南海北部陆缘构造属性研究进展

南海北部陆缘位于特提斯与古太平洋两大构造域的叠合部位,构造特征十分复杂,其构造属性一直是国内外学者争论的焦点,从主动陆缘到被动陆缘,火山型被动陆缘到非火山型被动陆缘等均有表述。南海复杂的形成机制以及东、西部构造差异性所引起的地球物理、岩浆活动等认识的异同,是造成南海北部陆缘构造属性认识差异的主要原因。通过与全球典型地区的比较研究,进一步加强对南海形成演化过程分析,开展大洋钻探与多学科综合分析,揭示南海海盆的多期扩张与多盆张裂特征,是认识南海北部陆缘构造属性的关键。探讨了南海三叉裂谷张裂模式,初步认为南海第1次扩张具有非火山型被动陆缘性质,第2次扩张具有火山型陆缘性质。     

南海北部新生代盆地群构造特征及其成因

南海北部陆缘自西向东分布有北部湾、琼东南、珠江口和台西南等新生代盆地。前人认为这些盆地是华南大陆东南缘裂解直至南海北部被动陆缘形成过程中逐渐形成的,但大量地震剖面揭示,南海北缘主控盆断裂倾向陆地,与典型的被动陆缘的主断裂倾向海盆的特征明显不符。因而,南海北部陆架盆地成因显然不是被动大陆边缘的Mckenzie伸展机制。为此,基于大量陆地调查和海域地震剖面资料的对比,揭示了南海北部陆缘至少在34Ma之前不是被动大陆边缘,早期陆缘断裂十分发育,主控断层为NE—NNE走向,和陆地同期走滑断层具有连续性。这些NNE—NE向断裂右行右阶走滑控制了拉分盆地内的EW或NEE方向的次级断裂,并控制了盆地内部近EW向的次级构造单元展布。因此,新生代南海北部陆缘的一系列盆地是动力学成因上具有密切联系的右行右阶拉分盆地群。这个拉分成因模式与南海北部陆缘新生代盆地内部沉积沉降中心迁移、构造跃迁、岩浆展布等特征非常一致。而南海北部真正成为典型被动大陆边缘的时间是在15Ma之后,但此时南海却停止了扩张,而且大约在10～5Ma由于菲律宾海板块沿吕宋岛弧-台湾造山带逐步楔入欧亚板块导致最后的弥散性NWW向断裂切割南海北部所有构造。从盆地动力学考虑,南海北部陆架盆地的成因主要与太平洋板块的动力学联系较为紧密。     

南海大陆边缘动力学研究进展:从陆缘裂解到海底扩张

南海处于印度—澳大利亚、欧亚和太平洋三大板块汇聚中心,地理位置独特,地质作用复杂,是研究大陆裂解—海底扩张过程以及大陆边缘动力学的天然地质实验室。通过总结近年来对南海大陆边缘动力学研究的最新进展认为:①南海北部陆缘为非火山型被动陆缘,其东段虽有岩浆底侵活动,但其为海底扩张结束后的产物,南部陆缘未发现下地壳高速层,也为非火山型陆缘;②南海陆缘上下地壳的拉张因子存在差异,表明陆缘的张裂变形在纵向上并非是均一的,而具有随深度变化的特点;③南海海盆是通过2期扩张形成的,具有由NE向SW渐进式扩张的特点,其东侧表现为成熟的洋盆,而西侧保留了更多陆缘裂谷的特征;④南海海盆形成的动力学模型存在碰撞挤出模型和古南海拖曳模型2种争论,2种模型各有优缺点,需要今后进一步综合研究。     

南海北部陆缘地壳结构探测结果分析

深部地震和重力资料反演揭示了南海北部陆缘地壳结构在总体上由北部的华南沿海（厚约30km）向南部的洋盆（5──8km）逐渐减薄。南海的近SN向拉张不仅造成南北方向地壳结构的巨大变化，也造成东西向的明显变化。在南海北部陆缘的西部，局部拉张产生了一系列裂谷构造。西沙海槽作为一条狭窄的陆内裂谷向西延伸，海槽南北两侧地壳厚度超过25km，海槽中部地壳减薄至不足10km。西端的莺歌海盆地地壳厚仅5km，缺少明显的壳内反射－折射。在珠江口盆地中部，地壳厚度在下陆坡明显减薄，地壳下部存在较薄的（3──4km）高速层（地震波速7．2──7．5km·s－1）；在珠江口盆地东部，地壳底部存在约 10km厚、300km宽的高速层。在台湾地区，由于弧陆碰撞，曾经减薄的陆壳在碰撞带增厚，莫霍面深度超过30km。南海北部陆缘在裂谷拉张和海底扩张期间岩浆活动平静，表明南海北部陆缘为非火山型陆缘。     

