从南海与大洋磁异常的相关性探讨南海的成因

对南海与大洋盆地条带状磁异常的相关分析表明，相关系数为０．１１—０．６５９，存在弱线性关系。这从一个侧面反映出边缘海洋壳与大洋壳本质上的差异。边缘海洋壳是玄武岩浆侵入并吞蚀破碎的大陆边缘地壳后形成的不同于大洋壳的新型洋壳。南海盆地磁异常显示其洋壳是新老不一、厚薄不匀的新洋壳的拼合叠覆体。     

Crustal velocity structure off SW Taiwan in the northernmost South China Sea imaged from TAIGER OBS and MCS data

During TAiwan Integrated GEodynamics Research of 2009, we investigated data from thirty-seven ocean-bottom seismometers (OBS) and three multi-channel seismic (MCS) profiles across the deformation front in the northernmost South China Sea (SCS) off SW Taiwan. Initial velocity-interface models were built from horizon velocity analysis and pre-stack depth migration of MCS data. Subsequently, we used refracted, head-wave and reflected arrivals from OBS data to forward model and then invert the velocity-interface structures layer-by-layer. Based on OBS velocity models west of the deformation front, possible Mesozoic sedimentary rocks, revealed by large variation of the lateral velocity (3.1–4.8 km/s) and the thickness (5.0–10.0 km), below the rift-onset unconformity and above the continental crust extended southward to the NW limit of the continent–ocean boundary (COB). The interpreted Mesozoic sedimentary rocks NW of the COB and the oceanic layer 2 SE of the COB imaged from OBS and gravity data were incorporated into the overriding wedge below the deformation front because the transitional crust subducted beneath the overriding wedge of the southern Taiwan. East of the deformation front, the thickness of the overriding wedge (1.7–5.0 km/s) from the sea floor to the décollement decreases toward the WSW direction from 20.0 km off SW Taiwan to 8.0 km at the deformation front. In particular, near a turn in the orientation of the deformation front, the crustal thickness (7.0–12.0 km) is abruptly thinner and the free-air (?20 to 10 mGal) and Bouguer (30–50 mGal) gravity anomalies are relatively low due to plate warping from an ongoing transition from subduction to collision. West of the deformation front, intra-crustal interfaces dipping landward were observed owing to subduction of the extended continent toward the deformation front. However, the intra-crustal interface near the turn in the orientation of the deformation front dipping seaward caused by the transition from subduction to collision. SE of the COB, the oceanic crust, with a crustal thickness of about 10.0–17.0 km, was thickened due to late magmatic underplating or partially serpentinized mantle after SCS seafloor spreading. The thick oceanic crust may have subducted beneath the overriding wedge observed from the low anomalies of the free-air (?50 to ?20 mGal) and Bouguer (40–80 mGal) gravities across the deformation front.     

Crustal Structures of the Northernmost South China Sea: Seismic Reflection and Gravity Modeling

The South China Sea (SCS) is a marginal sea off shore Southeast Asia. Based on magnetic study, oceanic crust has been suggested in the northernmost SCS. However, the crustal structure of the northernmost SCS was poorly known. To elaborate the crustal structures in the northernmost SCS and off southwest Taiwan, we have analyzed 20 multi-channel seismic profiles of the region. We have also performed gravity modeling to understand the Moho depth variation. The volcanic basement deepens southeastwards while the Moho depth shoals southeastwards. Except for the continental margin, the northernmost SCS can be divided into three tectonic regions: the disturbed and undisturbed oceanic crust (8–12 km thick) in the southwest, a trapped oceanic crust (8 km thick) between the Luzon-Ryukyu Transform Plate Boundary (LRTPB) and Formosa Canyon, and the area to the north of the Formosa Canyon which has the thickest sediments. Instead of faulting, the sediments across the LRTPB have only displayed differential subsidence offset of about 0.5–1 s in the northeast side, indicating that the LRTPB is no longer active. The gravity modeling has shown a relatively thin crust beneath the LRTPB, demonstrating the sheared zone character along the LRTPB. However, probably because of post-spreading volcanism, only the transtension-shearing phenomenon of volcanic basement in the northwest and southeast ends of the LRTPB can be observed. These two basement-fractured sites coincide with low gravity anomalies. Intensive erosion has prevailed over the whole channel of the Formosa Canyon.     

