The Moho transition zone in the Oman ophiolite-relation with wehrlites in the crust and dunites in the mantle

Field data in the Oman ophiolite show that the Moho transition zone (MTZ), which is on average 300 m thick above mantle diapirs, reduces to 50 m away from diapirs, with a sharp transition at the outskirts of the diapirs. We show here that this reduction is dominantly due to compaction of a dunitic mush present above diapirs in the MTZ, with upward injection of a wehrlitic magma in the crust, and, to a lesser extent, due to tectonic stretching. In order to explain the fraction of wehrlites injected into the crust, which is in the range of 25%, it is necessary that mantle upwelling is active, with a mantle flow velocity away from diapirs several times faster than the spreading velocity. If this velocity exceeds 5 times the ridge spreading-rate, a significant part of the MTZ may be entrained down into the mantle, flowing away from the diapir as tabular dunites.     

Deformation mechanisms in a continental rift up to mantle exhumation. Field evidence from the western Betics,Spain

The identification of the structures and deformation patterns in magma-poor continental rifted margins is essential to characterize the processes of continental lithosphere necking. Brittle faults, often termed mantle detachments, are believed to play an essential role in the rifting processes that lead to mantle exhumation. However, ductile shear zones in the deep crust and mantle are rarely identified and their mechanical role remains to be established. The western Betics (Southern Spain) provide an exceptional exposure of a strongly thinned continental lithosphere, formed in a supra-subduction setting during Oligocene-Lower Miocene. A full section of the entire crust and the upper part of the mantle is investigated. Variations in crustal thickness are used to quantify crustal stretching that may reach values larger than 2000% where the ductile crust almost disappears, defining a stage of hyper-stretching. Opposite senses of shear top-to-W and top-to-E are observed in two extensional shear zones located close to the crust-mantle boundary and along the brittle-ductile transition in the crust, respectively. Where the ductile crust almost disappears, concordant top-to-E-NE senses of shear are observed in both upper crust and serpentinized mantle. Late high-angle normal faults with ages of ca. 21 Ma or older (⁴⁰Ar/³⁹Ar on white mica) crosscut the previously hyper-stretched domain, involving both crust and mantle in tilted blocks. The western Betics exemplify, probably better than any previous field example, the changes in deformation processes that accommodate the progressive necking of a continental lithosphere. Three successive steps can be identified: i/a mid-crustal shear zone and a crust-mantle shear zone, acting synchronously but with opposite senses of shear, accommodate ductile crust thinning and ascent of subcontinental mantle; ii/hyper-stretching localizes in the neck, leading to an almost disappearance of the ductile crust and bringing the upper crust in contact with the subcontinental mantle, each of them with their already acquired opposite senses of shear; and iii/high-angle normal faulting, cutting through the Moho, with related block tilting, ends the full exhumation of the mantle in the zone of localized stretching. The presence of a high strength sub-Moho mantle is responsible for the change in sense of shear with depth. Whereas mantle exhumation in the western Betics occurred in a backarc setting, this deformation pattern controlled by a high-strength layer at the top of the lithosphere mantle makes it directly comparable to most passive margins whose formation lead to mantle exhumation. This unique field analogue has therefore a strong potential for the seismic interpretation of the so-called “hyper-extended margins”.     

地幔柱与洋脊分段成因探讨

地幔柱产生岩浆的核幔交界部位相当于"岩浆洋",下地幔与上地幔、上地幔与软流圈之间的岩浆流通道相当于"岩浆河流",岩浆由软流圈经岩石圈喷出地表的通道相当于"岩浆小溪",在它们之间都会有许多的"岩浆支流",其中有的"支流"将这些"河流"与"河流"或"河流"与"小溪"联系起来.洋脊分段现象是由于地幔柱中存在横向的"岩浆支流"...     

