南海东北部东沙海域海水层反射特征分析

本文基于对南海东北部东沙海域近期采集的多道反射地震资料进行重新处理获得新的地震海洋学数据,分析了该海域内孤立波/内孤立波包、沙丘上方和陡坎附近特殊反射结构特征,从而提供了新的海水层与海底相互作用依据.研究结果表明,除之前已发表文章中地震海洋学资料显示存在的第一模态内孤立波/波包和沙丘上方常见的反射样式-披毛状发射外,地震海洋学资料上还发现了第二模态内孤立波、陡坎上方的上抬型波动反射结构样式.在新的地震海洋学数据中,第一模态内孤立波振幅均小于50 m,宽度上都小于5 km,单个内孤立波的最大振幅约为45 m.内孤立波包的内孤立波振幅都相对较小,均小于40 m,并且与之前不同的是,彼此之间振幅相差不大,没有明显的排列规律.此次地震海洋学数据记录到的第二模态内孤立波,形态较为完整,上层和下层反射的振幅相差不大,在30 m左右;中间层大约在水深130 m位置处,垂向结构的整体大小大于200 m.沙丘上方反射结构普遍存在弱反射层,可能是湍流边界层,并且存在特殊反射样式-披毛状反射.但并不是沙丘上方都存在披毛状反射样式,本文分析它出现在地震海洋学资料上可能是受测线与沙丘走向之间夹角的影响.陡坎区域...     

南海北部陆坡稳定性定量分析

随着海洋工程的发展,海底滑坡作为一种潜在的地质灾害逐渐成为人们关注的热点.本文采用二维极限平衡法计算并分析了海底斜坡稳定性问题.通过对斜坡模型在各种条件下安全系数的计算,定量分析了斜坡内在因素(如斜坡角度、主要土力学参数)和主要触发机制(地震、快速堆积等)对安全系数的影响.理论计算表明,静态条件下,均质斜坡角度小于20°时,均处于稳定状态;对于含软弱层的斜坡,快速堆积等引起的不排水状态下斜坡安全系数明显降低,斜坡角度大于14°时就会发生失稳.拟静态条件下,当地震动峰值加速度(PGA)小于0.15g时,对于角度小于20°的均质斜坡处于稳定状态,但PGA大于0.25g时,角度大于13°的斜坡即处于失稳状态;对于含软弱层斜坡,PGA为0.1g时,角度大于10°的斜坡即处于不稳定状态;当PGA大于0.3g时,3°以上的海底斜坡即处于失稳状态,发生海底滑坡.结合南海北部陆坡海底地形、地貌特征,在静态条件下,均处于稳定状态;但在地震加载的拟静态下,根据南海北部地震动峰值加速度分布,台湾浅滩段则处于不稳定状态.这解释了该区域大陆坡折带处海底滑坡广泛发育的原因,也表明了地震是引发南海北部滑坡最主要的触发机制之一.     

南海东北部陆坡冷泉系统的浅地层剖面特征与分析

本文通过对南海东北部陆坡区浅地层剖面资料的处理,结合横跨陆坡区的长地震测线,总结了该区浅层气聚集、海底流体运移、泥火山和冷泉活动在浅地层剖面中的特征,并对剖面中出现的声学空白带现象做了分类.研究认为浊反射、增强反射等现象,反映了该区域存在广泛的浅层气聚集.对声学空白带成因的分析,认为窄空白带很可能对应海底流体运移的通道,而宽空白带则可能由沿地层分布的浅层气引起,同时提出了自生碳酸盐岩体散落分布造成声学空白的可能性.我们在南海东北部陆坡区发现了泥火山,并讨论了其运移通道及对BSR的破坏.同时发现研究区水体中存在声学异常体,推断为冷泉羽状流,指示该区存在活动的冷泉.本文依托大量的浅地层剖面数据,对南海东北部陆坡区的冷泉系统有了较全面的认识,对冷泉系统剖面特征的成因进行了初步探讨,可为南海冷泉系统研究的进一步开展提供有益参考.     

