Intraplate Volcanism Research Articles

Fani Maoré, a new “young HIMU” volcano with extreme geochemistry

Intraplate volcanism provides remarkable insight into diverse sources in the mantle because its source can be quite shallow, or as deep as the core-mantle boundary, and its origin can be as diverse as recycled crustal material or undifferentiated mantle. While geophysical approaches can in some cases locate the source of magmas, a geochemical approach is necessary to characterize both the nature of the source and the way it melts to produce the erupted lavas.Here we present geochemical and isotopic data obtained on a new submarine volcano (Fani Maoré) that was discovered in 2019 next to Mayotte Island in the Comoros. The radiogenic isotope data are remarkably uniform at subdued values intermediate between HIMU and EM1 compositions but trace element contents are unusual with a marked enrichment in Ba (Ba/Th ≈ 370 compared with the ocean island basalt (OIB) average of 100) and depletion in Pb (Ce/Pb ≈ 70 versus 25 for average OIB). This unique data set suggests that the basanites formed by melting of a carbonated mantle source that was highly enriched in Ba and volatiles. A similar source is also present under the East African Rift where contemporaneous basanite and carbonatite eruptions are known. This establishes a possible link between the volcanic activity of the Comoros and the East African Rift zone. More generally, it demonstrates that carbonated sources are more common in the mantle than previously thought and can be traced using trace element geochemistry. Other volcanoes in the world carry similar characteristics and we suggest that carbonated mantle sources explain the geochemical peculiarities of not only Fani Maoré in the Comoros but also those of Cape Verde volcanics and more generally those of many HIMU-like OIBs, in particular the so-called ‘young HIMU’ OIBs.

Thermochemical Structure and Melting Distribution of the Upper Mantle Beneath Intraplate Volcanic Areas in Eastern South China Block

AbstractThe Eastern South China Block (SCB) has experienced complex Mesozoic‐Cenozoic tectonism and intraplate volcanism. However, due to a lack of exhaustive exploration of the upper mantle's thermochemical structure, it is difficult to determine the extent of the lithospheric modification and the mechanisms by which the volcanism generate. Here, we jointly invert Rayleigh wave dispersion, surface heat flow, geoid height, and elevation data to provide a comprehensive thermal and compositional structure of the upper mantle beneath eastern SCB and infer regions of partial melting. Our model reveals widespread lithospheric thinning in the eastern SCB and large variations of lithospheric composition with a more fertile eastern Lower Yangtze lithosphere than the lithosphere elsewhere, suggesting the lithosphere of the eastern Lower Yangtze is more severely modified than the rest of the SCB. Moreover, two high‐temperature anomalies are revealed: one beneath the eastern Lower Yangtze and the other beneath the Pearl River Delta region, associated with the Pacific plate subduction and Hainan plume, respectively. The high‐degree partial melting (∼6%) in the asthenosphere beneath the Lower Yangtze is responsible for the strong ongoing lithospheric modification and the young intraplate volcanism in the Nvshan and Subei areas. Small‐scale upper mantle convections triggered by the large mantle upwellings created a low value of ∼3% melts, possibly responsible for the intraplate volcanism in the coastal CB and less severe lithospheric modification. We demonstrate that the lithospheric thickness and its thermochemical state are the key factors that influence the composition and evolution of intraplate volcanism in the eastern SCB.

Characterisation, axial anisotropy, and formation conditions of celestine minerals from the Jabal Eghei (Nuqay) late Neogene – Pleistocene volcanic province, southeastern edge of the Sirt Basin, southern Libya: Constraints on the mineralogical geothermometer