Morphology and tectonics of the Australian-Antarctic Discordance between 123° E and 128° E

The Australian-Antarctic Discordance (AAD) is an anomalously deep and rugged zone of the Southeast Indian Ridge (SEIR) between 120° E and 128° E. The AAD contains the boundary between the Indian Ocean and Pacific Ocean isotopic provinces. We have analyzed SeaMarc II bathymetric and sidescan sonar data along the SEIR between 123° E and 128° E. The spreading center in the AAD, previously known to be divided into several transform-bounded sections, is further segmented by nontransform discontinuities which separate distinct spreading cells. Near the transform which bounds the AAD to the east, there is a marked change in the morphology of the spreading center, as well as in virtually every measured geochemical parameter. The spreading axis within the Discordance lies in a prominent rift valley similar to that observed along the Mid-Atlantic Ridge, although the full spreading rate within the AAD is somewhat faster than that of slow-spreading centers (~ 74 mm a^–1 vs. 0–40 mm a^–1). The AAD rift valleys show a marked contrast with the axial high that characterizes the SEIR east of the AAD. This change in axial morphology is coincident with a large (~ 1 km) deepening of the spreading axis. The segmentation characteristics of the AAD are analogous to those of the slow-spreading Mid-Atlantic Ridge, as opposed to the SEIR east of the AAD, which exhibits segmentation characteristics typical of fast-spreading centers. Thus, the spreading center within and east of the AAD contains much of the range of global variability in accretionary processes, yet it is a region free from spreading rate variations and the volumetric and chemical influences of hotspots. We suggest that the axial morphology and segmentation characteristics of the AAD spreading centers are the result of the presence of cooler than normal mantle. The presence of a cool mantle and the subsequent diminution of magma supply at a constant spreading rate may engender the creation of anomalously thick brittle lithosphere within the AAD, a condition which favor, the creation of an axial rift valley and of thin oceanic crust, in agreement with petrologic studies. The morphologies of transform and non-transform discontinuities within the Discordance also possess characteristics consistent with the creation of anomalously thick lithosphere in the region. The upper mantle viscosity structure which results from lower mantle temperatures and melt production rates may account for the similarity in segmentation characteristics between the AAD and slow-spreading centers. The section of the AAD which overlies the isotopic boundary is associated with chaotic seafloor which may be caused by an erratic pattern of magmatism and/or complex deformation associated with mantle convergence. Finally, the pattern of abyssal hill terrain within a portion of the AAD supports previous models for the formation of abyssal hills at intermediate- and slow-spreading ridges, and provides insights into how asymmetric spreading is achieved in this region.     

Oceanic ridge off-axis deep structure in the Mansah region (Sumail massif,Oman ophiolite)

Large scale structural mapping of the Oman ophiolite indicates that the Mansah area (Sumail massif) was a ridge off-axis region at the time the ophiolite was detached. This paper presents a detailed structural mapping of the region. We show that, as opposed to other off-axis areas, it contains plunging lineations, correlated with a thick Moho transition zone and chromite pods, indicative of a mantle diapir. However this diapir has a discontinuous structure, it is bounded by shear zones and types of diabases that are not found elsewhere in Oman; it also has a broken crust, strongly affected by hydrothermal alteration. This suggests that Mansah fossilized an off-axis diapir intruding a cooling lithosphere. It may be a good candidate to be the root of an off-axis seamount such as those found in the East Pacific, and may bring new insights into this particular volcanism which we are only beginning to explore.     

Inversion of teleseismic waves at Shidao Seismographic Station

1 IntroductionThe temporary Shidao seismographic station,the farthest one from China s Mainland (except Tai-wan Province) supported by a national fundamentalresearch project for the study of the evolution ofcontinental margin, is located at Shidao island(…     

