拉萨地体东南缘地壳深部结构

doi:10.13745/j.esf.sf.2023.2.10

地学前缘 ›› 2023, Vol. 30 ›› Issue (3): 221-232.DOI: 10.13745/j.esf.sf.2023.2.10

• “印度-欧亚大陆碰撞及其远程效应”专栏之七 • 上一篇下一篇

拉萨地体东南缘地壳深部结构

徐啸¹^,²(), 余嘉豪¹^,², 向波¹^,²^,^*(), 郭晓玉¹^,², 李春森¹^,², 罗旭聪¹^,², 仝霄飞¹^,², 袁梓昭¹^,², 林燕琪¹^,², 时宏城¹^,²

1.中山大学地球科学与工程学院, 广东珠海 519082
2.南方海洋科学与工程广东省实验室(珠海), 广东珠海 519082

收稿日期:2023-01-20 修回日期:2023-02-01 出版日期:2023-05-25 发布日期:2023-04-27
通讯作者: *向波(1990—),男,博士研究生,主要从事青藏高原构造地球物理学研究。E-mail: xiangb5@mail2.sysu.edu.cn
作者简介:徐啸(1980—),男,副教授,硕士研究生导师,主要从事岩石圈尺度构造地球物理研究。E-mail: xuxiao8@mail.sysu.edu.cn
基金资助:
国家自然科学基金项目(41974097);国家自然科学基金项目(41874102);国家自然科学基金项目(42274120);国家自然科学基金项目(U1901214);科学技术部第二次青藏高原科学考察与研究计划(2019QZKK0701);广东省“珠江人才计划”创新创业团队引进项目(2017ZT07Z066);南方海洋科学与工程广东省实验室(珠海)创新团队项目(311021003)

Deep crustal structure of the southeastern Lhasa Terrane

XU Xiao¹^,²(), YU Jiahao¹^,², XIANG Bo¹^,²^,^*(), GUO Xiaoyu¹^,², LI Chunsen¹^,², LUO Xucong¹^,², TONG Xiaofei¹^,², YUAN Zizhao¹^,², LIN Yanqi¹^,², SHI Hongcheng¹^,²

1. School of Earth Sciences and Engineering, Sun Yat-sen University, Zhuhai 519082, China
2. Southern Marine Science and Engineering Guangdong Laboratory (Zhuhai), Zhuhai 519082, China

Received:2023-01-20 Revised:2023-02-01 Online:2023-05-25 Published:2023-04-27

摘要/Abstract

摘要：

拉萨地体南缘作为印度-欧亚大陆碰撞的最前缘是研究碰撞带地壳接触关系的最佳区域。拉萨地体东南部在新生代陆-陆碰撞前还经历了中生代特提斯大洋岩石圈的俯冲演化阶段,多期次构造演化导致地壳结构复杂,致使不同分辨率地球物理方法观测地壳深部结构不清晰,宽频带地震观测和深地震反射数据对于俯冲的印度地壳前缘位置的解释并不一致。本研究在前人布设的测线周围平行布设了一条短周期密集台阵测线。通过本次研究的高分辨远震P波接收函数计算结果发现,前人通过宽频带数据所界定持续向北延伸的印度下地壳实际并不连续,印度地壳仅存在于雅鲁藏布江缝合带下方,这一认识与深地震反射结果一致。拉萨地体的中上地壳存在对应南北向挤压形成的逆冲构造。宽频带数据观测结果中看到的下地壳榴辉岩化区域位于雅鲁藏布江缝合带以北,主要位于中拉萨以及北拉萨南部。

关键词: 拉萨地体, 雅鲁藏布江缝合带, 短周期密集台阵列, 接收函数

Abstract:

The southern Lhasa Terrane is the foremost area to investigate plate boundary interactions in the India-Eurasia collision zone. Prior to Cenozoic continental collision, the southeastern Lhasa Terrane underwent multi-stage tectonic evolution under Mesozoic subduction of the Tethyan oceanic lithosphere, which resulted in a complex regional crustal structure that renders inconsistent observations of its deep structure using different geophysical methods with different resolving power. Presently, broadband seismic observation and deep seismic reflection data disagree on the location of the subduction front at the Indian plate. To gain further insight, a short-period dense array survey line was laid parallel to the previous survey line. By using high-resolution P-wave receiver functions, we discovered that the previously defined northward extension of the Indian lower crust based on broadband data is actually not continuous. In fact, the Indian crust exists only beneath the Yarlung Zangbo suture zone, which is consistent with deep seismic reflection results. The middle and upper crust of the Lhasa Terrane develops a thrust fault under south-north compression, while lower crustal eclogitization observed in the broadband seismic data is mainly located in the north of the Yarlung Zangbo suture zone in the middle Lhasa Terrane and southern part of the North Lhasa Terrane.

Key words: Lhasa Terrane, Yarlung Zangbo suture zone, short-period dense array, receiver functions

中图分类号:

P545

徐啸, 余嘉豪, 向波, 郭晓玉, 李春森, 罗旭聪, 仝霄飞, 袁梓昭, 林燕琪, 时宏城. 拉萨地体东南缘地壳深部结构[J]. 地学前缘, 2023, 30(3): 221-232.

