排序方式： 共有10条查询结果，搜索用时 31 毫秒
1
1.
The Western Pacific Triangular Zone (WPTZ) is the frontier of a future supercontinent to be formed at 250 Ma after present. The WPTZ is characterized by double-sided subduction zones to the east and south, and is a region dominated by extensive refrigeration and water supply into the mantle wedge since at least 200 Ma. Long stagnant slabs extending over 1200 km are present in the mid-Mantle Boundary Layer (MBL, 410–660 km) under the WPTZ, whereas on the Core–Mantle Boundary (CMB, 2700–2900 km depth), there is a thick high-V anomaly, presumably representing a slab graveyard. To explain the D″ layer cold anomaly, catastrophic collapse of once stagnant slabs in MBL is necessary, which could have occurred at 30–20 Ma, acting as a trigger to open a series of back-arc basins, hot regions, small ocean basins, and presumably formation of a series of microplates in both ocean and continent. These events were the result of replacement of upper mantle by hotter and more fertile materials from the lower mantle.The thermal structure of the solid Earth was estimated by the phase diagrams of Mid Oceanic Ridge Basalt (MORB) and pyrolite combined with seismic discontinuity planes at 410–660 km, thickness of the D″ layers, and distribution of the ultra-low velocity zone (ULVZ). The result clearly shows the presence of two major superplumes and one downwelling. Thermal structure of the Earth seems to be controlled by the subduction history back to 180 Ma, except in the D″ layer. The thermal structure of the D″ layer seems to be controlled by older slab-graveyards, as expected by paleogeographic reconstructions for Laurasia, Gondwana and Rodinia back to 700 Ma.Comparison of mantle tomography between the Pacific superplume and underneath the WPTZ suggests the transformation of a cold slab graveyard to a large-scale mantle upwelling with time. The Pacific superplume was born from the coldest CMB underneath the 1.0–0.75 Ga supercontinent Rodinia where huge amounts of cold slabs had accumulated through collision-amalgamation of more than 12 continents. A high velocity P-wave anomaly on a whole-mantle scale shows stagnant slabs restricted to the MBL of circum-Pacific and Tethyan regions. The high velocity zones can be clearly identified within the Pacific domain, suggesting the presence of slab graveyards formed at geological periods much older than the breakup of Rodinia. We speculate that the predominant subduction occurred through the formation period of Gondwana, presumably very active during 600 to 540 Ma period, and again from 400 to 300 Ma during the formation of the northern half of Pangea (Laurasia). We correlate the three dominant slab graveyards with three major orogenies in earth history, with the emerging picture suggesting that the present-day Pacific superplume is located at the center of the Rodinian slab graveyard.We speculate the mechanism of superplume formation through a comparison of the thermal structure of the mantle combined with seismic tomography under the Western Pacific Triangular Zone (WPTZ), Laurasia (Asia), Gondwana (Africa), and Rodinia (Pacific). The coldest mantle formed by extensive subduction to generate a supercontinent, changes with time of the order of several hundreds of million years to the hottest mantle underneath the supercontinent. The Pacific superplume is tightly defined by a steep velocity gradient on the margin, particularly well documented by S-wave velocity. The outermost region of the superplume is characterized by the Rodinia slab graveyard forming a donut-shape. We develop a petrologic model for the Pacific superplume and show how larger plumes are generated at shallower depths in the mantle. We link the mechanism of formation of the superplume to the presence of the mineral post-perovskite, the phase transformation of which to perovskite is exothermic, and thus aids in transporting core heat to mantle, and finally to planetary space by plumes.We summarize the characteristics of tectonic processes operating at the CMB to propose the existence of an “anti-crust” generated through “anti-plate tectonics” at the bottom of the mantle. The chemistry of the anti-crust markedly contrasts with that of the continental crust overlying the mantle. Both the crust and the anti-crust must have increased in volume through geologic time, in close relation with the geochemical reservoirs of the Earth. The process of formation of a new superplume closely accompanies the process of development of anti-crust at the bottom of mantle, through the production of dense melt from the partial melting of recycled MORB, observed now as the ULVZ. When CMB temperature is recovered to near 4000 K through phase transformation, the recycled MORB is partially melted imparting chemical buoyancy of the andesitic residual solid which rises up from CMB, leaving behind the dense melt to sink to CMB and thus increase the mass of anti-crust. These small-scale plumes develop to a large-scale superplume through collision and amalgamation with time. When all recycled MORBs are consumed, it is the time of demise of superplume. Immediately above the CMB, anti-plate tectonics operates to develop anti-crust through the horizontal movement of accumulated slab and their partial melting. Thus, we speculate that another continent, or even a supercontinent, has developed through geologic time at the bottom of the mantle.We also evaluate the heating vs. cooling models in relation to mantle dynamics. Rising plumes control not only the rifting of supercontinents and continents, but also the Atlantic stage as seen by anchored ridge by hotspots in the last 200 Ma in the Atlantic. Therefore, we propose that the major driving force for the mantle dynamics is the heat supplied from the high-T core, and not the slab pull force by cooling. The best analogy for this is the atmospheric circulation driven by the energy from Sun. 相似文献
2.