南海及邻区岩浆岩时空分布特征及机制

南海的岩浆作用自始至终高度活跃,岩浆活动规模远远超出以前的想象。根据地壳发展阶段及其产生的岩浆作用强度和类型的差异,结合岩石同位素年龄资料,将南海及邻区划分为前吕梁期、吕梁期、晋宁期、加里东期、海西期、印支期、燕山期、喜马拉雅期等8个岩浆作用时期,主要分布于南海及周缘的广东、广西、海南岛、台湾岛、中南半岛、加里曼丹岛、菲律宾群岛,时代从前吕梁期至喜马拉雅期均有出露。南海及邻区最老的岩浆岩是在中南半岛出现的太古代黑云母花岗岩、紫苏花岗岩和辉长岩;最新的现代岩浆岩海陆均有发现,南海西南部和菲律宾等地区至今还有火山喷发岩浆活动。南海海区岩浆岩以燕山期和喜马拉雅期为主,燕山期以中酸性侵入岩为主,广泛分布于南海陆缘,尤其南海北部和西南部最甚;喜马拉雅期以强烈的基性、超基性岩浆活动为主,遍布于整个南海海区,以玄武岩为主。总体上,海区岩浆活动要比陆区晚。     

南海北部晚新生代岩浆活动的发育特征与构造意义

南海北部陆缘具有极其独特的岩浆活动特征,其岩浆活动在大陆张裂和破裂期间表现的较为薄弱,而在裂后期尤其是海底扩张停止之后的晚新生代时期却变得极为强烈。通过总结南海北部晚新生代玄武岩浆的岩石学、年代学、地球物理学等方面的研究成果,从发育规模、物质属性、构造模式以及通道特征等方面系统揭示了岩浆活动的发育特征及其隐含的构造意义。结果显示:(1)南海北部晚新生代玄武岩均显示洋岛玄武岩(OIB)的物质属性,且与世界典型热点火山OIB具有相似的同位素分布范围;(2)南海北部海底火山的侵入和喷出体积量与世界典型的大火成岩省具有可对比性;(3)岩石圈伸展过程中所形成的张性断裂可能为后期玄武岩浆的活动提供了良好的通道;(4)沉积地层中所发现的岩脉与岩墙复合体与海底火山活动应该具有相同的岩浆来源;(5)全球和区域地震层析成像结果均显示了一条清晰的深部低速通道,暗示了南海北部晚新生代玄武岩浆活动可能与深部地幔柱存在紧密联系。     

南海北部陆缘张裂—岩石圈折沉的地壳响应

南海北部陆缘在中生代晚期曾形成宏伟的华夏陆缘造山带。火成岩岩石学、岩相古地理学和地球物理学证据显示，该造山带不仅具有巨厚（50－60km)的陆壳，而且还有巨厚（160－180km)的岩石圈根，在地势上曾出现过高3500－4000m的华夏山系。陆缘裂陷盆地的形成发育历史、地壳－岩石圈深部结构、火成岩地球化学特征及理论计算均表明，南海北部陆缘从晚白垩世以来发生的张裂作用起始于华夏陆缘造山带的拉伸塌陷，岩石圈折沉是南海北部陆缘张裂的重要的引发机制。因此，南海北部陆缘张裂既不同于弧后扩张，也不受控于大西洋式的海底扩张，而是该区大陆构造演化和深部壳幔相互作用的结果。     

南海北部陆缘地热状态及其成因探讨

对收集、整理的462组大地热流和地温梯度数据进行统计分析,结果表明南海北部陆缘具有普遍偏高的大地热流和地温梯度,大地热流总体表现为由陆架向洋盆方向递增的趋势。海底热流资料经稳态温度场计算南海北部随深度变化的热流和温度分布,获取热居里面深度,与地磁资料反演的居里面深度进行对比,发现南海北部中、下陆坡磁居里面深度浅于热居里面深度,处于地热不平衡状态。通过对地壳结构、拉张因子、莫霍面埋深、断裂带及火山活动的综合分析,表明南海北部陆缘地热状态受控于地壳拉张减薄和莫霍面抬升的构造格局,裂后晚期局部岩浆活动对地热状态亦有影响。     

