运动学涡度在藏南冈底斯岩浆带中段谢通门-曲水韧性剪切带中的应用

Application of the Kinematic Vorticity in the Xaitongmoin-Quxu Ductile Shear Zone in the Middle Gangdese Magmatic Belt,Southern Tibet

一般剪切带主要由纯剪切和简单剪切共同作用，不同的剪切带及不同的构造位置两者所占有的比例不同。利用运动学涡度（wk）可以定量地分析两者间的比值大小。本文通过极莫尔圆法和有限应变轴率Rs／石英C轴组构法对冈底斯岩浆带中段谢通门一啦水滑覆型韧性剪切带的运动学涡度进行了计算，两种方法获得了较为一致的结果。通过极莫尔圆法，对剪切带中的9组糜棱岩样品进行了运动学涡度计算，获得了Wk=0．73～0．96，平均值Wk=0．83。运用有限应变轴率Rs／石英c轴组构法对4个样品进行了分析，得到Wk=0．85～0．93，均值为0．88，两种方法获得了较为一致的应变结果。还根据极莫尔圆图解，计算了该韧性剪切带的减薄量s=0．09～0．35，平均减薄量为0．20。研究表明该韧性剪切带为典型的以简单剪切为主伴有部分纯剪切的一般剪切，该剪切带的形成可能与拉萨地体在中新世时从挤压到侧向伸展的转换有关。该剪切带变形特征和运动学涡度的确定深化了对藏南冈底斯地区的构造演化过程的理解，并对青藏高原中南部的地质研究具有推进作用。

Generally ductile shear zone （DSZ） is a combination of simple and pure strain. The only difference is which factor （simple shear or pure shear） plays a significant role. With this theory, decomposing the strain and defining shearing types are very effective in quantitative study of a ductile shear zone. Two common methods of kinematic vorticity （Wk） measurements, the polar Mohr diagrams and quartz c-axis fabrics were applied for the first time in the Xaitongmoin-Quxu ductile shear zone, which show the similar results with uncertainties. By means of nine mylonitic samples analyzed with the polar Mohr diagram, the analyzed results suggest that kinematic vorticityranges from 0.73 to 0.96, mean value Wk around 0.83. In addition, according to polar Mohr diagrams, we calculated the thinning ratios （S） of the shear zone, showing that the thinning ratio ranges from 0. 09 to 0. 35, with a mean value around 0. 20. We also calculate the kinematic vorticity of the four samples through the quartz c-axis fabrics and strain ratio method, and the results show that Wk ranges from 0.85 to 0.93, with a mean value Wk around 0.88. Above integrated results of kinematic vorticity analyses indicate that the Xaitongmoin-Quxu ductile shear zone is general shear. The results show that the simple shear plays a dominate role in assessing formation mechanism of the Xaitongmoin-Quxu shear zone, which should be related to the tectonic transition of the Lhasa terrain from tectonic compression to extension during Miocene. The structural deformational characteristics and kinematic vorticity determines the tectonic evolution process of the Gangdese region and further advances geologic studies of the middle-south Tibetan plateau.