三江构造带新生代变形构造的时-空变化:研究综述及新数据

在印度-欧亚陆陆碰撞造山过程中,位于青藏高原东南部、喜玛拉雅东构造结东侧的西南三江地区在新生代经历了强烈变形,形成了复杂的构造样式。构造样式的时空变化可以有效限定印度-欧亚陆陆碰撞过程,而近年发表的大量数据为揭示三江构造带新生代构造样式时空变化提供了可能。通过综合前人研究数据,结合本文的新观察,提出三江构造带不同构造部位的变形特点显示规律性变化:处于碰撞前缘位置的腾冲、保山地块在印度地块向北迁移过程中最早(50~45Ma)与印度大陆发生碰撞,在挤压作用下形成褶皱+逆断层组合,以及块体边界压扭性剪切变形;随着印度地块持续向北运动,该变形样式逐渐向东、北部扩展,并使兰坪-思茅地块、扬子地块西缘剑川盆地沉积环境发生改变,地块两侧发生剪切变形;其中兰坪-思茅地块东侧(30Ma)剪切带的启动时间晚于西侧(34Ma)。地块两侧剪切带均大致经历了纯剪(挤压)-简单剪切(走滑)-纯剪(伸展)变形历史;剪切带各阶段变形的启动时间均具有南早北晚之特点。发生塑性变形的下地壳物质的剥露过程同样表现出时空不均匀性,结合古地磁研究成果,这种现象可能与地块内部的不均匀旋转有关。各剪切带最北端不但变形启动时间最晚,而且基本没有记录走滑变形。这种变形样式的时空变化表明,印度与欧亚大陆的碰撞变形效应在三江构造带内主要表现为陆块内部的弥散状挤压变形与块体刚性旋转形成的剪切带,块体向南逃逸规模较小。

Temporal and spatial variation in the Cenozoic strain of the Sanjiang orogenic belt, SW China:A brief review and new observations.

Locating in southeastern Tibetan Plateau, east of the Eastern Himalayan Syntaxis, the Sanjiang orogenic belt records a complicated Cenozoic deformation as responding to the India-Eurasia continent-continent collision and orogenic process. The temporal and spatial variation in structural pattern may provide a positive constrain on the continent-continent collision. A growing accumulation of detail structural and thermochronological studies makes it possible to reveal this variation. Synthesizing available data, we find that the Baoshan and Tengchong blocks were firstly (50~45Ma) deformed displaying fold-and-reversal fault pattern; this structural pattern gradually expanded northeastward into the Lanping-Simao block, causing a change in basin-pattern and two ductile shear zones along its western and eastern boundaries, respectively. Of them, the western shear zone (34Ma) has an earlier starting time than its eastern (<30Ma) counterpart. Both shear zones have similar strain-history:pure-shear (compression), simple shear (lateral slipping), and/then pure-shear (uplifting). The starting time of southern segments of both shear zones is earlier than their northern ones. The ductile-deformed lower crustal rocks along the shear zones uplifted heterogeneously likely due to a complicated block rotation according to paleomagnetic data. It is worthy pointing out that the northernmost tips of all shear zones didn't record any lateral slipping; they formed at the latest time. Such a Cenozoic strain pattern suggests that, during the continent-continent collision, the Sanjiang orogenic belt deformed mainly by pervasive shortening and heterogeneous block rotation, while as the scale of southeastward extrusion of block is limited.