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1.
东昆仑、玉树、汶川地震的发生规律和形成机理:兼论大陆地震成因与预测 总被引：6，自引：3，他引：3
青藏高原东北部东昆仑、汶川、玉树等强震的同震地表破裂不对称发育,伴随余震有规律地分别向东、南东和北北东方向迁移,很可能是源于恒河盆地流经亚东、当雄、安多、库赛湖、治多、玉树、甘孜、汶川的弧形下地壳“热河”的流速和流向变化形成的,下地壳热流物质正在向云南及邻区汇聚形成下地壳“热海”,导致长时间跨季度构造热干旱,其影响超过大气环流的作用。地表破裂不一定受断层控制,震源也不在断层面上,下地壳流动导致中地壳发震并进一步影响上地壳形成同震脆性破裂系统。大陆板内盆山过渡带地震密集,大陆板内地震是在下地壳层流的热动力作用下导致活动地壳分层变形的产物。在大陆盆山耦合、圈层耦合的非线性开放系统中,从大洋底部的软流圈层流进入大陆底部使得地幔软流圈加厚,底辟上升为大陆下地壳流动,为地震活动提供了巨量热能;热软化的下地壳缓慢的韧性流动孕育了大陆板内地震;中地壳韧 脆性剪切带易于积累能量,发生热能与应变能的转化,产生地震,形成震源层;上地壳脆性断层活动和地表破裂是地震释放深部能量的载体和方式之一。地壳稳定性评价的依据应当是地壳的活动性而不是断层的活动性。大陆活动构造区地震活跃期与平静期交替实际上是下地壳地震能量的聚散过程,体现在下地壳热主导的韧性流动构造与上地壳应力主导的脆性破裂构造之间的相互作用。下地壳热软化物质流动过程中流速、流向等突然改变触发地震,并产生共振波。大陆下地壳流层在厚度、温度、粘度、流速、流向上的变化产生一定程度的温度异常、流体异常及与其相关的大气层、电场、磁场、重力场、地球化学场、应力场、应变场、生物场等异常。合理布置天空网、地面网、地下网,综合立体监测有效的地震前兆,系统地开展长期、中期和短临地震预测,能够不断地提高地震预测水平。 相似文献
2.
The existence of peridotitic komatiites in the Archaean suggests that the Archaean mantle was significantly hotter than the modern mantle. This evidence is contradicted by estimates of Archaean continental geothermal gradients, based on the pressure and temperature recorded in metamorphic rocks, which suggest that there is no marked difference between Archaean and modern continental geothermal gradients.Numerical modelling shows that small changes in the mantle temperature can have an important influence on convection. If the average temperature of the upper mantle is increased by 200°C, convection within the mantle becomes chaotic and an upper mantle partial melt zone encircles the globe. The crust formed during this period will be komatiitic in composition but will be unstable and will be mixed back into the mantle by subduction. Later, when the mantle temperature falls to 100°C above its present level, the upper mantle partial melt zone contracts away from subduction areas.It is suggested that the first primitive felsic magmas were generated at subduction zones. The appearance of these magmas at ～3.8 Ga permitted the formation of buoyant continents and eventually led to crustal thickening. As a consequence of this thickening the proto-continents, consisting of a bimodal suite of basalts and sodic granodiorites, contained two types of latent energy: (1) radioactive energy held in elements such as Th, K and U; and (2) potential energy resulting from the elevation of the continents above sea level. The potential energy of the continents led to sedimentation. The increase in the rate of sedimentation during the Archaean resulted from increased crustal buoyancy. At the same time heat released by radioactive elements in the deep crust built up under the insulating blanket of the upper crust. This caused a major metamorphic, metasomatic and crustal melting event which produced the potassic granites of the late Archaean. Once the radioactive elements had been removed from the lower crust, that region of the continent become tectonically stable. The Proterozoic shelf sediments were deposited at the margins of these stable cratons.Convection models of the Archaean mantle show hot diapirs rising from the boundary layer above the core—mantle interface. We suggest that these diapirs began to melt at a depth of ～ 450 km, giving rise to komatiitic magmas. This model requires the average temperature of the Archaean upper mantle to be ～ 100°C above that of the modern mantle. The similarity between Archaean and modern continental geothermal gradients can be explained if Archaean continents formed above subduction zones.Raising the temperature of the Archaean mantle by 100°C (1) halves the thickness of the oceanic lithosphere, (2) increases the oceanic geothermal gradient at the mid-point of a convection cell, (3) decreases the viscosity of the mantle by at least an order of magnitude. The combination of these effects produces a marked decrease in the strength of the Archaean lithosphere and mantle. Thus the form of Archaean tectonics can be expected to have been very different from modern tectonics. 相似文献
3.
