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1.
The evolution of oceanic crust on the Kolbeinsey Ridge, north of Iceland, is discussed on the basis of a crustal transect obtained by seismic experiment from the Kolbeinsey Ridge to the Jan Mayen Basin. The crustal model indicates a relatively uniform structure; no significant lateral velocity variations are observed, especially in the lower crust. The uniform velocity structure suggests that the postulated extinct axis does not exist over the oceanic crust formed at the Kolbeinsey Ridge, but supports a model of continuous spreading along the ridge after oceanic spreading started west of the Jan Mayen Basin. The oceanic crust formed at Kolbeinsey Ridge is 1–2.5 km thicker than normal oceanic crust due to hotter-than-normal mantle from the Iceland Mantle Plume. The observed generally uniform thickness throughout the transect might also indicate that the temperatures of the astheno-spheric mantle ascending along the Kolbeinsey Ridge have not changed significantly since the age of magnetic anomaly 6B. 相似文献
2.
A.V. Jakovlev N.A. Bushenkova I.Yu. Koulakov N.L. Dobretsov 《Russian Geology and Geophysics》2012,53(10):963-971
We present a new three-dimensional model of P-velocity anomalies in the upper mantle beneath the Circum-Arctic region based on tomographic inversion of global data from the catalogues of the International Seismological Center (ISC, 2007). We used travel times of seismic waves from events located in the study area which were recorded by the worldwide network, as well as data from remote events registered by stations in the study region. The obtained mantle seismic anomalies clearly correlate with the main lithosphere structures in the Circum-Arctic region. High-velocity anomalies down to 250–300 km depth correspond to Precambrian thick lithosphere plates, such as the East European Platform with the adjacent shelf areas, Siberian Plate, Canadian Shield, and Greenland. It should be noted that lithosphere beneath the central part of Greenland appears to be strongly thinned, which can be explained by the effect of the Iceland plume which passed under Greenland 50–60 million years ago. Beneath Chukotka, Yakutia, and Alaska we observe low-velocity anomalies which represent weak and relatively thin actively deformed lithosphere. Some of these low-velocity areas coincide with manifestations of Cenozoic volcanism. A high-velocity anomaly at 500–700 km depth beneath Chukotka may be a relic of the subduction zone which occurred here about 100 million years ago. In the oceanic areas, the tomography results are strongly inhomogeneous. Beneath the North Atlantic, we observe very strong low-velocity anomalies which indicate an important role of the Iceland plume and active rifting in the opening of the oceanic basin. On the contrary, beneath the central part of the Arctic Ocean, no significant anomalies are observed, which implies a passive character of rifting. 相似文献
3.
Plume–ridge interaction in the Reykjanes Ridge and Icelandregion is graphically demonstrated by several V-shaped ridgessurrounding the spreading axis, indicating mantle flow awayfrom Iceland. It also has significant geochemical effects. Regionally,incompatible element concentrations increase northwards coincidingwith decreasing depth and increasing crustal thickness, depthof melting and proximity to Iceland. Major and trace elementdata show that isolated magma bodies feed individual volcanicsystems along the ridge. Fractionation within these systemsincreases towards 60–61°N, where it coincides withthe intersection of a V-shaped ridge, thicker crust and moreabundant seamounts. Trace element, Nd and Sr isotopic data revealdynamic melting and mixing within a southward-thinning, heterogeneousmantle wedge beneath the Reykjanes Ridge. Melting is variableand locally enhanced at 58°N, 59°N, 60°N and 61°N.A total of six mantle components are identified. Some are specificto Iceland whereas others are found only beneath the ridge axis.The geographical distribution of these components reflects theirorigin within the deep upper and lower mantle and subsequenttranslation by plume outflow along the entire length of theridge. KEY WORDS: plume–ridge interaction; Iceland; Reykjanes Ridge; dynamic mantle mixing and melting 相似文献
4.
