Tomographic structure of East Asia:Ⅱ.Stagnant slab above 660 km discontinuity and its geodynamic implications

P-wave arrival times of both regional and teleseismic earthquakes were inverted to obtain mantle structures of East Asia.No fast(slab) velocity anomalies was not find beneath the 660-km discontinuity through tomography besides a stagnant slab within the transition zone.Slow P-wave velocity anomalies are present at depths of 100-250 km below the active volcanic arc and East Asia.The western end of the flat stagnant slab is about 1 500 km west to active trench and may also be correlated with prominent surface topographic break in eastern China.We suggested that active mantle convection might be operating within this horizontally expanded "mantle wedge" above both the active subducting slabs and the stagnant flat slabs beneath much of the North China plain.Both the widespread Cenozoic volcanism and associated extensional basins in East Asia could be the manifestation of this vigorous upper mantle convection.Cold or thermal anomalies associated with the stagnant slabs above the 660-km discontinuity have not only caused a broad depression of the boundary due to its negative Clapeyron slope but also effectively shielded the asthenosphere and continental lithosphere above from any possible influence of mantle plumes in the lower mantle.     

华北克拉通东北部及邻区地壳和地幔转换带厚度研究

本文利用宽频流动台阵记录的远震波形资料和接收函数波动方程叠后偏移方法,获得了华北克拉通东北部边界及其邻近地区的地壳和地幔转换带的间断面结构图像.结果显示研究区域的地壳厚度存在显著的横向变化:以南北重力梯度带为界,西北部的兴蒙造山带地壳较厚(~40 km),东南部的燕山带、松辽盆地和辽东台隆地壳明显较薄(30~35 km).这有可能反映,研究区南北重力梯度带两侧地壳在中-新生代区域构造伸展过程中经历了不同程度的改造和减薄.地幔转换带成像结果显示,研究区410 km和660 km间断面结构存在横向差异.经度121°E-122°E之间,上地幔底部出现双重间断面,深度分别为660 km和690 km.经度122.5°E以东(北黄海地区),410 km间断面有5~20 km幅度的下沉,660 km间断面有5~15 km幅度的抬升;该地区地幔转换带厚度相对全球平均偏薄10~20 km,指示着该地区较热的上地幔底部温度环境.我们认为太平洋俯冲板块可能停滞在研究区119°E-122°E经度范围的地幔转换带中,但未延伸至118°E以西;而俯冲板块在124°E以东可能局部穿透了上地幔底部而进入下地幔,同时引起小尺度的地幔对流,导致北黄海地区下地幔物质的上涌.     

Dynamics of thinning and destruction of the continental cratonic lithosphere: Numerical modeling

