Studies Of Tectonic Evolution And Thermochronology In The Northern Upper Yangtze Region

The upper Yangtze region is a representative, intense intraplate deformation zone in China. Itexperienced complicated, long-term shortening deformation. The region also show important, oilprospective in marine deposits in China. Therefore, understanding of the Mesozoic deformationprocesses and evolution have important theoretical and actual implications in this region. Usingstructural and isotopic methods, this work undertaken detailed studies on the western Hunan-Hubei,eastern Sichuan and southern Dabashan fold belts in the upper Yangtze region.After structural analysis in the basement rocks in the Xuefeng uplift,7slate samples from theBanxi Group in the basement were chosen for elite XRD and K-Ar analyses. The7samples gavecooling ages of419-389Ma, i.e. Late Silurian to Middle Devonian, suggesting that the upliftexperienced a Caledonian tectono-thermal event and uplifting. It is proposed therefore that theCaledonian deformation event in South China affected the Xuefeng uplift and reached westwards tothe Zhangjiajie-Huayuan-Kaili fault zone.Field investigation and microstructural analysis were made along the Fangxian-Qingfengsegment of the Chengkou-Fangxian fault zone in this work. This fault zone was subjected to twophases of top-to-the-SSW thrusting. Ductile deformation was involved in the first phase of thethrusting whereas brittle deformation was involved in the second phase of the thrusting. Sevenmylonitized siltstones from metamorphosed strata of Lower Silurian in the footwall of the fault zonewere chosen for sericite separation and⁴⁰Ar/³⁹Ar dating. Five sericite samples yielded good plateauages of225-218Ma (Late Triassic) which were interpreted as cooling ages of the thrustingdeformation. It is proposed that the first thrusting along the Chengkou-Fangxian fault zone tookplace at the end of Middle Triassic when the Yangtze Plate collided with the Qinling Block. It isinferred from the newly-found angular unconformity at the Indosinian interfaces between the Pingbaand Chengkou-Fangxian fault zones that the Indosinian foreland deformation zone on the YangtzePlate occurred between the two fault zones.It is found in this field work that highly angular unconformity at the Indosinian interfacesexists in the Xuefeng uplift whereas slightly angular unconformity at the Indosinian interfacesoccurs between the uplift and Zhangjiajie-Huayuan-Kaili fault zone. The region between the Pingbaand Zhangjiajie-Huayuan-Kaili fault zones show disconformity at the Indosinian interfaces at15localities. These demonstrate that the Indosinian shortening deformation affected the Xuefeng upliftand reached westwards to the Zhangjiajie-Huayuan-Kaili fault zone, and there is no Indosinianfolding in the region between the Pingba and Zhangjiajie-Huayuan-Kaili fault zones. Field investigation shows that interfaces among upper Triassic and upper Jurassic showconformity or disconformity in the study region, indicating that no folding took place between LateTriassic and Late Jurassic in this region. Several angular unconformities between Lower Cretaceousand underlying strata as well as folding of upper Jurassic and older strata in the westernHunan-Hubei Fold Belt indicate that the fold belt developed at the end of the Late Jurassic. Thefolding reached westwards to the Qiyaoshan fault zone. The fold belt is an arc-shaped beltprotruding NW. Folds in the belt are trough-like with box anticlines and chevron synclines. Dipdirections of axial planes and thrusts in the belt all show pushing from SE to NW during the folding.The youngest folded strata and angular unconformities between upper Cretaceous andunderlying strata demonstrate that the eastern Sichuan Fold Belt formed at the end of EarlyCretaceous. Superposition of many NNE-trending folds also happened in the western Hunan-HubeiFold Belt east of the eastern Sichuan Fold Belt. The eastern Sichuan Fold Belt between theHuayinshan and Qiyaoshan fault zones show typical comb-like folds with chevron anticline and boxsynclines. Axial planes of the anticlines mostly dip westwards, and still suggest pushing from east towest. It is inferred that predominant detachment layers are the base of Sinian and Silurianmudstones.The youngest folded strata in the southern Dabashan Fold Belt are lower Cretaceous.Shortening of the fold belt caused clear fold superposition in the northern segments of the easternSichuan and western Hubei and Hunan fold belts. These indicate that the southern Dabashan FoldBelt developed at the beginning of Late Cretaceous. This is a arc-shaped fold belt protruding SSW,which can be divided into a root belt between the Chengkou-Fangxian and Pingba fault zones, amiddle zone between the Pingba and Tiexi-Wuxi fault zones and a front zone just south of theTiexi-Wuxi fault zone. The root zone is characterized by thrust sheets experiencing the Indosinianthrusting deformation, the middle zone by comb-like, box folds and the front zone by open foldswith short axes. The fold belt was developed by pushing SSW. The second phase of thrusting alongthe Chengkou-Fangxian fault zone took place at this stage. The northern Dabashan Belt in thesouthern of the Qinling orogenic belt thrusted over the northern margin of the Yangtze Platesouthwards on a large-scale.Many fault-slip data measurements on thrusts in the folded strata give113sets of stress data,which can be divided into three phases of stress fields. By comparison with the above-mentionedfolding evolution, it is proposed that a compression direction at the end of Late Jurassic is NW-SWrelated to the formation of the western Hubei-Hunan Fold Belt, the compression direction at the endof Early Cretaceous is nearly E-W related to the development of the eastern Sichuan Fold Belt, andthe compression direction at the beginning of Late Cretaceous is NE-SW related to formation of thesouthern Dabashan Fold belt. This paleo-stress analysis further proves the above division of the structural evolution stages.It is proposed from these studies that the region west of the Zhangjiajie-Huayuan-Kaili faultzone did not experienced the Caledonian folding. Four phases of folding in the Mesozoic arerecognized in this region. The region evolved from the Indosinian folding along the northern marginof the northern Dabashan Belt as well as the Xuefeng uplift and its western margin at the end ofMiddle Triassic, formation of the western Hubei-Hunan arc-shaped, through-like Fold Belt at the endof Late Jurassic, development of the eastern Sichuan comb-like Fold Belt at the end of EarlyCretaceous to formation of the southern Dabashan arc-shaped Fold Belt at the beginning of LateCretaceous. Three dynamics from closure of the paleo-Tethys and neo-Tethys as well as subductionin the Pacific Ocean acted in this region respectively, leading to long-term, complicated evolution ofthe intraplate deformation in the upper Yangtze region.