Assimilation of continental crust by komatiites in the Precambrian basement of the Carswell structure (Saskatchewan,Canada)

The isotope geochemistry (Sm-Nd, Pb-Pb and Rb-Sr) of mafic gneisses from the basement of the Carswell structure (Saskatchewan, Canada), rich both in Mg and incompatible elements (K, Rb, REE) has been investigated. A good Sm-Nd alignment gives a slope corresponding to an age of 3.7 Ga. However, comparison with major elements data strongly suggests that this alignment is a mixing line between Mg-rich, high CaO/Al₂O₃ magmas and the local felsic crust older than 2.9 Ga. The mafic magmas were probably of komatiitic affinity (MgO > 20 percent) but, nevertheless, were extracted from a source with nearly chondritic to slightly enriched light REE distribution. The age of the komatiite emplacement (1.9–2.9 Ga) is only loosely constrained by the oldest crustal residence age in the series and the subsequent metamorphic events. The granulite facies climax is dated at ca. 1.9 Ga by concordant whole rock Pb-Pb and Sm-Nd garnet-whole rock isochrons. The Rb-Sr systematics have been disturbed by later event(s) younger than 1.5–1.7 Ga, but do not permit a more precise assessment of the perturbation age.     

远震数据显示贝加尔裂谷带北穆伊斯克(Severomuysk)段地壳和上地幔的精细结构

通过贝加尔裂谷系统北穆伊斯克(Severomuysk)段的密集地震台站线性网络获得的远距离强震记录,并利用P波接收函数技术,揭示了地壳和上地幔顶部的复杂分层块状结构。横波速度的分布表明构成北穆伊斯克地壳的地块具有不同性质。这些地块的西部聚散和地壳下部的分层证实了该地区隆起的堆积-碰撞起源。位于西伯利亚克拉通变薄的倾斜边缘上的北穆伊斯克段解释了该地区地壳碰撞效应的强度。2015年地震的震中深度与Muyakan 凹陷地壳上部的明显速度差异存在令人信服的相关性。     

History of Earth's crust and isostasy

Isostatic equilibrium occurs in many parts of the Earth that differ in history and current crustal structure, indicating that major changes,in crustal composition and thickness during development have tended to preserve the state of equilibrium. A state of equilibrium disturbed by tectonic processes is soon restored. Processes that disturb the equilibrium development of the crust are called causative; those that arise as a consequence and tend to restore equilibrium are compensative. There is evidence for these two processes, which are responsible for displacements of opposite sign in the base and surface of the consolidated crust when equilibrium is regained. These processes indicate changes in different parts of the crust and enable us to say that the crust shows two types of development. The structure of the crust corresponds to the Airy-Plan general model, and this structure may be described by isostatic equations when the two types of processes are in balance. The same equations can be used when the state is not in equilibrium if there is introduced a correction to be deduced from gravity anomalies and data on the deep structure of the crust. These equations can be used to describe changes in the crust, and isostatic equations enable one to combine ideas with new data to describe changes in thickness and composition in some layers of the crust during development as well as some aspects of block movement. Examples of the use of these isostatic equations are given.—C. E. Sears     

Limits of stress measurements in the Earth's crust

Summary Limits of Stress Measurements in the Earth's Crust. Accurate measurements of stress in the Earth's crust cannot be performed at deep levels with currently available recording procedures. The maximum stress recordable is about 100MPa, and the maximum depth 1000–1200 m.

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Zusammenfassung Grenzen der Bestimmbarkeit der Spannungen in der Erdkruste. In großen Tiefen kann mit derzeit verfügbaren Untersuchungsmethoden die tatsächliche Spannung in der Erdkruste nicht bestimmt werden. Die größte meßbare Spannung beträgt ungefähr 100 MPa, die größte Tiefe 1000–1200 m.