Light carbon isotope events of foraminifera attributed to methane release from gas hydrates on the continental slope,northeastern South China Sea

In 2013, the China Geological Survey and Guangzhou Marine Geological Survey conducted the second Chinese gas hydrate expedition in the northern South China Sea(SCS) and successfully obtained visible gas hydrate samples. Five of the thirteen drilling sites were cored for further research. In this work, Site GMGS2-08 is selected for the stable isotopic analysis of foraminifera present in the boreholes in order to reveal the carbon isotopic characteristics of the foraminifera and their response to methane release in the gas hydrate geological system. Our results show that the methane content at Site GMGS2-08 is extremely high, with headspace methane concentrations up to 39300 μmol L~(-1). The hydrocarbon δ~(13)C values, ranging from-69.4‰ to-72.3‰ PDB, distinctly indicate biogenic generation. Based on the δD analytical results(~(-1)83‰ to~(-1)85‰ SMOW), headspace methane is further discriminated to be microbial gas, derived from CO_2 reduction. By isotopic measurement, five light δ~(13)C events are found in the boreholes from Site GMGS2-08, with foraminiferal δ~(13)C values being apparently lower than the normal variation range found in the glacial-interglacial cycles of the SCS. The δ~(13)C values of benthic Uvigerina peregrina are extremely depleted(as low as~(-1)5.85‰ PDB), while those of planktonic Globigerinoides ruber reach-5.68‰ PDB. Scanning electron micrograph(SEM) studies show that foraminiferal tests have experienced post-depositional alteration, infilled with authigenic carbonate, and the diagenetic mineralization is unlikely to be related to the burial depths. The correlation calculation suggests that the anaerobic oxidation of organic matter has only weak influences on the δ~(13)C composition of benthic foraminifera. This means that the anomalous δ~(13)C depletions are predominantly attributed to the overprinting of secondary carbonates derived from the anaerobic oxidation of methane(AOM). Furthermore, the negative δ~(13)C anomalies, coupled with the positive δ18O anomalies observed at Site GMGS2-08, are most likely the critical pieces of evidence for gas hydrate dissociation in the geological history of the study area.     

南海神狐峡谷群海底沉积物摩擦特性

海底滑坡区沉积物的摩擦特性对滑坡发育具有控制作用, 是海底边坡稳定性评估、滑坡过程中温压场演化及与滑坡相关的水合物成藏规律研究的基础参数.我国南海北部陆坡区不仅赋存着丰富水合物资源, 而且在地质历史上也曾多次发生滑坡作用, 目前仍是潜在的海底滑坡区.为了尽可能了解南海北部滑坡区沉积物的原位摩擦特性, 本文在围压P_c=20 MPa、孔隙压P_p=10 MPa及温度T≈20 ℃条件下, 对采自南海神狐峡谷群的4个沉积物样品开展了三轴准静态摩擦滑动实验.实验结果表明神狐峡谷群浅层沉积物: (1) 均呈现速度强化及位移强化特征; (2) 最大静摩擦系数μ_max为0.460~0.510, 稳态摩擦系数μ_ss为0.455~0.554, 二者的变化趋势较一致; (3) 内聚力c为0.30~0.57 MPa、摩擦角φ为24.5°~27.0°.而神狐峡谷群地形坡度较缓(＜6.8°).现今神狐峡谷群仅在沉积物自重作用下不会发生失稳, 总体上比较稳定.结合峡谷群多期次的滑坡特征以及滑坡体与水合物稳定域底界(通常与似海底反射层(Bottom simulating reflector, 简写BSR)重叠)、气烟囱等构造的空间分布关系, 我们推测: 神狐峡谷群滑坡的形成主要是由于BSR附近地层孔隙压升高、强度降低, 进而导致失稳.BSR附近地层孔隙压升高则可能是由于深部热成因游离气的聚集、BSR附近的水合物在地震等对其温、压场的扰动下分解所致.