石岛地震台远震记录反演研究

利用石岛地震台的远震体波记录,采用旋转相关函数法和接收函数法分别反演了台站下方介质的各向异性特征和速度结构.(1)对震中距25°~35°且记录良好的5次地震的ScS震相,采用旋转相关函数法反演了岩石圈的剪切波分裂参数.对深源地震的反演结果表明,石岛地震台快波偏振方向为N94°E,这意味着西沙附近处于近东西向微偏南的拉张或地壳下方的地幔流方向为近东西微偏南,西沙地区地壳是过渡性的,其底部的驱动力主要来自与欧亚板块运动一致的物质流.快慢波时间延迟为1.3 s,估算各向异性层厚度为100 km左右.(2)对震中距20°~60°的9次远震P波波形三分向记录,采用接收函数法反演了地壳和上地幔的S波速度结构.反演结果表明,石岛地震台下方地壳分为3层:约5 km以上有一速度梯度带,S波速度从1.5 km/s逐渐增加到3.5 km/s,其间有若干小的分层;在5~16 km的平均速度为3.8 km/s左右,其间有若干小的分层;在16.0~26.5 km的速度为3.6 km/s左右,这是一个明显的低速层;莫霍面埋深为26.5 km,莫霍面以下平均速度为4.7 km/s,也有若干小的分层,尤其是在莫霍面之下有一个明显的低速层.根据转换波到时分析和速度剖面左右摆动现象,认为反演结果中的小分层可能是不真实的,但在16.0~26.5 km的低速层的真实程度还是较高的,表明下地壳具有一定的塑性.     

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.     

A seismic refraction study of cretaceous oceanic lithosphere in the Northwest Pacific Basin

A seismic refraction study on old (110 Myr) lithosphere in the northwest Pacific Basin has placed constraints on crustal and uppermantle seismic structure of old oceanic lithosphere, and lithospheric aging processes. No significant lateral variation in structure other than azimuthally anisotropic mantle velocities was found, allowing the application of powerful amplitude modeling techniques. The anisotropy observed is in an opposite sense to that expected, suggesting the tectonic setting of the area may be more complex than originally thought. Upper crustal velocities are generally larger than for younger crust, supporting current theories of decreased porosity with crustal aging. However, there is no evidence for significant thickening of the oceanic crust with age, nor is there any evidence of a lower crustal layer of high or low velocity relative to the velocity of the rest of Layer 3. The compressional and shear wave velocities rule out a large component of serpentinization of mantle materials. The only evidence for a basal crustal layer of olivine gabbro cumulates is a 1.5 km thick Moho transition zone. In the slow direction of anisotropy, upper mantle velocities increase from 8.0 km s^-1 to 8.35 km s^-1 in the upper 15 km below the Moho. This increase is inconsistent with an homogeneous upper mantle and suggests that compositinal or phase changes occur near the Moho.     

Regional shape and tectonics of the equatorial East Pacific Rise

The geography of the East Pacific Rise (EPR) between 10°N and 6°S, redetermined by new surface ship surveys, is characterized by long spreading axes orthogonal to infrequent transform faults. Near 2°10N the EPR is intersected by the Cocos-Nazca spreading center at the Galapagos triple junction. The present pattern was established 27-5.5 m.y.b.p. by a complex sequence of rise-crest jumps and reorientations from a section of the Pacific-Farallon plate boundary. Transverse profiles of the rise flanks can be matched by thermal contraction curves for aging lithosphere, except between the triple junction and 4°S, where the east flank is anomalously shallow and almost horizontal. Most sections of spreading axis have the 10–30 km wide, 100–400 m high, axial ridge that is characteristic of fast spreading centers. However, within 60 km of the triple junction the rise crest structure is atypical, with an axial rift valley and elevated rift mountains, despite a spreading rate of 140 mm/yr. With the exception of this atypical section, the bathymetric profile along the spreading axis is remarkably even, with continuous, gentle slopes for hundreds of kilometers between major transform faults, where step-like offsets in axial depths occur. Most of the observations can be accommodated by a model in which the long spreading axes are underlain by continuous crustal magma chambers that allow easy longitudinal flow of magma, and whose size controls the style and dimensions of EPR crestal topography.Contribution of the Scripps Institution of Oceanography, new series.     

