Asia: a frontier for a future supercontinent Amasia

Asia is the world’s largest but youngest continent, in which Pacific-type (P-type) and collision-type (C-type) orogenic belts coexist with numerous amalgamated continental blocks. P-type orogens represent major sites of continental growth through tonalite-trondhjemite-granodiorite type (TTG-type) juvenile granitoid magmatism and accretion of oceanic crust and intra-oceanic arcs. The Asian continent includes several P-type orogenic belts, of which the largest are the Central Asian and Western Pacific. The Central Asian Orogenic Belt is dominated by P-type fossil orogens arranged with a regular northward subduction polarity. The Western Pacific is characterized by ongoing P-type orogeny related to the westward subduction of the Pacific plate. Asia has a multi-cratonic structure and its post-Palaeozoic history has witnessed amalgamation of the Laurasia composite continent and Pangaea supercontinent. Nowadays, Asia is surrounded by double-sided subduction zones, which generate new TTG-type crust and supply oceanic crust and microcontinents to its active margins. The TTG-crust can be tectonically eroded and subducted down to the mantle transition zone to form a ‘second’ continent, which may generate mantle upwelling, plumes, and extensive intra-plate volcanism. Moreover, recent plate movements around Asia are dominated by northward directions, which resulted in the India–Eurasia and Arabia–Eurasia collisions beginning at 50–45 and 23–20 Ma, respectively, and will result in Africa–Eurasia collision in the near future. Therefore, Asia is the best candidate to serve as the nucleus for a future supercontinent ‘Amasia’, likely to form 200–250 Ma in the future. In this paper we unravel a puzzle of continental growth in Asia through P-type orogeny by discussing its tectonic history and geological structure, subduction polarity in P-type orogens, tectonic erosion of TTG-type crust and arc subduction at convergent margins, generation of mantle plumes, and prospects of Asia growth and overgrowth.     

Continental growth and reworking on the edge of the Columbia and Rodinia supercontinents; 1.86–0.9 Ga accretionary orogeny in southwest Fennoscandia

Geological history from the late Palaeoproterozoic to early Neoproterozoic is dominated by the formation of the supercontinent Columbia, and its break-up and re-amalgamation into the next supercontinent, Rodinia. On a global scale, major orogenic events have been tied to the formation of either of these supercontinents, and records of extension are commonly linked to break-up events. Presented here is a synopsis of the geological evolution of southwest Fennoscandia during the ca. 1.9–0.9 Ga period. This region records a protracted history of continental growth and reworking in a long-lived accretionary orogen. Three major periods of continental growth are defined by the Transscandinavian Igneous Belt (1.86–1.66 Ga), Gothian (1.66–1.52 Ga), and Telemarkian (1.52–1.48 Ga) domains. The 1.47–1.38 Ga Hallandian–Danopolonian period featured reorganization of the subduction zone and over-riding plates, with limited evidence for continental collision. During the subsequent 1.38–1.15 Ga interval, the region is interpreted as being located inboard of a convergent margin that is not preserved today and hosted magmatism and sedimentation related to inboard extensional events. The 1.15–0.9 Ga period is host to Sveconorwegian orogenesis that marks the end of this long-lived accretionary orogen and features significant crustal deformation, metamorphism, and magmatism. Collision of an indenter, typically Amazonia, is commonly inferred for the cause of widespread Sveconorwegian orogenesis, but this remains inconclusive. An alternative is that orogenesis merely represents subduction, terrane accretion, crustal thickening, and burial and exhumation of continental crust, along an accretionary margin. During the Mesoproterozoic, southwest Fennoscandia was part of a much larger accretionary orogen that grew on the edge of the Columbia supercontinent and included Laurentia and Amazonia amongst other cratons. The chain of convergent margins along the western Pacific is the best analogue for this setting of Proterozoic crustal growth and tectonism.     