南海北部陆缘张裂--岩石圈拆沉的地壳响应

南海北部陆缘在中生代晚期曾形成宏伟的华夏陆缘造山带。火成岩岩石学、岩相古地理学和地球物理学证据显示，该造山带不仅具有巨厚(50～60 km)的陆壳，而且还有巨厚(160～180 km)的岩石圈根，在地势上曾出现过高3 500～4 000 m 的华夏山系。陆缘裂陷盆地的形成发育历史、地壳-岩石圈深部结构、火成岩地球化学特征及理论计算均表明，南海北部陆缘从晚白垩世以来发生的张裂作用起始于华夏陆缘造山带的拉伸塌陷，岩石圈拆沉是南海北部陆缘张裂的重要的引发机制。因此，南海北部陆缘张裂既不同于弧后扩张，也不受控于大西洋式的海底扩张，而是该区大陆构造演化和深部壳幔相互作用的结果。     

IODP367和368航次——南海张裂陆缘

正IODP 367和368航次将解决大陆解体期间岩石圈的减薄过程。据深反射地震数据揭示,南海北部(SCS)岩浆匮乏的张裂陆缘是一处优良的钻探地点。南海北部陆缘与伊比利亚纽芬兰岛裂谷陆缘相似,可能包括洋陆过渡带(COT)内已剥露及蛇纹石化的地幔。然而,最近的模拟研究表明,板弱化机制与其他的次大陆存在的岩石圈地幔的蛇纹石化不同。板块破裂的两个模式(在很热的软流圈地幔的情况下)可以有所     

The role of rifting in the development of the continental margins of the southwest subbasin,South China Sea: Insights from an OBS experiment

The continental margins of the southwest subbasin in the South China Sea mark a unique transition from multi-stages magma-poor continental rifting to seafloor spreading. We used reflection and refraction profiles across the margins to investigate the rifting process of the crust. Combining with the other seismic profiles acquired earlier, we focused on the comparative geological interpretation from the result of multichannel seismic analysis and wide-angle seismic tomography. Our result provides the evidence of upper crustal layer with abundant fractures below the acoustic basement with a P-wave velocity from 4.0 to 5.5 km s^?1. It indicates extensive deformation of the brittle crust during the continental rifting and can make a good explanation for the observed extension discrepancy in the rift margins of the South China Sea. The seismic chronostratigraphic result shows the possibility of the intra-continental extension center stayed focused for quite a long time in Eocene. Additionally, our evidence suggested that continental margin of the southwest subbasin had experienced at least three rifting stages and the existence of the rigid blocks is an appropriate explanation to the asymmetric rifting of the South China Sea.     

中生代晚期到新生代拉张应力下的华南大陆边缘

主要论述中生代晚期到新生代期间拉张应力作用下珠江口盆地的张裂和南海海盆的扩张,并根据拉张破裂理论论述它们之间的相互关系,进而探讨华南大陆边缘在这期间内产生上述形变的动力学机制。     

Structural evolution of Malay Basin,its link to Sunda Block tectonics

The Malay Basin is located offshore West Malaysia in the South China Sea, within north central region of 1st order Sunda Block. The basin developed partly as a result of tectonic collisions and strike-slip shear of the Southeast Asia continental slabs, as the Indian Plate collided into Eurasia, and subsequent extrusion of lithospheric blocks towards Indochina. The Sunda Block epicontinental earliest rift margins were manifested by the Palaeogene W–E rift valleys, which formed during NW–SE sinistral shear of the region. Later Eocene NW–SE dextral shear of (2nd order) Indochina Block against East Malaya Block rifted open a 3rd order Malay Basin. Developed within it is a series of 4th order N–S en-echelon ridges and grabens. The grabens and some ridges, sequentially, host W–E trending 5th order folds of later compressional episodes. The Malay Basin Ridge and Graben Model explains the multi-phased structural deformation which started with, the a) Pre-Rift Palaeo/Mesozoic crystalline/metamorphic Basement, b) Synrift phase during Paleogene, c) Fast Subsidence from Late Oligocene to Middle Miocene, d) Compressional inversion of first Sunda fold during Late Miocene, and e) Basin Sag during Plio-Pleistocene with mild compressional episodes. The subsequent Mio-Pliocene folding history of Malay Basin is connected to the collision of Sunda Block against subducting Indian–Australian Plate. This Neogene Sunda tectonics, to some degree after the cessation of South China Sea spreading, is due to the diachronous collision along the 1st order plate margins between SE Asia and Australia.     

