华北克拉通及东邻西太平洋活动大陆边缘地区的P波速度结构：对岩石圈减薄动力学过程的探讨

本文通过地震层析成像研究获得了华北克拉通及其东邻地区（30°N-50°N,95°E -145°E）1°×1°的P波速度扰动图像.结果显示,在西太平洋俯冲带地区,上地幔中西倾的板片状高速异常体与其上方的低速异常区构成俯冲带与上覆地幔楔的典型速度结构式样.俯冲板片高速体在约300~400 km深度范围内被低速物质充填,暗示俯冲板片可能发生了断离.在华北克拉通地区的上地幔中发现三个东倾排列的高速异常带.在此基础上,本文构建了华北克拉通及其东邻西太平洋活动大陆边缘地区的上地幔速度结构模式图,并据此探讨克拉通岩石圈减薄与西太平洋活动大陆边缘的深部动力学联系.本文认为,太平洋板片的俯冲（断离）,触发热地幔物质上涌并在上覆地幔楔中形成对流,使克拉通岩石圈受到改造（底侵与弱化）.随着俯冲板片后撤,地幔楔中的对流场以及对岩石圈改造的影响范围均随之东移,最终导致华北克拉通岩石圈自下而上、从西向东分三个阶段依次拆沉减薄.这一模式能很好地解释现今克拉通岩石圈自西向东呈台阶状减薄的深部现象.     

Joint bulk-sound and shear tomography for Western Pacific subduction zones

Detailed regional body wave tomographic inversion of the Western Pacific region has been performed using P and S travel times from common sources and receivers, with a joint inversion in terms of bulk-sound and shear wave-speed variations in the mantle. This technique allows the separation of the influence of bulk and shear moduli, and hence a more direct comparison with mineral physics information. The study region is parameterized with cells of side 0.5° to 2° and 19 layers to a depth of 1500 km, while the rest of the mantle was parameterized with 5×5° cells with 16 layers between the surface and the core–mantle boundary. A simultaneous inversion is made for regional and global structures to minimize the influence of surrounding structures on the regional image. A nested iterative inversion scheme is employed with local linearization and three-dimensional ray tracing through the successive model updates. The results of the regional tomographic inversion reveal the penetration of a subducted slab below the 660 km discontinuity at the Kurile–Kamchatka trench, while flattening of slabs above this depth is observed in the Japan and Izu–Bonin subduction zones on both the bulk-sound and shear wave-speed images. The penetration of a subducted slab down to a depth of at least 1200 km is seen below the southern part of the Bonin trench, Mariana, Philippine, and Java subduction zones. Fast shear wave-speed perturbations associated with the subducted slabs, down to the 410 km transition zone, are larger than the comparable bulk-sound perturbations for all these subduction zones except the Philippines. The bulk-sound signature for the subducted slab is more pronounced than for shear in the Philippines, Talaud, New Guinea, Solomon, and Tonga subduction zones, where penetration of the slab into the middle mantle is observed. Variation in the amplitude ratio between bulk-sound and shear wave-speed anomalies correlates well with the subduction parameters of the descending slab. Slabs younger than 90 Ma at the trench show bulk-sound dominance in the upper mantle, while older slabs have a stronger shear wave-speed signature. Spreading of the fast shear wave-speed zone between 800 and 1000 km is observed in the areas of deep subducted slab penetration, but has no comparable expression in the bulk-sound images. This high-velocity feature may reflect physical or chemical disequilibria introduced to the lower mantle by subducted slabs.     