罗迪尼亚超大陆聚合在华南陆块北缘的镁铁质岩浆岩记录

超大陆的聚合必然伴随着从大洋俯冲、弧陆碰撞到陆陆碰撞等一系列板块汇聚和造山过程,这些不同阶段的俯冲和汇聚过程会产生不同特征的岩浆岩记录.华南陆块是新元古代罗迪尼亚超大陆的重要组成部分,在这个超大陆聚合过程中有格林维尔期洋壳俯冲及其伴随的壳幔相互作用.总结了华南陆块北缘记录的罗迪尼亚超大陆聚合不同阶段发生的岩浆活动,比较了其产物的地球化学特征,探讨了它们对应的构造环境.华南陆块北缘900~950 Ma的岩浆活动产物以镁铁质岩浆岩为主,伴随有少量斜长花岗岩,为洋壳俯冲作用的产物.当洋壳俯冲到大陆边缘之下形成安第斯型俯冲带,古老陆源沉积物也被携带进入俯冲带,由此部分熔融产生的含水熔体交代上覆地幔楔形成极度富集的造山带岩石圈地幔,其在新元古代中期发生部分熔融形成具有极负锆石ε_Hf（t）值的镁铁质岩浆岩.因此,在罗迪尼亚超大陆聚合过程中地幔楔被交代形成镁铁质-超镁铁质交代岩,其中一部分在俯冲阶段就发生部分熔融形成大洋弧或大陆弧镁铁质岩浆岩,另一部分在俯冲之后由于大陆裂断引起造山带岩石圈拉张使其与上覆地壳一起部分熔融形成双峰式岩浆岩.      

The supercontinent cycle: A retrospective essay

The recognition that Earth history has been punctuated by supercontinents, the assembly and breakup of which have profoundly influenced the evolution of the geosphere, hydrosphere, atmosphere and biosphere, is arguably the most important development in Earth Science since the advent of plate tectonics. But whereas the widespread recognition of the importance of supercontinents is quite recent, the concept of a supercontinent cycle is not new and advocacy of episodicity in tectonic processes predates plate tectonics. In order to give current deliberations on the supercontinent cycle some historical perspective, we trace the development of ideas concerning long-term episodicity in tectonic processes from early views on episodic orogeny and continental crust formation, such as those embodied in the chelogenic cycle, through the first realization that such episodicity was the manifestation of the cyclic assembly and breakup of supercontinents, to the surge in interest in supercontinent reconstructions. We then chronicle some of the key contributions that led to the cycle's widespread recognition and the rapidly expanding developments of the past ten years.     

Contrasting modes of supercontinent formation and the conundrum of Pangea

Repeated cycles of supercontinent amalgamation and dispersal have occurred since the Late Archean and have had a profound influence on the evolution of the Earth's crust, atmosphere, hydrosphere, and life. When a supercontinent breaks up, two geodynamically distinct tracts of oceanic lithosphere exist: relatively young interior ocean floor that develops between the dispersing continents, and relatively old exterior ocean floor, which surrounded the supercontinent before breakup. The geologic and Sm/Nd isotopic record suggests that supercontinents may form by two end-member mechanisms: introversion (e.g. Pangea), in which interior ocean floor is preferentially subducted, and extroversion (e.g. Pannotia), in which exterior ocean floor is preferentially subducted.The mechanisms responsible remain elusive. Top–down geodynamic models predict that supercontinents form by extroversion, explaining the formation of Pannotia in the latest Neoproterozoic, but not the formation of Pangea. Preliminary analysis indicates that the onset of subduction in the interior (Rheic) ocean in the early Paleozoic, which culminated in the amalgamation of Pangea, was coeval with a major change in the tectonic regime in the exterior (paleo-Pacific) ocean, suggesting a geodynamic linkage between these events. Sea level fall from the Late Ordovician to the Carboniferous suggests that the average elevation of the oceanic crust decreased in this time interval, implying that the average age of the oceanic lithosphere increased as the Rheic Ocean was contracting, and that subduction of relatively new Rheic Ocean lithosphere was favoured over the subduction of relatively old, paleo-Pacific lithosphere. A coeval increase in the rate of sea floor spreading is suggested by the relatively low initial ⁸⁷Sr/⁸⁶Sr in late Paleozoic ocean waters. We speculate that superplumes, perhaps driven by slab avalanche events, can occasionally overwhelm top–down geodynamics, imposing a geoid high over a pre-existing geoid low and causing the dispersing continents to reverse their directions to produce an introverted supercontinent.     

