Formation and erosional decay of peneplains in the northern Gandise belt, Tibetan Plateau, revealed by low-temperature thermochronology and cosmogenic nuclides

大尺度准平原是青藏高原最重要的地貌组成部分，但对其形成和演化的认识却很少。通过低温热年代学、沉积物物源分析和宇宙成因核素分析，我们旨在(1)确定西藏中南部的夷平期；(2)重建造山后侵蚀和盆地形成时期的古地理和地质背景；(3)量化低起伏面第四纪的侵蚀速率和切割准平原的河流切割速率；(4)评估准平原第四纪是否长期被冰覆盖。我们的初步结果表明，不同的构造块体具有不同的热史。暴露在准平原的基底岩石在磷灰石FT和He闭合温度（分别约110℃和70℃）89 - 35 Ma以下冷却。花岗岩基岩和砾石样品中的be浓度产生了6-12 m Ma-1的局部侵蚀速率，而整个流域的侵蚀速率仅略高（11-18 m Ma-1）。这表明，准平原地区非常稳定，在过去的数百万年里几乎没有受到侵蚀的影响。

Large-scale peneplains constitute the most prominent geomorphic element of the Tibetan Plateau, but little is known about their formation and subsequent evolution. By applying lowtemperature thermochronology, sediment provenance analysis, and cosmogenic nuclides we aim to (1) date the planation phase in south-central Tibet, (2) reconstruct the paleogeography and geologic setting during post-orogenic erosion and basin formation, (3) quantify the Quaternary erosion rate of the low-relief surfaces and the incision rate of rivers dissecting the peneplains, and (4) evaluate whether the peneplains were covered by ice for extended periods in the Quaternary. Our preliminary results suggest that different structural blocks have variable thermal histories. Basement rocks exposed at the peneplains cooled below apatite FT and He closure temperatures (ca. 110 and 70 °C, respectively) between 89 and 35 Ma. 10Be concentrations in granitic bedrock and grus samples yield local erosion rates of 6-12 m Ma-1, whereas catchment-wide erosion rates are only slightly higher (11-18 m Ma-1). This demonstrates that the peneplain region is extremely stable and has barely been modified by erosion in the last millions of years.