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Heat flow across oceanic transform faults (TFs) and fracture zones (FZs) has rarely been studied in detail, despite these features representing distinct thermal boundaries within the oceanic lithosphere. Here, we present heat flow measurements across the St Paul fracture zone (SPFZ) in the equatorial Atlantic Ocean, from 48 Ma crust in the south to 71 Ma in the north. To the north of the FZ we find a basal heat flow of 63 mWm−2, and to the south a basal heat flow of 79 mWm−2, both in agreement with plate cooling models. However, within the SPFZ we find a heat flow of 83 mWm−2, greater than the values of the adjacent crust and 10–15 mWm−2 higher than predicted from conductive cooling models, suggesting that the thermal structure of the FZ has been modified. Evidence from seismic and sub‐bottom profiler data indicate recent active deformation within the SPFZ, potentially driven by lithospheric flexure across the FZ or temporal changes in TF configuration. We propose that this deformation may enable fluid circulation and heat advection within the basement, creating the seafloor heat flow anomaly within the FZ. These findings suggest that FZs may remain important zones predisposed to host deformation and fluid flow in the oceanic lithosphere, despite not being active plate boundaries., Key Points: A high heat flow anomaly of 10–15 mWm−2 is observed in the St Paul fracture zone compared to the adjacent oceanic crust. The heat flow anomaly is likely due to tectonically driven fluid flow but thermal rejuvenation at the transform fault could also contribute. Fracture zones may act as high permeability pathways for fluid flow, and form and evolve differently to standard oceanic lithosphere., Deutsche Forschungsgemeinschaft http://dx.doi.org/10.13039/501100001659, FP7 Ideas: European Research Council http://dx.doi.org/10.13039/100011199, https://doi.pangaea.de/10.1594/PANGAEA.950419, https://doi.pangaea.de/10.1594/PANGAEA.950420

Volcanism is the surface expression of extensive magmatic systems, with their intrusive counterpart representing ~80% of the total magma budget. Our knowledge of igneous processes therefore largely relies on our understanding of deep plutonic processes. In continental or oceanic environments, most of the intrusive igneous rocks bear geochemical cumulate signatures (e.g. depletion in incompatible elements and enrichment in compatible ones) that are commonly explained by mineral-melt segregation during differentiation. Deformation-assisted compaction aided by melt buoyancy is usually referred to as the main process involved in melt extraction. However, buoyancy alone is not sufficient, and a number of cumulative rocks are lacking any compaction evidence, opening the potential for the involvement of other processes. In addition, our view of magmatic systems has shifted in the last decades from large melt-rich bodies to crystal-rich magma reservoirs. This paradigm shift challenges some of the long-established first-order igneous concepts like the idea that melt differentiation at depth is mainly governed by (fractional) crystallization; alternatively, the presence of mush potentially favors additional processes such as melt-mush reactions. We propose a novel igneous process for the formation of igneous cumulates, consistent with the mushy nature of oceanic igneous reservoirs, their continuous/cyclic replenishment by primitive melts, and the widespread occurrence of reactive porous flow (RPF) during magma differentiation identified in a growing number of magmatic systems. The melt flush process relies on melt-mush reactions between the primitive recharge melt(s) and crystal mush. Replacement of the more evolved interstitial melt by the primitive recharge melt leading to reactions (dissolution+crystallization) and concomitant extraction of the more evolved melt from the cumulate by buoyancy participate in the acquisition of the final cumulate signature. This process relying on oceanic igneous systems considers for the first time melt inputs and not only melt extraction and matches the petrographic (e.g. mineral dissolution evidence) and geochemical constraints (trace element signatures) brought by natural oceanic samples. We tested various melt-mush reactions likely involved in the early stages of the melt flush process during RPF to investigate their thermodynamic feasibility with the Magma Chamber Simulator. First-order results show that one-step equilibration of primitive melts with primitive to moderately differentiated mush crystals triggers mineral assimilation. Together with the constraints established from the natural rock record, it strengthens the idea that RPF is a potential key process for magma differentiation in magma reservoirs at different evolution stages. The proposed melt flush process eventually adds to other processes involved in cumulate formation like magma compaction or crystal settling and is likely to apply to any other magmatic system from various settings sharing similar reservoir characteristics.

SUMMARYMid-plate upward mantle flow is a key component of global mantle convection, but its patterns are poorly constrained. Seismic anisotropy is the most direct way to infer mantle flow as well as melt distribution, yet the convection patterns associated with plume-like mantle upwelling are understudied due to limited seismic data coverage. Here, we investigate seismic anisotropy beneath the Madeira and Canary hotspots using a dense set of shear wave splitting observations and combining teleseismic and local events recorded by three-component broad-band and short-period seismic stations. Using a total of 26 stations in the Madeira archipelago and 43 stations around the Canary Islands, we obtain 655 high-quality measurements that reveal heterogeneous flow patterns. Although local event results are sparse around most islands, we can observe a small average of S-wave splitting times of 0.16 ± 0.01 s, which significantly increase with source depth beneath El Hierro (>20 km) and Tenerife (>38 km) up to 0.58 ± 0.01 and 0.47 ± 0.05 s. This suggests an influence of melt pocket orientation in magma reservoirs developed at uppermost-mantle depths. Likewise, anisotropy increases significantly beneath the islands with shield stage volcanism (up to 9.81 ± 1.78 per cent at El Hierro, western Canaries, against values up to 1.76 ± 0.73 per cent at Lanzarote, eastern Canaries). On average, teleseismic SKS-wave splitting delay times are large (2.19 ± 0.05 s), indicating sublithospheric mantle flow as the primary source for anisotropy in the region. In the Canaries, the western islands show significantly smaller average SKS delay times (1.93 ± 0.07 s) than the eastern ones (2.25 ± 0.11 s), which could be explained by destructive interference above the mantle upwelling. Despite complex patterns of fast polarization directions throughout both regions, some azimuthal pattern across close stations can be observed and related to present-day mantle flow and anisotropy frozen in the lithosphere since before 60 Ma. Additionally, we infer that the current presence of a mantle plume beneath the archipelagos leads to the associated complex, small-scale heterogeneous anisotropy observations.

Nucleation of earthquake slip at the plate boundary fault (décollement) in subduction zones has been widely linked to the frictional properties of subducting sedimentary facies. However, recent seismological and geological observations suggest that the décollement develops in the subducting oceanic crust in the depth range of the seismogenic zone, at least in some cases. To understand the frictional properties of oceanic crustal material and their influence on seismogenesis, we performed hydrothermal friction experiments on simulated fault gouges of altered basalt, at temperatures of 100–550°C. The friction coefficient (μ) lies around 0.6 at most temperature conditions but a low μ down to 0.3 was observed at the highest temperature and lowest velocity condition. The velocity dependence of μ, (a−b), changes with increasing temperature from positive to negative at ∼100°C and from negative to positive at ∼450°C. Compared to gouges derived from sedimentary facies, the altered basalt gouge showed potentially unstable velocity weakening over a wider temperature range. Microstructural observations and microphysical interpretation infer that competition between dilatant granular flow and viscous compaction through pressure-solution creep of albite contributed to the observed transition in (a−b). Alteration of oceanic crust during subduction produces fine grains of albite and chlorite through interactions with interstitial water, leading to reduction in its frictional strength and an increase in its seismogenic potential. Therefore, shear deformation possibly localizes within the altered oceanic crust leading to a larger potential for the nucleation of a megathrust earthquake in the depth range of the seismogenic zone.