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Résumé Limites de la possibilité de la détermination des contraintes dans l'écorce terrestre. Il est impossible de faire des déterminations exactes des contraintes dans l'écorce terrestre en utilisant les méthodes courantes. La contrainte la plus grande qui peut être mesurée est à peu près 100 MPa et la profondeur maximale 1000–1200 m.

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A lecture delivered at the Joint General Assemblies of the IASPEI and IAVCEI in Durham, August 1977.

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With 4 Figures     

Electric conductivity anomaly in the Earth's crust and mantle

Much evidence that reflects anomalous distributions of electrical conductivity within the earth's crust and mantle has been put forward in recent years through geomagnetic and geoelectric observations. This anomaly is called the “CA” (conductivity anomaly), and the study of such heterogeneity has become one of the important topics of present-day geophysics.     

Volcanic products in building of Earth's crust

A highly tentative analysis suggests that volcanism contributed less than 42 x 10¹⁸ but more than 14 x 10⁸ tons solid substance to the lithosphere and at least 7.2 x 10¹⁷ tons water to the hydrosphere during the entire geologic history of the Earth (4.5 x 10⁹ years). Volcanic contributions to the atmosphere are believed to be “infinitesimally small,” since the bulk of volcanic gases has been converted into solids (carbon, sulfur, etc.) or into solutes. -- V.P. Sokoloff.     

3D structure of the Earth's crust beneath the northern part of the Bohemian Massif

The SUDETES 2003 wide-angle refraction/reflection experiment covered the area of the south-western Poland and the northern Bohemian Massif. The good quality data that were gathered combined with the data from previous experiments (POLONAISE'97, CELEBRATION 2000) allowed us to prepare a 3D seismic model of the crust and uppermost mantle for this area. We inverted travel times of both refracted and reflected P waves using the JIVE3D package. This allowed us to obtain a model of P-wave velocity distribution as well as the shape of major boundaries in the crust. We also present a detailed uncertainty analysis for both the boundary depths and the velocity field. In doing the uncertainty analysis we found an interesting, strong dependence between uncertainty and inversion scheme (order of used phases). We also compared the model with surface geology and found good correlation between velocity inhomogeneities in the uppermost crust (down to 2 km) and major geological units. The higher velocity lower crust (6.9–7.2 km/s) could result from remelting of the lower crust or magmatic underplating.     

Formation of hydrocarbon reservoirs in the deep Earth's crust

Formation of reservoirs in crystalline rocks is associated with the development of rifts, with the periodic axial plunge of the rift floor during the extension of the Earth's crust, and with the elevation of consolidated basement masses during a compression phase.     

前寒武纪早期朝鲜半岛地壳的形成与演化

通过对朝鲜半岛前寒武纪结晶基底变质岩的岩石学、年代学和大地构造学分析, 提出太古宙-古元古代是古陆的形成阶段(Ar-Pt₁ ) , 可分为3个阶段: 古陆核的形成阶段(Ar1 -Ar₃ ) 、狼林微陆块的形成阶段(Ar₄ ) 和古陆的形成阶段( Pt₁ ) 。在朝鲜半岛发现的古老地壳岩石位于南浦-甑山和松原地区, 其中最老的岩石位于甑山地区, 年龄为3.4 ～3.6 Ga ( 3 503 ±123 Ma) , 相当于古太古代 (Ar₂ ) ; 松原地区分布着稍年轻的岩石(219～311 Ga) , 相当于中太古代(Ar₃ ) 。古陆核形成于朝鲜半岛北部的南浦-甑山和松原地区, 包括狼林地块和京畿地块的朝鲜半岛太古宙岩石大部分形成于新太古代, 中朝古陆的主体形成于太古宙末。古元古代末(约1 600或1 800 Ma) , 随着摩天岭海闭合, 狼林微陆块与冠帽微陆块通过碰撞聚合而形成一个整体, 称为摩天岭运动(可以对比中国的吕梁运动) 。摩天岭运动结束后, 朝鲜半岛作为中朝古陆的一部分, 整体进入相对稳定的地壳演化阶段。     

地球轨道参数在前寒武纪地层中的记录及研究进展

前寒武纪(4.6 Ga～541 Ma)占据约90％的地球发展历史.该时期大气成分、海洋氧化还原条件、全球气候和生命演化历程等均发生极大程度的改变,为现在的地球系统奠定了基础.地球轨道参数是描述地球系统演化过程的重要指标,对于研究日地系统、地月系统及地球本身演化具有重要意义.近年来一些学者在全球范围内2650～550 M...     

