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    湘西沅麻盆地中新生代构造变形特征及区域地质背景

    Mesozoic-Cenozoic structural deformation characteristics of Yuanling-Mayang basin and regional tectonic setting

    • 摘要: 提要:沅麻盆地是叠加在雪峰造山带中段的一个中生代大规模陆相盆地,盆地中新生代断裂、褶皱发育特征尚缺乏系统、深入的研究。本文通过构造剖面测制、野外路线调查并结合区域地质资料,对沅麻盆地中新生代构造格架、褶皱和断裂的发育特征、变形序列、时代及区域构造背景等进行了详细的分析研究,取得以下主要认识和进展。(1)NNE-NE向正断裂、逆断裂和平缓褶皱(主要为向斜)组成沅麻盆地的主体构造格架,同时发育其他多组不同方向、性质的褶皱和断裂。盆地西部和中部大部以正断裂为主,形成复杂的堑-垒构造格局,褶皱变形强度低;盆地东缘因区域怀化—沅陵断裂带控制而变形强烈,以多为东倾、少量西倾的逆断裂为主,常伴有倒转紧闭褶皱。(2)中三叠世以来沅麻盆地经历多期构造变形事件:①中三叠世晚期印支运动中受区域NW—NWW向挤压而形成NE—NNE向褶皱和逆断裂;②晚三叠世—早侏罗世期间受区域SN向挤压,形成EW向褶皱和膝折;③中侏罗世晚期早燕山运动中受区域NWW—近EW向挤压,形成盆地东缘上三叠统—中侏罗统卷入的SN向和NNE向褶皱和逆断裂;④早白垩世区域NW-SE向伸展作用下形成不同规模的NE—NNE向正断裂;⑤晚白垩世在区域SN向伸展体制下形成EW向正断裂;⑥古近纪中晚期受区域NE向挤压,形成卷入白垩系及先期地层的NW向褶皱、NW向逆断裂、NEE—NE向左行走滑(或兼逆冲)断裂、NNE向右行走滑断裂、NEE向与NNE向共轭剪节理等;⑦古近纪末—新近纪初在区域NW向挤压下,形成最新卷入地层为白垩系的NE向褶皱、NE-NNE向逆断裂、SN向逆断裂。上述多期变形中以早白垩世NW-SE伸展形成的正断裂和古近纪末—新近纪初NW-SE向挤压形成的褶皱和逆断裂最为重要,其次为中侏罗世晚期NWW—近EW向挤压下形成的褶皱和逆断裂。(3)前人提出的沅麻盆地东部及东侧外围地区发育的王炳坡、黄金坳、潭湾、麻阳等“飞来峰”构造并不存在,所谓“飞来峰”实为向下“生根”的断夹块,组成“飞来峰”的板溪群座立于古生界和中生界之中与逆冲断裂或正断裂活动有关。

       

      Abstract: Abstract: The Yuanling-Mayang basin is a giant Mesozoic continental basin in the middle segment of Xuefeng orogen. At present, the features of the Mesozoic-Cenozoic faults and folds in the basin is lack of systematic and deep research. The authors studied such aspects of this problem as Mesozoic-Cenozoic structural framework, characteristics of the faults and folds, deformation sequences and epochs and regional tectonic settings through tectonic profile and field geological investigation, and reached the following conclusions: (1) The principal tectonic framework of the Yuanling-Mayang basin is composed of NNE- to NE-trending normal faults, thrust faults and gentle folds, accompanied by other folds and faults of different trends and natures. In the western and middle basin, main deformations are normal faults which formed complex horst-graben structure, with weak fold deformation. On the east margin of the basin, controlled by the Huaihua-Yuanling fault belt, there occur strong deformations, such as thrust faults which mostly dip east and subordinately dip west, often accompanied by overturned tight folds. (2) The basin has undergone several tectonic events since Middle Triassic: ① Regional NW to NWW-striking compression in late Middle Triassic Indosinian movement which formed NE to NNE-trending folds and thrust faults. ② Regional NS-striking compression during Late Triassic-Early Jurassic which formed EW-trending folds and kinks. ③ Regional NWW to EW-striking compression in late Middle Jurassic Yanshanian movement which formed NS- and NNE-trending folds and thrust faults involving Upper Triassic and Lower-Middle Jurassic on the east margin of the basin. ④Regional NW-SE striking extension in Early Cretaceous which formed NE- to NNE-trending normal faults of different scales. ⑤ Regional NS-striking extension in Late Cretaceous which formed EW-trending normal fault. ⑥ Regional NE-striking compression during middle-late Paleogene which formed NW-trending folds and thrust faults, NEE- to NE-trending sinistral strike-slip faults, NNE-trending dextral strike-slip faults, and NEE- or NNE-trending conjugated shear joints involving Cretaceous and early layers. ⑦Regional NW compression during late Late Paleogene-Early Neogene which formed NE-trending folds, NE- to NNE-trending thrust faults and NS-trending thrust faults involving Cretaceous and early layers. Among them, the most important deformations are normal faults which originated from the NW-SE-striking extension in Early Cretaceous, the folds and thrust faults originated from the NW-striking compression during late Late Paleogene-Early Neogene. (3) The klippes located in the eastern basin and east periphery, defined by Previous researchers, such as Wangbinpo, Huangjin’ao, Tanwan and Mayang, do not exist. The so-called “klippes” are actually faulted blocks with deep roots, and the Banxi Group located in Paleozoic and Mesozoic were related to the movement of thrust faults or normal faults.

       

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