In this work, we calculated the Curie Point Depth (CPD) for the Brazilian Equatorial Margin using the Fourier and continuous wavelet (CWT) transforms considering a linear inversion for random and fractal magnetization models with four different magnetic datasets. After comparing our results with Curie maps reported by previous studies, we concluded that the more consistent Curie isotherm was obtained by applying the random model based on CWT to the Brazilian Equatorial Margin Magnetic Map dataset, a compilation of airborne and marine magnetic surveys complemented with magnetic anomaly data extracted from the World Digital Magnetic Anomaly Map in the deep-water oceanic domain. In addition, we estimated heat flow using CPD results and compare with a global heat flow model based on geothermal measurements. In the continental domain, Curie values range from 18 to 35 km, shallowing westwards until reaching anomalously 10 km in the Tocantins Province. In turn, the heat flow decreases eastwards from 88 to 58 mW/m2 as the Curie depth increases within the continental crust. Meanwhile, in the deep-water oceanic domain, the heat flow is overall higher (between 72 and 87 mW/m2), indicating a younger and warmer oceanic crust, whereas the Curie isotherm is shallower, varying between 10 and 30 km. Even so, it is deeper than Moho, suggesting a magnetized lithospheric mantle.

A three-dimensional tomographic image of the seismic velocity structure in the crust and upper mantle of northwestern Nicaragua was performed using the back-projection technique. The study is based on ~ 15,000 earthquakes located between 1975 and 1982 by the network installed by the USGS and the Institute of Seismic Research of Nicaragua. This data represents the longest continuous recording of seismicity by a homogeneous seismic network in Nicaragua. Data from this network has not been used before. Initially, a one-dimensional model of the seismic velocity structure (1-D) was determined in order to relocate the seismicity and to obtain a first estimate of the seismic velocity model of the continental crust. This 1-D crustal model was complemented with the upper mantle structure of the IASPEI91 Earth model. This initial 1-D model is used to iterate for a 3-D tomographic imaging of Nicaragua. The results show three distinct features. There is clear evidence of a low seismic velocity plume rising from the subducted slab at depths of between 60 and 120 km. This low-velocity zone may be interpreted as the high temperature melts originating in the slab and feeding the volcanic arc. In northern Nicaragua, a high-velocity region is identified beneath the coast that presumably reflects the thickness of the subducting oceanic plate, estimated to be approximately 50 to 60 km thick. Beneath the volcanic arc, there is a low seismic velocity zone extending from 10 to 20 km. This low-velocity zone is interpreted as high-temperature material in the crust feeding the magma chambers of the volcanic arc.

Earth’s magnetic field is recorded as oceanic crust cools, generating lineated magnetic anomalies that provide the pattern of polarity reversals for the past 160 million years1. In the lower (gabbroic) crust, polarity interval boundaries are proxies for isotherms that constrain cooling and hence crustal accretion. Seismic observations2–4, geospeedometry5–7 and thermal modelling8–10 of fast-spread crust yield conflicting interpretations of where and how heat is lost near the ridge, a sensitive indicator of processes of melt transport and crystallization within the crust. Here we show that the magnetic structure of magmatically robust fast-spread crust requires that crustal temperatures near the dike–gabbro transition remain at approximately 500 degrees Celsius for 0.1 million years. Near-bottom magnetization solutions over two areas, separated by approximately 8 kilometres, highlight subhorizontal polarity boundaries within 200 metres of the dike–gabbro transition that extend 7–8 kilometres off-axis. Oriented samples with multiple polarity components provide direct confirmation of a corresponding horizontal polarity boundary across an area approximately one kilometre wide, and indicate slow cooling over three polarity intervals. Our results are incompatible with deep hydrothermal cooling within a few kilometres of the axis2,7 and instead suggest a broad, hot axial zone that extends roughly 8 kilometres off-axis in magmatically robust fast-spread ocean crust. A record of Earth’s magnetic field constructed from near-bottom magnetization observations and oriented samples provides three-dimensional imaging of magnetic stripes in fast-spread crust, and suggests slow cooling off-axis, as opposed to deep hydrothermal cooling close to the spreading ridge.

The subduction of an oceanic plate is studied as the motion of a high-viscosity Newtonian fluid. The subducting plate spreads along the 670-km depth boundary under the influence of oppositely directed horizontal forces. These forces are due to oppositely directed horizontal temperature gradients. We consider the flow structure and heat transfer in the layer that includes both the oceanic lithosphere and the crust and moves underneath a continent. The heat flow is estimated at the contact between the subducting plate and the surrounding mantle in the continental limb of the subduction zone. Our study results show that the crustal layer of the subducting plate can melt and a thermochemical plume can form at the 670-km boundary. Our model of a thermochemical plume in the subduction zone shows the following: (1) formation of a plume conduit in the crustal layer of the subducting plate; (2) formation of a primary magmatic chamber in the area wherein the melting rate equals the rate of subduction; (3) origination of a vertical plume conduit from the primary chamber melting through the continent; (4) plume eruption through the crustal layer to the surface, i.e. formation of a volcano. Our experiments are aimed to model the plume conduit melting in an inclined flat layer above a local heat source. The melt flow structure in the plume conduit is described. Laboratory modeling have revealed that the mechanisms of melt eruption from the plume conduit differ depending on whether a gas cushion is present or absent at the plume roof.

The nature of compositional heterogeneity in Earth's lower mantle remains a long-standing puzzle that can inform about the long-term thermochemical evolution and dynamics of our planet. Here, we use global-scale 2D models of thermochemical mantle convection to investigate the coupled evolution and mixing of (intrinsically dense) recycled and (intrinsically strong) primordial heterogeneity in the mantle. We explore the effects of ancient compositional layering of the mantle, as motivated by magma ocean solidification studies, and of the physical parameters of primordial material. Depending on these physical parameters, our models predict various regimes of mantle evolution and heterogeneity preservation over 4.5 Gyr. Over a wide parameter range, primordial and recycled heterogeneity are predicted to co-exist with each other in the lower mantle of Earth-like planets. Primordial material usually survives as medium- to large-scale blobs (or streaks) in the mid-mantle, around 1000–2000 km depth, and this preservation is largely independent of the initial primordial-material volume. In turn, recycled oceanic crust (ROC) persists as large piles at the base of the mantle and as small streaks everywhere else. In models with an additional dense FeO-rich layer initially present at the base of the mantle, the ancient dense material partially survives at the top of ROC piles, causing the piles to be compositionally stratified. Moreover, the addition of such an ancient FeO-rich basal layer significantly aids the preservation of the viscous domains in the mid-mantle. Finally, we find that primordial blobs are commonly directly underlain by thick ROC piles and aid their longevity and stability. Based on our results, we propose an integrated style of mantle heterogeneity for the Earth involving the preservation of primordial domains along with recycled piles. This style has important implications for early Earth evolution and has the potential to reconcile geophysical and geochemical discrepancies on present-day lower-mantle heterogeneity.

