THE CRUSTAL COMPOSITIONS, STRUCTURES AND TECTONIC EVOLUTION OF THE QINLING OROGENIC BELT
Zhang Guowei; Dong Yunpeng; Yao Anping(Department of Geology, Northwest University, Xi'an, 710069)
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.
【CateGory Index】： P542
【CateGory Index】： P542