The Norilsk mining district is located at the northwest margin of the Tunguska basin, in the centre of the 3,000×4,000 km Siberian continental flood basalt (CFB) province. This CFB province was formed at the Permo-Triassic boundary from a superplume that ascended into the geometric centre of the Laurasian continent, which was surrounded by subducting slabs of oceanic crust. We suggest that these slabs could have reached the core–mantle boundary, and they may have controlled the geometric focus of the superplume. The resulting voluminous magma intruded and erupted in continental rifts and related extensive flood basalt events over a 2–4 Ma period. Cu–Ni–PGE sulfide mineralization is found in olivine-bearing differentiated mafic intrusions beneath the flood basalts at the northwestern margin of the Siberian craton and also in the Taimyr Peninsula, some 300 km east of a triple junction of continental rifts, now buried beneath the Mesozoic–Cenozoic sedimentary basin of western Siberia. The Norilsk-I and Talnakh-Oktyabrsky deposits occur in the Norilsk–Kharaelakh trough of the Tunguska CFB basin. The Cu–Ni–PGE-bearing mineralized intrusions are 2–3 km-wide and 20 km-long differentiated chonoliths. Previous studies suggested that parts of the magma remained in intermediate-level crustal chambers where sulfide saturation and accumulation took place before emplacement. The 5–7-km-thick Neoproterozoic to Palaeozoic country rocks, containing sedimentary Cu mineralization and evaporites, may have contributed additional metal and sulfur to this magma. Classic tectonomagmatic models for these deposits proposed that subvertical crustal faults, such as the northeast-trending Norilsk–Kharaelakh fault, were major trough-parallel conduits providing access for magmas to the final chambers. However, geological maps of the Norilsk region show that the Norilsk–Kharaelakh fault offsets the mineralization, which was deformed into folds and offset by related reverse faults, indicating compressional deformation after mineralization in the Late Triassic to Early Jurassic. In addition, most of the intrusions are sills, not dykes as should be expected if the vertical faults were major conduits. A revised tectonic model for the Norilsk region takes into account the fold structure and sill morphology of the dominant intrusions, indicating a lateral rather than vertical emplacement direction for the magma into final chambers. Taking into account the fold structure of the country rocks, the present distribution of the differentiated intrusions hosting the Norilsk-I and Talnakh–Oktyabrsky deposits may represent the remnants of a single, >60 km long, deformed and eroded palm-shaped cluster of mineralized intrusions, which are perceived as separate intrusions at the present erosional level. The original direction of sill emplacement may have been controlled by a northeast-trending paleo-rise, which we suggest is present at the southeastern border of the Norilsk–Kharaelakh trough based on analysis of the unconformity at the base of the CFB. The mineralized intrusions extend along this rise, which we interpret as a structure that formed above the extensionally tilted block in the metamorphic basement. Geophysical data indicate the presence of an intermediate magma chamber that could be linked with the Talnakh intrusion. In turn, this T-shaped flat chamber may link with the Yenisei–Khatanga rift along the northwest-trending Pyasina transform fault, which may have served as the principal magma conduit to the intermediate chamber. It then produced the differentiated mineralized intrusions that melted through the evaporites with in situ precipitation of massive, disseminated, and copper sulfide ore. The Norilsk–Kharaelakh crustal fault may not relate to mineralization and possibly formed in response to late Mesozoic spreading in the Arctic Ocean.Editorial handling: P. Lightfoot 相似文献
3.