K. R. Richardson J. R. Smallwood R. S. White D. B. Snyder P. K. H. Maguire 《Tectonophysics》1998,300(1-4):159-180
We have mapped the transition from the continental Faroe block (the Faroe Islands and surrounding shelf) to the thickened oceanic crust of the Faroe–Iceland Ridge in the North Atlantic using the results of a detailed sea-to-land seismic profile with wide-angle to normal-incidence recordings of explosive and airgun shots fired at sea along the Faroe–Iceland Ridge. Interpretation of all available seismic and gravity data indicates that this aseismic ridge is composed of 30±3-km-thick oceanic crust, with a gradual transition to ancient continental crust from 100 to 40 km northwest of the Faroe Islands, close to the shelf edge. This confirms that the crust beneath the Faroe Islands, which may be up to 46 km thick, comprises continental material in agreement with previous seismic and geochemical results. Results suggest that the upper 5.2±0.7 km of the Faroe crust consists of Tertiary basalts generated during continental breakup, overlying the continental crust beneath. The lower crust, where seismic constraint is poor, may exhibit high seismic velocities (7.1–7.6 km s−1) which we attribute to underplating or intrusion by mafic melts during continental breakup in the early Tertiary. 相似文献
5.
Karsten M. Haase Colin W. Devey Martin Wieneke 《Contributions to Mineralogy and Petrology》2003,144(4):428-448
We present new data on mineralogical, major and trace element compositions of lavas from the northernmost segment of the Kolbeinsey Ridge (North Kolbeinsey Ridge, NKR). The incompatible element enriched North Kolbeinsey basalts lie on a crystal fractionation trend which differs from that of the other Kolbeinsey segments, most likely due to higher water contents (~0.2%) in the NKR basalts. The most evolved NKR magmas erupt close to the Jan Mayen Fracture Zone, implying increased cooling and fractionation of the ascending magmas. Mainly incompatible element-enriched basalts, as well as some slightly depleted lavas, erupt on the NKR. They show evidence for mixing between different mantle sources and magma mixing. North Kolbeinsey Ridge magmas probably formed by similar degrees of melting to other Kolbeinsey basalts, implying that no lateral variation in mantle potential temperature occurs on the spreading axis north of the Iceland plume and that the Jan Mayen Fracture Zone does not have a cooling effect on the mantle. Residual garnet from deep melting in garnet peridotite or from enriched garnet pyroxenite veins does not play a role. The incompatible element-enriched source has high Ba/La and Nb/Zr, but must be depleted in iron. The iron-depleted mantle is less dense than surrounding mantle and leads to the formation of the North Kolbeinsey segment and its shallow bathymetry. The enriched NKR source formed from a relatively refractory mantle, enriched by a small degree melt rather than by recycling of enriched basaltic crust. The depleted mantle source resembles the mantle of the Middle Kolbeinsey segment with a depletion in incompatible elements, but a fertile major element composition. 相似文献
6.
Seismic images under 60 hotspots: Search for mantle plumes 总被引：10，自引：0，他引：10
The mantle plume hypothesis is now widely known to explain hotspot volcanoes, but direct evidence for actual plumes is weak, and seismic images are available for only a few hotspots. In this work, we present whole-mantle tomographic images under 60 major hotspots on Earth. The lateral resolution of the tomographic images is about 300 km under the continental hotspots and 400–600 under the oceanic hotspots. Twelve plume-like, continuous low-velocity (low-V) anomalies in both the upper and lower mantle are visible under Hawaii, Tahiti, Louisville, Iceland, Cape Verde, Reunion, Kerguelen, Amsterdam, Afar, Eifel, Hainan, and Cobb hotspots, suggesting that they may be 12 whole-mantle plumes originating from the core–mantle boundary (CMB). Clear upper-mantle low-V anomalies are visible under Easter, Azores, Vema, East Australia, and Erebus hotspots, which may be 5 upper-mantle plumes. A mid-mantle plume may exist under the San Felix hotspot. The active intra-plate volcanoes in Northeast Asia (e.g., Changbai, Wudalianchi, etc.) are related to the stagnant Pacific slab in the mantle transition zone. The Tengchong volcano in Southwest China is related to the subduction of the Burma microplate under the Eurasian plate. Although low-V anomalies are generally visible in some depth range in the mantle under other hotspots, their plume features are not clear, and their origins are still unknown. The 12 whole-mantle plumes show tilted images, suggesting that plumes are not fixed in the mantle but can be deflected by the mantle flow. In most cases, the seismic images under the hotspots are complex, particularly around the mantle transition zone. A thin low-V layer is visible right beneath the 660-km discontinuity under some hotspots, while under a few other hotspots, low-V anomalies spread laterally just above the 660-km discontinuity. These may reflect ponding of plume materials in the top part of the lower mantle or the bottom of the upper mantle. The variety of behaviors of the low-V anomalies under hotspots reflects strong lateral variations in temperature and viscosity of the mantle, which control the generation and ascending of mantle plumes as well as the flow pattern of mantle convection. 相似文献
7.