3D model of deep structure of the Early Precambrian crust in the East European Craton and paleogeodynamic implications

The integral 3D model of the deep structure of the Early Precambrian crust in the East European Craton is based on interpretation of the 1-EU, 4B, and TATSEIS seismic CDP profiles in Russia and the adjacent territory of Finland (FIRE project). The geological interpretation of seismic images of the crust is carried out in combination with consideration of geological and geophysical data on the structure of the Fennoscandian Shield and the basement of the East European platform. The model displays tectonically delaminated crust with a predominance of low-angle boundaries between the main tectonic units and the complex structure of the crust-mantle interface, allowing correlation of the deep structure of the Archean Kola, Karelian, and Kursk granite-greenstone terrane with the Volgo-Uralia granulite-gneiss terrane, as well as the Paleoproterozoic intracontinental collision orogens (the Lapland-Mid-Russia-South Baltia orogen and the East Voronezh and Ryazan-Saratov orogens) with the Svecofennian accretionary orogen. The lower crustal “layer” at the base of the Paleoproterozoic orogens and Archean cratons was formed in the Early Paleoproterozoic as a result of underplating and intraplating by mantle-plume mafic magmas and granulite-facies metamorphism. The increase in the thickness of this “layer” was related to hummocking of the lower crustal sheets along with reverse and thrust faulting in the upper crust. The middle crust was distinguished by lower rigidity and affected by ductile deformation. The crust of the Svecofennian Orogen is composed of tectonic sheets plunging to the northeast and consisting of island-arc, backarc, and other types of rocks. These sheets are traced in seismic sections to the crust-mantle interface.     

The reactivation of the ancient structure and corresponding changes in the Earth's crust. An example of the Bohemian Massif

The geological reconstructions suggest that prior to the Upper-Proterozoic sedimentation the Bohemian Massif was formed by a relatively uniform, stable crust with highgrade metamorphites near the surface. This ancient Moldanubian Formation is thought to be 1000–1800 m. y. old. The lowest Earth's crust layer of the initial Moldanubian structure is inferred to be composed by partially hydrated ultramafics.Three principal units showing different development of the initial structure are briefly discussed. The Moldanubicum represents a rigid block. The old Moldanubian sequence was transformed mainly due to the Paleozoic periplutonic metamorphism. An Upper-Proterozoic graben-type collapse generated the Teplá-Barrandian and Labe basins. Attention is given to the possible causes, mechanism and consequences of the longlasting geosynclinal subsidence. In Erzgebirge and Sudeten the initial Moldanubian structure was transformed in a combined way due to the geosynclinal development and Paleozoic metamorphism. Corresponding changes in the Earth's crust are discussed.

