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1.
The event across the Paleozoic–Mesozoic transition involved the greatest mass extinction in history together with other unique geologic phenomena of global context, such as the onset of Pangean rifting and the development of superanoxia. The detailed stratigraphic analyses on the Permo-Triassic sedimentary rocks documented a two-stepped nature both of the extinction and relevant global environmental changes at the Guadalupian–Lopingian (Middle and Upper Permian) boundary (G-LB, ca. 260 Ma) and at the Permo-Triassic boundary (P-TB, ca. 252 Ma), suggesting two independent triggers for the global catastrophe. Despite the entire loss of the Permian–Triassic ocean floors by successive subduction, some fragments of mid-oceanic rocks were accreted to and preserved along active continental margins. These provide particularly important dataset for deciphering the Permo-Triassic paleo-environments of the extensive superocean Panthalassa that occupied nearly two thirds of the Earth’s surface. The accreted deep-sea pelagic cherts recorded the double-phased remarkable faunal reorganization in radiolarians (major marine plankton in the Paleozoic) both across the G-LB and the P-TB, and the prolonged deep-sea anoxia (superanoxia) from the Late Permian to early Middle Triassic with a peak around the P-TB. In contrast, the accreted mid-oceanic paleo-atoll carbonates deposited on seamounts recorded clear double-phased changes of fusuline (representative Late Paleozoic shallow marine benthos) diversity and of negative shift of stable carbon isotope ratio at the G-LB and the P-TB, in addition to the Paleozoic minimum in 87Sr/86Sr isotope ratio in the Capitanian (Late Guadalupian) and the paleomagnetic Illawarra Reversal in the late Guadalupian. These bio-, chemo-, and magneto-stratigraphical signatures are concordant with those reported from the coeval shallow marine shelf sequences around Pangea. The mid-oceanic, deep- and shallow-water Permian records indicate that significant changes have appeared twice in the second half of the Permian in a global extent. It is emphasized here that everything geologically unusual started in the Late Guadalupian; i.e., (1) the first mass extinction, (2) onset of the superanoxia, (3) sea-level drop down to the Phanerozoic minimum, (4) onset of volatile fluctuation in carbon isotope ratio, 5) 87Sr/86Sr ratio of the Paleozoic minimum, (6) extensive felsic alkaline volcanism, and (7) Illawarra Reversal.The felsic alkaline volcanism and the concurrent formation of several large igneous provinces (LIPs) in the eastern Pangea suggest that the Permian biosphere was involved in severe volcanic hazards twice at the G-LB and the P-TB. This episodic magmatism was likely related to the activity of a mantle superplume that initially rifted Pangea. The supercontinent-dividing superplume branched into several secondary plumes in the mantle transition zone (410–660 km deep) beneath Pangea. These secondary plumes induced the decompressional melting of mantle peridotite and pre-existing Pangean crust to form several LIPs that likely caused a “plume winter” with global cooling by dust/aerosol screens in the stratosphere, gas poisoning, acid rain damage to surface vegetation etc. After the main eruption of plume-derived flood basalt, global warming (plume summer) took over cooling, delayed the recovery of biodiversity, and intensified the ocean stratification. It was repeated twice at the G-LB and P-TB.A unique geomagnetic episode called the Illawarra Reversal around the Wordian–Capitanian boundary (ca. 265 Ma) recorded the appearance of a large instability in the geomagnetic dipole in the Earth’s outer core. This rapid change was triggered likely by the episodic fall-down of a cold megalith (subducted oceanic slabs) from the upper mantle to the D″ layer above the 2900 km-deep core-mantle boundary, in tight association with the launching of a mantle superplume. The initial changes in the surface environment in the Capitanian, i.e., the Kamura cooling event and the first biodiversity decline, were probably led by the weakened geomagnetic intensity due to unstable dipole of geodynamo. Under the low geomagnetic intensity, the flux of galactic cosmic radiation increased to cause extensive cloud coverage over the planet. The resultant high albedo likely drove the Kamura cooling event that also triggered the unusually high productivity in the superocean and also the expansion of O2 minimum zone to start the superanoxia.The “plume winter” scenario is integrated here to explain the “triple-double” during the Paleozoic–Mesozoic transition interval, i.e., double-phased cause, process, and consequence of the greatest global catastrophe in the Phanerozoic, in terms of mantle superplume activity that involved the whole Earth from the core to the surface biosphere. 相似文献
2.