Flat-Slab Thermal Structure and Evolution Beneath Central Mexico

Recent seismic and magnetotelluric experiments, aimed at better characterizing the shape and state of the subducting slab and continental crust beneath Central Mexico, exposed significant differences with conclusions of previous studies. A new slab geometry is revealed in which the subducting Cocos slab is perfectly flat between 120 to 290?km from the trench, after which it plunges into the asthenosphere at a dip angle of ~65°, in sharp contrast with the previously proposed ~20° dip angle. Seismic tomography studies show negative P-wave velocity anomalies (?2 to ?4%) in the mantle wedge beneath the Mexican Volcanic Belt, and positive anomalies (+2 to +3%) for the subducted Cocos slab. Magnetotelluric experiments exposed a very low-resistivity area (1?C10? ??m) located within the continental crust just below the Mexican Volcanic Arc. Finally, several spots of non-volcanic tremors (NVTs) have been recorded inside the continental crust above the flat-slab segment. While all these experiments provide a better picture of the subduction system beneath Central Mexico, several key processes need further investigation. In this study, we take advantage of these new observations to better constrain the thermal structure beneath Central Mexico. Two different thermal models are computed for a mantle potential temperature (T _p) of 1,350 and 1,450°C, respectively. The new thermal structures are then converted into P-wave velocity anomalies and compared with the observed V _p anomalies. We found that a T _p of 1,450°C produced larger V _p anomalies that do not fit the observations. However, using a T _p of only 1,350°C, our predicted V _p anomalies are positive (+2 to +3%) for the cold slab and negative (?2 to ?4%) in the mantle wedge. These V _p estimates are consistent with the observed seismic tomography from P-wave arrivals, and therefore we conclude that a T _p of 1,350°C is a better estimate for the mantle potential temperature beneath Central Mexico. The new thermal model, in conjunction with phase diagrams for sediments, hydrated basalt and lithospheric mantle, have been used to estimate the amount and location of fluids released from the subducting Cocos slab. Several dehydration pulses have been identified along the slab interface where most of the fluids stored in sediments and oceanic crust are released into the overlying continental crust above the flat-slab. We found a good correlation between the pattern of these dehydration pulses and the location of NVTs, suggesting that slab dehydration is responsible for triggering the tremors. We suggest that NVT bursts localized above the flat slab segment represent the manifestation of ongoing continental crust hydration and weakening, a process that has been going on since 15?Ma ago when the Cocos slab entered into a flat-slab regime. Such continuous weakening would have reduced the suction forces that kept the slab in a flat regime in the last 15?Ma, allowing the slab to easily roll back. The continuous low-resistivity region recorded beneath the volcanic front in Central Mexico might represent the evidence of slab dehydration and crust weakening over time.     

Prospecting for dead slabs

At subduction zones that have only recently ceased to be active, the lithospheric slab may retain a seismic velocity greater than that of the surrounding mantle even after the slab becomes seismically dead. To seek the subduction zone thought to have been recently active along the western margin of North America, we examined the variation with propagation direction of P-wave travel time residuals from sources at various distances and azimuths to seismograph stations in Washington and California. The uncertainty in source location and origin time was removed by referring the travel-time delay to a nearby station overlying presumably more uniform mantle. An eastward-dipping band of anomalously early arrivals at several stations on the western flank of the Sierra Nevada and California Cascades may imply that a dead slab is present beneath northern California, though a definitive conclusion is premature at present because of a paucity of seismic sources in eastern North America. The position of the dead slab speculatively suggested by the travel-time data is roughly consistent with that predicted by others on the basis of heat flow and geochemistry in the Sierra Nevada, and the southward decrease in the magnitude of the travel-time advance associated with such a slab is in agreement with the history of subduction of the Farallon plate as reconstructed from ocean floor magnetic anomalies and continental tectonic activity.     

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．      

Is there a first-order discontinuity in the lowermost mantle?

