青藏高原P波速度层析成像与岩石圈结构

利用中国西部地震台网的数据,通过体波层析成像反演了青藏高原及邻域的三维P波速度结构.根据地壳和上地幔的速度变化和构造特征,重点讨论了下地壳流动、地幔上涌、岩石圈减薄以及与藏北新生代火山岩和藏南裂谷系的关系等问题.分析表明,青藏高原中、下地壳平均速度偏低,低速区主要分布在拉萨和羌塘块体内部,随着深度的增加逐渐扩大到松潘—甘孜块体.上述低速区之间多被高速带分隔,暗示地壳中、下部的韧性变形被限制在特定的区域,不太适于产生贯穿整个青藏高原的大规模横向流动.此外,地幔上涌也并非普遍发生于整个青藏高原,而是集中在羌塘、松潘—甘孜以及喜马拉雅东构造结附近,导致上述区域的岩石圈地幔较薄,并且伴生火山活动和岩浆作用.此外,由于印度大陆岩石圈在向北俯冲,板片下沉过程中引起地幔上涌,热流物质有可能上升进入地壳,这一作用对藏北新生代火山岩和藏南裂谷系的形成以及中、下地壳的韧性变形产生了明显的影响.     

The influence of crustal structure on compositions of subduction-related magmas

A survey of Sr isotopic ratios and other compositional features of subduction-related magma suites reveals significant correlations between these averaged parameters and characteristics of the underlying crust (i.e., thickness, composition, and age). These observations lead to the conclusion that crust and(or) mantle rocks in the hanging walls of subduction zones are involved in modification of primary mafic magmas (typically basalt or basaltic andesite). It is proposed that mafic magmas will stagnate within the crust or uppermost mantle where they may differentiate and react with wall rocks. The extent to which such processes manifest themselves will depend upon details of the local crustal structure. In particular, the composition and age of the crust will strongly influence such parameters as Sr, Nd and Pb isotopic compositions. Such data strongly indicate the involvement of crustal rocks in locales underlain by old sialic crust (e.g., central Andes). Depending upon the level of magma stagnation and evolution within the crust, different trends in isotopic composition may result. These isotopic trends may be enhanced by partial melting of the wall rocks to produce relatively silicic anatectic magmas, and locally they may reflect subduction of continental sediments. Interpretation of the isotopic data may be more ambiguous in locales underlain by younger and more mafic continental crust (Cascades, E Eleutians) and those underlain by oceanic crust owing to the similarity in isotopic composition of primary magmas and the latter crustal materials. Yet some degree of crustal involvement in magmatic evolution seems highly probable even in these more primitive terranes. Consequently, most island arc magmas, and especially those more evolved than basalt, are probably not primary in the sense that they do not represent direct melts of the upper mantle. Studies of arc volcanic rocks may yield misleading conclusions concerning processes of magma generation related to subduction unless evolutionary processes are defined and their effects considered. It appears that modern volcanic arcs provide a poor analog for models of early crustal development because the modern mantle-derived magmatic components are more mafic in composition than average continental crust.     

利用地下流体氦同位素比值估算大陆壳幔热流比例

地下流体中的氦同位素 3He来自地幔的排气作用 ,4He则是铀、钍衰变的产物 .由于铀、钍元素在大陆地壳中富集 ,4He通量与地壳热流呈正相关关系 ;同时 3He通量与地幔热流之间呈正相关 .所以地下流体的氦同位素比值 （3He / 4 He）与大陆壳幔热流比值 （qc/qm）呈反相关关系 .根据欧亚大陆和加拿大地盾的地下流体氦同位素比值数据和相应的壳幔热流比值数据 ,统计出 qc/ qm 与 3He / 4 He之间的回归关系 :qc/ qm =0 81 5- 0 30 0ln（3He / 4 He） ;此处 3He/ 4 He的单位是RA（大气的 3He/ 4 He比值 ） .有了地表热流值和壳幔热流比值即可得到地壳热流和地幔热流 .利用该公式以及热流值估算了中国主要盆地的壳幔热流值 ;根据这些数值得出的热岩石圈厚度和地壳平均生热率结果与地震学研究成果一致 .氦同位素比值是区分大陆热流中地壳热流值和地幔热流值的有用参数 .     

