西秦岭北缘武山—鸳鸯镇构造带磁组构特征

构造与磁组特征揭示出武山-鸳鸯镇构造带为一条复合性断裂带,变形样式表现为花状构造形态。野外及显微构造特征表明,先期韧性变形为右行剪切,发育于中、深构造层次;66个构造岩样品的磁化率椭球形态分析表明,其以平面和压扁应变为主,总体较高的磁化率各向异性度表现了构造带的强变形特征;磁化率椭球主轴方位显示NW和NEE走向两组磁面理的存在,暗示高应变剪切带在平面上可能以共轭或网格状形态出露,锐夹角分线近EW向;高角度磁面理及较为发育的低倾伏角磁线理暗示了沿构造带近EW向的走滑剪切,部分高倾伏角磁线理可能与构造带的挤压和(或)转换挤压相关,而相对集中的磁面理与相对分散的磁线理也表明了构造带的平面及压扁应变体制。强烈的右行转换挤压奠定了西秦岭北缘现今的反"S"型区域构造,表明碰撞造山过程中,西秦岭诸中、小块体一定程度的向西挤逸。中、新生代沿构造带继承性的发育以西秦岭北缘(渭河)断裂为中心的一系列正花状左行走滑构造,构成青藏高原东北边缘物质逃逸及应力释放与调整的重要边界。

Magnetic fabric study of Wushan-Yuanyangzhen tectonic zone in the northern margin of West Qinling, China

The Tangzang-Guanzizhen-Wushan tectonic zone (TGWTZ) in the northern margin of West Qinling is a complex boundary which is interpreted to be a suture with ophiolites involved. In order to better understand the collisional orogeny of West Qinling and the westward extension of Shangdan suture, this paper presents new geometric and kinematic data about the Wushan-Yuanyangzhen tectonic zone (WYTZ) which is the western segment of the TGWTZ. The structural and microstructural observations demonstrate that the deformation in the dextral ductile shear zone of WYTZ is developed under amphibolite-grade conditions. The AMS fabric(66 specimens from 20 sites)study indicate the shearing and flattening regime, and higher values of anisotropy degree, which imply that the shear zone had been subjected to strong deformation. The directions of principal susceptibilities are closely related to the deformation, and the magnetic foliations are divided into two groups that are conjugated, which imply the gridded configuration of high strain shear zone. The clustered foliation and more scattered lineation also imply the shearing and flattening tectonic regime. Most of the samples have steeper magnetic foliations and subhorizontal magnetic lineations and others have steeper lineation, which demonstrate the transpressional characteristics of the shear zone. The predominant dextral strike-slip shear system of D1 resulted in the reverse "S"-type structure in the northern margin of West Qinling, which revealed the great displacement of the strike-slip system in the collisional processing of West Qinling. The flower structures that comprised Weihe sinistral strike-slip faults systems and thrust faults (D2) from the Mesozoic to the Cenozoic superposed on the D1 ductile deformation, form one part of the northeast boundary of the Tibet escape tectonics.