Magmatic Ni-Cu sulfide deposits in orogenic belts have distinct petrological and geochemical features compared with world-class intraplate magmatic deposits. One such feature is the presence of abundant primary hydrous minerals such as phlogopite and amphibole, indicating a H2O-rich parental magma. However, the origin and nature of the fluids have not been established. In the southern Central Asian Orogenic Belt, the Permian Baishiquan and Tianyu mafic–ultramafic intrusions host magmatic Ni-Cu sulfide deposits. Both intrusions are composed of lherzolite, olivine websterite, gabbronorite, gabbro, and diorite. The main ore-host rocks are lherzolite and websterite, with disseminated, net-textured, and massive ores comprising the dominant primary ore types. Low zircon eHf (− 6.9 to 7.9 ‰) and varying δ18O values (5.2 to 6.6 ‰), as well as both arc-type and MOR-type zircon Nb/Yb (0.0013–0.011) and U/Yb (0.33–10.22), identify an altered lower oceanic crust component in the mantle source. A combination of altered lower oceanic crust and upper crust can account for the zircon Hf–O isotopic characteristics. A mixing model further reveals that the Baishiquan and Tianyu magmatic sulfide deposits originated from a depleted mantle source that experienced two stages of subduction metasomatism with a 30% addition of altered oceanic crust in the first stage and 20% in the second. In the second stage, > 10% oceanic sediment with high δ18O must also have been added to result in olivine δ18O values higher than those in mantle. This staged evolution is reflected in both the olivine and zircon signatures with olivine from disseminated and net-textured ores at the Tianyu deposit having high δ18O (4.6 to 6.3 ‰) with a similar range to zircon and high δ7Li isotopic compositions (12.03 to 19.46 ‰). Contamination in the source by oceanic sediment with high δ18O values can account for the olivine δ18O values (4.8 to 5.7 ‰ at Baishiquan) being higher than those in the mantle, while the addition of brine (5% at the Baishiquan deposit) can explain the olivine δ7Li values (20.96 to 31.01 ‰). The results suggest that H2O content, Li content, and δ7Li and δ18O isotopic signatures in olivine and zircon are good indicators of mantle metasomatism and olivine fluid processes in magmatic Ni-Cu sulfide deposits.

The Circum-Pacific Late Albian–Cenomanian orogenic belts (including the Sikhote-Alin–Western Sakhalin belt) were formed as a result of the deformation of mainly epioceanic terranes as fragments of Jurassic–Early Cretaceous accretionary wedges with ophiolites and other fragments of oceanic crust, turbidite basins, and island-arc systems. To the west of the Sikhote-Alin–Northern Sakhalin belt and orthogonally, the previously consolidation structures include the Bureya–Jiamusi–Khanka fragment of the orogenic belt of the Late Cambrian–Early Ordovician consolidation of the Late Proterozoic–Cambrian complexes. Within this belt, four isolated outcrops of the Heilongjiang complex are mapped. This complex combines metamorphic rocks of the epidote–amphibolite and glaucophane–schist facies and represents a fragment of a Jurassic accretionary wedge. It was assumed that these outcrops marked a suture; in particular, they represent the remains of the closed Mudanjiang paleoocean, separating the original Jiamusi terrane (and the Bureya–Jiamusi–Khanka belt) located to the west of Central Asia structures. This paper provides data that indicate that the Heilongjiang complex does not mark a suture, but is an underground near-horizontal continuation of the marginal continental accretionary wedge of the Nadanhada–Bikin terrane (flat subduction model) brought to the surface at the antiform bending site. The unity of the compared parts of the accretionary wedge is emphasized by the close matrix ages, the similarity of detritus zircon populations, and similar composition and age of allochthonous inclusions (limestone, chert, Late Paleozoic, and Early Mesozoic basalt). One important common feature is that Late Paleozoic and Early Mesozoic basalts from allochthonous inclusions occur as the N-MORB and OIB types in both cases, without any suprasubduction volcanism traces in the matrix. The Heilongjiang complex forms, according to this interpretation, a tectonic window among the more ancient structures of the Jiamusi terrane. There is no need to assume the existence of a Mudanjiang Ocean to explain the formation of the Heilongjiang complex. The structural features of this complex and its bedding conditions can be explained by the flat subduction processes of the Pacific slab in the Jurassic and its deformation in the Early Cretaceous.

Since marine seismic studies are relatively sparse and unevenly distributed, detailed tomographic images of the Moho geometry under large parts of the world’s oceans and marginal seas are not yet available. Marine gravity data is, therefore, often used to detect the Moho depth in these regions. Alternatively, Airy’s isostatic theory can be applied for this purpose. In this study, we compare different isostatic and gravimetric methods for a Moho recovery under the oceanic crust and continental margins, particularly focusing on a numerical performance of Airy, Vening Meinesz–Moritz (VMM), direct gravity inversion, and generalized (for the Earth’s spherical approximation) Parker–Oldenburg methods. Numerical experiments are conducted to estimate the Moho depth beneath the Indian Ocean. Results reveal that, among these investigated methods, the VMM model is probably the most suitable for a gravimetric Moho recovery beneath the oceanic crust and continental margins, when taking into consideration the lithospheric mantle density information. This method could to some extent model realistically a Moho geometry beneath mid-oceanic spreading ridges, oceanic subductions, most of oceanic volcanic formations, and marine sediment deposits. Nonetheless, this model still cannot fully reproduce a gradual Moho deepening caused by a conductive cooling and a subsequent isostatic rebalance of the oceanic lithosphere, which can functionally be described by a Moho deepening with the increasing ocean-floor age. Results also indicate that the Airy method typically overestimates the Moho depth under oceanic volcanic formations, while the direct gravity inversion and generalized Parker–Oldenburg methods could not reproduce more detailed features in the Moho geometry. Since Pratt’s theory better describes a large-scale isostatic mechanism of the oceanic lithosphere by means of compensation density variations, but does not account for additional changes in compensation depth (i.e., Moho depth) that are caused by these density changes, we tested a possibility of combining Pratt and Airy’s isostatic theories in order to estimate the Moho depth under the oceanic crust. Even this combined model cannot fully reproduce a gradual Moho deepening with the increasing ocean-floor age.