Recycling of the continental crust

In order to understand the evolution of the crust-mantle system, it is important to recognize the role played by the recycling of continental crust. Crustal recycling can be considered as two fundamentally distinct processes: 1) intracrustal recycling and 2) crust-mantle recycling. Intracrustal recycling is the turnover of crustal material by processes taking place wholly within the crust and includes most sedimentary recycling, isotopic resetting (metamorphism), intracrustal melting and assimilation. Crust-mantle recycling is the transfer of crustal material to the mantle with possible subsequent return to the crust. Intracrustal recycling is important in interpreting secular changes in sediment composition through time. It also explains differences found in crustal area-age patterns measured by different isotopic systems and may also play a role in modeling crustal growth curves based on Nd-model ages. Crustal-mantle recycling, for the most part, is a subduction process and may be considered on three levels. The first is recycling with only short periods of time in the mantle (<10 m.y.). This may be important in explaining the origin of island-arc and related igneous rocks; there is growing agreement that 1–3% recycled sediment is involved in their origin. Components of recycled crustal material, with long-term storage (up to 2.5 b.y.) in the mantle as distinct entities, has been suggested for the origin of ocean island and ultrapotassic volcanics but there is considerably less agreement on this interpretation. A third proposal calls for the return of crustal material to the mantle with efficient remixing in order to swamp the geochemical and isotopic signature of the recycled component by the mantle. This type of recycling is required for steady-state models of crustal evolution where the mass of the continents remains constant over geological time. It is unlikely if crust-mantle recycling has exceeded 0.75 km³/yr over the past 1–2 Ga.Good evidence exists that selective recycling is an important process. Sedimentary rocks preserved in different tectonic settings are apparently recycled at different rates, resulting in a bias in the sediment types preserved in the geologic record. Selective recycling has important implications for the interpretation of Nd model ages of old sedimentary rocks and in the analysis of accreted terranes. Although there is evidence that continental crust was formed prior to 3.8 Ga, the oldest preserved rocks do not exceed this age. It is likely that the intense meteorite bombardment, which affected the earth during the period 4.56–3.8 Ga, coupled with rapid mantle convection, which resulted from greater heat production, caused the destruction and probable recycling into the mantle of any early formed crust.Although crust-mantle recycling is seen as a viable process, it is concluded that crustal growth has exceeded crust-mantle recycling since at least 3.8 Ga. Intracrustal recycling has not been given adequate consideration in models of crustal growth based on isotopic data (particularly Nd model ages). It is concluded that crustal growth curves based on Nd model ages, while vastly superior to those based on K/Ar or Rb/Sr, tend to underestimate the volume of old crust, due to crust-mantle and/or intracrustal recycling.     

Thickness and thermal history of continental crust and root zones

The survival to the present of the Archean nuclei of Precambrian shields requires special explanation if, as seems likely, the rate of heat flow out of the earth was two or three times greater in the late Archean (2.5 b.y. ago) than at present, since such a high heat flux would have melted the base of the Archean crust. It is proposed that there must have existed beneath stable continental crust a root zone (or lithosphere, or tectosphere) at least 200 km thick which has acted as a thermal buffer between the crust and the convecting mantle; this is virtually the same model as has been proposed to explain the present distribution of heat flow between continents and oceans. The strong temperature dependence of silicate rheology insures that the mantle temperature at the base of the root zone was no more than about 150°C higher in the late Archean than at present; the greater Archean heat flux would have been removed mainly through faster sea-floor spreading. To have survived, the root zone must be mechanically and chemically distinct from the rest of the mantle, and its formation was probably intimately related to the differentiation and stabilization of the continental crust.     

