Stress and viscosity in the asthenosphere

Stresses and effective viscosities in the asthenosphere to a depth of 400 km are calculated on the basis of Weertmans “temperature method” i.e., on relating viscosity to the ratio of the temperature to the melting point (=homologous temperature). Some oceanic and continental geotherms and two melting point—depth curves, the dry pyrolite solidus and the forsterite₉₀ melting curve are used for the conversion of the homologous temperature to the effective viscosity. Two creep laws are considered, the linear, grain-size-dependent Nabarro—Herring (NH) creep law, and a power creep law, in which the creep rate is proportional to the third power of the stress. A plate tectonic model yields creep rates of 2 · 10⁻¹⁴ s⁻¹ for the oceanic and 3 · 10⁻¹⁵ s⁻¹ for the continental asthenosphere. These values are held constant for the calculations and may be valid for regions inside plates.The dry pyrolite mantle model results in high homologous temperatures in the asthenosphere below oceans (0.9), very low stresses (a few bars and lower) and shows a low viscosity “layer” of about 200-km thickness. Below continental shields the homologous temperature has a maximum value of 0.73, stresses are around 5–20 bar and the low-viscosity region is thicker and less pronounced than in the oceanic case. The Fo₉₀ mantle model generally gives lower homologous temperatures (maximum value below oceans beside active ridges 0.75). The stresses in the asthenosphere beneath oceans vary from a few bars to about 50 bar and below continents to about 100 bar. The low-viscosity region seems to reach great depths without forming a “channel”. The Figs. 1 and 2 show the approximate viscosity—depth distribution for the two mantle models under study.Assuming a completely dry mantle and a mean grain size of 5 mm, power law creep will be the dominating creep process in the asthenosphere. However, grains may grow in a high-temperature—low-stress regime (i.e., below younger oceans), an effect which will further diminish the influence of NH creep. In the upper 100–150 km of the earth some fluid phases may affect considerably creep processes.     

从高分辨率地震层析成像看青藏高原软流圈的物质运动

通过分辨率达到0.5°×0.5°×10 km的青藏高原地壳与上地幔三维成像,为研究青藏高原在新生代的动力学作用提供了新的认识.软流圈的波速扰动数据证实,特提斯大洋板块在拆沉后只俯冲到410 km的间断面之上,并不是所有的大洋板块都会俯冲到上地幔底部.这种大洋板块在软流圈拆沉后激发的热流体上涌,造成高原中部大规模的火山喷发,是青藏高原隆升的主要动力来源之一.根据上地幔三维地震层析成像结果定量计算了岩石圈-软流圈界面（LAB）的深度,揭示了软流圈地幔物质的上涌或者岩石圈地块下沉的作用布局,表明青藏高原的东部在新生代动力学作用过程中是一个相对独立的岩石圈地幔块体.      

Late Quaternary glacio- and hydro-isostasy, on a layered earth

Isostatic response of the Earth to changes in Quaternary Times of ice and water loads is partly elastic, and partly involves viscous mantle flow. The relaxation spectrum of the Earth, critical for estimation of the mantle flow component, is estimated from published determinations of Fennoscandian and Laurentide rebound, and of the nontidal acceleration of the Earth's rotation. The spectrum is consistent with an asthenosphere viscosity around 10²¹P, and a viscosity around 10²³P below 400 km depth. Calculation of relaxation effects is done by convoluting the load history with the response function in spherical harmonics for global effects, and in rectangular or cylindrical transforms for smaller regional effects. Broad-scale deformation of the globe, resulting from the last deglaciation and sea level rise, is calculated to have involved an average depression of ocean basins of about 8 m, and mean upward movement of continents of about 16 m, relative to the center of the Earth, in the last 7000 yr. Deflection in the ocean margin “hinge zone” varies with continental shelf geometry and rigidity of the underlying lithosphere: predictions are made for different model cases. The computational methods is checked by predicting Fennoscandian and Laurentide postglacial warping, from published estimates of icecap histories, with good results. The depth variations of shorelines formed around 17,000 BP (e.g., North America, 90–130 m; Australia, 130–170 m), are largely explainable in terms of combined elastic and relaxation isostasy. Differences between Holocene eustatic records from oceanic islands (Micronesia, Bermuda), and continental coasts (eastern North America, Australia), are largely but not entirely explained in the same terms.     

