Thin and layered subcontinental crust of the great Basin western north America inherited from Paleozoic marginal ocean basins?

The seismic profile of the crust of the northern part of the Basin and Range province by its thinness and layering is intermediate between typical continental and oceanic crust and resembles that of marginal ocean basins, especially those with thick sedimentary fill. The geologic history of the Great Basin indicates that it was the site of a succession of marginal ocean basins opening and closing behind volcanic arcs during much of Paleozoic time. A long process of sedimentation and deformation followed throughout the Mesozoic modifying, but possibly not completely transforming the originally oceanic crust to continental crust. In the Cenozoic, after at least 40 m.y. of quiescence and stable conditions, substantial crustal and upper-mantle changes are recorded by elevation of the entire region in isostatic equilibrium, crustal extension resulting in Basin and Range faulting, extensive volcanism, high heat flow and a low-velocity mantle. These phenomena, apparently the result of plate tectonics, are superimposed on the inherited subcontinental crust that developed from an oceanic origin in Paleozoic time and possibly retained some of its thin and layered characteristics. The present anomalous crust in the Great Basin represents an accretion of oceanic geosynclinal material to a Precambrian continental nucleus apparently as an intermediate step in the process of conversion of oceanic crust into a stable continental landmass or craton.     

灵山断褶带地质构造基本特征及其大地构造性质的探讨

灵山断褶带地处桂东南灵山、钦州和东兴等县,东北和西南两侧分别受六万大山和十万大山所夹峙,北端截于浦北县寨圩北西向断裂,南段延出国境以外,在我国长约240公里,宽约30公里。断褶带北东-南西向,主要由古生代、中生代和新生代地层以及酸性岩浆岩构成,断裂与褶皱发育,具明显的多期性和继承性。     

地球中的流体和穿越层圈构造

地球中的流体是当前科学研究的重点。从地球科学的角度来说,流体应包括气体、液体（水和石油）、熔体和地球中受应力作用而移动的物体。在半经为6378 km的固体地球中可分为7个层圈。目前对地球内部流体的了解很少,为探索流体在各层圈中的成分,物理化学性质和分布,以现阶段对地球层圈和流体研究程度来看,其重点应放在地球中穿越层圈的构造部分和地壳。地球中穿越层圈的构造主要有三个:板块构造的俯冲带是由上到下的穿越层圈构造,向下俯冲的大洋岩石圈可以抵达地幔过渡带;大洋中脊的扩张引起的由下而上的穿越层圈构造,使岩石圈和地幔的熔流体从下向上运移;地幔柱引起的由下而上的穿越层圈构造,使地幔的熔流体从下向上迁移。通过对三个穿越层圈构造和地壳中流体的研究,可以得出地壳、岩石圈、上地幔、过渡带、下地幔和核幔边界层流体的种类和成分、流动和演化。这是至今为至能鉴定到地球中深部流体的方法。这四个方面的研究是当前地球中流体科学研究的重点,并对开展深部找矿有实际意义。      

Characteristics of junctions of differently oriented structures in the Altay-Sayan region,as in the Shapshal' tectonic complex

Mechanical heterogeneities of the regional folded complex, intricate junctions and intersections of three different systems of deep fractures, impositions of younger structures onto the arcs of the ancient structural Plan, relics of the still more ancient “subcrustal” fractures, and other complexities, lead to a wide variety of the reactions of different parts of the earth's crust to the over-all tectonic tensions and hence to a highly varied assortment of structural expressions of such tensions, notwithstanding a unity of the genesis of all the deformations. – IGR Staff.     