Jean H.Bédard 《地学前缘(英文版)》2018,(1)
The lower plate is the dominant agent in modern convergent margins characterized by active subduction,as negatively buoyant oceanic lithosphere sinks into the asthenosphere under its own weight.This is a strong plate-driving force because the slab-pull force is transmitted through the stiff sub-oceanic lithospheric mantle.As geological and geochemical data seem inconsistent with the existence of modernstyle ridges and arcs in the Archaean,a periodically-destabilized stagnant-lid crust system is proposed instead.Stagnant-lid intervals may correspond to periods of layered mantle convection where efficient cooling was restricted to the upper mantle,perturbing Earth's heat generation/loss balance,eventually triggering mantle overturns.Archaean basalts were derived from fertile mantle in overturn upwelling zones(OUZOs),which were larger and longer-lived than post-Archaean plumes.Early cratons/continents probably formed above OUZOs as large volumes of basalt and komatiite were delivered for protracted periods,allowing basal crustal cannibalism,garnetiferous crustal restite delamination,and coupled development of continental crust and sub-continental lithospheric mantle.Periodic mixing and rehomogenization during overturns retarded development of isotopically depleted MORB(mid-ocean ridge basalt)mantle.Only after the start of true subduction did sequestration of subducted slabs at the coremantle boundary lead to the development of the depleted MORB mantle source.During Archaean mantle overturns,pre-existing continents located above OUZOs would be strongly reworked;whereas OUZOdistal continents would drift in response to mantle currents.The leading edge of drifting Archaean continents would be convergent margins characterized by terrane accretion,imbrication,subcretion and anatexis of unsubductable oceanic lithosphere.As Earth cooled and the background oceanic lithosphere became denser and stiffer,there would be an increasing probability that oceanic crustal segments could founder in an organized way,producing a gradual evolution of pre-subduction convergent margins into modern-style active subduction systems around 2.5 Ga.Plate tectonics today is constituted of:(1)a continental drift system that started in the Early Archaean,driven by deep mantle currents pressing against the Archaean-age sub-continental lithospheric mantle keels that underlie Archaean cratons;(2)a subduction-driven system that started near the end of the Archaean. 相似文献
4.
THINNING OF THE THICKENED LITHOSPHERE AND ITS GEODYNAMIC CONSEQUENCE: APPLICATION FOR TIBETAN PLATEAU1 ParsonsB ,McKenzieD .Mantleconvectionandthethermalstructureoftheplates[J] .JGeophysRes,1978,83;4 4 85～4 4 96 .
2 HousemanGA ,McKenzieDP ,MolnarP .Convectiveinstabilityofathickenedboundarylayeranditsrelevanceforthethermalevolutionofcontinentalconvergentbelts[J] .JGeophysRes 1981,86 :6 115～ 6 132 .
3 OwensTJ,ZandtG ,Theimplicationsofc… 相似文献
5.