Lithosphere thermal structure at the eastern margin of the Bohemian Massif: a case petrological and geophysical study of the Nied?wied? amphibolite massif (SW Poland)

The crustal section beneath amphibolite Nied?wied? Massif (Fore-Sudetic Block in NE Bohemian Massif), modelled on the basis of geological and seismic data, is dominated by gneisses with subordinate granites (upper and middle crust) and melagabbros (lower crust). The geotherm was calculated based on the chemical analyses of the heat-producing elements in the rocks forming the crust and the measurements of their density and heat conductivity. The results were verified by heat flow calculations based on temperature measurements from 1,600?m deep well in the Nied?wied? Massif and by temperature–depth estimates in mantle xenoliths coming from the nearby ca. 4.5?My basanite plug in Lutynia. The paleoclimate-corrected heat flow in the Nied?wied? Massif is 69.5?mW?m^?2, and the mantle heat flow is 28?mW?m^?2. The mantle beneath the Massif was located marginally relative to the areas of intense Cenozoic thermal rejuvenation connected with alkaline volcanism. This results in geotherm which is representative for lithosphere parts located at the margins of zones of continental alkaline volcanism and at its waning stages. The lithosphere–asthenosphere boundary (LAB) beneath Nied?wied? is located between 90 and 100?km depth and supposedly the rheological change at LAB is not related to the appearance of melt.     

http://www.sciencedirect.com/science/article/pii/S1674987112001065

It has been thought that granitic crust,having been formed on the surface,must have survived through the Earth’s evolution because of its buoyancy.At subduction zones continental crust is predominantly created by arc magmatism and is returned to the mantle via sediment subduction,subduction erosion, and continental subduction.Granitic rocks,the major constituent of the continental crust,are lighter than the mantle at depths shallower than 270 km,but we show here,based on first principles calculations, that beneath 270 km they have negative buoyancy compared to the surrounding material in the upper mantle and transition zone,and thus can be subducted in the depth range of 270-660 km.This suggests that there can be two reservoirs of granitic material in the Earth,one on the surface and the other at the base of the mantle transition zone(MTZ).The accumulated volume of subducted granitic material at the base of the MTZ might amount to about six times the present volume of the continental crust.Our calculations also show that the seismic velocities of granitic material in the depth range from 270 to 660 km are faster than those of the surrounding mantle.This could explain the anomalous seismic-wave velocities observed around 660 km depth.The observed seismic scatterers and reported splitting of the 660 km discontinuity could be due to jadeite dissociation,chemical discontinuities between granitic material and the surrounding mantle,or a combination thereof.     

Upper mantle structure of the Northern Eurasia from peaceful nuclear explosion data

Several long-range seismic profiles were carried out in Russia with Peaceful Nuclear Explosions (PNE). The data from 25 PNEs recorded along these profiles were used to compile a 3-D upper mantle velocity model for the central part of the Northern Eurasia. 2-D crust and upper mantle models were also constructed for all profiles using a common methodology for wavefield interpretation. Five basic boundaries were traced over the study area: N1 boundary (velocity level, V = 8.35 km/s; depth interval, D = 60–130 km), N2 (V = 8.4 km/s; D = 100–140 km), L (V = 8.5 km/s; D = 180–240 km) and H (V = 8.6 km/s; D = 300–330 km) and structural maps were compiled for each boundary. Together these boundaries describe a 3-D upper mantle model for northern Eurasia. A map characterised the velocity distribution in the uppermost mantle down to a depth of 60 km is also presented. Mostly horizontal inhomogeneity is observed in the uppermost mantle, and the velocities range from the average 8.0–8.1 km/s to 8.3–8.4 km/s in some blocks of the Siberian Craton. At a depth of 100–200 km, the local high velocity blocks disappear and only three large anomalies are observed: lower velocities in West Siberia and higher velocities in the East-European platform and in the central part of the Siberian Craton. In contrast, the depths to the H boundary are greater beneath the craton and lower beneath in the West Siberian Platform. A correlation between tectonics, geophysical fields and crustal structure is observed. In general, the old and cold cratons have higher velocities in the mantle than the young platforms with higher heat flows.Structural peculiarities of the upper mantle are difficult to describe in form of classical lithosphere–asthenosphere system. The asthenosphere cannot be traced from the seismic data; in contrary the lithosphere is suggested to be rheologically stratified. All the lithospheric boundaries are not simple discontinuities, they are heterogeneous (thin layering) zones which generate multiphase reflections. Many of them may be a result of fluids concentrated at some critical P–T conditions which produce rheologically weak zones. The most visible rheological variations are observed at depths of around 100 and 250 km.     