Numerical models of subduction of the oceanic crust with basaltic plateaus

Light continents and islands characterized by a crustal thickness of more than 30 km float over a convective mantle, while the thin basaltic oceanic crust sinks completely in subduction zones. The normal oceanic crust is 7 km thick. However, anomalously thick basaltic plateaus forming as a result of emplacement of mantle plumes into moving oceanic lithospheric plates are also pulled into the mantle. One of the largest basaltic plateaus is the Ontong Java plateau on the Pacific plate, which arose during the intrusion of a giant superplume into the plate ～100 Myr ago. Notwithstanding its large thickness (averaging ～30 km), the Ontong Java plateau is still experiencing slow subduction. On the basis of numerical modeling, the paper analyzes the oceanic crust subduction process as a function of the mantle convection vigorousness and the density, thickness, viscosity, and shape of the crust. Even a simplified model of thermocompositional convection in the upper mantle is capable of explaining the observed facts indicating that the oceanic crust and sediments are pulled into the mantle and the continental crust is floating on the mantle.     

Fluid/melt in continental deep subduction zones: Compositions and related geochemical fractionations

Plate subduction is the most magnificent process in the Earth. Subduction zones are important sites for proceeding matter- and energy- transports between the Earth's surface and the interior, continental crust growth, and crust-mantle interactions. Besides, a number of geological processes in subduction zones are closely related to human beings' daily life, such as volcanic eruptions and earthquakes, formation of mineral deposits. Subduction process thus has long been the centric topic of Earth sciences. The finding in 1980 s that continental crust could be subducted to mantle depths is a revolutionary progress in plate tectonic theory. Compared to oceanic crust, continental crust is colder, drier, lighter, and much more geochemically/isotopically heterogeneous. Hence, continental subduction process would affect the structure, compositions and evolutions of the overlying mantle wedge even more. During continental subduction and subsequent exhumation, fluids and melts can be generated in the(de)hydration process and partial melting process, respectively. These melts/fluids play important roles in crust-mantle interactions, elemental migrations, isotopic fractionations, and mantle metasomatism. By summarizing recent research works on subduction zones in this paper, we present a review on the types, physicochemical conditions and compositions of fluids/melts, as well as the migration behaviors of fluid-related characteristic elements(Nb-Ta-V) and the fractionation behaviors of non-traditional stable isotopes(Li-Mg) in subduction zones. The aim of this paper is to provide the readers an update comprehensive overview of the melt/fluid activities in subduction zones and of Li-Mg isotope systematics in subduction-related rocks and minerals.     

Thermal regime of the continental lithosphere

Thermally, the lithosphere may be defined as that outer portion of the earth in which heat is transferred primarily by conduction. It generally includes the crust and part of the mantle. The thermal regime of continental lithosphere is determined by many factors including heat flow from the asthenosphere, the vertical and lateral variation of both thermal conductivity and radiogenic heat production, tectonic history, and such superficial processes as climatic history and the shallow hydrothermal regime. From studies of the global heat flow data set, two generalizations regarding continental lithosphere have arisen, namely that: 1) there is a negative correlation between heat flow and tectonic age of continental lithosphere; and 2) the thermal evolution of continental lithosphere is similar to that of ocean basins with the result that the “stable geotherm” is similar in both environments. When continental heat-flow data are studied from a regional rather than a global point of view, considerable doubt arises as to the general applicability of either statement. R. U. M. Rao and his associates have demonstrated that while Precambrian terranes do have demonstrably lower heat flows than, say, Tertiary terranes, the data are not normally distributed and it is not possible to establish a negative correlation between heat flow and age in any rigorous statistical way. The scatter in the relation may be explained in terms of the variations in the duration, intensity and even the sign of continental thermotectonic events in contrast to the simple situation (creation of new oceanic lithosphere at mid-ocean ridges) which prevails in the oceans. The scatter also is partially attributable to the large and laterally variable radiogenic component of heat flow on continents. For a province for which a heat flow-heat production relation has been established, much of the scatter in surface heat flow due to crustal radiogenic heat production versus age is eliminated by determining reduced heat flow (surface heat flow minus radiogenic component) as a function of tectonic age, but much scatter remains, and it is still not possible to establish a heat flux-age relation in a rigorous way. Primarily because of the spatial variability in radiogenic heat production, no single geotherm can be used to characterize the thermal regime of a stable continental terrane. Thus, while some sites on stable continental blocks may have a geotherm fortuitously similar to that for old ocean basins, there is no reason to expect that this will be true generally, and many stable continental terranes will be characterized by geotherms markedly different from the geotherm for old ocean basins.     

