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青藏高原侏罗纪羌塘盆地构造—古地理和盆地演化

Brief Study on Tectonic-Palaeogeography and Its Evolution of Jurassic Qiangtang Basin in Tibetan Plateau, West China

【作者】 吕杰

【导师】 毛晓冬;

【作者基本信息】 成都理工大学 , 矿物学、岩石学、矿床学, 2010, 硕士

【摘要】 本论文以侏罗纪羌塘盆地形成演化的主要控盆构造系统研究为切入点,以实际区域地质调查资料为基础,以沉积学、大地构造、层序地层、地球化学等地质基础理论为指导,充分运用岩石地层、生物地层、年代地层、层序地层、事件地层分析以及沉积地球化学和同位素地球化学分析等方法相结合,系统分析盆地内沉积剖面结构和层序地层特征;同时对控盆构造特别是结合带岩石-构造地质特征开展系统分析,初步建立侏罗纪羌塘盆地演化模式。羌塘盆地位于青藏高原北部,夹于可可西里-金沙江结合带与班公湖-怒江结合带之间,南北边界断裂系统控制了盆地形态和地质构造特征。作为南羌塘地层区与北羌塘地层分区的界线龙木错-双湖结合带,根据对已有洋脊型蛇绿岩-蛇绿混杂岩和洋内弧-洋岛-海山增生楔混杂岩的地质条件、同位素年代学数据、地球化学特征及其深海放射虫硅质岩的确定等众多地质资料的综合分析,推论龙木错-双湖结合带、南羌塘古生代增生楔和班公湖-怒江结合带共同构成了青藏高原特提斯大洋最终消亡的巨型结合带(或特提斯对接带)。本论文在前人对羌塘地区区域地质调查报告和大量有关盆地、沉积作用、沉积相和古地理研究成果基础上,以岩石地层的系统划分为基础,生物地层为依据,通过对羌塘地区侏罗纪沉积地层的界面的分析和识别,对羌塘盆地侏罗纪地层进行客观的划分和对比。羌塘地层区内侏罗系分布广泛,厚度大,根据地层岩性组合特征,将羌塘构造-地层大区划分为两个地层分区,即北羌塘地层分区和南羌塘地层分区。北羌塘地层分区内侏罗纪地层发育齐全,“三砂夹两灰”组合特征明显,底栖的腕足、双壳类化石丰富,自下而上划分为:上三叠统-下侏罗统那底岗日组(T3J1n),中侏罗统雀莫错组(J2q)、布曲组(J2b)、夏里组(J2x),上侏罗统索瓦组(J3s)和雪山组(J3x)。南羌塘地层分区内侏罗纪地层不具有“三砂夹两灰”的特征,总体以海相碳酸盐岩及泥、页岩沉积为主,侏罗纪地层自下而上划分为下侏罗统曲色组(J1q),中侏罗统色哇组(J2s)、布曲组(J2b)、夏里组(J2x),上侏罗统索瓦组(J3s)和雪山组(J3x)。通过对地层不整合界面识别和对沉积层序划分与对比,对侏罗纪盆地不同演化阶段的岩相、古地理空间展布特征进行了系统的研究和分析,识别不同演化阶段的盆地充填沉积物特征。羌塘盆地侏罗系多数层段为碎屑岩和碳酸盐岩共生环境,少数层段为纯碳酸盐岩,因此,在相类型划分上按陆相环境、海陆过渡相环境、碎屑岩海相环境、碳酸盐岩海相环境和火山沉积环境来划分,整个盆地共鉴别出相类型16个、亚相类型37个。侏罗纪羌塘盆地层序地层研究共识别了6个二级层序,16个三级层序,其中1个I型层序,15个II型层序。