The Possible Role of Anoxic Alkaline High Subcritical Water in the Formation of Ferric Minerals, Methane and Disordered Graphitic Carbon in a BARB3 Drilled Sample of the 3.4Ga Buck Reef Chert.

Abundances of Re, Ir, and Au have been determined in a suite of oceanic basalts using radiochemical neutron activation analysis. Mean abundances of highly siderophile elements in the oceanic crust have been estimated. Siderophile elements are fractionated during basalt genesis. Iridium, Os, Pd, Pt, Ru, and Rh are depleted in basalts relative to peridotites. Gold is only slightly enriched in basalts. Rhenium is favorably partitioned into the liquid during partial melting of peridotites. Concentrations of siderophile elements in the earth's upper mantle are higher than that expected from core‐mantle equilibrium. Six platinum‐group elements, Re, and Au have chondritic ratios in the upper mantle, indicating the presence of a meteoritic component. The abundances of eight highly siderophile elements can be accounted for by 0.74% of CI‐chondritelike material in the upper mantle. The low‐Ti mare basalts have lower Re contents and lower Re/Ir ratios than oceanic basalts, which may be interpreted by a lower partition ratio of Re under extremely dry and reducing conditions in the lunar interior.

An alternative hypothesis for the origin of the banded iron formations and the synthesis of prebiotic molecules is presented here. I show the importance of considering water near its supercritical point and at alkaline pH. It is based on the chemical equation for the anoxic oxidation of ferrous iron into ferric iron at high-subcritical conditions of water and high pH, that I extract from E-pH diagrams drawn for corrosion purposes (Geophysical Research s Vol 15, EGU2013–22 Bassez 2013, Orig Life Evol Biosph 45(1):5-13, Bassez 2015, Procedia Earth Planet Sci 17, 492-495, Bassez 2017a, Orig Life Evol Biosph 47:453-480, Bassez 2017b). The sudden change in solubility of silica, SiO2, at the critical point of water is also considered. It is shown that under these temperatures and pressures, ferric oxides and ferric silicates can form in anoxic terrains. No FeII oxidation by UV light, neither by oxygen is needed to explain the minerals of the Banded Iron Formations. The intervention of any kind of microorganisms, either sulfate-reducing, or FeII-oxidizing or O2-producing, is not required. The chemical equation for the anoxic oxidation of ferrous iron is applied to the hydrolyses of fayalite, Fe2SiO4 and ferrosilite, FeSiO3. It is shown that the BIF minerals of the Hamersley Group, Western Australia, and the Transvaal Supergroup, South Africa, are those of fayalite and ferrosilite hydrolyses and carbonations. The dissolution of crustal fayalite and ferrosilite during water-rock interaction needs to occur at T&P just below the critical point of water and in a rising water which is undersaturated in SiO2. Minerals of BIFs which can then be ejected at the surface from venting arcs are ferric oxide hydroxides, hematite, FeIII-greenalite, siderite. The greenalite dehydrated product minnesotaite forms when rising water becomes supersaturated in SiO2, as also riebeckite and stilpnomelane. Long lengths of siderite without ferric oxides neither ferric silicates can occur since the exothermic siderite formation is not so much dependent in T&P. It is also shown that the H2 which is released during hydrolysis/oxidation of fayalite/ferrosilite can lead to components of life, such as macromolecules of amino acids which are synthesized from mixtures of (CO, N2, H2O) in Sabatier-Senderens/Fischer-Tropsch & Haber-Bosch reactions or microwave or gamma-ray excitation reactions. I propose that such geobiotropic synthesis may occur inside fluid inclusions of BIFs, in the silica chert, hematite, FeIII-greenalite or siderite. Therefore, the combination of high-subcritical conditions of water, high solubility of SiO2 at these T&P values, formation of CO also at these T&P, high pH and anoxic water, leads to the formation of ferric minerals and prebiotic molecules in the process of geobiotropy.