In 1983, Lay and Helmberger [Geophys. J. R. Astron. Soc. 75 (1983) 799–837] reported the detection of a precursor to the seismic phase ScS. They attributed this precursor to a sharp seismic discontinuity located several hundred kilometers above the core–mantle boundary. Such a lowermost mantle discontinuity implies the existence of a sharp phase change or a chemical boundary. Precursors to ScS and, less frequently, PcP have since been observed in numerous locations, but are not a global phenomenon. Frequently, PcP precursors are weak or absent when ScS precursors are observed in the same location, and vice versa. There can be significant variations in the amplitude and arrival time of the precursor relative to the main phase. The presence or absence of these precursors has led to speculations about the nature of the lowermost mantle. Here we demonstrate that ScS or PcP precursors may be produced by gradients in seismic wave speed associated with large-scale lowermost mantle heterogeneity. Rather than a phase or chemical boundary with substantial topography, such gradients require lateral variations in temperature and, close to the core–mantle boundary, composition.     

The influence of trench migration on slab penetration into the lower mantle

A two-dimensional numerical convection model in cartesian geometry is used to study the influence of trench migration on the ability of subducted slabs to penetrate an endothermic phase boundary at 660 km depth. The transient subduction history of an oceanic plate is modelled by imposing plate and trench motion at the surface. The viscosity depends on temperature and depth. A variety of styles of slab behaviour is found, depending predominantly on the trench velocity. When trench retreat is faster than 2–4 cm/a, the descending slab flattens above the phase boundary. At slower rates it penetrates straight into the lower mantle, although flattening in the transition zone may occur later, leading to a complex slab morphology. The slab can buckle, independent of whether it penetrates or not, especially when there is a localised increase in viscosity at the phase boundary. Flattened slabs are only temporarily arrested in the transition zone and sink ultimately into the lower mantle. The results offer a framework for understanding the variety in slab geometry revealed by seismic tomography.     

Lowermost mantle anisotropy beneath the Pacific: Imaging the source of the Hawaiian plume

We utilized recordings of seismic shear phases provided by several North American broadband seismometer arrays to provide unique constraints on shear wave anisotropy beneath the northern and central Pacific Ocean. Using a new analysis method that reduces measurement errors and enables the analysis of a larger number of available waveforms, we examined relative travel times of teleseismic S and Sdiff that sample a large area of lowermost mantle structure. The results of this study provide evidence for small-scale lateral and depth variations in shear wave anisotropy for a broad region of the lowermost mantle beneath the Pacific Ocean. In particular, we image a localized zone of anomalously strong anisotropy whose strength increases toward the top of D″ beneath Hawaii. Our results, combined with a previous study of V_P/V_SH ratios, indicate that ancient subducted slab material may be responsible for observations beneath the northern Pacific, while lenses or layers of core–mantle boundary reaction products or partial melt, oriented by horizontal inflow of mantle material to the Hawaiian plume source, can explain observations beneath the central Pacific.     

Suboceanic earthquake location and seismic structure in the Kanto district,central Japan

We present a combined method, using sP depth-phase data and double-difference arrival times, to determine the precise hypocenter locations of earthquakes that occur under the Pacific Ocean outside of the area covered by the land-based seismic network. We assess the effectiveness of the combined method using a data set of P- and S-wave arrival times and sP depth phase from suboceanic earthquakes recorded by both land-based seismic stations and offshore seismic stations (OFS). The hypocenters of the offshore earthquakes relocated using the combined method are consistent with those determined using the standard location method and OFS data. The differences in the hypocenters relocated by the two methods are less than 4 km. We applied the method to the subduction region that underlies the Kanto district, central Japan, and located a large number of earthquakes that occurred beneath the Pacific Ocean. We then determined the detailed 3D seismic velocity structure by inverting a large number of arrival times of P- and S-waves and sP depth phase from the relocated earthquakes in the study region. High-velocity anomalies related to the cold subducting Pacific slab and low-velocity anomalies related to the hot mantle wedge are clearly imaged. Beneath active volcanoes, low-velocity zones are visible from the surface to a depth of 100 km, reflecting fluids released by dehydration of the subducting Pacific slab. Strong lateral heterogeneities are revealed on the upper boundary of the Pacific slab beneath the forearc region. The low-velocity areas under the offshore region are associated with low seismicity and weak interplate coupling. A low-velocity layer is imaged along the upper boundary of the Philippine Sea slab in the northern part of Kanto district, which may reflect dehydration of the slab. Our tomographic images indicate that the overlaying Philippine Sea plate has effects on the spatial distribution of active volcanoes related to the subducting Pacific slab in the study region.     