Shinji Yamamoto Hiroki Senshu Shuji Rino Soichi Omori Shigenori Maruyama 《Gondwana Research》2009,15(3-4):443-453
Continental growth has been episodic, reflecting the episodic nature of mantle dynamics as well as surface dynamics of the Earth, the net result of which is exhibited by the present mantle with two huge reservoirs of TTG rocks, one on the surface continents and the other on the D″ layer on the Core-Mantle Boundary (CMB). During the early half of the Earth history, the felsic continental crust on the surface which formed in an intra-oceanic environment has mostly been subducted into the deep mantle, except in the rare case of parallel arc collision. The growth history of continental crust shows that with its simultaneous formation, a considerable amount must have also been subducted. Such ongoing subduction processes can be seen in the western Pacific region, through tectonic erosion, arc subduction, and sediment-trapped subduction. 相似文献
6.
青藏高原及邻区三角形发震构造域是全球大陆最显著的地震多发区.脆性活动断层及其弹性回跳模式无法合理解释该区深度集中分布在10~40 km的点状震源.针对发震构造和地震机理不明确这一重大科学问题, 以大陆动力学和地球系统动力学新思想为指导, 对青藏高原及邻区发震构造系统进行域、层、带、点相关研究, 阐明大陆地震构造系统的结构型式, 认为下地壳固态流变及其韧性剪切带是提供地震能量的孕震构造, 中地壳韧-脆性剪切带是累积地震能量的发震构造, 上地壳脆性断裂是释放地震能量的释震构造.在研究青藏高原及邻区地震构造系统及其形成背景的基础上, 进一步论证了大陆地震热流体撞击的形成机理: 地幔墙导致大洋中脊之下的软流圈热流物质层流到大陆特定部位汇聚加厚并底辟上升, 造成大陆下地壳部分熔融和固态流变, 并改变莫霍面的产状, 固态流变物质侧向非均匀流动, 形成大陆盆山体系, 流动的韧性下地壳与脆性上地壳之间具有韧-脆性剪切滑脱性质的中地壳不断积累由下地壳热能转换而来的应变能, 形成发震层, 震源定位于下地壳热流物质富集带("热河")中的固态-半固态流变物质撞击到强弱层块之间的构造边界, 不同热构造环境和撞击角度产生5种不同类型的地震.从而为大陆地震的科学预测奠定了全新的理论基础. 相似文献
7.
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.
Lithospheric evolution of the Andean fold–thrust belt, Bolivia, and the origin of the central Andean plateau 总被引：1，自引：1，他引：1
Nadine McQuarrie Brian K. Horton George Zandt Susan Beck Peter G. DeCelles 《Tectonophysics》2005,399(1-4):15
We combine geological and geophysical data to develop a generalized model for the lithospheric evolution of the central Andean plateau between 18° and 20° S from Late Cretaceous to present. By integrating geophysical results of upper mantle structure, crustal thickness, and composition with recently published structural, stratigraphic, and thermochronologic data, we emphasize the importance of both the crust and upper mantle in the evolution of the central Andean plateau. Four key steps in the evolution of the Andean plateau are as follows. 1) Initiation of mountain building by 70 Ma suggested by the associated foreland basin depositional history. 2) Eastward jump of a narrow, early fold–thrust belt at 40 Ma through the eastward propagation of a 200–400-km-long basement thrust sheet. 3) Continued shortening within the Eastern Cordillera from 40 to 15 Ma, which thickened the crust and mantle and established the eastern boundary of the modern central Andean plateau. Removal of excess mantle through lithospheric delamination at the Eastern Cordillera–Altiplano boundary during the early Miocene appears necessary to accommodate underthrusting of the Brazilian shield. Replacement of mantle lithosphere by hot asthenosphere may have provided the heat source for a pulse of mafic volcanism in the Eastern Cordillera and Altiplano at 24–23 Ma, and further volcanism recorded by 12–7 Ma crustal ignimbrites. 4) After 20 Ma, deformation waned in the Eastern Cordillera and Interandean zone and began to be transferred into the Subandean zone. Long-term rates of shortening in the fold–thrust belt indicate that the average shortening rate has remained fairly constant (8–10 mm/year) through time with possible slowing (5–7 mm/year) in the last 15–20 myr. We suggest that Cenozoic deformation within the mantle lithosphere has been focused at the Eastern Cordillera–Altiplano boundary where the mantle most likely continues to be removed through piecemeal delamination. 相似文献
10.