西北太平洋岛弧系列成因的探讨

西北太平洋岛弧系列中的各岛弧(除马里亚纳岛弧为洋壳型弧外)均由陆壳型弧和洋壳型弧组成,并且左端与大陆相连,右端被后形成的岛弧所截。整个岛弧系列,从中生代末期开始发育至今,由北向南依次发展,规律明显。     

DEEP STRUCTURE OF EARTH AND PROBLEMS OF SUPER-DEEP DRILLING

Implementation of a proposed program of super-deep drilling in the U.S.S.R. would solve critical problems in contemporary geology including: a) structure and composition of the basaltic layer, lower granitic layer, definition of the Conrad and Mohorovi?i? discontinuities and relation of forces in the mantle to tectonic processes in the crust, b) differentiation processes leading to earth layering, c) study of granitization and basification, and their relation to ore-deposition, d) hydrothermal solutions and their mineralization and metamorphic effects; gas, fluid, and heat migration within and between mantle and crust, e) pre-Archeozoic development of continents and ocean basins, discovery of pre-Archeozoic rocks, f) definition of representative type sections through the crust. Five sites are proposed: 1) In the epi-Caspian depression to penetrate a sequence of 13–15 kilometers of sedimentaries in an oil province with a steep geothermic gradient, 2) in a polymetallic-ore eugeosyncline of the Urals through 10–12 kilometers of geosynclinal metamorphics and possibly a 3–8 kilometer granitic layer, 3) In the Kem' region of Karelia where the Conrad discontinuity is 8–10 km down in Archeozoic, and possibly older, rocks, 4) in the Kurina depression of Azerbaydzhan where the basaltic.layer occurs beneath a sedimentary-effusive miogeosyncline with a basal granitic layer, all only 5–8 km thick, and 5) on Kunashir in the Kuriles where the Mohorovi?i? discontinuity occurs beneath young volcanics, folded and metamorphic geosynclinal complexes, and a very thin, if any, granitic layer, all only 12 km thick. Super-drilling in the epi-Caspian depression should follow completion of a 7-kilometer hole started in 1961. Projects in Azerbaydzhan, the Urals, and Karelia present the fewest problems of drilling and logistics and should be undertaken as soon as possible. Remoteness and difficult drilling problems will necessitate drilling on Kunashir last. An extensive program of geophysical surveying and development of equipment must be part of the program. —M. Russell     

DEVELOPMENT OF THE EARTH AND ITS TECTOGENESIS

It is assumed that the earth having been formed in a cold state was warmed up by radioactive heat. Its further evolution was determined by differentiation of its constituent material through successive smelting from the mantle of relatively light components and subsequent displacement upward. The most intensive differentiation in the upper “layer” (evidently at depths of from 100 to 200km) causes strong vertical movements of the crust. After that is exhausted slower differentiation of the deeper “layer” (evidently at depths of from 200-300 km) is manifested on the surface in slow oscillatory movements forming platforms. Further heating activates the material of much deeper layers (as deep as 900 km) and causes large masses of basalt to rise to the surface. This ascent of basic material induces post-platform activity (observed in Central and East Asia), extrusions of plateau basalts and, filially, destruction of the continental crust through melting, metasomatic replacement and dissolution into the large volume of superheated basalt. As a result of destruction of the continental crust, large grabens, mediterranean seas and ocean basins have been formed. The development of tectonic processes is profoundly influenced by 1) the formation of deep faults in the crust and upper mantle which determine the routes of material distribution and 2) by the “the -lid-on-the-kettle-with-boiling-water” phenomenon. The last involves periodical accumulation of heat at depth, expansion of the mantle, the opening of deep faults, and quick removal of heat to the surface, together with heated material, along the faults. The periodicity of tectogenesis may be connected with this mechanism. It is assumed that up to now the earth has been warming up. Traces of tension of the crust accompanying the process of formation and expansion of the oceanic deeps are therefore considered to be an expression of a more general tension of whole crus and the upper mantle of the expanding earth. The expansion process expressed in the opening of defaults and uplift of deep-seated material to the surface along faults may be nonuniform, affecting some regions earlier than others, which causes the simultaneous existence of several regions in different stages of development. The author accepts the idea of deep-seated origin of sea-water, which rises from the mantle during subsidence of the oceanic basins and destruction of the continental crust. The author divides the history of the earth into two partly overlapping stages: granitic and basaltic. — Auth English summ.     