XU Xiao, YU Jiahao, XIANG Bo, GUO Xiaoyu, LI Chunsen, LUO Xucong, TONG Xiaofei, YUAN Zizhao, LIN Yanqi, SHI Hongcheng. Deep crustal structure of the southeastern Lhasa Terrane[J]. Earth Science Frontiers, 2023, 30(3): 221-232.

图/表 8

图1 藏南构造简图及前人地球物理观测测线分布(b引自文献[4⇓⇓⇓⇓⇓⇓⇓⇓-13]) a—青藏高原主要构造单元;b—拉萨地体中东部构造简图,并展示了该区域主要的宽频带地震观测、深地震反射观测、大地电磁观测的分布。观测结果主要表现为拉萨地体的壳内含有低速或者低阻体,且宽频带与深反射数据对于印度地壳向北延伸的最终位置以及下地壳成分存在比较大的差异。

Fig.1 Tectonic units of southern Tibet (a) and distribution of previous geophysical observation lines (b, adapted from [4⇓⇓⇓⇓⇓⇓⇓⇓-13])

图2 藏南东部南北向岩石圈结构(引自文献[31-32]) 根据图1中的地球物理观测的综合结果以及地表超钾岩和He同位素的分布显示,向北俯冲的印度大陆与欧亚大陆的接触面从浅部往深部并不连续,不同的研究方法给出了不同的缝合带位置分布。图中构造信息仅作为定性示意,不做具体量化。

Fig.2 Lithospheric structure beneath the south-north trending rift zone of southeastern Tibet. Adapted from [31-32].

图3 短周期密集台阵位置及地震事件分布图(引自文献[4,7]) 图中蓝色三角形为本次研究的节点式地震仪,黄色和绿色为前人布设的宽频带地震仪;本次研究测线由南向北横跨了特提斯喜马拉雅北部、南拉萨、中拉萨和北拉萨南部等地质体;研究区包括冈底斯岩基、林子宗火山岩、洛巴堆—米拉山断裂、亚东—谷露裂谷、念青唐古拉山、狮泉河—纳木错断裂等构造单元。

Fig.3 Location of short-period dense station array and distribution of seismic events. Adapted from [4,7].

图4 节点式短周期台站数据结果和宽频带地震仪数据结果的对比分析(引自文献[4]) a—短周期和宽频带地震仪位置和70 km深度穿刺点分布图;b和d—短周期(红色振幅)和宽频带(蓝色振幅)接收函数结果按反方位角排列,对应台站位置见图a,黄色虚线为莫霍面,绿色虚线为下地壳顶界面;c和e—放大显示反方位角116°到120°范围内的结果,主要是清晰显示本次研究结果;f—将图b和d中提取的两个界面按照南北向展示,不同区域位置显示在图a。

Fig.4 Comparative analysis of interpreted nodal short-period station and broadband seismogaphic data. Adapted from [4].

图5 短周期密集台站数据叠加结果 a,b和c分别为高斯系数1.5、2.5和3.5的叠加结果。

Fig.5 Short-period dense station data improvement results using Gaussian overlay

图6 高斯系数2.5的CCP叠加成像结果与Gangdese 92°E对比(据文献[7]修改) a—Gangdese 92°E CCP成像结果,红色虚线为前人解释的印度地壳前缘位置,黑色粗虚线为莫霍面,黑色细虚线为与本次研究对比相似的壳内信息;b—测线位置图;由于本次研究布设台站在东西上有大约1°的跨度,因此,为了准确对应地表构造单元,按照实际台站分布分别分成3段测线分别成像展示,这样也更好地显示了深部结构的东西向变化;c,d和e—为短周期台站数据CCP叠加成像结果,测线位置见图b,图中虚线标识与图a相同。

Fig.6 Results of CCP superimposed imaging of Gangdese 92°E with Gaussian factor 2.5 (c-e) compared with previous result (a), and location of seismic profiles (b). Modified after [7].

图7 高斯系数3.5的CCP叠加成像结果(a据文献[44]修改) a—拉萨地体和高喜马拉雅地区中、高压变质岩带形成的p-T条件;b,c和d—为高斯系数3.5的短周期台站数据CCP叠加成像结果,测线位置见图6b。

Fig.7 (a) p-T curve for the metamorphic belts of the Lhasa Terrane and Himalayas (a, modified from [44]) and (b-d) results of CCP superimposed imaging of Gangdese 92°E with Gaussian factor 3.5

图8 特提斯喜马拉雅北缘及拉萨地体地壳结构无比例简单示意图有关印度俯冲板片的位置仅定性展示东部和中部俯冲的岩石圈地幔存在角度差异并有板片撕裂现象,俯冲板片前缘仅以黑色实曲线加以问号标识,不代表实际位置。

Fig.8 Crustal structural model of the Tethys Himalayan northern margin and Lhasa Terrane (not to scale)

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拉萨地体东南缘地壳深部结构

Deep crustal structure of the southeastern Lhasa Terrane

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