秦岭造山带基本组成与结构及其构造演化

THE CRUSTAL COMPOSITIONS, STRUCTURES AND TECTONIC EVOLUTION OF THE QINLING OROGENIC BELT

秦岭造山带主要由三大套构造岩石地层单元组成，经历了三个主要演化阶段：1．前寒武纪古老基底形成演化阶段，2．主造山期（Pt3—T2）板块构造演化阶段，3．中新生代陆内构造演化阶段。在早中元古代以扩张构造体制占主导，形成裂谷与小洋盆兼杂并存的基本构造格局，经10—8亿年晋宁期从扩张垂向加积增生构造体制为主向以侧向增生为主的板块构造体制的过渡，于晚元古代中晚期开始进入板块构造演化阶段。在晚古生代早期由于东古特提斯洋的形成，扬子板块北缘沿秦岭南部扩张打开，形成华北板块、扬子板块及其间的秦岭微板块，沿商丹和勉略二缝合带自南向北俯冲消减碰撞，于中三叠世最后全面陆陆碰撞造山，而后又发生了强烈陆内造山作用，终成今日之秦岭山脉面貌。现今的秦岭造山带岩石圈结构是一正在调整演化中的具流变学分层的“立交桥式”三维结构，上部地壳呈多层逆冲推覆迭置的不对称扇形几何学模式，岩石圈中部则是成水平状流变层，而深部地幔则是最新调整的近南北向的地球物理异常状态与结构，形成从下到上构造方向近乎正交的圈层非耦合关系。

The basic facts of the Qinling orogenic belt are that it is composed of three distinct teetonstratigraphic units and under went three main tectonic evolutionary stages. The firststage is the formation of the orogenic basements in Precambrian. The second stage is theplate tectonic evolution in Late Proterozoic-Middle Triassic, which is the main orogenic stageand characterized by plate tectonic regime. The last is the Mesozoic-Cenozoic intracontinental tectonic evolutionary stage. The most fundmental geological facts suggest that theQinling orogen was typified by mosaic rifts and small oceans with vertical accretion growthand intensive mantle intrusion, and formed in an extensional setting during Early--MiddleProterozoic. The Jiningian epoch (1000--800 Ma) is the transitional evolutionary stage fromthe extional and vertical accretion and extional tectonic regime to plate tectonic regime. TheQinling orogen was evolved in a plate tectonic regime from Middle Late to Proterozoic. Asshown by Paleozoic geological records, the present-day tectonic framework of the Qinlingwas constructed during the main orogenic stage. The Qinling was originally a branch on thenorthern flank of the paleo-Tethys during the periods from late Proterozoic to the early andmiddle Paleozoic, and two limited oceans then developed which separated North China plate,Yangtze platform and Qinling microplate and eventually marked two major suture zones,called Shangdan and Mianlue, respectively. The subduction between them led to continentcontinent obliquely diachronous collision in late Hercynian and indosinian times. Intensivemagmatism and crustal deformation indicate strong postorogenic intracontinental orogenesisduring the Mesozoic-Cenozoic eras. The fundmental composition and evolution of the Qinlingwere built up by different orogenesis under different mechanisms throughout its history. Thedetailed studies of geology and geophysics indicate that the present 3-D model of the Qinlingis a " flyover-like" framework. Deep geophysical field is featured by nearly S-N trendinganomalies, while the upper crust is dominated by E-W treading structures. Between themare the middle and lower crusts which are in a rheological stage of the horizontally flattening. Fundmental structures of the upper crust was bunt during the main orognic phase,which contains residual structures and is intensively superposed by the late intracontinentaltectonism. This tectonic model for the Qinling orogen is distinct from the existing ones inthat it represents a complicated development of diverse tectonic regimes.