克拉通演化的超大陆背景与克拉通盆地的成因机制

研究表明克拉通的形成与超大陆的汇聚和裂解有着重要关系。本文对近年来超大陆重建的研究进行了分析对比,对克拉通发展与超大陆事件的关系做出了总结。前人对超大陆的研究表明,其形成与地幔动力有直接联系,地幔柱重组的旋回导致了超大陆的旋回。Phillips and Bunge（2007）在前人三维球体地幔对流模型的基础上加入大陆进行了模拟实验,结果显示周期性的超大陆旋回只发生在理想模型中,而Senshu et al.（2009）对代表陆壳的英云闪长岩-奥长花岗岩-花岗岩（TTG）地壳进行了研究,提出随着俯冲的TTG地壳产热速率的下降,超大陆旋回的周期随之变长;更有许多学者指出,历史上哥伦比亚超大陆存在时间明显较长,因此超大陆的旋回并不具有周期性。对近年来不同学者提出的哥伦比亚、 罗迪尼亚、 冈瓦纳、 潘基亚4个超大陆新的重建证据进行分析,大致确定出上述4个超大陆的形成时间、 格局及演化过程。此外,对华北、 东欧、 西伯利亚、 亚马孙、 刚果、 西非6个克拉通各自的演化进行分析,也显示出克拉通演化与超大陆汇聚及裂解在时间与空间上有对应关系。通过分析得出克拉通演化与超大陆旋回有关,且确定出克拉通演化的4个超大陆旋回。本文最后讨论了克拉通盆地的成因机制以及3种端元类型,并将盆地的发育与超大陆演化的巨旋回相联系。     

http://www.sciencedirect.com/science/article/pii/S1674987115000948

The formation of continents involves a combination of magmatic and metamorphic processes. These processes become indistinguishable at the crust-mantle interface, where the pressure-temperature(P-T)conditions of(ultra) high-temperature granulites and magmatic rocks are similar. Continents grow laterally, by magmatic activity above oceanic subduction zones(high-pressure metamorphic setting), and vertically by accumulation of mantle-derived magmas at the base of the crust(high-temperature metamorphic setting). Both events are separated from each other in time; the vertical accretion postdating lateral growth by several tens of millions of years. Fluid inclusion data indicate that during the high-temperature metamorphic episode the granulite lower crust is invaded by large amounts of low H_2O-activity fluids including high-density CO_2 and concentrated saline solutions(brines). These fluids are expelled from the lower crust to higher crustal levels at the end of the high-grade metamorphic event. The final amalgamation of supercontinents corresponds to episodes of ultra-high temperature metamorphism involving large-scale accumulation of these low-water activity fluids in the lower crust.This accumulation causes tectonic instability, which together with the heat input from the subcontinental lithospheric mantle, leads to the disruption of supercontinents. Thus, the fragmentation of a supercontinent is already programmed at the time of its amalgamation.     

Superplume, supercontinent, and post-perovskite: Mantle dynamics and anti-plate tectonics on the Core–Mantle Boundary

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.     

Role of tonalite-trodhjemite-granite (TTG) crust subduction on the mechanism of supercontinent breakup

The tonalite-trondhjemite-granite (TTG) crust has been considered to be buoyant and hence impossible to be subducted into the deep mantle. However, recent studies on the juvenile arc in the western Pacific region indicate that immature island arcs subduct into the deep mantle in most cases, except in the case of parallel arc collision. Moreover, sediment trapped subduction and tectonic erosion are also common. This has important implications in evaluating the role of TTG crust in the deep mantle and probably on the bottom of the mantle. Because the TTG crust is enriched in K, U and Th, ca. 20 times more than that of CI chondrite, the accumulated TTG on the Core Mantle Boundary (CMB) would have played a critical role to initiate plumes or superplumes radiating from the thermal boundary layer, particularly after 2.0 Ga, related to the origin of superplume-supercontinent cycle. This is because selective subduction of oceanic lithosphere including sediment-trapped subduction, tectonic erosion and arc- and microcontinent-subduction proceeded under the supercontinent before the final amalgamation ca. 200-300 million years after the formation of the nuclei. We speculate the mechanism of superplume evolution through the subduction of TTG-crust and propose that this process might have played a dominant role in supercontinent breakup.     

