南海中南—司令断裂带的延伸特征及其与南海扩张演化的关系

为了确定中南—司令断裂带在南海海盆及其在南部陆缘的延伸位置,并探讨其与南海扩张的关系,本文利用重磁异常、地震、莫霍面深度、P波速度特征、钻井拖网资料,对中南—司令断裂带的延伸位置进行了综合地质和地球物理研究,厘定了中南—司令断裂带在东部次海盆与西南、西北次海盆之间呈NS向延伸,并南延至南海南部陆缘之上,深度上切割至莫霍面。根据南海海盆中磁异常条带走向的变化,及磁异常条带、走滑/转换断裂、扩张方向的印证关系,结合前人对古南海"剪刀状"碰撞闭合、南海扩张演化、构造应力场的研究,提出在32~25 Ma,伴随着南海东部次海盆的NNW向扩张,南海海盆及南沙地块整体发生顺时针旋转,使中南—司令断裂走向由形成初期的NNW向转变为N—S向;23.5 Ma之后,顺时针旋转停止,南海东部次海盆继续NNW向扩张,西南次海盆呈NW—SE向渐进式扩张。作为一条切穿地壳的深大断裂,中南—司令断裂与红河-越东断裂、马尼拉海沟断裂三条深大断裂一起组成区域"滑线场",制约南海海盆的扩张与南沙地块的南移。     

渔山列岛海洋资源保护与开发利用设想

一、渔山列岛概况 渔山列岛位于浙江省沿海中部,隶属于宁波市象山县石浦镇(见图1),在象山半岛东南、猫头洋东北,距石浦铜瓦门山47.5 km,即28°51.4'N～28°56.4'N.122°13.5'E～122°17.5'E之间,列岛由13岛41礁组成,呈NE-SW向排列,东西向宽约4.5 km,NE-SW向长约7.5km,岛礁总面积约2 km2,其中可计量面积的岛礁29个,面积约1.603 km2,滩地面积0.503 km2,岸线约20.845 km.     

南海海盆地形与NW向断裂

新的2'×2'卫星测高获得的水深数据表明,除了在南海中央海盆扩张脊附近分布有高耸、断续的近东西向海山链外,在南海深海平原上还存在一些北西向的连续线状凸起特征.这些线状特征高约500m,宽10-30km,绵延数百至近千公里.反射地震数据则显示,这些海底线状隆起实际上是宽50-100km的走滑断裂带,在该断裂带内还有一些低幅和隐伏褶皱,它们代表了海盆内部的压性走滑断裂带,反映了海盆扩张停止后台湾-吕宋岛弧向西的构造挤压应力对南海海盆的持续作用.其中一条穿过116°E的北北西向断裂带构成了中央海盆与西南海盆的边界断裂.     

琼东南盆地深水区长昌凹陷地壳结构特征

长昌凹陷位于琼东南盆地深水区,向东通过西沙海槽与南海西北次海盆相通,其近东西向的展布形态明显异于深水区其他凹陷的NE-NEE向形态,为了弄清其地壳结构,从而更好地分析凹陷的结构和演化机制,这里根据深反射地震资料、VSP资料和最新重力资料对长昌凹陷的地壳结构进行了综合地球物理模拟.结果显示:长昌凹陷北侧地壳厚度为22～24 km,南侧地壳厚度约20～22 km,从两侧向长昌凹陷中央地壳厚度逐渐减薄,最薄处只有2.8 km;莫霍面深度与沉积基底呈镜像关系,沉积基底最深的地方莫霍面深度最浅,最浅深度距海平面13.8 km;凹陷中央东部存在一层厚约4 km的下地壳高速层,该层在地震剖面和层速度剖面上均可识别.     

南海的岩石圈结构与均衡模型

本文用四种方法计算了南海的岩石圈厚度,并建立了南海海盆的岩石圈均衡模型。在此基础上,分析了南海海盆的岩石圈结构特征:即从海盆中部向南、北两侧,层3厚度、地壳厚度和岩石圈厚度逐渐增大,与地壳年龄呈正向关系。这表明,南海海盆有如大洋(大西洋)一样的形成演化机制—由正常的裂谷和扩张过程发育而成。     

南海西北海盆的构造特征及南海新生代的海底扩张

分析了南海西北海盆及其邻区的地形地貌、重磁场异常和地壳结构特征,并对穿过西北海盆和中央海盆的地震剖面进行精细解释。结果发现,西北海盆的地球物理场异常、地质构造和地壳厚度均呈ＮＥ走向展布,而中央海盆则表现为ＥＷ向特征;西北海盆中的新生代沉积比中央海盆多一套地层（Ｔ４－Ｔｇ）,说明西北海盆的年龄比中央海盆老。联系到南海西南海盆和西北海盆在区域构造、地球物理场异常和地形地貌特征等方面的相似,以及西南海盆和中央海盆由磁异常条带对比得出的年龄差异,我们认为,西北海盆和西南海盆是在第一次海底扩张时（４２－３５ＭａＢ．Ｐ．）形成的,中央海盆是在第二次海底扩张时形成的。     