Possible contribution of the asthenosphere, below the subducted oceanic lithosphere, to the genesis of arc magmas: Geochemical evidence from the andean southern volcanic zone (33–46°S)

The application of the Sr/Ca-Ba/Ca systematics to volcanic rocks of the Andean Southern Volcanic Zone (33°S–46°S) has revealed a good correlation between the estimated degree of partial melting required to generate primary magmas and the projected extensions of the oceanic Nazca plate fracture zones under the continental South American plate. Magmas erupted at volcanic centers situated above these projections are thought to have been derived from primary magmas generated by relatively high degrees of melting, whereas those erupted at other centers are thought to have evolved from magmas produced by comparatively low degree of fusion. We interpret this relationship to reflect the facilitation of heat and mass transfer from the asthenosphere below the subducted oceanic lithosphere to the subarc mantle by the fracture zones. This contribution enhances the degree of melting of the subarc mantle source as well as the fraction of material derived from the subducted oceanic crust. This model predicts the predominance of basalts depleted in incompatible trace elements in centers located above the Nazca plate fracture zone extensions and of basalts enriched in incompatible trace elements in centers situated between boundaries of fracture extensions.     

Tectonic controls on magma genesis and evolution in the northwestern United States

Field, chronologic, chemical, and isotopic data for late Cenozoic basaltic rocks from the northwestern United States illustrate the relationship between crustal structure and tectonic forces in controlling the genesis and evolution of continental volcanism. In the northwestern U.S., the first major episode of basaltic volcanism was triggered by crustal rifting in a “back-arc” environment, east of the westward-migrating volcanic arc created by the subduction of the Juan-de-Fuca plate beneath the North American plate. Rifting and volcanism were concentrated by pre-existing zones of crustal weakness associated with boundaries between the old Archean core of the continent and newly accreted terranes. Basalts erupted during this time (Columbia River, Steens Mountain) show evidence of significant fractionation histories including contamination by crust of varying age depending on the crustal structure at the eruption site. Presumably this reflects ponding and stagnation of primary magmas in the crust or at the crust-mantle interface due to their encounter with thick crust, not yet extended and still containing its low-density, easily fusible component. Continued rifting of this crust, and modification of its composition through extraction of rhyolitic partial melts and deposition of the fractionation products from primary basaltic melts, coupled with a shift in stress orientation roughly 10.5 Ma ago, allowed relatively unfractionated and uncontaminated magmas to begin reaching the surface. In the western part of the region (Oregon Plateau), these magmas tapped a mantle source similar to that which produced most of the ocean island basalts of the northern hemisphere. To the east (Snake River Plain), however, the mantle sampled by basaltic volcanism has isotopic characteristics suggesting it has preserved a record of incompatible element enrichment processes associated with the formation of the overlying Archean crustal section some 2.6 Ga ago.     