The crustal structure beneath the Central Andean forearc and magmatic arc as derived from seismic studies — the PISCO 94 experiment in northern Chile (21°–23°S)

In this study, we present an interpretation of seismic refraction profiles from the PISCO 94 experiment in northern Chile. As the PISCO experiment was a combined active and passive seismological study, we also discuss results of the passive part in the context of the seismic refraction model. Previous seismic refraction and gravimetric studies indicate a maximum crustal thickness of about 70 km beneath the Pre- and Western Cordillera. The new seismic refraction data lead to a differentiated image of the Andean crust which shows strong varying characteristics. The crustal discontinuities (up to five are detected) dip from W to E. The upper crust has a thickness of 18 km (Precordillera) to 23 km (magmatic arc) underlain by the recent middle crust down to 35–45 km where the velocity increases to about 7 km/s at its base. This crustal level is interpreted as old continental lower crust and its base as blurred continental (paleo) Moho. Beneath the Precordillera, a strong discontinuity at 70 km depth with a velocity increase to about 8 km/s was detected, interpreted as the recent geophysical Moho. For the magmatic arc, this deep discontinuity could not be found by active seismic measurements. The tomographic models of the seismological studies, in general, confirm the seismic refraction results. Anomalously high v_p/v_s ratios in the deeper part of the forearc indicate a hydrated mantle wedge consisting of serpentine and amphibole-bearing peridotite and the 70 km discontinuity is interpreted as the boundary between these two different stages of the hydrated mantle wedge. A zone of high attenuation (Qp) and high v_p/v_s ratios beneath the magmatic arc coincides with the low velocity zones and indicates partially molten rocks from a depth of 20 km down to the asthenospheric wedge.     

Phase diagrams of the FeO-MgO-SiO2 system and the structure of the mantle discontinuities

Internally consistent thermodynamic computation of equilibria in the FeO-MgO-SiO₂ system up to 300 kbar is carried out and phase diagrams and profiles of the elastic properties and density are constructed at the depths of 300–800 km. Comparisons of calculated thermodynamic properties for different petrological models with seismic velocity profiles have been used to constrain the mineralogy of the mantle discontinuities. The 400-km discontinuity may represent the univariant or divariant transition in the olivine component of pyrolite as well as a chemical boundary. For the pyrolite composition at the depth of 650 km there are two different spinel + perovskite + stishovite (640 km) and magnesiowustite + spinel + perovskite (650 km) divariant loops (1–2 km wide) separated by a Invariant zone spinel + perovskite (4–6 km wide). The results indicate that phase changes in pyrolite do not explain the 650-km discontinuity. It is also shown that it is impossible to match the seismic properties observed at the depths of 600–800 km and through the discontinuity with any isochemical petrological model considered in the FMS system. However, increasing the iron content or silica and iron contents across the 650-km discontinuity can produce thermodynamic properties in the lower mantle that are more consistent with those inferred from seismic observations. Constraints on the SiO₂ and iron contents in the mantle are inferred from the comparison of thermodynamic and seismological data.     

