柴达木盆地北缘东段察汗河右行转换挤压剪切带的变形样式及构造意义

Structural style and tectonic implication of the Chahanhe dextral transpressional shear zone in the eastern North Qaidam basin

转换挤压剪切带普遍发育在汇聚板块边界及碰撞造山带中,对调节造山过程中的增生、碰撞及物质侧向挤出等起重要作用。位于青藏高原东北缘的柴达木盆地北缘构造带(柴北缘构造带)被认为是早古生代原特提斯洋闭合、大陆深俯冲、陆-陆碰撞和造山后伸展垮塌作用的产物。最近,在柴北缘构造带东段乌兰北部察汗河地区,笔者新厘定出NWW—SEE向展布的右行转换挤压韧性剪切带。本文通过对该剪切带内宏微观构造特征、石英c轴组构、运动学涡度等研究,结合锆石U-Pb年代学数据,来探该讨剪切带的转换挤压样式及对柴达木盆地北缘早古生代造山作用的启示意义。察汗河韧性剪切带内X-Z面上发育的宏微观构造以及石英c轴组构共同指示了右行走滑剪切指向,结合糜棱岩的轻微压扁—压扁的应变椭球体形态,共同指示了其具有转换挤压的构造性质;估算的平均运动学涡度限定了其转换挤压变形样式。根据石英颗粒的重结晶机制、c轴组构滑移系以及其开角温度计,确定其变形温度为500~553℃,指示了中地壳变形层次,并结合剪切带内石英动态重结晶颗粒大小,估算其差异应力为28.5~30.0 MPa,古应变速率为10^(-12)/s。通过锆石U-Pb定年,获得卷入韧性剪切变形的闪长岩时代为432±3 Ma,结合柴达木盆地北缘已有区域地质资料,推断韧性剪切作用的活动时代为中志留世—早泥盆世(432~396 Ma)。以上资料显示,察汗河韧性剪切带表现出垂直于造山带的水平收缩和平行于造山带的侧向挤出的构造变形样式,伴随着部分由北向南的逆冲分量,形成于柴达木盆地北缘早古生代晚期的碰撞造山阶段,并指示了其造山作用具有斜向汇聚及碰撞特征。

The transpressional shear zone,commonly recognized in the convergence plate margins and collision orogens,has played a significant role for accommodating accretion,collision lateral extrusion parallel to orogens.The North Qaidam tectonic belt,which is located in northern Tibet,was regarded as resulting from closure of Proto-Tethys,continental deep subduction and post-orogenic extension collapse during early Paleozoic era.Recently,an NWW—SEE trending dextral transpression ductile shear zone was newly recognized in the Chahanhe area,eastern North Qaidam Mountains.In this contribution,we present macro-microstructural observations,quartz C-axis fabric analyses,quantitative structural data(including kinematic vorticity,paleo-stress and strain rate),and combine with zircon U-Pb dating data,to investigate the type of the transpression of the ductile shear zone and discuss its tectonic evolution.The macro-to micro-structural feature and quartz C-axis fabric in X-Z planes show that the Chahanhe ductile shear zone is characterized by dextral strike-slip shear sense.Finite strain and kinematic vorticity in deformed rocks indicate a slightly prolate ellipsoid near plane strain,suggesting a transpressional style.Mineral recrystallization mechanism,quartz c-axis fabrics slip system and opening-angle thermometer reveal that the Chahanhe ductile shear zone developed under 500 to 553℃,corresponding to a mid-crust level(16~18 km).Combining with dynamic recrystallization particle size of quartz,the differential stress and strain rate were estimated to be 29~30 MPa and 10^(-12)/s,respectively.Zircon U-Pb dating of diorite involved in ductile shearing gave an age of 432±3 Ma.Combining with previously geological data suggests the timing of the ductile shearing deformation between middle Silurian and early Devonian(432~396 Ma).The integrated data indicate that the Chahanhe ductile shear zone is characterized by combination of horizontal contraction perpendicular to the orogenic belt and lateral extrusion parallel to the orogenic belt,accompanied by a part of top-to-the south thrust component,resulted from oblique convergence and collision during late early Paleozoic orogeny.