The Early Paleozoic Structural Deformation Mechanism And Evolution Proccess In East Guizhou And Its Neighbor Area

The interaction between Yangtze Block and Cathaysia Block within the South China Block had been the main focus for the domestic and oversea researcher,including the long-termed complicated tectonic evolution in the South China Block.Among those issues,it is still in debate that whether the orogenic process caused by the interaction between Yangtze and Cathaysia in Early Paleozoic belongs to the typical continental-oceanic subduction or collision or the intraplate orogeny far away from the continental margins.And the magmatisms,deformation,metamorphism and provenance research are limited in the Wuyi-Yunkai area in Cathaysia,rarely reported in the southeastern Yangtze.Meanwhile it is still unclear about the link between the South China and the Gondwana supercontinent: where is the exact position of South China in the reconstruction model of Gondwana;Why the Early Paleozoic magmatisms in South China postponed so long after those in Gondwana? Both the questions are critical to understand the Early Paleozoic events in South China and to rebuild the Gondwana model.The Neoproterozoic to Early Paleozoic strata widely outcrop within the East Guizhou and its neighbor area along the southeastern margin of Yangtze Block.It is indicated by the strata unconformities that at least there events could be identified.And abundant structural deformation such as folds,faults and ductile shear could be observed in the Neoproterozoic to Early Paleozoic strata in the study area.However,it is still unclear for the stage division,overlap relationship,evolution stage,forming background and its link to the Gondwana for these Early Paleozoic deformations.In recent years,the study on the Early Paleozoic strata provenance in South China likely recorded the material from Gondwanaland.Thus,the Late Neoproterozoic to Early Paleozoic strata and structural deformations along the southeastern margin of Yangtze Block could provide new information to constrain the evolution of Yangtze and Gondwana,especially to constrain the tectonic nature of the Early Paleozoic deformation in Yangtze.This thesis aims to study the Neoproterozoic to Early Paleozoic strata in southeastern margin of Yangtze Block,by detailed investigating the field outcrop and typical corridors to analyze its deformation and structural assembly.The indoor research aims to acquire the cooling age of the syn-defomation mineral in the strong strain zone like ductile shear zone by Ar-Ar method,and to acquire the deformation temperature by analyze the micro-structural and quartz c-axis fabric to constrain the deformation time of the ductile shear in the area.Meanwhile,the U-Pb dating and Hf isotope of detrital zircon are used to constrain the provenance of each strata in the study area and to reveal the tectonic evolution of the basin,to define the provenance reverse time of the Late Neoproterozoic to Early PaleozoicOur work is guided by analytical tectonics and plate tectonics.Integrated with the latest research in this area,we further make it clear that the nature of the Late Neoproterozoic to Early Paleozoic magmatic,sedimentary,and metamorphic events along the southeastern margin of Yangtze by compare those in different area.Moreover,we proposed the position of South China in the Gondwana reconstruction by comparing the magmatic and metamorphic events between different blocks.The main conclusions in this thesis are listed below: 1,The structural deformation is obvious zoning characteristic in the study area.The whole study area is located to the west of the Jiangshao-chenzhou-linwu fault,the subduction area of the southeastern Yangtze block.The thin-skin type NW and SE directed thrust fault and folds,and the capping-sliding type folds and faults are developed.There tectonic belts can be divided by the main regional boundary faults: the west Guangxi basement-cap thrust fold belt to the east of the Jingzhou-Qianyang fault,is defined by the SE directed thrust fault and several thrust fault forming the nappe type with the triangle structural belt,traction structure and etc.The strata is usually steep dip and developed inner-layer tight cuspidal edge fold,defined as the strong structural belts close to the root area.The East Guizhou basement uplift belt to the east of Sandu-Kaili fault and west of Jingzhou-Qianyang fault,is defined by wide fold and forming the core of the duplex anticline with the Neoproterozoic basement strata,the intensity of this belt is obviously lighter than the previous belt.The middle Guizhou cap detachment fault to the east of the Sandu-Kaili fault,is defined as the box detachment fold,thrust front belt resulting in the NW-directed fault and the normal fault resulted by the gravity detachment.The crushing stress is relaxed by the detachment fold deformation,thus the intensity of this belt is obvious slighter.In total,the intensity of the stress is relaxed continuously from SE to NW,rather than the symmetry flower type structural type.Hence we can conclude that the main stress direction forming the tectonic style is coming from the SE side and the structural initiated from the southeastern area.2,The Congjiang-Gaowu ductile shear zone,Yongli-Lingli ductile shear zone and Zaibian shear zone are both NNE-NEE directed steep-dip brittle-ductile shear zone that transported into the surrounding rocks.Mylonitize is common in the ductile shear zone.The NW to SE thrust ductile shear signature can be indicated by the augen asymmetry quartz feldspar rotational porphyroclast,the S-C fabric.The micro structural reveal that the quartz in the ductile shear zone mainly display the character of bulging recrystalization,subparticle rotation recrystalization,suggesting it underwent high green-shit to low amphibolite grade(400-500?)and low green-shit grade(300-400?)metamorphism.The EBSD quartz c-axis fabric also indicate that the study area underwent the mediate to low temperature deformation(400-550?)to the low temperature deformation(<400?).3,The Ar-Ar dating on the sericite,biotite and plagioclase from the Congjiang-Gaowu ductile shear zone,Yongli-Linli shear zone and Nanjia shear zone is 416 to 102 Ma,representing the cooling age of exhumation after the thrust shear.Integrated with the regional data,we suggest the South China was in NW-SE press filed during the late Ordovician to middle Silurian(453 to 426Ma),resulting the NW to SE reverse thrust in the east Guizhou and its neighbor area.The regional stress field turn from press to stretch during middle Silurian to early Devonian(426-419Ma)and exhumed at early Devonian(416-402Ma).During the late Early Paleozoic along the southeastern Yangtze and Cathaysia blocks,a series of synchronal NW-SE directed thrust and NE-trended strike-slip shear were formed during the transpressing during the oblique collision.4,In this contribution,we present new zircon U–Pb ages and Hf isotope data for detrital zircons from the Neoproterozoic to Silurian sedimentary sequences in the southeastern Yangtze Block.Samples from Neoproterozoic rocks generally display a major peak at 900–560 Ma,whereas samples from Lower Paleozoic rocks are characterized by several broader peaks within the age ranges of 600–410 Ma,1100–780 Ma,1.6–1.2 Ga and 2.8–2.5 Ga.Provenance analysis indicates that the 900–630 Ma detritus in Cryogenian to Ediacaran samples were derived from 900–820 Ma syn-collisional igneous rocks and from 820–630 Ma syn-rift igneous rocks in South China that acted as an internal source.The occurrence of 620–560 Ma detritus indicates southeast Yangtze was associated with the Late Neoproterozoic arc volcanism along the north margin of East Gondwana.The change of provenance resulted in the deposition of 550–520 Ma and 1.1–0.9 Ga detrital zircons in the Cambrian–Ordovician sedimentary rocks.The ?Hf(t)values of these detrital zircons are similar to those of the zircons from the northwestern Australia–Antarctica and South India.This change of provenance in the Cambrian can be attributed to the intracontinental collision between South China and South Qiangtang,and the convergence of India and Australia when East Gondwana finally amalgamated.5,Combined with the previous research,the east Guizhou and its neighbor area mainly underwent three stages:(1)Cambrian to middle Ordovician(540-460Ma)initial intraplate subduction;(2)middle Ordovician to late Silurian(460-420Ma)intraplate subduction to collision;(3)late Silurian to early Devonian(420-390Ma)postoroginc exhumation.