Helium, volatile fluxes and the development of continental crust

Mantle-derived helium has a substantial primordial component and is readily distinguished from radiogenic “crustal” He by its isotopic composition. For some years it has been known to be escaping at mid-ocean ridges and more recently it has been shown to be escaping through the continental lithosphere in tectonically active areas, particularly those undergoing extension or volcanism. The C/³He value observed in ocean ridge basalts and continental gases that contain only mantle He, is close to 10⁹. This is believed to be a typical value for the upper mantle. Other continental gases have ratios that vary widely and are diluted with crustal carbon. The ratio C/⁴He decreases with time through the production of radiogenic⁴He, and depends on the C/(U + Th) value. Departures from the average may result from exceptional concentrations of U and Th or from C/He fractionation.There is circumstantial evidence for a steady-state flux of He through the continents that may be estimated from He accumulations in lakes and aquifers. The mantle component of such fluxes is calculated from their³He content. If the mantle component is accompanied by C in the proportion indicated above, and extensional areas make up as little as 10% of the crust at any one time, then about 10% of the present inventory of crustal C would have been added to the crust every Ga by this means. C/K values for the crust and mantle are today very similar, and K may therefore scale as C. K/U and K/Th vary within narrow limits and they may scale with C also.The most plausible means of scavenging He from the mantle is by partial melting: He is expected to enter the first few percent of liquid formed, and the loss of mantle He and C at the surface is associated with the emplacement of basaltic bodies in the lower crust carrying K, U and Th. Some limits are placed on the thickness of basalt added in extensional areas.Mantle-derived CO₂ has often been invoked as a means of dehydrating continental crust to produce granulites. However, the amounts of CO₂, estimated from mantle He fluxes, entering the crust in those active tectonic areas studied so far appears too small to produce dehydration on a regional scale. The addition of mantle-derived material to the crust in extensional zones is a first-order crustal growth process the importance of which has previously been underestimated.     

Oceanic subcrustal flow toward the continents and constancy of global sea-level

Summary From the conservation of the mass of the earth including the hydrosphere it can be concluded that continental growth has been connected with subcrustal flow from the ocean toward the continents. Calculations show that the volume of ocean bottom subsidence nearly equals to the volume of the uplifted continental masses above the level of the primeval ocean bottom. The sea level has not changed appreciably since Precambrian. Change of ocean bottom topography and emergence of continents do not effect global sea level. Transgression and regression are figurative terms and really indicate subsidence resp. uplift of the continental crust blocks around the shoreline.     

Two types of alkaline rocks-two types of upper mantle

1) Petrochemical studies of volcanic rocks shows that alkaline rocks of continents and oceans are different genetically in spite of their mineralogical and chemical similarity. 2) Oceanic rocks develop according to the following type: tholeiitic basalt — olivine basalt — alkaline rocks. 3) Continental alkaline rocks are derivatives of initially alkaline basalts and are connected by gradual transitions with calc-alkaline rocks of island arcs. 4) The source of all volcanic rocks is the upper mantle. Therefore the existence of two main types of rocks — oceanic and continental — reflects basic heterogeneities in composition and structure of the upper mantle.     

Three-dimensional Mapping of Magma Source and Transport Regions from Seismic Data: The Mantle Wedge beneath Northeastern Japan

—We investigate the distribution of partial melt in island arc using the seismic velocity structure of the mantle wedge beneath northeastern Japan. The comparison of the seismic tomography with laboratory velocity data on a partially-molten mantle rock yields estimates of melting zones in three dimensions. We employ experimental data on the degree of partial melt in hydrous peridotite to give constraints on the melt fraction and temperature. Melting and magma-rich zones derived from the velocity structure coincide with observed low Q zones. The results of the three-dimensional mapping indicate that the source of magma in island arc is diapir-like melting patches localized within the low velocity zones of the mantle wedge. Extensive volcanic activity along the volcanic front is due to the presence of vast magma-rich zones just beneath the Moho. Those melting zones in the uppermost mantle may, in turn, cause melting of lower crustal materials and produce felsic magma. Melt appears to stay at and beneath the Moho, where crystallization fractionation may proceed. Melt exists at greater depths in the back-arc region, which may correlate with across-arc variations of chemical compositions of the volcanic rocks observed in northeastern Japan. We suggest that magma migration in the ductile lower crust may cause low-frequency microearthquakes, and magma penetration into the brittle upper crust may produce mid-crustal S-wave reflectors.     