南海西北海盆的构造特征及南海新古生代的海底扩张

分析了南海西北海盆及其邻区的地形地貌,重磁场异常和地壳结构特征,并对穿过西北海盆和中央海盆的地震剖面进行精确解释。结果发现,西北海盆的地球物理场异常,地质构造和地壳厚度均呈ＮＥ走向展布,而中央海盆则表现为ＥＷ向特征,西北海盆中的新生代沉积比中央海盆多一套地层（Ｔ４－Ｔ８）,说明西北海盆的年龄比中央海盆老,联系到南海西南海盆和西北海盆在区域构造,地球物理场异常和地形地貌特征等方面的相似,以及西南海盆     

滨海断裂带珠江口段的重磁资料解释

通过重磁资料系统处理、研究、解释,确认滨海断裂带珠江口段的存在,且是一条规模巨大特征明显的断裂带,深度在30 km以上,呈NEE向展布,是由滨岸断裂带和滨海断裂带界限的高重力地质块体带,重力、航磁梯度带或灰度带图像反映清楚。在浅部,它与NW和NE向断裂互有切割,不过,NW向断裂切割滨海断裂带和NE向断裂带的频度更高,图像更清晰。在10 km以深,NE、NW向断裂基本上消失,阻截在滨海断裂带内,某些资料认为,NE向莲花山断裂带往西南入海(本区)转为NEE走向,从重磁资料分析,这些NEE走向断裂其实是滨海断裂带的一部分。莲花山断裂带进入本区后,仍以NE45°左右方向,经深圳、香港,至万山群岛被滨海断裂带截断,图像清楚,不存在NE向转NEE向现象。滨海断裂带是一条活动性较强的断裂带,在珠江口段,强势不改,它与NW、NE向断裂交汇并相互切割,具备5级以上地震构造条件,尤其是与形成时代新、活动性较强的NW向断裂交切部位(如镇海湾、广海湾、崖门、澳门、担杆列岛地区)。但是,NW向断裂在本区较浅,不如粤闽、粤桂交界地区深(≥30 km),所以本研究区(段)地震强度应低于后者。     

基于GIS的近30年长江口及其邻近海域赤潮时空分布特征研究

收集了1990年—2019年长江口及其邻近海域(120°30'E～123°30'E,29°00'N～32°30'N)记载的赤潮事件,基于GIS软件对所有赤潮事件进行整理,分析了赤潮的时空分布规律,并绘制长江口及邻近海域赤潮分布图.结果表明:近30年来,长江口及其邻近海域赤潮经历了先升高后下降的过程,赤潮次数共计144次,赤潮面积>1000 km2有28次.赤潮发生核心区集中在长江口外、花鸟山-嵊泗列岛、岱山岛-中街山列岛、舟山岛-朱家尖岛海域;长江口及邻近海域赤潮多发期为5—8月,5、6、7、8月发生的赤潮次数分别占总数的28.37％、34.75％、17.78和9.29％;东海原甲藻(Prorocenrum donghaiense)、中肋骨条藻(Skeletonema costatum)是长江口及其邻近海域最常见赤潮肇事种,发生次数分别为55次和40次,占统计总次数的38.19％和27.78％,2000年以来,东海原甲藻赤潮发生频率呈上升态势.     

南海南部海底地震仪试验及初步结果

采用德国SedisIV型海底地震仪(OBS)和中国科学院地质与地球物理研究所自主研发的OBS,以4×24.5L的大容量气枪阵列为震源,于2009年4～6月在南海南部开展了OBS试验,获得了两条勘测线,其中OBS2009-1测线(剖面1)从南海西南次海盆南部陆缘延伸到海盆中央,另一条OBS2009-2测线(剖面2)穿过礼乐滩东部向西北延伸进入海盆。由剖面2的14台OBS采集的广角地震反射、折射勘测地震数据可知,此次试验,OBS地震记录清晰、震相丰富,所使用的气枪有足够的能量输出,显示了其良好的工作能力,是一次比较成功的地震勘测。数据初步处理和初至波层析成像结果表明,礼乐滩地块的基底较高,很有可能与南海北部陆缘存在共轭关系,但与南海北部陆缘不同的是,北部陆缘有较厚的沉积层覆盖,而礼乐滩块体上的沉积层很薄;东部次海盆地壳明显被拉薄,海盆内的地壳也很薄,莫霍面埋深较浅。     