Intraplate volcanism during the Late Cenozoic in the Leiqiong area of southernmost China, with basaltic lava flows covering a total of more than 7000 km2, has been attributed to an underlying Hainan plume. To clarify detailed features of the Hainan plume, such as the morphology of its magmatic conduits, the depth of its magmatic pool in the upper mantle and the pattern of mantle upwelling, we determined tomographic images of the mantle down to a depth of 1100 km beneath southern China using 18,503 high-quality arrival-time data of 392 distant earthquakes recorded by a dense seismic array. Our results show a mushroom-like continuous low-velocity anomaly characterized by a columnar tail with a diameter of 200–300 km extending down to the lower mantle beneath north of the Hainan hotspot and a head spreading laterally in and around the mantle transition zone, indicating a magmatic pool in the upper mantle. Further upward, the plume head is decomposed into smaller patches, and when reaching the base of the lithosphere, a pancake-like anomaly has formed to feed the Hainan hotspot. This result challenges the classical model of a fixed thermal plume that rises vertically to the surface. Hence we propose a new layering-style model for the magmatic upwelling of the Hainan plume. Our results indicate spatial complexities and variations of mantle plumes probably due to heterogeneous compositions and thermochemical structures of the deep mantle. 相似文献
8.
We present a new three-dimensional SV-wave velocity model for the upper mantle beneath South America and the surrounding oceans, built from the waveform inversion of 5850 Rayleigh wave seismograms. The dense path coverage and the use of higher modes to supplement the fundamental mode of surface waves allow us to constrain seismic heterogeneities with horizontal wavelengths of a few hundred kilometres in the uppermost 400 km of the mantle.The large scale features of our tomographic model confirm previous results from global and regional tomographic studies (e.g. the depth extent of the high velocity cratonic roots down to about 200–250 km).Several new features are highlighted in our model. Down to 100 km depth, the high velocity lid beneath the Amazonian craton is separated in two parts associated with the Guyana and Guapore shields, suggesting that the rifting episode responsible for the formation of the Amazon basin has involved a significant part of the lithosphere. Along the Andean subduction belt, the structure of the high velocity anomaly associated with the sudbduction of the Nazca plate beneath the South American plate reflects the along-strike variation in dip of the subducting plate. Slow velocities are observed down to about 100 km and 150 km at the intersection of the Carnegie and Chile ridges with the continent and are likely to represent the thermal anomalies associated with the subducted ridges. These lowered velocities might correspond to zones of weakness in the subducted plate and may have led to the formation of “slab windows” developed through unzipping of the subducted ridges; these windows might accommodate a transfer of asthenospheric mantle from the Pacific to the Atlantic ocean. From 150 to 250 km depth, the subducting Nazca plate is associated with high seismic velocities between 5°S and 37°S. We find high seismic velocities beneath the Paraná basin down to about 200 km depth, underlain by a low velocity anomaly in the depth range 200–400 km located beneath the Ponta Grossa arc at the southern tip of the basin. This high velocity anomaly is located southward of a narrow S-wave low velocity structure observed between 200 and 500–600 km depth in body wave studies, but irresolvable with our long period datasets. Both anomalies point to a model in which several, possibly diachronous, plumes have risen to the surface to generate the Paraná large igneous province (LIP). 相似文献
9.