东海莫霍面起伏与地壳减薄特征初步分析

收集、整理大量由地震剖面提供的沉积层厚度资料,得到东海沉积层等厚图。对完全布格重力异常进行沉积层重力效应改正后,得到剩余重力异常,利用地震资料揭示的莫霍面深度值来约束界面反演得到东海莫霍面埋深。结果表明,东海陆架盆地莫霍面深度在25～28 km之间平缓变化,地壳厚度为14～26 km,西厚东薄;冲绳海槽盆地莫霍面深度为16～26 km,地壳厚度为12～22 km,北厚南薄。东海陆架盆地东部与冲绳海槽盆地南部地壳减薄明显,拉张因子分别达到2.6和3。初步分析认为冲绳海槽地壳以过渡壳为主,并未形成洋壳。     

Crustal Thinning of the Northern Continental Margin of the South China Sea

Magnetic data suggest that the distribution of the oceanic crust in the northern South China Sea (SCS) may extend to about 21 °N and 118.5 °E. To examine the crustal features of the corresponding continent–ocean transition zone, we have studied the crustal structures of the northern continental margin of the SCS. We have also performed gravity modeling by using a simple four-layer crustal model to understand the geometry of the Moho surface and the crustal thicknesses beneath this transition zone. In general, we can distinguish the crustal structures of the study area into the continental crust, the thinned continental crust, and the oceanic crust. However, some volcanic intrusions or extrusions exist. Our results indicate the existence of oceanic crust in the northernmost SCS as observed by magnetic data. Accordingly, we have moved the continent–ocean boundary (COB) in the northeastern SCS from about 19 °N and 119.5 °E to 21 °N and 118.5 °E. Morphologically, the new COB is located along the base of the continental slope. The southeastward thinning of the continental crust in the study area is prominent. The average value of crustal thinning factor of the thinned continental crust zone is about 1.3–1.5. In the study region, the Moho depths generally vary from ca. 28 km to ca. 12 km and the crustal thicknesses vary from ca. 24 km to ca. 6 km; a regional maximum exists around the Dongsha Island. Our gravity modeling has shown that the oceanic crust in the northern SCS is slightly thicker than normal oceanic crust. This situation could be ascribed to the post-spreading volcanism or underplating in this region.     

Lithospheric structure below the eastern Arabian Sea and adjoining West Coast of India based on integrated analysis of gravity and seismic data

Four uniformly spaced regional gravity traverses and the available seismic data across the western continental margin of India, starting from the western Indian shield extending into the deep oceanic areas of the eastern Arabian Sea, have been utilized to delineate the lithospheric structure. The seismically constrained gravity models along these four traverses suggest that the crustal structure below the northern part of the margin within the Deccan Volcanic Province (DVP) is significantly different from the margin outside the DVP. The lithosphere thickness, in general, varies from 110–120 km in the central and southern part of the margin to as much as 85–90 km below the Deccan Plateau and Cambay rift basin in the north. The Eastern basin is characterised by thinned rift stage continental crust which extends as far as Laxmi basin in the north and the Laccadive ridge in the south. At the ocean–continent transition (OCT), crustal density differences between the Laxmi ridge and the Laxmi basin are not sufficient to distinguish continental as against an oceanic crust through gravity modeling. However, 5-6 km thick oceanic crust below the Laxmi basin is a consistent gravity option. Significantly, the models indicate the presence of a high density layer of 3.0 g/cm³ in the lower crust in almost whole of the northern part of the region between the Laxmi ridge and the pericontinental northwest shield region in the DVP, and also below Laccadive ridge in the southern part. The Laxmi ridge is underlain by continental crust upto a depth of 11 km and a thick high density material (3.0 g/cm³) between 11–26 km. The Pratap ridge is indicated as a shallow basement high in the upper part of the crust formed during rifting. The 15 –17 km thick oceanic crust below Laccadive ridge is seen further thickened by high density underplated material down to Moho depths of 24–25 km which indicate formation of the ridge along Reunion hotspot trace.     