The spatial distribution of recent (under 2 Ma) volcanism has been studied in relation to mantle hotspots and the evolution of the present-day supercontinent which we named Northern Pangea. Recent volcanism is observed in Eurasia, North and South America, Africa, Greenland, the Arctic, and the Atlantic, Indian, and Pacific Oceans. Several types of volcanism are distinguished: mid-ocean ridge (MOR) volcanism; subduction volcanism of island arcs and active continental margins (IA + ACM); continental collision (CC) volcanism; intraplate (IP) volcanism related to mantle hotspots, continental rifts, and transcontinental belts. Continental volcanism is obviously related to the evolution of Northern Pangea, which comprises Eurasia, North and South America, India, Australia, and Africa. The supercontinent is large, with predominant continental crust. The geodynamic setting and recent volcanism of Northern Pangea are determined by two opposite processes. On one hand, subduction from the Pacific Ocean, India, the Arabian Peninsula, and Africa consolidates the supercontinent. On the other hand, the spreading of oceanic plates from the Atlantic splits Northern Pangea, changes its shape as compared with Wegener’s Pangea, and causes the Atlantic geodynamics to spread to the Arctic. The long-lasting steady subduction beneath Eurasia and North America favored intense IA + ACM volcanism. Also, it caused cold lithosphere to accumulate in the deep mantle in northern Northern Pangea and replace the hot deep mantle, which was pressed to the supercontinental margins. Later on, this mantle rose as plumes (IP mafic magma sources), which were the ascending currents of global mantle convection and minor convection systems at convergent plate boundaries. Wegener’s Pangea broke up because of the African superplume, which occupied consecutively the Central Atlantic, the South Atlantic, and the Indian Ocean and expanded toward the Arctic. Intraplate plume magmatism in Eurasia and North America was accompanied by surface collisional or subduction magmatism. In the Atlantic, Arctic, Indian, and Pacific Oceans, deep-level plume magmatism (high-alkali mafic rocks) was accompanied by surface spreading magmatism (tholeiitic basalts). 相似文献
3.
白垩纪大火成岩省与地幔对流 总被引：2，自引：0，他引：2
白垩纪事件是全球非常明显和重要的一次地质突发事件,包括洋壳的超巨量形成,地磁正超时达41Ma之久(124~83 Ma),海水温度大幅度升高,黑色页岩沉积和石油形成的大量增长,海平面的快速上升,大气CO2水平的急剧升高,以及伴生的生物灭绝事件等。中—新生代的大火成岩省与冈瓦纳超大陆的裂解伴生,是超级地幔热柱产生的结果,而与欧亚超大陆的形成伴生分散火成岩省,是超级冷地幔下降流的结果,两者的联合构成全地幔对流的格局。全地幔对流模型为白垩纪地质演化、生物演化和环境演化的突变提供地球深部过程的约束。 相似文献
4.
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
5.
The mean depth of the mantle transition zone depends on the global mantle temperature through the Clapeyron slope. Numerical models of mantle convection with endothermic phase change at 670 km are characterized by quiet periods of partial layering alternating with catastrophic events. During an avalanche, global thermal anomalies, reaching several tens of degrees, and lasting several tens of million years occur that induce a global deepening of the mantle transition zone. The resulting inertia tensor perturbations lead to acceleration of the Earth’s rotation. The bottom heat fluxes remains strong as a consequence of the steep thermal gradient at the CMB level; whereas it decreases to a low level during quiet periods. The upper surface heat flux displays a more complex behavior due to the re-heating of upper mantle by plumes generated at the transition zone and at the CMB. Following these numerical results, two significantly different regimes of mantle convection may have alternated several times during the Earth’s history. The avalanche periods are also associated with a global increase of plate tectonic activity (ridge accretion and continental break-up) due to an increase of the surface velocity during the avalanche process. This work shows the richness of the behavior in surfaces and bottom heat fluxes that arises from the alternation of quiet periods of partial layering with catastrophic avalanche events. Complexities in the real Earth displays are surprisingly simular to those caused by the consequences of mantle avalanches. 相似文献
6.