Fluid dynamical simulations were carried out in order to investigate the effect of the large-scale mantle flow field and the depth of the plume source on the structure of the Iceland plume through time. The time-dependent location and shape of the plume in the Earth's mantle was calculated in a global model and it was refined in the upper mantle using a 3D Cartesian model box. Global flow was computed based on density heterogeneities derived from seismic tomography. Plate motion history served as a velocity boundary condition in both models. Hotspot tracks of the plume conduits and the plume head were calculated and compared to actual bathymetry of the North Atlantic. If a plume source in the lowermost mantle is assumed, the calculated surface position of the plume conduit has a southward component of motion due to southward flow in the lower mantle. Depending on tomography model, assumed plume age and buoyancy the southward component is more or less dominating. Plume models having a source at the 660 km discontinuity are only influenced by flow in the upper mantle and transition zone and hence rather yield westward hotspot motion. Many whole-mantle plume models result in a V-shaped track, which does not match the straight Greenland–Iceland–Faroe ridge. Models without strong southward motion, such as for a plume source at 660 km depth, match actual bathymetry better. Plume tracks were calculated from both plume conduits and plume heads. A plume head of 120 K anomalous temperature gives the best match between plume head track and bathymetry. 相似文献
10.
《International Geology Review》2012,54(10):1213-1225
P- and S-wave tomography of the upper mantle beneath the Cape Verde hotspot is determined using arrival-time data measured precisely from three-component seismograms of 106 distant earthquakes recorded by a local seismic network. Our results show a prominent low-velocity anomaly imaged as a continuous column <100 km wide from the uppermost mantle down to about 500 km beneath Cape Verde, especially below the Fogo active volcano, which erupted in 1995. The low-velocity anomaly may reflect a hot mantle plume feeding the Cape Verde hotspot. 相似文献
11.
From April to July 2002 we carried out a deployment of 6 ocean bottom seismometers and 4 ocean bottom hydrophones in the North Atlantic south of Iceland. During the deployment period we recorded clear Rayleigh waves from 2 regional and 14 teleseismic earthquakes. This corresponds to a Rayleigh wave detection rate of nearly 92% for events with MW ≥ 6.06.0 and epicentral distance less than 110°, close to detection rate estimates based on noise level variability. We measured Rayleigh wave event-station group dispersion and inter-station phase dispersion for one Mid-Atlantic Ridge event. The group dispersion curve is sensitive to the structure of the North-East Atlantic with an average age of 39 Myr. The phase dispersion curve is sensitive to the structure just south of Iceland (average plate age 33 Myr). Both dispersion curves indicate faster velocities than previously postulated for oceanic plate generated at the Reykjanes Ridge. A grid search approach was used to constrain the range of models fitting the data. The high velocity seismic lid just south of Iceland in the model for the phase dispersion path is slower or thinner than in the group dispersion model, which averages over a larger area and a somewhat older plate age, but the velocities in the low velocity half space are similar. We further consider the residual bathymetry in the experimental area. The residual anomaly decreases by 300–400 m from the Reykjanes Ridge to the 30 Myr old plate south of Iceland. This decrease can be explained by the disappearance of a mantle thermal anomaly associated with the Iceland plume. Both the residual bathymetry and the surface wave data are thus consistent with the notion that the southward spreading of the Icelandic plume is channelised underneath the Reykjanes Ridge and does not spread far outside this channel.Based on the experience from the pilot experiment, we estimate that a minimum recording time of 13–15 months in favourable weather conditions (April–September) is required to record enough data to map the spreading plume with surface waves, and to produce a tomographic image to a depth of 1000 km using body waves. This can be achieved by a continuous deployment of at least 20 months, or by two or three deployments during the spring and summer of consecutive years. 相似文献
12.