通过对控制盆地的大地构造要素及整个青藏高原及邻区大地构造演化的综合分析,归纳侏罗纪羌塘盆地的发生、发展的大地构造演化信息,认识到侏罗纪羌塘盆地中央隆起的南、北两侧盆地的性质存在较大差异:侏罗纪北羌塘盆地是在晚三叠世前陆盆地基础上进一步演化而成的弧后前陆盆地,岩浆弧位于北羌塘南缘的那底岗日-各拉丹东一带;早侏罗世土阿辛期-中侏罗世巴柔期,南羌塘盆地性质完全继承了晚三叠世残余盆地的特征,形态上为北浅南深的单斜式盆地。晚侏罗世班一怒洋向北俯冲,南羌塘地区逐渐抬升成陆。南侧班公湖-怒江洋盆由离散板块边缘逐步向汇聚边缘演化,海水逐渐南移,在南羌塘盆地近班公湖-怒江洋盆一侧沉积了厚度较大、面积宽广的侏罗系。晚侏罗世末期班公湖-怒江洋向北俯冲,南羌塘地区逐渐抬升成陆。通过探讨沉积盆地构造沉降-沉积充填-海平面变化和构造隆升-沉积间断-海平面变化之间的关系,结合构造-岩浆事件分析,建立了侏罗纪羌塘盆地的形成演化模式。在早侏罗世期间,火山碎屑-陆源碎屑滨海-浅海相沉积区广泛分布于羌北弧后前陆盆地,并有少量三角洲、河流向沉积,在羌被盆地东北缘蒂让碧错一土门一索县一带发育了一套近海海陆交互相沉积;中央隆起带则主要表现出岛链的性质,部分地区为水下隆起,同时有少量海相碳酸盐岩台地沉积发育;中央隆起南缘羌南地区,则因为当时其南侧的班公湖-怒江洋盆的不断扩张,而继承晚三叠世残余盆地特征,相继发育了从陆缘碎屑滨岸的成熟度较高的石英砂岩或长石砂岩,到浅海直至半深海相带沉积特点;滨岸带沿中央隆起带南侧分布,河流相沉积不如羌北盆地发育,说明其地势较中央隆起带北侧的羌北盆地平缓;浅海-半深海沉积以陆源碎屑泥-页岩沉积为主的岩相带为特点,逐渐向南过渡为班公湖-怒江洋盆;羌北盆地的物源供给一部分来自北面的木孜塔格-巴颜喀拉造山剥蚀区,另一部分来自羌塘中央隆起;而羌南盆地物源则主要来自北面的中央隆起区。海侵作用在中侏罗世不断扩大,羌北地区的中侏罗世沉积分布广泛,总体上有陆源碎屑岩和碳酸盐岩组成,并且构成了一套底部碎屑岩组合-下部碳酸盐岩组合-中部碎屑岩组合-上部碳酸盐岩组合的沉积旋回,其中夹有多层膏盐等蒸发岩类,生物礁也较为发育。由于中、晚三叠世中特提斯的逐步形成,继日干配错群局限浅海环境之后,至早侏罗世时,整个羌南分区进入新的海侵阶段,处于正常的陆缘浅海环境,而多玛地区则成为一较为宽阔的浅海环境,总体属浅海碳酸盐台地沉积。多玛地区在进入中侏罗世莎巧木期之后,仍继承了早侏罗世的特点,持续保持浅海环境,发育一套以碳酸盐岩为主的沉积组合。羌塘地区晚侏罗世地层主要分布在北羌塘,南羌塘西南缘日土县发育有火山岩地层五峰尖组,在尼玛县北部靠近班公湖-怒江缝合带的部分地层面貌与北羌塘一致,晚侏罗世地层包括索瓦组、雪山组。侏罗纪羌塘盆地的发育经历了四个阶段:①大陆边缘裂陷阶段:在早侏罗世早-中期,为沉积盆地的孕育、产生期;②盆地快速拗陷扩展阶段:早侏罗世晚期羌北地区进入快速坳陷沉降扩展期,前期的大陆边缘裂陷区快速坳陷沉降,演化为羌北坳陷盆地。羌南盆地继续作为残余盆地的性质存在;③盆地持续沉降充填阶段:中侏罗世中期至晚侏罗世早期,羌塘盆地表现为相对稳定的持续沉降、充填阶段,为沉积盆地的发展期;④盆地萎缩阶段:在晚侏罗世晚期至早白垩世早期,羌南盆地抬升,发生了最后一次大规模的海退,羌北盆地为海平面下降过程中的沉积,反映了盆地逐步萎缩的特征。

