地学前缘 ›› 2021, Vol. 28 ›› Issue (6): 205-226.DOI: 10.13745/j.esf.sf.2021.11.12

• “印度-欧亚大陆碰撞及其远程效应”专栏之二 • 上一篇    下一篇

西藏南羌塘增生杂岩中俯冲反向驱动高压变质岩折返:来自猫耳山大型拆离断层的启示

李典1, 王根厚2,*, 刘正勇1, 李鹏胜1, 冯翼鹏2, 唐宇2, 李超3, 李阳1   

  1. 1.成都理工大学 地球科学学院, 四川 成都 610059;
    2.中国地质大学(北京) 地球科学与资源学院, 北京 100083;
    3.中国地质科学院 地质研究所, 北京 100037
  • 收稿日期:2021-10-31 修回日期:2021-11-10 出版日期:2021-11-25 发布日期:2021-11-25
  • 通讯作者: *王根厚(1963—),男,教授,博士生导师,从事构造地质学和区域构造地质研究。E-mail:wgh@cugb.edu.cn
  • 作者简介:李 典(1985—),男,博士,讲师,主要从事构造地质学研究。E-mail:lidian19@cdut.edu.cn
  • 基金资助:
    国家自然科学基金项目(42172259); 中国地质调查局专项调查项目(1212011221115)

Subduction reversal in the accretion complex drives the exhumation of deep subducted mélange in southern Qiangtang, Tibet: Insights from the Mao'ershan detachment fault

LI Dian1, WANG Genhou2,*, LIU Zhengyong1, LI Pengsheng1, FENG Yipeng2, TANG Yu2, LI Chao3, LI Yang1   

  1. 1. College of Earth Sciences, Chengdu University of Technology, Chengdu 610059, China;
    2. School of Earth Sciences and Resources, China University of Geosciences(Beijing), Beijing 100083, China;
    3. Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China;
  • Received:2021-10-31 Revised:2021-11-10 Online:2021-11-25 Published:2021-11-25

摘要: 大洋岩石圈俯冲增生过程中可能伴随着复杂的深部板片运动过程。高压变质岩无疑是记录这些深部过程的良好载体。最近的研究提出,在特定情况下,双向俯冲中占主导的俯冲板块拖曳另一侧板块发生反向运动,从而短板片可能被另一侧长板片拖出。该研究提示我们关注俯冲增生过程中这种可能的“不正常”的板片运动方式,从而客观而全面地剖析碰撞造山带。现有高压变质岩折返模式中,除了俯冲隧道流模式,其余模式均强调单次快速折返。然而,俯冲反向运动导致的折返过程有所不同:对单个高压变质岩来说仍是快速折返,但是对整体高压变质岩带来说,整个俯冲反向期间必然都存在高压变质岩折返,从而形成较长的折返过程持续时间。对上地壳层次的折返相关构造变形的研究有助于揭示上述过程。

关键词: southern Qiangtang accretionary complex, exhumation mechanism of HP rocks, detachment

Abstract: The subduction and accretion of oceanic lithosphere may be accompanied by a complex slab motion in depth, and HP rocks are certainly a good proxy to reflect this deep process. Recent studies show that subduction reversal can occur, in particular, in a divergent double subduction zone when a slab pull of one slab exceeds that of a shorter slab, the shorter slab then experiences a net upward pull. This study prompted us to pay attention to this ‘abnormal’ plate movement during oceanic subduction and accretion in objective and comprehensive analysis of collisional orogen belt. Most of the current mechanisms of the exhumation of HP rocks emphasize a single rapid exhumation process, except the ‘corner flow’ model. It can be expected that the exhumation process should be different in the case of subduction reversal. Exhumation of a single HP rock is still a rapid process; but exhumation of an entire HP rock belt must occur during the entire subduction reversal period, so the exhumation process lasts longer. Research on the exhumation related upper crustal structural deformation has the potential to unveil the above process.|||This subduction reversal hypothesis is first proposed for Triassic HP rocks exposed in the southern Qiangtang mélange belt in central Tibet. Therefore, we chose to study the Mao'ershan accretionary complex located in the northernmost part of the Qiangtang mélange belt. We analyzed its crustal structural characteristics, geometric structure, movement style and active period related to subduction reversal, based on geological mapping and structural and chronological studies. The field geological mapping results show that the Mao'ershan complex has similar characteristics as the metamorphic core complex. Subduction-accretion complex forms its core that is surrounded by Late Paleozoic strata from the top, with a detachment fault system separating the two. Beneath the brittle detachment fault, a shear zone develops southward from the top of the subduction-accretion complex. Three-dimensional strain and kinematic vorticity results indicate that the strain type in the Mao'ershan shear zone is elongate strain, dominated by simple shear strain. Mineral deformation analysis and fractal dimension measurements show that the shear temperature is associated with low greenschist and lower amphibolite facies. Based on the new 40Ar-39Ar geochronology data, we conclude that the Mao'ershan shear zone was active at ~260 Ma. Based on the above study and combined with the geological features of the central Qiangtang mélange belt, we believe that the Mao'ershan complex was exhumed during the early stage of the subduction reversal. We thus infer that the movement rate of subduction reversal is about 3.5 mm/a, which is similar to the exhumation rate of HP rocks in central Qiangtang mélange belt. Our research may provide a new perspective on the exhumation mechanism for other HP rocks around the world.

Key words: southern Qiangtang accretionary complex, exhumation mechanism of HP rocks, detachment

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