We present a thermophysical model for interaction between the conduit of a thermochemical plume and horizontal free convection flows in the mantle: The mantle flow incident on the plume conduit melts at the conduit boundary (front part) and crystallizes at its back. Geological data on the intensity of plume magmatism over the last 150 Myr are used to estimate the total thermal power of mantle plumes. A possible scenario for plume-related mantle recrystallization is proposed. Over the lifespan of a thermochemical plume, mantle melts and recrystallizes owing to the motion of the plume source and interaction between the plume conduit and horizontal free convection flows. The plume conduits can melt and recrystallize the entire mantle over a certain period of time. The model for the interaction of drifting plume conduits with mantle flows and the estimated total thermal power of mantle plumes are used to estimate the duration of plume-related melting and recrystallization of the entire mantle. The influence of mantle plumes on the convective structure of the mantle through melting is judged from the model for plume interaction with horizontal mantle flows. 相似文献
7.
The supercontinental status of the contemporary aggregation of continents called North Pangea is substantiated. This supercontinent
comprises all continents with the probable exception of Antarctica. In addition to the spatial contiguity of continents, the
supercontinent is characterized by the prevalence of the continental crust that combines North America and Eurasia, Eurasia
and Africa, and Eurasia and Australia. Over the course of the 300–250-Ma evolution from Wegener’s Pangea to contemporary North
Pangea, the aggregation of continents has not lost its supercontinental status, despite modification of the supercontinent
shape and opening and closure of the newly formed Paleotethys, Tethys, Atlantic, and Indian oceans. Over the last 250–300
Ma, all movements of the lithospheric plates have most likely occurred within the Indo-Atlantic segment of the Earth, whereas
the Pacific segment has remained oceanic. In short, the formation of the North Pangea supercontinent can be outlined in the
following terms. The long and deep subduction of the lithospheric plates beneath Eurasia and North America gave rise to the
stabilization of the continents and accumulation of huge bodies of the cold lithosphere commensurable in volume with the upper
mantle at the deeper mantle levels. This brought about compensation ascent of hot mantle (mantle plumes) near the convergent
plate boundaries and far from them. A special geodynamic setting develops beneath the supercontinent. Due to encircling subduction
of the lithospheric plates and related squeezing of the hot mantle, an ascending flow, or plume (superplume) formed beneath
the central part of the supercontinent. In our view, the African superplume broke up Wegener’s Pangea in the Atlantic region,
caused the opening of the Atlantic and Indian oceans, and migrated to the Arctic Region 53 Ma ago. 相似文献
8.
Plate tectonic evolution of the southern margin of Eurasia in the Mesozoic and Cenozoic 总被引：4，自引：0，他引：4
Thirteen time interval maps were constructed, which depict the Triassic to Neogene plate tectonic configuration, paleogeography and general lithofacies of the southern margin of Eurasia. The aim of this paper is to provide an outline of the geodynamic evolution and position of the major tectonic elements of the area within a global framework. The Hercynian Orogeny was completed by the collision of Gondwana and Laurussia, whereas the Tethys Ocean formed the embayment between the Eurasian and Gondwanian branches of Pangea. During Late Triassic–Early Jurassic times, several microplates were sutured to the Eurasian margin, closing the Paleotethys Ocean. A Jurassic–Cretaceous north-dipping subduction boundary was developed along this new continental margin south of the Pontides, Transcaucasus and Iranian plates. The subduction zone trench-pulling effect caused rifting, creating the back-arc basin of the Greater Caucasus–proto South Caspian Sea, which achieved its maximum width during the Late Cretaceous. In the western Tethys, separation of Eurasia from Gondwana resulted in the formation of the Ligurian–Penninic–Pieniny–Magura Ocean (Alpine Tethys) as an extension of Middle Atlantic system and a part of the Pangean breakup tectonic system. During Late Jurassic–Early Cretaceous times, the Outer Carpathian rift developed. The opening of the western Black Sea occurred by rifting and drifting of the western–central Pontides away from the Moesian and Scythian platforms of Eurasia during the Early Cretaceous–Cenomanian. The latest Cretaceous–Paleogene was the time of the closure of the Ligurian–Pieniny Ocean. Adria–Alcapa terranes continued their northward movement during Eocene–Early Miocene times. Their oblique collision with the North European plate led to the development of the accretionary wedge of the Outer Carpathians and its foreland basin. The formation of the West Carpathian thrusts was completed by the Miocene. The thrust front was still propagating eastwards in the eastern Carpathians.During the Late Cretaceous, the Lesser Caucasus, Sanandaj–Sirjan and Makran plates were sutured to the Iranian–Afghanistan plates in the Caucasus–Caspian Sea area. A north-dipping subduction zone jumped during Paleogene to the Scythian–Turan Platform. The Shatski terrane moved northward, closing the Greater Caucasus Basin and opening the eastern Black Sea. The South Caspian underwent reorganization during Oligocene–Neogene times. The southwestern part of the South Caspian Basin was reopened, while the northwestern part was gradually reduced in size. The collision of India and the Lut plate with Eurasia caused the deformation of Central Asia and created a system of NW–SE wrench faults. The remnants of Jurassic–Cretaceous back-arc systems, oceanic and attenuated crust, as well as Tertiary oceanic and attenuated crust were locked between adjacent continental plates and orogenic systems. 相似文献
9.
