盖层中的韧性剪切带——以长江中下游为例

DUCTILE SHEAR ZONE DEVELOPED IN SEDIMENTARY COVER-BASED ON AN EXAMPLE FROM MIDDLE-LOWER YANGTZE RIVER REACHES

一般认为韧性剪切带发育在中深一深层次地壳内,是岩石在塑性状态下发生连续变形的结果,这与绝大多数韧性剪切带出现在变质基底和造山带根带的事实是吻合的。然而,在长江中下游地区沉积盖层中的局部地段同样发育着相当深层次温压条件下出现的韧性剪切带,这是以往所忽视的问题。在长江中下游特殊地区的古生界,甚至三叠系沉积岩系中鉴别出两种类型韧性剪切带:一是发育在主动侵位的花岗岩接触带及附近的滑覆带中,是局部热-动力事件的产物,多形成滑覆型韧性剪切带;二是形成于高应变速率条件下的平移型韧性剪切带。这两种剪切带虽然形成环境不同,但都具有典型的塑性变形特征,表现出与深层次韧性剪切带相似的结构和构造。这对研究韧性剪切带形成机理和环境,以及对长江中下游地区构造规律的认识具有重要意义。

The ductile shear zone is developed normaly in depth of crust, as a result of that rocks have been suffered continuous plastic deformation. It is proved by the facts that most of ductile shear zones were found in metamorphosed basement and the root belt of orogen. However, in the sedimentary cover of the middle-lower Yangtze River reaches, the ductile shear zone is also observed locally in the same state as from lower crust with high P-T condition. Nobody paid enough attention to it in this area before 1988. The two types of ductile shear zones are recognized in the Paleozoic, even the Triassic sedimenta- ry sequences in the middle-lower Yangtze River reaches. They are listed as follows:1) One exists within the sliding thrusts developed in or near contact zone of granitoid diapir as a product of 'thermal-dy- namic 'event in local areas, and forms the sliding ductile shear zone dominantly; 2) The other is trans- forming ductile shear zone resulted from the special condition of high strain rate. Although, the two shear zones are different in forming environment, both of them have typical behaviours of plastic defor- mation showing similar textures and structures occurred in depth of crust. It is of significance to re- search the forming mechanism of ductile shear zone and explore the evolution process of the tectonics in the middle-lower Yangtze River reaches。