     

南海北部陆坡块体搬运沉积体系的地震响应与成因机制

块体搬运沉积体系是一种重力作用下形成的深水沉积体系,在全球大陆边缘沉积体系中扮演着重要角色.块体搬运沉积作用不仅造成危害极大的深水地质灾害,而且与海洋天然气水合物和深水油气的形成过程与富集有密切联系.根据新获得的海底地形地貌资料和地震资料,揭示了块体搬运沉积体系识别特征,明确了其分布范围.这些块体搬运沉积体系具有明显的滑坡形态和沉积构造;头部表现张性断裂构造,内部有旋转断块和流动构造,趾部表现为挤压和逆冲构造.块体搬运沉积体系表现出不同的成因类型.通过白云凹陷天然气水合物的分布模式和白云块体搬运沉积体系有限元数值模拟研究,认为白云块体搬运沉积体系与天然气水合物分解密切相关,进一步给出6种水合物分解模型情况下海底稳定性的安全因子,随着水合物分解量的增加,斜坡的稳定性逐渐减弱,当水合物的分解达到一定数量时,就会发育海底块体搬运沉积体系.     

The 3D magnetic structure beneath the continental margin of the northeastern South China Sea

Understanding the continental margin of the Northeastern South China Sea is critical to the study of deep structures, tectonic evolution, and dynamics of the region. One set of important data for this endeavor is the total-field magnetic data. Given the challenges associated with the magnetic data at low latitudes and with remanent magnetism in this area, we combine the equivalent-source technique and magnetic amplitude inversion to recover 3D subsurface magnetic structures. The inversion results show that this area is characterized by a north-south block division and east-west zonation. Magnetic regions strike in EW, NE and NW direction and are consistent with major tectonic trends in the region. The highly magnetic zone recovered from inversion in the continental margin differs visibly from that of the magnetically quiet zones to the south. The magnetic anomaly zone strikes in NE direction, covering an area of about 500 km × 60 km, and extending downward to a depth of 25 km or more. In combination with other geophysical data, we suggest that this strongly magnetic zone was produced by deep underplating of magma associated with plate subduction in Mesozoic period. The magnetically quiet zone in the south is an EW trending unit underlain by broad and gentle magnetic layers of lower crust. Its magnetic structure bears a clear resemblance to oceanic crust, assumed to be related to the presence of ancient oceanic crust there.     

Phytoplankton growth and microzooplankton grazing in the continental shelf area of northeastern South China Sea after Typhoon Fengshen

Dilution experiments were used to investigate the phytoplankton growth and microzooplankton grazing in the continental shelf area of northeastern South China Sea during 30 June and 7 July, 2008, occurring about a week after Typhoon Fengshen. We detected negative phytoplankton growth rates (−0.03 to −2.02 d⁻¹) and measured grazing rates of microzooplankton on phytoplankton in size-fractionations of 20-200 μm (1.25±0.44 d⁻¹), 3-20 μm (1.48±0.63 d⁻¹) and <3 μm (1.02±0.42 d⁻¹). Results showed significant correlations between phytoplankton growth and microzooplankton grazing rates, between phytoplankton and ciliate abundance, and between the dominant phytoplankton Thalassionema nitzschioides and the dominant ciliate Helicostomella longa (p<0.05). Phytoplankton decay, due to nutrient-limited conditions occurring with the fading of upwelling and spreading of freshwater plume after Typhoon Fengshen, may account for negative phytoplankton growth rates in this study. Synergism in the specific size-selective grazing of various species, including ciliates and heterotrophic dinoflagellates, may contribute to similar grazing rate on phytoplankton in different size-fractionations, at the integrated level. Interactions between phytoplankton and microzooplankton, including grazing selectivity, top-down and bottom-up control between phytoplankton and microzooplankton may contribute to these findings. Our results indicate that under conditions of negative phytoplankton growth microzooplankton grazing may reduce energy loss from the epipelagic waters by retrieving energy from the decaying phytoplankton community.     

