南海成因：岩石圈破裂与俯冲带相互作用新认识

南海深部计划与国际大洋钻探航次取得了一系列创新进展与重大突破:1)发现南海陆缘岩石圈减薄之初未出现地幔蛇纹岩出露,且岩浆迅速出现;2)新提出南海不是"小大西洋",而是"板缘张裂"盆地,与经典的大西洋型"板内张裂"陆缘模式不同;3)揭示南海受到俯冲带的强烈控制,提出俯冲诱发地幔上涌并影响南海岩浆活动。     

南海深海盆的沉积充填

依据大洋钻探井及地震剖面资料,定量确定南海同扩张期和后扩张期深海盆沉积充填差异及沉积物来源变化。研究显示,南海深海盆自渐新世(32 Ma)开始形成,随着南海二次扩张海盆范围逐渐扩大,海盆内主要充填火山碎屑角砾岩及火山灰,碳酸盐岩、超微化石软泥,泥质粘土、粉砂质粘土、泥岩及粉细砂岩。深海盆充填主要沉积物为晚中新世(11.6Ma)以来的陆源碎屑沉积,丰富陆源碎屑的供给与南海闭合过程中同期区域构造事件(如青藏高原快速隆升、菲律宾板块俯冲)密切相关,也与晚中新世以来东亚季风增强以及源区强烈的风化剥蚀有关。     

Rifting process and formation mechanisms of syn-rift stage prolongation in the deepwater basin,northern South China Sea

Based on the latest seismic and geological data, tectonic subsidence of three seismic lines in the deepwater area of Pearl River Mouth Basin （PRMB）, the northern South China Sea （SCS）, is calculated. The result shows that the rifting process of study area is different from the typical passive continental margin basin. Although the seafloor spreading of SCS initiated at 32 Ma, the tectonic subsidence rate does not decrease but increases instead, and then decreases at about 23 Ma, which indicates that the rifting continued after the onset of seafloor spreading until about 23 Ma. The formation thickness ex- hibits the same phenomenon, that is the syn-rift stage prolonged and the post-rift thermal subsidence delayed. The formation mechanisms are supposed to be three： （1） the lithospheric rigidity of the northern SCS is weak and its ductility is relatively strong, which delayed the strain relaxation resulting from the seafloor spreading; （2） the differential layered independent extension of the lithosphere may be one reason for the delay of post-rift stage; and （3） the southward transition of SCS spreading ridge during 24 to 21 Ma and the corresponding acceleration of seafloor spreading rate then triggered the initiation of large-scale thermal subsidence in the study area at about 23 Ma.     

南海表面海温异常对南海季风影响的数值模拟

采用P-σ混合坐标系区域气候模式模拟了4－7月南海季风的爆发、演变过程，并进行了3组敏感性数值试验，研究南海表面海温异常对南海季风的影响，得到以下结论：（1）南海4月份海温异常对南海季风的爆发日期影响不大，但对季风爆发后的强度有所影响，异常增温造成南海季风增强，异常降温则南海季风减弱。（2）南海季风爆发和强度的变化与南海本身的海温变化情况有密切的关系，尤其是5月份南海海温异常。5月份南海异常增温可以使南海季风提前爆发，季风增强，南海海温异常降低时，南海季风爆发的时间推迟，季风减弱。（3）南海海温持续异常可以影响南海及中国大陆的高低空环流变化，海温持续异常增温可以使南海季风提前爆发，显地加强南海季风，并有利于南海季风向北推进，但当海温在6月份进一步持续增温时，则有利于季风维持在较南地区，阻碍季风向北发展；当海温持续异常降低时，南海季风推迟爆发，且明显减弱。     

南海深部过程的探索

2011—2018年,国家自然科学基金委员会设立"南海深部过程演变"重大研究计划,同时推动实施了三次半的大洋钻探航次,使南海进入国际深海研究的前列,在深海盆洋陆相互作用、边缘海盆地的板缘张裂和气候演变的低纬驱动等3方面取得了突破性进展,挑战了源自西欧、北大西洋的传统观念。深海探索的进展,确立了中国在南海科学上的引导地位,下一轮的深入研究可望将南海建成世界海洋科学的天然实验室,进一步提升中国在海洋基础研究中的国际地位。     