南海北部地球物理特征及地壳结构

为了研究南海地壳结构，中国和日本合作在南海北部首次进行了以炸药为震源的综合地球物理调查。经初步分析其地壳结构主要特征为：南海北部地壳分为沉积层、上地壳层、中地壳层及下地壳层。大陆架及上陆坡地壳厚度大、稳定。下陆坡地壳厚度除中地壳外，其他壳层厚度减薄且不稳定。深海盆地壳分３层，厚度虽薄但相对稳定，其底部缺失７．３ｋｍ·ｓ－１的高速层。测区内地壳总厚度：陆壳２６—３０ｋｍ，过渡壳１３—２２ｋｍ，洋壳为８ｋｍ。     

尼日利亚南部大陆边缘重力场特征及解释

利用中国-尼日利亚国际合作航次获得的船载重力数据,分析了尼日利亚南部大陆边缘的自由空间重力异常和布格重力异常特征,并通过两条从陆架—陆坡—陆隆一直延伸到深海盆地的重力剖面拟合出地壳密度结构。研究结果表明,地壳厚度总体上从陆架、陆坡至深海平原呈现阶梯状减薄的趋势,东侧的减薄幅度大于西侧,东侧从24 km减小到10 km,西侧从21 km减小到14 km。     

南海西北部与红河地区地球物理场及其地壳深部结构特征

分析了南海西北部与红河地区地球物理场特征，计算了研究区重、磁资料的一阶小波细节变换、四阶小波逼近变换。根据重力场资料以及南海北部盆地钻井取样的测试结果，同时参考在研究区进行的地震勘探结果，对研究区的地壳结构进行了反演计算。结果表明，研究区域地壳结构较为复杂，地壳厚度在17—38km之间，总的趋势由陆向洋地壳厚度逐渐减薄，反映出该区域地壳具有陆壳、过渡壳的性质，同时存在上地幔隆起区及凹陷区。用地震层折成像结果与重力资料计算出的地壳分布趋势进行了对比验证。根据地幔对流结果探讨了研究区深部地球动力学特征及其与深部地壳结构的关系。     

Deep structures of the Palawan and Sulu Sea and their implications for opening of the South China Sea

Compared to the northern South China Sea continental margin, the deep structures and tectonic evolution of the Palawan and Sulu Sea and ambient regions are not well understood so far. However, this part of the southern continental margin and adjacent areas embed critical information on the opening of the South China Sea (SCS). In this paper, we carry out geophysical investigations using regional magnetic, gravity and reflection seismic data. Analytical signal amplitudes (ASA) of magnetic anomalies are calculated to depict the boundaries of different tectonic units. Curie-point depths are estimated from magnetic anomalies using a windowed wavenumber-domain algorithm. Application of the Parker–Oldenburg algorithm to Bouguer gravity anomalies yields a 3D Moho topography. The Palawan Continental Block (PCB) is defined by quiet magnetic anomalies, low ASA, moderate depths to the top and bottom of the magnetic layer, and its northern boundary is further constrained by reflection seismic data and Moho interpretation. The PCB is found to be a favorable area for hydrocarbon exploration. However, the continent–ocean transition zone between the PCB and the SCS is characterized by hyper-extended continental crust intruded with magmatic bodies. The NW Sulu Sea is interpreted as a relict oceanic slice and the geometry and position of extinct trench of the Proto South China Sea (PSCS) is further constrained. With additional age constraints from inverted Moho and Curie-point depths, we confirm that the spreading of the SE Sulu Sea started in the Early Oligocene/Late Eocene due to the subduction of the PSCS, and terminated in the Middle Miocene by the obduction of the NW Sulu Sea onto the PCB.     

The Crustal Structure Character of East China Sea

This paper presents actuality of investigation and study of the crustal structure characters of East China Sea at home and abroad. Based on lots of investigation and study achievements and the difference of the crustal velocity structure from west to east, the East China Sea is divided into three parts - East China Sea shelf zone, Okinawa Trough zone and Ryukyu arc-trench zone. The East China Sea shelf zone mostly has three velocity layers, i.e., the sediment blanket layer （the velocity is 5.8-5.9 km/s）, the basement layer （the velocity is 6.0-6.3 km/s）, and the lower crustal layer （the velocity is 6.8-7.6 km/s）. So the East China Sea shelf zone belongs to the typical continental crust. The Okinawa Trough zone is located at the transitional belt between the continental crust and the oceanic crust. It still has the structural characters of the continental crust, and no formation of the oceanic crust, but the crust of the central trough has become to thinning down. The Ryukyu arc-trench zone belongs to the transitional type crust as a whole, but the ocean side of the trench already belongs to the oceanic crust. And the northwest Philippine Basin to the east of the Ryukyu Trench absolutely belongs to the typical oceanic crust.     