AbstractFive celestine crystals were sampled from the (palaeo)surface intervening between the late Miocene to Pleistocene basaltic sequences of the Jabal Eghei (Nuqay) volcanic province in southern Libya and then characterised by applying a combination of the SEM–WDS, ICP/OES, PXRD and IR methods. Colour variations and related minerogenetic frameworks were also investigated. Three samples have greenish-blue-to-blue colour (480.4–482.5 nm), whereas the other two samples have blue–green colour (cyan; 489.1–494.1 nm). The colour purity ranges from 1.36–7.16. Their composition is similar, end-member celestine, in which only 1.6–4.1 at.% of Sr2+ content was substituted by Pb2+ (0.7–0.9 at.%), Ba2+ (0.5–0.7 at.%) and Ca2+ (0.2–0.8 at.%). Three samples contained vacancies, from 1.0 to 1.9 at.%. The content of other chemical elements is minor. The resulting unit-cell parameters have the ranges: a0 = 8.3578(9)–8.3705(6) Å; b0 = 5.3510(5)–5.3568(4) Å; c0 = 6.8683(7)–6.8767(2) Å and V0 = 307.17(5)–308.34(4) Å3. The PXRD and IR results are mainly in accordance with the SEM–WDS results, with a high level of correlation. However, a few discrepancies were found, producing several possible interpretations, the primary cause being a slight unit-cell axial anisotropy i.e. thermal expansion. As a consequence these results yield a new geothermometric tool that is based on the unit-cell axial anisotropy. The celestines investigated were formed during a Miocene intraplate volcanism with basaltic magmas, and associated brines lifted by the structural conduits (normal faults crosscutting the Sirt basin). The Sr-bearing fluids then poured into and over the faulted and fractured lagoon-type gypsum, anhydrite Eocene sediments. The celestine mineralisation formed within a ~368–430 K (~95–157°C) temperature range. The celestine formed at slightly elevated temperature and pressure conditions, close to the shallow subsurface environment (over 250 bars).

Multi-scale anisotropy in NE China: Evidence for localized mantle upwelling

It is commonly proposed that the subduction of the Pacific plate has been responsible for widespread Holocene intraplate volcanism across NE China and the Korean Peninsula. Yet, how this process drives volcanism and even if it plays a critical role remains a topic of vigorous debate. In this study, using seismic data from four networks across NE China and northern Democratic People's Republic of Korea (DPRK), we analyze shear wave splitting in converted P to S-waves at the Moho (Pms), S-waves from the subducted slab interface (local S), and SKS phases. The Pms phases show a relatively weak crustal anisotropy (<0.25 s), with fast polarization directions aligned sub-parallel to major tectonic features. For the local S and SKS phases, fast polarization directions show significant lateral variations. We further perform a quantitative inversion to show that the depth of the anisotropy is ∼150 km, thus driven by flow within the asthenosphere associated with Pacific subduction. However, the presence of many null SKS splitting phases, together with scattered local S anisotropy across a wide range of incidence angles suggests a localized region of vertical flow directly beneath Changbaishan volcano. Such patterns correspond well to regional upper-mantle seismic velocity structure, and suggest that a localized upwelling with a relatively deep origin drives volcanism in the Changbaishan region. Furthermore, we infer that this mantle upwelling is deflected to the SW beneath Changbaishan and spreads asymmetrically at the base of the lithosphere, possibly because of the long history of volcanism in the region.

Decoding subcontinental lithosphere processes: The key role of fractional crystallization in Central Andes monogenetic volcanism - Insight from El Negrillar volcanic field, Chile

Subduction zones are complex geodynamic settings in which volcanic arcs form due to the interaction between subducting slabs, water, and continental materials. Investigating this extensive Earth recycling system offers valuable insights into the exchange of materials between the crust and mantle. While previous research has primarily focused on analyzing polygenetic volcanoes to explore this interaction, monogenetic volcanic fields (MVF) present an opportunity to delve into the mechanisms operating within the upper and lower mantle. Monogenetic volcanoes are a class of volcanoes that are typically smaller in size but showcase a range of eruptive styles and compositions. These volcanoes can be found in various locations along the arc, including the main Central Volcanic Zone (CVZ) (arc-front volcanoes), and even far from the tectonic plate boundaries in the back-arc region (intraplate volcanism).We carried out a detailed geochemical study of El Negrillar (EN) MVF, located in the Central Andes main-arc, on the Altiplano-Puna Plateau's southwestern border. The volcanic field has 35 eruptive centers and a total of 86 eruptive phases originating from its three clusters: Northern, Central, and Southern. EN magmas range in composition from basaltic andesite to dacite and have produced over 6.8 km3 (DRE) of lava and pyroclastic material, which makes it the most voluminous volcanic field in the Central Andes, and therefore, an excellent example to study MVF in arc-systems. We have conducted an extensive study encompassing major and trace elements, as well as isotopic analysis using whole rock Sr-Nd-Pb. We also present an analysis of our results with available geochemical data from the back-arc and main-arc volcanoes, including other monogenetic volcanoes near EN (<80 km, EN's neighbors). The results revealed that EN magmas and their neighbors are strongly related and exhibit a clear similarity in major and trace element compositions and isotopic ratios, indicating a common source and origin of their melts. Our comparison across the arc surprisingly reveals similarities between EN and back-arc monogenetic volcanism. Both regions indicate geochemical signatures that do not support melting via slab metasomatism, typical of subduction zones. Instead, they show low Th/Ce (<0.1 ppm) and Ba/La (<30 ppm) and high Ce/Pb (>6 ppm), Ba/Th (>100 ppm), high 143Nd/144Nd > 0.51240, and La/Yb ratios at a given La/Sm compared to the main-arc polygenetic volcanoes. Interestingly, EN also exhibits adakite-like signatures with increasing SiO2 content (high Sr > 600 ppm, Sr/Y > 40, and high La/Yb > 20), a term used for igneous rock suites with chemical characteristics that are identical to those of adakites but produced through petrogenetic processes that do not include a slab component. We explore how fractional crystallization and crustal assimilation participate in the development of monogenetic volcanoes, and our findings underscore the significant influence of fractional crystallization on the monogenetic volcanic processes in the Salar de Atacama region. The geochemical changes observed in this area can be attributed to a combination of varying degrees of fractional crystallization within magmas generated through the partial melting of a garnet-enriched environment, which could be primary lower crust or partial melting of a peridotite mantle that has later equilibrated with garnet-bearing crustal melts.