Global trends in the evolution of metallogenic processes as a reflection of supercontinent cyclicity

The worldwide distribution of large and superlarge mineral deposits (LSLDs) on a geological time scale is analyzed. It has been established that their formation from Eoarchean to Cenozoic was nonuniform in time. The maxima and minima of ore generation intensity correlate well with global cyclical processes, eventually resulting in the assembly and breakup of supercontinents. The periods of supercontinent amalgamation are characterized by the highest rate of continental crust growth due to the contribution of juvenile sources, a maximum of orogenic activity, and the most intense deposit formation. Periods close to betweencycle boundaries are distinguished by a low intensity of both endogenic and ore-forming processes. As follows from the available data, the number of known LSLDs slightly decreases from the Kenoran to Columbian cycle, significantly decreases in the next Rodinian cycle, which, in turn, is followed by abrupt growth in the Pangaean and Amasian cycles, especially as concerns LSLDs of the granitoid-related class. The intensification of metallogenic activity correlates with a commensurable increase in orogenic activity of the Earth’s crust probably caused by continental crust expansion, an increase in the number of sialic blocks participating in the formation of accretionary and collisional orogens, and acceleration of lithospheric plate motion. Some trends are also described for other LSLD classes (basic–alkaline, volcanic-hosted massive sulfide, sedimentary, epigenetic sediment-hosted), caused to a certain extent by supercontinent cycles and their evolutionary variations.     

West Pacific and North Indian Ocean Seafloor and Their Ocean-Continent Connection Zones: Evolution and Debates

The Indian Ocean and the West Pacific Ocean and their ocean-continent connection zones are the core area of "the Belt and Road". Scientific and in-depth recognition to the natural environment, disaster distribution, resources, energy potential of “the Belt and Road” development, is the cut-in point of the current Earth science community to serve urgent national needs. This paper mainly discusses the following key tectonic problems in the West Pacific and North Indian oceans and their ocean-continent connection zones (OCCZs): 1. modern marine geodynamic problems related to the two oceans. Based on the research and development needs to the two oceans and the ocean-continent transition zones, this item includes the following questions. (1) Plate origin, growth, death and evolution in the two oceans, for example, 1) The initial origin and process of the triangle Pacific Plate including causes and difference of the Galapagos and West Shatsky microplates; 2) spatial and temporal process, present status and trends of the plates within the Paleo- or Present-day Pacific Ocean to the evolution of the East Asian Continental Domain; 3) origin and evolution of the Indian Ocean and assembly and dispersal of supercontinents. (2) Latest research progress and problems of mid-oceanic ridges: 1) the ridge-hot spot interaction and ridge accretion, how to think about the relationship between vertical accretion behavior of thousands years or tens of thousands years and lateral spreading of millions years at 0 Ma mid-oceanic ridges; 2) the difference of formation mechanisms between the back-arc basin extension and the normal mid-oceanic ridge spreading; 3) the differentials between ultra-slow dian Ocean and the rapid Pacific spreading, whether there are active and passive spreading, and a push force in the mid-oceanic ridge; 4) mid-oceanic ridge jumping and termination: causes of the intra-oceanic plate reorganization, termination, and spatial jumps; 5) interaction of mantle plume and mid-oceanic ridge. (3) On the intra-oceanic subduction and tectonics: 1) the origin of intra-oceanic arc and subduction, ridge subduction and slab window on continental margins, transform faults and transform-type continental margin; 2) causes of the large igneous provinces, oceanic plateaus and seamount chains. (4) The oceanic core complex and rheology of oceanic crust in the Indian Ocean. (5) Advances on the driving force within oceanic plates, including mantle convection, negative buoyancy, trench suction and mid-oceanic ridge push, is reviewed and discussed. 2. The ocean-continent connection zones near the two oceans, including: (1) Property of continental margin basement: the crusts of the Okinawa Trough, the Okhotsk Sea, and east of New Zealand are the continental crusts or oceanic crusts, and origin of micro-continent within the oceans; (2) the ocean-continent transition and coupling process, revealing from the comparison of the major events between the West Pacific Ocean seamount chains and the continental margins, mantle exhumation and the ocean-continent transition zones, causes of transform fault within back-arc basin, formation and subduction of transform-type continental margin; (3) strike-slip faulting between the West Pacific Ocean and the East Asian Continent and its temporal and spatial range and scale; (4) connection between deep and surface processes within the two ocean and their connection zones, namely the assembly among the Eurasian, Pacific and India-Australia plates and the related effect from the deep mantle, lithosphere, to crust and surface Earth system, and some related issues within the connection zones of the two oceans under the super-convergent background. 3. On the relationship, especially their present relations and evolutionary trends, between the Paleo- or Present-day Pacific plates and the Tethyan Belt, the Eurasian Plate or the plates within the Indian Ocean. At last, this paper makes a perspective of the related marine geology, ocean-continent connection zone and in-depth geology for the two oceans and one zone.     