Diatoms from the southwestern continental slope, South China Sea, and their paleoenvironmental significance since the last glacial times

A quantitative study was undertaken on diatoms from cores (SA08-34) obtained from the southwestern continental slope of the South China Sea (SCS). A total of 165 diatom species belonging to 45 genera were identified. We constructed a stratigraphic subdivision and correlation according to the characteristics of diatom assemblages together with ¹⁴C dating and carbonate analysis. We also discuss the sedimentary environment in the sea area since the last glacial times. The research shows that the diatom assemblages coincide with interglacial and glacial times, and changes in diatom abundance reflect the instability of the climate in the southern part of the SCS, such that short-term, temperature descending events correlate with the interglacial interval. The abundance of diatoms is relevant to interglacial and glacial times, since high abundance values were associated with an interglacial interval, and low abundance values with the last glacial maximum. We assume that strong upwelling developed in the interglacial interval, the development of which was influenced by variations of monsoons in the East Asian region.     

Opening and evolution of the South China Sea constrained by studies on volcanic rocks: Preliminary results and a research design

The South China Sea (SCS) is characterized by abundant seamounts, which provide important information about the evolution of the SCS and related deep processes. Cenozoic volcanism in the SCS and its surroundings comprises three stages relative to the spreading of the SCS:prespreading (>32 Ma), syn-spreading (32-16 Ma), and post-spreading (<16 Ma). The pre-spreading magmatism predominantly occurs on the northern margin of the SCS and in South China coastal areas and shows a bi-modal affinity. The syn-spreading magmatic activity was very limited on the periphery of the SCS, but may be concentrated in the SCS. However, seafloor samples of this stage are not available yet because of overlying thick sedimentary deposits. Post-spreading magmatism is widespread in the central and southwest sub-basins of the SCS, Hainan Island, Leizhou Peninsula, Thailand, and Vietnam. These are mainly alkali basalts with subordinate tholeiites, and display OIB-type geochemical characteristics. The Dupal isotope anomaly and presence of high-magnesian olivine phenocrysts suggests their possible derivation from the Hainan mantle plume. The temporal and spatial distribution of Cenozoic volcanism in the SCS and its surroundings may be accounted for either by plate stress re-organization before and after SCS spreading, or by ridge suction of plume flow during opening of the SCS. If the latter is the case, the volcanic rocks within the SCS basin may not be typical mid-ocean ridge basalts (MORB). It remains puz-zling, however, that the transition between the South China continental margin and the SCS basin does not have features typical of a volcanic rifted margin. Clearly, the relationship between mantle plume and SCS opening needs further evaluation. A better un-derstanding of the link between deep processes and opening of the SCS not only requires enhanced studies on igneous petrogene-sis, but also is heavily dependent on systematic sampling of seafloor rocks.     

Characteristics of hydrocarbon accumulation and exploration potential of the northern South China Sea deepwater basins

Although the huge potential of the northern South China Sea deepwater basins has been proven by a series of discoveries that followed the exploration breakthrough of well LW 3-1-1, recent drilling and other studies have demonstrated the uniqueness and complicated nature of hydrocarbon accumulations of the deepwater basins there. Based on a review of previous work and the latest exploration activities and studies, the purpose of this paper is to discuss the critical controls for hydrocarbon accumulations in the deepwater basins of the northern South China Sea. A terrestrial-marine transitional coal-bearing source rock is proposed to be the primary source rock for the deepwater basins. A marine source rock, which was first identified as contributing to hydro-carbon generation in this region, probably plays a significant role in the deep-and ultra-deep water basins south to the Pearl River Mouth and Qingdongnan basins. The shelf margin delta depositional systems in the Baiyun Sag, sourced from the Pearl River, are currently primary exploration targets in the deepwater part of the Pearl River Mouth Basin, whereas the western Red River delta-ic-submarine fan depositional systems, initially proven by drilling, are the possible major exploration reservoirs in the Qing-dongnan deepwater areas. Current deepwater exploration targets at the large-sized structural traps and deep and ultra-deep areas in the south of the Pearl River Mouth and Qingdongnan basins will be the future exploration focus. Deepwater exploration activities and relevant fundamental studies, supporting and promoting each other, are of great importance to the national energy supply of China, the basic regional studies of the South China Sea, advancements in technology, and development of related deepwater industries, and will safeguard national sovereignty and territorial integrity.     