Timing of Izanagi-Pacific ridge subduction beneath Eurasia: Constraints from metamorphic soles in Hokkaido, Japan

The subduction of the Pacific Plate beneath Eurasia controls not only Cenozoic intraplate volcanism, but also deep-focus earthquakes along the continental margin of eastern Asia. However, the timing of subduction of the Paleo-Pacific (Izanagi)−Pacific ridge and the initial subduction of the Pacific Plate beneath Eurasia remain subjects of debate. Metamorphic soles provide key evidence for ridge subduction, and together with coeval igneous activity, they are widely used to constrain the timing of ridge subduction. Here, we present the results of a study of amphibolites from the Hidaka metamorphic belt, Hokkaido, northern Japan, which are interpreted to be metamorphic soles. Our integrated study involved secondary ion mass spectrometry (SIMS) U-Pb zircon dating, mineral chemistry, whole-rock geochemistry, and Sr-Nd-Pb-Hf isotopic analyses. SIMS U-Pb zircon dating indicates that the amphibolite-facies sole metamorphism occurred in the late Eocene. Phase equilibria modeling suggests that the peak P−T conditions of metamorphism were 8.0−9.5 kbar/700−730 °C. The amphibolites belong to the tholeiitic series, and they are relatively enriched in heavy rare earth elements relative to light rare earth elements, with depletions in Nb, Ta, P, and Ti. They have (87Sr/86Sr)i = 0.704207−0.704998, εNd(t) = +11.65 to +11.96, εHf(t) = +14.28 to +16.32, (206Pb/204Pb)i = 18.240−18.255, and (207Pb/204Pb)i = 15.522−15.525. The geochemistry of these amphibolites reveals that their protoliths had normal mid-oceanic-ridge basalt affinities, and their Pb isotopic signatures indicate Indian-type mantle rather than Pacific-type mantle. Considering the geochemistry of coeval igneous rocks, we conclude that the late Eocene metamorphism in the Hidaka metamorphic belt records the intraoceanic subduction that followed heat transfer from the incipient mantle wedge toward the top of the subducting plate, and it provides a key constraint on the timing of subduction of the Izanagi-Pacific ridge.

Controls of Tellurium enrichment in the rare earth elements and yttrium (REY) rich deep-sea sediments from the Central Indian Ocean Basin (CIOB)

