西藏阿里雅鲁藏布江缝合带韧性剪切带的磁组构特征

The magnetic fabric characteristics of ductile shear zones in Yarlungzangbojiang suture belt,Ali,Tibet

应用磁组构测量方法,厘定、划分了札达地区印度河—雅鲁藏布江缝合带内的韧性剪切带,两条强韧性带分别位于缝合带北缘和南缘,均具有南盘(下盘)俯冲、平面右旋扭动运动特征和压扁型应变特征。韧性剪切发生于65Ma以前,它们应是印—欧两大板块俯冲-碰撞剪切应变产物,是板块缝合带的重要组成部分。

Field geological surveys demonstrate that mylonitization accompanied greenschist facies retrograde metamorphism widely occurs in late Cretaceous granites （γ3^（3-4）） and in sandstone of the middle Carboniferous Dachang formation for about 20 km across the Dajiweng-Naburu geological section in the Zanda-Ayilariju mountain area, All, Tibet. The section included steeply north-dipping penetrative schist（attitude 60°∠55°） and pointing-to-north lineation- Tensile strain concentrated in the north slope of Ayilariju and Zanda-Ayilariju mountain, formed two northwest ductile shear fracture zones about 10-20 km in width. This paper tests and analyzes the samples of tectonic rocks in the section of Indus river-Yaluzangbujiang river suture belt in the Zanda basin, All, Tibet by use of magnetic fabric method. Magnetic fabric characteristics show that ductile deformation widely takes place in the suture belt. There are 10 measurement points in the Dajiweng ductile shear belt; the magnetic inhomogeneity P≥ 1.10 at seven points, and P≤ 1.05 at three points. The granites deformed most strongly, and their P values are more than 1.20, the highest up to 1.42. The schist deformed second strongly, presenting sudden changes between strong and weak deformation. The serpentines and Eocene conglomerates did not experience ductile deformation. There are 12 measurement points in the Zanda ductile shear helt; P≥1. 05 at nine points and P≥1. 10 at four points, reflecting the ductile deformation alternately strong and weak; but the deformation is less intense than the former. The ductile shear zones in the Indus River-Yaluzanghujiang suture belt of the Zanda area have been rectified and divided. The study shows that there are two ductile shear zones on the northern and southern sides of the suture zone. They exhibit features related to suhduction of the southern （lower） plate, dextral rotation and flattening strains, which show that the plate convergence led the Earth＇s crustal rocks to a strongly compressed environment, rather than simply sheared. The ductile shear formed 65 Ma ago, resulted from Indian-Eurasian plate collision and shearing. Theyare important parts of the plate suture zone, which led the fractured south side of Tethys Himalayas belt （northern India passive continental margin deposition） to be affixed to Zhongba-Zanda terrane tectonic mélange whose attitude is steeper and northeast dipping.