青藏高原东北部隆升:来自宁夏同心小洪沟剖面的证据

Uplift of the Northeastern Tibetan Plateau:Evidences from the Xiaohonggou Section in Tongxin,Ningxia

青藏高原边界地区的研究,尤其是砾石研究,对探讨青藏高原的隆升过程及隆升机制具有重要意义。本文选取青藏高原东北部香山山前小洪沟剖面,对出露的新生界各层位的砾石进行统计。统计结果显示,该剖面寺口子组上段、红柳沟组下段、红柳沟组上段、第四系以及现今河床出露的砾石成分主要为砂岩和石英砂岩,这与香山地区岩性相符合;砾石主要呈次圆状和次棱角状;长短轴比主要为1～2之间,为近圆状;砾石主要集中在中砾和小砾类别;分选好至中等好。砾石粒径分布显示出向细粒成分偏的特征,主要呈尖峰正态分布。这些特征表明各层位砾石相似的搬运过程,为中距离山前河流冲积砾石。沉积分析表明该砾石与气候振荡无必然联系,为构造隆升的产物。砾石沉积年龄由邻区磁性地层定年结果来限定。砾石特征结合邻区沉积分析表明香山地区在寺口子组沉积时(始新世)已具有相当大的高程;至清水营组沉积时(渐新世),该山体被剥蚀夷平;到红柳沟组沉积时(中新世早、中期),香山经历了再次的隆升;至干河沟组沉积时(中新世晚期到上新世),构造趋于稳定;到更新世时,再次出现隆升事件。始新世香山山体可能与晚白垩世至新生代早期的构造事件有关,中新世的隆升时间可以作为印-藏碰撞效应到达香山地区的时限,显示青藏高原东北边界新生代的变形隆升时间较前人研究结果早,且存在多期隆升。

The study of marginal areas of Tibetan Plateau, especially gravels, can help to understand and constrain the evolution and mechanism of the uplift of the plateau. In this study, the Cenozoic gravels in the Xiaohonggou section, which is located in the piedmont of Xiangshan, northeast of Tibetan Plateau, has been chosen to conduct gravel counting. Based on the counting data, gravels in the upper segment of Sikouzi Formation, lower and upper segments of Hongliugou Formation, Quaternary and present riverbed, appear to be mainly sandstone and quartz sandstone, which is similar to the lithology in Xiangshan mountain, indicating that they came from Xiangshan area. The gravels are mainly pebbles and cobbles, sub-rounded and sub-angular, and well to moderately well sorted. The ratio of the length of maximum axis to the minimum axis ranges 1 to 2. The grain-size distribution is mainly fine-skewed and leptokurtic normal distributing. These characteristics show all the gravels have experienced similar transport processes. It is interpreted that the gravels was formed by the alluvial fans transported with moderate distance far from the Xiangshan. The ages of the gravels have been constrained by magnetostratigraphical work conducted in adjacent Hejiakouzi section. This indicates that there has been a relatively high relief in the Xiangshan, northeast of Tibetan Plateau before the Sikouzi Formation deposited （Eocene）. During Oligoeene, when the Qingshuiying Formation deposited, the existing high relief had been eroded. The second high relief of Xiangshan occurred during early to middle Miocene, when the Hongliugou Formation deposited. During the deposition of Ganhegou Formation （later Miocene to Pliocene）, Xiangshan area experienced another tectonically stable period. The latest uplift event happened in Pleistocene. The high relief in Xiangshan during Eocene might be caused by the impact of tectonic events during the later Cretaceous and early Cenozoic. The second high relief started from middle Miocene can be related to the impact of India-Asian collision. These imply that the uplift of the northeast of Tibetan Plateau happened much earlier than previously proposed, and this area has experienced several periods of uplift.