As a potential Te reservoir for the global budget, the occurrence, material sources, migration and enrichment mechanism of Te in deep-sea sediments remain unclear. Based on the major and trace elements measurements of bulk sediments, this study proposed a procedure to quantify the contribution of Te by main host minerals, combining with TIMA and LA-ICP-MS in two cores (GC04 and GC11) from the Central Indian Ocean Basin (CIOB) and a core from east of the 90°E Ridge (GC03). Te shows the highest enrichment coefficient of 650 referred to UCC (Upper Continental Crust) in the REY (Rare Earth Elements and Yittrium) rich sediments in GC04, GC11 and GC02 (a core at the south of GC04 and GC11). Clinoptilolite-K and osumilite are indicated as the main host minerals, which correlates Te sources with volcanogenic materials and weathering products of terrigenous rocks. The inter elements correlation, cluster analysis and factors analysis further indicate distinct hydrothermal genetic Te. So, we put forward a hypothesis that Te is mainly sourced from the submarine volcanogenic hydrothermal activities, which are closely associated with local intraplate volcanism episodes from late Miocene with spatial and temporal differences. This study is the first attempt to quantify Te from different minerals and to reveal its geochemical behavior in the deep sea sediments, which will be benefit to the understanding of Te reservoir and cycle in deep sea and the comprehensive utilization of REY-rich deep-sea sediments. However, for the abundant presence of undetectable nanoscale minerals in these sediments, the contribution of Fe-Mn (oxyhydr) oxides and clay minerals of Te need more investigation.

Tectonic transition revealed by upper mantle heterogeneities and anisotropy of the SE margin of the Tibetan Plateau: Insights into the Cenozoic intraplate volcanisms

The SE margin of the Tibetan Plateau is a key channel for understanding the plateau's evolution. However, the links between mantle dynamics, surface deformation, and associated intraplate volcanism remain elusive. Here, 3D radially anisotropic seismic structures of the upper mantle beneath the SE margin of the Tibetan Plateau are presented based on adjoint waveform tomography using dense arrays. The seismic image shows a prominent tectonic transition along 26°N, characterized by strong heterogeneities. In the north of 26°N, lithospheric delamination is revealed by high-velocity anomalies between the Tibetan Plateau and the Yangtze Craton at the 150–250 km depth, which may drive the early Cenozoic potassic volcanism. In the south of 26°N, asthenospheric flow is suggested by significantly low velocity and positive radial anisotropy at the uppermost mantle, which probably accounts for the late Cenozoic sodic volcanism. We propose that such upper mantle dynamics shape the surface expression of plateau uplift and intraplate volcanism. Plain language summaryThe mechanism controlling the growth and volcanism of the Tibetan Plateau remains ambiguous. We present clues from subsurface structures using earthquake seismology. We observe strong heterogeneities of velocity and anisotropy in the upper mantle of the SE margin of the Tibetan Plateau. The results suggest that lithospheric delamination and asthenospheric flow control the generation of the early Cenozoic K-rich volcanism and late Cenozoic Na-rich volcanism, respectively. Our study indicates that tectonic transition might exist in the subsurface of the SE margin of the Tibetan Plateau.

Basanite cobbles in Pleistocene sediments in Central Otago and their implications for intraplate volcanism and Clutha River paleo-drainage

ABSTRACT The occurrence of volcanic basanite cobbles in Pleistocene terraces at Galloway and in the upper Clutha valley in Central Otago, in an area devoid of known volcanic edifices, has implications for Cenozoic intraplate volcanism and Pleistocene drainage in the region. We present petrographic, mineralogical, whole rock geochemical and Sr-Nd-Pb isotopic data, as well as an 39Ar-40Ar date of 24.4 ± 0.06 Ma, together with mineral analyses of enclosed peridotite xenoliths. We conclude from this dataset that these alkaline basanite cobbles are most likely derived from the Alpine Dike Swarm in northwest Otago, probably in the Lake Wanaka-Hawea-Luggate area, which extends the volcanic field east of its known extent. The pilotaxitic textures and presence of intersertal glass, along with the volume of basanitic material, suggests that the source rocks were likely effusive, which would make this the first such known component of the Alpine Dike Swarm and requires emergent land at that time. The occurrence of the Galloway basanite cobbles implies that Pleistocene Clutha River flowed through or over what is now the Dunstan Range. The young (late Pleistocene) age of all the basanite-bearing gravels means that the basanite cobbles have likely been reworked several times before final deposition.

Decratonised lithosphere associated with mantle plume upwelling in the Singhbhum craton, eastern India based on geophysical and geodynamic perspective