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Zusammenfassung Geologische Rekonstruktionen führen zur Ansicht, daß die Böhmische Masse in der Zeitperiode vor der oberproterozoischen Sedimentation aus verhältnismäßig gleichartiger, stabiler Erdkruste mit nahe der Erdoberfläche gelegenen, durch Tiefenmetamorphose umgewandelten Gesteinen bestand. Das Alter dieser »moldanubischen« Formation wird auf 1000 bis 1800 Mill. Jahre geschätzt. Der untere Teil der moldanubischen Ausgangskruste dürfte wahrscheinlich von partiell hydratisierten Ultramafiten aufgebaut worden sein.Im vorliegenden Aufsatz werden drei Gebiete besprochen, wo eine unterschiedliche Entwicklung des ursprünglichen Baues erfolgte. Das Moldanubikum bildet einen rigiden Block, worin der ursprüngliche Ausgangsbau vor allem durch die paläozoische periplutonische Metamorphose umgewandelt wurde. In dem Teplá-Barrandium- und dem Labe (Elbe)-Gebiet erfolgte im oberen Proterozoikum eine Grabeneinsenkung des alten Baues in die Tiefe. Im Text wird ein Modell von Ursachen, Mechanismus und Folgen der langfristigen geosynklinalen Subsidenz dieses Gebietes dargestellt. Im Erzgebirge und in den Sudeten wurde der moldanubische Ausgangsbau durch eine geosynklinale Entwicklung und die paläozoische Metamorphose umgewandelt. Die unterschiedliche Entwicklung führte zu Änderungen in der Zusammensetzung und Mächtigkeit der Erdkruste, die in erwähnten Einheiten durch seismische Charakteristik gekennzeichnet wird.

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Résumé Une reconstruction géologique montre que le Massif de Bohême était formé — avant la sédimentation du Protérozoique supérieur — par une écorce relativement stable et uniforme avec des métamorphites profondément transformée située près de la surface. L'âge de cette ancienne formation »Moldanubienne« est estimée à 1000–1800 millions d'années. La couche la plus profonde de la croûte originelle de la structure Moldanubienne initiale est considerée comme composée par des roches ultramafiques, hydratées en partie.L'auteur discute brièvement les trois unités principales d'ou découle un développement différent de la structure initiale. Le Moldanubien forme un bloc rigide dans lequel la structure initiale de départ a été transformée principalement par le métamorphisme paléozoïque périplutonique. Au Protérozoïque supérieur, un effondrement du type graben a créé les bassins Teplá-Barrandien et de Labe. Les causes possibles, le méchanisme et les conséquences de la subsidence géosynclinale de longue durée sont étudiées. Dans l'Erzgebirge et dans les Sudètes, la structure moldanubienne initiale a été transformée par le développement géosynclinal de même que par le métamorphisme paléozoïque. Les changements y correspondant dans l'écorce terrestre sont discutées.

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Vertical zonation and petrogenesis of the early Precambrian crust in Western Australia

Variations in metamorphic grade, structural style, isotopic ages and granite geochemistry observed within the Yilgarn craton, and between the Yilgarn and Pilbara cratons, Western Australia, are interpreted in terms of vertical zonation of the Archaean crust. We correlate the gneiss-granulite suite of the Wheat Belt (southwestern Yilgarn) with concealed coeval infracrustal roots of the low-grade granite—greenstone Kalgoorlie terrain (eastern Yilgarn). Differences between the Pilbara, Southern Cross and Laverton granite—greenstone blocks and the downfaulted linear greenstone belts of the Kalgoorlie block are interpreted in terms of deeper-level exposure in the first three blocks.Ultramafic—mafic volcanic sequences in the Yilgarn craton can be divided into at least two major groups — the lower greenstones, regarded as relicts of a once extensive simatic crust, and the significantly younger upper greenstones, believed to have formed within linear troughs following the intrusion of Na-rich granites.At least three major Archaean granite phases occur in Western Australia: (1) 3.1-2.9 b.y. old (recognized to date only in the western Yilgarn and in the Pilbara craton); (2) 2.8-2.7 b.y. old, and (3) 2.6 b.y. old (the two latter phases can only be separated from each other in the eastern Yilgarn, and phase (3) is also identified in the Pilbara). In the main, granites of phases (1) and (2) are Na-rich and those of phase (3) are K-rich. There is evidence for a secular increase in Rb levels and initial ⁸⁷Sr/⁸⁶Sr ratios. It is suggested that the K-rich granites grade down into Na-rich granites, and the former were generated by ensialic anatexis resulting in upward migration of K, Rb, U, and Th-enriched magmas.A review of data from several Archaean cratons in other continents suggests that evidence from these regions can be interpreted in terms of the general model of crustal evolution proposed for Western Australia. Implications of this model concerning petrogenesis of Archaean plutonic and volcanic suites, geothermal gradients and tectonic evolution of greenstone belts are discussed. Partial melting associated with mantle diapirism is thought to have given rise to the ultramafic—mafic volcanic cycles. Widespread subsidence and partial melting of this crust yielded Na-rich acid magmas. The development of the upper greenstones was confined to linear belts in a partly cratonized crustal environment. About 2.6 b.y. ago a rise in the geothermal gradient resulted in regional metamorphism and crusctal anatexis which gave rise to the K-rich granites.     

