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1.
We document strong seismic scattering from around the top of the mantle Transition Zone in all available high resolution explosion seismic profiles from Siberia and North America. This seismic reflectivity from around the 410 km discontinuity indicates the presence of pronounced heterogeneity in the depth interval between 320 and 450 km in the Earth’s mantle. We model the seismic observations by heterogeneity in the form of random seismic scatterers with typical scale lengths of kilometre size (10-40 km by 2-10 km) in a 100-140 km thick depth interval. The observed heterogeneity may be explained by changes in the depths to the α-β-γ spinel transformations caused by an unexpectedly high iron content at the top of the mantle Transition Zone. The phase transformation of pyroxenes into the garnet mineral majorite probably also contributes to the reflectivity, mainly below a depth of 400 km, whereas we find it unlikely that the presence of water or partial melt is the main cause of the observed strong seismic reflectivity. Subducted oceanic slabs that equilibrated at the top of the Transition Zone may also contribute to the observed reflectivity. If this is the main cause of the reflectivity, a substantial amount of young oceanic lithosphere has been subducted under Siberia and North America during their geologic evolution. Subducted slabs may have initiated metamorphic reactions in the original mantle rocks. 相似文献
2.
Mineral assemblages displayed by MORB and alkali-poor olivine tholeiites have been investigated over the pressure interval 4.6–18 GPa at 1200°C. Both compositions crystallize to form normal eclogites between 4.6 and 10 GPa and there is little change in the relative proportions of garnet and pyroxene over this range. However, the proportion of garnet increases rapidly above 10 GPa as pyroxene dissolves in the garnet structure and pyroxene-free garnetites (±stishovite) are produced by 14–15 GPa, dependent upon composition. The garnetite facies for both compositions possess zero-pressure densities of 3.75 g/cm3, implying that subducted oceanic crust remains appreciably denser than surrounding mantle to depths exceeding 600 km. It is demonstrated that the seismic velocity distributions in the mantle between 400 and 650 km are inconsistent with Anderson's hypothesis that this region is of eclogitic composition. 相似文献
3.
The mineralogy adopted by a depleted harzburgite composition has been studied over the pressure interval 5–26 GPa at temperatures of 1300–1400°C. The pyroxene-garnet component of the harzburgite composition (harzburgite minus 82 wt.% olivine) transforms to majorite garnet by 18–19 GPa, and further disproportionates to the assemblage of garnet + stishovite + Mg2SiO4 spinel above 20 GPa. At still higher pressures, first ilmenite (22–24 GPa) and then perovskite MgSiO3 (24–26 GPa) are found to coexist with garnet. Garnet disappears at 26 GPa and almost complete transition to perovskite is achieved at this pressure. The mineral proportions and density profiles in the subducting oceanic lithosphere, modelled by a combination of 80% harzburgite + 20% primitive MORB compositions are calculated as a function of depth under conditions isothermal with surrounding pyrolite mantle, and also for a temperature distribution in which the slab is substantially cooler than surrounding mantle to below 700 km. Under isothermal conditions, the slab has a density similar to surrounding mantle to a depth of 600 km. However, between 600 and 700 km, the slab is up to 0.08 g/cm3 denser than surrounding mantle. This is caused primarily by the higher alumina content in pyrolite as compared to harzburgite, which causes the transition to perovskite in pyrolite to occur at substantially higher pressures than in harzburgite. The presence of alumina also smears out the garnet-perovskite transition in pyrolite over a depth interval of 50 km, whereas this transformation is much sharper in the harzburgite composition. Calculations based on the observed phase equilibria also show that a subducted cool slab remains much denser (by 0.1–0.3 g/cm3) than surrounding mantle to a depth of 700 km but possesses a density similar to surrounding mantle below this depth. These results have important implications for the dynamical behaviour of slabs possessing different thermal regimes when they encounter the 670 km discontinuity and also for the nature of this discontinuity. 相似文献
4.
受俯冲残留体影响的410km间断面起伏形态的研究对于确定地球内部物质构成及地球动力学过程具有重要作用.帕米尔—兴都库什俯冲区域拥有全球少有的中、深源地震,为研究410km间断面起伏提供了良好的资源.利用日本Hi-net地震台网和美国TA台阵记录的帕米尔—兴都库什俯冲区域的6个震源深度为154.0~220.9km、震级为Mb5.6~6.4的中、深源地震的短周期/宽频带波形资料,经过4次根倾斜叠加处理,获得了36组Hi-net子台网和TA记录资料的倾斜叠加灰度图,从中提取了与410km间断面相关的次生转换震相SdP,发现受俯冲残留体影响下的410km间断面的深度位于372~398km.较之持续俯冲的西太平洋地区海洋岩石圈,研究区域俯冲滞留体对于410km间断面的相变线的影响要小得多. 相似文献
5.