The mode of crustal and lithospheric mantle evolution of cratonic blocks and their subsequent deformation and decratonisation due to regional plate tectonics and continued intraplate geodynamic process, are not well understood. This is especially true for the highly complex Eoarchean Singhbhum craton, located in the eastern part of the Indian shield, which underwent prolonged episodes of intraplate volcanism, mafic dyke intrusions, rifting, collision and widespread sedimentation lasting for over two billion years, leaving behind noticeable imprints in the crust and mantle lithosphere. Our detailed analysis of the geophysical characteristics indicates high crustal velocities and presence of a pronounced metasomatised zone between the depth 14 km and 30 km, which is characterised by drop in Vs from 3.93 km/s to 3.76 km/s. This zone is further followed by 14 km thick high velocity magmatic crust above the Moho. A substantial part of the upper crust has been eroded away from this craton due to thermal-induced uplifting and massive crustal exhumation, caused by the multiple hot mantle plume interactions during Precambrian period, in which mantle potential temperature and pressure reached as high as 1700 oC and 4.4 GPa respectively. This craton is conspicuously associated with highly positive gravity anomalies (up to +10mGal), which are well correlated with high elevation and the location of mafic volcanic suites. It is also characterised by an unusually thin lithosphere (∼ 60-128 km), high Moho temperatures (910°C) and high dose of heat flow (47mW/m2) emanating from the mantle, which conforms with the estimated low effective elastic thickness of ∼30 km. Transient effects of the paleo-thermal anomalies possibly still persist due to periodic rejuvenation of surrounding rift valleys and suture zones. Eventually, this craton now shows geophysical characters of more or less a mobile belt, not of a typical craton. Since, almost 150-200 km thick mantle lithosphere from the bottom has been eroded away from this region due to persistent thermo-geodynamic activities and mantle upwelling induced rise of mantle isotherms, our study does not favour the idea of cratonic stability and preservation of deep continental roots.

Cenozoic Uplift and Volcanism of Hangai Dome, Central Mongolia Triggered by Lower Mantle Upwellings

AbstractThe mechanisms of surface uplift of Hangai Dome (&gt;2 km) and formation of Cenozoic intraplate volcanism in central Mongolia remain vigorously debated. Here we directly investigate the mantle transition zone (MTZ) structure beneath central Mongolia to explore whether the deep mantle geodynamical processes have played a role. Our results reveal significant MTZ thinning of 8–13 km in three subareas mainly caused by the uplifted 660‐km discontinuity (∼10 km). In addition, the 410‐km discontinuity is depressed beneath the Hangai Dome (∼6 km) and the northern portion of the study area (10–15 km). These observations provide direct evidences for intracontinental mantle upwellings from the lower to the upper mantle, causing the Cenozoic uplift and basaltic volcanism of central Mongolia. Furthermore, these mantle upwellings are relatively sluggish compared to canonical mantle plumes based on the small magnitude of MTZ thinning and thus relatively moderate thermal anomaly within the MTZ (62–100 K).

Origin of Alkaline Basaltic Intrusive Rocks in an Exhumed Accretionary Complex: Implications for Past Petit‐Spot Volcanism in the Ocean

AbstractAccreted basaltic rocks are expected to provide information on intraplate volcanism in the oceans. Basaltic rocks, originating from mid‐ocean ridges, plateaus, and seamounts, have been reported from exhumed accretionary complexes. However, basaltic rocks related to petit‐spot volcanism remain poorly documented. We examined basaltic intrusive rocks in an exhumed accretionary complex on Amami‐Oshima Island, Ryukyu Arc. Basaltic sills intruded mélange composed of seamount‐derived micritic limestone and basaltic rocks, which deformed together with the mudstone‐dominated mélange in the subduction zone. A separate basaltic sill intruded pelagic chert before being incorporated as a mélange block in the mudstone matrix during deformation in the subduction zone. Geochemical discrimination diagrams and trace element patterns indicate that the accreted dolerites and nonintrusive basalts likely originate from mid‐ocean ridges and hotspots, respectively. On the other hand, the geochemical characteristics of basaltic sills are compatible with alkaline basalt with enriched in incompatible trace elements relative to other basaltic rocks of hotspot origin and may represent a low degree of partial melting from a deeper mantle source (&gt;90 km). A higher ratio of gadolinium to ytterbium in the basaltic sills relative to hotspot or mid‐ocean ridge‐related basalt indicates both a deeper melting depth, and a magma source which upwelled through thick oceanic lithosphere far from the mid‐ocean ridge. Based on the timing of basaltic intrusion and geochemical features, we suggest that the basaltic intrusive rocks could record past petit‐spot volcanism in the oceanic plate.