Seismic imaging of the entire arc of Tohoku and Hokkaido in Japan using P-wave, S-wave and sP depth-phase data

In order to better understand seismic structure and seismotectonics of the entire arc of Tohoku and Hokkaido in Japan, we combined arrival time data from earthquakes beneath Tohoku and Hokkaido land areas, and beneath the Pacific Ocean to determine the three-dimensional (3D) velocity structures (Vp and Vs) under the entire Northeast (NE) Japan-Kuril arc. We adopted 176,431 P-wave and 110,953 S-wave arrival times, from 5123 local earthquakes, and 2843 sP depth-phase data from 385 events that occurred beneath the Pacific Ocean. The 385 suboceanic events were accurately relocated by using P-wave, S-wave and sP depth-phase arrival time data jointly. The obtained results confirmed the major features delineated by previous studies and revealed some new features of the structural heterogeneity beneath NE Japan and the Kuril forearcs. High-velocity anomalies of the cold subducting Pacific slab and low-velocity anomalies in the hot mantle wedge were imaged clearly. Strong lateral heterogeneities were revealed on the upper boundary of the Pacific slab under the forearc region, which showed a good correlation with the spatial distribution of large interplate earthquakes. These results indicated that strong coupling sections (or asperities) and weak-coupled or decoupled patches might exist along the upper boundary of the Pacific slab. Widespread low-velocity anomalies were visible in the forearc mantle above the subducting Pacific slab, which might reflect serpentinization of the forearc mantle associated with the dehydration process of the subducting slab. Our results also showed a general tendency for seismic coupling in the asperities to be located around low-velocity areas on the slab boundary under the suboceanic region.     

Latent heat effects of the major mantle phase transitions on low-angle subduction

Very low to zero shallow dip angles are observed at several moderately young subduction zones with an active trenchward moving overriding plate. We have investigated the effects of latent heat for this situation, where mantle material is pushed through the major mantle phase transitions during shallow low-angle subduction below the overriding plate. The significance of the buoyancy forces, arising from the latent heat effects, on the dynamics of the shallowly subducting slab is examined by numerical modeling. When a 32-Ma-old slab is overridden with 2.5 cm/yr by a continent, flat subduction occurs with a 4–5 cm/yr convergence rate. When latent heat is included in the model, forced downwellings cause a thermal anomaly and consequently thermal and phase buoyancy forces. Under these circumstances, the flat slab segment subducts horizontally about 350 km further and for about 11 Ma longer than in the case without latent heat, before it breaks through the 400-km phase transition. The style of subduction strongly depends on the mantle rheology: increasing the mantle viscosity by one order of magnitude can change the style of subduction from steep to shallow. Similarly, an overriding velocity of less than 1 cm/yr leads to steep subduction, which gradually changes to flat subduction when increasing the overriding velocity. However, these model parameters do not change the aforementioned effect of the latent heat, provided that low-angle subduction occurs. In all models latent heat resulted in a substantial increase of the flat slab length by 300–400 km. Varying the olivine–spinel transition Clapeyron slope γ from 1 to 6 MPa/K reveals a roughly linear relation between γ and the horizontal length of the slab. Based on these results, we conclude that buoyancy forces due to latent heat of phase transitions play an important role in low-angle subduction below an overriding plate.     