伽师强震群的深部动力学条件 总被引：3，自引：1，他引：3
根据天山造山带及其两侧的塔里木盆地和准噶尔盆地的岩石圈二维速度结构、二维密度结构、二维电性结构、壳幔过渡带的详细结构以及大地热流和震源深度的分布，再结合对新疆西北部的蛇绿岩带、高压变质带和岩浆岩分布的综合分析，建立了天山造山带的地球动力学“层间插入消减”模型。该模型认为，塔里木板块的中上地壳在库尔勒断裂附近向天山造山带的中下地壳层间插入；而下地壳连同岩石圈地幔向天山造山带的上地幔俯冲消减。在天山的西段（哈萨克斯坦境内），费尔干纳地块由北向南插入到南天山之下约180km的深处，在其东段（中国境内），塔里木盆地由南向北插入南天山之下。这两个具有不同方向的下降板片的接触部位为费尔干纳走滑断裂。我国的伽师、喀什、乌恰地震区均落在这两个具有不同俯冲方向板块的结合部位附近，有着特殊的深部构造背景。南、北两大板块的双向挤压必定产生强大的应力，在地震区附近，这种应力的积累与释放具有4个显著的特点：（1）板片的俯冲消减速度约高达每年22mm左右；（2）应力的积累与释放速率加快；（3）应力释放较容易；（4）应力释放较为集中。这4个特点可能是伽师地区在较短的时间内连续发生数次强震的构造因素。 相似文献
6.
IntroductionGeologistsfirstlyfoundcoesite-bearingecologitesattheendof1980'sandthenthemicrodiamond(Xu,elal,1992)inDabieshanarea.Theultra-highpressure(UHP)metamorphismandthegeodynamicprocessesofDabieorogenhaveattractedmanygeoscientists(Wang,etal,1989,O... 相似文献
7.
Ample observational evidence shows that there is a northward crustal subduction zone underneath the Yarlung Zangbo suture between India and Eurasia. It penetrates Moho to a depth of about 100 km. There are probably multiple such crustal subductions under the Himalayas. They are different from lithosphere subduction during oceanic collisions. The detected slabs in the upper mantle north of the Yarlung Zangbo suture can be interpreted as remains of the Indian Plate’s mantle lithosphere. In contrary to ocean-continent subduction, the mantle lithosphere is delaminated from the crust as the Indian Plate subducts underneath Eurasia. Existing structural images of the crust and upper mantle of the Tibetan Plateau reveal that there were both northward and southward subductions over different geological periods, causing some seismic velocity anomalies around those subduction zones. 相似文献
8.
Catherine Chauvel Albrecht W. Hofmann Philippe Vidal 《Earth and Planetary Science Letters》1992,110(1-4)
himu, em i andem ii are three of the main geochemical mantle components that give rise to oceanic island basalts [1]. They represent the end members that produce the extreme isotopic compositions measured on intraplate volcanics. In French Polynesia, all three mantle components are represented in volcanic rocks. The characteristichimu signature is found in Tubuai, Mangaia and Rurutu,em i is present in the source of Rarotonga and Pitcairn volcanics andem ii dominates the composition of most Society Islands. Intermediate values between the three end members are found on most islands.We suggest that the three components are not independent but are physically related in the mantle. Thehimu component is thought to be recycled oceanic crust that lost part of its Pb through hydrothermal processes prior to and during subduction.em i andem ii are believed to acquire their isotopic and trace element characteristics through entrainment of sediments that were subducted together with the oceanic crust.The trace element pattern and the isotopic composition ofhimu lavas can be quantitatively modelled using a mixture of 25% old recycledmorb crust and 75% mantle peridotite. The extreme Pb composition is modelled assuming that Pb was lost from oceanic crust when hydrothermal alteration at the ridge leached Pb from the basalt to redeposit it as sulphides on top of and throughout the crust, followed by preferential dissolution of sulphides during dehydration in the subduction zone. These processes led to a drastic increase of theU/Pb ratio of the subducted material which evolved over 2 Ga to very radiogenic Pb isotopic compositions. Pb isotopic compositions similar to those ofem i andem ii are modelled assuming that sediments with average crustal Pb isotopic compositions were subducted and recycled into the mantle together with the underlyingmorb oceanic crust. Pelagic sediments (μ 5 andκ 6) account for the Pb isotopic composition ofem i whereas terrigenous sediments (μ 10 andκ 4.5) evolve towards theem ii end member. A few percent of sediment in the recycled crust-sediment mixture will destroy the characteristic Pb isotopic signature of thehimu component. This, together with the low probability of isolating oceanic crust in the mantle for 2 Ga, explains why the extremehimu composition, as seen on Tubuai and St Helena, is sampled so rarely by oceanic volcanism. 相似文献
9.