【Abstract】 In this thesis, the focal point is the main structural system which regulates the formation and evolution of the Jurassic Qiangtang basin. We analyze the structure of in-basin sedimentary section plane and the character of sequence stratigraphic, by means of using the data of regional geological surveying as basis, geological basic theories which include sedimentology, tectonics, sequence stratigraphy, geochemistry as guidance, and in association with stratigraphic analysis of rock stratigraphic unit, biostratum, chrono stratigraphic unit, sequence stratigraphic, event stratigraphic, sedimentary geochemistry analysis and isotope geochemistry analysis. We also initially establish the evolution model of Jurassic Qiangtang basin by means of analysis of basin-regulation structure, especially analyzing the suture zone’s character of litho-structure.Qiangtang basin, which is sandwiched between Hoh Xil-Jinsha River suture zone and Bangong-Nu suture zone, is situated in the northern Tibetan Plateau. Basin shape and geological features are regulated by North-South border fault system.Lungmuco-Shuanghu suture zone is the dividing line between South Qiangtang stratigraphic region and North Qiangtang stratigraphic region. Based on comprehensive analysis of existing geologic setting of ridge type ophiolite-ophiolitic mélange and intraoceanic arc-oceanic island-seamount accretionary wedge mélange, and the isotope chronologic data, the geochemistry features, and the confirmation of deep sea Radiolarla silicalite, we deduce that the Lungmuco-Shuanghu suture zone, South Qiangtang Palaeozoic accretionary wedge and Bangong-Nu suture zone constitute the giant suture zone (or Thetys suture zone) which represents the final demise of Tibetan Plateau Proto-Thetys ocean.In this thesis, we conduct the division and correlation of the Jurassic strata of Qiangtang basin objectively, by using the previous regional geological survey reports of Qiangtang and numerous research results about basin, sedimentation, sedimentary facies and palaeogeographic, and based on the systematical lithostratigraphic division and biostratum, and in association with analysis and recognition of the Qiangtang Jurassic formation boundary. Jurassic strata are thick and widely distributed in Qiangtang region. According to the strata-lithologic association, we divide Qiangtang structural-stratigraphic area into two stratigraphic regions called North Qiangtang region and South Qiangtang region respectively. Jurassic strata develop completely in North Qiangtang region. And it is obviously that two limestone layers are contained within three sandstone layers. Fossils of benthic brachiopod and bivalvia are abundant. The Jurassic strata in North Qiangtang region, from bottom to top, could be divided into six formations. They are called Upper Triassic-Lower Jurassic Nadigangri Formation(T3J1n), Middle Jurassic Quemocuo Formation(J2q), Buqu Formation(J2b), Xiali Formation(J2x), and Upper Jurassic Suowa Formation(J3s), Xueshan Formation(J3x), respectively. The Jurassic strata of South Qiangtang region is marine facies carbonate, mudstone and shale as a whole, and does not have the character that two limestone layers are contained within three sandstone layers. It could be divided into six formations. They are called Lower Jurassic Quse Formation(J1q), Middle Jurassic Sewa Formation(J2s), Buqu Formation(J2b), Xiali Formation(J2x), and Upper Jurassic Suowa Formation(J3s), Xueshan Formation(J3x), respectively.Via identification of the unconformity interface and division and correlation of the depositional sequence, we study and analyze the lithofacies and palaeogeographic spatial distribution, which could represent the different evolution stage of the Jurassic basin, and then identify the filled deposit’s feature in different evolution stage. Most Qiangtang Jurassic layers form in the symbiotic environment including clastic rock and carbonate rock. Few are pure carbonate rock. We identify 16 facies types and 37 subfacies types in the entire basin, based on the facies classification including terrestrial facies environment, transitional facies environment, clastic marine facies environment, carbonate marine facies environment and volcanogenic sedimentation environment. We also identify 2 second order sequence and 16 third order sequence in sequence stratigraphic research of Jurassic Qiangtang basin. In 16 third order sequence, 1 is Type I sequence, other 15 are Type II sequence.By comprehensive analysis of the tectonic elements which regulate the basin and the tectonic evolution of Tibetan Plateau and its adjacent areas, and by generalization of the information of the Jurassic Qiangtang basin’s generation and evolution, we recognize that the North Qiangtang basin is differ from the Southern part in basin characteristic. The Jurassic North Qiangtang basin is a back arc foreland basin, which evolved from the Late Triassic foreland basin. The magmatic arc is located in Nadigangri-Geladandong, southern margin of the North Qiangtang. From Early Jurassic