Bimodal tholeiitic—dacitic magmatism and the Early Precambrian crust

Interlayered plagioclase-quartz gneisses and amphibolites from 2.7 to more than 3.6 b.y. old form much of the basement underlying Precambrian greenstone belts of the world; they are especially well-developed and preserved in the Transvaal and Rhodesian cratons. We postulate that these basement rocks are largely a metamorphosed, volcanic, bimodal suite of tholeiite and high-silica low-potash dacite—compositionally similar to the 1.8-b.y.-old Twilight Gneiss — and partly intrusive equivalents injected into the lower parts of such volcanic piles.We speculate that magmatism in the Early Precambrian involved higher heat flow and more hydrous conditions than in the Phanerozoic. Specifically, we suggest that the early degassing of the Earth produced a basaltic crust and pyrolitic upper mantle that contained much amphibole, serpentine, and other hydrous minerals. Dehydration of the lower parts of a downgoing slab of such hydrous crust and upper mantle would release sufficient water to prohibit formation of andesitic liquid in the upper part of the slab. Instead, a dacitic liquid and a residuum of amphibole and other silica-poor phases would form, according to Green and Ringwood's experimental results. Higher temperatures farther down the slab would cause total melting of basalt and generation of the tholeiitic member of the suite. This type of magma generation and volcanism persisted until the early hydrous lithosphere was consumed.An implication of this hypothesis is that about half the present volume of the oceans formed before about 2.6 b.y. ago.     

High-pressure phase transitions of anorthosite crust in the Earth's deep mantle

We investigated phase relations, mineral chemistry, and density of lunar highland anorthosite at conditions up to 125 GPa and 2000 K. We used a multi-anvil apparatus and a laser-heated diamond-anvil cell for this purpose. In-situ X-ray diffraction measurements at high pressures and composition analysis of recovered samples using an analytical transmission electron microscope showed that anorthosite consists of garnet, CaAl₄Si₂O₁₁-rich phase (CAS phase), and SiO₂ phases in the upper mantle and the mantle transition zone. Under lower mantle conditions, these minerals transform to the assemblage of bridgmanite, Ca-perovskite, corundum, stishovite, and calcium ferrite-type aluminous phase through the decomposition of garnet and CAS phase at around 700 km depth. Anorthosite has a higher density than PREM and pyrolite in the upper mantle, while its density becomes comparable or lower under lower mantle conditions. Our results suggest that ancient anorthosite crust subducted down to the deep mantle was likely to have accumulated at 660–720 km in depth without coming back to the Earth's surface. Some portions of the anorthosite crust might have circulated continuously in the Earth's deep interior by mantle convection and potentially subducted to the bottom of the lower mantle when carried within layers of dense basaltic rocks.     

Deep structure of the Amur lithospheric Plate border zone

Based on the study of the deep Amur Plate border structure, seismogravimetric (density) and geoelectric sections of the plate lithosphere crossing geologically and seismically defined plate boundaries are compiled to construct a three-dimensional model of its lithosphere. The model demonstrates the almost ring structure of the plate with its inner part dipping for 50 km and uplifted fringing. The plate boundary zone consists of orogenic belts (Mongol-Okhotsk, Selenga-Stanovoi, Transbaikal, and Sikhote-Alin) and margins of the Siberian and North China platforms. The plate boundaries are described by mantle diapirism models with a largely bilateral inclined dip of its bordering belts.