南海区域岩石圈的壳-幔耦合关系和纵向演化

南海区域岩石圈由地壳层和上地幔固结层两部分组成。具典型大洋型地壳结构的南海海盆区莫霍面深度为９～１３ｋｍ,并向四周经陆坡、陆架至陆区逐渐加深;陆缘区莫霍面一般为１５～２８ｋｍ,局部区段深达３０～３２ｋｍ,总体呈与水深变化反相关的梯度带;东南沿海莫霍面深约２８～３０ｋｍ,往西北方向逐渐增厚,最大逾３６ｋｍ。南海区域上地幔天然地震面波速度结构明显存在横向分块和纵向分层特征。岩石圈底界深度变化与地幔速度变化正相关;地幔岩石圈厚度与地壳厚度呈互补性变化,莫霍面和岩石圈底界呈立交桥式结构,具有陆区厚壳薄幔—洋区薄壳厚幔的岩石圈壳－幔耦合模式。南海区域白垩纪末以来的岩石圈演化主要表现为陆缘裂离—海底扩张—区域沉降的过程,现存的壳－幔耦合模式显然为岩石圈纵向演化产物,其过程大致可分为白垩纪末至中始新世的陆缘裂离、中始新世晚期至中新世早期的海底扩张和中新世晚期以来的区域沉降等三个阶段。     

Mantle flow patterns and magma chambers at ocean ridges: Evidence from the Oman ophiolite

As a result of an extensive program of structural mapping in the ultramafic section of the Oman ophiolite, maps of mantle flow below the spreading center of origin have been drawn. They reveal a mantle diapiric system in which the uppermost mantle flow diverges from diapirs 10–15 km across, which could have been spaced by an average distance of 50 km. Some diapirs could have been located off-axis. The rotation of flow lines in the diapirs occurs within the few hundred meters of the transition zone separating the mantle and crustal formations. The importance of this zone is stressed. The structure of the layered gabbros of the crustal unit in most places reflects a large magmatic flow induced by the solid state flow in the underlying peridotites. The magmatic foliation of the gabbros steepens upsection and becomes parallel to the sheeted dike attitude. A new model of a tent-shaped magma chamber is derived from these structural data.     

Detailed study of the Brunhes/Matuyama reversal boundary on the east Pacific rise at 19°30′ S: Implications for crustal emplacement processes at an ultra fast spreading center

We have conducted the first detailed survey of the recording of a geomagnetic reversal at an ultra-fast spreading center. The survey straddles the Brunhes/Matuyama reversal boundary at 19°30 S on the east flank of the East Pacific Rise (EPR), which spreads at the half rate of 82 mm yr^-1. In the vicinity of the reversal boundary, we performed a three-dimensional inversion of the surface magnetic field and two-dimensional inversions of several near-bottom profiles including the effects of bathymetry. The surface inversion solution shows that the polarity transition is sharp and linear, and less than 3–4 km wide. These values constitute an upper bound because the interpretation of marine magnetic anomalies observed at the sea surface is limited to wavelengths greater than 3–4 km. The polarity transition width, which represents the distance over which 90% of the change in polarity occurs, is narrow (1.5–2.1 km) as measured on individual 2-D inversion profiles of near-bottom data. This suggests a crustal zone of accretion only 3.0–4.2 km wide. Our method offers little control on accretionary processes below layer 2B because the pillow and the dike layers in young oceanic crust are by far the most significant contributors to the generation of marine magnetic anomalies. The Deep-Tow instrument package was used to determine in situ the polarity of individual volcanoes and fault scarps in the same area. We were able to make 96 in situ polarity determinations which allowed us to locate the scafloor transition boundary which separates positively and negatively magnetized lava flows. The shift between the inversion transition boundary and the seafloor transition boundary can be used to obtain an estimate of the width of the neovolcanic zone of 4–10 km. This width is significantly larger than the present width of the neovolcanic zone at 19°30 S as documented from near-bottom bathymetric and photographic data (Bicknell et al., 1987), and also larger than the width of the neovolcanic zone at 21° N on the EPR as inferred by the three-dimensional inversion of near-bottom magnetic data (Macdonald et al., 1983). The eruption of positively magnetized lava flows over negatively magnetized crust from the numerous volcanoes present in the survey area and episodic flooding of the flanks of the ridge axis by extensive outpourings of lava erupting from a particularly robust magma chamber may result in a widened neovolcanic zone. We studied the relationship between spreading rate and polarity transition widths obtained from 2-D inversions of the near-bottom magnetic field over various spreading centers. The mean transition width corrected for the time necessary for the reversal to occur decreases with increasing spreading rate but our data set is still too sparse to draw firm conclusions from these observations. Perhaps more interesting is the fact that the range of the measured transition widths also decreases with spreading rate. In the light of these results, we propose a new model for the spreading rate dependency of polarity transition widths. At slow spreading centers, the zone of dike injection is narrow but the locus of crustal accretion is prone to small lateral shifts depending on the availability of magmatic sources, and the resulting polarity transition widths can be narrow or wide. At intermediate spreading centers, the zone of crustal accretion is narrow and does not shift laterally, which leads to narrower transition widths on the average than at slow spreading centers. An intermediate, or even a slow spreading center, may behave like a fast or hot-spot dominated ridge for short periods of time when its magmatic budget is increased due to melting events in the upper mantle. At fast spreading centers, the zone of dike injection is narrow, but the large magmatic budget of fast spreading centers may result in occasional extensive flows less than a few tens of meters thick from the axis and off-axis volcanic cones. These thin flows will not significantly contribute to the polarity transition widths, which remain narrow, but they may greatly increase the width of the neovolcanic zone. Finally the gabbro layer in the lower section of oceanic crust may also contribute to the observed polarity transition widths but this contribution will only become significant in older oceanic crust (50–100 m.y.).     