ABSTRACTNew analysis of the geophysical data of the ultraslow-spreading Mohns Ridge and its off-axis structure reveals a distinctive asymmetric structure. We calculate residual bathymetry (RB) and residual mantle Bouguer gravity anomaly (RMBA) and decompose the anomalies into symmetric and asymmetric components between the ridge conjugates. The western flank of the Mohns Ridge at crustal age of ~50–15 Ma is characterized by a broad zone of elevated RB and more negative RMBA, which we term the Vesteris Plateau (VP). The VP anomaly has a surface area of ~1.12 × 105 km2 and an excess crust volume of ~2.33 × 105 km3, making it a significant anomaly comparable to other anomalies such as the Bermuda Rise. Extending north of the Kolbeinsey Ridge for more than 500 km, the VP lies above an anomalous upper mantle region of low shear-wave seismic velocity, indicating that the VP might represent the northernmost reach of the Iceland-Jan Mayen mantle anomaly. In addition, the western ridge flank of the Mohns Ridge at crustal age of 6–0 Ma is associated with higher RB and more positive RMBA relative to the eastern conjugate, indicating tectonic uplift and associated exposure of lower crust and upper mantle near the ridge axis. 相似文献
13.
Major hypotheses on the formation of the Iceland region are considered. It is noted that plate- and plume-tectonic genesis
is the most substantiated hypothesis for this region. Model estimations of the effect of hot plume on the formation of genetically
different oceanic ridges are obtained. Computer calculations are performed for the thermal subsidence rate of aseismic ridges
(Ninetyeast and Hawaiian-Emperor) in the asthenosphere of the Indian and Pacific oceans. Comparative analysis of the calculated
subsidence rates of these ridges with those in the Iceland region (Reykjanes and Kolbeinsey ridges) is performed. The results
suggest that the thermophysical processes of formation of the spreading Reykjanes and Kolbeinsey ridges were similar to those
of the aseismic Ninetyeast and Hawaiian-Emperor ridges: the genesis of all these ridges is related to the functioning of a
hotspot. Analysis of the heat flux distribution in the Iceland Island and Hawaiian Rise areas is carried out. Analysis and
numerical calculations indicate that the genesis of Iceland was initially characterized by the plume-tectonic transformation
of a continental rather than oceanic lithosphere. The level of geothermal regime near Iceland was two times higher (100 mW/m2) relative to the Hawaiian Rise area (50 mW/m2) because the average lithosphere thickness of the Reykjanes and Kolbeinsey ridges near the Iceland was approximately two
times less (40 km) relative to the thickness of the Pacific Plate (80 km) in the Hawaiian area. The main stages of evolution
of the Iceland region are based on geological and geothermal data and numerical thermophysical modeling. The Cenozoic tectonic
evolution of the region is considered. Paleogeodynamic reconstructions of the North Atlantic in the hotspot system at 60,
50, and 20 Ma are obtained. 相似文献
14.
海南地幔柱与南海形成演化 总被引：15，自引：0，他引：15
东南亚上地幔地震层析成像表明,海南岛周围之下存在地幔柱,近垂直的低波速柱体位于海南岛及南海之下,从浅部向下穿越660 km的不连续面处（上下地幔的分界面）并一直延伸到1 900 km。南海及周边地区包括雷琼半岛、海南岛、北部湾盆地、广西北海涠洲岛、以及中南半岛等地,分布有一定量的新生代碱性玄武岩,它们的地球化学数据显示出OIB的特点并具有DUPAL异常,表明其源区较深。此外,由南海新生代碱性玄武岩中的橄榄石-流体平衡所推导的南海底地幔潜在温度（ 1 661℃）位于夏威夷（1 688℃）与冰岛热点（1 637℃）相应值之间,为海南岛地幔柱的存在提供了岩石学及矿物化学方面的约束。基于以上地球物理学、地球化学及矿物化学方面的证据,结合数字模拟实验资料,表明在海南岛及邻近区域之下存在地幔柱。建立了一个南海形成演化的初步模型：（1）50~32 Ma,印度洋板块-欧亚板块碰撞及其所导致的太平洋板块后退的综合效应为南海地区提供了一个伸展环境,进而为地幔柱物质的上升提供了通道;（2）32~21 Ma,当地幔柱柱头到达软流圈时, 由于侧向物质流与扩张中心发生相互作用,促进了南海的扩张,并在26~24 Ma期间发生了洋脊重新就位,使扩张中心从原来的18°N附近（即现今西北海盆的中心）调整到15.5°N附近（即现今的东部亚盆）;（3）21~15.5 Ma, 随着地幔柱效应的逐渐增强,热点-洋脊相互作用越来越强烈,在大约21 Ma发生了洋脊的再次重新就位事件,诱发了西南海盆的扩张;（4）15.5 Ma~现在,由于印澳板块前缘与巽他大陆碰撞,使得南海大约在15.5 Ma停止扩张,并沿着南沙海槽及吕宋海沟向菲律宾岛弧及巴拉望地块之下俯冲,而南海热点继续活动,直到第四纪还有碱性玄武岩喷出
地表。 相似文献
15.