Thinning of the cratonic lithosphere is common in nature, but its destruction is not. In either case, the mechanisms for both thinning and destruction are still widely under debate. In this study, we have made a review on the processes and mechanisms of thinning and destruction of cratonic lithosphere according to previous studies of geological/geophysical observations and numerical simulations, with specific application to the North China Craton (NCC). Two main models are suggested for the thinning and destruction of the NCC, both of which are related to subduction of the oceanic lithosphere. One is the “bottom-up” model, in which the deeply subducting slab perturbs and induces upwelling from the hydrous mantle transition zone (MTZ). The upwelling produces mantle convection and erodes the bottom of the overriding lithosphere by the fluid-melt-peridotite reaction. Mineral compositions and rheological properties of the overriding lithospheric mantle are changed, allowing downward dripping of lithospheric components into the asthenosphere. Consequently, lithospheric thinning or even destruction occurs. The other is the “top-down” model, characterized by the flat subduction of oceanic slab beneath the overriding cratonic lithosphere. Dehydration reactions from the subducting slab would significantly hydrate the lithospheric mantle and decrease its rheological strength. Then the subduction angle may be changed from shallow to steep, inducing lateral upwelling of the asthenosphere. This upwelling would heat and weaken the overriding lithospheric mantle, which led to the weakened lithospheric mantle dripping into the asthenosphere. These two models have some similarities, in that both take the subducting oceanic slab and relevant fluid migration as the major driving mechanism for thinning or destruction of the overriding cratonic lithosphere. The key difference between the two models is the effective depth of the subducting oceanic slab. One is stagnation and flattening in the MTZ, whereas the other is flat subduction at the bottom of the cratonic lithosphere. In the NCC, the eastern lithosphere was likely affected by subduction of the Izanagi slab during the Mesozoic, which would have perturbed the asthenosphere and the MTZ, and induced fluid migration beneath the NCC lithosphere. The upwelling fluid may largely have controlled the reworking of the NCC lithosphere. In order to discuss and analyze these two models further, it is crucial to understand the role of fluids in the subduction zone and the MTZ. Here, we systematically discuss phase transformations of hydrous minerals and the transport processes of water in the subduction system. Furthermore, we analyze possible modes of fluid activity and the problems to explore the applied feasibility of each model. In order to achieve a comprehensive understanding of the mechanisms for thinning and destruction of cratonic lithosphere, we also consider four additional possible dynamic models: extension-induced lithospheric thinning, compression-induced lithospheric thickening and delamination, large-scale mantle convection and thermal erosion, and mantle plume erosion. Compared to the subduction-related models presented here, these four models are primarily controlled by the relatively simple and single process and mechanism (extension, compression, convection, and mantle plume, respectively), which could be the secondary driving mechanisms for the thinning and destruction of lithosphere.     

The existence of a thin low-viscosity layer beneath the lithosphere

The horizontal temperature gradient at the base of the lithosphere at an oceanic fracture zone, where plate of different ages is juxtaposed, is expected to drive a local circulation, the characteristics of which can be constrained by the amplitude, wavelength and age-dependence of the geoid. Two-dimensional numerical models of convection in a fluid layer overlain by a solid conducting lid have been used to generate theoretical geoid profiles at right angles to the fracture zone. Only a thin, low-viscosity layer provides a reasonable fit to the data. The best model so far obtained has a fluid layer 150 km thick with viscosity 1.5 × 10¹⁹ Pa s under a 75 km lid. Such a layer, which is incapable of transmitting strong horizontal shear stresses, could provide the decoupling mechanism between plate and deep mantle flow required to balance the forces on the plates.     

Crust and upper mantle structures beneath Northeast China from receiver function studies

P-wave and S-wave receiver function analyses have been performed along a profile consisted of 27 broadband seismic stations to image the crustal and upper mantle discontinuities across Northeast China. The results show that the average Moho depth varies from about 37 km beneath the Daxing’anling orogenic belt in the west to about 33 km beneath the Songliao Basin, and to about 35 km beneath the Changbai mountain region in the east. Our results reveal that the Moho is generally flat beneath the Daxing’anling region and a remarkable Moho offset (about 4 km) exists beneath the basin-mountain boundary, the Daxing’anling-Taihang Gravity Line. Beneath the Tanlu faults zone, which seperates the Songliao Basin and Changbai region, the Moho is uplift and the crustal thickness changes rapidly. We interpret this feature as that the Tanlu faults might deeply penetrate into the upper mantle, and facilitate the mantle upwelling along the faults during the Cenozoic era. The average depth of the lithosphere-asthenosphere boundary (LAB) is ~80 km along the profile which is thinner than an average thickness of a continental lithosphere. The LAB shows an arc-like shape in the basin, with the shallowest part approximately beneath the center of the basin. The uplift LAB beneath the basin might be related to the extensive lithospheric stretching in the Mesozoic. In the mantle transition zone, a structurally complicated 660 km discontinuity with a maximum 35 km depression beneath the Changbai region is observed. The 35 km depression is roughly coincident with the location of the stagnant western pacific slab on top of the 660 km discontinuity revealed by the recent P wave tomography.     