冲绳海槽中段地球物理场及对其新生洋壳的认识

通过对冲绳海槽中的磁场进行分析，在海槽轴部追踪到了线性磁条带异常。另外，在扩张轴附近也拖到了新鲜拉斑玄武岩。重力自由空气异常值较低。根据这是由于深部地幔大幅上升所致。地震剖面及磁力资料显示在海槽轴部有强磁性浅层侵入体及海底山，这些侵入体很可能来自于深部地幔。高温地幔物质上涌在海槽轴部形成高热流、强磁异常、多火山以及热液活动。上述现象说明冲绳海槽中段张裂轴部大陆岩石圈已经破裂，并可能已有新生洋壳形成。     

Variations in Moho and Curie depths and heat flow in Eastern and Southeastern Asia

The Eastern and Southeastern Asian regions witness the strongest land–ocean and lithosphere–asthenosphere interactions. The extreme diversity of geological features warrants a unified study for a better understanding of their geodynamic uniqueness and/or ubiquity from a regional perspective. In this paper we have explored a large coverage of potential field data and have detected high resolution Moho and Curie depths in the aforementioned regions. The oldest continental and oceanic domains, i.e. the North China craton and the Pacific and Indian Ocean have been found thermally perturbed by events probably linked to small-scale convection or serpentinization in the mantle and to numerous volcanic seamounts and ridges. The thermal perturbation has also been observed in proximity of the fossil ridge of the western Philippine Sea Basin, which shows anomalously small Curie depths. The western Pacific marginal seas have the lowest Moho temperature, with Curie depths generally larger than Moho depths. The contrary is true in most parts of easternmost Eurasian continent. Magmatic processes feeding the Permian Emeishan large igneous province could have also been genetically linked to deep mantle/crustal processes beneath the Sichuan Basin. The regionally elongated magnetic features and small Curie depths along the Triassic Yangtze-Indochina plate boundary suggest that the igneous province could be caused by tectonic processes along plate margins, rather than by a deep mantle plume. At the same time, we interpret the Caroline Ridge, the boundary between the Pacific and the Caroline Sea, as a structure having a continental origin, rather than as hotspot or arc volcanism. The surface heat flow is primarily modulated by a deep isotherm through thermal conduction. This concordance is emphasized along many subduction trenches, where zones of large Curie depths often correspond with low heat flow. Local or regional surface heat flow variations cannot be faithfully used in inferring deep thermal structures, which can be better constrained overall through Curie depths detected from surface magnetic anomalies.     