Transformation of the oceanic crust into the continental one in orogenic belts is an important problem in petrological studies. In the paleocontinental sector of the Urals, a key object for tracing the stages of metamorphism and investigating the origin of anatectic granites is the Murzinka-Adui metamorphic complex. We have analyzed trace elements in zircons and established their genesis, sources, crystallization conditions, and stages of metamorphic events and granite generation in this complex. Zircons compositions were determined by the LA-ICP-MS method. Temperatures were calculated from Ti contents in the zircons. We distinguish three geochemical types of zircons, which differ in the ratios of light and heavy REE, U, Th, Ti, Y and show different values of Ce- and Eu-anomalies and Zr/Hf ratios, which are indicative of different crystallization conditions, as follows. Type I: minimal total LREE content; clear negative Eu- and Ce- anomalies; features of magmatic genesis; crystallization temperatures from 629 to 782 °C. Type II: higher contents of Ti, La, and LREE; low Ce-anomaly; assumed crystallization from highly fluidized melts or solutions. Type III: low positive Eu-anomaly; high REE content; low Th/U-ratio; zircons are assumed to originate from a specific fluidized melt with a high Eu-concentration. Ancient relict zircons (2300–330 Ma) in gneisses and granites show features of magma genesis and belong to types I and II. Such grains were possibly inherited from granitoid sources with different SiO2 contents and different degrees of metamorphism. Based on the geological and petrogeochemical features and zircon geochemistry of the Murzinka-Adui complex, there are grounds to conclude that the material composing this complex was generated from the sialic crust. The main stages of metamorphism and/or granite generation, which are traceable from the changes in types and compositions of the zircons, are dated at 1639, 380–370, 330, and 276–246 Ma. Thus, transformation of the oceanic crust into the continental one was a long-term and complicated process, and, as a result, the thickness of the sialic crust is increased in the study area.

SUMMARY The Davie Fracture Zone (DFZ) evolved during the Jurassic and Cretaceous breakup and subsequent drift of Gondwana off East Africa. This old weak zone has been reactivated during the evolution of the East African Rift System. Recent faulting of Cenozoic sediments in the Kerimbas Basin off northern Mozambique shows that they are affected by the neotectonics. The question is if and how the crustal fabric in our research area has been modified by the rifting process. We present two seismic refraction profiles acquired offshore northern Mozambique to investigate its regional crustal structure and tectonic history. The profiles show a continent–ocean transition zone that widens from around 40 km at 13°S to more than 100 km at 11°S. In the west the transitional crust is up to 12 km thick. To the east, around 150 km off the Mozambique coast lies oceanic crust whose thickness varies from 4.9 to 6.5 km along the northern line and from 6.5 to 7.5 km along the southern one. The latter presents an unusual high-velocity lower crustal body (7.0–7.2 km s−1), about 40 km wide and 3.8 km thick, underlying the oceanic crust. The body may consist of underplated melt with the same source as the nearby Paisley Seamount, which has not yet reached its isostatic equilibrium. Despite well documented recent seismicity along the margin, neither of the profiles reveal significant crustal modifications or reduced crustal seismic velocities that might be related to ongoing extensional tectonics as part of of the East African Rift System. Neither profile reveals seismic evidence for the presence of a major fracture zone or sheared continental margin parallel to the margin. Instead, the profiles’ broad continent–ocean transitions are consistent with their formation during an early Jurassic stage of plate divergence oblique to the margin. Later, after 157 Ma, the azimuth of relative plate motion between East and West Gondwana changed to be parallel to the margin, and parts of the continent–ocean transitions may have been locally reactivated in a strike-slip sense. However, details on the plate movements during the directional change of the seafloor spreading between 157 and 144 Ma are not available. The oceanic crust formed by the initial divergent oblique extension became faulted/modified by the strike-slip movements between both plates. Instead of a narrow deformation zone, the DFZ is charcaterized by a broad, diffuse zone of transtensional deformation.

The recycling of oceanic crust, with distinct isotopic and chemical signature from the pyrolite mantle, plays a critical role in the chemical evolution of the Earth with insights into mantle circulation. However, the role of the mantle transition zone during this recycling remains ambiguous. We here combine the unique resolution reflected body waves (P410P and P660P) retrieved from ambient noise interferometry with mineral physics modeling, to shed new light on transition zone physics. Our joint analysis reveals a generally sharp 660-km discontinuity and the existence of a localized accumulation of oceanic crust at the bottom mantle transition zone just ahead of the stagnant Pacific slab. The basalt accumulation is plausibly derived from the segregation of oceanic crust and depleted mantle of the adjacent stagnant slab. Our findings provide direct evidence of segregated oceanic crust trapped within the mantle transition zone and new insights into imperfect whole mantle circulation., By combining ambient noise interferometry with mineral physics modeling, this work sheds new light on mantle transition zone physics. Their findings provide new evidence of segregated oceanic crust subducted and trapped within the mantle transition zone, implying complex mantle circulation modes.

Several interplate seismic events, such as short-term slow slip events (S-SSEs) and low-frequency earthquakes (LFEs), have been identified in the Ryukyu Trench, southwestern Japan. As one of the specific characteristics of this seismicity, the depths at which S-SSEs occur at the plate interface beneath Okinawa Island are approximately 5–10 km shallower than those beneath the Yaeyama Islands. To elucidate the cause of this difference in depth, we constructed a three-dimensional, Cartesian thermomechanical subduction model and applied the subduction history of the Philippine Sea (PHS) plate in the model region. As a result, the interplate temperatures at which S-SSEs take place were estimated to range from 350 to 450 °C beneath Okinawa Island and from 500 to 600 °C beneath the Yaeyama Islands. The former temperature range is consistent with previous thermal modelling studies for the occurrence of slow earthquakes, but the latter temperature range is by approximately 150 °C higher than the former. Therefore, explaining how the depth difference in S-SSEs could be caused from the aspect of only the thermal regime is difficult. Using phase diagrams for hydrous minerals in the oceanic crust and mantle wedge, we also estimated the water content distribution on and above the plate interface of the PHS plate. Near the S-SSE fault planes, almost the same amount of dehydration associated with phase transformations of hydrous minerals from blueschist to amphibolite and from amphibolite to amphibole eclogite within the oceanic crust were inferred along Okinawa Island and the Yaeyama Islands, respectively. On the other hand, the phase transformations within the mantle wedge were inferred only beneath the Yaeyama Islands, whereas no specific phase transformation was inferred beneath Okinawa Island around the S-SSE occurrence region. Therefore, we conclude that dehydrated fluid derived from the oceanic crust at the plate interface would play a key role in the occurrence of S-SSEs.

On the assumption that the 2D heat flux anomaly in the territory of Caucasus is provided by the convective heat transfer from the mantle wedge, the angle and velocity of subduction of the paleo-lithospheric plate Tethys (Mediterranean and East Black Sea paleo-lithospheric plates) under the East-European paleo-lithospheric plate (Caucasus region) are estimated numerically. Within the framework of the geodynamical model constructed the horizontal extent of the 2D heat flux anomaly observed in the rear of the Caucasus mountain belt corresponds to subduction velocity of ~ 40 mm per year which is close to that observed with the help of geodetic means. The upwelling convective mantle flows can transport the mantle calc-alkali magmas (containing metals) to the subsurface Earth’s crust layers, thus making the ore deposits to be attached to the upper lithospheric locations above the ascending flows of the Karig-Richter vortices.

