共查询到20条相似文献，搜索用时 31 毫秒
1.
The shapes and orientations of Benioff zones beneath island arcs, interpreted as marking the location of subducted lithosphere, provide the best presently available constraints on the global convective flow pattern associated with plate motions. This global flow influences the dynamics of subduction. Subduction zone phenomena therefore provide powerful tests for models of mantle flow. We compute global flow models which, while simple, include those features which are best constrained, namely the observed plate velocities, applied as boundary conditions, and the density contrasts given by thermal models of the lithosphere and subducted slabs. Two viscosity structures are used; for one, flow is confined to the upper mantle, while for the other, flow extends throughout the mantle.Instantaneous flow velocity vectors match observed Benioff zone dips and shapes for the model which allows mantle-wide flow but not for the upper mantle model, which has a highly contorted flow pattern. The effect of trench migration on particle trajectories is calculated; it is not important if subduction velocities are greater than migration rates. Two-dimensional finite element models show that including a coherent high viscosity slab does not change these conclusions. A coherent high viscosity slab extending deep into the upper mantle would significantly slow subduction if flow were confined to the upper mantle. The maximum earthquake magnitude, Mw, for island arcs correlates well with the age of the subducted slab and pressure gradient between the trench and back-arc region for the whole mantle, but not the upper mantle, flow model. The correlations with orientations of Benioff zones and seismic coupling strongly suggest that the global return flow associated with plate motions extends below 700 km. For both models, regions of back-arc spreading have asthenospheric shear pulling the back-arc toward the trench; regions without back-arc spreading have the opposite sense of shear, suggesting global flow strongly influences back-arc spreading. 相似文献
2.
青藏高原及邻区大地水准面异常场源结构 总被引：1，自引：0，他引：1
以青藏高原及邻区地形、地震层析成像、沉积层底面、Moho面及岩石层底面资料为基础 ,讨论了由地形起伏、地球内部各界面以及物质密度不均匀引起的大地水准面异常的计算方法 ,正演计算出青藏高原及邻区岩石圈内部物质不均匀产生的大地水准面异常 ,并把从全阶大地水准面异常中扣除正演模拟得到的岩石圈大地水准面异常与不同阶次波段的大地水准面进行比较 ,以寻求表示青藏高原及邻区地幔物质不均匀的大地水准面异常球谐函数的最佳阶次。结合地震层析成像资料分析结果得出 ,表示青藏高原及邻区下地幔、上地幔及岩石圈物质不均匀的大地水准面异常球谐函数的阶次范围分别为 2 6阶 ,7 60阶和 61 3 60阶。 相似文献
3.
《Gondwana Research》2010,17(3-4):545-562
This article reviews the electrical conductivity structures of the oceanic upper mantle, subduction zones, and the mantle transition zone beneath the northwestern Pacific, the Japanese Islands, and continental East Asia, which have particularly large potential of water circulation in the global upper mantle. The oceanic upper mantle consists of an electrically resistive lid and a conductive layer underlying the lid. The depth of the top of the conductive layer is related to lithospheric cooling in the older mantle, whereas it is attributable to the difference in water distribution beneath the vicinity of the seafloor spreading-axis. The location of a lower crustal conductor in a subduction zone changes according to the subduction type. The difference can be explained by the characteristic dehydration from the subducting slab in each subduction zone and by advection from the backarc spreading. The latest one-dimensional electrical conductivity model of the mantle transition zone beneath the Pacific Ocean predicts values of 0.1–1.0 S/m. These values support a considerably dry oceanic mantle transition zone. However, one-dimensional electrical profiles may not be representative of the mantle transition zone there, since there exists a three-dimensional structure caused by the stagnant slab. Three-dimensional electromagnetic modeling should be made in future studies. 相似文献
4.
This article reviews the electrical conductivity structures of the oceanic upper mantle, subduction zones, and the mantle transition zone beneath the northwestern Pacific, the Japanese Islands, and continental East Asia, which have particularly large potential of water circulation in the global upper mantle. The oceanic upper mantle consists of an electrically resistive lid and a conductive layer underlying the lid. The depth of the top of the conductive layer is related to lithospheric cooling in the older mantle, whereas it is attributable to the difference in water distribution beneath the vicinity of the seafloor spreading-axis. The location of a lower crustal conductor in a subduction zone changes according to the subduction type. The difference can be explained by the characteristic dehydration from the subducting slab in each subduction zone and by advection from the backarc spreading. The latest one-dimensional electrical conductivity model of the mantle transition zone beneath the Pacific Ocean predicts values of 0.1–1.0 S/m. These values support a considerably dry oceanic mantle transition zone. However, one-dimensional electrical profiles may not be representative of the mantle transition zone there, since there exists a three-dimensional structure caused by the stagnant slab. Three-dimensional electromagnetic modeling should be made in future studies. 相似文献
5.
