华北克拉通北缘医巫闾山韧性剪切拆离带的运动学涡度与韧性减薄量

The Kinematic Vorticity and the Ductile Thinning of the YiwulüShan Ductile Shear Detachment Zone in North China Craton

医巫闾山韧性剪切拆离带位于华北克拉通北缘,为一走向NE、倾向WNW的低角度正断层系,由下盘的韧性剪切带(糜棱岩带)、未变形中生代的花岗岩体;脆性拆离断层面及上盘未变质的沉积岩系组成。以拆离断层带下盘韧性剪切带内糜棱岩长石碎斑为标志体的三维有限应变测量显示,应变主轴X轴伸长,Y轴不变,Z轴缩短。以极莫尔圆法估算拆离带内糜棱岩的运动学涡度值介于0.61到0.96,平均为0.80(涡度值是无量纲数),表明医巫闾山韧性剪切拆离带形成机制为以简单剪切为主的一般剪切作用。结合三维有限应变测量,医巫闾山韧性剪切拆离带为一加长减薄型剪切带。以有限应变测量与运动学涡度估算为基础,初步估算了该韧性剪切拆离带的韧性减薄量沿剪切拆离方向,减薄量从10%增加到40%,且减薄量与应变强度正相关、与运动学涡度负相关。

Objectives：The Yiwulüshan ductile detachment shear zone is located on the northern North China Craton,and strike NE,dips to W—NW,and consists of footwall which composed of ductile shear zone（mylonite zone） and no deformation Mesozoic granite pluton,brittle detachment fault and hangwall sedimentary rocks. In this paper,we focus on the ductile thinning with the Yiwulüshan ductile detachment shear zone. Methods： we use the strain measurement and Polar Mohr diagram to estimate the strain and the kinematic vorticity （Wk） values,and estimate the values of ductile thinning of the Yiwulüshan ductile detachment shear zone. Result： The results of strain measurement using the feldspar porphyroclasts of mylonite within the ductile zone shows the principal axes X〉Y （≈1）〉Z of the strain ellipsoids and that the X axis is lengthening,the Y axes is remaining unchanging and the Z axes is shortening. The kinematic vorticity （Wk） values,estimated by the Polar Mohr diagram,changed from 0.61 to 0.96 with an average of 0.80,which suggest that the Yiwulüshan ductile detachment zone experienced simple dominated general shearing.  Conclusions： Based on The strain measurements and kinematics vorticity estimation,the ductile thinning of Yiwulüshan ductile detachment shear zone ranged from 10% to 40% alone the sense of detachment. It suggests that it has a positive correlativity between the ductile thinning and strain intensity has,and a negative correlativity with the kinematic vorticity.