The formation of continental crust in the Mongolia-Transbaikalia region is researched to identify the mechanisms of interactions between the crust and the mantle in the development of the Neoarchean, Proterozoic and Paleozoic magmatic and sedimentary complexes in the study area. Using the results of his own studies conducted for many years and other published data on this vast region of Central Asia, the author have analysed compositions, ages and conditions for the formation of Karelian, Baikalian, Caledonian and Hercynian structure-formational complexes in a variety of geodynamic settings. Based on the geostructural, petrological, geochemical, geochronological and Sm-Nd isotope data, he determines the crustal and mantle sources of magmatism, conducts the identification and mapping of isotopic provinces, and reveals the role of island-arc oceanic, accretion-collision and intraplate magmatism in the formation of continental crust. Considering the formation of the bulk continental crust, three main stages are distinguished: (1) Neoarchean and Paleoproterozoic (Karelian) (almost 30% of the crust volume), (2) Meso-Neoproterozoic (Baikalian) (50%), and (3) Paleozoic (Caledonian and Hercynian) (over 20%). This sequence of the evolution stages shows the predominance of the ancient crustal material in igneous rocks sources at the early stage. During the subsequent stages, tectonic structures created earlier were repeatedly reworked, and mixed crustal-mantle and juvenile sources were widely involved in the formation of the bulk continental crust in the study area.

The ubiquitous hydrothermal circulation within the newly formed oceanic crust plays an important role in the transport of heat, water and chemical constituents in the earth-ocean-atmosphere system. One enigma during this process is whether seawater-derived sulfate participates in the deep hydrothermal circulation. Our results showed that homogeneous sulfate-bearing fluid separated to a sulfate-rich dense liquid phase and a sulfate-poor light liquid phase. The liquid–liquid phase separation (LLPS) temperature increased with rising fluid pressure but decreased with the addition of low dielectric constant (D) components. In situ Raman spectroscopic characterization revealed that the Zn2+-SO42- association increased with the addition of low D components. All these observations confirmed our previous conclusions that strong ion complexation triggered the LLPS. The formation of a sulfate-rich dense liquid phase could postpone the precipitation of sulfate, resulting in the participation of seawater-derived sulfate in the deep hydrothermal circulation. Sulfate would be reduced to S2− and mixed with that leached from the basalt, leading to the elevated δ34S value of the sulfide precipitated on the seafloor.

The Andaman–Nicobar–Sumatra subduction margin, with a well-developed Benioff zone down to ~ 200 km depth, extends over 1300 km along strike and has a lateral extent of ~ 200 km. Two-dimensional (2D) profiles based on generalized inversion of free-air gravity anomaly data across different segments of the Andaman–Nicobar sector of the margin were analyzed by reconstructing the geometry of the converging India–Eurasia lithospheric plates. Detailed 2D structures of the Ninetyeast Ridge (NER), fore-arc basin, volcanic arc, back-arc basin, spreading ridge, Sewell Rise, Mergui Ridge and Mergui Basin, and depths of Moho and the lithosphere–asthenosphere boundary were delineated. The volcanic arc is located at a distance of ~ 150–200 km from the trench and is marked by a zone of positive gravity anomaly separated from the trench by the fore-arc basin with a zone of gravity low. The topographic and gravity anomaly patterns show complex geometrical patterns over the volcanic arc due to the presence of a number of regional faults striking parallel to the trench axis developing a series of pull-apart (transtensional) basins as well as the interaction of the spreading ridges on the overriding plate. A narrow zone of negative gravity anomaly (~ − 185 to − 110 mGal) observed in the gravity model of the margin is interpreted to be associated with fore-arc shear fault above a zone of fractured oceanic crust with a thick sedimentary layer, located above a zone of sharp bending of the eastward converging Indian lithosphere in the fore-arc basin. A high concentration of seismicity and a trench-parallel band of moderate moment energy release along this fault might have been caused by concentrated deformation within the zone of flexing of the descending plate. A wide fore-arc (> 200 km) and the enhanced deformation of the subducting Indian oceanic plate in central sectors possibly resulted from increased interaction between the NER and the Andaman trench and extension in the Andaman back-arc. Appearance of a second moderate energy band and shifting of seismicity toward the trench axis following the 2004 MW 9.2 mega-event are apparently caused by the migration of the stress field from deeper to shallower part of the Indian lithosphere. A similar type of seismicity migration toward shallower part of the descending plate has been recorded along other subduction margins around the globe.

The article presents new views on volcanism and earthquakes. The problem of the scientific basis for the interpretation of geotectonic theories was discussed. The interpretation of the mantle plume phenomenon according to the theory of expansion and the theory of primal forces of nature is presented. The heat source in the Earth’s core is thermonuclear reactions. The cause of earthquakes is not collisions of lithospheric plates, but plasma eruptions. The stresses in the rock mass, causing global fractures, arise due to the increase in the size of nuclides in the process of plasma recombination. The oldest basalts that make up the oceanic plates remain invariably along the shores of continents. The widespread view of the subduction of lithospheric plates and vertical convection cells of rock matter should be regarded as a scientific fiction.

Three different-age series of granitoid veins and dikes of the Baikal–Muya fold belt were studied. Two of them, plagiogranites of the ophiolite complex and the first postophiolitic plagiogranites, are associated with the suprasubduction ophiolites of the eastern Baikal–Muya belt. The third series is represented by hypabyssal tonalite–plagiogranite–leucogranite complex of the Kichera zone in the western Baikal–Muya belt. The composition and isotope-geochemical characteristics (eNd(T) = –0.9; –1.3) of the plagiogranite veins no more than 60 cm thick, and eNd(T) values (–1.8…+ 0.2) of host layered leucocratic gabbros in the Sredne-Mamakan ophiolitic complex are consistent with the previously established suprasubduction nature of the ophiolite association. Tonalites and plagiogranites of the dyke system of the post-ophiolitic magmatic series intersect the dunite–pyroxenite–gabbro banded series of the Sredne-Mamakan ophiolites of the eastern Baikal–Muya fold zone. High Sr/Y ratios and low concentrations of Y and heavy REE indicate that these granitoids are ascribed to the adakite series. LA-ICP-MS study of zircon from post-ophiolitic plagiogranites yields the crystallization age of 629 ± 5 Ma. Sm–Nd isotope-geochemical characteristics of plagiogranitoids (eNd(T) = +2.5; +4.0) in combination with geochemical data confirm their origin during partial melting of a mafic protolith corresponding to the Neoproterozoic oceanic crust. The adakitic granitoids in the Kichera zone of the western Baikal–Muya belt belong to the tonalite–leucogranite differentiated series of the hypabyssal complex, which has no a direct spatial relationship with unambiguous ophiolite associations. The chemical composition and Sm–Nd isotope-geochemical characteristics of these rocks (eNd(T) = +3.2…+7.1) indicate the heterogeneity of the predominantly juvenile island-arc or oceanic Neoproterozoic crust, which experienced partial melting at 595 ± 5 Ma.