南海处于欧亚板块、 菲律宾海板块、 太平洋板块和印度-澳大利亚板块的交汇处, 其地质和构造作用十分复杂．通过面波群速度成像, 给出了南海及邻区的三维横波速度分布并分析了其地球动力学意义．南海西部和南部新布设的地震台站使得利用单台法时路径覆盖比过去更好. 特别是在华南地区, 新的台站分布能够弥补该地区地震少且台站少造成的射线密度不够的缺点． 首先运用多重滤波法得到南海周边48个台站周期为14——130 s范围内的基阶瑞雷波频散曲线图; 接着通过子空间反演得到整个区域在不同周期时的群速度分布; 最后通过阻尼最小二乘反演得到不同深度切片上的横波速度分布及不同纵剖面上的横波速度分布． 结果显示: ① 海盆速度较高, 且速度分布很好地勾勒出海盆的轮廓. 浅层较高的横波速度说明海盆都具有洋壳性质, 而深部较高的横波速度则可能对应扩张中心生成洋壳后残留的高速物质． 不同海盆速度上的差异与它们的热流值和年龄大小一致．海盆下的高速异常在60 km以下消失, 且在一定深度范围内由低速区替代． 在低速区下200 km深度, 在南海海盆观测到一条NE-SW走向的高速异常, 可能与古俯冲带有关. ② 环南海出现明显的高速区, 对应俯冲带特征, 且这些高速区速度差异明显且有间断, 说明俯冲带的非均质性和俯冲角度的差异. ③ 在环南海高速区内侧（向南海侧）观测到不连续的低速区． 在浅层, 这些低速区反映了沉积层和地壳的厚度特征. 在地幔, 这些低速区可能对应于古太平洋俯冲带的地幔楔或者也可能反映了南海海盆停止扩张后残留的地幔熔融物质. ④ 南海海盆岩石圈的厚度为60——85 km． 相似文献
10.
Introduction Bohai Bay, along with its adjacent areas, is one of the seismically active areas in North China. Understanding its crust/upper-mantle structural characteristics and lateral heterogeneity of the medium in this area is of great significance to the study of seismogenic environment, thus improvimg the level of earthquake prediction. For years, scientists have studied the area by gravity and magnetic methods (FENG, et al, 1989), geothermal field (WU, et al, 1988; TIAN, ZHANG, 19… 相似文献
11.
A new model is proposed for the structure of the Kaapvaal craton lithosphere. Based on chemical thermodynamics methods, profiles of the chemical composition, temperature, density, and S wave velocities are constructed for depths of 100–300 km. A solid-state zone of lower velocities is discovered on the S velocity profile in the depth interval 150–260 km. The temperature profiles are obtained from absolute values of P and S velocities, taking into account phase transformations, anharmonicity, and anelastic effects. The examination of the sensitivity of seismic models to the chemical composition showed that relatively small variations in the composition of South African xenoliths result in lateral temperature variations of ～200°C. Inversion of some seismic profiles (including IASP91) with a fixed bulk composition of garnet peridotites (the primitive mantle material) leads to a temperature inversion at depths of 200–250 km, which is physically meaningless. It is supposed that the temperature inversion can be removed by gradual fertilization of the mantle with depth. In this case, the craton lithosphere should be stratified in chemical composition. The depleted lithosphere composed by garnet peridotites exists to depths of 175–200 km. The lithospheric material at depths of 200–250 km is enriched in basaltoid components (FeO, Al2O3, and CaO) as compared with the material of garnet peridotites but is depleted in the same components as compared with the fertile substance of the underlying primitive mantle. The material composing the craton root at a depth of ～275 km does not differ in its physical and chemical characteristics from the composition of the normal mantle, and this allows one to estimate the thickness of the lithosphere at 275 km. The results of this work are compared with data of seismology, thermal investigations, and thermobarometry. 相似文献
12.
Introduction Major tectonic activities occur in collisions zones between plates or intra-plate continental blocks. Therefore, it is significant to investigate collision processes. We know that orogenic and seismic belts in plate margins are closely relate… 相似文献
13.