The deep structure of the Iceland-Faeroe Ridge

Two long seismic refraction lines along the crest of the Iceland-Faeroe Ridge reveal a layered crust resembling the crust beneath Iceland but differing from normal continental or oceanic crust. The Moho was recognised at the south-eastern end of the lines at an apparent depth of 16–18 km. A refraction line in deeper water west of the ridge and south of Iceland indicates a thin oceanic type crust underlain by a 7.1 km/s layer which may be anomalous upper mantle.An extensive gravity survey of the ridge shows that it is in approximate isostatic equilibrium; the steep gravity gradient between the Norwegian Sea and the ridge indicates that the ridge is supported by a crust thickened to about 20 km rather than by anomalous low density rocks in the underlying upper mantle, in agreement with the seismic results. An increase in Bouguer anomaly of about 140 mgal between the centre of Iceland and the ridge is attributed to lateral variation in upper mantle density from an anomalous low value beneath Iceland to a more normal value beneath the ridge. Local gravity anomalies of medium amplitude which are characteristic of the ridge are caused by sediment troughs and by lateral variations in the upper crust beneath the sediments. A steep drop in Bouguer anomaly of about 80 mgal between the ridge and the Faeroe block is attributed partly to lateral change in crustal density and partly to slight thickening of the crust towards the Faeroe Islands; this crustal boundary may represent an anomalous type of continental margin formed when Greenland started to separate from the Faeroe Islands about 60 million years ago.We conclude that the Iceland-Faeroe Ridge formed during ocean floor spreading by an anomalous hot spot type of differentiation from the upper mantle such as is still active beneath Iceland. This suggests that the ridge may have stood some 2 km higher than at present when it was being formed in the early Tertiary, and that it has subsequently subsided as the spreading centre moved away and the underlying mantle became more normal; this interpretation is supported by recognition of a V-shaped sediment filled trough across the south-eastern end of the ridge, which may be a swamped sub-aerial valley.     

Crustal analysis of the Ulleung Basin in the East Sea (Japan Sea) from enhanced gravity mapping

To facilitate geological analyses of the Ulleung Basin in the East Sea (Japan Sea) between Korea and Japan, shipborne and satellite altimetry-derived gravity data are combined to derive a regionally coherent anomaly field. The 2-min gridded satellite altimetry-based gravity predicted by Sandwell and Smith [Sandwell DT, Smith WHF (1997) J Geophys Res 102(B5):10,039–10,054] are used for making cross-over adjustments that reduce the errors between track segments and at the cross-over points of shipborne gravity profiles. Relative to the regionally more homogeneous satellite gravity anomalies, the longer wavelength components of the shipborne anomalies are significantly improved with minimal distortion of their shorter wavelength components. The resulting free-air gravity anomaly map yields a more coherent integration of short and long wavelength anomalies compared to that obtained from either the shipborne or satellite data sets separately. The derived free-air anomalies range over about 140 mGals or more in amplitude and regionally correspond with bathymetric undulations in the Ulleung Basin. The gravity lows and highs along the basin’s margin indicate the transition from continental to oceanic crust. However, in the northeastern and central Ulleung Basin, the negative regional correlation between the central gravity high and bathymetric low suggests the presence of shallow denser mantle beneath thinned oceanic crust. A series of gravity highs mark seamounts or volcanic terranes from the Korean Plateau to Oki Island. Gravity modeling suggests underplating by mafic igneous rocks of the northwestern margin of the Ulleung Basin and the transition between continental and oceanic crust. The crust of the central Ulleung Basin is about a 14–15 km thick with a 4–5 km thick sediment cover. It may also include a relatively weakly developed buried fossil spreading ridge with approximately 2 km of relief.     