Lithospheric thermal-rheological structures of the continental margin in the northern South China Sea

Thermal structures of three deep seismic profiles in the continental margin in the northern South China Sea are calculated, their "thermal" lithospheric thicknesses are evaluated based on the basalt dry solidus, and their rheological structures are evaluated with linear frictional failure criterion and power-law creep equation. "Thermal" lithosphere is about 90 km in thickness in shelf area, and thins toward the slope, lowers to 60-65 km in the lower slope, ocean crust and Xisha Trough. In the mid-west of the studied area, the lithospheric rheological structure in shelf area and Xisha Islands is of four layers: brittle, ductile, brittle and ductile. Because of uprising of heat mantle and thinning of crust and lithosphere in Xisha Trough, the bottom of the upper brittle layer is only buried at 16 km. In the eastern area, the bottom of the upper brittle layer in the north is buried at 20 km or so, while in lower slope and ocean crust, the rheological structure is of two layers of brittle and ductile, and crust and uppermost mantle form one whole brittle layer whose bottom is buried at 30-32 km. Analyses show that the characteristics of rheological structure accord with the seismic result observed. The character of rheological stratification implies that before the extension of the continent margin, there likely was a ductile layer in mid-lower crust. The influence of the existence of ductile layer to the evolution of the continent margin and the different extensions of ductile layer and brittle layer should not be overlooked. Its thickness, depth and extent in influencing continent margin's extension and evolution should be well evaluated in building a dynamic model for the area.     

Carbonate dissolution and deep-water paleoceanography of the South China Sea since the Middle Pleistocene

Based on the data of oxygen isotope, micropaleontology and paleomagnetism, we set up the detailed chronology of Core 17957-2 from the southern South China Sea and discuss the change of carbonate dissolution over the last 800 ka. Down-core variation of carbonate content records the “Pacific-type” cycle of higher glacial values and lower interglacial values, though the core lies above the modern lysocline. Carbonate dissolution indices indicate that several severe dissolution of CaCO₃happened during the transition from interglacial to glacial stages. Spectral analyses of these indices show that the carbonate dissolution periodicities are mainly made up of 500 ka and 100 ka. Compared with the cycles of carbonate dissolution of the Indian Ocean, the long-term (500-ka) periodicity reflects the characteristic of the deep-water circulation of the oceans.     

南海海盆演变与深部海流

南海在距今34 Ma之前的始新世从陆地变为海洋,古水深不断加深,至距今24 Ma之前的中新世/渐新世之交,由于T60构造运动,南海海盆整体进入深海环境。但是,自中新世以来随着吕宋岛弧向欧亚板块碰撞,南海海盆的半封闭程度在距今10.0、6.5、3.0和1.2 Ma之前加剧,导致南海深部海水只能来自巴士海峡海槛深度2600 m以浅的太平洋。此后,巴士海峡两侧的南海与太平洋深部海水交换,由于全球海平面变化,呈现冰期/间冰期模式。     

Benthic foraminiferal fauna turnover at 2.1 Ma in the northern South China Sea

Quantitative analysis of benthic foraminifera from ODP Site 1146 in the northern South China Sea (SCS) shows that abundance of Bulimina alazanensis, sometimes up to about 90%, decreased gradually since 3.2 Ma, especially at 2.1 Ma. Abundance of other benthic foraminiferal species, Globobulimina sub-globosa and Cibicidoides wuellerstorfi, increased after 2.1 Ma. Comparison with changes in oxygen and carbon isotopes of planktonic and benthic foraminifera shows that high abundance values of B. alazanensis corresponded with lower values of oxygen isotope, but for carbon isotope, high values of the species were consistent with heavier carbon isotope of benthic foraminifera and lighter carbon isotope of planktonic foraminifera, respectively, and vice versa. Considering factors such as uplift of Bashi Strait, expansion of the North Hemisphere Glaciation, strengthening of East Asian winter mon-soon and variations in oxygen and carbon isotope of foraminifera, changes of B. alazanensis in ODP Site 1146 suggest that the source of deep water masses of the northern South China Sea changed from the warm Pacific deep water with high oxygen content to Pacific Intermediate water with low oxygen content at 2.1 Ma. In addition, the strengthened East Asian winter monsoon resulted in increased pri-mary productivity, high nutrient and suboxic bottom water. Variations in species of B. alazanensis seemed to be unable to tolerate environmental stress induced by deep water masses and productivity changes.     