藏北新生代玄武质火山岩起源的深部机制——大陆俯冲和板片断离驱动的地幔对流上涌模式

近年来地震层析成像揭示出可可西里-西昆仑中新世-第四纪钾质火山岩带下方存在一个深达900km的巨型地幔低速体,空间上与新特提斯洋和印度大陆俯冲断离板片沉降形成的冷地幔下降流共存(Replumaz et al.,2010a,b),两者构成统一的地幔对流体系。研究表明,羌塘古近纪(60~34Ma)钠质玄武岩和高钾钙碱性玄武岩均以富含Ti O2、P2O5和大离子亲石元素为特征,主体具有与OIB相近的微量元素组成和弱亏损的Sr、Nd同位素特征,指示岩浆起源于软流圈的上涌熔融,但Nb、Ta的弱亏损表明岩浆源区有岩石圈地幔熔融组分的贡献。羌塘(32~26Ma)碱性钾质玄武岩与可可西里和西昆仑中新世以来喷发的钾质玄武岩的地球化学性质相近,不相容元素比值和Sr、Nd同位素组成指示岩浆起源于古俯冲地幔楔的低程度熔融。这些特征表明藏北软流圈上涌作用始于古近纪,初始上涌中心位于羌塘地体之下。计算表明藏北古近纪火山岩距离当时的印度大陆北缘的最大和最小距离约为1250km和700km,与现今可可西里地幔低速体的南、北边界与印度大陆北缘的距离相近,支持羌塘古近纪地幔上涌作用也是受藏南冷地幔下降流所驱动。青藏高原在南北缩短过程中不仅表现为软流圈自西向东挤出流动,地幔垂向对流也是其重要的运动形式,在地幔上升流形成的藏北热幔区内,地壳的水平缩短增厚与岩石圈地幔的伸展减薄呈脉动式共存。藏南冷地幔下降流和藏北热地幔上升流的持续北移是导致藏北后碰撞火山岩时空迁移的主要控制因素。     

Petrological-geophysical models of the internal structure of the lithospheric mantle of the Siberian Craton

Based on the simultaneous inversion of unique ultralong-range seismic profiles Craton, Kimberlite, Meteorite, and Rift, sourced by peaceful nuclear and chemical explosions, and petrological and geochemical data on the composition of xenoliths of garnet peridotite and fertile primitive mantle material, the first reconstruction was obtained for the thermal state and density of the lithospheric mantle of the Siberian craton at depths of 100–300 km accounting for the effects of phase transformation, anharmonicity, and anelasticity. The upper mantle beneath Siberia is characterized by significant variations in seismic velocities, relief of seismic boundaries, degree of layering, and distribution of temperature and density. The mapping of the present-day lateral and vertical variations in the thermal state of the mantle showed that temperatures in the central part of the craton at depths of 100–200 km are somewhat lower than those at the periphery and 300–400°C lower than the mean temperature of tectonically younger mantle surrounding the craton. The temperature profiles derived from the seismic models lie between the 32.5 and 35 mW/m² conductive geotherms, and the mantle heat flow was estimated as 11–17 mW/m². The depth of the base of the cratonic thermal lithosphere (thermal boundary layer) is close to the 1450 ± 100°C isotherm at 300 ± 30 km, which is consistent with published heat flow, thermobarometry, and seismic tomography data. It was shown that the density distribution in the Siberian cratonic mantle cannot be described by a single homogeneous composition, either depleted or enriched. In addition to thermal anomalies, the mantle density heterogeneities must be related to variations in chemical composition with depth. This implies significant fertilization at depths greater than 180–200 km and is compatible with the existence of chemical stratification in the lithospheric mantle of the craton. In the asthenosphere-lithosphere transition zone, the craton root material is not very different in chemical composition, thermal regime, and density from the underlying asthenosphere. It was shown that minor variations in the chemical composition of the cratonic mantle and position of chemical (petrological) boundaries and the lithosphere-asthenosphere boundary cannot be reliably determined from the interpretation of seismic velocity models only.     

The Ivrea—Verbano and Strona-Ceneri Zones, Northern Italy: A cross-section of the continental crust—New evidence from seismic velocities of rock samples