Principal stages in geological evolution of Ukrainian shield

The first attempt at coordination of the newest geochronological findings with identifiable petrographic complexes within the Ukrainian Shield, here seen as products of geosynclinal evolution and of granitization (in the modern sense), suggests the following characteristic traits of the Precambrian evolution of the earth's crust: an arrest of the upper Archean geosynclinal cycle (the second one of the three) in its initial or early stages; a general non-inversion of geosyncline; a very high geothermal gradient and hence the cardinal role of granitization and the "universality" - involvement of practically all kinds of rocks - of the regional metamorphism. The Shield has behaved as a platfomal entity ever since the end of its last (lower Proterozoic) geosynclinal cycle. The author's historical analysis of the Shield, as distinct from the still current stratigraphic representations, is based among other things on the recognized independence of metamorphic facies from stratigraphic as well as structural boundaries, identifications of formations corresponding to definite stages and zones of the geosynclinal evolution, correlations of geochronological data with petrographic varieties of rocks and with their spatial distribution, examination of migmatites and granitoids from the viewpoints of ultrametamorphism and granitization, proofs of the function of deep fissuring in the character of sedimentation and intrusive activities, and plane analysis of folded structures. Metallogenic analysis of the Shield is facilitated by such identifications and correlations, inasmuch as migrations and accumulations of ore constituents and others were definitely associated with granitization activities. — V. P. Sokoloff     

地球系统多圈层构造观的基本内涵

地球系统多圈层构造观的基本点是,把地球作为一个活的天体放在宇宙系统之中,更多地考虑地球深部壳-幔-核之间的相互作用,考虑地外天体对地球运动的作用和影响。这一构造观认为:构造运动并不仅仅是岩石圈板块之间的相互作用,而是地球系统的全球动力作用过程;陆与洋是对立统一相互转化的,单纯的大陆增生说是不正确的;地幔对流说至今未被证实,陆块是活动的,但不能大规模漂移;大陆地壳不是单纯地侧向或垂向增生,而是多旋回构造-岩浆作用叠合的产物;地球的构造不是均变式向前发展,而是非均变、非线性、旋回式向前演化的;地球表层在不同地史阶段,均有其受相应深断裂体系控制的不同的构造格局,大西洋-印度洋-太平洋式大洋盆体制,只是在中生代晚期以来才出现的。      

THE USE OF SCALE MODELS IN TECTONOPHYSICS

The author, using the theory of physical similarity as developed in the U. S. S. R. and equations describing the development of folds and faults in rocks, theoretically proves the possibility of using scale models in tectonophysics.

New instruments necessary for investigation of equivalent materials (which are necessary for conditions of similarity) have been created in the U. S. S. R. Some substances having properties meeting model-material requirements have been known for a long time. New materials with the required properties have also been created. As a result, scale models can be practically used to study tectonic deformation and fractures.

The fundamental principles of the optical method of investigation of stress state of elastic and plastic transparent models are described, indicating that the scale-model method may be used for the investigation of the tectonic-stress fields in the earth's crust.

Three examples demonstrate the ability of the scale-model method to help solve different geological and geophysical problems. The hypothetical physical conditions of two types of folds - longitudinal bending and longitudinal thickening - were checked.

The notions about the distribution of tectonic faults formed during the growth of transversal bending anticlines were made more precise with the aid of transparent models.

Transparent plastic models are used to study the ratio of the magnitude of tangential stresses in the region of earthquake foci to the velocity of the movement of the earth's crust. In elastic transparent models it is possible to see changes in the character of earthquake foci with time due to the development of tectonic faults In such models, the influence of a type of tectonic deformation and fault magnitude on the value of seismic energy generating from the earthquake focus can be studied. All these data cannot be obtained by only field investigations. Therefore, even experimental information obtained from scale models facilitates the development of geological criteria of seismicity.     