The thermal structure of subduction zones constrained by seismic imaging: Implications for slab dehydration and wedge flow

Thermal models of subduction zones often base their slab–wedge geometry from seismicity at mantle depths and, consequently, cannot be used to evaluate the relationship between seismicity and structure. Here, high-resolution seismic observations from the recent Broadband Experiment Across the Alaska Range (BEAAR) constrain, in a rare instance, the subducting slab geometry and mantle wedge temperature independent of seismicity. Receiver functions reveal that the subducting crust descends less steeply than the Wadati-Benioff Zone. Attenuation tomography of the mantle wedge reveals a high Q and presumably cold region where the slab is less than 80 km deep. To understand these two observations, we generate thermal models that use the improved wedge geometry from receiver functions and that incorporate temperature- and strain-rate-dependent olivine rheology. These calculations show that seismicity within the subducting crust falls in a narrow belt of pressure–temperature conditions, illuminating an effective Clapeyron slope of 0.1 K/MPa at temperatures of 450–750 °C. These conditions typify the breakdown of high-pressure hydrous minerals such as lawsonite and suggest that a single set of dehydration reactions may trigger intermediate-depth seismicity. The models also require that the upper, cold nose of the mantle wedge be isolated from the main flow in the mantle wedge in order to sustain the cold temperatures inferred from the Q tomography. Possibly, sufficient mechanical decoupling occurs at the top of the downgoing slab along a localized shear zone to 80 km depth, considerably deeper than inferred from thrust zone seismicity.     

Core–mantle boundary topography as a possible constraint on lower mantle chemistry and dynamics

The origin of large low shear-wave velocity provinces (LLSVPs) in the lowermost mantle beneath the central Pacific and Africa is not well constrained. We explore numerical convection calculations for two proposed hypotheses for these anomalies, namely, thermal upwellings (e.g., plume clusters) and large intrinsically dense piles of mantle material (e.g., thermochemical piles), each of which uniquely affects the topography on Earth's core–mantle boundary (CMB). The thermochemical pile models predict a relatively flat but elevated CMB beneath piles (presumed LLSVPs), with strong upwarping along LLSVP margins. The plume cluster models predict CMB upwarping beneath upwellings that are less geographically organized. Both models display CMB depressions beneath subduction related downwelling. While each of the two models produces a unique, characteristic style of CMB topography, we find that seismic models will require shorter length scales than are currently being employed in order to distinguish between the end-member dynamic models presented here.     

南阿拉斯加地壳及上地幔结构成像研究

通过反演562891个纵波和156321个横波走时数据,第一次同时获得了阿拉斯加地区地壳及上地幔的纵波与横波速度以及泊松比图像,为更好地认识阿拉斯加地区的深部地震结构、太平洋板块与亚库塔特板块的俯冲几何形态提供了科学依据.成像结果表明P波和S波速度图像与泊松比结构具有很好的一致性,强的高速度和低泊松比异常沿着阿拉斯加俯冲带延伸至200 km深度,该高速度和低泊松比异常体与俯冲带的地震空间分布吻合,因此,我们认为该高速体为俯冲的太平洋板块和亚库塔特板块.从地震空间分布发现,大部分大地震（M>6.5）发生在高速度与低速度异常交界处,可能反映了俯冲板块之间强耦合作用.在俯冲带的地幔楔显示出广泛的低速度和高泊松比异常,并且这些异常与岛弧火山的位置相对应,这与大洋板块俯冲所形成的岩浆入侵作用有关.研究结果表明在南阿拉斯加俯冲带,俯冲板块的俯冲角度从兰格尔块体下方的平坦变成在布里斯托尔湾下方的陡峭,这与亚库塔特板块俯冲在兰格尔块体下方和太平洋板块俯冲在布里斯托尔湾下方有关.在基奈半岛和科迪亚克岛连接处的上地幔位置存在强烈的低速与高泊松比异常体,使该处的大洋俯冲板块变薄.这一现象可能与亚库塔特板块和太平洋板块相互碰撞作用以及软流圈强烈的上升流入侵有关.     