The true partitioning between continental and oceanic lithosphere in oceans is unclear. According to early models, oceanic-type accretion generates pairs of linear magnetic anomalies, which are indicators of oceanic lithosphere and can be used as isochrons formed by seafloor spreading. However, seaward-dipping reflectors at conjugate volcanic passive margins also generate linear magnetic anomalies. The thick wedges of the inner seaward-dipping reflectors are associated with magnetic anomalies that are clearly distinct in shape and amplitude from those recorded in the distal oceanic realm. However, linear magnetic anomalies indistinguishable from those related to oceanic crust exist in the outer seaward-dipping reflector domain of many volcanic passive margins. Located seaward of the inner seaward-dipping reflectors, the crust of outer seaward-dipping reflectors is thus generally considered to be “oceanic.” However, the outer seaward-dipping reflector crust may be interpreted as tectonically exhumed mid-to-lower magma-intruded continental crust covered with syntectonic basalts. Although both oceanic crust and outer seaward-dipping reflector crust are associated with thick lava sections, the linear magnetic anomalies of outer seaward-dipping reflectors represent pre-oceanization magnetic anomalies that develop along extended continental lithosphere. We illustrate the consequence of these uncertainties on the type of lithosphere between Greenland and Europe. Here, depending on latitude, 20%–100% of the lithosphere previously thought to be oceanic might, on the contrary, be continental. Since more than 50% of passive margins worldwide are volcanic, poor mapping of seaward-dipping reflector–bearing crust types, and misinterpretation of linear magnetic anomaly–bearing distal volcanic passive-margin crust, could have led to widespread overestimation of the age of continental breakup and the extent of oceanic lithosphere in oceans.

SUMMARY We present models of crustal and uppermost mantle structure beneath the Hawaiian Swell and surrounding region. The models were derived from ambient-noise intermediate-period Rayleigh-wave phase velocities and from seafloor compliance that were estimated from continuous seismic and pressure recordings collected during the Hawaiian Plume-Lithosphere Undersea Mantle Experiment (PLUME). We jointly inverted these data at the locations of over 50 ocean-bottom instruments, after accounting for variations in local bathymetry and sediment properties. Our results suggest that the crystalline crust is up to 15 km thick beneath the swell and up to 23 km thick closer to the islands. Anomalously thick crust extends towards the older seamounts, downstream of Hawaii. In a second region, anomalies immediately to the south of Hawaii may be associated with the leading edge of the shallow Hawaiian magma conduit. In a third region, thickened crust to the immediate west of Hawaii may be related to Cretaceous seamounts. Low seismic velocities identified in the uppermost mantle to the northeast of Hawaii may be linked to the Molokai fracture zone and may be manifest of complex non-vertical pathways of melt through the upper lithosphere. Velocity anomalies decrease in amplitude towards the surface, suggesting that melt becomes focused into conduits at depths between 20 and 40 km that escape the resolution capabilities of our data set.

SUMMARYThe Philippine Sea plate (PHS) is subducting beneath southwest Japan from the Nankai Trough. In this region, deep low-frequency earthquakes/tremors (LFEs) are active and their epicentres are distributed along the downdip of the source region of M8-class earthquakes that have occurred every 100–150 yr. The LFE activity may potentially be strongly related to the occurrence of great earthquakes between tectonic plates. To investigate the structural features around the LFEs, we applied teleseismic receiver function (RF) analysis to the seismograms observed at permanent and temporal seismograph stations in western Shikoku and we detected seismological evidence of a slab dehydration reaction linked to LFE activity. Based on the first-order harmonic decomposition of RFs, we first estimated the average plunge azimuth of the PHS beneath western Shikoku. Considering the backazimuth dependence of converted phase amplitude, we constructed the cross-section of the radial component RFs, excluding the incoming waveform data from the updip directions of the dipping slab. In this profile, the parallel negative and positive P-to-S converted phase alignment within a distance of 10 km were imaged. These phase alignments corresponded to the top of the slab and the slab Moho discontinuity, respectively. At the northern side of the profile, the landward (continental) Moho was also detected. In the region where LFEs were actively distributed, both the landward Moho and slab surface were unclear. The second-order harmonic decomposition of RFs for several kilometres above the slab Moho indicated that the anisotropic symmetry axes suddenly changed at the southern limit of the LFE active region; the fast axes in the region were normal to the trench though axes in the southern area were parallel. According to the thermal and pressure condition, a phase transition with a dehydration reaction can occur in the oceanic crust near the southern edge of the LFE active region. Once the dehydration process advances, released water causes the serpentinization of the mantle material near the slab surface and the corresponding seismic velocity decreases. The impedance contrast decreases at the boundary between the lower crust and the mantle wedge, as well as that between the mantle wedge and oceanic crust; therefore, the amplitudes of the converted phases at these boundaries also become small. Considering that serpentinite ordinarily has strong anisotropy with a seismic fast axis direct to the shear direction, all features observed are evidence of the dehydration process in the flat slab.

The aim of this work is to present an easy-to-use Python code to calculate the gravitational effect due to lateral variations in the thermal structure of oceanic lithospheric plates, necessary to appropriately isolate the contribution of the variations in crustal thickness. The model is applied to calculate the Moho depth in the West Philippine Basin, and the results are compared with seismic data. The estimations of the Moho depth taking into account the thermal correction presented a better fit with the seismic data and smoother geographical variations than the model without thermal correction.

The SW Japan arc built by subduction of the Philippine Sea (PHS) plate exhibits uneven distribution of volcanoes: thirteen Quaternary composite volcanoes form in the western half of this arc, Kyushu Island, while only two in the eastern half, Chugoku district. Reconstruction of the PHS plate back to 14 Ma, together with examinations based on thermal structure models constrained by high-density heat flow data and a petrological model for dehydration reactions suggest that fluids are discharged actively at depths of 90–100 km in the hydrous layer at the top of the old (> 50 Ma), hence, cold lithosphere sinking beneath Kyushu Island. In contrast, the young (15–25 Ma) oceanic crust downgoing beneath Chugoku district releases fluids largely at shallower depths, i.e. beneath the non-volcanic forearc, to cause characteristic tectonic tremors and low-frequency earthquakes (LFEs) and be the source of specific brine springs. Much larger amounts of fluids supplied to the magma source region in the western SW Japan arc could build more densely-distributed volcanoes.

—Homogenized melt inclusions in olivine were studied in Archean komatiites from the Barberton Greenstone Belt, Weltevreden Formation in South Africa (3.3 Ga), Abitibi Greenstone Belt in Canada (2.72 Ga), and the Belingwe Greenstone Belt in Zimbabwe (2.69 Ga). Contamination of the komatiite melts with crustal material enriched in Rb, Cl, and H2O during the crystallization of olivine is demonstrated. Uncontaminated melts have mantle Rb/Nb ratios but are significantly enriched in Cl and H2O relative to K and Ce, respectively, exhibiting similar incompatibility during crystallization and partial mantle melting. These observations suggest the presence of a chlorine- and water-enriched mantle source before 3.3 Ga. The excess Cl and H2O contents in the komatiites are assumed to result from the interaction of partially molten mantle plumes with the mantle transition zone. The most likely source of Cl and H2O enriching the deep mantle is the oceanic lithosphere that endured a seafloor alteration. We conclude that the recycling of the altered oceanic lithosphere into the mantle, probably via subduction, began in the first billion years of the Earth’s history. Delamination of the Archean crust could not cause transport of chlorine and water into the deep mantle.

