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Submarine landslides exhibiting extreme geometrical and run-out characteristics have been identified and mapped along most continental margins; raising concerns about potential risks to populations should similar events occur. Hazards associated with such events have frequently been assessed using approximations, resulting in data unsuitable for mitigation strategies. Three approximations appear consequential: (i) addressing the problem in two dimensions, thereby neglecting the effect of complex morphology; (ii) employing a deterministic approach that disregards uncertainty related to the heterogeneity of sediment properties; and (iii) treating the sediment as a perfectly elastic–plastic material, simplifying the mechanical behaviour and overlooking the degradation of sediment mechanical properties (strain softening) during different phases of slope movement. Here, we introduced the strain-softening behaviour into a 3D slope stability model. Identification of the critical failure surface was conducted in terms of the probability of failure, considering the influence of sediment parameter variability and uncertainty on the likelihood of failure. The developed model was then used to assess the slope stability of a well-studied example from the literature, the Nice slope (SE France). Our findings indicate that neglecting lateral morphological changes leads to an overestimation of the probability of failure. Additionally, we demonstrated that strain-softening behaviour could significantly affect the factor of safety and the probability of failure for the studied slopes. We argue that a risk assessment and definition of a mitigation strategy require well-advanced characterisation of the mechanical behaviour of sedimentary layers and an analysis incorporating the complex morphology of submarine slopes. [ABSTRACT FROM AUTHOR]

ABSTRACT The tectonic evolution of the Palaeocene–Eocene Tethys Himalayan foreland basin plays a crucial role in reshaping the collisional orogenic process of the Yarlung–Tsangpo oceanic basin. However, studies examining the sedimentary response during the tectonic evolution of the foreland basin are lacking. In this study, through a detailed field investigation and analysis of Palaeocene–Eocene strata in the Tingri area, we clarified the evolution of the Tethys Himalayan tectonic regions and its sedimentary response. Carbon and oxygen isotopes, geochemistry and detrital zircon U–Pb dating demonstrated that the lower Palaeocene Jidula Formation and upper Palaeocene–lower Eocene Zongpu Formation were deposited in a coastal–shallow marine environment, with the detritus sourced from the northern Indian passive margin. The upper Eocene Pengqu Formation was deposited in a deltaic environment, with its detritus sourced from the Gangdese arc and the Yarlung–Tsangpo suture zone at the active continental margin. Combined with the nearshore subaqueous fan branch channel of the Jidula Formation and the slump deformation of the Zongpu Formation, the Palaeocene–Early Eocene southern Tethys Himalaya in the Tingri area was located in the forebulge of the peripheral foreland basin. The marine–continental interactive delta of the Pengqu Formation and its provenance from the Gangdese magmatic arc indicate that the Tingri area was situated in the foredeep of the peripheral foreland basin during the Late Eocene. The study provides valuable insights into the collisional orogenic processes between the Indian and Eurasian plates. [ABSTRACT FROM AUTHOR]

ABSTRACT The northern continental margin of the South China Sea (SCS) is an important component of deep‐water circulation, providing excellent conditions for studying bottom currents in a marginal sea. Seismic data were employed to discern the sedimentary patterns prevalent in the deep‐water continental slope sediments on the northern continental margin of the SCS, encompassing gravity flow, contourite and mixed depositional systems. The contourite depositional system includes various types of deposits (such as separated mounded drifts, patch or channel‐related drifts, deformed sheeted drifts, composite drifts, bottom current sediment waves, plastered contourite drifts) and various morphologic erosional features eroded by the bottom current (such as moats, non‐depositional surfaces, troughs and scarps). These contourite features are related to the continental slope's morphology and its sources. The Dongsha slope exhibits distinctive characteristics marked by intense bottom current erosion and deposition, featuring separated mounded drifts and deformed sheeted drifts along its lower slope. The lower slope of the Pearl River showcases a spectrum of bottom current‐induced features, including sediment wave fields, erosion fields and contourite drifts. The southern flank of the Shenhu slope is characterised by a bottom current erosion field, a non‐depositional surface, a sediment wave field and isolated mounded drifts. On the Yingqiong slope, the contourite drifts are limited to its southern flank where gravity flow action is absent, and the complex geomorphology interacts with the bottom current, forming a complex contourite depositional system. The results of this study serve as a foundational framework for further global research on bottom current circulation and hydrodynamics. [ABSTRACT FROM AUTHOR]