Geodynamic modeling of thermal structure of subduction zones

During subduction processes, slabs continuously have heat exchange with the ambient mantle, including both conduction and advection effects. The evolution of slab thermal structure is one of the dominant factors in controlling physical and chemical property changes in subduction zones. It also affects our understanding of many key geological processes, such as mineral dehydration, rock partial melting, arc volcanism, and seismic activities in subduction zones. There are mainly two ways for studying thermal structure of subduction zones with geodynamic models: analytical model and numerical model. Analytical model provides insights into the most dominant controlling physical parameters on the thermal structure, such as slab age, velocity and dip angle, shear stress and thermal conductivity, etc. Numerical model can further deal with more complicated environments, such as viscosity change in the mantle wedge, coupling process between slabs and the ambient mantle, and incorporation of petrology and mineralogy. When applying geodynamic modeling results to specific subduction zones on the Earth, there are many factors which may complicate the process, therefore it is difficult to precisely constrain the thermal structure of subduction zones. With the development of new quantitative methods in geophysics and geochemistry, we may obtain more observational constraints for thermal structure of subduction zones, thus providing more reasonable explanations for geological processes related to subduction zones.     

日本海沟俯冲带热结构与深源地震

本文利用有限差分方法,计算了全地幔对流模式和双层地幔对流模式下日本海沟俯冲带热结构、浮力及P波速度异常分布,基于亚稳态橄榄石相变模型推测亚稳态橄榄石的存在范围,同时分析了热传导系数、热膨胀系数和热源对俯冲带热结构的影响,以及俯冲带所受浮力与俯冲带形态的关系.结果表明,双层地幔对流模式下模拟的P波速度异常分布与层析成像结果更为相符,也与深源地震的分布有较好的相关性.板块内部亚稳态橄榄石的存在范围随热传导系数和热膨胀系数的减小而增大,同时忽略相变潜热和剪切生热的影响也会造成模型所预测的亚稳态橄榄石范围偏大.俯冲带所受负浮力在400 km深度附近达到最大值,亚稳态橄榄石的存在使负浮力逐渐减小,甚至在板块内部产生正浮力,不利于俯冲带穿透660 km相变界面.     

西北太平洋和汤加俯冲地区深震特殊聚簇的特性及成因探究

对比研究了具有不同热参数、不同俯冲形态的西北太平洋俯冲地区和汤加俯冲地区的深震特殊聚簇的地震学特性和成因.利用单键群方法探测到两个特殊的深震聚簇G1N和G1T.聚簇G1N位于地震空区下方,具有极低的b值（~0.54）,完全不同于具有高b值（~1.04）的汤加俯冲地区聚簇G1T.通过对聚簇地区板块形态、地震主应力轴、地震深度分布特征的分析,以及和汤加典型的板片折曲处地震活动性的对比,我们认为深震聚簇G1N附近的板块表现出板片折曲的特征,板块俯冲到地幔过渡带底部受到下地幔的黏性阻力,板片局部向上凸起发生折曲,产生局部的拉张应力,叠加在俯冲造成的压缩构造背景上,应力状态发生改变,从而影响该深震聚簇的地震活动性.汤加地区G1T聚簇深震的成因则完全不同,没有体现出板片折曲、应力变化的特征;相反,这些深震发生在较冷的Vitiza-Fiji俯冲板块上,该板块在5~8 Ma年前先行俯冲到G1T区域并与Tonga板块发生拆离,G1T聚簇深震就发生在这些温度依然很低、滞留于~500 km深度处的高速板片残留体上.     