The Ivrea Zone of northwest Italy exposes a section of the lower crust and lithospheric mantle deformed during the Variscan Orogeny, where a series of carbonated alkaline pipes was emplaced over a time span of ca. 40 Ma, following a crustal underplating event at ca. 288 Ma. We use a coupled 2D petrological-thermomechanical approach to geodynamically model the processes that led to the Variscan continental collision and subsequent emplacement of the alkaline pipes with application to craton margin metasomatic events. The following criteria are assessed to develop a generic model: 1) closure of Rheic Ocean through subduction of oceanic crust and subduction-related metasomatism of the lithospheric mantle, 2) high-temperature metamorphism of the lower crust associated with continental collision, preceding the 3) emplacement of a crustal underplating event, and 4) partial melting of metasomatized lithospheric mantle domains during gravitational collapse of the orogen to source the alkaline pipes. Our model investigates the collision process by varying thermal ages of the intervening oceanic crust (40, 60, and 80 Ma) and by changing the rheological strength of the lower crust. We employ two ideal end-member cases, wet quartzite and plagioclase An75, an intermediate dry quartz case to represent a migmatized lower crust, and two polymineralic cases of felsic granulite and mafic granulite as possible representations to the Ivrea Zone specifically. We discuss the results through three scenarios controlled by the age of the oceanic crust and rheology of the lower crust that can reproduce tectonic scale features that are observed in the Ivrea Zone. Scenario 1: the wet quartzite 40 Ma model features widespread H2O and CO2 fluxes within the continental lithospheric mantle. The model displays little melt productivity due to the presence of an overthickened crust and the continental lithospheric mantle decouples and peels off beneath the lower crust of the colliding terrane. Scenario 2: the plagioclase An75 60 Ma model shows high-temperature metamorphism in the lower crust and reproduces the metasomatism of the continental lithospheric mantle that is a precursor to the genesis of the alkaline pipes. The model is permissive for the emplacement of the crustal underplating event, but the relative timings are misplaced. Scenario 3: the mafic granulite 80 Ma model recreates the tectonic relationships between lower crustal high-temperature metamorphism followed by initiation of the magmatic underplating event, and lastly by localized melting of metasomatized domains of the lithospheric mantle. This model can potentially explain two of the prevailing hypotheses for the formation of the alkaline pipes. Whereas all models reproduced different stages of the tectono-magmatic evolution of continental collision, only scenario 3 could sufficiently recreate the relative sequence of events observed in the Ivrea Zone. Scenarios 1 and 2 may provide precious information about the nature and timing of metasomatic processes in the lithospheric mantle, with implications for the enhanced prospectivity for a wide range of ore systems in specific domains along the margins of lithospheric blocks.

To quantify the intensity of oceanic oxygen isotope buffering through hydrothermal alteration of the oceanic crust, a 2D hydrothermal circulation model was coupled with a 2D reactive transport model of oxygen isotopes. The coupled model calculates steady-state distributions of temperature, water flow and oxygen isotopes of solid rock and porewater given the physicochemical conditions of oceanic crust alteration and seawater δ18O. Using the present-day seawater δ18O under plausible modern alteration conditions, the model yields δ18O profiles for solid rock and porewater and fluxes of heat, water and 18O that are consistent with modern observations, confirming the model's validity. The model was then run with different assumed seawater δ18O values to evaluate oxygen isotopic buffering at the midocean ridges. The buffering intensity shown by the model is significantly weaker than previously assumed, and calculated δ18O profiles of oceanic crust are consistently relatively insensitive to seawater δ18O. These results are attributed to the fact that isotope exchange at shallow depths does not reach equilibrium due to the relatively low temperatures, and 18O supply via spreading solid rocks overwhelms that through water flow at deeper depths. Further model simulations under plausible alteration conditions during the Precambrian showed essentially the same results. Therefore, δ18O records of ophiolites that are invariant at different Earth ages can be explained by the relative insensitivity of oceanic rocks to seawater δ18O and do not require constant seawater δ18O through time.

In subduction zones, landward dipping regions of low shear wave velocity and elevated Poisson’s ratio, which can extend to at least 120 km depth, are interpreted to be all or part of the subducting igneous oceanic crust. This crust is considered to be overpressured, because fluids within it are trapped beneath an impermeable seal along the overlying inter-plate boundary. Here we show that during slow slip on the plate boundary beneath southern Vancouver Island, low frequency earthquakes occur immediately below both the landward dipping region of high Poisson’s ratio and a 6–10 km thick shear zone revealed by seismic reflections. The plate boundary here either corresponds to the low frequency earthquakes or to the anomalous elastic properties in the lower 3–5 km of the shear zone immediately above them. This zone of high Poisson’s ratio, which approximately coincides with an electrically conductive layer, can be explained by slab-derived fluids trapped at near-lithostatic pore pressures., Regions of the subducting oceanic crust are often considered to be overpressured, owing to fluid trapped beneath an impermeable seal along the overlying inter-plate boundary. Here, the authors show that slow slip earthquakes at the Cascadia subduction zone occur immediately below a 6-10 km-thick shear zone, in which slab-derived fluids are likely trapped at near-lithostatic pore pressures.

This study explores the spatial distribution of seismicity, the pattern of b-values, gravity and magnetic anomalies, heat flow data, and focal mechanism solutions to more thoroughly understand the present-day stress distribution and the nature of the crust, which characterize the rifting process in the northern Red Sea region. The region shows alternating low and high b-values in congruence with low and high cumulative seismic moment releases, respectively, and with negative and positive gravity anomalies, respectively. In general, except for larger-sized earthquake locations that characterized by high stress accumulation, low b-values, low seismic moment release, and low-gravity anomalies dominate the zones of the Gulf of Aqaba and the Gulf of Suez, thereby implying low stress accumulation consistent with an old crust of no differential development. Conversely, the rest of the Northern Red Sea region exhibited heterogeneities in the spatial distribution of b-values, cumulative seismic moment releases, and gravity anomalies, thereby implying stress heterogeneities. The stress heterogeneities may impute to the differences in material properties of the upper crust in the region. Zones of a positive Bouguer anomaly and high stress are notably associated with a relatively weak crust characterized by high rate of seismicity, while zones of the negative Bouguer anomaly and low stress characterized an older and more stable crust. However, details on the nature of crust required additional geophysical and geological data of high resolution in future studies.