Three-dimensional seismic mapping of the upper mantle beneath Fennoscandia (Baltic Shield) using an ACH-type of inversion technique in combination with P-wave travel-time residual observations from the local seismograph network gave the following results. The central parts of the Baltic Shield are characterized by relatively high seismic velocities down to approximately 300 km. Those parts of the shield most affected by the Caledonide orogeny exhibit relatively low velocities particularly in the uppermost 100 km depth interval. The lower part of the upper mantle (300–600 km) does not exhibit pronounced seismic velocity anomalies and in this respect is in contrast to results from similar studies in regions subjected to neotectonic processes like parts of central and southeastern Europe. The seismic anomaly pattern in the presumed thickened lithosphere is in quantitative agreement with similar ones derived from surface wave dispersion analysis and inversion of electrical measurements. The general orientation of these anomalies coincides with that of the glacial uplift. 相似文献
16.
Teleseismic P arrivals at seismological stations are inverted into a model of velocity perturbations down to a depth of about 470 km. Directionally independent average residuals, computed from steeply inciding waves, are transformed into a model of lithospheric thickness. Both models show a good correspondence with the main tectonic features of the Italian Peninsula. Positive velocity perturbations are observed beneath the Alps and in depths over 200 km also beneath the Po Basin. A high-velocity anomaly of the Tyrrhenian subduction is less pronounced, probably due to a directional dependence of P velocities in the mantle. Negative velocity perturbations indicate several low-velocity regions, e.g. beneath the Northern Apennines, the Sicily region and in the upper 100 km beneath the Po Basin. The amplitudes of velocity perturbations beneath the depth of 200 km are smaller on the average than those in the upper two layers. The whole region is characterized by large undulations of the lithosphere base which reaches depths from less than 60 km to more than 150 km. The most prominent lithospheric root beneath the Alps is a product of the collision between the European and the Adriatic plates while the lithospheric thickening beneath the Calabrian coast is likely to be connected with the eastern wing of the Tyrrhenian subduction. The dramatic changes of lithosphere thickness between the northern and the southern Apenninic arcs and northern Calabria as well as the thinnings at the western closure of the Po Basin, indicate important deep-seated boundaries of lithospheric blocks of autonomous geodynamic development. 相似文献
17.