http://dx.doi.org/10.1016/j.gsf.2016.07.005 总被引：1，自引：1，他引：0
The Hadean history of Earth is shrouded in mystery and it is considered that the planet was born dry with no water or atmosphere. The Earth-Moon system had many features in common during the birth stage. Solidification of the dry magma ocean at 4.53 Ga generated primordial continents with komatiite. We speculate that the upper crust was composed of fractionated gabbros and the middle felsic crust by anorthosite at ca. 21 km depth boundary, underlain by meta-anorthosite (grossular + kyanite + quartz) down to 50–60 km in depth. The thickness of the mafic KREEP basalt in the lower crust, separating it from the underlying upper mantle is not well-constrained and might have been up to ca. 100–200 km depending on the degree of fractionation and gravitational stability versus surrounding mantle density. The primordial continents must have been composed of the final residue of dry magma ocean and enriched in several critical elements including Ca, Mg, Fe, Mn, P, K, and Cl which were exposed on the surface of the dry Earth. Around 190 million years after the solidification of the magma ocean, “ABEL bombardment” delivered volatiles including H2O, CO2, N2 as well as silicate components through the addition of icy asteroids. This event continued for 200 Myr with subordinate bombardments until 3.9 Ga, preparing the Earth for the prebiotic chemical evolution and as the cradle of first life. Due to vigorous convection arising from high mantle potential temperatures, the primordial continents disintegrated and were dragged down to the deep mantle, marking the onset of Hadean plate tectonics. 相似文献
11.
Crustal accretion and dynamic feedback on mantle melting of a ridge centred plume: The Iceland case 总被引：1，自引：0，他引：1
A major consequence of the interaction of a plume with an oceanic ridge is the enhanced melt production and associated crust generation. In the case of Iceland crustal thickness as large as 20 to 40 km has been reported. Crustal seismic velocities are high, and have to be explained by thermal or chemical effects. In the first part of the paper we address the question whether extraction of melt out of the plume beneath a slowly spreading ridge and deposition of extracted basalt volumes at the surface produces a dynamic feedback mechanism on mantle melting. To study this question we solve the convection equations for a ridge centred plume with non-Newtonian rheology including melting, melt extraction associated with deposition of cold crust at the surface of the model, and using a simplified approach for compaction. The assumption of cold crust is justified if the thickness of each deposited basaltic layer is less than roughly 1 km. Depending on the buoyancy flux of the plume, crustal thicknesses between 10 and 40 km are modelled, showing characteristic dipping structures resembling the rift-ward dipping basaltic layers of East- and Western Iceland. Comparing the resulting crustal thickness and magma generation rate with models in which the dynamic effect of crust deposition has been suppressed indicates, that melt generation beneath a slowly spreading ridge is considerably damped by the dynamic feedback mechanism if the plume buoyancy flux exceeds 400 to 600 kg/s. Based on the observed crustal thickness of Iceland our models predict a plume buoyancy flux of 1140 kg/s.In the second part we study the accretion of the Icelandic crust by a thermo-mechanical model in more detail based on the Navier–Stokes-, the heat transport and the mass conservation equations including volumetric sources. Hot (1200 °C) molten crustal material is injected into the newly forming crust with a constant rate at different crustal source regions: a) deep, widespread emplacement of dykes and sills including crustal underplating, b) magma chambers at shallow to mid-crustal level, and c) surface extrusions and intrusions in fissure swarms at shallow depth connected to volcanic centres. We identify the material from the different source regions by a marker approach. Varying the relative dominance of these source regions, characteristic crustal structures evolve, showing shallow dipping upper crustal layers with dip angles between 10 and 15°. The thermal structure of the crust varies between cold crust (shallow-source region dominating) and hot crust (deep-source region dominating). We use observations of maximum depth of seismicity to constrain the depth of the 650 °C isotherm and seismological inferences on the lower crust to constrain temperatures in that region. The best agreement with our models is achieved for crust formation dominated by deep dykes and underplating with a considerable influence of magma chamber accretion. 相似文献
12.