冲绳海槽海底地形的补偿模式初步研究

从区域补偿模式和实验均衡理论出发,利用重力和地形资料计算了冲绳海槽的均衡响应函数,结果表明：冲绳海槽南段弹性板有效厚度和补偿深度明显大于中段,结合其它地质地球物理资料解释认为,产生这种差异的原因主要是南,中两段岩石圈温度和补偿机制的不同所致。     

琼东南盆地深水区东区凹陷带结构构造及其演化特征

琼东南盆地深水区东区凹陷带,即松南—宝岛—长昌凹陷,位于琼东南盆地中央坳陷东端。在大量地震资料解释的基础上,对38条主要断层进行了详细分析。获得以下认识:(1)琼东南盆地深水区东区凹陷带平面上表现为近EW向展布的平行四边形,剖面结构表现为自西向东由半地堑—不对称的地堑—半地堑有规律变化。(2)琼东南盆地深水区东区凹陷带断裂系统可划分控制凹陷边界断层、控制洼陷沉积中心断层和调节性断层3类。(3)琼东南盆地深水区东区凹陷带古近纪时期受到太平洋板块俯冲和南海海盆扩张的双重影响,构造应力场发生NW—SE→SN转变。构造演化可划分为3个阶段:～32Ma,应力场以区域性NW—SE向伸展为主,断裂系统以NE—SW向为主,控制凹陷边界;32～26Ma,以南海海盆近SN向拉张应力场为主,断裂系统以NWW—SEE向为主,断层活动控制凹陷沉积中心;26～Ma,区域性伸展与南海海盆扩张应力均逐渐减弱,NE—SW向和NWW—SEE向断裂继承性发育。(4)琼东南盆地深水区东区凹陷带内部主要断层在渐新统崖城组和陵水组沉积时期活动速率快,地形高差大、沉积水体深、沉积厚度大,控制了崖城组和陵水组的大规模沉积,有利于烃源岩的发育。圈闭以受断层控制的断鼻和断块为主,长昌主洼凹中隆起带发育2个最为理想的构造圈闭。     

Anomalous travel times of teleseismic P-waves reflected under Arctic marine areas

The travel-time residuals of teleseismic PP waves can indicate velocity anomalies near the reflection points. Deep structure can thus be inferred in areas which are not well observed by conventional techniques. In this paper, published data are used to examine parts of the Arctic Ocean, the adjacent shelves and Greenland.Late reflections occur under part of the Alpha Ridge, and the Lomonosov Ridge near the North Pole, and early reflections tend to occur under much of the Nansen spreading ridge. It is suggested that a hotter than normal region underlies a section of the Lomonosov Ridge, while extensive mantle upwelling does not appear to occur directly under the Nansen Ridge, which may be a passive spreading center. A zone of very late reflections occupies much of the East Siberian Shelf. This may be due to intrusion of the asthenosphere into lithospheric sutures or thickening of the asthenosphere. travel-time delays under the rest of the Eurasian shelf are consistent with the known structure of the area. Late reflections in east Greenland and Baffin Bay could be due to Tertiary hot spot activity in these locations.     

南海海盆及其周缘岩浆岩地球化学特征对岩浆过程及地幔结构的指示意义

本文通过对昆嵩高原,三水盆地以及玳瑁海山岩浆岩的地球化学研究来揭示南海海盆以及周边地区的岩浆过程以及地幔源区的性质。昆嵩,三水及玳瑁海山玄武质岩浆的地球化学特征均反应这些岩浆并非来源于一个起源于核幔边界的深部地幔柱,而是来源于较浅的岩浆源区,例如岩石圈下地幔或者软流圈地幔。此外,南海海盆岩浆岩的地幔源区较昆嵩高原和三水盆地岩浆岩的地幔源区更加亏损并且表现出较低的地幔温度。这些差异可能表明三水盆地内的岩浆活动被南海的开张所抑制,而之后又在昆嵩高原复苏。研究区的地幔表现出不均一性,其中的富集地幔端元表现出EM2的特征。这个EM2富集端员是由于伴随着大陆沉积物的中生代古南海板块俯冲到研究区地幔中所形成的。     

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.