Electromagnetic Studies Of The Lithosphere And Asthenosphere

In geodynamic models of the Earth's interior, the lithosphere and asthenosphere are defined in terms of their rheology. Lithosphere has high viscosity, and can be divided into an elastic region at temperatures below 350 °C and an anelastic region above 650 °C. Beneath the lithosphere lies the ductile asthenosphere, with one- to two-orders of magnitude lower viscosity. Asthenosphere represents the location in the mantle where the melting point (solidus) is most closely approached, and sometimes intersected. Seismic, gravity and isostatic observations provide constraints on lithosphere-asthenosphere structure in terms of shear-rigidity, density and viscosity, which are all rheological properties. In particular, seismic shear- and surface-wave analyses produce estimates of a low-velocity zone (LVZ) asthenosphere at depths comparable to the predicted rheological transitions. Heat flow measurements on the ocean floor also provide a measure of the thermal structure of the lithosphere.Electromagnetic (EM) observations provide complementary information on lithosphere-asthenosphere structure in terms of electrical conductivity. Laboratory studies of mantle minerals show that EM observations are very sensitive to the presence of melt or volatiles. A high conductivity zone (HCZ) in the upper mantle therefore represents an electrical asthenosphere (containing melt and/or volatile) that may be distinct from a rheological asthenosphere and the LVZ. Additionally, the vector propagation of EM fields in the Earth provides information on anisotropic conduction in the lithosphere and asthenosphere. In the last decade, numerous EM studies have focussed on the delineation of an HCZ in the upper mantle, and the determination of melt/volatile fractions and the dynamics of the lithosphere-asthenosphere. Such HCZs have been imaged under a variety of tectonic zones, including mid-ocean ridges and continental rifts, but Archaean shields show little evidence of an HCZ, implying that the geotherm is always below the mantle solidus. Anisotropy in the conductivity of oceanic and continental lithosphere has also been detected, but it is not clear if the HCZ is also anisotropic. Although much progress has been made, these results have raised new and interesting questions of asthenosphere melt/volatiles porosity and permeability, and lithosphere-upper mantle heterogeneity. It is likely that in the next decade EM will continue to make a significant contribution to our understanding of plate tectonic processes.     

Receiver functions in northeast China - implications for slab penetration into the lower mantle in northwest Pacific subduction zone

Seismic studies of the subducting lithosphere and the upper mantle discontinuities in the northwest Pacific subduction zone beneath Japan and northeast China have suggested contrary subduction scenarios. There was little consensus on the issue whether the subducting slab penetrates the upper mantle discontinuities into the lower mantle or it is deflected atop of the 660-km discontinuity over several hundred kilometers. We calculate receiver functions from a recent seismic broadband station network located in northeast China and find topographic variations of the upper mantle discontinuities. A deeper-than-normal 660-km discontinuity is observed over an area of 400 km and it coincides with the stagnant slab imaged by seismic tomography. The 660-km discontinuity is locally depressed by more than 35 km and the transition zone is thickened by more than 20 km in the east of the region where it encounters the slab. These observations provide evidence of the slab accumulating in the mantle transition zone and locally penetrating into the lower mantle.     

基于三重震相方法探测日本海俯冲区地幔转换带的速度结构

本文基于中国数字地震台网记录的发生于日本北海道地区的一次中源地震的三重震相资料研究了日本海俯冲区地幔转换带的速度结构．结果表明,该区域P波速度结构与S波速度结构的一致性整体上较强．冷的西太平洋俯冲板块导致410 km间断面出现了10 km的抬升,660 km间断面出现了25 km的下沉;410 km和660 km间断面之上均存在与俯冲板块相关的高速层;660 km间断面下方存在厚度为65 km的低速异常．纵横波波速比vP/vS值在210—400 km深度范围内偏低,约为1.827,体现出海洋板块低泊松比的特征;在560—685 km深度范围内,该值偏高,约为1.831,可能预示地幔转换带底部含有一定量的水．      

呼包盆地周缘壳、幔结构研究

对呼包盆地周边七个台站的远震接收函数研究表明：研究区地壳厚度为43～46 km,地壳速度比和S波速度结构均无异常;阴山造山带岩石圈埋深为65～85 km,且上地幔S波平均速度偏低,是典型的异常地幔区.根据S波速度结构和地壳厚度随地形高程反比的变化关系,以及地表广泛出露的幔源玄武岩分布,推测该区地幔深部热物质上涌是阴山造山带隆升的原因之一.上升的软流层物质与地幔发生交代、侵蚀作用导致岩石圈减薄,S波速度降低.呼包盆地的形成可能与深部物质上涌造成的拉张效应有关.     