It has been thought that granitic crust,having been formed on the surface,must have survived through the Earth’s evolution because of its buoyancy.At subduction zones continental crust is predominantly created by arc magmatism and is returned to the mantle via sediment subduction,subduction erosion, and continental subduction.Granitic rocks,the major constituent of the continental crust,are lighter than the mantle at depths shallower than 270 km,but we show here,based on first principles calculations, that beneath 270 km they have negative buoyancy compared to the surrounding material in the upper mantle and transition zone,and thus can be subducted in the depth range of 270-660 km.This suggests that there can be two reservoirs of granitic material in the Earth,one on the surface and the other at the base of the mantle transition zone(MTZ).The accumulated volume of subducted granitic material at the base of the MTZ might amount to about six times the present volume of the continental crust.Our calculations also show that the seismic velocities of granitic material in the depth range from 270 to 660 km are faster than those of the surrounding mantle.This could explain the anomalous seismic-wave velocities observed around 660 km depth.The observed seismic scatterers and reported splitting of the 660 km discontinuity could be due to jadeite dissociation,chemical discontinuities between granitic material and the surrounding mantle,or a combination thereof. 相似文献
6.
Transient surface deflections associated with non-continuous subduction are studied through the use of a sequence of idealized numerical models of time dependent mantle convection. The major concerns of this study are the magnitude and duration of dynamically supported topographic fluctuations resulting from either the initiation or termination of a subduction zone at the Earth's surface. The former is modelled by prescribing an initial temperature structure which mimics the presence of a cold slab of lithosphere immersed in a warm uniform-temperature mantle, while the latter is induced by thermally detaching this slab from the upper surface of the convection cell. The model “slabs” are thus defined thermally, rather than mechanically, and their negative buoyancy induces convective flow in the neighbouring mantle. The full hydrodynamic equations governing natural convection are solved numerically in order to follow the evolution of the temperature and velocity fields with time. The resulting model flows are, therefore, dynamically self-consistent, and differ from previous kinematic models in that the flow velocities are determined at each instant from the evolving thermal field, rather than prescribed as boundary conditions. As the lithospheric slab sinks into the mantle, the induced flow produces normal stresses, which in turn result in a broad topographic depression. Subsequently, as the slab-surface distance increases, the topography rebounds at a rate which is strongly dependent upon the imposed temperature contrast between the slab and the surrounding mantle material. Assuming that these depressions are filled with sediments, if a subsequent episode of subduction were to begin before the initial depression was eliminated, new sediments would be superimposed upon the old. Repeated episodes of subduction at a continental margin may, therefore, be an important factor contributing to repeated cycles of platform sedimentation. 相似文献
7.
Glyn M. Jones 《Tectonophysics》1983,99(2-4)
Altimeter geoid profiles crossing trenches and island arcs typically exhibit a long-wavelength increase in geoidal height approaching the trench which, in many cases, reaches a maximum over the back-arc area. Bathymetry profiles across trenches show a similar regional increase in the mean depth of the ocean floor behind the trench. Filters corresponding to Airy and Pratt models of isostatic compensation were applied to bathymetry profiles crossing seven different trench systems in order to estimate how much of the observed geoid variation in these regions could be attributed to isostatically-compensated ocean-floor topography. The results indicate that short-wavelength, high-amplitude variations in the geoid, which in some cases account for over 50% of the total observed amplitude variation, can be reasonably reproduced assuming a Pratt model of isostatic compensation. An additional component of the geoid arises from the uncompensated outer rise seaward of the trench. It is therefore concluded that a large part of the geoid signal over trenches and island arcs may be related to variations in sea-floor topography. This topographic component should be removed from altimeter geoid profiles before using the data to infer details of the deeper structure of subduction zones. 相似文献
8.
