小波分析与解析延拓方法相结合研究东海深部构造

东海是环太平洋活动构造带的重要组成部分,是探讨西太平洋边缘及边缘海形成演化的重要窗口。向上延拓主要用来削弱局部异常,突出深部异常,小波分析是重磁场位场分解的有效工具,不仅能提取深部异常,还能更好地将区域异常和局部异常分离。本文利用东海及邻域最新的布格重力异常数据资料,将小波分析与解析延拓方法相结合,分析对比向上延拓20 km的布格重力异常结果与小波分析4阶逼近布格重力异常结果,结合东海及邻域莫霍面的深度,对重力场及莫霍面深度进行初步研究。其中小波4阶逼近结果代表莫霍面形态,小波4阶细节结果代表东海及邻域凹陷凸起形态。     

台湾地区重力场特征及莫霍面反演

台湾地区空间重力异常幅值在-240~340mGal之间变化,布格重力异常幅值在-140~380mGal之间变化,重力异常及圈闭走向呈现北东、北北东向。将小波分析方法引入台湾地区的重力异常数据处理,经过分析比对,台湾地区布格重力异常小波分析三阶逼近结果代表莫霍面起伏形态,利用重力数据反演了深部界面莫霍面,研究区莫霍面深度为12~32km,莫霍面展布呈现北东走向,台湾岛区莫霍面深,在24~31km之间变化,由西北往东南为厚-薄-厚分布,台湾东部海区莫霍面深度浅,在12~17km之间变化,台湾岛属于陆壳结构,靠近菲律宾海的台湾岛外海地区,属于海洋性地壳结构。     

南海东北部深部构造与中新生代沉积盆地

利用OBS资料作约束条件对南海东北部的地球物理资料，主要是重力资料和多道反射地震资料进行反演，获取比较理想的莫霍面深度，地壳厚度，中生代沉积基底面，新生代沉积底界面等地壳结构信息，研究发现该区中生代沉积盆地形成模式与新生代沉积盆地的形成模式不同，中生代沉积基底与莫霍面呈正相关，而新生代沉积基底则与莫霍面呈明显的镜像关系。中生仝层不受边界断层控制，中生代沉积基底与莫霍面呈正相关，而新生代沉积基底则与莫霍面呈明显的镜像关系。中生代地层不受边界断层控制。中生代沉积坳陷边界实质上是残留的中生代地层的边界，中生代沉积盆地具有大型坳陷沉积特征，而新生代盆地为断陷盆地。     

南极罗斯海重力场特征及莫霍面深度反演

本文基于中国南极考察第30航次、第32航次所获得的实测重力资料,结合NGDC资料,开展12个航次重力场数据的平差融合工作,全部386个交点平差后标准差减小为±1.53×10−5 m/s2,与卫星重力差值平均值为1.49×10−5 m/s2,均方差为±3.81×10−5 m/s2,并在此基础上采用频率域界面反演法计算莫霍面深度。研究发现,与沉积盆地对应重力异常低值相悖,在罗斯海北部盆地、维多利亚地盆地、中央海槽、东部盆地4个主要盆地腹地却表现为重力异常高值,跨度达100 km以上。莫霍面深度分布整体呈南深北浅之势,范围为10~28 km。伴随着罗斯海西部盆地的多次拉张及岩浆活动,该区域的地壳厚度和莫霍面深度高值和低值相间分布,并表现出越来越大的差异性。综合剖面结果表明,罗斯海重力异常值的长波长变化与莫霍面的起伏呈正相关关系,但是反演的莫霍面深度与区域重力场特征并非完全对应,所以岩浆底侵和地壳侵入仍不足以导致罗斯海盆地的重力异常或盆地几何形状。     

南海北部磁静区重磁特征及成因

南海磁静区位于南海北部的洋陆结合带上,在磁异常图上位于陆架高值正磁异常带以南,海盆磁异常条带区以北。收集了南海北部的地质和地球物理相关资料,总结了南海北部磁静区的研究现状,对ΔT磁异常数据进行了低纬度化极处理,参考磁静区周围的自由空间重力异常分布,结合区域地质背景划分了南海磁静区的分布;采用小波多尺度分解方法讨论了南海北部磁静区及周围区域的重磁场特征,对主要界面的反演发现磁静区内存在磁性基底深度增加、居里等温面隆升、磁性层厚度减薄和莫霍面抬升的现象,认为南海磁静区形成的直接原因是区域内磁性层厚度的减薄,包括中生代末期地壳的拉张沉降使区域老地层断陷,磁性基底深度增加,磁异常减弱;拉张减薄促使深部地幔热物质向上运移,莫霍面抬升,磁性层发生热退磁,居里等温面抬升,磁性层厚度减薄,磁异常减弱;南海扩张期和张裂以后磁静区的热活动剧烈,深部高温物质底侵,形成高速层,进一步减弱了区域磁异常。     

