedimentary sources throughout the Mesozoic, with only minor evidence for contributions from magmatic sources. Detrital zircon with Mesozoic ages are rare (≲10% of all ages), with no <160 Ma zircon observed; instead detrital zircon ages form five main age populations (230–330 Ma; 400–500 Ma; 650–1,200 Ma; 1,600–2,100 Ma and 2,400–2,600 Ma). These results indicate that during Late Triassic‐Early Jurassic time, sediment was transported from sources in the Yidun terrane across the Jinsha suture into the Qamdo depocenter as it formed a south‐facing foreland basin. Middle‐Upper Jurassic strata of the central Qamdo basin record local input from volcanic arcs of the Bangong‐Nujiang suture zone as the Meso‐Tethys ocean closed and a north‐facing foreland developed in the Qamdo region. The youngest Cretaceous deposits reflect coarse‐grained alluvial deposition synchronous with development of east‐vergent growth‐strata, reflecting intermontane basin formation resulting from inboard stress transmitted from closure of Neo‐Tethys oceanic lithosphere due to convergence with the Lhasa block. By this time, the Qamdo basin region was a high‐standing depocenter developed within orogenic uplands that preceded formation of the high topography of the "Lhasaplano," and is consistent with models suggesting proto‐plateau formation prior to Himalayan orogenesis. Plain Language Summary: The Qamdo basin represents a rare opportunity to investigate sediment deposition that records Late Triassic‐Cretaceous tectonic development of the southeastern Tibet plateau region. Using ages of zircon sand grains as well as the mineralogical composition of sand grains, we interpret that the Qamdo basin received sediment shed mainly from mountains to the east as the ocean closed during continental collision. Later in the Jurassic, this area received minor and localized sediment input from volcanic arc systems which we interpret to indicate on‐going closure of the Bangong‐Nujiang suture zone. The youngest sediment in the Qamdo basin is Cretaceous and records sediment accumulation in an intermontane basin formed due to tectonism to the south. Together these results indicate the region of southeast Tibet evolved into an elevated region that could represent a plateau that formed prior to the collision of India with Asia. Key Points: Qamdo basin records convergent tectonism along the Jinsha suture during Late Triassic and Bangong‐Nujiang suture zone during CretaceousMost sediment was derived from recycling sedimentary rocks of the Yidun terrane and eastern Qiangtang block, with limited volcanogenic inputBy Cretaceous, Qamdo deposition occurred within an intermontane setting that may represent development of pre‐Himalayan proto‐plateau uplands [ABSTRACT FROM AUTHOR]