Free conductive flows in the asthenosphere, layer C, and subduction zone are considered on the basis of experimental and theoretical simulation. The main forces acting on the oceanic lithospheric plate in the subduction zone are described. The horizontally directed forces arising due to free convection in the asthenosphere and transferring the oceanic lithospheric plate toward subduction zone have been estimated. These are friction force Fa and force of gravitational sliding F rd. Thermogravitational force F tg, which is created because the subducting lithospheric plate has a lower average temperature than the ambient mantle, is estimated. The force created owing to phase transitions in the subducted plate has been estimated as well. The tangential stress at the contact of the subducting plate with the continental lithosphere and underlying upper mantle has been determined. The horizontal force arising due to different lateral temperature gradients in the upper mantle on the left and on the right of the subducting plate has been estimated. The results of experimental modeling of the effect exerted upon subduction by counter free convective flows developing in the asthenosphere are considered. The experiments show that the position of the descending free convective flow and thus of the subduction zone depends on the ratio of the thermal power of astehnospheric countercurrents. The pressure arising near the 670 km boundary gives rise to spreading of the subducting plate over this boundary. 相似文献
9.
We present the first seismic image of the upper mantle beneath the active intraplate Changbai volcano in Northeast Asia determined by teleseismic travel time tomography. The data are measured at a new seismic network consisting of 19 portable stations and 3 permanent stations. Our results show a columnar low-velocity anomaly extending to 400-km depth with a P-wave velocity reduction of up to 3%. High velocity anomalies are visible in the mantle transition zone, and deep-focus earthquakes occur at depths of 500–600 km under the region, suggesting that the subducting Pacific slab is stagnant in the transition zone, as imaged clearly by global tomography. These results suggest that the intraplate Changbai volcano is not a hotspot like Hawaii but a kind of back-arc volcano related to the deep subduction and stagnancy of the Pacific slab under Northeast Asia. 相似文献
10.
Carlo Doglioni 《Tectonophysics》2009,463(1-4):208-213
The Schellart's [Schellart, W.P., 2007, The potential influence of subduction zone polarity on overriding plate deformation, trench migration and slab dip angle. Tectonophysics, 445, 363–372.] paper uses slab dip and upper plate extension for testing the westward drift. His analysis and discussion are misleading for the study of the net rotation of the lithosphere since the first 125 km of subduction zones are sensitive also to other parameters such upper plate thickness, geometry and obliquity of the subduction zone with respect to the convergence direction. The deeper (> 125 km) part cannot easily be compared as well because E- or NE-directed subduction zones have seismic gaps between 270–630 km. Moreover the velocity of subduction hinge cannot be precisely estimated and it does not equal to backarc spreading due to accretionary prism growth and asthenospheric intrusion at the subduction hinge. It is shown here that hinge migration in the upper plate or lower plate reference frames supports a general global polarization of the lithosphere in agreement with the westward drift of the lithosphere. The W-directed subduction zones appear controlled by the slab–mantle interaction with slab retreat imposed by the eastward mantle flow. The opposite E-NE-directed subduction zones seem rather mainly controlled by the convergence rate, plus density, thickness and viscosity of the upper and lower plates. Finally, the geological and geophysical asymmetries recorded along subduction and rift zones as a function of their polarity with respect to the tectonic mainstream are not questioned in the Schellart's paper, but they rather represent the basic evidence for the westward drift of the lithosphere. 相似文献
11.
D.H. Chung 《Tectonophysics》1977,42(1):T35-T42
The seismologically observed Pn velocity anomalies in the conterminous United States are restricted to the lithosphere, but the observed teleseismic delay-time variations are due principally to the regional variations in the physical state (i.e., thickness of lowvelocity zone and/or percent melt, etc.) of the asthenosphere. The observed low Pn velocity has been attributed to partial melting in the upper mantle, but it is shown that the partial-melting model alone cannot explain the seismologically observed Pn velocities in such an anomalous region as the Basin and Range Province. The present structure of the Basin and Range Province is possibly a result of rifting in the western conterminous United States; under it there may lie a mixed structure of old crust and mantle materials. The low-velocity zone under the Basin and Range Province would then be caused by downward chemical transition from the sub-Moho pyrolitic mantle material into a plagioclase-rich ophiolitic (old oceanic crust and upper mantle) composition and associated meltingand then into a peridotitic composition at the bottom of the lowvelocity zone. This mixed material model, with partial melting, would explain the low Pn velocity and low seismic Q in the region, as well as other geophysical observations. 相似文献
12.