Zur Entwicklung von Riftsystemen im Frühstadium (Hebung — Spaltung — Vulkanismus — Spreading) am Beispiel der Afar Senke

A combination of palaeomagnetic, seismological, gravitational, aeromagnetic and geochemical observations, as well as geological and regional considerations are strongly indicative of anticlockwise rotational movements of the Danakil Alps and formation of new oceanic crust in the Northern Afar Triangle. The decreasing amount of spreading in the Southern Red Sea is compensated by en chelon crustal spreading (formation of oceanic crust in a continental environment) in the Danakil-Afar Depression. Here, the geophysical properties are generally intermediate between the more typical continental (Ethiopia) and oceanic (Red Sea, Gulf of Aden) data. Such intermediate type crust is proposed to be caused by “oceanization” of formerly continental crust, i. e. fragmentation and basification through massive dyke injections (mantle diapirism). The structure and evolution of the wider Afar Triangle, East-African Rift System, Red Sea and Gulf of Aden are used to derive a model for possible stages during initial continental break-up and compared with selected, similarly structured parts of the n-Atlantik. The continental break-up probably develops in the following stages: 1. general uplift associated with surface fracturing above an asthenospheric diapir (uplift), 2. development of linear “Scheitel”-Grabensystems along the crest of the uplift or uplift chains (rupture), 3. graben with (contaminated) volcanism stage (volcanism), 4. “oceanization” of the developing depression through fragmentation and basification by massive oceanic and/or contaminated dyke-injections of the former continental crust along several sporadically active lineaments, 5. “crustal spreading” on land or concentration of mantle derived, oceanic crust-injections along one major lineament in a dry, continental environment, 6. “evaporit-stage of sea-floor spreading” with sporadic seawater connections to an open marine basin and 7. “ocean-floor spreading” in the deep-sea environment of advanced oceanic troughs. The derivation of these stages basically involves the addition of “sea-floor spreading” processes (oceanization, crustal-, sea- and ocean-floor spreading) to the well known sequence: Hebung — Spaltung — Vulkanismus (Cloos, 1939) and relate it to mantle-diapirism processes. All the above stages are recognizable along the Afro-Arabian Rifts and seem to have morphological equivalents in the Atlantic.     

Certain problems of the structure and evolution of transition zones between continents and oceans

A type of continental-oceanic transition zone, referred to as the Columbian transition zone, is distinguished from two other commonly known types of these zones. The subsidence of the Earth's crust, typical of all transition zones, is shown to be connected (by geophysical properties) to the transformation of continental crust into intermediate crust and later into oceanic. The most likely mechanisms of such changes are the basification of continental crust, its foundering, block by block, into the heated upper mantle, and its substitution by new oceanic crust. The evolution of transition zones of the Pacific type is largely influenced by deep faults, which reach down to the level of undepleted mantle. From this level, the volatile products rise to the surface which results in the formation of calc-alkali magmas on island arcs. The Benioff zones are deep faults, whose inclinations are dependent on the density contrasts in the upper mantle on either side of the Benioff zones. The denser mantle flows beneath the mantle of lower density. This phenomenon is depicted by plate tectonics as subduction.On the whole, the evolution of transition zones gives rise to the growth of the oceans at the expense of the continents, though oceanic crust becomes thicker by addition of volcanogenic layers composed of andesite, in the transition zones (type two) of the Pacific type at island arcs.     