变质作用、板块构造及超级大陆旋回

麻粒岩相超高温变质作用（GUHTM）主要发育于新太古代至寒武纪岩石中;推测在深部较年轻的,特别是新生代造山带岩石中也会有GUHTM存在。岩石中最初出现GUHTM记录意味着产生瞬时极高热流处的地球动力学发生了改变。许多GUHTM带可能发育于类似现代大陆弧后的构造背景中。在较热的地球上,超大陆及其裂解形成的循环组合,尤其是经岩石圈减薄的洋盆卷入到其外翻过程中可能产生比现代太平洋边缘更热的大陆弧后。中温榴辉岩 高压麻粒岩相变质作用（EHPGM）也是最先发现于新太古代岩石记录中,并发育于从元古宙至古生代岩石中。EHPGM带是对GUHTM带的补充,并经常认为是记录了从俯冲至碰撞造山作用的过程。在元古宙岩石记录中的蓝片岩明显记录了与现代俯冲作用相关的低热流梯度。以发育柯石英(±硬柱石)或金刚石为特征的硬柱石蓝片岩和榴辉岩（高压变质作用,HPM）及超高压变质岩（UHPM）主要是在显生宙形成。HPMUHPM记录了显生宙俯冲碰撞造山带早期碰撞过程中的低热流梯度及陆壳的深俯冲作用。尽管与直觉不同,在超级大陆聚敛期（Wilson旋回洋盆打开和关闭）的大陆地块增生过程,许多HPMUHPM带看来确实是通过小洋盆关闭而发育起来的,反映双重热体制的双重变质带仅发育于新太古代以来的岩石记录中。双重热体制是现代板块构造的特点,而双重变质作用则是板块构造在岩石记录中的特征性标志。尽管构造样式很可能不同,新太古代以来GUHTM和EHPGM带的发育证明“元古宙板块构造体制”的开始。以冷俯冲和大陆地壳深俯冲至地幔,以及其中的部分又从深达300 km处发生折返为标志,“元古宙板块构造体制”在新元古代进化为“现代板块构造体制”,这个转变可由岩石中的HPMUHPM证明。记录这种极端条件的变质带年龄是不一致的,而变质作用发生时间与各大陆岩石圈聚合到超级克拉通（如Superia/Sclavia）或超级大陆（如Nuna (Columbia), Rodinia, Gondwana, 和Pangea）的时间却是一致的。     

Black shales and other sediments with high organic matter contents in Phanerozoic climatic cycles: Communication 2. Black shales during the Pangea existence

According to recent concepts, the Earth surface was permanently transformed during its geological history. Some stages of its evolution were marked by the convergence of separate continental blocks to result in the formation of supercontinents, which resisted successfully centrifugal processes. Other stages were characterized by the opposite tendency: after their long existence, the supercontinents became disintegrated into several large and small blacks, the motion of which was accompanied by opening of new sea basins and closure of former basins with the oceanic crust. The second half of the Paleozoic was marked by amalgamation of large continental blocks. In the Devonian, collision between Laurentia and Baltica culminated in the formation of the Euroamerica continent. After the closure of the Ural paleocean in the terminal Carboniferous–initial Permian, it was united with the Siberian and Kazakhstan continental blocks. These events provided the prerequisites for the formation of a new supercontinent (Pangea), which acquired its final configuration at the end of the Permian. One of its segments located mainly south of the equator included Gondwana. Another segment located northward included Euroamerica, Kazakhstan, Siberian, and two China continental blocks. During its geological history, Pangea suffered many dramatic events including several extinctions of organisms. The most significant event took place in the terminal Permian–initial Triassic and at the transition between the Triassic and Jurassic periods.     