地壳根、造山热与岩浆作用 总被引：8，自引：1，他引：7
简要讨论了近年来造山带及其岩浆作用研究的主要进展。造山带流变学结构与造山热和岩浆作用有着密切的耦合关系。年轻的山带往往存在地壳根,但古老的山带地壳根是否存在,取决于造山带的热状态和榴辉岩化的强度,只有缺乏流体和冷的造山带才保留有地壳根,例如古生代的南乌拉尔山和北美前寒武纪的南Trans-Hudson造山带。造山带的伸展塌陷往往伴随着幔源岩浆底侵、地壳软化、隆升和强烈岩浆作用。由于地幔浮力和造山热的作用,一些山带具有高的海拔和薄而热的地壳,属于具有长期活动性的构造带。研究表明,这些具有长期活动性的构造带,是建立在以前形成的热的、软化了的弧后区内。中国昆仑—秦岭—大别造山系北缘,古生代时期发育了与俯冲有关的弧岩浆带,而南缘发育了相近时代的与弧后伸展有关的双峰式岩浆带,构成古生代双岩浆带。该造山系早中生代的造山作用,就是在南缘的古生代弧后岩浆带基础上发展起来的。因此,该双岩浆带提供了造山热控制复合造山作用的实例。 相似文献
13.
14.
This paper presents the results of the study of the free mantle surface (FMS) depth beneath continents and oceans. The reasons
for the observed dependence of the FMS depth on the crustal thickness in the continental lithosphere are discussed. The influence
of radial variations in the mantle’s density is evaluated. The calculations performed have indicated that the observed dependence
of the FMS depth on the crustal thickness is caused mostly by lateral inhomogeneities in the lithospheric mantle, and the
size of these inhomogeneities is proportional to the thickness of the crust. The origin of such inhomogeneities can be related
to the process of continental crust formation. 相似文献
15.
Crustal thinning beneath the Rwenzori region, Albertine rift, Uganda, from receiver-function analysis 总被引：1，自引：1，他引：0
Ingo Wölbern G. Rümpker A. Schumann A. Muwanga 《International Journal of Earth Sciences》2010,99(7):1545-1557
The Rwenzori mountains in western Uganda, with a maximum elevation of more than 5,000 m, are located within the Albertine
rift valley. We have deployed a temporary seismic network on the Ugandan side of the mountain range to study the seismic velocity
structure of the crust and upper mantle beneath this section of the rift. We present results from a receiver-function study
revealing a simple crustal structure along the eastern rift flank with a more or less uniform crustal thickness of about 30 km.
The complexity of inner-crustal structures increases drastically within the Rwenzori block. We apply different inversion techniques
to obtain reliable results for the thickness of the crust. The observations expose a significantly thinner crust beneath the
Rwenzori range with thickness values ranging from about 20–28 km beneath northern and central parts of the mountains. Our
study therefore indicates the absence of a crustal root beneath the Rwenzori block. Beneath the Lake Edward and Lake George
basins we detect the top of a layer of significantly reduced S-wave velocity at 15 km depth. This low-velocity layer may be
attributed to the presence of partial melt beneath a region of recent volcanic activity. 相似文献
16.