In the available numerical models, mantle plumes are represented by homogeneous ascending streams of thermal convection. Pulses are considered to be possible only in thermochemical plumes within the compositionally inhomogeneous mantle. We show that pulses can also occur under regular thermal convection in the homogeneous mantle. As the intensity grows, the flow in the tail of a thermal plume first begins pulsing and then the plume breaks up into a set of sequentially emerging thermals. For the present-day mantle, the pulsation periods for plumes in the lower mantle can range up to 10 Ma and about 1 Ma in the upper mantle. 相似文献
10.
F. Pirajno 《Mineralogy and Petrology》2004,82(3-4):183-216
Summary Intraplate tectono-magmatic phenomena, including the emplacement of layered intrusions, and the giant dyke swarms, anorogenic (hotspot) volcanism, oceanic plataeux, rifting processes, basin formation, and geomorphological features are discussed in the context of the mantle plume theory. A review of the relationships between mantle plumes and ore deposits focuses on direct links, proxied by the emplacement of mafic-ultramafic magmas (e.g. PGE and Ni–Cu sulphides associated with flood basalts) and indirectly in rift systems where high geothermal gradients are set up in the crust above the plume, induce large scale circulation of hydrothermal fluids, which result in the generation of a wide range of ore deposits. Peak periods in the deposition of iron formations coincide with plume events in the Archeaen and Proterozoic. Passive margins, which evolve from continental breakups and triple junctions, host abundant mineral and hydrocarbon resources. 相似文献
11.
Nicolas Coltice Hervé Bertrand Patrice Rey Fred Jourdan Benjamin R. Phillips Yanick Ricard 《Gondwana Research》2009,15(3-4):254-266
Throughout its history, the Earth has experienced global magmatic events that correlate with the formation of supercontinents. This suggests that the distribution of continents at the Earth's surface is fundamental in regulating mantle temperature. Nevertheless, most large igneous provinces (LIPs) are explained in terms of the interaction of a hot plume with the lithosphere, even though some do not show evidence for such a mechanism. The aggregation of continents impacts on the temperature and flow of the underlying mantle through thermal insulation and enlargement of the convection wavelength. Both processes tend to increase the temperature below the continental lithosphere, eventually triggering melting events without the involvement of hot plumes. This model, called mantle global warming, has been tested using 3D numerical simulations of mantle convection [Coltice, N., Phillips, B.R., Bertrand, H., Ricard, Y., Rey, P. (2007) Global warming of the mantle at the origin of flood basalts over supercontinents. Geology 35, 391–394.]. Here, we apply this model to several continental flood basalts (CFBs) ranging in age from the Mesozoic to the Archaean. Our numerical simulations show that the mantle global warming model could account for the peculiarities of magmatic provinces that developed during the formation of Pangea and Rodinia, as well as putative Archaean supercontinents such as Kenorland and Zimvaalbara. 相似文献
12.
Philippe Machetel 《Comptes Rendus Geoscience》2003,335(1):91-97
The numerical models of mantle convection agree to depict avalanches behaviour according to the level of endothermicity of the spinel → perovskite phase change. Their potential effects on the global thermal and dynamical states of the mantle have been computed thanks to a numerical code, which takes into account both the 400-km exothermic and the 660-km endothermic phase changes. The cycle followed by the avalanches is: local layering, destabilization of the 660-km thermal layer, travelling and spreading on the core, and reappearing of the local layering. Therefore, mantle convection is characterized by quiet periods of partial layering embedded in catastrophic events. During the avalanche, the amplitude of the surface velocity is multiplied by two, which would imply an enhanced plate tectonic and ridge activities. The global thermal effects of the avalanche are compatible with a high mantle temperature and an acceleration of Earth's rotation during the Cretaceous. They also offer a coherent explanation to locate the origin of mantle plumes both within the CMB and just below the transition zone. 相似文献
13.