东经九十度海岭有效弹性厚度计算及其对构造运动的解释

有效弹性厚度(T_e)表示岩石圈抵抗变形的能力,其大小主要取决于岩石圈内部的温度结构和地壳物质组成。作为全球最长的海岭之一,东经九十度海岭(NER)来源与形成过程一直是国内外科学家研究的热点,然而受到该地区复杂构造活动的影响,研究者对海岭的形成过程仍缺乏清晰认识。本文从T_e的角度出发,通过空间褶积方法计算了沿着NER不同位置处T_e的空间分布特征。计算结果表明,整个海岭的T_e主要在0~35 km之间变化,表现为北(8°N~1°N)高(平均值为20 km)、中(1°N~15°S)低(平均值在5 km以下)、南(15°S~30°S)高(平均值为30 km),变化趋势与凯尔盖朗热点的3期岩浆活动相对应。T_e的变化反映了NER形成过程中东南印度洋脊与热点的相对位置的调整,说明NER是凯尔盖朗热点、印度洋板块扩张与东南印度洋洋中脊迁移三者共同作用的结果。最后,结合T_e的结果与ROYER板块重构的结果,本文提出了NER形成过程的模式。     

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.     

P矢量方法在南海夏季环流诊断计算中的应用

基于1998年6～7月南海调查航次的CTD资料,对南海环流采用最近发展的P矢量方法进行诊断计算.计算结果:黑潮向西入侵南海,然后做反气旋弯曲向东北方向流动,最终有通过巴士海峡流出南海的趋势.在南海北部存在一个气旋性环流,这个环流的强度和范围随深度增加而减小.该环流的冷中心位置随深度增加稍向南移.南海中部、越南以东海域存在一个明显的气旋涡和反气旋涡,尤其在200m及其以上水层均相当稳定,反气旋涡位于越南以东,其中心位置在11°53'N,111°50'E,气旋涡的中心位置在13°17'N,112°55'E,两者的尺度皆约为250km.吕宋岛西侧存在一个反气旋涡.在计算海区南部、巴拉望岛西南海域,100m以上层存在一个反气旋式涡.从各层流场分布均可以显示海流在西部强化的现象.     

ON THE DISTRIBUTIONAL CHARACTERISTICS OF FOULING ORGANISMS IN DONGSHAN BAY

The species composition, attaching season and variation of amounts of fouling organ-isms are regulated by various environmental factors, of which a few factors always play theleading role, and represent the principal aspect of a contradiction. In previous studies, theauthors have concentrated on the relationship between tbe distance from shore, orsalinity gradients and the distribution of fouling organisms. The present paper providingsome basic information of fouling communities in Dongshan, lay stress on the relationshipbetween the fluency of water and the distribution of fouling organisms in the same bay orharbor. Dongshan Bay (23°50′N, 117°30′E) is situated near the juncture of Southern Fujianand Guangdong Province, having a length of 10 nautical miles from north to south and 6nautical miles from west to east. The Bay is surrounded by land on three sides. The mouth     

The northern boundary of the Jan Mayen microcontinent, North Atlantic determined from ocean bottom seismic, multichannel seismic, and gravity data

The Jan Mayen microcontinent was as a result of two major North Atlantic evolutionary cornerstones—the separation of Greenland from Norway (~54 Ma), accompanied by voluminous volcanic activity, and the jump of spreading from the Aegir to the Kolbeinsey ridge (~33 Ma), which resulted in the separation of the microcontinent itself from Eastern Greenland (~24 Ma). The resulting eastern and western sides of the Jan Mayen microcontinent are respectively volcanic and non-volcanic rifted margins. Until now the northern boundary of the microcontinent was not precisely known. In order to locate this boundary, two combined refraction and reflection seismic profiles were acquired in 2006: one trending S–N and consisting of two separate segments south and north of the island of Jan Mayen respectively, and the second one trending SW–NE east of the island. Crustal P-wave velocity models were derived and constrained using gravity data collected during the same expedition. North of the West Jan Mayen Fracture Zone (WJMFZ) the models show oceanic crust that thickens from west to east. This thickening is explained by an increase in volcanic activity expressed as a bathymetric high and most likely related to the proximity of the Mohn ridge. East of the island and south of the WJMFZ, oceanic Layers 2 and 3 have normal seismic velocities but above normal average crustal thickness (~11 km). The similarity of the crustal thickness and seismic velocities to those observed on the conjugate M?re margin confirm the volcanic origin of the eastern side of the microcontinent. Thick continental crust is observed in the southern parts of both profiles. The northern boundary of the microcontinent is a continuation of the northern lineament of the East Jan Mayen Fracture Zone. It is thus located farther north than previously assumed. The crust in the middle parts of both models, around Jan Mayen island, is more enigmatic as the data suggest two possible interpretations—Icelandic type of oceanic crust or thinned and heavily intruded continental crust. We prefer the first interpretation but the latter cannot be completely ruled out. We infer that the volcanism on Jan Mayen is related to the Icelandic plume.     