当今中国大陆地壳移动的动力

地幔圈形状周期性变化使地壳产生势能(位能)力导致地壳运动。当地幔圈由圆相对变扁,南、北半球的高纬度区地壳向低纬度区对挤;当地幔圈由扁相对变圆,恢复等位面平衡,南、北半球低纬度区的地壳区分别向高纬度区挤压。当今中国大陆地壳移动的动力是由于西部地区的地幔还在继续进行恢复等位面平衡产生的。      

Evolution of central massifs

In spite of the abundance of definitions of central massifs, there is no integration of data Khain and Sheynmann (1960) define the central massifs as a residual province of older cycles within a progressive folding system. They note such provinces tend to be remodeled by the younger movements, yet retain their basic “setup” while exerting influence on the surrounding rocks. Central massifs may be classified into: 1) blocks of ancient Precambrian platforms, 2) blocks of Paleozoic or Mesozoic folded structures within younger geosynclinal system and 3) provinces of early consolidation which serve as “growth centers” within a geosynclinal system. Despite the differences in origin and age of central massifs, they have many common features. All are within geosynclinal belts and serve to divide them into segments; they are polygonal to diamond-shaped and are bounded by deep rifts with “flows” of ultrabasic to basic magma. As a rule, there are three periods of development of a massif with a single tectonic cycle: 1) continental regimen with denudation 2) minor marine trangression, block deformations, volcanism and granite intrusion and 3) transition to intermontane low with associated volcanic activity. Central massifs show,a mosaic of variously trending faults caused largely by vertical movements. Some students attribute the much thinner crust within the central massifs, based on geophysical studies, to a redistribution of deep-seated substance from the massif toward the geosyncline. — W. D. Lowry.     

Modern volcanism in the Earth’s northern hemisphere and its relations with the evolution of the North Pangaea modern supercontinent and with the spatial distribution of hotspots on the Earth: The hypothesis of relations between mantle plumes and deep subduction

The paper reports results of the analysis of the spatial distribution of modern (younger than 2 Ma) volcanism in the Earth’s northern hemisphere and relations between this volcanism and the evolution of the North Pangaea modern supercontinent and with the spatial distribution of hotspots of the Earth’s mantle. Products of modern volcanism occur in the Earth’s northern hemisphere in Eurasia, North America, Greenland, in the Atlantic Ocean, Arctic, Africa, and the Pacific Ocean. As anywhere worldwide, volcanism in the northern hemisphere of the Earth occurs as (a) volcanism of mid-oceanic ridges (MOR), (b) subduction-related volcanism in island arcs and active continental margins (IA and ACM), (c) volcanism in continental collision (CC) zones, and (d) within-plate (WP) volcanism, which is related to mantle hotspots, continental rifts, and intercontinental belts. These types of volcanic areas are fairly often neighboring, and then mixed volcanic areas occur with the persistent participation of WP volcanism. Correspondingly, modern volcanism in the Earth’s northern hemisphere is of both oceanic and continental nature. The latter is obviously related to the evolution of the North Pangaea modern supercontinent, because it results from the Meso-Cenozoic evolution of Wegener’s Late Paleozoic Pangaea. North Pangaea in the Cenozoic comprises Eurasia, North and South America, India, and Africa and has, similar to other supercontinents, large sizes and a predominantly continental crust. The geodynamic setting and modern volcanism of North Pangaea are controlled by two differently acting processes: the subduction of lithospheric slabs from the Pacific Ocean, India, and the Arabia, a process leading to the consolidation of North Pangaea, and the spreading of oceanic plates on the side of the Atlantic Ocean, a process that “wedges” the supercontinent, modifies its morphology (compared to that of Wegener’s Pangaea), and results in the intervention of the Atlantic geodynamic regime into the Arctic. The long-lasting (for >200 Ma) preservation of tectonic stability and the supercontinental status of North Pangaea are controlled by subduction processes along its boundaries according to the predominant global compression environment. The long-lasting and stable subduction of lithospheric slabs beneath Eurasia and North America not only facilitated active IA + ACM volcanism but also resulted in the accumulation of cold lithospheric material in the deep mantle of the region. The latter replaced the hot mantle and forced this material toward the margins of the supercontinent; this material then ascended in the form of mantle plumes (which served as sources of WP basite magmas), which are diverging branches of global mantle convection, and ascending flows of subordinate convective systems at the convergent boundaries of plates. Subduction processes (compressional environments) likely suppressed the activity of mantle plumes, which acted in the northern polar region of the Earth (including the Siberian trap magmatism) starting at the latest Triassic until nowadays and periodically ascended to the Earth’s surface and gave rise to WP volcanism. Starting at the breakup time of Wegener’s Pangaea, which began with the opening of the central Atlantic and systematically propagated toward the Arctic, marine basins were formed in the place of the Arctic Ocean. However, the development of the oceanic crust (Eurasian basin) took place in the latter as late as the Cenozoic. Before the appearance of the Gakkel Ridge and, perhaps, also the oceanic portion of the Amerasian basin, this young ocean is thought to have been a typical basin developing in the central part of supercontinents. Wegener’s Pangaea broke up under the effect of mantle plumes that developed during their systematic propagation to the north and south of the Central Atlantic toward the North Pole. These mantle plumes were formed in relation with the development of global and local mantle convection systems, when hot deep mantle material was forced upward by cold subducted slabs, which descended down to the core-mantle boundary. The plume (WP) magmatism of Eurasia and North America was associated with surface collision- or subduction-related magmatism and, in the Atlantic and Arctic, also with surface spreading-related magmatism (tholeiite basalts).     

