Rock-magnetism, magnetic fabric and kinematic vorticity analysis of the Shagoujie shear zone, Qinling orogen
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摘要:
秦岭商丹构造带内发育的晚三叠世沙沟街韧性剪切带蕴含大量地质信息，很好地记录了秦岭印支期碰撞造山过程。为了探究该剪切带的运动学特征及其动力学背景，在野外观测、显微构造分析的基础上，对其中发育的糜棱岩进行了磁组构和运动学涡度研究。岩石磁学和磁组构分析结果显示：样品的平均磁化率Km值总体较高，载磁矿物主要为磁铁矿等铁磁性矿物；磁化率各向异性度PJ值较大，表明构造变形较为强烈；形态参数T值多大于0，反映磁化率椭球体以扁球体为主；磁线、面理优势产状与野外观测到的矿物线、面理较为一致。结合磁组构、边界断层以及C面理产状，认为沙沟街韧性剪切带具有左行走滑挤压的运动学特征。运动学涡度Wk值及其分布特征表明，沙沟街剪切带中纯剪切作用所占的比重总体大于简单剪切作用，并且剪切带的核部应位于北界断层附近。综合分析认为，沙沟街韧性剪切带的运动学特征反映了总体斜向汇聚背景下的局部走滑挤压，与商丹带西段发育的同期韧性剪切带具有完全反向的运动学指向，这可能与碰撞导致的侧向挤出构造有关。
Abstract:The Shagou shear zone, located in the Shangdan belt of East Qinling, is characterized by ductile deformation, which can contribute to constrain the Late Triassic tectonic setting of the Qinling orogen. In order to probe its kinematics and further to explore the dynamic mechanism, mylonite samples were analyzed in terms of anisotropy of magnetic susceptibility (AMS) and kinematic vorticity (Wk) on the basis of the field observations and microstructural analyses. The values of mean susceptibility (Km) show a wide variation range, which means that the ferromagnetic mineral like magnetite acted as the dominant magnetic carrier of the samples. The corrected anisotropy degree (PJ) are relatively large indicative of a strong structural deformation. Shape parameter (T) are generally more than zero, indicating that the susceptibility ellipsoid of the Shagoujie shear zone is mainly oblate. Magnetic fabrics, defined by AMS ellipsoids, is relatively well consistent with the mineral fabrics. Combined with the magnetic fabrics and the attitude of the boundary fault or the C-foliation, it can be concluded that the Shagoujie shear zone showing a sinistral transpressional kinematic. Furthermore, Wk and its distribution patterns reflect that the core of the Shagoujie shear zone is located in northern boundary fault, and it also demonstrates that pure shear component is the dominant shear force during the formation process of the Shagoujie shear zone. Taken together, the kinematics of the Shagoujie shear zone suggest that it was formed in a local transpressional setting under oblique convergence. What's more, Late Triassic bilateral extrusion tectonics of the Qinling orogen may explain the plain fact that the Shagoujie shear zone has a completely opposite kinematic with the coeval shear zone located in the western Shangdan belt.
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Key words:
- Magnetic fabric /
- Kinematic vorticity /
- Microstructure /
- Rock magnetism /
- Shagoujie shear zone /
- Qinling orogen
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图 2
沙沟街韧性剪切带露头与显微照片
Figure 2.
Field photographs and photomicrographs of the Shagoujie ductile shear zone
图 3
沙沟街剪切带平均磁化率直方图(a)及磁化率值分布曲线(b)
Figure 3.
Histogram of mean susceptibility (a) and its distribution curve (b) of the Shagoujie shear zone
图 4
沙沟街剪切带代表性样品的磁滞回线
Figure 4.
Hysteresis loops of representative samples of the Shagoujie shear zone
图 5
沙沟街剪切带磁滞参数的Day图
Figure 5.
Hysteresis ratios on Day plot of the Shagoujie shear zone
图 6
沙沟街剪切带代表性样品的IRM曲线和反向场退磁曲线
Figure 6.
IRM acquisition curves and back-field demagnetization curves of SIRM for representative sample
图 7
沙沟街剪切带代表性样品的热磁曲线特征(黑色为升温曲线，灰色为降温曲线)
Figure 7.
Temperature dependence of magnetic susceptibility (dark line show heating, whereas grey lines show cooling)
图 11
沙沟街韧性剪切带磁组构分布特征
Figure 11.
Magnetic fabric patterns of the Shagou ductile shear zone
图 12
沙沟街剪切带运动学涡度值分布特征
Figure 12.
Kinematic vorticity patterns of the Shagoujie ductile shear zone
图 13
横过剪切带的变形分解图(据Lin et al., 2007)
Figure 13.
Diagram showing the localization and partitioning of deformation across shear zone (after Lin et al., 2007)
图 14
沙沟街韧性剪切带形成的动力学背景模式图
Figure 14.
Geodynamic model for the formation of the Shagoujie ductile shear zone
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