The formation of ferric minerals on the anoxic early Earth is usually explained with the action of microorganisms or UV light in acidic conditions. Results show that amorphous and crystalline ferric oxides and silicates can form in the absence of oxygen, microorganisms and UV light, when rocks, located in the upper crust of the Earth until ca 1 km depth, contain ferrous silicates which interact with water called high subcritical, at 300-350 °C and 10-25 MPa. This conclusion is established following the analyses of Eh-pH diagrams for the Fe-H2O system and solubility diagrams for quartz and amorphous silica. It is shown that water below the critical point and not above can lead to the formation of ferric iron in geological terrains on Earth and extraterrestrial objects where anoxic alkaline high subcritical water reacts with rocks containing ferrous silicates.

Lithospheric Structure of the Hikurangi Subduction Margin

<p><b>In this thesis, controlled-source seismic data acquired during two regional-scale experiments are analysed to determine the offshore lithospheric structure at the Hikurangi subduction margin in New Zealand. Subduction of the ∼120 Myr old Hikurangi Plateau occurs beneath the east coast of North Island, New Zealand. Because the plateau is an oceanic large igneous province, where the crustal thickness is about 50% greater than normal oceanic crust, there are different dynamics and seismicity patterns compared to the subduction of a regular oceanic crust. On interseismic time-scales, the plate interface in the south is locked down to depths of 30 km and experiences deep (30-45 km) slow-slip events (SSEs). In contrast, the plate interface is creeping in the north and experiences shallow (5-10 km) SSEs. It is important to understand the seismic velocity structure from the upper crust down to the base of the lithosphere in order to gain insights into the observed variations in subduction thrust slip behaviour, SSEs and the structure of an oceanic plateau in general. The thesis consists of three projects, each focusing on different aspects of the lithosphere at the Hikurangi subduction margin. </b></p><p>Project one investigates the crustal and upper mantle structure of the subducting Hikurangi Plateau at the southern Hikurangi margin. In this study, onshore-offshore seismic data acquired during the Seismic Array HiKurangi Experiment (SAHKE) are used. By forward-model raytracing the travel-times of observed refractions and wide-angle reflections in common receiver gathers, the Hikurangi Plateau crustal thickness is estimated to be 12±1 km. A ∼10% reduction in P-wave-speeds in the Hikurangi Plateau crust beneath the trough is observed. Refractions provide evidence for two upper mantle layers: an upper layer with regular upper mantle P-wave-speeds (8.0±0.2 km/s); and a deeper layer with a high P-wave-speed (8.7±0.2 km/s) at a depth of 50±2 km beneath the Hikurangi trough. Similarly fast upper mantle P-wave speeds are reported along margin-parallel azimuths under the North Island, about 100 km down-dip of the subduction zone at depths of ∼8-10 km from the Moho suggesting that the upper mantle of the Hikurangi Plateau is characterised by anomalously high P-wave-speeds along all azimuths. A velocity reduction of ∼10%, similar to that in the crust, is deduced to extend down to 25±2 km in the upper mantle beneath the trough, as a result of the formation of a low-velocity zone in the faster upper mantle layer. It is proposed that this is due to the serpentinisation of mantle peridotite by hydration through bending-induced normal faults and/or due to crack porosity introduced by thermal cracking, further enhanced by bending-related faulting. This implies that the “regular mantle” (VP ∼8 km/s) is not regular, but rather the faster upper mantle has mechanically bent, fractured and altered. The onset of seismicity in the lower band of the double seismic zone and high upper mantle VP under the North Island is observed at depths of ∼50 km. This is consistent with the hypothesis that the lower band of earthquakes in a double seismic zone is due to antigorite dehydration processes, a hydrous mineral in the low-velocity zone in the upper mantle beneath the trough. Despite the differences in crustal thickness and high upper mantle P-wave speeds, subduction-related upper mantle hydration and dehydration are analogous with other margins where regular oceanic crust is subducting. </p><p>The second project is focused on a series