南海海盆的形成演化探讨

根据“陆缘扩张”理论,利用古地磁数据,结合地质、地球物理资料,对南海海盆的成因机制和演化过程进行探讨。结果得出:南海的多期多轴扩张及其形成演化,是在欧亚板块、太平洋板块和印度板块的联合作用下,中、新生代南海周缘的微板块和岛弧的相互运动以及南海海盆构造应力场不断变化的情况下逐渐完成的。     

Structures of the northeasternmost South China Sea continental margin and ocean basin: geophysical constraints and tectonic implications

The northeastern part of the South China Sea is a special region in many aspects of its tectonics. Both recent drilling into the Mesozoic and new reflection seismic surveys in the area provide a huge amount of data, fostering new understanding of the continental margin basins and regional tectonic evolution. At least four half-grabens are developed within the Northern Depression of the Tainan Basin, and all are bounded on their southern edges by northwestward-dipping faults. One of the largest half-grabens is located immediately to the north of the Central Uplift and shows episodic uplift from the late Oligocene to late Miocene. Also during that period, the Central Uplift served in part as a material source to the Southern Depression of the Tainan Basin. The Southern Depression of the Tainan Basin is a trough structure with deep basement (up to 9 km below sealevel or 6 km beneath the sea bottom) and thick Cenozoic sedimentation (>6 km thick). Beneath the Southern Depression we identified a strong landward dipping reflector within the crustal layer that represents a significant crustal fault. This reflector coincides with a sharp boundary in crustal thicknesses and Moho depths. We show that the northeasternmost South China Sea basin, which may have undergone unique evolution since the late Mesozoic, is markedly different from the central South China Sea basin and the Huatung Basin, both geologically and geophysically. The Cenozoic evolution of the region was largely influenced by pre-existing weaknesses due to tectonic inheritance and transition. The South China Sea experienced multiple stages of Cenozoic extension.     

Marine mineral resources of the South China sea

Abstract

The South China Sea is located within the domain of a plate triple‐junction and can be divided into five major geotectonic blocks that control the formation and distribution of the mineral resources of the region: (1) the southern China faulted block, (2) the eastern Indochina faulted block, (3) the Nansha‐Borneo faulted block, (4) the Taiwan‐Luzon faulted block, and (5) the central ocean basin faulted block. Apart from oil and gas, the most intensively exploited mineral deposits in the South China Sea are near‐shore placer minerals of titaniferous magnetite, zircon, monazite, tin, gold, and chromite. Based on analyses of submarine morphology and sea level change during the past 15,000 years, the South China Sea continental shelves are considered to be highly prospective for additional placer occurrence associated with such submarine features as: submerged platforms and terraces, drowned rivers and sand bars, ancient beaches, and seafloors covered by relict sediments. Additionally, based on available data, polymetallic sulfides and manganese nodules and crusts are considered as speculative resources of the future in the South China Sea.     

东海陆架盆地中生代残留地层特征及其构造启示

东海陆架盆地是位于中国东部华南大陆边缘的一个中、新生代叠合盆地,具有较大油气潜力。目前东海陆架盆地油气的发现均来自于新生界,对中生代残留地层的各方面特征认识不足:在空间上通常集中于特定构造单元,且基本位于盆地西部;在时间上主要涉及白垩纪和侏罗纪,且多是定性或半定量的研究。本文在前人研究的基础上,收集、整理了研究区目前最新、最全的反射地震资料和钻井数据,从钻遇中生界井的标定出发,以地震资料的层序划分和解释为基础,进行残留地层的研究,空间上统一盆地东、西两大坳陷带,时间上统揽白垩纪、侏罗纪以及前侏罗纪三个时期。结果表明,东海陆架盆地中生代残留地层遭受了后期严重的剥蚀改造,总体呈现东厚西薄、南厚北薄的特征,残留地层范围随时间不断东扩。对比各时期残留地层平面展布特征,揭示了东海陆架盆地的演变过程:三叠纪时期盆地原型为被动大陆边缘坳陷型盆地,早、中侏罗世时期为活动大陆边缘弧前盆地,晚侏罗世—晚白垩世时期为大陆边缘弧后伸展盆地;与此相对应,古太平洋板块俯冲肇始于晚三叠世—早、中侏罗世时期,板块后撤始于晚侏罗世。东海陆架盆地在中生代的东侧边界位于钓鱼岛隆褶带的东侧。     