The structure of thermal convection and geotherms beneath a continent and ocean

The formation of the thermal cross section of the lithosphere and mantle upon the interaction between the mantle convection and the immobile continent surrounded by the oceanic lithosphere is studied by numerical modeling. The convective temperature and velocity fields and then the averaged geotherms for subcontinental and suboceanic regions up to the boundary with the core are calculated from the solution of convection equations with a jump in viscosity in the continental zone. Using the experimental data on the solidus temperature in the rocks of the upper mantle, the average thickness of the continental and oceanic lithosphere is estimated at 190 and 30 km, respectively. The effect of a hot spot formed in the subcontinental upper mantle at a depth of 250–500 km, which has not been previously noted, is revealed. Although the temperature in this zone is typically assumed to be close to adiabatic, the calculations show that it is actually higher than adiabatic by up to 200°C. The physical mechanism responsible for this effect is associated with the accumulation of convective heat beneath the thermally insulating layer of the continental lithosphere. The revealed anomalies can be important in studying the phase and mineral transformations at the base of the lithosphere and in the regional geodynamical reconstructions.     

Upper mantle structure in the ocean-continent transition zone: Kamchatka

We consider results from modeling the crustal and upper mantle velocity structure in Kamchatka by seismic tomography and compare these with gravity data and present-day tectonics. We found a well-pronounced (in the physical fields) vertical and lateral variation for the upper mantle and found that it is controlled by fault tectonics. Not only are individual lithosphere blocks moving along faults, but also parts of the Benioff zone. The East Kamchatka volcanic belt (EKVB) is confined to the asthenospheric layer (the asthenosphere lens) at a depth of 70–80 km; this lens is 10–20 km thick and seismic velocity in it is lower by 2–4%. The top of the asthenosphere lens has the shape of a dome uplift beneath the Klyuchevskoi group of volcanoes and its thickness is appreciably greater; overall, the upper mantle in this region is appreciably stratified. A low-velocity heterogeneity (asthenolith) at least 100 km thick has been identified beneath the Central Kamchatka depression; we have determined its extent in the upper mantle and how it is related to the EKVB heterogeneities. Gravity data suggest the development of a rift structure under the Sredinnyi Range volcanic belt. The Benioff zone was found to exhibit velocity inhomogeneity; the anomalous zones that have been identified within it are related to asthenosphere inhomogeneities in the continental and oceanic blocks of the mantle.     

欧亚地区均衡残差大地水准面和上地幔强度

首先计算了欧亚地区均衡残差大地水准面．基于地幔热对流的内负荷理论和最新全球层析成像结果，探讨了欧亚地区中波长均衡残差大地水准面的地球动力学意义．研究结果表明，中波长均衡残差大地水准面主要受上地幔粘滞度和岩石层强度的影响，进而得出欧亚地区一些古老地盾和构造稳定地区的上地幔与年轻山脉及构造活动地区的上地幔结构存在着差异．这个差异主要是占老地盾和构造稳定地区，如波罗的海地盾、中西伯利亚地台、东欧等区域，冷却的上地幔已穿透地幔较深，上地幔与岩石层之间耦合较好;而年轻山脉和构造活动区，如帕米尔、天山、贝加尔活动带、青藏高原、日本海周围地区，在上地幔可能存在着热物质即粘滞度很低的软流层，上地幔与岩石层耦合程度较差，甚至有可能解耦．从欧亚地区上地幔属性的差异，可以解释该地区的一些地球动力学问题．     