Contemporaneous assembly of Western Gondwana and final Rodinia break-up: Implications for the supercontinent cycle

Geological, geochronological and isotopic data are integrated in order to present a revised model for the Neoproterozoic evolution of Western Gondwana. Although the classical geodynamic scenario assumed for the period 800–700 Ma is related to Rodinia break-up and the consequent opening of major oceanic basins, a significantly different tectonic evolution can be inferred for most Western Gondwana cratons. These cratons occupied a marginal position in the southern hemisphere with respect to Rodinia and recorded subduction with back-arc extension, island arc development and limited formation of oceanic crust in internal oceans. This period was thus characterized by increased crustal growth in Western Gondwana, resulting from addition of juvenile continental crust along convergent margins. In contrast, crustal reworking and metacratonization were dominant during the subsequent assembly of Gondwana. The Río de la Plata, Congo-São Francisco, West African and Amazonian cratons collided at ca. 630–600 Ma along the West Gondwana Orogen. These events overlap in time with the onset of the opening of the Iapetus Ocean at ca. 610–600 Ma, which gave rise to the separation of Baltica, Laurentia and Amazonia and resulted from the final Rodinia break-up. The East African/Antarctic Orogen recorded the subsequent amalgamation of Western and Eastern Gondwana after ca. 580 Ma, contemporaneously with the beginning of subduction in the Terra Australis Orogen along the southern Gondwana margin. However, the Kalahari Craton was lately incorporated during the Late Ediacaran–Early Cambrian. The proposed Gondwana evolution rules out the existence of Pannotia, as the final Gondwana amalgamation postdates latest connections between Laurentia and Amazonia. Additionally, a combination of introversion and extroversion is proposed for the assembly of Gondwana. The contemporaneous record of final Rodinia break-up and Gondwana assembly has major implications for the supercontinent cycle, as supercontinent amalgamation and break-up do not necessarily represent alternating episodic processes but overlap in time.     

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.     

Supercontinent tectonics and biogeochemical cycle： A matter of ‘life and death＇

<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.     

The Grenvillian and Pan-African orogens: World's largest orogenies through geologic time, and their implications on the origin of superplume

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.     

Granite subduction: Arc subduction, tectonic erosion and sediment subduction

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.     

Geochemistry and tectonic significance of mafic volcanic rocks of the Hindoli belt, southeastern Rajasthan: Implications for continent assembly

Mafic volcanic rocks that occur within the sedimentary pile of the Hindoli Group were analyzed for major and trace elements (including REE) to establish tectonic setting of volcanism during the early Proterozoic history of the North Indian Craton. The mafic volcanics are sub-alkaline showing compositional variation from picrobasalt to basalt. They are LREE enriched with (La/Yb)_N ratio ranging from 4.67?C6.19 (avg.5.27) and exhibit slightly concave REE patterns relative to chondrite. The multi-element patterns of these mafic volcanic rocks display relative enrichment in Th and LREE and negative anomalies of Nb and P. These geochemical characteristics are consistent with a subduction related origin. Various variation diagrams, involving immobile trace elements, distinguish the Hindoli lavas as arc basalt. However, their Ti and Nb contents are higher than those of subduction related magmas. Probably the wedge melting, along with mixing of rising asthenosphere might have produced these characteristics. It is suggested that the Hindoli basin originated by rifting of island- arc lithosphere, caused by rising plume in an extensional back arc region. Based on the results of the present geochemical study, it is proposed that in the early Proterozoic the Mewar block had an active-type continental margin on its present eastern side. The continental magmatic arcs and intra-arc basins developed on this margin were subsequently incorporated into the Mewar protocontinent. Possibly, the plate carrying the Bundelkhand block subducted beneath the eastern margin of the Mewar block, resulting in the final amalgamation of the two blocks along Great Boundary Fault zone or Banas Dislocation Zone. The arc related volcanism of north Indian shield at about 1850?C1832 Ma, appears to represent the global subduction event, which resulted in the amalgamation and formation of Columbia supercontinent.     

俯冲带岩浆作用与大陆地壳生长

大陆地壳的起源、生长和改造一直都是国际地学界广泛关注的热点问题,目前仍存在一定的争议,特别体现在陆壳增生的方式和速率上.为了探讨大陆地壳的生长方式,简要综述了俯冲带及其岩浆作用和大陆地壳生长的研究成果.俯冲带可划分为洋洋俯冲带、洋陆俯冲带和陆陆俯冲带,其岩浆作用以产出弧岩浆岩为主要特征,被广泛接受为大陆地壳生长的主要方式.目前主要有两种陆壳生长的假说:玄武岩模式和安山岩模式.玄武岩模式主要通过拆沉和底垫过程来实现新生地壳向大陆地壳的演化;安山岩模式则强调陆壳直接形成于产出安山质岩浆的俯冲带岩浆弧环境.俯冲带和碰撞带等板块汇聚边界是显生宙大陆地壳生长和改造的主要位置,俯冲带岩浆作用对陆壳生长发挥着重要的作用.