Illawarra Reversal: The fingerprint of a superplume that triggered Pangean breakup and the end-Guadalupian (Permian) mass extinction 总被引：1，自引：1，他引：0
The Permian magnetostratigraphic record demonstrates that a remarkable change in geomagnetism occurred in the Late Guadalupian (Middle Permian; ca. 265 Ma) from the long-term stable Kiaman Reverse Superchron (throughout the Late Carboniferous and Early-Middle Permian) to the Permian–Triassic Mixed Superchron with frequent polarity changes (in the Late Permian and Triassic). This unique episode called the Illawarra Reversal probably reflects a significant change in the geodynamo in the outer core of the planet after a 50 million years of stable geomagnetism. The Illawarra Reversal was likely led by the appearance of a thermal instability at the 2900 km-deep core–mantle boundary in connection with mantle superplume activity. The Illawarra Reversal and the Guadalupian–Lopingian boundary event record the significant transition processes from the Paleozoic to Mesozoic–Modern world. One of the major global environmental changes in the Phanerozoic occurred almost simultaneously in the latest Guadalupian, as recorded in 1) mass extinction, 2) ocean redox change, 3) sharp isotopic excursions (C and Sr), 4) sea-level drop, and 5) plume-related volcanism. In addition to the claimed possible links between the above-listed environmental changes and mantle superplume activity, I propose here an extra explanation that a change in the core's geodynamo may have played an important role in determining the course of the Earth's surface climate and biotic extinction/evolution. When a superplume is launched from the core–mantle boundary, the resultant thermal instability makes the geodynamo's dipole of the outer core unstable, and lowers the geomagnetic intensity. Being modulated by the geo- and heliomagnetism, the galactic cosmic ray flux into the Earth's atmosphere changes with time. The more cosmic rays penetrate through the atmosphere, the more clouds develop to increase the albedo, thus enhancing cooling of the Earth's surface. The Illawarra Reversal, the Kamura cooling event, and other unique geologic phenomena in the Late Guadalupian are all concordantly explained as consequences of the superplume activity that initially triggered the breakup of Pangea. The secular change in cosmic radiation may explain not only the extinction-related global climatic changes in the end-Guadalupian but also the long-term global warming/cooling trend in Earth's history in terms of cloud coverage over the planet. 相似文献
4.
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
5.
6.
Nickolay L. Dobretsov Alexey A. Kirdyashkin Anatoliy G. Kirdyashkin Valery A. Vernikovsky Igor N. Gladkov 《Lithos》2008,100(1-4):66-92
Thermochemical plumes form at the base of the lower mantle as a consequence of heat flow from the outer core and the presence of local chemical doping that decreases the melting temperature. Theoretical and experimental modelling of thermochemical plumes show that the diameter of a plume conduit remains practically constant during plume ascent. However, when the top of a plume reaches a refractory layer, whose melting temperature is higher than the melt temperature in the plume conduit, a mushroom-shaped plume head develops. Main parameters (melt viscosity, ascent time, ascent velocity, temperature differences in the plume conduit, and thermal power) are presented for a thermochemical plume ascending from the core–mantle boundary. In addition, the following relationships are developed: the pressure distribution in the plume conduit during the ascent of a plume, conditions for eruption-conduit formation, the effect of the P–T conditions and controls on the shape and size of a plume top, heat transfer between a thermochemical plume and the lithosphere (when the plume reaches the bottom of a refractory layer in the lithosphere), and eruption volume versus the time interval t1 between plume formation and eruption. These relationships are used to determine thermal power and time t1 for the Tunguska syneclise and the Siberian traps as a whole.
