本文通过对临夏盆地长达30m.y.的连续沉积(临夏群)共计779个样品的粒度特征分析,划分出七大完整的沉积旋回。粒度曲线明显地表示出青藏高原的强烈隆升始于距今3.4Ma前。该文还初步确定出青藏高原地区两次夷平过程最终结束的年代。

Linxia Basin(102°45'～103°45'E, 35°15'～35°45'N) is a small basin in the northeastern margin of the Tibet.The Cenozoic Linxia Group, 620 m thick, spanning over 30 Myr, is very suitable for the study of uplift processes and geomorphological evolution of Tibet Plateau.Grain size is a very important indicator of sedimentary environment including climate and tectonics.At the sedimentary center of Linxia basin, 779 grain size samples with 0.25 m～1 m intervals were taken from Linxia Group which was dominanted by lacustrine siltstones and mudstones punctuated by fluvial conglomarates or sandstones to be analysed by SKG-2000 micro-photo sizer, except for the boulder beds and the 60 m thick loess at the top.The results demonstrated that the Linxia Group has seven macro-sedimentary tectonic cycles which occurred respectively at 30 Ma, 21.71 Ma, 14.71 Ma,11.86 Ma, 5.4 Ma, 3.4 Ma and around 2.5 Ma BP.Berore Miocene, much of the stratigraphic bore has a lower value of grain size and mild variations, suggesting the Tibet Plateau was a lower land at that time.But at the beginning of the Pliocene, the coarse particle contents increased dramatically, especially at the 3.4 Ma～2.5 Ma period, implying that the strongest tectonic movement happened for the first time since 30 Ma.This tectonic movement caused the Tibet Plateau to begin to uplift as a whole and Linxia Basin to be disintegrated.The grain size analysis results also show that the Tibet Plateau had undergone two planation processes.One occurred at about 24 Ma BP and the other occurred at about 4.0 Ma BP～3.4 Ma BP.