南海东北部陆缘地壳结构特征及下地壳高速层成因

南海东北部陆缘构造演化信息丰富,对于理解南海的演化过程至关重要。本文收集了南海东北部的深反射地震和海底广角地震成果剖面,提取地壳和下地壳高速层的厚度结果,并结合水深、重磁异常和岩石圈的流变学等地质地球物理资料,对南海东北部的地壳减薄特征、吕宋-琉球转换板块边界的性质和下地壳高速层的分布及成因进行了分析和讨论。南海东北部的地壳减薄在横向和垂向上都存在不均匀性,以下地壳减薄为主,在台西南盆地存在极端减薄地壳;南海北缘的白云凹陷、西沙海槽和西缘的中建南盆地也存在类似的极端减薄地壳,且都与刚性地块共轭或邻近,推测刚性地块的存在导致地壳初始破裂时下地壳流动和地幔上隆是局部出现地壳极端减薄的主要原因。吕宋-琉球转换板块边界两侧在海底地形、新生代反射和重磁异常等方面均存在差异,与中生代岛弧引起的高磁异常大角度相交,其可能是中生代古特提斯构造域向太平洋构造域转换的边界断裂。下地壳高速层在南海东北部广泛发育,结合其分布特征和波速比V_p/V_s的分布区间,认为其是多期次岩浆底侵形成的铁镁质基性岩。     

Magnetic zoning and seismic structure of the South China Sea ocean basin

We made a systematic investigation on major structures and tectonic units in the South China Sea basin based on a large magnetic and seismic data set. For enhanced magnetic data interpretation, we carried out various data reduction procedures, including upward continuation, reduction to the pole, 3D analytic signal and power spectrum analyses, and magnetic depth estimation. Magnetic data suggest that the South China Sea basin can be divided into five magnetic zones, each with a unique magnetic pattern. Zone A corresponds roughly to the area between Taiwan Island and a relict transform fault, zone B is roughly a circular feature between the relict transform fault and the northwest sub-basin, and zones C, D, and E are the northwest sub-basin, the east sub-basin, and the southwest sub-basin, respectively. This complexity in basement magnetization suggests that the South China Sea evolved from multiple stages of opening under different tectonic settings. Magnetic reduction also fosters improved interpretation on continental margin structures, such as Mesozoic and Cenozoic sedimentary basins and the offshore south China magnetic anomaly. We also present, for the first time, interpretations of three new 2D reflection seismic traverses, which are of ~2,000 km in total length and across all five magnetic zones. Integration of magnetic and seismic data enables us to gain a better 3D mapping on the basin structures. It is shown that the transition from the southwest sub-basin to the east sub-basin is characterized by a major ridge formed probably along a pre-existing fracture zone, and by a group of primarily west-dipping faults forming an exact magnetic boundary between zones D and E. The northwest sub-basin has the deepest basement among the three main sub-basins (i.e., the northwest sub-basin, the southwest sub-basin, and the east sub-basin). Our seismic data also reveal a strongly faulted continent–ocean transition zone of about 100 km wide, which may become wider and dominated with magmatism or transit to an oceanic crust further to the northeast.     

南海北部陆坡区海底峡谷地貌、沉积特征及控制因素

南海北部陆坡发育众多海底峡谷,其形成、发育、演化过程都存在较大差异。本文选取南海北部陆坡典型的珠江口外海底峡谷群、东沙海底峡谷、台湾浅滩南海底峡谷和澎湖海底峡谷进行研究,通过高分辨率多道地震数据和多波束测深数据,结合前人研究成果,对4条典型海底峡谷的形态特征、沉积充填特征及结构、形成发育过程及控制因素进行研究。结果表明,南海北部陆缘各个海底峡谷的形成受多个控制因素的影响,其影响程度及方式都有差别。构造活动、海平面变化及沉积物重力流与海底峡谷的演化密切相关,而陆地河流和局部构造因素也以不同方式影响着海底峡谷的发育。对于发育在主动大陆边缘的台湾岛东南侧的澎湖海底峡谷,其板块运动和岩浆活动活跃,其上发育的海底峡谷的控制因素以内营力地质作用为主。而具有被动大陆边缘属性的其他3条峡谷,由于构造运动较少或停止,其上发育的海底峡谷的控制因素以外营力地质作用为主。