The Siberian and other trap provinces are characterized by giant volumes of lavas and sills formed a very short time period. Data permit a model for superplumes with three stages of formation: early (variable picrites and alkali basalts), main (tholeiite plateau basalts), and final (ultrabasic and alkaline lavas and intrusions). These stages reflect the evolution of a superplume from the ascent of one or several independent plumes, through the formation of thick lenses of mantle melts underplating the lithosphere and, finally, intrusion and extrusion of differentiated mantle melts. Synchronous syenite–granite intrusions and bimodal volcanism abundant in the margins of the Siberian traps are the result of melting of the lower crust at depths of 65–70 km under the effect of plume melts. 相似文献
7.
N.L. Dobretsov 《Russian Geology and Geophysics》2011,52(12):1539-1552
There were two key stages in the history of Paleozoids that formed in the place of the Paleoasian ocean, one in the Cambrian–Ordovician and the other in the Permian–Triassic. Both time spans were characterized by a combination of similar geodynamic, magmatic, and geomagnetic events: closure and opening of oceanic basins, intense plume magmatism associated with Earth's core cooling, and absence of geomagnetic reversals (superchrons). Three superchrons about 490–460, 260–300, and 124–86 Ma correlate with major events of plume magmatism. Plume reconstructions have to be updated for the period 490–460 Ma, which corresponded to the third superchron and was marked by ocean opening. The previous superplume, about 800–740 Ma, requires further justification but fits the global periodicity with 240 Ma major cycles and smaller ones of 120 (or also 30) Ma.In the Late Cambrian–Ordovician, large-scale accretion and collision events acted, in similar tectonic settings, upon the vast territory that currently extends from the Polar Urals to Lake Baikal (and was times larger in the past). As a result, Gondwanian microcontinents (Kokchetav, Altai–Mongolia, Tuva–Mongolia, etc.) and island arcs joined into the Kazakhstan–Tuva–Mongolia system. The formation of the Late Cambrian–Ordovician orogen in Central Asia was synchronous with opening of the Ural, Ob–Zaisan, Turkestan, and Paleotethys oceans. The plume pulses (520–500 and 490–460 Ma) may have been responsible for opening of new oceans, accelerated amalgamation of terranes, and synchronicity in geodynamic events from the Urals to Transbaikalia. 相似文献
8.
The making and breaking of supercontinents: Some speculations based on superplumes, super downwelling and the role of tectosphere 总被引：13，自引：7，他引：6
The mechanisms of formation and disruption of supercontinents have been topics of debate. Based on the Y-shaped topology, we identify two major types of subduction zones on the globe: the Circum-Pacific subduction zone and the Tethyan subduction zone. We propose that the process of formation of supercontinents is controlled by super downwelling that develops through double-sided subduction zones as seen in the present day western Pacific, and also as endorsed by both geologic history and P-wave whole mantle tomography. The super-downwelling swallows all material like a black hole in the outer space, pulling together continents into a tight assembly. The fate of supercontinents is dictated by superplumes (super-upwelling) which break apart the continental assemblies. We evaluate the configuration of major supercontinents through Earth history and propose the tectonic framework leading to the future supercontinent Amasia 250 million years from present, with the present day Western Pacific region as its frontier. We propose that the tectosphere which functions as the buoyant keel of continental crust plays a crucial role in the supercontinental cycle, including continental fragmentation, dispersion and amalgamation. The continental crust is generally very thin, only about one tenth of the thickness of the tectosphere. If the rigidity and buoyancy is derived from the tectosphere, with the granitic upper crust playing only a negligible role, then supercontinent cycle may reflect the dispersion and amalgamation of the tectosphere. Therefore, supercontinent cycle may correspond to super-tectosphere cycle. 相似文献
9.