of long-offset, late-arriving wide-angle seismic reflections observed in the onshore-offshore common receiver gathers from the first project. Results from modelling these wide-angle reflections using forward-model raytracing, amplitude versus offset modelling and synthetic waveform modelling, are consistent with a series of reflective horizons approaching sub-lithospheric depths of the subducting Pacific Plate. A ∼3-5 km thick, azimuthally anisotropic layer with a P-wave anisotropy of 13-15% is proposed to exist at a depth of 70 km. A 5 km thick layer with low P-wave velocity (7.6 km/s) and a high VP/VS ratio (>>2.5) is then required below the anisotropic layer. It is followed by another ∼3-5 km thick layer with slightly lower (7.4 km/s) or higher (7.8 km/s) P-wave velocity and a regular VP /VS ratio (∼1.85). The higher VP/VS ratio in the upper layer indicates that it contains either melt or volatiles, whereas the relatively low VP/VS ratio in the lower layer may indicate a relatively lower fluid content. These two layers comprise a composite low VP layer with a thickness of ∼8-10 km beneath the anisotropic layer, and is interpreted to be the lithosphere-asthenosphere boundary (LAB) channel. It is consistent with the down-dip projected depths of the LAB channel found from an earlier study. The most prominent discovery here is the azimuthally anisotropic layer whose fast azimuth is subparallel to the direction of absolute plate motion (perpendicular to the margin). Strong shearing occurring at the LAB channel due to the differential movement of the lithosphere on top of the asthenosphere is suggested to give the preferential alignment of olivine crystals along the direction of maximum finite shear strain and produce the observed azimuthal anisotropy. Results from this study show, therefore, that it is not a single low-velocity channel that makes up the LAB, but a series of layers that make up an LAB zone. In addition, the study highlights the key role that wide-angle reflections from controlled-sources can play in investigating the fine-scale structure of the LAB boundary zone due to the short wavelengths and the generation of enhanced amplitudes when the reflections approach the critical angles (∼55◦). </p><p>The primary objective of the third research project is to estimate the VP/VS ratio of the Hikurangi margin forearc using mode-converted seismic phases from airgun shots recorded by an array of multi-component ocean bottom seismographs (OBS) deployed as a part of the Seismogenesis at Hikurangi Integrated Research Experiment (SHIRE). PPS mode-conversions at the sediment-basement interface and the top of the subducting crust are identified. Estimated average VP/VS ratios for the topmost sediments range from 2.5±1.0 to 6.0±2.5 and are consistent with water-saturated, unconsolidated sediments. Average VP/VS ratios for the entire column of sediments and sedimentary rocks above the subducting crust range from ∼1.55±0.08 to 2.20±0.08. Low-average VP/VS ratios between ∼1.55±0.08 and ∼1.78±0.12 are estimated for a region of higher sediment thickness in the southern Hikurangi margin. The thick sediments may result in a higher degree of compaction. The low VP/VS ratios are also coincident with the offshore extension of the Pahau Torlesse Terrane which consists mainly of low-porosity, highly compacted, Cretaceous greywackes. In contrast, high VP/VS ratios between ∼1.85±0.10 and 2.22±0.08 are observed in regions with lower sediment thickness, which may reflect effects of lower degree of compaction and lithology. Furthermore, the average VP/VS of the rocks and sediments above the subducting crust show a weak correlation with the slip-rate deficits on the subduction thrust. Shear-wave splitting results indicate an anisotropy of ∼3.5% localised in the top layer (∼1-2 km) of sediments beneath the seafloor. Fast polarisation directions are oriented perpendicular to the plate interface contours, suggesting stress-aligned, fluid-filled cracks. </p><p>The work presented in this thesis provides constraints on the lithospheric structure of the Hikurangi subduction margin, from the upper plate down to the base of the lithosphere, using controlled-source seismology. The results provide insights on the physical properties of the materials and their association with geodynamic processes. The outcomes of this thesis advance our knowledge of the Hikurangi subduction margin and contribute to our knowledge of plate tectonics in general. </p>