Seismic velocities under confining pressures to 10 kbar have been measured for rocks of the Ivrea—Verbano and Strona—Ceneri Zones of northern Italy, a metamorphic complex thought to represent a cross-section of the continental crust and crust—mantle boundary. Laboratory-determined compressional wave velocities for schists and gneisses of the amphibolite facies found in the upper levels of the section (having an average density of 2.74 g/cm³) average 6.45 km/sec at pressures between 6 and 10 kbar. These increase with depth to values greater than 7.1 km/sec for amphibolites and rocks of the amphibolite—granulite facies transition and to 7.5 km/sec. (average density 3.06 g/cm³) in intermediate and mafic granulite facies rocks near the base of the section. Compressional wave velocities then abruptly increase to 8.5 km/sec in ultramafic complexes near the Insubric Line. Regional geophysical surveys show that P_g is 6.0 km/sec (density of 2.7 g/cm³), P* is 7.2–7.4 km/sec (density of 3.1 g/cm³) and P_n is 8.1 km/sec, values which are in agreement with the laboratory data when effects of temperature are taken into consideration. Estimated thicknesses of exposed rock units are in reasonable agreement with thicknesses determined for crustal layers in seismic refraction experiments. The agreement between the regional crustal structure and the laboratory-determined values of velocity and density provides strong evidence for the hypothesis that the rocks of this metamorphic complex represent a cross-section of the continental crust of the Po Basin.Using the Ivrea—Verbano and Strona—Ceneri sequence as a model of the continental crust, the crust of northern Italy is shown to consist of a thick series of metamorphic rocks with greenschist facies rocks occupying the uppermost levels. These grade downward into amphibolite facies gneisses and schists with occasional granitic intrusives. The Conrad discontinuity is marked by a change from silicic and intermediate amphibolite facies gneisses to intermediate and mafic granulite facies rocks in which hydrous minerals diminish in abundance and thus represents a distinct transition in terms of both composition and metamorphic grade. The lower crust is dominated by a heterogeneous series of mafic and metapelitic rocks in the granulite facies. Importantly, metasedimentary rocks of intermediate silica content found in the complex can have compressional wave velocities equivalent to velocities in mafic rocks suggesting that the lower continental crust everywhere is not necessarily mafic in composition. Ultramafic complexes near the Insubric Line may represent the upper mantle of the continent and their setting suggests that the continental crust-upper mantle boundary is sharp and is not isochemical.     

Thermal Structure and Rheology of the Upper Mantle beneath Guangdong and Hainan Provinces, China

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The present rate of uplift of Fennoscandia implies a low-viscosity asthenosphere

The rate of displacement in Fennoscandia has been intensively discussed for many years. It is now widely accepted to be an isostatic response of the glacial history of the area. The Earth's present response to deglaciation in Fennoscandia is simulated using a three-dimensional (3D), viscoelastic model in which the asthenosphere and mantle viscosity are allowed to vary so that the maximum rate of present uplift matches its observed value. The deglaciation history is considered to be known, and the C¹⁴-datings are converted to sidereal years. The pattern of the present uplift gives a firm match with the observed data when a low-viscosity asthenosphere is introduced. Assuming a 15,000 years load cycle, i.e. the glacier was applied to the surface for 15,000 years before the melting started, the best fitting earth viscosity model is a 10²⁴ Nm lithosphere overlying a 75 km-thick 2.0 × 10¹⁹ Pas asthenosphere and a 1.2 × 10²¹ Pas mantle. The simulations suggest a remaining maximum uplift of 40 m.     

Flow in upper-mantle rocks: Some geophysical and geodynamic consequences

Flow mechanisms effective in the upper mantle and some of the parameters of the creep equation are determined from the study of peridotites from basalt and kimberlite xenoliths and alpine-type massifs. Creep controlled by dislocation climb, as inferred by Weertman, is the dominant mechanism. Evidence for superplastic flow is found in the deepest kimberlite xenoliths. Flow in the alpine-type massifs is ascribed either to intrusion in the crust when continental plates collide (lherzolite massifs) or to sea-floor spreading (harzburgite massifs included in ophiolites). The consideration of textures, crystal substructures and preferred orientations connected with P,T equilibrium conditions derived from pyroxenes, helps in deciphering the large-scale structure and flow of peridotites in the crust and in the mantle down to 200 km. For the first 150 km, the representative structures are those of the basalt xenoliths and the kimberlite xenoliths with a coarsegrained texture. They have many features in common and probably represent a static lithosphere with, in basalt xenoliths, possible evidence for the transition to the shear flowing asthenosphere. The porphyroclastic and mosaic-textured xenoliths, in kimberlites equilibrated at depth between 150 and 200 km and a few more superficial basalt xenoliths, reflect a much larger strain rate and applied stress and might be connected to vertical instabilities also responsible for magma genesis.     