利用重磁计算解释南海海盆中部地壳结构特征

以南海海盆中部3个海盆的交汇处为研究区，通过对重磁异常场的向上延拓计算及对重力异常的重反演拟合计算，并结合地质资料，进行综合分析和解释来揭示此区域的地壳结构特征。结果表明研究区地壳结构复杂，三种性质的地壳均有分布。各海山形成过程和岩石组成及地壳结构不尽相同，区内有年轻的大洋玄武岩海山，也有包含了大陆玄武岩的“双性”海山。综合各种地质、地球物理现象，认为南海海盆可能是多种机制联合作用的结果，西南、西北两海盆的形成时间要早于中央海盆。     

Study on the distribution characteristics of faults and their control over petroliferous basins in the China seas and its adjacent areas

As one of the main controlling factors of oil and gas accumulation, faults are closely related to the distribution of oil and gas reservoirs. Studying how faults control petroliferous basins is particularly important. In this work, we investigated the plane positions of major faults in the China seas and its adjacent areas using the normalized vertical derivative of the total horizontal derivative (NVDR-THDR) of the Bouguer gravity anomaly, the fusion results of gravity and magnetic anomalies, and the residual Bouguer gravity anomaly. The apparent depths of major faults in the China seas and its adjacent areas were inverted using the Tilt-Euler method based on the Bouguer gravity anomaly. The results show that the strikes of the faults in the China seas and its adjacent areas are mainly NE and NW, followed by EW, and near-SN. Among them, the lengths of most ultra-crustal faults are in the range of 1 000–3 000 km, and their apparent depths lie between 10 km and 40 km. The lengths of crustal faults lie between 300 km and 1 000 km, and their apparent depths are between 0 km and 20 km. According to the plane positions and apparent depths of the faults, we put forward the concept of fault influence factor for the first time. Based on this factor, the key areas for oil and gas exploration were found as follows: the east of South North China Basin in the intracontinental rift basins; the southeast region of East China Sea Shelf Basin, the Taixinan and Qiongdongnan basins in the continental margin rift basins; Zhongjiannan Basin in the strike-slip pull-apart basins; the Liyue, Beikang, and the Nanweixi basins in the rifted continental basins. This work provides valuable insights into oil and gas exploration, mineral resource exploration, and deep geological structure research in the China seas and its adjacent areas.     

Basement-involved faults and deep structures in the West Philippine Basin: constrains from gravity field

To reveal the basement-involved faults and deep structures of the West Philippine Basin (WPB), the gravitational responses caused by these faults are observed and analyzed based on the latest spherical gravity model: WGM2012 Model. By mapping the free-air and Bouguer gravity anomalies, several main faults and some other linear structures are located and observed in the WPB. Then, by conducting a 2D discrete multi-scale wavelet decomposition, the Bouguer anomalies are decomposed into the first- to eighth-order detail and approximation fields (the first- to eighth-order Details and Approximations). The first- to third-order Details reflect detailed and localized geological information of the crust at different depths, and of which the higher-order reflects gravity field of the deeper depth. The first- to fourth-order Approximations represent the regional gravity fields at different depths of the crust, respectively. The fourth-order Approximation represents the regional gravity fluctuation caused by the density inhomogeneity of Moho interface. Therefore, taking the fourth-order Approximation as input, and adopting Parker-Oldenburg interactive inversion, We calculated the depth of Moho interface in the WPB. Results show that the Moho interface depth in the WPB ranges approximately from 8 to 12 km, indicating that there is typical oceanic crust in the basin. In the Urdaneta Plateau and the Benham Rise, the Moho interface depths are about 14 and 16 km, respectively, which provides a piece of evidence to support that the Banham Rise could be a transitional crust caused by a large igneous province. The second-order vertical derivative and the horizontal derivatives in direction 0° and 90° are computed based on the data of the third-order Detail, and most of the basement-involved faults and structures in the WPB, such as the Central Basin Fault Zone, the Gagua Ridge, the Luzon-Okinawa Fault Zone, and the Mindanao Fault Zone are interpreted by the gravity derivatives.     