本文利用重力和地震P波到时数据反演得到了中国东北地区地壳上地幔三维密度结构.与单一的重力或地震反演相比,重震反演一方面有效地克服了重力反演结果垂向分辨率低的问题,另一方面也提高了地震反演结果的可靠性.结果显示:中国东北地区的地壳及上地幔剩余密度异常分布与构造单元具有明显的相关性,造山带对应低密度异常,盆地对应高密度异常;区域内火山下方有明显的低密度体存在,可能是由于太平洋板块俯冲进入上地幔并部分滞留,在滞留板块深部脱水和软流圈热物质共同作用下产生了上涌岩浆,喷发后形成了火山. 相似文献
14.
Takeshi Tsuji Yasuyuki Nakamura Hidekazu Tokuyama Millard F. Coffin Keita Koda 《Island Arc》2007,16(3):361-373
Abstract To show the structure of oceanic crust and Moho around the eastern Ogasawara Plateau, we have analyzed industry-standard two-dimensional multichannel seismic reflection data. To obtain improved velocity models, phase information of seismic signals was used for velocity analysis and velocity models for oceanic crust above Moho were determined. We apply this velocity analysis technique to seismic reflection data around the eastern Ogasawara Plateau, with the result of clear images of structures within oceanic crust and Moho. South of the Ogasawara Plateau, Moho deepens proximal to the Plateau. Moho distal to the Plateau is ca 7 km below sea floor (bsf), whereas it is ca 10 km bsf near the Plateau. The characters of oceanic crust and Moho differ significantly north and south of the Plateau. To the north, the structure of oceanic crust is ambiguous, the sea floor is shallower and less smooth, and Moho is discontinuous. To the south, structures within oceanic crust and Moho are imaged clearly, and the sea floor is deeper. A strong Moho reflection south of the Plateau might represent a sharp boundary between layered gabbro and peridotite. However, discontinuous Moho reflections north of the Plateau might represent rough topography because of intensive magmatism or a gradual downward increase in velocity within a thick Moho transition zone. A fracture zone north of the Plateau also appears to separate oceanic crust and Moho of different characters, suggesting vigorous magmatism between the Plateau and the fracture zone, and that the Ogasawara Plateau and the fracture zone influenced the genesis of oceanic crust and upper mantle. Differences in acoustic characteristics to the north and south of the Plateau are apparent in profiles illuminated by seismic attributes. 相似文献
15.
Eiji Ohtani 《Earth and Planetary Science Letters》1984,67(2):261-272
The densities of silicate liquids with basic, picritic, and ultrabasic compositions have been estimated from the melting curves of minerals at high pressures. Silicate liquids generated by partial melting of the upper mantle are denser than olivine and pyroxenes at pressures higher than 70 kbar, and garnet is the only phase which is denser than the liquid at pressures from 70 kbar to at least 170 kbar. In this pressure range, garnet and some fraction of liquid separate from ascending partially molten diapirs. It is therefore suggested that aluminium-depleted komatiite with a high Ca/OAl2O3 ratio may be derived from diapirs which originated in the deep upper mantle at pressures from 70 kbar to at least 140 kbar (200–400 km in depth), where selective separation of pyropic garnet occurs effectively. On the other hand, aluminium-undepleted komatiite is probably derived from diapirs originating at shallower depths (< 200 km). Enrichment of pyropic garnet is expected at depths greater than 200 km by selective separation of garnet from ascending diapirs. The 200-km discontinuity in the seismic wave velocity profile may be explained by a relatively high concentration of pyropic garnet at depths greater than 200 km. 相似文献
16.