Subduction‐Induced Asthenospheric Flow Around the Songliao Basin in NE China Revealed by Shear Wave Splitting Measurements of Dense Seismic Arrays

AbstractThe traditional mantle plume model fails to explain late Cenozoic intraplate volcanism in Northeast China (NEC). We constrained the pattern of upper mantle deformation to study the origin of intraplate volcanism by quantifying the shear wave splitting parameters captured by three NW‐SE linear seismic arrays in NEC. The dense station spacing (10 km) allowed us to image the small‐scale variations in anisotropic structures in unprecedented detail. The WNW‐ESE oriented subduction‐parallel anisotropy is likely induced by the asthenospheric return flow in the big mantle wedge convection associated with Pacific subduction. Therein the most notable feature is a toroidal pattern of anisotropy beneath the Songliao Basin, which is distinguished by a larger splitting time (approximately 1.0 s) relative to weak anisotropy in the center. The toroidal anisotropy pattern coincides with a high‐velocity anomaly extending from uppermost mantle down to ∼300 km depth, presumably indicating early stage foundering lithosphere. The asthenospheric return flow then moved around the foundering lithosphere and induced the corresponding toroidal anisotropy. The decompression partial melting of upwelling asthenospheric materials erupted along weak zones generating late Cenozoic intraplate basalts on both flanks of the Songliao Basin. The high‐density seismic array reveals that the Pacific subduction and early stage lithospheric foundering processes controlled the genesis of late Cenozoic volcanism in NEC.

The upper-mantle density variation beneath the South Atlantic Ocean and its implications for hotspots and large igneous provinces

The South Mid-Atlantic Ridge (SMAR), the most typical slow-spreading ridge on the earth, is a significant area to study the oceanic crust-mantle evolution. We used gravity data and the S-wave velocity model to derive a 3D density model of the upper-mantle beneath the South Atlantic Ocean to better understand the relationship between ridges, hotspots, and large igneous provinces. The density variations in the upper-mantle are related to thermal and compositional changes. The segment near the Agulhas-Falkland Fracture Zone (AFFZ) is characterized by low-density but inconspicuous low-velocity anomalies reflecting the depleted composition. Its composition is different from the northern part of the SMAR. A low-density layer at 100 km that extends 200 km from the ridge to the Discovery hotspot and that disappears near the AFFZ, is also present. The higher degree of mantle melting beneath the segment near the AFFZ is most probably influenced by the residual plume materials from the Discovery hotspot. Deep low densities beneath the Rio Grande Rise (RGR) and the Walvis Ridge (WR) indicate a higher degree of melting and their formation is probably influenced by the Tristan da Cunha plume. When the RGR is driven away from the plume, its original hot materials cool down and present as cold and dense uppermost mantle deposits. As the plume is located in the African plate, the WR was continuously formed as a seamount chain by intraplate volcanism of mantle plume.

Structure and evolution of the Australian plate and underlying upper mantle from waveform tomography with massive data sets

SUMMARY We present a new S-wave velocity tomographic model of the upper mantle beneath the Australian Plate and its boundaries that we call Aus22. It includes azimuthal anisotropy and was constrained by waveforms from 0.9 million vertical-component seismograms, with the densest data sampling in the hemisphere centred on the Australian continent, using all available data covering this hemisphere. Waveform inversion extracted structural information from surface waves, S- and multiple S-waves and constrained S- and P-wave speeds and S-wave azimuthal anisotropy of the crust and upper mantle, down to the 660-km discontinuity. The model was validated by resolution tests and, for particular locations in Australia with notable differences from previous models, independent inter-station measurements of surface-wave phase velocities. Aus22 can be used to constrain the structure and evolution of the Australian Plate and its boundaries in fine detail at the regional scale. Thick, high-velocity (and, by inference, cold) cratonic lithosphere occupies nearly all of western and central Australia but shows substantial lateral heterogeneity. It extends up to the northern edge of the plate, where it collides with island arcs, without subducting. Diamondiferous kimberlites and lamproite deposits are underlain by cratonic lithosphere, except for the most recent diamondiferous lamproites in the King Leopold Orogen. The rugged eastern boundary of the cratonic lithosphere resolved by the model provides a lithospheric definition of the Tasman Line. Just east of the Tasman Line, an area of intermediate-thick lithosphere is observed in the southern part of the continent. The eastern part of Australia is underlain by thin, warm lithosphere, evidenced by low seismic velocities. All the sites of Cenozoic intraplate volcanism in eastern Australia are located on thin lithosphere. A low-velocity anomaly is present in the mantle transition zone (410–660 km depths) beneath the Lord Howe and Tasmanid hotspots, indicative of anomalously high temperature and consistent with a deep mantle upwelling feeding these hotspots and, possibly, also the East Australia hotspot. High seismic velocities at 200–410 km depth below New Guinea indicate the presence of slab fragments, probably linked to the subduction of the Australian Plate. High seismic velocities are observed in the transition zone below northeast Australia and indicate the presence of subducted lithospheric fragments trapped in the transition zone, possibly parts of the former northern continental margin of Australia.