The composition of Earth's mantle, continental crust, and oceanic crust continuously evolve in response to the dynamic forces of plate tectonics and mantle convection. The classical view of terrestrial geochemistry, where mid‐ocean ridges sample mantle previously depleted by continental crust extraction, broadly explains the composition of the oceanic and continental crust but is potentially inconsistent with observed slab subduction to the lower mantle and oceanic crust accumulation in the deep mantle. We develop a box model to explore the key processes controlling crust‐mantle chemical evolution. The model mimics behaviors observed in thermochemical convection simulations including subducted oceanic crust separating and accumulating in the deep mantle. We demonstrate that oceanic crust accumulation strongly depletes the mantle independently of continental crust extraction. Slab stalling depths and continental crust recycling rates also affect the extent and location of mantle depletion. We constrain model regimes that reproduce oceanic and continental crust compositions using Markov chain Monte Carlo sampling. Some regimes deplete the lower mantle more than the upper mantle, contradicting the assumption of a more enriched lower mantle. All regimes require oceanic crust accumulation in the mantle. Though a small fraction of the mantle mass, accumulated oceanic crust can sequester trace element budgets exceeding the continental crust, depleting the mantle more than continental crust extraction alone. Oceanic crust accumulation may therefore be as important as continental crust extraction in depleting the mantle, contradicting the paradigmatic complementarity of depleted mantle and continental crust. Instead, depleted mantle is complementary to continental crust plus sequestered oceanic crust.

Our knowledge of the oceanic lithosphere largely comes from analogy with ophiolite complexes and the direct scientific drilling of the present‐day oceanic crust (e.g., Christensen and Salisbury, 1975, 1989; Smith and Vine, 1989; Dilek and Furnes, 2011, 2014). In this study, we summarized previous experimental results on seismic properties of oceanic lower crust and upper mantle according to different tectonic settings. The results are used to highlight the compositional heterogeneity and the nature of the oceanic Moho. Observation in different ophiolites reveal an ideal oceanic lithosphere profile with ideal petrologic units and seismic units (Dilek and Furnes, 2011, 2014). The lithospheric mantle beneath ocean basins is composed of tectonized peridotites, which include layered lherzolites and harzburgites and lenses of dunites with chromitites and nearly correspond to the seismic Layer 4. The overlying layered gabbros and mafic sheeted dike complex equal to the seismic Layer 3, as a result of crystallization from a magma chamber. The transitional unit between the former two petrologic units consists of layered ultramafic and mafic rocks, corresponds to the petrological Moho. The seismic Layer 2 and 1 are well defined by pillow lavas and massive flows, and the overlying abyssal sediments, respectively. Compared these results with the refraction seismic profiles, the oceanic crust and upper mantle show different composition and structure. The P‐wave velocities of the Layer 3 gabbros varies from 6.7 to 7.0 km s‐1 and have low velocity gradients of <0.1 km s‐1. Although the gradual increase of P‐ and S‐wave velocities with depth can be attributed to the increasing proportion of mafic minerals from the top to the bottom, prehnite‐pumpellyite facies alteration of basalts, greenschist‐faces metamorphism to epidote‐amphibolite facies metamorphism of gabbros will decrease the velocities of the Layer 2 and Layer 3 (Christensen and Salisbury, 1975, 1989), because the P‐wave velocities of chlorite and hornblende are 6.00 and 7.00 km s‐1, respectively, lower than those of plagioclase and pyroxene, respectively (Carlson, 2004). In addition, local velocity anomalies near the petrologic Moho can be related serpentinization of ultramafic rocks (Salisbury and Christensen, 1978;Carlson et al., 2009). In the Layer 4, the characteristic P‐wave velocities of the upper mantle should fall in the range of 7.8 to 8.2 km s‐1. Poisson's ratios of chrysotile and lizardite, which are stable in oceanic crustal environments according to the phase diagram, is 0.267 and 0.359, respectively, higher than those of olivine and pyroxene (Wang et al., 2013). Serpentinization will significantly decreased velocities and densities of peridotites and is the main reason for the variation of the Moho reflectivity beneath oceans.