Temperature-depth profiles in Czechoslovakia and some adjacent areas derived from heat-flow measurements, deep seismic sounding and other geophysical data

The results of seismic measurements along three deep seismic sounding (DSS) profiles on the territory of Czechoslovakia and in adjacent countries have provided sufficient material about the crustal structure and the depth of the Moho discontinuity. These data, together with gravity and aeromagnetic data and the determinations of heat-flow values, were used to select several locations where the temperature—depth profiles were calculated. The Moho temperature of about 500 C beneath the Bohemian Massif increases to 800–1000 C and even more beneath the inner Neogene depressions of the Carpathian system. The regional differences in mantle heat-flow contribution between both these provinces may reach 1 μcal. cm⁻² sec⁻¹; such a variation in energy inflow may then be the driving force for the geological evolution. The geophysical implications of different thermal structure of the crust are discussed. Because of high subsurface temperatures in the Hungarian basin, partial melting at a depth of about 30 km may not be excluded.     

Lower crust indentation or horizontal ductile flow during continental collision?

Conditions for indentation and channelised flow are investigated with two-dimensional thermomechanical models of Alpine-type continental collision. The models mimic the development of an orogen at an initial central portion of weakened lithosphere 150 km wide, coherent with several geological reconstructions. We study in particular the role of lower crustal strength in developing peculiar geometries after 20 Ma of shortening at 1 cm/year. Crustal layers produce geometries of imbricate layers, which result from two contrasted mechanisms of either channelised ductile lateral flow or horizontal rigid-like indentation:

– Channelised lateral flow develops when the lateral lower crust has a viscosity less than 10²¹ Pa s, exhibiting velocities opposite to the direction of convergence. This mechanism of deformation produces subhorizontal shear zones at the boundaries between the lower crust and the more competent upper crust and lithospheric mantle. It is also associated with a topographic plateau that equilibrates with a wide (about 200 km) but quasi-constant crustal root about 50 km deep.

– In contrast, indentation occurs with lateral lower crust layers that have a viscosity greater than about 10²³ Pa s, producing significant shortening and thickening of the central crust. In this case topography develops steep and narrow (around 100 km wide), associated with a thickened crust exceeding 60 km depth. A crustal-scale pop-up forms bounded by subvertical shear zones that root into the mantle lithosphere.

A revised estimation of the steady-state geotherm for the continental lithosphere and its implication for mantle melting

An analytical solution has been derived for the steady-state geotherm of the continental lithosphere, using an empirical thermal conductivity model that incorporates the experimentally observed temperature and pressure effect. Based on recent global compilations of crustal thickness and heat flow data, a standard continental lithosphere is re-defined by a global mean model with total crustal thickness of 40 km and surface heat flow of 55 mWm-² (within which 28 mWm^-2 is assumed to be derived from deep mantle source). The thickness of the continental lithosphere (≅125 km), consistent with previous models, is given by the depth at which the geotherm intersects the potential asthenosphere temperature of 1280°C. It is shown that the new steady-state geotherm is much hotter than that based on the previously adopted model where material thermal conductivity is assumed to be constant (≅3.14 W/m/k) throughout the lithosphere. The consequence of this re-evaluation of pre-extension thermal structure in the lithosphere is that the minimum stretching factor required to cause the onset of dry mantle partial melting can be 15–20% lower than the previous estimate. Also, if minor amounts of water or other volatile element or dry basalt are present in the upper mantle, melting of the subcontirfental mantle is very likely to occur for any geotherms constructed using surface heat flows > 30 mWm-².     