Trace elements in ocean ridge basalts

A correlary of sea floor spreading is that the production rate of ocean ridge basalts exceeds that of all other volcanic rocks on the earth combined. Basalts of the ocean ridges bring with them a continuous record in space and time of the chemical characteristics of the underlying mantle. The chemical record is once removed, due to chemical fractionation during partial melting. Chemical fractionations can be evaluated by assuming that peridotite melting has proceeded to an olivine-orthopyroxene stage, in which case the ratios of a number of magmaphile elements in the extracted melt closely match the ratios in the mantle. Comparison of ocean ridge basalts and chondritic meteorites reveals systematic patterns of element fractionation, and what is probably a double depletion in some elements. The first depletion is in volatile elements and is due to high accretion temperatures of a large percentage of the earth from the solar nebula. The second depletion is in the largest, most highly charged lithophile elements (“incompatible elements”), probably because the mantle source of the basalts was melted previously, and the melt, enriched in these elements, was removed. Migration of melt relative to solid under ocean ridges and oceanic plates, element fractionation at subduction zones, and fractional melting of amphibolite in the Precambrian are possible mechanisms for depleting the mantle in incompatible elements. Ratios of transition metals in the mantle source of ocean ridge basalts are close to chondritic, and contrast to the extreme depletion of refractory siderophile elements, the reason for which remains uncertain. Variation of ocean ridge basalt chemistry along the length of the ridge has been correlated with ridge elevation. Thus chemically anomalous ridge segments up to 1000 km long appear to broadly coincide with regions of high magma production (plumes, hot spots). Basalt heterogeneity at a single location indicates mantle heterogeneity on a smaller scale. Variation of ocean ridge basalt chemistry with time has not been established, in fact, criteria for recognizing old oceanic crust in ophiolite terrains are currently under debate. The similarity of rare earth element patterns in basalt from ocean ridges, back-arc basins, some young island arcs, and some continental flood basalts illustrates the dangers of tectonic labeling by rare earth element pattern.     

Three-dimensional numerical simulations of crustal deformation and subcontinental mantle convection

3-D simulations of mantle convection allowing for continental crust are explored to study the effects of crustal thickening on lithosphere stability and of continents on large-scale mantle flow. Simulations begin with a crustal layer within the upper thermal boundary layer of a mantle convection roll in a 1 × 1 × 1 Cartesian domain. Convective stresses cause crust to thicken above a sheet-like mantle downwelling. For mild convective vigor an initial crustal thickness variation is required to induce 3-D lithospheric instability below the zone of crustal convergence. The amplitude of the required variation decreases with increasing convective vigor. Morphologically, instability is manifest in formation of drip-like thermals that exist within the large-scale roll associated with initial crustal thickening. A strong surface signature of the drips is their ability to cause deviations from local Airy compensation of topography. After the initial thickening phase, the crustal accumulation that forms serves as a model analog to a continent. Its presence leads to mantle flow patterns distinctly different from the steady-state roll that results in its absence. Large lateral thermal gradients are generated at its edge allowing this region to be the initiation site for continued small-scale thermal instabilities. Eventually these instabilities induce a restructuring of large-scale mantle flow, with the roll pattern being replaced by a square cell. Although preliminary and idealized, the simulations do show the fluid dynamical plausibility behind the idea that significant mantle variations can be generated along the strike of a largely 2-D mountain chain by the formation of the chain itself. The ability of a model continent to cause a change in fundamental convective planform also suggests that the effects of continental crust on mantle convection may be low-order despite the seemingly trivial volume of crust relative to mantle.     

Geothermal effects in the formation of electrically conducting zones and temperature distribution in the earth

Pollack and Chapman have shown that the surface heat flow in continental regions is dependent not only on the earth's crust below the observation site, but also on the upper mantle there. Therefore heat flow can be used to investigate the role of the thermal conditions in the creation of the electrically conductive zones in both the crust and mantle.Empirical exponential formulas describe the depth to the conductivity increase in the crust corresponding to granitization, the depth to the conductive zone at the top of the asthenosphere (SLVZ), as a function of heat flow. Comparing the latter with temperature estimations in the asthenosphere it is concluded that partial melting of the upper mantle occurs only where $\text{q ? 42}\text{m W m}{}^{\mathrm{?2}}\text{? 1}\text{HFU}$.The depth to the conductivity increase corresponding to the mineralogic phase transition in the upper mantle is increased with high temperatures. Such a conductive zone shows that the maximum temperature difference between stable platform areas and active zones is about 1000°C.     