西太平洋侏罗纪海洋地壳断裂特征及其成因机制*

侏罗纪洋壳为现存最古老的海洋地壳, 残留在地球表面上很少, 目前对于侏罗纪洋壳的断裂特征和构造变形了解很少。本文利用高分辨率的反射地震剖面精细解释了位于西太平洋的侏罗纪洋壳基底、沉积地层和断裂结构, 发现在研究区存在基底断层、沉积断层和垮塌断层三种类型的断裂构造, 并对其走向、倾角、断距等几何参数与变形特征进行了推测和定量研究。研究还发现, 基底断层是洋壳受到板块伸展拉张而产生的, 在后期海底沉积过程中持续发育并错断上覆沉积物, 在海底形成明显的断层陡坎。沉积断层是沉积地层自身重力作用的产物,受到沉积地层岩石性质的控制。垮塌断层是岩浆侵出或者侵入形成海山, 导致洋壳及其上覆沉积局部抬升并向两侧推移, 引起先存的基底断层和沉积断层重新错动产生的。研究区内切断洋壳基底和上覆沉积的活动断层的推测走向大体符合侏罗纪洋壳基底面起伏、重力异常骤变界面以及地磁异常条带等的走向, 表明这些断裂从侏罗纪洋中脊的海底扩张中演变而来, 并且持续活动至今。这些发育在古老洋壳上的断层能够长时间让水进入岩石圈并进入俯冲带及地球内部, 从而促进地球水循环。尽管目前尚未发现这些断裂产生大地震, 但这些断层可能随着板块俯冲而演变成俯冲带地震大断裂, 今后研究应该关注这类断层在靠近海沟之前的演化规律和潜在地震风险。     

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。     

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.     

重力水平梯度矢量法在琼东南盆地基底断裂划分上的应用

基底断裂与盆地是一对相互影响的伴生构造,断裂活动控制盆地内沉积填充和构造样式以及后期资源的分布。本文利用琼东南盆地2′×2′的自由空间重力异常,在进行地形校正、布格异常校正得到布格重力异常的基础上求得重力水平梯度矢量。尝试利用重力水平梯度矢量对基底断裂进行划分并取得良好效果:在盆地基底划分出48条断裂,并将其分为3个等级,其中一级断裂5条,二级断裂8条,三级断裂35条;将确定的断裂与其他地质、地球物理方法(地震剖面)确定的断裂进行比较,发现在主要格架上具有一致性;重力水平梯度矢量法与其他解释方法比较具有成本低廉、方法简单、结果直观的优点。     

海区沉积盆地的三维变密度基底反演及其应用

建立了适合海区沉积盆地基底反演的密度模式，推导了对应的重力异常精确公式（近区计算公式）和近似公式（远区计算公式），提出模拟沉积盆地复杂变化的密度差－深度曲线的分段拟合方法。建立了一种利用地震波速度谱、重力和水深资料反演沉积盆地基底的方法。利用地震波速度谱的资料对厦澎凹陷的密度－深度关系进行了统计分析，并反演了厦澎凹陷的重力基底，认为整个厦澎凹陷可能由不同构造格局的东北和西南两部分组成，并分别讨论了这两部分的基底特征。     

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.     

Crustal thickness variations in the Eastern Mediterranean and southern Aegean region

In this paper, regional analog gravity anomaly map obtained from the General Directorate of Mineral Research and Exploration (MTA) was digitized and used for the calculation of the crustal thickness (Moho depth) variations in the Eastern Mediterranean and the southern part of the Aegean Region. In the gravity anomaly map, there are mainly E–W trending apparent gravity anomalies represented by the contours up to 150 mGal. They are generally parallel to the shorelines of Africa, Turkey and Crete. Crustal thickness variations were calculated from the gravity anomalies, using an empirical equation in this study. Obtained thicknesses (Moho depths) were mapped and correlated with the previous investigations and seismological findings. According to the estimations, crustal thicknesses are about 25–30 km along the coastal regions and more than 30 km on the onshore part of Turkey increasing up to 42 km through the eastern Anatolia. However, there are thin crustal zones around 17 km in the offshore Egypt, to the NW part of Cyprus and about 19 km to the north of Crete. They may be related with the main tectonic trends in this region except the circular thinning to the south of Kas (southwestern part of Turkey). In order to determine the locations and boundaries of prominent tectonic elements, Analytic Signal (AS) and maxspots maps of the gravity anomalies were also prepared in this study. All produced maps are generally consistent to each other and the boundaries of main tectonic units were apparently illustrated in the maxspots map from the horizontal gradient of Bouguer anomalies.     

