Early to middle Eocene (50−43 Ma) Xianqian Basin in central Tibet: Archives of arid paleoenvironments and a southward plateau uplift trend

Sedimentary basins provide valuable records of local surface uplift and paleoenvironment. While previous studies have inferred that the Qiangtang terrane in the Tibetan interior has been at high elevation since at least the Eocene, direct evidence from well-dated basins within the Qiangtang terrane remains scarce. Here, we present integrated stratigraphic, sedimentologic, paleontologic, and provenance data from the Maerguo Formation in the Xianqian Basin, southwestern Qiangtang. U-Pb dating of tuff zircons and lacustrine carbonate indicates that the Maerguo Formation was deposited between ca. 50 Ma and 43 Ma, establishing an Eocene record for central Tibet’s paleoenvironmental, tectonic, and topographic evolution. The Maerguo Formation consists of >1900 m of well-exposed strata that comprise five distinct units from bottom to top: Em1 (lower conglomerates); Em2 (fluvial red beds); Em3 (lower fine-grained lacustrine red beds); Em4 (upper lacustrine unit); and Em5 (upper conglomerates). Growth strata developed synchronously with a north-dipping thrust along the northern margin of the Xianqian Basin, while the southern part of the basin dips gently and lies unconformably above Mesozoic strata. Multiproxy provenance data indicate sediment sources from surrounding sedimentary and volcanic rocks, with sediments derived from more localized metamorphic rocks and Jurassic granitoids to the north becoming dominant during the deposition of units Em4 and Em5 in the northern basin. The presence of abundant gypsum in Em3, lacustrine carbonate in Em3 and Em4, and small, narrow, elongate leaf fossils in Em4 indicates an arid environment. These findings indicate that the Xianqian Basin was a short-lived, arid, internally drained system that terminated well before the peak expansion of Cenozoic basins along the Bangong-Nujiang suture to the south. The integrated paleogeographic reconstruction suggests that surface uplift of the Tibetan Plateau progressed southward from its interior since the Eocene. This Tibetan uplift pattern may have been driven by lithospheric removal and associated crustal shortening and thickening, beginning in the central Qiangtang terrane.