摘要
青藏高原东南缘新生代期间响应印度与欧亚板块碰撞发生了顺时针旋转及东南向逃逸,这个过程在调节高原内部隆升变形等方面起到重要作用.但目前关于青藏高原东南缘旋转变形,东南向逃逸变形等构造变形过程之间的相互关系的认识仍不清楚.本研究选择青藏高原东南缘北部囊谦盆地渐新世地层开展了详细的古地磁旋转变形研究,共获得25个采点225个古地磁样品.系统的热退磁分析共分离出188个特征剩磁方向,得到21个有效采点,矫正前的平均方向为 Dg=168.1.±10.8°,Ig=-21.1.±19.1°,α95=11.8°,k=8.3,校正后为 Ds=171.0.±7.4°,Is=-33.3°±10.8°,α95=6.8°,k=22.7.详细的岩石磁学和野外检验表明上述特征剩磁方向是原生特征剩磁.通过与相应欧亚参考极对比,上述磁偏角显示囊谦盆地渐新世地层记录了 17.2°±8.1°的逆时针旋转变形.结合弯山构造分析和其他地质证据,本研究认为囊谦盆地与南部的贡觉、芒康和兰坪盆地在晚始新世后响应印度-欧亚板块碰撞而发生了弯山构造变形,顺时针旋转幅度自北至南逐渐增加,囊谦盆地新近纪以来还叠加了局部逆时针旋转变形.
Abstract
The southeastern Tibetan Plateau has undergone clockwise rotation and southeastward extrusion in response to the India-Eurasia collision during the Cenozoic.This process has played a crucial role in accommodating the uplift and deformation of the plateau.However,the relationship between these tectonic deformation processes,such as rotation and southeastward extrusion,remains unclear.To gain better insight into the Cenozoic tectonic deformation of the southeastern Tibetan Plateau,we conducted a new paleomagnetic study in the Nangqian Basin,located in the northern part of this region.We set up two sampling sections,Baerqu(BEQ:32.2°N,96.0°E)and Tuotuohe(TTH:32.0°N,59.9°E),in the western part of the Nangqian Basin,from which we collected a total of 225 paleomagnetic samples from 25 sampling sites.Rock magnetic analysis indicates that the main magnetic carriers are pseudo-single domain(PSD)magnetite and single domain(SD)hematite.Through detailed thermal demagnetization analysis,we successfully isolated 188 characteristic remanent magnetic directions from 21 sites.Their mean direction is Dg=168.1°±10.8°,Ig=-21.1°±19.1°,α95=11.8°,k=8.3 in geographic coordinate,and Ds=171.0°±7.4°,Is=-33.3°±10.8°,α95=6.8°,k=22.7 in stratigraphic coordinate.Detailed rock magnetic and field test analyses indicated that this mean direction is likely primary magnetization.The studied strata are speculated to be Oligocene based on comparisons of lithology and spores with strata from adjacent areas.Compared with the Eurasian apparent polar wander path reference pole,this paleomagnetic declination yields a 17.2°±8.1° counterclockwise rotation of the Nangqian Basin since the Oligocene.Combined with oroclinal analysis and other geological evidence,this study proposes that the Nangqian,Gonjo,Mangkang and Lanping basins have exhibited an increasing degree of clockwise rotation from north to south in an oroclinal bending manner since the Late Eocene.Additionally,the Nangqian Basin has experienced a superimposed counterclockwise local rotation after the Neogene.
维普
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