Contribution of global potential field data to the tectonic reconstruction of the Rio Grande Rise in the South Atlantic

The application of advanced enhancement techniques for geophysical anomalies to global gravity (WGM2012) and magnetic (EMAG2) models sheds light on the complex tectonic evolution of the Rio Grande Rise (RGR) in the southern South Atlantic. Long wavelength Bouguer gravity lows indicate a thicker crust beneath of the ridge, whose nature can be related to a microcontinent or an excess of volcanism within the oceanic realm. Recently dredged continental rocks reinforce the hypothesis of a microcontinent or, at least, slivers of continental crust. However, the reserval magnetic pattern of the processed magnetic anomalies provide no evidence of aborted spreading center similar to the well-studied Jan Mayen microcontinent and the surrounding (inactive) Aegir and (active) Kolbeinsey ridges in the North Atlantic Ocean. The reversal magnetic anomalies show a series N-S trending parallel stripes roughly follow the current South American coastline and segmented by E-W oriented oceanic fracture zones (FZs). The magnetic stripes are bended eastwards at the RGR, showing a more complex magnetic pattern similar to that in the Iceland. The aborted Cruzeiro do Sul Rift (CSR) and the Jean Charcot Chain (JCC) are structures that cross the RGR and contribute to the understanding of the tectonic evolution of the South Atlantic Ocean. NW-SE oriented extensive gravity lows and bathymetric valleys, which mark the CSR, are segmented by E-W trending magnetic lineaments related to FZs. This structural configuration suggests that the extensional event, which formed the rift and the seamounts chain, was replaced by strike-slip movements along the FZs. In addition, we constructed a plate kinematic model for the evolution of the RGR based on bathymetric, free-air and Bouguer gravity and magnetic data. Our model comprises five main stages of the RGR formation and evolution between late Cretaceous and Paleocene (ca. 95 - 60 Ma), separated by published seafloor isochrones. The proposed model suggests that the RGR was built at the mid-Atlantic ridge by increased magmatism probably related to the Tristan da Cunha hotspot.     

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。     

Crustal structure of the South Florida Platform,eastern Gulf of Mexico: An ocean-bottom seismograph refraction study

Five seismic refraction lines, 70–90 km long, were shot in the South Florida Platform region of the Gulf of Mexico using digital ocean-bottom seismographs. Apparent velocities and depths were calculated from the refracted arrivals using a flat-layer model for the region. The two dominant refractors have apparent compressional-wave velocity ranges of 5.6 to 5.9 km s^–1 and 6.2 to 6.7 km s^–1. On the Sarasota Arch, the depth to the top of a 5.8–5.9km/s layer is 3–4 km below sea level. This depth corresponds to the depth to the crystalline basement. The basement dips to the north and to the south from the arch, with velocity of the upper crust increasing from 5.8–5.9 km s^–1 to a maximum of 6.7 km s^–1 at a depth of 6.3 km. Under the continental slope, the crust has presumably been thinned and extended. The deepest refractor has an apparent velocity of about 7.5 km s^–1 at a depth of 25 km. The thickness of the crustal section and the absence of any mantle arrivals in these long refraction profiles on the platform suggest that thick continental crust underlies the South Florida Platform. A north-south cross-section through the platform suggests the presence of two structural highs separated by a portion of the South Florida Basin, which contains at least 5 km of sediment.     

我国南海历史性水域线的地质特征

40a的海洋地质、地球物理实测研究表明,九段线不仅是显示我国南海主权的历史性水域线,而且总体上也是南海与东部、南部和西部陆区及岛区的巨型地质边界线。根据实测数据,本文将从地质成因、来源、演化的角度论述此南海历史性水域线的合理性。主要结论包括:历史性水域线的东段在地形上基本与马尼拉海沟一致,海沟西侧为南海中央海盆洋壳区,东侧为菲律宾群岛。根据国际地质研究的资料,菲律宾群岛始新世以前位于较偏南的纬度,后来于中晚中新世(距今16~10Ma)仰冲于南海中央海盆之上,因此菲律宾群岛是一个外来群岛。而黄岩岛在马尼拉海沟以西,是中央海盆洋壳区的一个岛礁,与菲律宾群岛成因不同。南海历史性水域线的南段在地形上基本与南沙海槽一致,伴随南沙地块由北部陆缘向南裂离,古南海洋壳沿此海槽以南俯冲至加里曼丹岛陆壳之下,因此南沙地块与加里曼丹陆块为两个来历不同的地块。南海历史性水域线西段的分布在地形上与越东巨型走滑断裂带基本一致,可能与西沙地块、中沙地块、南沙地块从南海北部陆缘向南滑移有关。南沙地块北缘陡直的正断层结构,突显中央海盆是拉裂形成,其基底和中新生代地层与北部珠江口盆地的地层结构可以对比,说明南沙岛礁原属我国华南大陆南缘,后因南海的形成裂离至现今的位置。