摘要
龙木错-双湖缝合带是青藏高原内部一条极为重要的印支期构造带，分隔了两侧的北羌塘地块和南羌塘增生杂岩带，并保留着古、中、新特提斯洋演化的重要信息。1:5万、1:1万地质填图及构造解析表明：缝合带以南为三叠纪古特提斯洋俯冲、消减形成的增生杂岩带，区域上与藏东的聂荣、嘉玉桥、吉塘变质杂岩系，以及滇川西部的云岭、昌宁-孟连变质杂岩系相连，构成了青藏高原及周缘印支期的巨型增生造山带。增生杂岩主要经历了三期构造变形，两期区域变质作用；第一期变形（D1）与三叠纪古特提斯洋沿海沟发生的俯冲消减作用相关，为右行剪切下的塑性流变，塑造了增生杂岩的总体构造样式，形成了区域上的E-W及N-S向构造带，发育透入性的面理S1时代为244-202Ma。200Ma左右，古特提斯洋闭合，俯冲-增生事件结束，第二期挤压变形（D2）引起了强烈的纵向构造置换，发育纵弯褶皱和小型逆冲断层，形成了区域性的轴面褶劈理S2，指示增生杂岩早侏罗世被剥露到浅构造层次所经历的一期水平收缩变形。第三期变形（D3）表现为浅-表构造层次形成的中常-开阔型纵弯褶皱，指示羌塘盆地上新世发生的一次NW-SE向收缩变形，轴面劈理S3具有非透入性特点，变质作用微弱。
侏罗纪以后，南、北羌塘对接成为统一地块，北羌塘地块在燕山晚期及新生代经历了中、新特提斯洋闭合引起的浅-表层次板内变形。燕山晚期形成的NW-SE向逆冲断层带、褶皱及相关断裂叠加在印支期的纵弯褶皱之上。新生代以来羌塘盆地存在两个变形体制的转折，构造变形可分为三个阶段：Ⅰ幕（50-40Ma至8-4Ma），最大主应力轴N-S向近水平，最小主应力轴近垂直，地壳发生南北向大规模水平缩短，近东西向纵弯褶皱和逆冲断层造成了羌塘盆地地壳的持续加厚。Ⅱ幕（8-4Ma至86.9Ka），最大主应力轴仍为近N-S向，但中间应力轴垂直地表。地壳厚度剧烈增加后重力作用凸显，E-W向被动伸展形成了共轭状的走滑断裂系和拉分盆地。羌塘盆地向东逃逸，次生有南北向的纵弯褶皱，河流冲积层的水平收缩与下部固体地壳走滑变形存在不耦合滑脱面。Ⅲ幕（更新世至今），应力场发生了反转，最大主应力垂直地面，N-S向挤压应力退缩为中间主应力，羌塘盆地发生E-W向的主动伸展，形成共轭状正断层。
The Longmu Co-Shuanghu suture, which is located in the Central Qiangtang, isan extremely important Indosinian tectonic zone in the Tibetan Plateau that separatesthe South Qiangtang accretionary complex belt and the North Qiangtang terrane. Thissuture retains important information regarding the evolution of the Palaeo-TethysOcean and its deformations after Jurassic reflect a portion of the closure of the Meso-and Neo-Tethys ocean.1:50000and1:10000geological mappings indicate that theaccretionary complex belt on th south of the suture formed during the northwestwardsubduction of the Paleo-Tethys Ocean in Triassic and tectonically linked with Nierong,Jiayuqiao and Jitang metamorphic complexes in east Tibet and Changning-Mengliancomplex in west Yunnan. Generally, the above metamorphic complexes constitute alarge-scale Indosinian accretionary orogen in Tibetan Plateau. Based on structuralanalyses on both macroscopic and microscopic scales, it’s concluded that theaccretionary complex mainly experienced three stages of deformation and two stagesof regional metamorphism. The earliest deformation(D1) bearing dextral shearexhibites intense rheology and shaped the general structural style, formed the regionalE-W-and N-S-trending tectonic belts and developed a penetrative foliation S1. The D1deformation is resulted from the subduction of the Paleo-Tethys Ocean dated at244-202Ma. Around200Ma，The Paleo-Tethys Ocean closed and the subduction andaccretion event finished. Under the convergence background, the accretionary complexexperienced the second contractional deformation(D2) and formed flexural folds, smallthrust faults and an axial crenulation foliation S2overpinting S1, reflecting that theaccretionary complex experienced a subsequent compressional deformation whenexhumed onto an shallower tectonic level. The last deformation(D3) recorded by nearlyN-S-trending buckle folds and nonpenetrative axial cleavage S3is deduced to beassociated with an contraction deformation of Qiangtang basin in Pliocene.
After the Jurassic, the South Qiangtang accretionary complex belt and the NorthQiangtang terrane merged into one unified terrane which exhibits intraplate deformation. The North Qiangtang terrane experienced Late Yanshanian, and Cenozoicdeformations on shallow-superficial level resulting from the closure of the Meso-andNeo-Tethys Oceans. The structural mapping of the Permian-Triassic structural layerindicates that the Mesozoic structural style was dominated by Late Yanshanianhorizontal contractional deformation characterised by NW-SE trending thrust faultsand flexural-slip folds. These folds and thrusts were superimposed on Indosinianbuckle folds. Structural analysis on deformations of the Neogene structural layerindicates that there were two transitions in the tectonic evolution of North Qiangtangterrane dividing the Cenozoic deformation into three stages. In episode I (50-40Ma to8-4Ma), the crust experienced large-scale N-S horizontal shortening and verticalthickening showing by buckle folds and thrust faults. In episode II (8-4Ma to86.9Ka), E-W passive extension formed conjugate strike-slip fault systems and pull-apartbasins. During this episode, the Qiangtang basin was extruded eastwards, and a numberof superficial N-S trending buckle folds were formed. In episode III (86.9Ka to thepresent), the stress field was reversed, and the Qiangtang basin experienced E-W activeextension, forming conjugate normal faults.
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