Continental recycling and true continental growth 总被引：1，自引：0，他引：1
Tsuyoshi Komiya 《Russian Geology and Geophysics》2011,52(12):1516-1529
Continental crust is very important for evolution of life because most bioessential elements are supplied from continent to ocean. In addition, the distribution of continent affects climate because continents have much higher albedo than ocean, equivalent to cloud. Conventional views suggest that continental crust is gradually growing through the geologic time and that most continental crust was formed in the Phanerozoic and late Proterozoic. However, the thermal evolution of the Earth implies that much amounts of continental crust should be formed in the early Earth. This is “Continental crust paradox”.Continental crust comprises granitoid, accretionary complex, and sedimentary and metamorphic rocks. The latter three components originate from erosion of continental crust because the accretionary and metamorphic complexes mainly consist of clastic materials. Granitoid has two components: a juvenile component through slab-melting and a recycling component by remelting of continental materials. Namely, only the juvenile component contributes to net continental growth. The remains originate from recycling of continental crust. Continental recycling has three components: intracrustal recycling, crustal reworking, and crust–mantle recycling, respectively. The estimate of continental growth is highly varied. Thermal history implied the rapid growth in the early Earth, whereas the present distribution of continental crust suggests the slow growth. The former regards continental recycling as important whereas the latter regarded as insignificant, suggesting that the variation of estimate for the continental growth is due to involvement of continental recycling.We estimated erosion rate of continental crust and calculated secular changes of continental formation and destruction to fit four conditions: present distribution of continental crust (no continental recycling), geochronology of zircons (intracontinental recycling), Hf isotope ratios of zircons (crustal reworking) and secular change of mantle temperature. The calculation suggests some important insights. (1) The distribution of continental crust around at 2.7 Ga is equivalent to the modern amounts. (2) Especially, the distribution of continental crust from 2.7 to 1.6 Ga was much larger than at present, and the sizes of the total continental crust around 2.4, 1.7, and 0.8 Ga became maximum. The distribution of continental crust has been decreasing since then. More amounts of continental crust were formed at higher mantle temperatures at 2.7, 1.9, and 0.9 Ga, and more amounts were destructed after then. As a result, the mantle overturns led to both the abrupt continental formation and destruction, and extinguished older continental crust. The timing of large distribution of continental crust apparently corresponds to the timing of icehouse periods in Precambrian. 相似文献
17.
A. V. Zubovich V. I. Makarov S. I. Kuzikov O. I. Mosienko G. G. Shchelochkov 《Geotectonics》2007,41(1):13-25
The results of longstanding GPS measurements in the northwestern part of Central Asia are discussed. These results impose certain constraints for modeling of intraplate tectonic processes. In the territory covered by observations, the velocity vectors of recent motions of the Earth’s surface relative to the stable portion of Eurasia decrease northward. The plane field of velocities, which rules out the development of extension zones, indicates the impossibility of the mountain building driven by ascending mantle flows beneath the lithosphere of these regions. The nonuniform spatial distribution of the motions is suggestive of the discrete character of the Earth’s crust and its deformation. The crust is brittle, at least in its upper part, and capable of breaking into blocks. The blocks, which move at different velocities, interact with one another and change their original orientation and position, while experiencing independent deformations. This phenomenon has been exemplified in the Tarim Block and the Tien Shan. Within the limits of the constraints imposed by the GPS measurements, the mechanism of intracontinental mountain building related to the lateral flow of asthenospheric material and to the drag of the overlying lithospheric layers is discussed. This mechanism springs from Argand’s ideas [2, 29] and the plate tectonic concept [10, 23]. The upper-mantle convective flow in the direction of the Indian Plate’s motion was the main cause of the crustal deformation. The detachment of the lithospheric mantle from the Indian Plate approximately 25 Ma ago and its subduction beneath the Himalayas and Tibet, along with simultaneous ascent of the remaining crust and uplift of the Tibetan Plateau, allowed the mantle flow to spread far northward beneath the Asian continent. This process is accompanied by consecutive separation and sinking of the cooling asthenospheric material over the entire area from the Himalayas to Siberia as the subcrustal material cools. As a result, the flow velocity decreases, the roof of the active flow plunges, and the lithosphere becomes thicker. The motion and deformation of the lithospheric layers dragged by deep flow cannot follow the asthenospheric flow strictly, owing to the rigidity of the layers. Therefore, a difference of tangential velocities originates between the flow and the lithosphere, thus giving rise to horizontal shear stresses. These stresses affect the overlying lithospheric layers, including the crustal ones, and bring about their drag and tectonic delamination. Simultaneously, the decreasing velocity in the direction of the mantle flow results in bending of the lithospheric layers that is accompanied by local warping of the crust and its stacking and fragmentation into blocks. The different velocities of block motions lead to their mechanical interactions. This scenario of intracontinental mountain building allows an explanation of the many specific features of tectonic processes and orogeny in within-plate mountainous regions. 相似文献
18.