Else-Ragnhild Neumann Jean-Guy Schilling 《Contributions to Mineralogy and Petrology》1984,85(3):209-223
Major element compositions of submarine basalts, quenched glasses, and contained phenocrysts are reported for samples from 25 dredge stations along the Mohns-Knipovich Ridge between the Jan Mayen fracture zone and 77°30N. Most of the basalts collected on the Jan Mayen platform have a subaerial appearance, are nepheline normative, rich in incompatible elements, and have REE-patterns strongly enriched in light-REE. The other basalts (with one exception) are tholeiitic pillow basalts, many of which have fresh quenched glass rims. From the Jan Mayen platform northeastwards the phenocryst assemblage changes from olivine±plagioclase±clinopyroxene±magnetite to olivine +plagioclase±chrome-spinel. This change is accompanied by a progressive decrease in the content of incompatible elements, light-REE enrichments and elevation of the ridge that are similar to those observed south of the Azores and Iceland hotspots. Pillow basalts and glasses collected along the esternmost part of the Mohns Ridge (450 to 675 km east of Jan Mayen) have low K2O, TiO2, and P2O5 contents, light-REE depleted patterns relative to chondrites, and Mg/(Mg+Fe2+) ratios between 0.64 and 0.60. Pillow basalts and glasses from the Knipovich Ridge have similar (Mg/Mg+Fe2+) ratios, but along the entire ridge have slightly higher concentrations of incompatible elements and chondritic to slightly light-REE enriched patterns. The incompatible element enrichment increases slightly northward. Plagioclase phenocrysts show normal and reverse zoning on all parts of the ridge whereas olivines are unzoned or show only weak normal zoning. Olivine-liquid equilibrium temperatures are calculated to be in the range of 1,060–1,206° C with a mean around 1,180° C.Rocks and glasses collected on the Jan Mayen Platform are compositionally similar to Jan Mayen volcanic products, suggesting that off-ridge alkali volcanism on the Jan Mayen Platform is more widespread than so far suspected. There is also evidence to suggest that the alkali basalts from the Jan Mayen Platform are derived from deeper levels and by smaller degrees of partial melting of a mantle significantly more enriched in light-REE and other incompatible elements than are the tholeiitic basalts from the Eastern Mohns and Knipovich Ridge. The possibility of the presence of another hitherto unsuspected enriched mantle region north of 77° 30 N is also briefly considered.It remains uncertain whether geochemical gradients revealed in this study reflect: (1) the dynamics of mixing during mantle advection and magma emplacement into the crust along the Mid-Atlantic Ridge (MAR) spreading axis, (e.g. such as in the mantle plume — large-ion-lithophile element depleted asthenosphere mixing model previously proposed); or (2) a horizontal gradation of the mantle beneath the MAR axis similar to that observed in the overlying crust; or (3) a vertical gradation of the mantle in incompatible elements with their contents increasing with depth and derivations of melts from progressively greater depth towards the Jan Mayen Platform. 相似文献
18.
Three long, strike-parallel, seismic-refraction profiles were made on the continental shelf edge, slope and upper rise off New Jersey during 1975. The shelf edge line lies along the axis of the East Coast Magnetic Anomaly (ECMA), while the continental rise line lies 80 km seaward of the shelf edge. Below the unconsolidated sediments (1.7–3.6 km/sec), high-velocity sedimentary rocks (4.2–6.2 km/sec) were found at depths of 2.6–8.2 km and are inferred to be cemented carbonates. Although multichannel seismic-reflection profiles and magnetic depth-to-source data predicted the top of oceanic basement at 6–8 km beneath the shelf edge and 10–11 km beneath the rise, no refracted events occurred as first arrivals from either oceanic basement (layer 2, approximately 5.5 km/ sec) or the upper oceanic crust (layer 3A, approximately 6.8 km/sec). Second arrivals from 10.5 km depth beneath the shelf edge are interpreted as events from a 5.9 km/sec refractor within igneous basement. Other refracted events from either layers 2 or 3A could not be resolved within the complex second arrivals. A well-defined crustal layer with a compressional velocity of 7.1–7.2 km/sec, which can be interpreted as oceanic layer 3B, occurred at 15.8 km depth beneath the shelf and 12.9 km beneath the upper rise. A well-reversed mantle velocity of 8.3 km/sec was measured at 18–22 km depth beneath the upper continental rise. Comparison with other deep-crustal profiles along the continental edge of the Atlantic margin off the United States, specifically in the inner magnetically quiet zone, indicates that the compressional wave velocities and layer depths determined on the U.S.G.S. profiles are very similar to those of nearby profiles. This suggests that the layers are continuous and that the interpretation of the oceanic layer 3B under the shelf edge east of New Jersey implies progradation of the shelf outward over the oceanic crust in that area. This agrees with magnetic anomaly evidence which shows the East Coast Magnetic Anomaly landward of the shelf edge off New Jersey and with previous seismic reflection data which reveal extensive outbuilding of the shelf edge during the Jurassic and Lower Cretaceous, probably by carbonate bank-margin accretion. 相似文献
19.