南海大陆边缘构造活动与重磁场特征研究

南海是研究大陆边缘类型、大陆边缘成因机制的理想场所.本文对南海重力和磁力数据及其向上延拓处理结果所反映的地球物理场特征进行了分析.在综合前人关于南海地质、地球物理研究工作的基础上,论述了南海大陆边缘构造特征与重磁场特征、基底断裂体系及火成岩分布与性质.南海周边的大陆边缘类型各不相同,重磁场特征、基底断裂和火成岩分布直接...     

南海北部东沙海域巨型水下沙丘的分布及特征

本文基于多波束测深和高分辨率多道反射地震数据研究了东沙海域深水巨型水下沙丘的特征.巨型水下沙丘发育在230~830 m水深的上陆坡范围内,呈斑块状分布.NW-SE向的近海底流体运动不仅冲蚀地层,形成了三条与水下沙丘间隔分布的冲蚀带,为水下沙丘提供了沉积物来源,同时也为水下沙丘的形成提供了动力源.研究区水下沙丘波长（L）范围55~510 m,波高（h）范围1.5~20 m,二者呈指数关系分布.沙丘的波长随水深增大而增大,波高则在500~700 m水深范围内最大.水下沙丘NE-SW向展布的脊线和几何参数关系是与现今水动力条件相平衡的结果.

     

Stretching characteristics and its dynamic significance of the northern continental margin of South China Sea

To investigate the thinning of the whole crust, and the contribution of the upper versus lower crust to the stretching since Cenozoic, we calculated the stretching factor of the northern margin of South China Sea with data such as whole crustal thickness, depth of Cenozoic sedimentary basement and the horizontal displacement of faults. An isometric line drawing on whole crustal stretching factor is then obtained. Along the seismic Line 1530 in Baiyun sag, we also calculated the stretching factors of the upper and lower crust. The results suggest that the whole crustal thickness decreases seaward while the whole crustal stretching factor increases from shelf to slope. The Moho upwells highest beneath where the crust is thinned most. The value of the whole crustal stretching factor ranges from 1.5 to 6. Two areas were thinned intensely: the center of Yinggehai Basin, and the Baiyun sag in the Pearl River Mouth Basin. The calculation of the upper and lower crustal stretching factors from DSP1530 in Baiyun sag shows that the original crust of Baiyun sag should be thinned before deformation. Its pre-Cenozoic evolution as well as tectonic position during Cenozoic might be responsible for that.     

Controlling factors on the submarine canyon system: A case study of the Central Canyon System in the Qiongdongnan Basin,northern South China Sea

Based on an integrated analysis of high-resolution 2D/3D seismic data and drilling results, this study analyzes the tectonic-sedimentary evolution of the Qiongdongnan Basin (QDNB) since the late Miocene, and discusses the controlling factors on the formation and development of the Central Canyon System (CCS). The sediment failures caused by the relative sea level falling might have discharged deposits from the slope to the canyon. The two suits of the infillings, i.e., turbidites and mass transport complex (MTC), were derived from the northwestern source and northern source, respectively. The sediment supplies, which differ significantly among different areas, might have led to the variations observed in the internal architectures. Tectonic transformation around 11.6 Ma had provided the tectonic setting for the CCS and formed an axial sub-basin in the central part of the Changchang Depression, which could be called the rudiment of the CCS. The tectonic activity of the Red River Fault (RRF) at about 5.7 Ma might have strengthened the hydrodynamics of the deposits at the junction of the Yinggehai Basin (YGHB) and the QDNB to trigger a high-energy turbidity current. The MTC from the northern continental slope system might have been constrained by the Southern Uplift, functioning as a barrier for the infillings of the CCS. Thanks to a sufficient sediment supply during the Holocene period and the paleo-seafloor morphology, the relief of modern central canyon with the starving landform in the eastern Changchang Depression might have been accentuated by deposition of sediments and vertical growth along the canyon flanks, where collapse deposits were widely developed. Corresponding to the segmentation of the CCS, the forming mechanisms of the canyon between the three segments would be different. The turbidite channel in the head area had likely been triggered by the abundant sediment supply from the northwestern source together with the fault activity at about 5.7 Ma of the RRF. The formation and evolution of the canyon in the western segment were caused by combined effects of the turbidite channel from the northwestern source, the MTC from the northern continental slope, and the paleo-seafloor geomorphology. In the eastern segment, the canyon was constrained by the tectonic transformation occurring at approximately 11.6 Ma and the insufficient sediment supply from the wide-gentle slope.     