华北地区壳内低速层与地壳流变的关系及其对强震孕育的影响

京津唐张地区普遍存在壳内低速层 ,鄂尔多斯块体内部没有发现低速层 ,壳内低速层的这种分布受新生代裂陷伸展的控制。华北地区中地壳下部和下地壳低速层是岩石塑性流变的结果 ,中地壳上部低速层是地壳裂陷伸展时形成的水平拆离带和韧性剪切带 ,岩石各向异性和流体作用可能是引起低速的原因。壳内软弱层 (低速和塑性流变层 )增强了块体层间的解耦作用 ,对地震孕育起着重要作用     

The geological diversity of island arc volcanism: Northeast Russia

Paleovolcanological and paleotectonic reconstructions developed for the continent-ocean transition zone in Northeast Asia demonstrate a high diversity of island arc volcanic settings. There are two main types of island arc volcanism recognized so far, (i) volcanic arcs of euliminary systems (VAES) and (ii) intrageosynclinal volcanic arcs, including areas of insular volcanism (IIV). The volcanic arcs of euliminary systems include the present-day Kuril-Kamchatka, Aleutian, and the Paleozoic- Early Cretaceous Taigonos volcanic arcs. The latter is considered to be a part of the Talovka-Taigonos euliminary system (TTES), an old double island arc system analogous to present-day systems, the Kuril-Kamchatka and Aleutian ones. Both the TTES and similar present-day euliminary systems are structural complexes that confine concentrically-zoned geosyncline areas on the side of the Pacific. The characteristic features of the VAES include a long history of evolution, stable (calc-alkaline) basalt-andesite composition of volcanic products, and transverse geochemical zonation. Geophysical evidence reveals the complicated processes of endogenous crustal accretion and destruction of continental crust within the VAES zones. The IIV follow the structural pattern of the corresponding geosynclinal system. Their evolution is relatively short, while the spatial position and the composition of their magmatic bodies may considerably vary at different stages of evolution of the geosynclinal systems. Most island arc zones are characterized by calc-alkaline volcanism, but potassium alkaline, alkali-ultrabasic, and ultrabasic rocks also occur in some structures. The settings of intrageosynclinal insular volcanism are diverse and include (a) volcanic overcompensation, (b) geoanticlinal uplift, and (c) volcanotectonic downwarping during the orogenic stage of geosynclines. The calc-alkaline volcanism of island environments in geoanticline zones is likely related to the endogenous accretion of continental crust within a geosyncline system. Intrageosynclinal island-arc volcanism is still very poorly understood. Investigation of this phenomenon is one of the urgent tasks of paleovolcanology.     

Recent geodynamic evolution of convergent plate margins and the reconstruction of fossil plate boundaries

Summary The discovery of paleoplates buried in the upper mantle leads to an interpretation of the subduction as a discontinuous process running in cycles and shifting the place of its operation in or against the direction of ocean floor spreading. This mechanism explains the distribution of calc-alkaline volcanism of different age in fossil convergent plate boundaries. The establishment of regular spatial correlation of the aseismic gap in the Wadati-Benioff zones with the distribution of calc-alkaline volcanism enables to reconstruct fossil plate boundaries and to define allochtonous terranes in apparently homogeneous continental plates. The hampering effect of the ocean floor morphology and of the fragments of continental plates approaching the trench, which substantially influences the rates of subduction and the geodynamic history of active continental margins in different domains along the trench, allows us to understand the complicated geological development of continental wedges in fossil convergent plate margins. The establishment of the segmented nature of active subduction zones and the dramatic morphology of the lower limit of the active subducted slab along the trench help us to interpret extensive lateral gaps in volcanic chains overlying active as well as fossil subduction zones.