柴北缘早新生代旋转变形特征及其构造意义  

Early Cenozoic rotation feature in the northern Qaidam marginal thrust belt and its tectonic implications

作　　者：栗兵帅 颜茂都[2] 张伟林[2] LI Bingshuai;YAN Maodu;ZHANG Weilin(School of Earth Sciences,East China University of Technology,Nanchang 330013,China;State Key Laboratory of Tibetan Plateau Earth System and Resources Environment(TPESRE),Institute of Tibetan Plateau Research,Chinese Academy of Sciences,Beijing 100101,China)

机构地区：[1]东华理工大学地球科学学院,江西南昌330013 [2]中国科学院青藏高原研究所青藏高原地球系统与资源环境国家重点实验室,北京100101

出　　处：《地学前缘》2022年第4期249-264,共16页Earth Science Frontiers

基　　金：国家自然科学基金项目(42164005,41974080,41804065);科学技术部第二次青藏高原综合科学考察研究项目(2019QZKK0707)。

摘　　要：青藏高原东北部作为高原北东向扩展的前缘地带,新生代以来变形十分强烈,是研究青藏高原隆升变形过程和生长模式的关键地区之一。然而高原东北部何时卷入印度-欧亚大陆碰撞挤压变形系统以及高原扩展的运动学、动力学过程和机制等仍存在很大争议。大陆碰撞及持续挤压过程往往会伴随块体及其内部的旋转变形,而古地磁磁偏角可以定量恢复块体绕垂直轴发生的旋转变形,在研究块体旋转变形方面具有其独特优势。高原东北部,尤其是柴达木盆地,缺乏早新生代的细致旋转变形研究,制约了我们对高原东北部地区早新生代的旋转变形特征及其对印度-欧亚大陆碰撞远程响应的理解。柴北缘地区出露有近乎连续完整的早新生代路乐河组-下干柴沟组地层,为研究青藏高原东北部早新生代旋转变形提供了理想场所。本文对柴北缘逆冲带北中部的驼南和高泉两剖面早新生代路乐河组和下干柴沟组地层开展精细古地磁旋转变形研究:包括在驼南剖面布设4个时间节点、24个采点260个古地磁岩心样品,高泉剖面布设2个时间节点、14个采点150个古地磁岩心样品。通过系统岩石磁学和热退磁实验分析,揭示两剖面早新生代样品的载磁矿物主要是赤铁矿,并含有少量磁铁矿;所获得31个有效采点的高温特征剩磁方向通过褶皱检验和倒转检验,指示可能是岩石沉积时期记录的原生剩磁方向。结合柴北缘中部红柳沟剖面已有古地磁数据,三剖面古地磁结果一致表明柴北缘地区在45~35 Ma期间发生了显著(约20°)逆时针旋转变形。结合东部陇中盆地同时期古地磁旋转变形记录,发现二者具有反向的共轭旋转变形关系。综合青藏高原东部早新生代(52~46 Ma)旋转变形和渐新世以来走滑断裂活动等证据,我们认为:(1)高原东北部的共轭旋转变形是该地区对印度-欧亚碰撞的远程响应,其时�The northeastern Tibetan Plateau(NETP)is the frontal region of the northeastward propagation of the Tibetan Plateau with intensive deformation during the Cenozoic.It is one of the key regions to study the uplift and deformation processes and decipher the growth pattern of the Tibetan Plateau.However,controversies still exist regarding the time of NETP involvement with the India-Eurasia convergent deformational system,the kinematic and geodynamic processes as well as the growth mechanism of the Tibetan Plateau.Continental collision and continuous indentation are generally accompanied by vertical-axis rotation(VAR)of blocks and their internal structures.Paleomagnetic declination has its unique advantage to quantitative determination of block rotation about a vertical axis.However,the lack of Early Cenozoic paleomagnetic rotation records in NETP,especially in the Qaidam Basin,limited our understanding of the rotation patterns in NETP as well as the far-field effect of India-Eurasia collision since the Early Cenozoic.The northern Qaidam Basin contains well exposed near successive Lulehe and Xiaganchaigou Formations and is an ideal place to study Early Cenozoic VARs of NETP.Here,we conducted detailed paleomagnetic rotation study on the Lulehe and Xiaganchaigou Formations at the Tuonan and Gaoquan localities in the northern-middle part of the northern Qaidam Basin.In total,260 drill cores from 24 sites within 4 time-intervals from Tuonan,and 150 drill cores from 14 sites within 2 time-intervals from Gaoquan were collected.Detailed rock magnetic and thermal demagnetization experiments indicated that hematite is the dominant while magnetite the subordinate magnetic carriers.The obtained total of 31 site-mean characteristic remanent magnetization directions were validated by both fold and reversal tests,indicating they were likely primary magnetization directions.The obtained paleomagnetic results,together with results from the Hongliugou locality in the mid-northern Qaidam Basin,revealed a remarkable(~20°)counterclockwi

关 键 词：青藏高原东北部 古地磁旋转变形 早新生代 柴北缘 喜马拉雅东构造结 

分 类 号：P318.4[天文地球—固体地球物理学] P542[天文地球—地球物理学]