Electrical resistivity structure across the Late Cenozoic Abaga-Dalinor volcanic field, eastern China, from 3-D magnetotelluric imaging and its tectonic implications

The Abaga-Dalinor volcanic field (ADVF) in Inner Mongolia, China, located to the west of the North-South Gravity Lineament (NSGL), is the largest and less known area of Late Cenozoic intraplate volcanism in Northeast China. Knowledge of the subsurface structure beneath the ADVF will improve our understanding of its evolution process and formation mechanisms. New broadband magnetotelluric (MT) data were collected along a profile of about 300 km that crosses the major basaltic areas exposed at the surface in the ADVF and a series of east-north-east (ENE) trending faults. The electrical resistivity structure of the crust and uppermost mantle across the ADVF was generated by 3-D inversion of full impedance tensor and tipper data. This model reveals a thick high-resistivity layer (>1000 Ωm) in the upper crust that may represent the Precambrian basement and Late Paleozoic-Mesozoic granitic plutons. In contrast, high conductivity anomalies dominate the middle-lower crust. The northward-dipping high-conductivity feature at depths of 20–40 km below sea level under the Chagan Obo fault is suggested to be the involvement of the Early Paleozoic subducted oceanic crust. A remarkably high-conductivity zone (1–20 Ωm) exists in the middle-lower crust beneath the Abaga and Dalinor volcanic areas. It is explained to be a saline fluid zone of ∼0.45%–0.48% that exsolved from the partial melts as magma ascent rapidly. The impermeable upper crust traps these fluids in the middle-lower crust. A moderately high-conductivity anomaly in the uppermost mantle below near the Solonker Suture Zone (SSZ) is attributed to a minimum partial melt of ∼3–4% associated with the localized asthenospheric upwelling, which feeds the Abaga and Dalinor volcanos as a common magma source. Integrating our resistivity model with previous geophysical and geochemical studies, we suggest that intraplate volcanism in the ADVF is mainly controlled by the pre-existing tectonic weak zones. The lithosphere within the SSZ is susceptible to thermal perturbation from the upwelling mantle, providing the preconditions for melt generation. A set of ENE trending faults and an inferred NW-SE trending fault jointly dominated the vertical transport of magma and lateral extension along the faults, which may have influenced the spatial distribution of basalts and volcanic cones at the surface in the ADVF. In addition, we tend to suggest that Late Cenozoic intraplate volcanism in the ADVF could be the result of decompressing melting within the localized small-scale asthenospheric upwelling induced by the edge-driven convection near the NSGL, which is indirectly related to the westward subduction and retreat of the (Paleo-)Pacific plate during the Late Mesozoic to Cenozoic.

Long‐Distance Asthenospheric Transport of Plume‐Influenced Mantle From Afar to Anatolia

AbstractThe origin of widespread volcanism far from plate boundaries and mantle plumes remains a fundamental unsolved question. An example of this puzzle is the Anatolian region, where abundant intraplate volcanism has occurred since 10 Ma, but a nearby underlying plume structure in the deep mantle is lacking. We employed a combination of seismic and geochemical data to link intraplate volcanism in Anatolia to a trail of magmatic centers leading back to East Africa and its mantle plume, consistent with northward asthenospheric transport over a ∼2,500 km distance. Joint modeling of seismic imaging and petrological data indicates that the east Anatolian mantle potential temperature is higher than the ambient mantle (∼1,420°C). Based on multiple seismic tomography models, the Anatolian upper mantle is likely connected to East Africa by an asthenospheric channel with low seismic velocities. Along the channel, isotopic signatures among volcanoes are consistent with a common mantle source, and petrological data demonstrate similar elevated mantle temperatures, consistent with little cooling in the channel during the long‐distance transport. Horizontal asthenospheric pressure gradients originating from mantle plume upwelling beneath East Africa provide a mechanism for high lateral transport rates that match the relatively constant mantle potential temperatures along the channel. Rapid long‐distance asthenospheric flow helps explain the widespread occurrence of global intraplate magmatism in regions far from deeply‐rooted mantle plumes throughout Earth history.