Toarcian stage to Middle Jurassic Bajocian stage, the South Qiangtang basin totally inherited from the features of Late Triassic residual basin, was a monocline basin that had shallow northern and deep southern, morphologically. In Late Jurassic, with the northward subduction of Bangong-Nu Ocean, South Qiangtang region gradually uplifted in land. Bangong-Nu oceanic basin had been transformed divergence plate boundary into convergence plate boundary. Sea water had transferred to the Southern part. Thick and wide Jurassic strata had been deposited at South Qiangtang basin near Bangong-Nu oceanic basin.Through investigation of the relationship between tectonic descents, sediment fill, eustatic sea level change of sedimentary basin and tectonic uplift, cessation of deposition, eustatic sea level change of sedimentary basin, and in association with tectonic-magmatic event analysis, evolutionary model and form pattern of Jurassic Qiangtang basin are established. In Early Jurassic, pyroclast-terrigenous detrital offshore-shallow sea facies sedimentary areas were widely distributed in North Qiangtang back arc foreland basin. And few delta facies and fluvial facies deposit existed. There is a set of offshore transitional facies deposit developed in the Dirangbicuo-Tumen-Suoxian area, Northeastern margin of the North Qiangtang basin. The central uplift belt mainly manifested the feature of island chain, and was partly underwater uplift. And few marine facies carbonate platform developed at the underwater uplift areas. Because of its southern Bangong-Nu oceanic basin’s constantly augmentation, the South Qiangtang basin inherited from the features of Late Triassic residual basin, and developed from epicontinental clastic shoreland facies sandstone to shallow sea facies and hemipelagic deposit in succession. Shoreland facies developed along the southern central uplift belt. Fluvial facies deposit in South Qiangtang basin was less than in the Northern part. This shows that the topography of South Qiangtang basin is smoother than North Qiangtang basin. Epeiric sea-hemipelagic deposit were formed by terrigenous clastic mudstone-shale, and gradually transited southward to Bangong-Nu oceanic basin. The provenances of North Qiangtang basin were the Muzitage-Bayanhar orogenic denuded zone and the Qiangtang central uplift belt. The provenance of South Qiangtang basin was mainly the Qiangtang central uplift belt. In Middle Jurassic, Marine transgression expanded gradually. Middle Jurassic deposit was widely distributed, and was made up of terrigenous clastic rocks and carbonate rocks. These two types of rocks formed a cycle of sedimentation including the basal clastic rock combination, the lower part carbonate rock combination, the middle part clastic rock combination and the upper part carbonate rock combination. There were also some layers of evaporate held within, and well developed bioherm. In Middle-Late Triassic, because of the formation of the meso-Tethys, after the Riganpeicuo Group which is restricted shallow sea environment, the entire South Qiangtang region entered a new marine transgressing stage in Early Jurassic, and was in the normal epicontinental marginal sea environment. But the Duoma area had become a broad epicontinental sea environment, and mainly formed by shallow sea carbonate deposit. Duoma area, in Middle Jurassic, was the same environment and deposit as in Early Jurassic. Late Jurassic Strata in Qiangtang area were mainly distributed in North Qiangtang. The volcanic rock strata, Wujianfeng Formation, developed in Ritu, southwestern edge of the South Qiangtang. The features of some strata developed at north Ritu, near the Bangong-Nu suture zone, are same as North Qiangtang. Late Jurassic strata include Suowa Formation and Xueshan Formation.Jurassic Qiangtang basin has experienced four stages of development. The first stage is continental margin chasmic stage. In early-middle stage of Early Jurassic, the basin entered into the stage of gestation and generation. The second stage is rapid expansion of basin depression stage. In late stage of Early Jurassic, North Qiangtang area entered into the stage of rapid expansion of basin depression, the previous continental margin faulted depression depressed rapidly, and finally evolved into the North Qiangtang basin. South Qiangtang basin was still a residual basin. The third stage is continued basin subsidence and filling stage. From middle stage of the Middle Jurassic to early stage of the Late Jurassic, Qiangtang basin manifested the relative stable and persistent depression and fill. This stage was the developing stage of the basin. The forth stage is basin atrophy stage. From late stage of the Late Jurassic to early stage of the Early Crataceous, South Qiangtang basin uplifted and underwent the last extensive marine regression. At the same time, North Qiangtang basin deposited a set of sediment which represented the process of sea level descent, and reflected the basin was gradually atrophied.

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