Structure of the Upper Mantle and Transition Zone Beneath the South China Block Imaged by Finite Frequency Tomography 总被引：1，自引：0，他引：1
We applied the finite frequency tomography method to S wave data recorded by 350 broadband stations beneath the South China Block(SCB) and its surroundings from earthquakes occurring between July 2007 and July 2010,to better understand upper mantle deformation.Differential travel-times in the pair of stations with appropriate weighting for each station are used in the inversion.Our results are consistent with previous tomography that show a high velocity anomaly beneath the Sichuan basin and a high velocity anomaly in the transition zone beneath the Yangtze Craton.However,the resolution of mantle heterogeneity provides new insight into the tectonic framework of subduction of Burmese lithosphere in the west part of the study region and subduction of oceanic lithosphere in the east.In the subduction realm,west of 107°E,a significant fast S-wave anomaly is located on the southeast of Sichuan Basin.East of 107°E,and two narrow and discontinuous fast S-wave anomalies occur at a depth of 400-600 km beneath the middle of the South China block overlain by the pronounced low S-wave anomalies at a depth of 100 and 400 km.If the fast anomalies located in the mantle transition zone represent stagnant slabs,their fragmented nature may suggest that they could be produced by different episodes of subduction beneath western Pacific island and the above slow velocity anomaly may associated with the back-arc regions of ongoing subduction.In addition,tomography also reveals an anomalously high S-wave velocity continental root extends eastward to a depth 400 km beneath the eastern Sichuan Basin.This anomaly may be related to eastern extrusion of Indian lithosphere associated with the collision of India and Eurasia.Moreover,our results also show large slow anomalies beneath the Red River fault region connected to deeper anomalies beneath the South China Fold Belt and South China Sea.AH these observations are consistent with the scenario that the South China block has been built by both of subduction of Paleopacific plate and eastward subduction of Burma microplate. 相似文献
13.
《Physics and Chemistry of the Earth, Part A: Solid Earth and Geodesy》2000,25(4):343-353
Details of the Earth's geoid and gravity fields are summarized and examined. A set of 9274 centerpoints of 5 ° cubes (referred to as bloblets) represents subducted slab locations. This set, developed from reconstructed plate history, was provided by the first author of Lithgow-Berttelloni et. al. [1998] and is the best available estimate of locations of subduction material in the Earth's mantle. Two global mass solutions offered here utilize 1) only those bloblets in the outer 800 km, and 2) only those bloblets in the outer 1400 km. Since each bloblet location represents the center of a 5-degree cube [a larger volume than appropriate for a fragment of subducted lithosphere] it was necessary in the 800 km depth limit model to reduce their density to 0.004 grams/cc, and by increasing bloblet density six times at 797.5 km depth to simulate the piling up of slab material beneath the 670 km boundary. The 1400 km depth limit model [commensurate with evidence of slab penetration into the lower mantle from seismic tomography] required estimating densities for the bloblets at nine different mantle depths. An additional four point-masses at 3000 km depth (to simulate CMB topography, unrelated to dynamic topography) completes the mass models. Both these models show reasonable agreement to patterns and magnitudes for degrees 2–10, 3–10, 4–10, 2–3, 3, and 2 geoid fields with both geometric and hydrostatic flattening. These models support an assessment that topography at the core mantle boundary (CMB) may be produced by processes within the core rather than from within the mantle. Possible causes for the CMB topography are discussed. 相似文献
14.
蛇纹石脱水与大洋俯冲带中源地震(70~300km)的关系 总被引：4，自引：2，他引：4
蛇纹石脱水致裂作用是诱发大洋俯冲带中源地震(70~300km)的一种重要成因机制,它与中等深度双地震带的形成有很密切的关系。双地震带在冷俯冲带中是一种常见现象,它由上下相距20~40km的两个平行地震层组成。上地震层位于俯冲洋壳中,可能是洋壳蓝片岩脱水形成榴辉岩的系列脱水反应诱发了地震;下地震层位于大洋俯冲地幔中,可能是部分交代的地幔橄榄岩脱水控制着中源地震的分布。蛇纹岩在高温高压条件下的变形实验证实蛇纹石在脱水过程中引起岩石弱化和脆性破裂,这已经得到了对蛇纹石脱水过程中岩石物理性质和变形后样品的显微构造等理论研究上的支持。在蛇纹石脱水过程中,产生的流体与固体残留物分离,形成了大量的I型(张性)微裂隙,最终导致岩石破裂和形成断层。根据叶蛇纹石脱水反应相图,理论上在大洋俯冲带中蛇纹石脱水位置会出现双层结构,但只有平行于俯冲板块顶层等温线的一支才可能脱水诱发地震,并对应于双地震带的下地震层。下地震层所处的位置具有低的vp/vs值,暗示岩石圈大洋地幔顶层发生了部分交代。但它的交代机制尚不清楚,可能是海水通过洋底转换断层和/或沿着在外海沟隆起中形成的断层渗入大洋地幔顶层,并发生了洋壳和大洋地幔交代。双地震带在120~200km深度合一以后,冷俯冲带中所发生的中源地震可能与蛇纹石脱水有关,在热俯冲带中更可能与“湿”榴辉岩脱水有关。 相似文献
15.