SUMMARYSeismic attenuation is measured from a swarm of 50 earthquakes in Kīlauea volcano in 2018, associated with caldera collapse. The traverse extends at nearly constant azimuth to the saddle between Mauna Loa and Mauna Kea, continuing to Maui beneath the distal flanks of three dormant volcanos. From Maui the traverse then extends seaward to the Aloha Cabled Observatory (ACO) on the seafloor north of O‘ahu. The effective attenuation is measured with respect to an ${\omega ^{ - 2}}$ earthquake source model. Frequency dependent ${Q_P}$ and ${Q_S}$ are derived. The initial path is shallow and uphill, the path to Maui propagates at mid-crustal depths, and the path to ACO extends through oceanic crust. The observations of ${Q_P} \le {Q_S}$ over each traverse are modelled as bulk attenuation ${Q_K}$. Several attenuation processes are observed, including ${Q_\mu }$, ${Q_K}$, $Q\sim f$, constant Q and scattering. The observation of bulk attenuation is ascribed to contrasting physical properties between basalt and water saturated vesicles. The ratio of Q values between shallow and mid-crustal propagation is used to derive an activation energy E* for the undetermined shear attenuation mechanism. A Debye relaxation peak is fit to the ${Q_S}( f )$ and ${Q_K}( f )$ observed for the mid-crustal pathway. A prior high-frequency attenuation study near Wake Island compares well with this Hawaiian Q data set, which in general shows lower values of Q than observed for Wake.

Seismicity, heat flow, seismic tomography data, prerift and synrift magmatism are considered as intensity indicators of geodynamic processes along the Atlantic-Arctic rift system (AARS). In this rift system, several large (over 100 km ) sub-latitudinal displacements of the rift axis are due to left-lateral strike-slip faulting. The AARS segments are distinguished by the age of splitting of continental plates from each other. A dependence is revealed between the current thermal state of the mantle under the AARS and the age of spreading start. This dependence is established from both seismic tomography and heat flow data. In section δ ( Vp/Vs ), the locations of the main segmenting faults and ‘cold’ anomalies in the upper mantle are coincident. Distributions of total seismic moments are practically synchronous in the depth intervals of 0–13, 13–35, and >35 km. The maximum values above the plumes are represented by higher seismic moments in the surface layer. The main demarcation zones differ in maximum energy release values in the AARS with shearing features. Comparison of these values against the age of the start of spreading processes shows trends of heat flow and medium field tomography in the AARS segments. The trends confirm the thermal interpretation of the seismic tomography data and suggest mantle cooling with age and a decrease in the mean temperatures of the mantle. The main factor causing the sublatitudinal asymmetry of heat flow in the AARS is the impact of Coriolis forces on the magma in the asthenospheric source. Most of the synrift igneous formations seem to be related to the influence of long-lived anomalies in the mantle, which had lower rates of magma generation than those typical of the formation of magmatic provinces. In conditions for spreading and the formation of the oceanic crust, the process followed the principle of energy cost minimization, and the prerift magmatic provinces with the pre-processed crust contributed to the choice and positioning of the AARS trajectory. The plume branches are imposed in the tomographic section and thus ‘concealing’ the relationship between the age and the thermal state. However, that does not change the trend to cooling of the mantle beneath the AARS, proportionally to the time since the start of spreading.

This paper presents new data on rocks dredged from the Hahajima Seamount during the cruise KH03-3 of R/V Hakuho-Maru in 2003. It is shown that the seamount is a tectonically displaced block of the Earth’s crust composed of island-arc ophiolites. The basement of the ophiolite complex is made up of highly depleted mantle residue (dunites and harzburgites). Upward, they grade into cumulate lherzolites, various gabbroids, dolerites, and basalts. This complex marks the initial stages of island-arc evolution: formation of an oceanic crust of the marginal sea. Upward, the section is built up of the island-arc rock association consisting of tholeiites, boninites, and adakites. New data on mineralogy and geochemistry of the studied rocks are reported. The ophiolitic complex served as the basement for island-arc association and was affected by the powerful heat flow that resulted in repeated melting of mantle residue and metamorphism of the above rocks.

The Oman-United Arab Emirates ophiolite has been used extensively to document the geological processes that form oceanic crust. The geometry of the ophiolite, its extension into the Gulf of Oman, and the nature of the crust that underlies it are, however, unknown. Here, we show the ophiolite forms a high velocity, high density, >15 km thick east-dipping body that during emplacement flexed down a previously rifted continental margin thereby contributing to subsidence of flanking sedimentary basins. The western limit of the ophiolite is defined onshore by the Semail thrust while the eastern limit extends several km offshore, where it is defined seismically by a ~40–45°, east-dipping, normal fault. The fault is interpreted as the southwestern margin of an incipient suture zone that separates the Arabian plate from in situ Gulf of Oman oceanic crust and mantle presently subducting northwards beneath the Eurasian plate along the Makran trench., The Semail ophiolite provides evidence for geological processes that form oceanic crust, however, its deep structure remains debated. Here, the authors use geophysical imaging to determine that the ophiolite is bound by a thrust fault in the west, and a normal fault in the east, bounding a rapidly subsiding basin, implying the ophiolite may not be rooted in the Gulf of Oman crust.

Oxygen isotope geochemistry is a powerful tool for investigating rocks that interacted with fluids, to assess fluid sources and quantify the conditions of fluid–rock interaction. We present an integrated modelling approach and the computer program PTLoop that combine thermodynamic and oxygen isotope fractionation modelling for multi-rock open systems. The strategy involves a robust petrological model performing on-the-fly Gibbs energy minimizations coupled to an oxygen fractionation model for a given chemical and isotopic bulk rock composition; both models are based on internally consistent databases. This approach is applied to subduction zone metamorphism to predict the possible range of δ18O values for stable phases and aqueous fluids at various pressure (P) and temperature (T) conditions in the subducting slab. The modelled system is composed of a mafic oceanic crust with a sedimentary cover of known initial chemical composition and bulk δ18O. The evolution of mineral assemblages and δ18O values of each phase is calculated along a defined P–T path for two typical compositions of basalts and sediments. In a closed system, the dehydration reactions, fluid loss and mineral fractionation produce minor to negligible variations (i.e. within 1 ‰) in the bulk δ18O values of the rocks, which are likely to remain representative of the protolith composition. In an open system, fluid–rock interaction may occur (1) in the metasediment, as a consequence of infiltration of the fluid liberated by dehydration reactions occurring in the metamorphosed mafic oceanic crust, and (2) in the metabasalt, as a consequence of infiltration of an external fluid originated by dehydration of underlying serpentinites. In each rock type, the interaction with external fluids may lead to shifts in δ18O up to 1 order of magnitude larger than those calculated for closed systems. Such variations can be detected by analysing in situ oxygen isotopes in key metamorphic minerals such as garnet, white mica and quartz. The simulations show that when the water released by the slab infiltrates the forearc mantle wedge, it can cause extensive serpentinization within fractions of 1 Myr and significant oxygen isotope variation at the interface. The approach presented here opens new perspectives for tracking fluid pathways in subduction zones, to distinguish porous from channelled fluid flows, and to determine the P–T conditions and the extent of fluid–rock interaction.