Results of lithospheric studies from long-range profiles in Siberia

Several long-range seismic profiles, obtained during the last ten years in Siberia, show the complicated lithospheric structure of the Siberian platforms. The three component observations, conducted at distances up to 3000 km, made it possible to obtain information on P- and S-velocities in the crust, on P-velocity and Q-factor for the upper mantle, and on the seismic boundaries responsible for reflected, refracted and converted waves down to a depth of 400–700 km.The crustal models are typical of old platforms of Eurasia: the average thickness of 40 km, three layers with P-velocities 6.2, 6.5, 7.0 km/s and thicknesses of 10–15 km are distinguished. The depth to the M discontinuity varies from 45–50 km beneath the old Tunguss depression, to 35–40 km beneath the younger Vilyui basin. The most complicated Moho structure is observed in the boundary between the West Siberian and the Siberian platforms.A strong inhomogeneity of P-velocity models was revealed for the upper mantle. The horizontal inhomogeneities are more larger in the uppermost mantle to depths of 80–100 km, where P-velocities vary from 8.0–8.2 km/s beneath the young West Siberian plate to 8.4–8.6 km/s beneath some blocks of the Siberian craton. The fine vertical inhomogeneity was studied with reflections correlated after computer processing of seismograms. They outlined several low-velocity layers 20–50 km thick. The layers were characterized by low Q as well.Intensive waves were recorded from the transition zone between the upper and lower mantle. The top of the zone is nearly horizontal in the area; its depth is 400 ± 25 km. The bottom of the zone lies at about 700 km.     

Conductive structure of Ukrainian Carpathians from EM observations

Geomagnetic variation observations in the Carpathian region gave the data for tracing the axis of a 1200 km long Carpatian electrical conductivity anomaly (CA) and estimation of its integral longitudinal conductivity (~2 × 10⁸ S × m). We made also 35 magnetotelluric soundings (MTS) in the south-east part of the Ukrainian Carpathians. The shape of MTS curves regularly changes from south-west to north-east forming 6 zones of identical behaviour. Most interesting MTS curves are above the CA. The longitudinal curves define the CA at a depth of 10 km; the transverse ones are not sensitive to crustal CA but they define a mantle conductor at a depth of 100–200 km with conductance ~5000 S which can be identified with the asthenosphere. The principal crustal conductors manifested by MTS data in Carpatians are CA subducting in south-west direction from moderately conductive sediments and a conductive zone of Transcarpathian deep fault. Correlation of electrical conductivity structure with seismicity is discussed.     

A general model for the intrusion and evolution of 'mantle' garnet peridotites in high-pressure and ultra-high-pressure metamorphic terranes

Garnet‐bearing peridotite lenses are minor but significant components of most metamorphic terranes characterized by high‐temperature eclogite facies assemblages. Most peridotite intrudes when slabs of continental crust are subducted deeply (60–120 km) into the mantle, usually by following oceanic lithosphere down an established subduction zone. Peridotite is transferred from the resulting mantle wedge into the crustal footwall through brittle and/or ductile mechanisms. These ‘mantle’ peridotites vary petrographically, chemically, isotopically, chronologically and thermobarometrically from orogen to orogen, within orogens and even within individual terranes. The variations reflect: (1) derivation from different mantle sources (oceanic or continental lithosphere, asthenosphere); (2) perturbations while the mantle wedges were above subducting oceanic lithosphere; and (3) changes within the host crustal slabs during intrusion, subduction and exhumation. Peridotite caught within mantle wedges above oceanic subduction zones will tend to recrystallize and be contaminated by fluids derived from the subducting oceanic crust. These ‘subduction zone peridotites’ intrude during the subsequent subduction of continental crust. Low‐pressure protoliths introduced at shallow (serpentinite, plagioclase peridotite) and intermediate (spinel peridotite) mantle depths (20–50 km) may be carried to deeper levels within the host slab and undergo high‐pressure metamorphism along with the enclosing rocks. If subducted deeply enough, the peridotites will develop garnet‐bearing assemblages that are isofacial with, and give the same recrystallization ages as, the eclogite facies country rocks. Peridotites introduced at deeper levels (50–120 km) may already contain garnet when they intrude and will not necessarily be isofacial or isochronous with the enclosing crustal rocks. Some garnet peridotites recrystallize from spinel peridotite precursors at very high temperatures (c. 1200 °C) and may derive ultimately from the asthenosphere. Other peridotites are from old (>1 Ga), cold (c. 850 °C), subcontinental mantle (‘relict peridotites’) and seem to require the development of major intra‐cratonic faults to effect their intrusion.