基于P波三重震相研究青藏高原上地幔速度结构及其动力学意义

青藏高原因其复杂的结构和演化历史,一直都是研究大陆碰撞、构造运动及其动力学的热点区域。本文采用三重震相波形拟合技术,基于中国地震观测台网和大型流动台阵记录到的某地震P波垂向记录,获得了包括拉萨、南羌塘和松潘甘孜地块在内的青藏高原上地幔P波速度结构。结果表明:①拉萨和南羌塘地块下方地幔过渡带存在高速异常,推测是俯冲的印度板片滞留体,过渡带底部的板片残余温度较低,使得660-km相变滞后约3～8km。而松潘甘孜地块下方过渡带同样存在高速异常,可能是欧亚岩石圈发生拆沉进入地幔过渡带所致。这说明印度板块俯冲作用的影响已经到达地幔过渡带,其俯冲前缘位于班公怒江缝合带附近。②从拉萨、南羌塘到松潘甘孜地块,200km之上的地幔岩石圈高速盖层速度由南向北逐渐减小,松潘甘孜地块则出现盖层缺失。推测受小规模地幔对流或者热不稳定性的影响,在南羌塘和松潘甘孜地块,增厚的欧亚岩石圈发生拆沉作用,岩石圈被减薄和弱化,造成羌塘地块上地幔低速和松潘甘孜地块上地幔高速盖层缺失。拆沉的冷的欧亚岩石圈可能部分停留在410-km上方,使得410-km抬升约10km,部分沉入地幔过渡带,表现为松潘甘孜地块地幔过渡带中存在高速异常。低温造成660-km下沉约8km,导致地幔过渡带增厚。      

华南地区上地幔P波三维速度结构和动力学意义:来自有限频层析成像的证据

利用155个宽频带流动地震仪记录的连续地震波形数据,通过有限频层析成像技术,反演获得了华南地区上地幔的高分辨率P波三维速度模型.结果显示,大致以江南造山带为界,研究区域南部的华夏块体的大部分区域上地幔存在一个清晰的低速异常构造,而研究区北部的扬子克拉通的大部分区域上地幔则存在高速异常结构,并且这些速度异常体都向下延伸到地幔转换带.一个重要的结果是在(27°N,118°E)处观测到通过410-km界面的上涌流,并且在上升的过程中逐渐向西和向北扩展,显示为华夏块体深部200～400 km深度的大范围低速异常,可能为华夏块体广泛分布的新生代岩浆活动提供深部来源.更重要的是华夏块体通过410-km界面上涌流在上涌的过程中向北延伸,越过江南造山带"侵入"到扬子克拉通的南部地区,造成了扬子克拉通较厚的岩石圈对应的高速异常体向南倾斜的假象.最后,位于117°E(郯庐断裂的南端)以东的扬子克拉通岩石圈已经被"活化",即被来自南部热的软流圈物质替而代之.同时,推断在华夏块体下方地幔转换带内低速异常体可能是与海南地幔柱有关.海南地幔柱和(27°N,118°E)410-km界面上涌流的关系还有待于今后更大范围地震台阵反演研究进行验证.     

One-Dimensional Thermal Modeling of the Eastern Pontides Orogenic Belt (NE Turkey)

The eastern Pontides orogenic belt is one of the most complex geodynamic settings in the Alpine–Himalayan belt due to the lack of systematical geological and geophysical data. In this study, 1D crustal structure and P-wave velocity distribution obtained from gravity modeling and seismological data in the area has been used for the development of the thermal model of the eastern Pontides orogenic belt. The computed temperature-depth profiles suggest a temperature of 590?±?60°C at a Moho depth of 35?km indicates the presence of a brittle-ductile transition zone. This temperature value might be related to water in the subducted crust of the Tethys oceanic lithosphere. The Curie temperature depth value of 29?km, which may correspond to the crustal magma chambers, is found 5–7?km below the Moho depth. Surface heat flow density values vary from 66.5 and 104.7?mW?m^?2. High mantle heat flow density value of 48?mW?m^?2 is obtained for the area should be related to melting of the lithospheric mantle caused by upwelling of asthenosphere.     

