Abstract
The Cenozoic basins and ranges form the high topography of the northeastern Tibet that resulted from the India-Eurasia collision. Sedimentary rocks in the basins provide direct insight into the exhumation history of the ranges and the tectonic processes that led to the northeastward growth of the Tibetan Plateau. In this study, we analyzed and compared detrital zircon U-Pb ages from sands of modern rivers draining the Bei Shan, and North Qilian Shan and sandstones from the Yumen Basin. The zircon age distributions indicate that the strata dated to 24.2-16.7 Ma in the basin were derived from the Bei Shan, and the basin provenance changed rapidly to the North Qilian Shan terrane at ~16 Ma. These results suggest that an early stage of deformation along the Bei Shan at ~24 Ma was replaced by the growth of the North Qilian Shan at ~16 Ma. We conclude that the far-field effect associated with the Indo-Asian collision may result from Oligocene deformation in the Bei Shan, but the emergence of the North Qilian Shan at ~16 Ma could reflect the most recent outward growth of the Tibetan Plateau that may have been caused by the removal of some lithospheric mantle beneath central Tibet.
Highlights
We present ~1000 single-grain U-Pb zircon ages from the modern streams draining the North Qilian Shan, the Bei Shan and from late Oligocene to late Pliocene sediment sequence in the Yumen Basin that were dated using magnetostratigraphy
The post-15.8 Ma samples had relatively high P values (0.093–0.932) from the K-S test with sample QL1, low P values (0–0.008) with sample QL2 and zero P values compared with the pre-15.8 Ma strata and the Bei Shan samples, again implying the dominant volume of zircons in the post-15.8 Ma samples are remarkably different than the pre-16.7 Ma and the Bei Shan samples. These results clearly reveal that the detrital zircons from the 24.0–16.7 Ma strata in the basin are locally derived from the Bei Shan source, whereas zircons from the post15.8 Ma strata come from the North Qilian Shan source
Further south of the Qilian Shan, the Qaidam Basin initiated as a result of thrust faulting-related subsidence along its northwestern margin during the Paleocene or Eocene and a wedge of conglomerate filled in the accommodation created by the subsidence[9,22,23], suggesting that early Cenozoic high topography might lie in the North Qaidam-South Qilian Shan region
Summary
We present ~1000 single-grain U-Pb zircon ages from the modern streams draining the North Qilian Shan, the Bei Shan and from late Oligocene to late Pliocene sediment sequence in the Yumen Basin that were dated using magnetostratigraphy. The Cenozoic strata of the Yumen Basin are well exposed and consist of five primary stratigraphic units: the Oligocene Huoshaogou Formation, the late Oligocene to early Miocene Baiyanghe Formation, the middle Miocene to Pliocene Shulehe Formation, the Late Pliocene Yumen conglomerate, and the Quaternary Jiuquan conglomerate The age of these stratigraphic units is based on fossils of mammalian, ostracode, and palynological assemblages and magnetostratigraphy[8,17]. The density of modern sand samples is not large enough to account for all of the streams draining the North Qilian Shan and the Bei Shan, the sampled rivers cross most lithologic units of both ranges that were expected to yield zircons
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