The deep thermal characteristic of continental margin of the northern South China Sea

Heat flow plays an important role in the study of thermal structure and thermal evolution of continental margin of the northern South China Sea. The analysis of heat flow value shows that margin heat flow in the northern South China Sea is relatively high setting, but the percentage of crustal heat flow is lower than 35% in terrestrial heat flow. The terrestrial heat flow exhibited a current of rise from the Northern Continental Margin to the Southern Central Basin. However, the proportion of crustal heat flow in terrestrial heat flow slowly dropped down in the same direction. It is suggested that the main factor causing high heat flow setting is the moving up of hot material from asthenosphere.     

Oxygen and carbon isotope records of calcareous nannofossils for the past 1 Ma in the southern South China Sea

Oxygen and carbon isotopic analyses have been carried out on calcareous nannofossils from ODP Site 1143 in the southern South China Sea. The results demonstrate that the δ18O values of calcareous nannofossils for the past 1 Ma vary systematically with those of planktonic and benthic foraminifera from the same site. But the average δ18O value of nannofossils is 1.869‰ higher than that of planktonic foraminifera, and is 3.855‰ lower than that of benthic foarminifera. There are systematic differences between the δ13C values of nannofossils, planktonic foarminifera and benthic foraminifera. The average δ13C of nannofossils is 0.756‰ higher than that of benthic foraminifera, and is 0.460‰ lower than that of planktonic foraminifera. The δ13C values of nannofossils exhibit a significant positive shift beginning near isotope stage 14 and lasting until stage 8. This study also finds that there is a close ralationship between the δ13C of nannofossils and the percentage of Florisphaera profunda. This implies that the δ13C of nannofossils can be used as an indicator of sea water surface paleoproductivity.     

Cenozoic stratigraphy of Taiwan: Window into rifting, stratigraphy and paleoceanography of South China Sea

Shallow marine sequences of the northern South China Sea (SCS) are uplifted and exposed by plate convergence in the Taiwan mountain belt. These deposits provide detailed geological information about the rifting event, stratigraphy, sedimentology, paleoclimate and paleoceanography of the shallow SCS to compare with what are recorded in the ODP 1148 deep-sea core. Seismic surveys and marine micropalentological studies show that Eocene sequences in the offshore Taiwan Strait and onland Taiwan mountain belt are all deposited in rifting basins and are covered unconformably by the Late Oligocene-Neogene post-rifting strata. Between syn-rifting and post-rifting sequences, there is a regional break-up unconformity throughout the island. Early Oligocene and Late Eocene strata are missing along the break-up unconformity equivalent to the T7 unconformity in the Pearl River Mouth Basin off south China. This may suggest that the SCS oceanic crust could have initiated between 33 and 39 Ma. Neither obvious stratigraphic gap nor slumping features are found in the Oligocene-Miocene transition interval of Taiwan. This observation highly contrasts with what has been documented from the ODP 1148 deep-sea core. This suggests that the stratigraphic gap and slumping features could only be recorded in the SCS deep sea region, but not in the shallow shelf near Taiwan. Compared to the Middle Miocene paleoceanographic re-organization events in the SCS deep sea, the geological history of the Taiwan shallow sequence shows changes of in sedimentation and faunal composition. Due to the Antarctic glacial expansion at~14 Ma, Middle to late Miocene strata of the Western Foothills show progressive regression sedimentations associated with a decrease of benthic foraminif-eral abundance and a sharp faunal turnover event. Many Early-Middle Miocene endemic benthic foraminifers were extinct in 14-13 Ma and new benthic foraminifers of the Kuroshio Current fauna appeared from 10.2 Ma, comparable with new occurrence of Modern benthic foraminifers at 9 Ma in the Java Sea area. This reveals that the Western Boundary Kuroshio Current in the North Pacific could initiate from 10-9 Ma due to closures of the Indo-Pacific seaways by convergent tectonics between the Australian Continent and the Indonesian Arc in 12-8 Ma. Subduction of the SCS oceanic lithosphere since the Middle Miocene resulted in formation of the Hengchun Ridge accretionary prism and the North Luzon Arc. Occurrence of these two bathymetric highs ( 2400 m) since the Middle Miocene and closures of the inter-arc passages in the North Luzon arc in the last 3.5 Ma would control the water exchanges between the West Pacific and the deep SCS. Accordingly, the tectonic evolution in the Central Range-Hengchun Peninsula accretionary prism and the arc-forearc Coastal Range not only control directly the route for water exchanges between the West Pacific and the SCS, but also indirectly shows a great influence on the geochemistry of deep SCS waters. The latter is best shown by much negative carbon isotope values of benthic foraminifers in the ODP 1148 deep-sea core than the West Pacific records in the last 14 Ma.     

