新疆塔西河地区8Ma以来沉积环境演化及其构造意义

天山南北麓巨厚的新生代地层记录着天山构造隆升与沉积环境演化的历史。在对新疆塔西河剖面进行磁性地层研究的基础上,通过沉积学分析,确定自8.1M a以来塔西河剖面存在4个沉积相组合,对应的沉积环境演化分为4个阶段:1)在8.10⁵.75M aB.P.之前为浅湖沉积环境;2)在5.75⁴.27M aB.P.为扇三角洲沉积环境;3)在4.27³.44M aB.P.为滨浅湖沉积环境;4)在3.44M aB.P.之后为冲积扇沉积旋回期。塔西河剖面沉积环境演化、岩相与沉积速率的变化等共同揭示出天山自8.1M aB.P.以来先后经历了构造活动稳定期（>5.75M aB.P.）、过渡期（5.75³.44M aB.P.）以及快速强烈间歇性隆升期（<3.44M aB.P.）等3个构造活动阶段,并且自3.44M aB.P.,构造活动有增强的趋势。

SEDIMENTARY ENVIRONMENT EVOLUTION SINCE 8MaB.P. IN THE TAXIHE AREA,XINJIANG,NORTHWEST CHINA AND ITS TECTONIC SIGNIFICANCE

When and how the Tianshan Mountains began to uplift has been controversial topics.Some previous studies analyzed the history of tectonic uplift solely based on the increase in accumulation rate based on magnetostra￣tigraphy.Although the increase in accumulation rate could be resulted from tectonic uplift,the effect of climatic change should not be neglected.On the other hand,although the information on sedimentary environment is very useful to analyze the tectonic uplift,it was rarely used for the north piedmont of the Tianshan Mountains.The thick Cenozoic terrestrial sequence,which was deposited in southern and northern foreland basins of the Tianshan Mountains,provides us an opportunity to probe into the process of the sedimentary environment evolution and the history of tectonic uplift,and thus to understand the relationships between mountain building,erosion-deposition,and global or regional climate better.We thus carried out a magnetostratigraphic study with the hope to better understand the uplift history of the Tianshan Mountains and the process of the sedimentary environment in the region.In the north piedmont of the Tianshan Mountains,the Taxihe region is suitable for magnetostratigraphical and sedimentological study,as the north-flowing Taxihe River has cut across the Tugulu Anticline and the excellent outcrop in the valley provides us a unique opportunity to decipher the history of tectonic uplift since the terrigenous deposits.The studied section,about 1200m thick,lies about 28km southeast of Manasi and is located on the northern limb of the Tugulu Anticline.A total of 676 paleomagnetic samples were taken from the Taxihe section.One sample was selected from each sampled horizon.All selected samples（in total 301）were subject to stepwise thermal demagnetization in 13～17 steps in an ASC TD-48 oven with an internal residual field less than 10nT.Remnant magnetization was measured with a 2G cryogenic magnetometer at the Paleomagnetic Laboratory of the Institute of Geomechanics,Chinese Academy of Geological Sciences in Beijing.Based on the constructed magnetostratigraphic polarity sequence and correlated with GPTS of Cande and Kent,the Taxihe section is paleomagnetically dated to span from 8.10MaB.P.to 1.95MaB.P.Based on the above time scale,detailed sedimentary study of this sequence has revealed four sedimentary facies associations reflecting four stages of sedimentary environment evolution.They are:（1）the shallow lake facies before 5.75MaB.P.,（2）the fan delta facies between 5.75MaB.P.and 4.27MaB.P.,（3）the littoral lake facies between 4.27MaB.P.and 3.44MaB.P.,（4）the alluvial fan facies since 3.44MaB.P.The associated tectonic uplift of the northern Tianshan Mountains,based on the abrupt changes in accumulation rates and sedimentary facies,is suggested to have experienced three stages,that is,the stable stage（>5.75MaB.P.）,the transitional stage（5.75～3.44MaB.P.）,and the rapid intermittent uplift stage（<3.44MaB.P.）.The uplift at ca.3.44MaB.P.is of great importance in tectonics and geomorphology,resulting in the increase in accumulation rate from an average 0.165mm/a to 0.270mm/a.Although it is clear that this uplift had great influence on sedimentary environment,the role climatic changes played is still in the vague,namely,researchers have not fully understood how and to what extent climatic changes would impact the environment.