The dynamics of big mantle wedge, magma factory, and metamorphic–metasomatic factory in subduction zones 总被引：1，自引：1，他引：0
S. Maruyama A. Hasegawa M. Santosh T. Kogiso S. Omori H. Nakamura K. Kawai D. Zhao 《Gondwana Research》2009,16(3-4):414-430
The East Asian continental margin is underlain by stagnant slabs resulting from subduction of the Pacific plate from the east and the Philippine Sea plate from the south. We classify the upper mantle in this region into three major domains: (a) metasomatic–metamorphic factory (MMF), subduction zone magma factory (SZMF), and the ‘big mantle wedge’ (BMW). Whereas the convection pattern is anticlockwise in the MMF domain, it is predominantly clockwise in the SZMF and BMW, along a cross section from the south. Here we define the MMF as a small wedge corner which is driven by the subducting Pacific plate and dominated by H2O-rich fluids derived by dehydration reactions, and enriched in large ion lithophile elements (LILE) which cause the metasomatism. The SZMF is a zone intermediate between MMF and BMW domains and constitutes the main region of continental crust production by partial melting through wedge counter-corner flow. Large hydrous plume generated at about 200 km depth causes extensive reduction in viscosity and the smaller scale hydrous plumes between 60 km and 200 km also bring about an overall reduction in the viscosity of SZMF. More fertile and high temperature peridotites are supplied from the entrance to this domain. The domain extends obliquely to the volcanic front and then swings back to the deep mantle together with the subducting slab. The BMW occupies the major portion of upper mantle in the western Pacific and convects largely with a clockwise sense removing the eastern trench oceanward. Sporadic formation of hydrous plume at the depth of around 410 km and the curtain flow adjacent to the trench cause back arc spreading. We envisage that the heat source in BMW could be the accumulated TTG (tonalite–trondhjemite–granodiorite) crust on the bottom of the mantle transition zone. The ongoing process of transportation of granitic crust into the mantle transition zone is evident from the deep subduction of five intra-oceanic arcs on the subducting Philippine Sea plate from the south, in addition to the sediment trapped subduction by the Pacific plate and Philippine Sea plate. The dynamics of MMF, SZMF and BMW domains are controlled by the angle of subduction; a wide zone of MMF in SW Japan is caused by shallow angle subduction of the Philippine Sea plate and the markedly small MMF domain in the Mariana trench is due to the high angle subduction of Pacific plate. The domains in NE Japan and Kyushu region are intermediate between these two. During the Tertiary, a series of marginal basins were formed because of the nearly 2000 km northward shift of the subduction zone along the southern margin of Tethyan Asia, which may be related to the collision of India with Asia and the indentation. The volume of upper mantle under Asia was reduced extensively on the southern margin with a resultant oceanward trench retreat along the eastern margin of Asia, leading to the formation of a series of marginal basins. The western Pacific domain in general is characterized by double-sided subduction; from the east by the oldest Pacific plate and from the south by the oldest Indo-Australian plate. The old plates are hence hydrated extensively even in their central domains and therefore of low temperature. The cracks have allowed the transport of water into the deeper portions of the slab and these domains supply hydrous fluids even to the bottom of the upper mantle. Thus, a fluid dominated upper mantle in the western Pacific drives a number of microplates and promote the plate boundary processes. 相似文献
14.
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
15.