南海北部陆坡细粒沉积物天然气水合物系统的形成模式初探

我国在南海北部陆坡的细粒沉积物中首次钻获水合物样品,为深入了解这一特殊成藏体系的形成机制,本研究以钻探区域高精度的二维和三维多道地震资料为基础,通过精细的地球物理解释和地质分析,探讨了该天然气水合物系统的成藏控制因素.细粒沉积物天然气水合物系统中水合物的富集过程存在着三个特殊的控制因素：（1）大面积丰富的含甲烷流体的参与.南海北部深水盆地广泛存在的异常热流体活动促使深部的热解成因天然气或浅层的生物成因气运移到水合物稳定带,有利于水合物的发育.热流体活动表现为底辟和气烟囱构造,在地震剖面上,表现为地震声学模糊带、同相轴下拉或中断、顶部出现反射亮点,这些构造直抵水合物稳定带.在气烟囱或底辟构造顶部通常有BSR伴生,说明热流体活动与水合物成藏具有密切关系;（2）断裂构造的发育在水合物稳定带中细粒沉积物的成核和生长过程中具有重要的作用,一方面促进稳定带流体的垂向和侧向运移,另一方面,构造裂缝可能成为成核和生长过程的空间;（3)大型海底滑坡导致了地层中超压流体活动和构造裂缝的发育,从而促进了水合物富集.在上述研究基础上,提出了南海北部水合物钻探区高饱和度水合物成藏模式.     

南海北部陆坡水合物分解引起海底不稳定性的定量分析

分析了天然气水合物分解引起海底斜坡不稳定性的原因,探讨了水合物分解引起沉积物层孔隙压力变化规律.以南海北部神狐海域水合物样品钻获区为研究对象,应用极限平衡法探讨了海底无限斜坡的稳定性问题,计算了设定的不同滑动面的安全系数,建立了目标不稳定区相关参数条件下的水合物分解与海底斜坡不稳定的模式图,模拟计算了不同程度的水合物分解对海底斜坡失稳的影响作用.结果表明：在假设条件下,南海神狐海域当水深为1200 m,沉积物层厚度为200 m,对于20°的较大坡角,沉积物层中5%水合物分解将引起海底斜坡失稳;斜坡坡角为5°时,沉积物层中15%水合物分解后,将会引起海底斜坡失稳;当斜坡坡角为3°或更小时,沉积物层中25%水合物分解后,将会引起海底斜坡失稳.     

Paleomagnetism of sedimentary strata and the origin of the structures in the western slope of South Urals

Paleomagnetic data may contribute to studying the formation history of orogens; in particular, these data can promote identifying the pattern and scales of deformations at the final stages of orogeny. We have conducted paleomagnetic studies of the Paleozoic and Neoproterozoic sediments in the western part of the Western Ural Megazone in South Urals. The detailed thermal demagnetization revealed the intermediate temperature magnetization component in most samples. This magnetization has a reversed polarity and has been acquired before folding or at the early stages of the deformations. The directions of this component are narrowly grouped in rocks of a different age in all the segments of the studied part of South Urals, and the regional average direction closely agrees with the reference paleomagnetic direction of 270 Ma for the East European Platform. The results of our study suggest the following conclusions: (1) the main magnetization component in the studied sedimentary rocks has a secondary origin; (2) this component has an age of ~270 Ma and has been formed during the Kungur deformations (279–272 Ma ago) of the western part of South Urals; (3) neither a general rotation of the studied part of the Urals relative to the East European Platform nor local rotations of the individual tectonic blocks relative to each other are revealed; (4) the changes in the strike of the structures from NE within the Karatau uplift to the submeridional in the remaining part of the Urals is not an oroclinal bend.     

