Qinling Orogenic Belt Something The Qinling Handover Conversion Area Intracontinental Tectonic Characteristics And Evolution Of The Process

Most of the orogens in the world had not gone to cratonization after collision but a new tectonic stage characterized sorts of intensive intracontinental tectonics activities.Some of the activities,not well explained by plate tectonics,are unique process of continental tectonic evolution,and consequently become excellent materials which could be contributed to the establish of the continent dynamic theory.The great Qinling Mountains which geographically separates the North and South China today are an indosinian collisional orogen between NCB and SCB,the tectonic activities had not ceased after collision,but became more intensive even than the collision event.The conjunction area of East and West Qinling orogen(CAQ hereafter) which locate in the central part of the Qinling is the narrowest area along the strike,and also the crossing area of EW striking Qinling orogen and SN striking Helan-Chuandian tectonic belt.In Cenozoic,it also became the margin of the Tibet plateau.Consequently,various intracontinental tectonics activities were developed and became a critical area for the study of the intracontinental tectonic evolution of Qinling.This study presents much new structure date of the late indosinian granites,sedimentary data and structure analysis of the intramountain J₁₊₂ basins and Wenxian-Taibai tectonic belt,then summarized the characteristics of the intracontinental tectonics activities and discussed the process and mechanism of them.1.The CAQ gradually transferred into orogenic collapse stage(T₃-J₁₊₂) after collision which involved the entire crust and/or the lithosphere from the lower part to the upper part.The late indosinian granites in the CAQ display primarily magmatic structure,and their magnetic foliations and lineations are very different from the regional structure characterized the thrust tectonics,the emplacement mechanism also changed in different areas,all these evidence indicate that the tectonic settings is not compressional in late indosinian period when the magmatic activities developed,but a tectonic quiescence just after collision.Combine with the "Post-collisional" geochemical characteristics and the participation of mantle-derived composition;the emplacement events of granites maybe represent a transition from collision to orogenic collapse,and the magma maybe derives from the breakoff or delamination processes,then ascend to the upper crust along the strike-slip or transtensional fault. Sedimentary and structure date display that the J₁₊₂ intramoutain basin are half-graben basins, the sedimentation is controlled by the extensional bolder fault in the margin of the basins,and there are also a series of syn-sedimentary normal falut developed in the internal basin and form the horst and graben structure style,which indicate the upper part of the crust transfer from compressional regime to extensional regime,and the orogenic collapse reach the climax.Late indosinian magmatism,J₁₊₂ extensional basins and the synchronous extensional metamorphic dome in this area can be considered as the different reflection from the lower part of the crust to the upper part about the orogenic collapse.2.Intensive intracontinental orogeny had been developed in the CAQ during J₃-K₂,and the crust was greatly shortened again after collsion orogeny.Thrust and special conjugate structures had been developed in the CAQ to accommodate the convergent movement between NCB and SCB,especially the Ordos Block and Yangtze Block.Great crust-shortening after collision had been occurred during J₃-K₂ in the CAQ,and mainly focus on two aspects.First one is that the J₁₊₂ extensional basins that developed in the orogenic collapse stage ceased and suffered extensively compression,which display an intracontinental thrust inherited from the collision structures.The second one is that the development of the NEE striking Wenxian-Taibai sinistral strike-slip faults belt.The belt is developed mainly during J₃-K₂,and the tanstensional activities during K₁ controled the sedimentation and formation of the K₁ basins along the strike of the faults belt.There are two stage of the evolution of the K₁ basins,Fengxian basin and Chengxian basin formed in the early period of K₁,Shixia-Erlangmiao bansin,Moba basin and Qiaotou bansin in the late period,which indicate twice activities of the bolder faults.And the strata of the basins were then folded in K₂.Combine with the evolution of the Sichuan and Ordos basin that distributed in the two sides of the CAQ this period,we concluded that the NCB and SCB was intensively converged during J₃-K₂ again after collision,and the lithosphere of NCB and SCB hugely subducted beneath the orogen,led to the expansion of orogen to the foreland area and control the development of the foreland basins,and formed a conjugate NEE and NWW strike-slip faults belt in the CAQ which greatly accomadated the convergence of the Ordos and Yangtze Block.3.The CAQ became the margin of the Tibet plateau in Ccnozoic,and gradually formed new tectonic frameworks which control the development of the sedimentation and magmatism, which constitute part of the transitional margin of the Tibet plateau,the material of Tibet Plateau was limitedly escaped eastward along the CAQ.The CAQ became the margin of the Tibet plateau in Cenozoic,and the SN striking structures characterized distribution of thickness of the crust and geophysical features cross superimposed on the previous EW Qinling structures,three group sinistral faults belt,NEE, NWW and EW were developed and controlled the E-N sedimentary basins and magmtism, and also became the intensive earthquake belts,anastomosing in the east part of the CAQ,all of these combined together to form a "fence-like" escaping tectonic framework in the CAQ which result from the resistence of the NCB,SCB against the outward expansion of the Tibet plateau.