Recent advances in three-dimensional numerical simulations of mantle convection have aided in approximately reproducing continental movement since the Pangea breakup at 200 Ma. These have also led to a better understanding of the thermal and mechanical coupling between mantle convection and surface plate motion and predictions of the configuration of the next supercontinent. The simulations of mantle convection from 200 Ma to the present reveals that the development of large-scale cold mantle downwellings in the North Tethys Ocean at the earlier stage of the Pangea breakup triggered the northward movement of the Indian subcontinent. The model of high temperature anomaly region beneath Pangea resulting from the thermal insulation effect support the breakup of Pangea in the real Earth time scale, as also suggested in previous geological and geodynamic models. However, considering the low radioactive heat generation rate of the depleted upper mantle, the high temperature anomaly region might have been generated by upwelling plumes with contribution of deep subducted TTG(tonalite-trondhjemite-granite) materials enriched in radiogenic elements. Integrating the numerical results of mantle convection from 200 Ma to the present, and from the present to the future, it is considered that the mantle drag force acting on the base of continents may be comparable to the slab pull force, which implies that convection in the shallower part of the mantle is strongly coupled with surface plate motion. 相似文献
16.
地幔柱构造理论研究若干问题及研究进展 总被引：3，自引：0，他引：3
介绍了目前地幔柱构造理论研究中若干重要问题和最新进展,许多证据显示,地幔柱是严自于核幔边界附近的Ｄ″层发生热扰动并产生地幔柱的热动力源于外地核的不均匀加热作用;一个新启动的地幔柱在穿过整个地幔的缓慢上升过程会形成巨大球状顶冠和狭窄尾柱;地幔柱巨大球状顶冠会导致地壳发生上隆、区域变质作用、地壳深熔作用、构造变形作用和大规模火山作用,形成大陆或大洋溢流玄武岩;地幔柱狭窄尾柱的长期活动会在上覆运动板块上 相似文献
17.
Variable involvements of mantle plumes in the genesis of mid-Neoproterozoic basaltic rocks in South China: A review 总被引：5，自引：0，他引：5
Ca. 825–720 Ma global continental intraplate magmatism is generally linked to mantle plumes or a mantle superplume that caused rifting and fragmentation of the supercontinent Rodinia. Widespread Neoproterozoic igneous rocks in South China are dated at ca. 825–760 Ma. There is a hot debate on their petrogenesis and tectonic affiliations, i.e., mantle plume/rift settings or collision/arc settings. Such competing interpretations have contrasting implications to the position of South China in the supercontinent Rodinia and in Rodinia reconstruction models.Variations in the bulk-rock compositions of primary basaltic melts can provide first order constraints on the mantle thermal–chemical structure, and thus distinguish between the plume/rift and arc/collision models. Whole-rock geochemical data of 14 mid-Neoproterozoic (825–760 Ma) basaltic successions are reviewed here in order to (1) estimate the primary melts compositions; (2) calculate the melting conditions and mantle potential temperature; and (3) identify the contributions of subcontinental lithosphere mantle (SCLM) and asenthospheric mantles to the generation of these basaltic rocks.In order to quantify the mantle potential temperatures and percentages of decompression melting, the primary MgO, FeO, and SiO2 contents of basalts are calculated through carefully selecting less-evolved samples using a melting model based on the partitioning of FeO and MgO in olivine. The mid-Neoproterozoic (825–760 Ma) potential temperatures predicted from the primary melts range from 1390 °C to 1630 °C (mostly > 1480 °C), suggesting that most 825–760 Ma basaltic rocks in South China were generated by melting of anomalously hot mantle sources with potential temperatures 80–200 °C higher than the ambient Middle Ocean Ridge Basalt (MORB)-source mantle.The mantle source regions of these Neoproterozoic basaltic rocks have complex histories and heterogeneous compositions. Enriched mantle sources (e.g., pyroxenite and eclogite) are recognized as an important source for the Bikou and Suxiong basalts, suggesting that their generations may have involved recycled components. Trace elements variations show that interactions between asthenospheric mantle (OIB-type mantle) and SCLM played a very important role in generation of the 825–760 Ma basalts. Our results indicate that the SCLM metasomatized by subduction-induced melts/fluids during the 1.0–0.9 Ga orogenesis as a distinct geochemical reservoir that contributed significantly to the trace-elements and isotope inventory of these basalts.The continental intraplate geochemical signatures (e.g., OIB-type), high mantle potential temperatures and recycled components suggest the presence of a mantle plume beneath the Neoproterozoic South China block. We use the available data to develop an integrated plume-lithosphere interaction model for the ca. 825–760 Ma basalts. The early phases of basaltic rocks (825–810 Ma) were most likely formed by melting within the metasomatized SCLM heated by the rising mantle plume. The subsequent continental rift allowed adiabatic decompression partial melting of an upwelling mantle plumes at relatively shallow depth to form the widespread syn-rifting basaltic rocks at ca. 810–800 Ma and 790–760 Ma. 相似文献
18.