One of the most promising planetary bodies that might harbor extraterrestrial life is Mars, given the presence of liquid water in the deep subsurface. The upper crust of Mars is mainly composed of >3.7-billion-year-old basaltic lava where heat-driven fluid circulation is negligible. The analogous crustal environment to the Martian subsurface is found in the Earth's oceanic crust composed of basaltic lava. The basaltic crust tends to cool down for 10–20-million-years after formation. However, microbial life in old cold basaltic lava is largely unknown even in the Earth's oceanic crust, because the lack of vigorous circulation prevents sampling of pristine crustal fluid from boreholes. Alternatively, it is important to investigate deep microbial life using pristine drill cores obtained from basaltic lava. We investigated a basaltic rock core sample with mineral-filled fractures drilled during Integral Ocean Drilling Project Expedition 329 that targeted 104-million-year-old oceanic crust. Mineralogical characterizations of fracture-infilling minerals revealed that fractures/veins were filled with Mg-rich smectite called saponite and calcium carbonate. The organic carbon content from the saponite-rich clay fraction in the core sample was 23 times higher than that from the bulk counterpart, which appears to be sufficient to supply energy and carbon sources to saponite-hosted life. Furthermore, a newly developed method to detect microbial cells in a thin-section of the saponite-bearing fracture revealed the dense colonization of SYBR-Green-I stained microbial cells spatially associated with saponite. These results suggest that the presence of saponite in old cold basaltic crust is favorable for microbial life. In addition to carbonaceous chondrite, saponite is a common product of low-temperature reactions between water and mafic minerals on Earth and Mars. It is therefore expected that deep saponite-bearing fractures could host extant life and/or the past life on Mars.

Ultra-deep reflection seismic lines (American COCORP, diverse European projects such as ECORS, some industrial surveys such as those from ION-GXT) have conspicuously shown that the lower part of the continental crust is highly reflective. Strong, short, discontinuous, sub-horizontal, wavy reflections are characteristic, imparting an undulating highly reflective pattern to the lower continental crust. Most of the times, the Moho is interpreted at the base of such reflections, at the boundary with the seismically transparent upper mantle. The other important crustal discontinuity, the Conrad, is usually interpreted at the top of such reflectors, at the boundary with the seismically transparent upper crust. In this manner, ultra deep seismic sections usually display the lower continental crust as a strongly reflective wavy layer of varying thickness sandwiched between the transparent upper crust and the transparent subcontinental upper mantle. The reasons for such high reflectivity include the development of abundant subhorizontal ductile shear zones and the dominant subhorizontal foliation so characteristic of exposed highgrade metamorphic rocks of the lower crust. Introduction and Discussion Since the start of the recording of ultra-deep reflection seismic lines, first in Europe, later in the USA, it has been recognized that the lower crust is characterized by an anomalous high reflectivity when compared to the seismically transparent upper continental crust and mantle. Strong, short, discontinuous and undulating reflections form a wavy band of variable thickness at the base of the continental crust in most ultra-deep seismic sections that imaged several locations around the globe (Figure 1). Interestingly, this is exceptionally visible at places where the crust has undergone stretching and thinning due to extensional stresses in a breaking continent. Examples of this are known from the Rhine Graben in Europe (Figure 2), in the Basin and Range Province of Western USA and in the passive margins of the South Atlantic Ocean (Figure 1). In most cases, the Moho does not appear as a single, strong reflector. Usually its position is inferred at the base of the lower crust short and strong reflections. Below them the mantle is practically devoid of seismic reflections (Figures 1 and 2). The Conrad discontinuity, that marks a significant increase in the velocity of the compressional seismic waves, broadly coincides with the top of the reflective lower crust (Figure 2); but, interestingly enough, it appears much more frequently as a well marked seismic reflection than the Moho (Zalan et al., 2009). On the other hand, the thinner oceanic crust does not show such reflectivity layering throughout its 7 to 11 kilometers of thickness. Well exposed outcrops of obducted oceanic crust, such as those in the Oman Mountains, and high resolution seismic sections, point to a tripartite division of this type of crust consisting of a thin, weakly reflective, lower layer of banded gabbros, a thick middle layer composed of criss-crossing sheeted dykes and an upper seismically transparent layer of pillow lavas (Zalan et al., 2011). The sub-oceanic Moho, contrary to the sub-continental Moho, is almost invariably displayed as a discrete strong continuous to discontinuous seismic reflection. These distinct seismic behaviors between the continental and oceanic crusts point to differences in composition and depths of occurrence; thus, differences in confining pressure and temperature. All these converge to suggest that strong differences in the rheology of both types of crust may be responsible for their characteristic seismic response. The reason for such strong, sub-horizontal reflectivity of the lower continental crust has been historically attributed to ductile sub-horizontal shear zones, developed in response to the change in the rheology of quartz and feldspar, the two most abundant minerals in the continental crust, from brittle to ductile behavior. Their brittle friction and plastic flow laws indicate a change in rheology below the depths of 10-12 km for quartz and 2030 km for feldspars (Figure 3). So, above such depths the continental rocks tend to behave in an elastic manner, displaying mostly brittle deformation. Below these depths, the behavior is predominantly plastic, dominated by ductile deformation. The concept of sub-horizontal shear zones dominating the lower continental crust was derived from the exposures of stretched lower crustal rocks exhumed in the metamorphic core complexes of the Basin and Range Province of the Western USA (Figure 4) (Wernicke and Burchfiel, 1982). Recent AFTA studies of uplifted passive continental margins have shown tremendous amounts of rising and

