中天山北缘大型右旋走滑韧剪带研究

STUDY OF A LARGE SCALE PALEOZOIC DEXTRAL STRIKE- SLIP DUCTILE SHEAR ZONE ALONG THE NORTHERN MARGIN OF THE CENTRAL TIANSHAN ,XINJIANG

中天山北缘是一个近 EW向的大型右旋走滑韧剪带。宏、微观构造尺度的运动学研究表明 ,该带经历过至少二期韧性变形作用。第一期为从南向北的逆冲推覆韧剪变形 ,时代为中—晚志留世 ,以米什沟剖面为代表 ,对应于早古生代洋壳从北向南俯冲及稍后吐哈陆块朝中天山岛弧的碰撞事件。第二期为沿 EW方向的右旋走滑韧性变形 ,其构造形迹广泛分布于中天山北缘带各个地段 ;北天山石炭纪火山岩已卷入该期构造活动 ,走滑时代为晚石炭世—早二叠世 ,对应于晚石炭世塔里木与西伯利亚两大板块碰撞造山诱发的陆内变形、走滑剪切。走滑带中新生白云母 4 0 Ar/39Ar年龄为 ( 2 69± 5) Ma。剪切面理、拉伸线理、矿物韧剪构造、石英 C轴组构提供了构造运动学证据 ;地层不整合及同位素测年值提供了变形时间证据。二叠纪以后的构造事件也影响到中天山北缘带 ,但只有脆性变形形迹 ,无韧性剪切。

This paper presents some resluts from geometric and kinematic studies on the Paleozoic ductile shear zone along the northern margin of the central Tianshan belt.The ductile shear zone extends east- west for more than10 0 0 km in the Chinese Tianshan,and is composed of mylonitized Early Paleozoic sedimentary rocks,volcanic and volcaniclastic rocks,ophiolitic melange blocks and pre- Cambrian schists and gneisses. Macro and micro kinematic studies suggest that this zone recorded two stages of Paleozoic ductile shear deformation,a top- to north thrust shearing during the late Silurian followed by a dextral strike- slip ductile deformation during L ate Carboniferous- Early Permian.The first phase structures were strongly destroyed or replaced by the second phase strike- slip ductile shear structures.Shearing foliation,streching lineation, asymmetric rock or mineral sense criteria of shear and quartz C- axis fabrics provide some kinematic evidences.Stratigraphic unconformities and geochronological data provide some age evidences of ductile shear deformation.The northern marginal zone of the central Tianshan is a large scale dextral strike- slip ductile shear zone at surface now.Geodynamic evolution of plate tectonics and ductile shear deformation of the Paleozoic central and southern Tianshan belt was discussed in the text.We conclude that the early phase of top- to- north thursting event is corresponding to the southward subduction of the ancient Tianshan oceanic curst during the Ordovician- Silurian time and the collision ocurred between the Turpan- Hami continental block and the central Tianshan arc during the late Silurian;a late phase of dextral strike- slip ductile shear event corresponds to intracontinental deformation caused by a great collision between Tarim and Siberia continental plates during the late Carboniferous time.No ductile shear structure is found in the Post- Permian rocks,only brittle fracture and faulting were recorded in the study area,yielded by the Cenozoic collision event.