A self-consistent approach is proposed for the investigation of the thermal conditions, chemical composition, and internal structure of the upper mantle of the Earth. Using this approach, the thermal state of the lithospheric mantle beneath the Siberian Craton (SC) is reconstructed from P velocities, taking into account the phase transitions, anharmonicity, and the effects of anelasticity. The velocities of seismic waves are more sensitive to temperature than to the composition of the mantle rocks, which allows the velocity models to be effectively used for reconstruction of the thermal regime of the mantle. The temperature at depths 100–300 km is reconstructed by inversion of the Kraton and Kimberlit superlong seismic profiles for compositions of the garnet harzburgite, lherzolite, and intermediate composition of garnet peridotite. The averaged temperature in the normal continental mantle is reconstructed by inversion of the IASP91 reference model for depleted and fertile substance. One-dimensional models and two-dimensional thermal fields undergo a substantial fall in temperature (～300–600°C) beneath the Siberian Craton as compared to the temperatures of the continental mantle and paleotemperatures inferred from the thermobarometry of xenoliths. Temperature profiles of the Siberian Craton deduced from seismic data lie between the conductive geotherms of 32.5–40.0 mW/m2 and below the P(H)-T values obtained for low- and high-temperature xenoliths from the Mir, Udachnaya, and Obnazhennaya kimberlite pipes. The thickness of the thermal lithosphere estimated from the intersection with the potential adiabat is 300–320 km, which is consistent with the data on heat flows and seismotomographic observations. This provides grounds for the assumption that the low-temperature anomalies (thermal roots of continents) penetrate down to a depth of 300 km. The analysis of the sensitivity of seismic velocity and density to the variations in temperature, pressure, and chemical and phase composition of petrological models shows that recognition of fine differences in chemical composition of the lithospheric rocks by seismic methods is impossible. 相似文献
17.
The results of a controlled source seismic reflection–refraction experiment carried out in 1992 reveal the following characteristics of the northern Izu–Bonin (Ogasawara) oceanic island arc–trench system. (1) The crust rapidly thickens from the Shikoku back-arc basin to the arc, is thickest beneath the active rifts, and then gradually thins to the forearc. The thickness of the crust beneath the arc rift zone and the back-arc basin are ∼ 20 km and 8 km, respectively. (2) The Moho vanishes beneath the forearc. Velocities rapidly decrease eastwards beneath the inner trench wall. (3) The velocity of the lower crust of the arc and the back-arc basin is 7.1–7.3 km/s. This velocity is higher than the typical oceanic lower crust whose velocity is ∼ 6.7 km/s. (4) The velocity of the middle crust of the arc is ∼ 6 km/s. This layer does not exist beneath the back-arc basin. (5) A slight difference in the velocity gradient of the middle crust exists between the arc rift zone and the forearc. Based on these findings and previous studies, it is inferred that: (i) the middle crust is probably granitic rock and formed in more than two episodes; (ii) the lower crust formed by igneous underplating which may also have affected part of the back-arc basin; and (iii) the root of the serpentinite diapir on the inner trench wall is a low-velocity mantle wedge that was probably caused by large amounts of water released from the subducting Pacific plate at depths shallower than 30 km. 相似文献
18.
Sean C. Solomon 《Physics of the Earth and Planetary Interiors》1975,11(2):97-108
The top of the olivine-spinel phase change in subducted oceanic lithosphere can be located by the travel times of seismic waves which have propagated through the slab. P-wave travel-time residuals from deep earthquakes in the Tonga island are observed at Australian seismic stations are grouped according to the depth of the earthquake. The change in mean residual with a change in earthquake depth is related to the velocity contrast between slab and normal mantle at that depth. The curve mean residual versus earthquake depth displays a region of markedly increased slope between earthquake depths of about 250 and 350 km. The most probable explanation of this observation is an elevation by 100 km of the olivine-spinel phase change within the relatively cooler slab. No evidence was found for vertical displacements within the slab of any deeper phase changes.A temperature contrast between slab and normal mantle of about 1,000°C at 250 km depth is implied. This finding confirms current thermal models for subducted lithosphere but is inconsistent with the global intraplate stress field unless only a few percent of the negative buoyancy force at subduction zones is transmitted to the surface plates. 相似文献
19.
Xiaobin Cao Huiming Bao Caihong Gao Yun Liu Fang Huang Yongbo Peng Yining Zhang 《Acta Geochimica》2019,(3):327-334
Understanding the origin of ocean island basalts(OIB) has important bearings on Earth's deep mantle.Although it is widely accepted that subducted oceanic crust, as a consequence of plate tectonics, contributes material to OIB's formation, its exact fraction in OIB's mantle source remains ambiguous largely due to uncertainties associated with existing geochemical proxies. Here we show, through theoretical calculation, that unlike many known proxies, triple oxygen isotope compositions(i.e.D^(17 )O) in olivine samples are not affected by crystallization and partial melting. This unique feature, therefore, allows olivine D^(17 )O values to identify subducted oceanic crusts in OIB's mantle source. Furthermore, the fractions of subducted ocean sediments and hydrothermally altered oceanic crust in OIB's mantle source can be quantified using their characteristic D^(17 )O values. Based on published D^(17 )O data, we estimated the fraction of subducted oceanic crust to be as high as 22.3% in certain OIB, but the affected region in the respective mantle plume is likely to be limited. 相似文献
20.
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. 相似文献