利用冈底斯中-东部197个宽频带天然地震台站记录到的数据和远震P波走时层析成像方法,获得了该区域的P波速度扰动图像。层析成像结果显示研究区地壳和上地幔地震波速度结构存在着复杂的空间变化。首先,在藏南拆离系断层(STD)以北的特提斯喜马拉雅地壳中存在着较强的低速异常,但是该低速异常的北端在远离裂谷带的地方并没有明显越过雅鲁藏布江缝合线(YZS),这与前人的观测结果略有不同;在亚东-古露(YGR)和措美-桑日(CSR)裂谷带的下方存在低速异常,但异常强度都没有前者大;在两个裂谷带之间的拉萨地块中-南部,地壳表现为强高速特征。这些结果表明,影响青藏高原地壳构造演化的"地壳通道流(Crustal Channel Flow)"在藏南主要分布在特提斯喜马拉雅地区,在雅鲁藏布江缝合线以北的冈底斯地区,可能主要局限于沿裂谷带分布。其次,被解释为印度岩石圈地幔的上地幔高速异常,在研究区西部,抵达了雅鲁藏布江缝合线以北100km或更远的地方,而在研究区东部,并没有越过雅鲁藏布江缝合线,而是停留在缝合线以南~100km的高喜马拉雅下方,印证了前人给出的印度板块俯冲角度在研究区附近存在东西向变化的层析成像结果。此外,我们的层析成像结果还印证了冈底斯东南侧的上地幔低速异常根植于上地幔底部,我们认为该现象可能与巽他块体的顺时针旋转引起向东俯冲的缅甸弧向西后撤有关。 相似文献
16.
Three dimensional P-wave velocity structure beneath the Tohoku district, northeastern Japan arc, is investigated by an inversion of arrival times from local earthquakes using the method originally due to Aki and Lee (1976).In the crust (0–32 km depth) a low-velocity region is found along the volcanic front and its vicinity. Velocities at depths of 32–65 km are low beneath the regions where many Quaternary volcanoes and geothermal areas are distributed. In the region deeper than 65 km, the subduction of the Pacific plate is clearly revealed, and the mantle structure above the descending plate is rather uniform. These features suggest that volcanic activities have relation to the upper mantle structure. The results obtained in this study will be helpful in investigating the mechanism of magma generation in a subduction zone. 相似文献
17.
Seiichi Miura Narumi Takahashi Ayako Nakanishi Tetsuro Tsuru Shuichi Kodaira Yoshiyuki Kaneda 《Tectonophysics》2005,407(3-4):165-188
The Japan Trench subduction zone, located east of NE Japan, has regional variation in seismicity. Many large earthquakes occurred in the northern part of Japan Trench, but few in the southern part. Off Miyagi region is in the middle of the Japan Trench, where the large earthquakes (M > 7) with thrust mechanisms have occurred at an interval of about 40 years in two parts: inner trench slope and near land. A seismic experiment using 36 ocean bottom seismographs (OBS) and a 12,000 cu. in. airgun array was conducted to determine a detailed, 2D velocity structure in the forearc region off Miyagi. The depth to the Moho is 21 km, at 115 km from the trench axis, and becomes progressively deeper landward. The P-wave velocity of the mantle wedge is 7.9–8.1 km/s, which is typical velocity for uppermost mantle without large serpentinization. The dip angle of oceanic crust is increased from 5–6° near the trench axis to 23° 150 km landward from the trench axis. The P-wave velocity of the oceanic uppermost mantle is as small as 7.7 km/s. This low-velocity oceanic mantle seems to be caused by not a lateral anisotropy but some subduction process. By comparison with the seismicity off Miyagi, the subduction zone can be divided into four parts: 1) Seaward of the trench axis, the seismicity is low and normal fault-type earthquakes occur associated with the destruction of oceanic lithosphere. 2) Beneath the deformed zone landward of the trench axis, the plate boundary is characterized as a stable sliding fault plain. In case of earthquakes, this zone may be tsunamigenic. 3) Below forearc crust where P-wave velocity is almost 6 km/s and larger: this zone is the seismogenic zone below inner trench slope, which is a plate boundary between the forearc and oceanic crusts. 4) Below mantle wedge: the rupture zones of thrust large earthquakes near land (e.g. 1978 off Miyagi earthquake) are located beneath the mantle wedge. The depth of the rupture zones is 30–50 km below sea level. From the comparison, the rupture zones of large earthquakes off Miyagi are limited in two parts: plate boundary between the forearc and oceanic crusts and below mantle wedge. This limitation is a rare case for subduction zone. Although the seismogenic process beneath the mantle wedge is not fully clarified, our observation suggests the two possibilities: earthquake generation at the plate boundary overridden by the mantle wedge without serpentinization or that in the subducting slab. 相似文献
18.