The Grenvillian and Pan-African orogens: World's largest orogenies through geologic time, and their implications on the origin of superplume 总被引：11，自引：11，他引：0
The Neoproterozoic Earth was shaped largely by the Grenvillian and Pan-African orogenies. Out of these, the Grenvillian orogeny has long been regarded to be of minor nature in terms of global-scale orogenic episodes, whereas the Pan-African orogeny has been widely recognized in many continental fragments, although not in major parts of Asia. Based on chronological information in zircons from major river mouths across several important terrains of the globe, we show here that the Grenvillian orogeny contributed significantly to the formation of the continental crust. The time period between 0.6 Ga and 0.8 Ga marked the climax at the dawn of the Pan-African orogeny. Continental crust formed in this period is concentrated in the Pan-African orogenic belts widely across the globe. These regions were widespread over the half hemisphere of the globe, and were subsequently reduced in size after they moved to form Laurasia. The normalized frequency distribution of zircon ages from river-mouth sand over the world clearly demonstrates that Neoproterozoic and (0.9–0.6 Ga) and Grenvillian (1.3–1.0 Ga) peaks define the largest population. This means that extensive subduction, and hence active plate tectonics, might have operated through these periods. The zircon study has also brought to light new regions of the Grenvillian orogenic belts, particularly in the continents which are now covered by thick Phanerozoic sedimentary basins. Based on the new locations of Grenvillian orogens identified in this study, and using the distribution patterns as a marker bed, we propose revised paleogeographic configurations of the Rodinia and Gondwana supercontinents.Our results demonstrate that the Neoproterozoic was the most active period of crust formation in the Earth. The cold basins, formed right after the assembly of Rodinia, exhibit a basin chain fringing the northern periphery of Rodinia, which turned into sites of mantle upwellings and led to the rifting and separation of the supercontinental assembly. The continents then moved northwards after the formation of Gondwana at ca. 540 Ma, and enlarged the northern half of the supercontinent Pangea since 250 Ma.Based on the results, we also evaluate the role of supercontinents in the mechanism of generation of superplumes addressing the enigma that the coldest mantle right above the Core–Mantle Boundary turns to the hottest one over a period of several hundreds of million years. Slab graveyard formed by the Pan-African subduction can be imaged through P-wave tomography. We postulate that the high-velocity anomaly in the D” layer underneath Gondwana has now transformed to the low-V regions to generate the African superplume. The tectonic history of solid Earth in the Phanerozoic seems to be controlled by the slab graveyards formed by the Grenvillian orogeny ca. 1.0 Ga. 相似文献
10.
M. Santosh 《地学前缘(英文版)》2010,1(1):21-30
<正>The formation and disruption of supercontinents have significantly impacted mantle dynamics,solid earth processes,surface environments and the biogeochemical cycle.In the early history of the Earth,the collision of parallel intra-oceanic arcs was an important process in building embryonic continents.Superdownwelling along Y-shaped triple junctions might have been one of the important processes that aided in the rapid assembly of continental fragments into closely packed supercontinents. Various models have been proposed for the fragmentation of supercontinents including thermal blanket and superplume hypotheses.The reassembly of supercontinents after breakup and the ocean closure occurs through "introversion","extroversion" or a combination of both,and is characterized by either Pacific-type or Atlantic-type ocean closure.The breakup of supercontinents and development of hydrothermal system in rifts with granitic basement create anomalous chemical environments enriched in nutrients, which serve as the primary building blocks of the skeleton and bone of early modern life forms. A typical example is the rifting of the Rodinia supercontinent,which opened up an N—S oriented sea way along which nutrient enriched upwelling brought about a habitable geochemical environment.The assembly of supercontinents also had significant impact on life evolution.The role played by the Cambrian Gondwana assembly has been emphasized in many models,including the formation of 'Trans-gondwana Mountains' that might have provided an effective source of rich nutrients to the equatorial waters,thus aiding the rapid increase in biodiversity.The planet has witnessed several mass extinction events during its history,mostly connected with major climatic fluctuations including global cooling and warming events,major glaciations,fluctuations in sea level,global anoxia,volcanic eruptions, asteroid impacts and gamma radiation.Some recent models speculate a relationship between superplumes,supercontinent breakup and mass extinction.Upwelling plumes cause continental rifting and formation of large igneous provinces.Subsequent volcanic emissions and resultant plume-induced "winter" have catastrophic effect on the atmosphere that lead to mass extinctions and long term oceanic anoxia.The assembly and dispersal of continents appear to have influenced the biogeochemical cycle,but whether the individual stages of organic evolution and extinction on the planet are closely linked to Solid Earth processes remains to be investigated. 相似文献
1