Petrogenesis and chemogenesis of oceanic and continental orogens in <scp>A</scp>sia: Current topics, <scp>Part I</scp>

Olivine and orthopyroxene crystals composing the oceanic upper mantle align under progressive simple shear strain. Because the thermal diffusivities (κ) and viscosities of these minerals are anisotropic, mineral alignment affects vertical heat flow and upper mantle dynamics. The vertical thermal diffusivity of upper mantle peridotite decreases with progressive simple shear strain, leading to higher temperatures in the shallow upper mantle than predicted by an isotropic half‐space cooling model. This, in turn, causes higher surface heat flow, shallower ocean basins, weaker asthenosphere, and slightly thinner lithosphere. Viscosity associated with an oriented simple shear strain (ηsh), such as that caused by plate motion, also evolves with progressive strain, though ηsh at high strains has not been characterized. Regardless of the evolution of ηsh with strain, the effects of thermal diffusivity anisotropy on upper mantle temperatures, surface heat flow, lithosphere thickness, asthenosphere viscosity and shear stress at the plate bottom remain evident. Shear heating elevates the geotherm by more than κ anisotropy except in the youngest ocean and in cases with significant shear weakening or a slowly moving plate (less than ∼3 cm/yr). The magnitude of shear heating effects depends on how ηsh increases or decreases at high strains. For models in which ηsh increases as a function of strain, shear heating elevates temperatures beyond reasonable upper mantle estimates (i.e., above 1400°C), suggesting that such strain induced viscosity increases are not likely. We present a model for ocean upper mantle cooling and deformation that includes upper mantle thermal diffusivity anisotropy and shear heating. This model predicts heat flow, shear wave split times, lithosphere thicknesses, and basin depths which are consistent with observations but suggests that basal tractions on ocean plates are lower than previously thought.