Jun Korenaga 《地学学报》2008,20(6):419-439
The chemical composition of the bulk silicate Earth (BSE) indicates that the present‐day thermal budget of Earth is likely to be characterized by a significant excess of surface heat loss over internal heat generation, indicating an important role of secular cooling in Earth’s history. When combined with petrological constraints on the degree of secular cooling, this thermal budget places a tight constraint on permissible heat‐flow scaling for mantle convection, along with implications for the operation of plate tectonics on Earth, the history of mantle plumes and flood basalt magmatism, and the origin and evolution of Earth’s oceans. In the presence of plate tectonics, hotter mantle may have convected more slowly because it generates thicker dehydrated lithosphere, which could slow down subduction. The intervals of globally synchronous orogenies are consistent with the predicted variation of plate velocity for the last 3.6 Gyr. Hotter mantle also produces thicker, buoyant basaltic crust, and the subductability of oceanic lithosphere is a critical factor regarding the emergence of plate tectonics before the Proterozoic. Moreover, sluggish convection in the past is equivalent to reduced secular cooling, thus suggesting a more minor role of mantle plumes in the early Earth. Finally, deeper ocean basins are possible with slower plate motion in the past, and Earth’s oceans in the Archean is suggested to have had about twice as much water as today, and the mantle may have started as dry and have been gradually hydrated by subduction. The global water cycle may thus be dominated by regassing, rather than degassing, pointing towards the impact origin of Earth’s oceans, which is shown to be supported by the revised composition of the BSE. 相似文献
19.
Because of the strongly different conditions in the mantle of the early Earth regarding temperature and viscosity, present-day geodynamics cannot simply be extrapolated back to the early history of the Earth. We use numerical thermochemical convection models including partial melting and a simple mechanism for melt segregation and oceanic crust production to investigate an alternative suite of dynamics which may have been in operation in the early Earth. Our modelling results show three processes that may have played an important role in the production and recycling of oceanic crust: (1) Small-scale (x×100 km) convection involving the lower crust and shallow upper mantle. Partial melting and thus crustal production takes place in the upwelling limb and delamination of the eclogitic lower crust in the downwelling limb. (2) Large-scale resurfacing events in which (nearly) the complete crust sinks into the (eventually lower) mantle, thereby forming a stable reservoir enriched in incompatible elements in the deep mantle. New crust is simultaneously formed at the surface from segregating melt. (3) Intrusion of lower mantle diapirs with a high excess temperature (about 250 K) into the upper mantle, causing massive melting and crustal growth. This allows for plumes in the Archean upper mantle with a much higher excess temperature than previously expected from theoretical considerations. 相似文献
20.
The relative importance of the contribution of the lower crust and of the lithospheric mantle to the total strength of the continental lithosphere is assessed systematically for realistic ranges of layer thickness, composition, and temperature. Results are presented as relative strength maps, giving the ratio of the lower crust to upper mantle contribution in terms of crustal thickness and surface heat flow. The lithosphere shows a “jelly sandwich” rheological layering for low surface heat flow, thin to average crustal thickness, and felsic or wet mafic lower crustal compositions. On the other hand, most of the total strength resides in the seismogenic crust in regions of high surface heat flow, crust of any thickness, and dry mafic lower crustal composition. 相似文献