Lithospheric geoid anomalies record changes in elevation and potential energy experienced by continental lithosphere. Estimates of local isostatic equilibration and potential energy, in tandem with lithosphere-related geoid anomalies, can be used to estimate paleolithospheric thickness, providing a clearer understanding of how and why continental topography is developed. We employ several simplifying assumptions about the crustal and mantle lithosphere density and structure (and readily acknowledge that our results are therefore first-order approximations) to predict the pre-orogenic structure of the lithosphere. At the outset we emphasize that while this approach does not provide an exhaustive evaluation of the deformation mechanism, it does serve to quantify the relative role played by the variations in the crustal and upper mantle components of the lithosphere. In this way we are able to use independent measurement of lithospheric geoid anomalies, current (post-orogenic) elevation and lithospheric structure, and paleoelevation information to estimate topographic development and structural support over time. Application of this technique to the southwestern United States indicates that the uplift of the Colorado Plateau is the result of processes in both the crust and mantle lithosphere and that the lithosphere of the pre-orogenic Southern Basin and Range was thinned relative to the Northern Basin and Range and Colorado Plateau. Although we use the southwestern U.S. as an example, this method can help constrain uplift mechanisms for any region for which the structure and geoid anomaly of the modern lithosphere is well understood. 相似文献
19.
With the aim of investigating the P-wave velocity structure below the Tertiary volcano Vogelsberg, a network of 10 mobile short period seismograph stations was installed in May 1987 for a period of 20 months. P-Wave travel time residuals relative to the station Kleiner Feldberg/Taunus (TNS) were determined for 168 seismic events using the Jeffreys - Bullen travel time tables. At all stations the relative residuals showed a positive sign, indicating a low velocity zone beneath the Vogelsberg. Maxima were found in the northern part of the Vogelsberg (station VAD +0.5 s) and in the region of the Amöneburger Basin (station RAU +0.28 s).The travel time residuals were inverted using the tomographic inversion method of Aki et al. (1977). The slowness perturbations of the single blocks were calculated relative to a crustal and upper mantle model of the Rhenish Massif. The results show an intracrustal low velocity body (about –9%) striking in a Variscan direction and underlying the north-eastern part of the Vogelsberg, and another velocity minimum (about – 6%) in the region of the Am6neburger Basin. In the lower crust and the upper mantle the velocities are reduced by about 4% relative to the starting model.The Variscan alignment of the low velocity zone under the Vogelsberg correlates with results of other geological studies. It can be assumed that during the rifting phase of the Upper Rhinegraben Variscan lineations have been reactivated, favouring uprising of magma along these old structures. The position and extension of the low velocity zone correlate with the assumed sediment distributions in the area of investigation. This may account for about one-half of the observed anomaly. The reason for the velocity reduction of about 4% in the entire underground region of the Vogelsberg down to a depth of about 70 km can be explained by the intensive fracturing of the lithosphere, caused by thermal and pressure gradients during the magma eruption process. 相似文献
20.
名义无水矿物(NAMs)中的结构水在地球内部演化中扮演了重要角色.应用红外光谱对产自5个构造背景下的13个二辉橄榄岩进行了详细的结构水测定,探讨不同构造背景下橄榄岩结构水的含量和赋存机制.研究显示,全岩和橄榄石结构水含量按构造背景从高到低排序,依次为超高压地体、地幔柱、稳定克拉通、俯冲带和活化克拉通,反映了橄榄岩的水含量与构造环境具有显著相关性.在相同构造背景下,石榴石二辉橄榄岩比尖晶石二辉橄榄岩含更多结构水,表明上地幔水分布可能具有分层性.超高压地体和稳定克拉通样品中橄榄石具有[Si]空位导致的吸收峰3 611~3 613 cm-1,而缺乏由[Fe3+]引起的吸收峰3 325 cm-1和3 355 cm-1,表明地幔的还原性随深度增强. 相似文献