Rapid absolute plate motion changes inferred from high-resolution relative spreading reconstructions: A case study focusing on the South America plate and its Atlantic/Pacific neighbors

The reconstruction of past plate motions relative to a deemed–to–be–fixed hotspot reference frame relies on the sparse occurrence of intraplate volcanism. Consequently, this absolute reference frame often features a temporal resolution that exceeds the rapid kinematic changes observed in plate–to–plate spreading reconstructions, changes recently shown to occur within less than 5 Ma. In this work we put forward an alternative method based on the study of high–resolution relative plate motion data sets. By studying time periods featuring a relatively high probability of plate motion change across multiple spreading ridges, we are able to identify and quantify (likely) changes in absolute plate motion. Specifically, we implement such approach and provide well–defined estimates for the absolute plate motion changes of South America and neighboring plates Nubia, Antarctica, Somalia, North America and Pacific. We find that kinematic changes for all these plates occur between 9 and 5 Ma. For South America, we identify a change also between 14 and 10 Ma. Lastly, we estimate the torque–variations required upon these plates to generate the inferred kinematic changes, which we find to be between ∼5⋅1023 and ∼20⋅1024 N ⋅ m.

The composition of gas emissions at Petite Terre (Mayotte, Comoros): inference on magmatic fingerprints

The Comoros archipelago is an active geodynamic region located in the Mozambique Channel between East continental Africa and Madagascar. The archipelago results from intra-plate volcanism, the most recent eruptions having occurred on the youngest island of Grande Comore and on the oldest one of Mayotte. Since 2018, the eastern submarine flank of Mayotte has been the site of one of the largest recent eruptive events on Earth in terms of erupted lava volume. On land, the most recent volcanic activity occurred in Holocene on the eastern side of Mayotte, corresponding to the small Petite Terre Island, where two main and persistent gas seep areas are present (Airport Beach, namely BAS, and Dziani Dzaha intracrateric lake). The large submarine eruption at the feet of Mayotte (50 km offshore; 3.5 km b.s.l.) is associated with deep (mantle level) seismic activity closer to the coast (5–15 km offshore) possibly corresponding to a single and large magmatic plumbing system. Our study aims at characterizing the chemical and isotopic composition of gas seeps on land and assesses their potential link with the magmatic plumbing system feeding the Mayotte volcanic ridge and the recent submarine activity. Data from bubbling gases collected between 2018 and 2021 are discussed and compared with older datasets acquired between 2005 and 2016 from different research teams. The relation between 3 He/ 4 He and δ 13 C-CO 2 shows a clear magmatic origin for Mayotte bubbling gases, while the variable proportions and isotopic signature of CH 4 is related to the occurrence of both biogenic and abiogenic sources of methane. Our new dataset points to a time-decreasing influence of the recent seismo-volcanic activity at Mayotte on the composition of hydrothermal fluids on land, whose equilibrium temperature steadily decreases since 2018. The increased knowledge on the gas-geochemistry at Mayotte makes the results of this work of potential support for volcanic and environmental monitoring programs.

Triggering of episodic back-arc extensions in the northeast Asian continental margin by deep mantle flow

Back-arc extension has been well documented in subduction plate tectonic regimes. However, the reasons why back-arc extensions are associated with some subduction systems but not others have remained elusive. Here, spatio-temporal variations in the composition of Cenozoic basalts in the northeast Asian continental margin are used to constrain the mechanism for episodic back-arc extensions. Using geochemical data sensitive to tectonic affinity, we show that typical volcanic arc compositions are located in the eastern margin of northeast Asia, whereas coeval intraplate volcanic compositions are located in the western part of northeast Asia, and that the intraplate and arc volcanism exhibit two eastward shifts, from 52 Ma to 33 Ma and from 33 Ma to 21 Ma. Intraplate basalts dated at ca. 11 Ma display a weak, arc-like geochemical signature, which suggests that the upwelling of asthenospheric mantle resulted in the remelting of previously melt-extracted lithospheric mantle modified by slab-derived fluids and the cessation of back-arc extensions. Thus, we propose that the eastward mantle flow resulted in eastward shifts of back-arc extensions that led to the development of extensive Cenozoic arc and intraplate volcanism in the northeast Asian continental margin.