Thermal Structure of the Ionian Slab

We propose a thermal model of the subducting Ionian microplate. The slab sinks in an isothermal mantle, and for the boundary conditions we take into account the relation between the maximum depth of seismicity and the thermal parameter L_th of the slab, which is a product of the age of the subducted lithosphere and the vertical component of the convergence rate. The surface heat-flux dataset of the Ionian Sea is reviewed, and a convective geotherm is calculated in its undeformed part for a surface heat flux of 42 mW m^–2, an adiabatic gradient of 0.6 mK m^–1, a mantle kinematic viscosity of 10¹⁷ m² s^–1 and an asthenosphere potential temperature of 1300°C. The calculated temperature-depth distribution compared to the mantle melting temperature indicates the decoupling limit between lithosphere and asthenosphere occurs at a depth of 105 km and a temperature of 1260°C. A 70–km thick mechanical boundary layer is found. By considering that the maximum depth of the seismic events within the slab is 600 km, a L_th of 4725 km is inferred. For a subduction rate equal to the spreading rate, the corresponding assimilation and cooling times of the microplate are about 7 and 90 Myr, respectively. The thermal model assumes that the mantle flow above the slab is parallel and equal to the subducting plate velocity of 6 cm yr^–1, and ignores the heat conduction down the slab dip. The critical temperature, above which the subduced lithosphere cannot sustain the stress necessary to produce seismicity, is determined from the thermal conditions governing the rheology of the plate. The minimum potential temperature at the depth of the deepest earthquake in the slab is 730°C.     

中国境内天山地壳上地幔结构的地震层析成像

根据横跨中国境内天山的库车—奎屯宽频带流动地震台阵和区域地震台网记录的近震和远震P波走时数据,利用地震层析成像方法重建了沿该地震台阵剖面下方400 km深度范围内地壳上地幔的P波速度结构.结果表明：沿新疆库车—奎屯剖面,天山地壳具有明显的横向分块结构,且南、北天山地壳显示了较为强烈的横向变形特征,表明塔里木地块对天山地壳具有强烈的侧向挤压作用;在塔里木和准噶尔地块上地幔顶部有厚度约60～90 km的高速异常体,塔里木—南天山下方的高速异常体产生了较为明显的弯曲变形,而准噶尔—北天山下方的高速异常体向南一直俯冲到中天山南侧边界下方300 km的深度,两者形成了不对称对冲构造;在塔里木和准噶尔地块下方150～400 km深度存在上地幔低速体,其中塔里木地块一侧的上地幔低速物质上涌到南天山地块的下方;在塔里木—南天山200～300 km深度范围的上地幔存在高速异常体,它可能是地幔热物质向上迁移过程融断的塔里木岩石圈的拆离体. 上述结果表明,塔里木地块的俯冲可能涉及整个岩石圈深度,但其前缘仅限于南天山的北缘;青藏高原隆升的远程效应可能不但驱动塔里木岩石圈向北俯冲,同时还造成天山造山带南侧上地幔物质的涌入;天山造山带上地幔广泛存在的低速异常有助于其上地幔的变形,而上地幔物质的强烈非均匀性应有助于推动天山造山带上地幔小尺度地幔对流的形成;根据研究区地壳上地幔速度结构特征推断,新近纪以来天山快速隆升的主要力源来自青藏高原快速隆升的远程效应,相对软弱的上地幔为加速天山造山带的变形和隆升创造了必要条件.     

基于重力数据特征的东北地区深部动力机制解析

中国东北地区处于古亚洲构造域、蒙古—鄂霍茨克构造域和环太平洋构造域叠加作用最为显著的地区,是地学研究的热点区域.为了探析欧亚大陆下西太平洋板片的俯冲形态以及揭示该区域深部地球动力学机制,利用卫星重力数据通过预处理共轭梯度快速密度反演算法获得了包含东北地区、华北部分地区及日本海海域在内的研究区域上地幔三维密度结构,结合天然地震三维层析成像结果共同揭示太平洋板片的俯冲形态和深部动力机制.俯冲的太平洋板片在日本海沟处呈高密度异常,向西横向持续扩张,深度方向上有逐渐增加趋势.不连续的高密度体俯冲至地幔转换带(410～660km)后继续水平西向俯冲,部分滞留板片可能进入下地幔;在大兴安岭断裂带下面转换带中同样发现水平分布的高密度体,推断是大兴安岭断裂带下方地幔岩石圈拆沉的结果,横向不均匀分布的俯冲板片边缘已抵至大兴安岭造山带附近,这对于研究东北地区深部动力学机制具有重要的意义.