We have used Rayleigh wave dispersion analysis and inversion to produce a high resolution S-wave velocity imaging profile of the crust and uppermost mantle structure beneath the northeastern boundary regions of the North China Craton (NCC). Using waveform data from 45 broadband NCISP stations, Rayleigh wave phase velocities were measured at periods from 10 to 48 s and utilized in subsequent inversions to solve for the S-wave velocity structure from 15 km down to 120 km depth. The inverted lower crust and uppermost mantle velocities, about 3.75 km/s and 4.3 km/s on average, are low compared with the global average. The Moho was constrained in the depth range of 30–40 km, indicating a typical crustal thickness along the profile. However, a thin lithosphere of no more than 100 km was imaged under a large part of the profile, decreasing to only ~ 60 km under the Inner Mongolian Axis (IMA) where an abnormally slow anomaly was observed below 60 km depth. The overall structural features of the study region resemble those of typical continental rift zones and are probably associated with the lithospheric reactivation and tectonic extension widespread in the eastern NCC during Mesozoic–Cenozoic time. Distinctly high velocities, up to ~ 4.6 km/s, were found immediately to the south of the IMA beneath the northern Yanshan Belt (YSB), extending down to > 100-km depth. The anomalous velocities are interpreted as the cratonic lithospheric lid of the region, which may have not been affected by the Mesozoic–Cenozoic deformation process as strongly as other regions in the eastern NCC. Based on our S-wave velocity structural image and other geophysical observations, we propose a possible lithosphere–asthenosphere interaction scenario at the northeastern boundary of the NCC. We speculate that significant undulations of the base of the lithosphere, which might have resulted from the uneven Mesozoic–Cenozoic lithospheric thinning, may induce mantle flows concentrating beneath the weak IMA zone. The relatively thick lithospheric lid in the northern YSB may serve as a tectonic barrier separating the on-craton and off-craton regions into different upper mantle convection systems at the present time. 相似文献
20.
Christoph Beier João Mata Ferdinand Stöckhert Nadine Mattielli Philipp A. Brandl Pedro Madureira Felix S. Genske Sofia Martins José Madeira Karsten M. Haase 《Contributions to Mineralogy and Petrology》2013,165(5):823-841
The islands of the Azores archipelago emerge from an oceanic plateau built on lithosphere increasing in age with distance from the Mid-Atlantic Ridge from 10 to 45 Ma. Here, we present the first comprehensive major and trace element and Sr–Nd–Pb isotope data from Santa Maria, the easternmost island of the archipelago, along with published data from the other Azores islands situated much closer to the Mid-Atlantic Ridge axis. We can show that the distinctively more variable and more enriched trace element ratios at Santa Maria combined with a relatively small range in Sr–Nd–Pb isotope ratios are the result of low degrees of partial melting of a common Azores mantle plume source underneath thicker lithosphere. This implies that melt extraction processes and melting dynamics may be able to better preserve the trace element mantle source variability underneath thicker lithosphere. These conclusions may apply widely for oceanic melts erupted on relatively thick lithosphere. In addition, lower Ti/Sm and K/La ratios and SiO2 contents of Santa Maria lavas imply melting of a carbonated peridotite source. Mixing of variable portions of deep small-degree carbonated peridotite melts and shallow volatile-free garnet peridotite could explain the geochemical variability underneath Santa Maria in agreement with the volatile-rich nature of the Azores mantle source. However, Santa Maria is the Azores island where the CO2-rich nature of the mantle source is more evident, reflecting a combination of a smaller extent of partial melting and the positioning at the edge of the tilted Azores mantle plume. 相似文献