Multiscale seismic tomography and mantle dynamics 总被引：15，自引：10，他引：5
In this article we first introduce the methodology of multiscale seismic tomography and the way to interpret the obtained tomographic images, and then review the significant recent results of multiscale seismic tomography with emphasis on mantle plumes and subducting slabs. Global and regional tomography shows that most of the slab materials under Western Pacific and East Asia are stagnant in the mantle transition zone before finally collapsing down to the core–mantle boundary as a result of large gravitational instability from phase transitions. Local and teleseismic tomography studies have imaged clearly the subducting slabs and arc magma chambers in the upper-mantle wedge, indicating that geodynamic systems associated with arc magmatism and back-arc spreading are related to deep processes, such as convective circulation in the mantle wedge and dehydration reactions of the subducting slab. Because most hotspots are located in poorly instrumented continental and oceanic regions, 3-D crust and upper-mantle structure is determined for only a few hotspots such as Iceland, Yellowstone and Eifel which are covered by seismic networks, and plume-like slow anomalies are revealed under those hotspots. Global tomography has revealed deep mantle plumes under the major hotspots such as Hawaii, Iceland, Kerguelen, South Pacific and Africa. Strong lateral heterogeneities are revealed at the bottom of the mantle, which are associated with the deeply subducted slabs and the birth of mantle plumes. A thorough understanding of the deep Earth structure will only be achieved by a combination of more effective seismic imaging techniques and dense coverage of global seismic networks, particularly in the oceans. 相似文献
19.
The role of the uppermost mantle strength in the pattern of lithosphere rifting is investigated using a thermo-mechanical finite-element code. In the lithosphere, the mantle/crust strength ratio (SM/SC) that decreases with increasing Moho temperature TM allows two strength regimes to be defined: mantle dominated (SM > SC) and crust dominated (SM < SC). The transition between the two regimes corresponds to the disappearance of a high strength uppermost mantle for TM > 700 °C. 2D numerical simulations for different values of SM/SC show how the uppermost mantle strength controls the style of continental rifting. A high strength mantle leads to strain localisation at lithosphere scale, with two main patterns of narrow rifting: “coupled crust–mantle” at the lowest TM values and “deep crustal décollement” for increasing TM values, typical of some continental rifts and non-volcanic passive margins. The absence of a high strength mantle leads to distributed deformations and wide rifting in the upper crust. These numerical results are compared and discussed in relation with series of classical rift examples. 相似文献
20.
Permian komatiites and associated basalts from the marine sediments of Chhongtash Formation, southeast Karakoram, Ladakh, India 总被引：1，自引：0，他引：1
Summary The Karakoram micro-plate is the southern most sector of the Central Asian micro-plate mosaic which was separated by a narrow
rift basin. A major rifting phase started during Permian time, which lead to drift of not only Karakoram but of the entire
Eurasian (Asian) Plate from Gondwana land. This was at a time when a prominent sequence of black argillites occupied most
part of the Karakoram Tethys basin. The geodynamic setting for this sequence may be interpreted as the evolution of a passive
margin affected by extensional tectonics. The extensional activity is evident from the extrusion of basalts and komatiitic
rocks in the region.
In this paper the geochemical relations between komatiites and basalts of the Chhongtash, southeast Karakoram are investigated.
The basaltic and komatiitic (ultrabasic) flows are petrologically and geochemically distinct, yet they display a close spatial
and temporal association, and they are related to each other through olivine and clinopyroxene fractionation. The chemical
characteristics of the ultrabasic to basic magmatism in the region is consistent with formation above a mantle plume that
impinged on the continental lithosphere. Hence, a model of partial melting in a mantle plume and fractional crystallization
in a deep-seated magma chamber is envisaged to explain the evolution of these volcanic rocks. The komatiite melts are interpreted
to have been derived by high degree partial melting of mantle plumes in the tail region, while the basalts were interpreted
to be the result of interaction of source plume with cool mantle through which the plume head passed. This study is the first
of its kind, to suggest a rift related nature in the Chhongtash, southeast Karakoram, that represent the initial stage of
Mesozoic rifting along the southern margin of Eurasia when Gondwana started to drift away from Eurasia. 相似文献