Stability of the boundary layer between the lithosphere and convecting mantle and the steady-state lithospheric geotherm

In the steady state, the convective boundary layer (CBL) (the transition from the lithosphere to the convecting mantle, the lithosphere-asthenosphere boundary) is on the verge of stability. This determines its depth, thickness, and the steady-state temperature distribution in the lithosphere. Had the mantle been homogeneous, the base of the lithosphere at the current potential temperature would lie globally at the same depth H _rh of 50 to 70 km. Actually, the regime of interaction of the mantle convection with the lithosphere is determined by the relationship between this depth and the thickness H _depl of the chemical boundary layer including the crust and the layer of the depleted rock. If the thickness of the chemical boundary layer is small H _depl < H _rh, as it is the case in the present-day oceanic mantle, the suboceanic regime is established with the mantle convection that does not reach the base of the chemical boundary layer. In this case, the top of CBL is located at depth H _rh, while the oceanic heat flow and the depth of the seafloor only depend on the potential temperature T _p and, within the areas where the crust is older than 60 to 70 Ma, are the same everywhere far from the disturbed territories (the hot points and the subduction zones). The absence of noticeable distinctions between the heat flow in the different oceanic basins suggests a global constancy of the potential temperature. If H _depl > H _rh, the subcontinental regime of the interaction of the mantle convection with the lithosphere is established. In this case, the CBL is immediately adjacent to the depleted lithosphere, its top is located at depth H _depl, and the surface heat flow q(T _p, H _depl) not only depends on the potential temperature T _p but also on the the thickness of the depleted lithosphere H _depl; it decreases with increasing H _depl and, therefore, with the age of the lithosphere. Given the potential temperature, the dependence q(T _p, H _depl) agrees well with the envelope of the results of kimberlite xenolith thermobarometry presented in the diagram of the deepest xenolith depth as a function of the heat flow. It is likely that in the lowest part of the continental lithosphere there is a zone of horizontal shear deformation, from where kimberlites entrain the strongly deformed and, at the same time, the deepest xenoliths. Besides, the azimuthal anisotropy of seismic velocities can be associated with this zone. The change in its direction with depth can be observed as the Lehmann discontinuity.     

The mantle convection model with non-Newtonian rheology and phase transitions: The flow structure and stress fields

The mantle convection model with phase transitions, non-Newtonian viscosity, and internal heat sources is calculated for two-dimensional (2D) Cartesian geometry. The temperature dependence of viscosity is described by the Arrhenius law with a viscosity step of 50 at the boundary between the upper and lower mantle. The viscosity in the model ranges within 4.5 orders of magnitude. The use of the non-Newtonian rheology enabled us to model the processes of softening in the zone of bending and subduction of the oceanic plates. The yield stress in the model is assumed to be 50 MPa. Based on the obtained model, the structure of the mantle flows and the spatial fields of the stresses σ_xz and σ_xx in the Earth’s mantle are studied. The model demonstrates a stepwise migration of the subduction zones and reveals the sharp changes in the stress fields depending on the stage of the slab detachment. In contrast to the previous model (Bobrov and Baranov, 2014), the self-consistent appearance of the rigid moving lithospheric plates on the surface is observed. Here, the intense flows in the upper mantle cause the drift and bending of the top segments of the slabs and the displacement of the plumes. It is established that when the upwelling plume intersects the boundary between the lower and upper mantle, it assumes a characteristic two-level structure: in the upper mantle, the ascending jet of the mantle material gets thinner, whereas its velocity increases. This effect is caused by the jump in the viscosity at the boundary and is enhanced by the effect of the endothermic phase boundary which impedes the penetration of the plume material from the lower mantle to the upper mantle. The values and distribution of the shear stresses σ_xz and superlithostatic horizontal stresses σ_xx are calculated. In the model area of the subducting slabs the stresses are 60–80 MPa, which is by about an order of magnitude higher than in the other mantle regions. The character of the stress fields in the transition region of the phase boundaries and viscosity step by the plumes and slabs is analyzed. It is established that the viscosity step and endothermic phase boundary at a depth of 660 km induce heterogeneities in the stress fields at the upper/lower mantle boundary. With the assumed model parameters, the exothermic phase transition at 410 km barely affects the stress fields. The slab regions manifest themselves in the stress fields much stronger than the plume regions. This numerically demonstrates that it is the slabs, not the plumes that are the main drivers of the convection. The plumes partly drive the convection and are partly passively involved into the convection stirred by the sinking slabs.     