中朝陆台北侧褶皱带构造发展的几个问题

中朝与西伯利亚陆台之间的乌拉尔—蒙古—鄂霍茨克褶皱系是古亚洲洋演化的结果。其发展分为两个大的阶段:早期阶段从中元古代起在北部形成蒙古—鄂霍茨克洋,到寒武纪初封闭,形成兴凯褶皱带;晚期阶段从震旦—寒武纪初起在南部形成乌拉尔—蒙古洋,到泥盆纪大洋封闭,形成早及中古生代褶皱带。这就是中朝与西伯利亚陆台边缘褶皱带发育不对称的原因。 乌拉尔—蒙古洋最后封闭的缝合带在内蒙古中部形成中古生代褶皱带。它包括贺根山蛇绿岩带,二道井—查干诺尔混杂体带和它们之间的锡林浩特花岗岩—变质岩带。后者是造山碰撞的中心,上泥盆统法门阶的磨拉石堆积主要沿此带分布。 石炭—二叠纪时,本区广泛兴起裂谷活动。这些裂谷发育在不同的基底之上,多数发生于不同时期构造带的界线上。它们在新增生的年青陆壳上形成,其特征不同于红海或东非型裂谷;其岩浆活动和变形、变质作用使年青陆壳增厚并更加稳固。     

MAGMATISM OF THE EASTERN SAYAN

New geological and petrological data on the range of magmatic complexes and formations of the Eastern Sayan show two primary magmas: basic and granitoid. These magmas were formed through melting hard deep-seated layers of the earth crust: basaltic and sialic. During the geosynclinal stage the development of magmas belonging to the Archean, Proterozoic, and Salair [Cambrian] volcanic cycles proceeded consecutively from ultrabasic and basic formations formed in a pre-orogenic or earlier-orogenic geosynclinal development stage to granitoids set up in a synorogenic or later-synorogenic development stage. During the platform stage middle Paleozoic (Lower Devonian) and Mesozoic-Cenozoic cycles of magmatism proceeded directly, without the geosynclinal preparatory stage. Their development, accompanied by faulting, proceeded in reverse order from acidic and alkalic intrusions to predominantly basic eruptives. A further development of deep-seated basic and granitoid magmas was determined first by magmatic differentiation and later by assimilation phenomena which took place during the magma's passage into upper structural layers. The granitoids of geosynclinal magmatic complexes correspond petrochemically to the intermediate types of calc-alkalic rocks of the Pacific Ocean belt. The granitoids and alkalic rocks of the Lower Devonian platform magmatic complex resemble those of the Cenozoic East-Asia alkalic province. The composition of the granitoid magma belonging to the volcanic cycle is conditioned initially chiefly by the sial environment and geosynclinal strata. Magmatic complexes and formations are characterized by definite endogenic mineralizations. Chromium, nickel, cobalt, platinum, diamond, asbestos and other deposits are genetically connected with Proterozoic basic and ultrabasic rocks; gold, muscovite and tin-rare metal pegmatite with upper Proterozoic granitoids. Copper, galenaite and gold-ore occurrences are related to the postmagmatic manifestations of Salair granitoids. Deposits of pyrochlore carbonatites, molybdenite, graphite and others belong to Lower Devonian acidic and alkalic granitoids. — Auth. English summ.     