深化南海大陆边缘研究

南海作为中国重要的边缘海,在中生代期间经历西部特提斯域的构造演化,形成主动大陆边缘.在新生代期间受太平洋域构造演化作用的影响在南北两侧出现陆缘共轭张裂,形成发育多个大型新生代盆地的被动大陆边缘.中国南海从主动大陆边缘转化为被动大陆边缘,不仅是大陆边缘动力学研究的天然实验室,而且还蕴藏着极其丰富的石油和天然气资源,从而持续激发着科学家的广泛研究兴趣.     

南海东沙海域网状沙丘的发现及其成因探讨

基于多波束测深和高分辨率多道反射地震数据,以及遥感资料研究南海北部东沙海域深水沙丘,总结底形形态分布特征,分析其水动力成因.按照空间分布形态将沙丘分为平行沙丘、波状沙丘和网状沙丘,本文首次发现水下网状沙丘,重点讨论网状沙丘的特征分布和形成机制.网状沙丘发育在东沙环礁东北部水深约352~420 m之间,研究发现其延伸方向共有三个走向：NS、NE-SW和EW,前两个方向为西向传播的内波所致,而最后一个则很有可能是由洋盆西向传播的内波遇到东沙环礁后形成衍射,向北传播的内波引起了向南的近海底底流,冲刷海底底层,剥蚀出足够的沉积物颗粒,从而形成EW走向的水下沙丘.值得一提的是,在多边形网状沙丘的上方,反射地震剖面揭示水层中发育与下方沙丘相当波长的高频震荡内波,并且在遥感资料上也发现相关的衍射和内波干涉区域,而内波干涉很有可能导致高频震荡,从而形成网状沙丘.本研究丰富了水下沙丘形态类型,探讨了新型水下沙丘的形成机制,在陆坡底沙运动及工程应用中具有重要的价值.

     

南海东北部973剖面BSR及其热流特征

对南海东北部973项目采集的地震测线进行了处理,阐明了恒春海脊处天然气水合物似海底反射（BSR）的特征.不同类型剖面的分析表明,单道地震剖面可以揭示南海东北部地区BSR一定的分布,但地震处理对揭示BSR分布全貌起重要作用.南海东北部的BSR具有世界大陆边缘BSR典型的特征,切穿了沉积层理反射,与海底起伏大体平行,为一强振幅的负极性反射.该BSR特征明显,意味着南海东北部地区存在含天然气水合物沉积.利用甲烷水合物与多组分天然气水合物相平衡曲线,计算了稳定带底界的埋深,并与BSR深度进行了对比分析.无论是甲烷水合物稳定带底界还是多组分天然气水合物稳定带的底界,单一地温梯度计算的结果不可能与BSR深度在整个剖面上对应.可知本区横向上地温梯度变化较大.利用BSR资料估算了地温梯度并求得热流值.计算表明,地温梯度与热流值由西向东,随着离海沟距离的增大、离岛弧距离的减小而减小.BSR计算得到的热流值为28～64 mW/m2,与台湾西南实测热流值的结果基本可以对比.     

穿越东沙隆起和潮汕坳陷的OBS广角地震剖面

为了探明南海中北部陆缘深部地壳结构,使用2D射线追踪正演和反演方法,拟合了一条南海中北部陆缘的OBS广角地震剖面(OBS2006-3).该剖面穿越东沙隆起和潮汕坳陷,长319 km,NNW-SSE走向,共投放海底地震仪14台.速度结构模型表明:潮汕坳陷存在巨厚的中生代沉积,最大厚度达到8 km,速度从顶部的4.4 km/s向下增加到底部的5.3 km/s.莫霍面埋藏深度由陆向海方向减小,其埋深从东沙隆起下方的24~25 km减小到南部隆起带的17 km.东沙隆起下方的上地壳中存在一向上突起的高速异常,速度达到6.9 km/s,可能与地幔物质上涌和岩浆活动有关.在整个剖面的下地壳底部存在厚3~12 km,速度为7.1~7.4 km/s的高速层.根据各个时期岩浆活动特征,推测下地壳高速层是由海底扩张停止之后的岩浆底侵作用形成的.