赤道西太平洋 1°45''''S,156°E 海区的半日内潮波特性

通过对TOGA-COARE期间的一组锚系仪器阵列资料的分析得出:在赤道西太平洋1°45′S,156°E.海域存在显著的半日潮频内波,它的水平波数(波长)、垂向波数、水平传播速度和垂向传播速度分别约为:3.3×10-2 km-1 (210 km),-1.6×10-3 m-1,2.0 m/s,-3.8 cm/s.波形向斜下方传播,亦即波能向斜上方传输.它在观测点西南方生成后,向东北方向传播,到达观测海区.流速矢量旋转谱水平随深度的变化呈马鞍形,低谷及深处的峰所在深度分别与南赤道流及赤道潜流的南边界所在深度大体一致.旋转椭圆主轴方位角随深度变化,在浅层(40 m处)为北偏东30°,到深处(324 m)转为东偏南14°.总体上呈东北方向,表明波来自西南方向.     

Crustal structure of the Southwest Subbasin,South China Sea,from wide-angle seismic tomography and seismic reflection imaging

The Southwest Subbasin (SWSB) is an abyssal subbasin in the South China Sea (SCS), with many debates on its neotectonic process and crustal structure. Using two-dimensional seismic tomography in the SWSB, we derived a detailed P-wave velocity model of the basin area and the northern margin. The entire profile is approximately 311-km-long and consists of twelve oceanic bottom seismometers (OBSs). The average thickness of the crust beneath the basin is 5.3 km, and the Moho interface is relatively flat (10–12 km). No high velocity bodies are observed, and only two thin high-velocity structures (~7.3 km/s) in the layer 3 are identified beneath the northern continent-ocean transition (COT) and the extinct spreading center. By analyzing the P-wave velocity model, we believe that the crust of the basin is a typical oceanic crust. Combined with the high resolution multi-channel seismic profile (MCS), we conclude that the profile shows asymmetric structural characteristics in the basin area. The continental margin also shows asymmetric crust between the north and south sides, which may be related to the large scale detachment fault that has developed in the southern margin. The magma supply decreased as the expansion of the SWSB from the east to the west.     

Oceanic crust thickens approaching the Clipperton Fracture Zone

A multi-channel seismic reflection image shows the reflection Moho dipping toward the Clipperton Fracture Zone in crust 1.4 my old. This seismic line crosses the fracture zone at its eastern intersection with the East Pacific Rise. The seismic observations are made in travel time, not depth. To establish constraints on crustal structure despite the absence of direct velocity determinations in this region, the possible effects of temperature, tectonism, and anomalous lithospheric structure have been considered. Conductive, advective, and frictional heating of the old crust proximal to the ridge-transform intersection can explain <20% of the observed travel-time increase. Heating has a negligible effect on crustal seismic velocity beyond ~10 km from the ridge tip. The transform tectonized zone extends only 6 km from the ridge tip. Serpentinization is unlikely to have thickened the seafloor-to-reflection Moho section in this case. It is concluded that, contrary to conventional wisdom, the 1.4 my old Cocos Plate crust thickens approaching the eastern Clipperton Ridge-Transform Intersection. Increase in thickness must be at least 0.9 km between 22 and 3 km from the fracture zone.     

<Emphasis Type="Italic">S</Emphasis>-wave velocity structure and tectonic implications of the northwestern sub-basin and Macclesfield of the South China Sea

Based on the optimum P-wave model, the S-wave velocity structure of a wide angle seismic profile (OBS2006-1), across the northwestern sub-basin (NWSB) and the Macclesfield, is simulated by a 2-D ray-tracing method. The results indicate the S-wave velocities in the upper and lower crust of the NWSB are 3.2–3.6 km/s and 3.6–4.0 km/s, with Vp/Vs ratios of 1.82–1.88 and 1.74–1.82, respectively, which reflect typical oceanic crust characteristics. The S-wave velocity in the upper crust of the NWSB is a little higher in the NNW segment than that in the SSE segment, while the lateral variation of Vp/Vs ratio is in the opposite. We suggest that the NWSB might have experienced asymmetrical magma flows during sea floor spreading, which may have blurred the magnetic anomaly lineation. The comparison of S-wave velocities along the northern margin of the SCS shows that the west section is different from the east section, and the northwestern margin has a non-volcanic crust structure. The S-wave structures and P-wave velocity models along the northern margin, Macclesfield and Reed Bank show that the Macclesfield might have a conjugate relationship with the Reed Bank.