北祁连中段早古生代双向俯冲——碰撞造山模式剖析

在十余年野外考察的基础上，通过火山－沉积组合，高压变质带及俯冲杂岩带产出特征，花岗岩浆活动，同位素年龄值等综合分析研究，结合近年区调成果，提出北祁连中段地区旱古生代的构造演化模式，认为该区是在古陆壳基底上由震旦纪打开经海底扩张生成的留有微陆块的微洋盆，寒武－奥隐纪，以黑河－八宝河为轴发生海底扩张，同时分别向南北两侧发生了俯冲杂岩带也随之由南向北先后反弹回跳到地表，转化为汇聚过渡壳；南侧由早期被动陆     

东昆仑东段清水泉中酸性侵入岩地球化学特征及成因讨论

清水泉侵入体位于青海省沟里地区的北部,处于东昆南造山带的东段,为东昆仑岩浆岩带的组成部分.该侵入体主要为花岗闪长岩和斜长花岗岩,次为闪长岩.不同岩性之间具有清楚的接触界线,花岗闪长岩和斜长花岗岩中含有暗色闪长质包体.岩石化学特征显示,侵入体为富钙中钾的钙碱性系列岩石,是岩浆成因的Ⅰ型花岗岩,形成于与岛弧、大陆弧有关的大...     

The building blocks of continental crust: Evidence for a major change in the tectonic setting of continental growth at the end of the Archean

Oceanic arcs are commonly cited as primary building blocks of continents, yet modern oceanic arcs are mostly subducted. Also, lithosphere buoyancy considerations show that oceanic arcs (even those with a felsic component) should readily subduct. With the exception of the Arabian–Nubian orogen, terranes in post-Archean accretionary orogens comprise < 10% of accreted oceanic arcs, whereas continental arcs compose 40–80% of these orogens. Nd and Hf isotopic data suggest that accretionary orogens include 40–65% juvenile crustal components, with most of these (> 50%) produced in continental arcs.Felsic igneous rocks in oceanic arcs are depleted in incompatible elements compared to average continental crust and to felsic igneous rocks from continental arcs. They have lower Th/Yb, Nb/Yb, Sr/Y and La/Yb ratios, reflecting shallow mantle sources in which garnet did not exist in the restite during melting. The bottom line of these geochemical differences is that post-Archean continental crust does not begin life in oceanic arcs. On the other hand, the remarkable similarity of incompatible element distributions in granitoids and felsic volcanics from continental arcs is consistent with continental crust being produced in continental arcs.During the Archean, however, oceanic arcs may have been thicker due to higher degrees of melting in the mantle, and oceanic lithosphere would be more buoyant. These arcs may have accreted to each other and to oceanic plateaus, a process that eventually led to the production of Archean continental crust. After the Archean, oceanic crust was thinner due to cooling of the mantle and less melt production at ocean ridges, hence, oceanic lithosphere is more subductable. Widespread propagation of plate tectonics in the late Archean may have led not only to rapid production of continental crust, but to a change in the primary site of production of continental crust, from accreted oceanic arcs and oceanic plateaus in the Archean to primarily continental arcs thereafter.