桂北地区龙胜韧性剪切带的应变特征及构造意义

The strain characteristics and tectonic significance of the Longsheng ductile shear zone in Northern Guangxi,China

为探讨桂北龙胜地区韧性剪切带的应变特征和深化对华南早古生代构造事件的认识,本文对龙胜韧性剪切带开展宏-微观构造、磁组构和岩石流变学等进行了研究。龙胜韧性剪切带具有典型的宏-微观韧性变形构造,包括透入性片理、S-C组构、拉伸线理、旋转碎斑、A型褶皱和不对称压力影等。磁组构和宏-微观构造研究表明,龙胜韧性剪切带呈NNE向延伸超20 km,宽6~8 km。糜棱C面理的极密点产状294°∠56°,磁面理的极密点产状280°∠50°。磁面理相对于糜棱C面理产生了逆时针方向的旋转,表明龙胜韧性剪切带具有右旋剪切的性质。宏-微观构造和磁组构综合研究表明,龙胜韧性剪切带显示多期韧性变形的特征,总体呈现早期左旋逆冲剪切和压扁型应变,晚期右旋正滑剪切和拉伸型应变的特征。龙胜韧性剪切带具有相对较低的古差异应力和应变速率,分别为18.03~30.64 MPa和(0.10~0.83)×10^(-13) s^(-1)。龙胜韧性剪切带形成于早古生代华夏陆块自南东向扬子陆块挤压受阻而产生反冲作用的构造背景;早期造山挤压背景下,龙胜韧性剪切带发生左旋逆冲剪切和压扁型应变;晚期造山后伸展背景下,龙胜韧性剪切带发生右旋正滑剪切和拉伸型应变。

In this paper,the macro and micro structures,magnetic fabric,rock rheology and strain characteristics of Longsheng ductile shear zone developed in northern Guangxi are studied in order to deepen the understanding of early Paleozoic tectonic events in South China.It reveals that the typical macroscopic and microscopic ductile deformation fabrics of Longsheng ductile shear zone includes penetrative foliation,S-C fabric,stretching lineation,rotating porphyroclast,A-type fold and asymmetric pressure shadow.The study of magnetic fabric and macroscopic and microscopic structures shows that the Longsheng ductile shear zone extends more than 20 km in the NNE-SSW direction with a width of 6~8 km.Stereographic projection reveals that the maximum density occurs at 294°∠56°for mylonite C foliation and 280°∠50°for magnetic foliation,and the magnetic foliation rotates counterclockwise in relation to the mylonitic C-foliation,indicating that the Longsheng ductile shear zone has right-handed shear sense.Comprehensive research leads to the recognition that the Longsheng ductile shear zone experienced multi-stage ductile deformation,the early stage is characterized by left-lateral thrust shearing with flattening strain and the late stage is by right-lateral normal slip shearing with stretching strain.The calculated paleo-differential stress and strain rate values of the Longsheng ductile shear zone are 18.03~30.64 MPa and(0.10~0.83)×10^(-13) s^(-1),respectively.It is considered that the Longsheng ductile shear zone was formed in the early Paleozoic,and its tectonic background was the back thrust of the Cathaysian Block compressing from southeast to northwest to Yangtze Block.The early orogenic extrusion caused left lateral thrust shear and flattening strain and the late post-orogenic extension caused right-lateral normal slip shear and tensile strain in Longsheng ductile shear zone.