An Oligocene-Miocene intermontane narrow lowland in the central Tibetan Plateau: Insights from provenance analysis and palynological record of a Cenozoic sedimentary succession

Abstract

• The Niubao Formation in the Avengco Basin, the western part of the BNSZ, was deposited during Oligocene-Miocene with bidirectional sources. • The Oligocene-Miocene paleoelevation of the Avengco Basin was 3090-3290 m. • A 3 km high intermontane lowland on the BNSZ were formed during the Oligo-Miocene. • This intermontane lowland was probably formed by the reactivation of thrust belts and cloud extend westward to the Avengco Basin. A series of Cenozoic basins are distributed along the Bangong-Nujiang suture zone and amalgamate into an east-west trending topographic lowland. The reconstruction of the formation process for the lowland is critical to understanding the evolution of the Tibetan Plateau and paleoclimate. Here we present results from sedimentology, zircon U-Pb ages, and palynological records of the Niubao Formation in the Avengco Basin, the western part of central Tibet. The Niubao Formation consists of conglomerate, sandstone, siltstone, marl, and limestone, which were deposited in alluvial fan and shallow lake. The palynological assemblages suggest that the Niubao Formation in the Avengco Basin was deposited during the Oligocene-Miocene. Provenance analyses suggest that the Niubao Formation can have bidirectional sources, including the Southern Qiangtang terrane and/or Bangong-Nujiang suture zone in the north and the Northern Lhasa terrane in the south. By using the co-existence approach, palynological records from the Niubao Formation yielded a paleoelevation of 3090-3290 m after temperature and lapse rate correction, which was 1-2 km lower than those of the Lhasa and Qiangtang terranes during the Oligocene-Miocene. Based on the above observations and previous studies from the coeval sediments of the Lunpola and Nima Basins in central Tibet, we propose that the intermontane lowland along the Bangong-Nujiang suture zone was formed as a whole during the Oligocene at the latest and extended to the Avengco Basin in the west.