中国及其邻区地球三维结构初始模型的建立

对人工地震测深及天然地震面波体波三维层折反演数据进行统一处理，建立了中国及其邻区地球三维结构初始模型．此模型图像表明，中国及其邻区地球各圈层横向变化明显．岩石圈及软流圈内速度分布主要反映这一区域自古生代以来板块及地块拼合模式．各主要板块或地块（塔里木、扬子、中朝、青藏、哈萨克斯坦、印度、印度支那）岩石圈增厚或有很深的地慢根，板块或地块间的造山带岩石圈减薄，软流圈速度降低。下地幔底部及核幔边界D″层出现高速异常，表明古太平洋及古特提斯洋俯冲板块因重力坍塌已进入地球深层，形成亚洲超级下降地幔柱。这一下降地幔柱引起地球表层物质向中亚、东亚地区集中，印度半岛、青藏高原、新疆、蒙古至贝加尔一带，成为全球岩石圈最大的汇聚场所．     

基于P波三重震相的华南地区上地幔速度结构研究

华南块体是研究太平洋板块俯冲和岩石圈减薄机制等问题的最佳场所之一.本文基于中国地震观测台网和大型流动台阵记录到的震中距10°~30°之间的两个中深源地震P波记录,利用三重震相波形拟合技术,获得了中扬子克拉通和华夏地块上地幔高精度P波速度结构.研究结果表明:(1)中扬子克拉通过渡带底部存在高速异常,系太平洋俯冲板块的滞留体.俯冲的板块并没有进入下地幔,660-km间断面下沉约11 km,与后尖晶石相变的克拉伯龙斜率为负有关.而华夏地块过渡带底部并无明显高速异常,接近全球平均模型;(2)整个华南块体,410-km间断面上方普遍存在低速层,主要与上地幔部分熔融有关,与IASP91相比P波速度减小了1.38%~2.29%;(3)在研究区域内,中扬子克拉通和华夏地块都存在岩石圈减薄(80 km),推测可能与太平洋板块俯冲和快速回撤导致的岩石圈拆沉有关.且华夏地块减薄程度较明显,下伏软流圈速度较低,说明其上地幔强度较弱、温度较高.另外,中扬子克拉通过渡带中存在一个较宽的速度梯度带,可能与520-km间断面有关,其具体成因有待进一步研究.     

Electrical Structure in Marine Tectonic Settings

This review paper presents recent research on electrical conductivity structure in various marine tectonic settings. In at least three areas, marine electromagnetic studies for structural exploration have increasingly progressed: (1) data accumulations, (2) technical advances both for hardware and software, and (3) interpretations based on multidisciplinary approaches. The mid-ocean ridge system is the best-studied tectonic setting. Recent works have revealed evidence of conductive zones of hydrothermal circulation and axial magma chambers in the crust and partial melt zones of the mid-ocean ridge basalt source in the mantle. The role of water or dissolved hydrogen and its redistribution at mid-ocean ridges is emphasized for the conductivity pattern of the oceanic lithosphere and asthenosphere. Regions of mantle upwelling (hotspot or plume) and downwelling (subducting slab) are attracting attention. Evidence of heterogeneity exists not only in the crust and the upper mantle, but also in the mantle transition zone. Electrical conductive zones frequently overlap seismic low-velocity zones, but discrepancies are also apparent. Some studies have compared conductivity models with the results of seismic and other studies to investigate the physical properties or processes. A new laboratory-based conductivity model for matured oceanic lithosphere and asthenosphere is proposed. It takes account of both the water distribution in the mantle as well as the thermal structure. It explains observed conductivity patterns in the depth range of 60–200 km.