Constraining the properties of continental margin where dynamic interplay between the lithosphere and convective upper mantle actively occurs can provide implications to understand the evolution and modification process of cratonic lithosphere. Cratonic lithosphere in the eastern Eurasian plate has experienced multiple episodes of tectonic events at the continental margin during the Phanerozoic, which are characterized by collision and subduction of continental and oceanic plates with extensive magmatic intrusion, extension, and rifting. To better understand the complex tectonic processes and their influences on the continental lithosphere, we imaged upper mantle structure beneath the Archean-Proterozoic basement in the Korean Peninsula (KP) by seismic tomography. Our dataset is entirely based on relative arrival time of teleseismic body wave recorded by dense seismic array across the southern KP in order to extract velocity variations from the local area and reduce the effects from deeper upper mantle structures. The images show a sharp and large lateral velocity variation within a short distance (< 200 km) in KP, which indicates highly variable lithospheric structure. An anomalously thick high-velocity structure beneath the southwestern part suggests a fragment of a long-lasting cratonic lithosphere (~150 km) at the continental margin. The absence of deep lithosphere and mostly occupied by hot, buoyant, low-velocity upper mantle beneath the Gyeonggi massif, Gyeongsang continental arc-backarc system, and eastern continental margin of KP shows highly modified regions. In addition, there are clear spatial correlations of low-velocity upper mantle with partial melting zones and the localities of Cenozoic basalts, high surface heat flux, and high topography with recent uplift along the eastern mountain ranges. A strong variation in the upper mantle velocity structure without clear distinctions in surface geology across different massifs suggests heterogeneous modification of continental lithosphere by recent and transient processes, such as the opening of the East Sea (Sea of Japan) or a subduction of Philippine Sea Plate. Dynamic interaction of the prominent lithospheric structure and convective upper mantle has controlled reactivation and destabilization at the cratonic margin of KP.

The mantle convection model with phase transitions, non-Newtonian viscosity, and internal heat sources is calculated for two-dimensional (2D) Cartesian geometry. The temperature dependence of viscosity is described by the Arrhenius law with a viscosity step of 50 at the boundary between the upper and lower mantle. The viscosity in the model ranges within 4.5 orders of magnitude. The use of the non-Newtonian rheology enabled us to model the processes of softening in the zone of bending and subduction of the oceanic plates. The yield stress in the model is assumed to be 50 MPa. Based on the obtained model, the structure of the mantle flows and the spatial fields of the stresses σ xz and σ xx in the Earth's mantle are studied. The model demonstrates a stepwise migration of the subduction zones and reveals the sharp changes in the stress fields depending on the stage of the slab detachment. In contrast to the previous model (Bobrov and Baranov, 2014), the self-consistent appearance of the rigid moving lithospheric plates on the surface is observed. Here, the intense flows in the upper mantle cause the drift and bending of the top segments of the slabs and the displacement of the plumes. It is established that when the upwelling plume intersects the boundary between the lower and upper mantle, it assumes a characteristic two-level structure: in the upper mantle, the ascending jet of the mantle material gets thinner, whereas its velocity increases. This effect is caused by the jump in the viscosity at the boundary and is enhanced by the effect of the endothermic phase boundary which impedes the penetration of the plume material from the lower mantle to the upper mantle. The values and distribution of the shear stresses σ xz and superlithostatic horizontal stresses σ xx are calculated. In the model area of the subducting slabs the stresses are 60–80 MPa, which is by about an order of magnitude higher than in the other mantle regions. The character of the stress fields in the transition region of the phase boundaries and viscosity step by the plumes and slabs is analyzed. It is established that the viscosity step and endothermic phase boundary at a depth of 660 km induce heterogeneities in the stress fields at the upper/lower mantle boundary. With the assumed model parameters, the exothermic phase transition at 410 km barely affects the stress fields. The slab regions manifest themselves in the stress fields much stronger than the plume regions. This numerically demonstrates that it is the slabs, not the plumes that are the main drivers of the convection. The plumes partly drive the convection and are partly passively involved into the convection stirred by the sinking slabs.

Origin of dipping structures in fast-spreading oceanic lower crust offshore Alaska imaged by multichannel seismic data

Continent-ocean differences and a differentiation of the earth