中国东部新生代陆相断陷盆地层序的构成样式

选取我国东部跨不同古构造、古气候和古地理单元的多个陆相断陷盆地,如济阳凹陷、南阳和泌阳凹陷、南堡凹陷、歧口凹陷、江汉盆地、东濮凹陷、百色盆地和苏北盆地等,以层序界面和层序单元的精细划分为基础,对古近系-新近系的构造地层格架和层序结构特征进行对比分析,总结出犁式断坳型和陡倾式断坳型2种类型层序地层格架和断坡带型、弯折带型、枢纽带型(或斜坡型)3种典型的层序构成样式。不同级次的构造控制着层序的发育和层序样式特点,在此基础上建立了对储集体展布和烃源岩潜力具有重要预测功能的层序地层模式,从而为我国东部具有地域及地质特色的陆相含油气盆地内隐蔽油气藏的寻找和评价提供了一套新的思维框架。     

南海北部大陆架海洋生态系统Ecopath模型的应用与分析

过度捕捞和环境恶化导致渔业资源和海洋生态系统逆向发展。采用EwE5.1软件,对南海北部大陆架建立Ecopath(生态通道模型)模型。通过各级流量、生物量、生产量、捕捞量、系统总流量以及生态位和混和营养效应等方面的分析,得出营养流通主要有2种途径,肉食鱼类间饵料竞争非常激烈,低值鱼类间具相似的捕食压力,顶级捕食者对大部分鱼类负效应不明显。系统受过度捕捞渔业影响很大,并存在营养级I利用效率低和渔业资源小型化、低值化等不稳定的幼态特征。     

南海海盆东-西部地质特征存在巨大差异

南海中部存在巨大的中南断裂将南海海盆分割为东-西两部分,至少自1亿年以来,在沉积环境与沉积厚度、洋陆边界的属性与特征、大陆破裂的时代、岩浆活动的来源与程度、减薄大陆架和大陆坡的宽度、洋壳年龄与磁性层结构、磁异常条带特征、岩石化学等诸多方面都存在巨大差异,它们主要受控于早期地质构造背景的差异、东西部大陆减薄伸展速率的变化以及海底扩张的构造环境的不同,并深刻影响了之后的区域沉积分区和沉降特征。     

Formation and evolution of Precambrian continental crust in South China

The occurrence of zircons with U-Pb ages of ~3.8 Ga and Hf model ages of ~4.0 Ga in South China suggests the existence of the Hadean crustal remnants in South China. Furthermore, a detrital zircon with a U-Pb age as old as 4.1 Ga has been found in Tibet. This is the oldest zircon so far reported in China. These results imply that continental crust was more widespread than previously thought in the late Hadean, but its majority was efficiently reworked into Archean continental crust. On the basis of available zircon U-Pb age and Hf isotope data, it appears that the growth of continental crust in South China started since the early Archean, but a stable cratonic block through reworking did not occur until the Paleoproterozoic. Thus the operation of some form of plate tectonics may occur in China conti- nents since Eoarchean. The initial destruction of the South China craton was caused by intensive magmatic activity in association with the assembly and breakup of the supercontinent Rodinia during the Neoproterozoic. However, most of the Archean and Paleoproterozoic crustal materials in South China do not occur as surface rocks, but exist as sporadic crustal remnants. Nevertheless, the occur- rence of Neoproterozoic magmatism is still a signature to distinguish South China from North China.