印度洋无震海岭及海底高原的初步研究

根据１９８６年１０月－１９８７年５月第三次南极考察和首次环球科学考察所获的印度洋实测重力资料，对印度洋一些典型构造进行了分析研究。初步主人为：无震海岭，海底高原和大洋中脊都有着复杂的壳－幔结构，其上都伴有一个布格异常的低值带，但引种布格局异常低值原因却不尽相同。虽然上述３者都是大洋中的隆起地带，但前两者的地壳增厚，莫氏丰下拱，软流圈变深，影响布格异常的主要因素是其下存在着一个较大的负山根。相反，在     

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.     

Gravity anomalies of the ridge-transform system in the South Atlantic between 31 and 34.5° S: Upwelling centers and variations in crustal thickness

To decipher the distribution of mass anomalies near the earth's surface and their relation to the major tectonic elements of a spreading plate boundary, we have analyzed shipboard gravity data in the vicinity of the southern Mid-Atlantic Ridge at 31–34.5° S. The area of study covers six ridge segments, two major transforms, the Cox and Meteor, and three small offsets or discordant zones. One of these small offsets is an elongate, deep basin at 33.5° S that strikes at about 45° to the adjoining ridge axes.By subtracting from the free-air anomaly the three-dimensional (3-D) effects of the seafloor topography and Moho relief, assuming constant densities of the crust and mantle and constant crustal thickness, we generate the mantle Bouguer anomaly. The mantle Bouguer anomaly is caused by variations in crustal thickness and the temperature and density structure of the mantle. By subtracting from the mantle Bouguer anomaly the effects of the density variations due to the 3-D thermal structure predicted by a simple model of passive flow in the mantle, we calculate the residual gravity anomalies. We interpret residual gravity anomalies in terms of anomalous crustal thickness variations and/or mantle thermal structures that are not considered in the forward model. As inferred from the residual map, the deep, major fracture zone valleys and the median, rift valleys are not isostatically compensated by thin crust. Thin crust may be associated with the broad, inactive segment of the Meteor fracture zone but is not clearly detected in the narrow, active transform zone. On the other hand, the presence of high residual anomalies along the relict trace of the oblique offset at 33.5° S suggests that thin crust may have been generated at an oblique spreading center which has experienced a restricted magma supply. The two smaller offsets at 31.3° S and 32.5° S also show residual anomalies suggesting thin crust but the anomalies are less pronounced than that at the 33.5° S oblique offset. There is a distinct, circular-shaped mantle Bouguer low centered on the shallowest portion of the ridge segment at about 33° S, which may represent upwelling in the form of a mantle plume beneath this ridge, or the progressive, along-axis crustal thinning caused by a centered, localized magma supply zone. Both mantle Bouguer and residual anomalies show a distinct, local low to the west of the ridge south of the 33.5° S oblique offset and relatively high values at and to the east of this ridge segment. We interpret this pattern as an indication that the upwelling center in the mantle for this ridge is off-axis to the west of the ridge.     

台湾及其邻海的重力特征与构造、地震的关系

分析认为，在台湾及其邻海的重力场中，具有短波长特征的空间重力异常受地形与海深所制约．玉山的200×10     

Relation between Gravity Field Feature and Tectonics and Earthquakes in Taiwan and Its Adjacent Seas

Short wave gravity anomaly is correlated to sea floor topography in the gravity field of Taiwan and its adjacent seas. Gravity values of 200 × 10-5ms-2 at Yushang and -160 × 10-5ms-2 at Liuqiu sea trench are respectively the maximum and minimum gravity values in this area.Bouguer gravity anomaly reflects not only Moho interface undulation, but also fault distribution.The inflexion of gradient belt of Bouguer gravity anomaly is a spot liable to earthquakes. Middlelong wave geoid is the best data to invert crustal thickness. We calculate crustal thickness by using geoid data, and the maximum value is 38km; the minimum value is 12km in Taiwan and its adjacent seas.     

THE BOUGUER ANOMALIES AND DEPTHS OF MOHOROVICIC DISCONTINUITY IN THE SOUTH CHINA SEA REGION

The South China Sea is situated at the continsntal margin of South China. In this region, there are both continental and oceanic crusts. The values of Bouguer gravity anomalies on the continental shelf are low positive or low negative. Because the depth of the Mohorovicic discontinuity in this region is about 26-32 km below sea level, the crust belongs to the continental type. The values of Bouguer gravity anomalies in the deep-sea region are more than 250 mgal and the depth of the Moho-surface is about 10-15 km below sea level, so the crust is of oceanic type. The values of gravity anomalies and depths of the Moho-surface, obtained over the continental (and island) slope, range between those regions mentioned above, so the crust belongs to the transitional type. The continental crust is inferred to be directly in contact with the oceanic crust as a result of a lithospheric fault.