The current study reports the occurrence of a deformed Paleoproterozoic K-rich metasedimentary rock unit at the eastern margin of Bastar Craton, India. Geochemical studies of the metasedimentary rocks suggest the presence of an active continental margin at the eastern part of the Bastar Craton. Detailed petrological and mineralogical studies suggest that K-feldspar, quartz, chlorite, muscovite, kaolinite, and ± biotite represent the primary mineral assemblage in the rock unit. Combined mineral thermobarometric and P–T pseudosection estimate the peak metamorphic condition (i.e. M3 metamorphic) at T ~ 550 °C, P ~ 0.5 GPa. Furthermore, monazite grains (about 20–60 μm in diameter) were dated using the in-situ U–Th–Pbtotal dating method under EPMA. Based on composition and texture, monazite spot ages can be categorized into four groups: Group I, Group II, Group III, and Group IV. The mean age of the oldest monazite population (Group I) at ~ 2100 Ma suggests vertical sagging of the sedimentary basin and the associated M1 metamorphism. The M2 metamorphic event is represented by the Group II monazite population's mean age of ~ 1629 Ma. Rhyolite magmatism in the eastern margin of the Bastar Craton may be connected to this specific event. The Group III monazite mean age of ~ 1533 Ma represents the M3 metamorphic event (peak metamorphic condition). This event could be linked to the accretionary growth at the Bastar Craton's eastern boundary during the assembly of the Columbia supercontinent. Furthermore, the M2 and M3 events may correlate with orogenic events recorded in the Central Indian Tectonic Zone at ca 1.62 and 1.5 Ga, respectively. Finally, the mean age of ~ 528 Ma for the Group IV monazite population represents Pan-African orogenic activity along the eastern margin of the Bastar Craton. [ABSTRACT FROM AUTHOR]

The study is dedicated to geological interpretation of high-resolution seismoacoustic data acquired in the Central Atlantic during cruise 60 of the R/V Akademik Ioffe. Seismic facies of abyssal plane deposits, contourites, gravitites, and sediment waves have been identified based on specific characteristics of the seismic structure of the upper sediment cover. Geological interpretation was conducted considering the results of lithological studies of sediment cores collected during research cruises of the Shirshov Institute of Oceanology, Russian Academy of Sciences, as well as deep-sea drilling data. The role of gravity-driven processes in the formation of near-surface deposits is most significant in the region adjacent to the continental margin of South America. In the entire study area, bottom currents are considered as one of the main agents of sediment cover formation, responsible both for the deposition of contourite drifts and stratified hemipelagic sediments. [ABSTRACT FROM AUTHOR]

We investigate, using observations from seismic reflection data, the lateral variability in breakup extrusive magmatic addition along the strike of the Pelotas segment of the austral South Atlantic rifted margin and its control on post-rift accommodation space and sediment deposition. Our analysis of regional seismic reflection profiles shows that magmatic addition on the Pelotas margin varies substantially along strike from extremely magma-rich to magma-normal within a distance of ∼300 km. Using 2D flexural back-stripping, we determine the post-rift accommodation space above top volcanics. In the north, where volcanic seaward-dipping reflectors (SDRs) are thickest, the Torres High shows SDRs up to ∼20 km thick, and post-breakup water-loaded accommodation space is much less than in the south, where magmatic addition is normal and SDRs are thinner. We show that post-breakup accommodation space correlates inversely with SDR thickness, being less for magma-rich margins and more for magma-normal/intermediate margins. The Rio Grande Cone, with large sediment thickness, is underlain by small SDR thicknesses allowing large post-breakup accommodation space. A relationship is observed between the amount of volcanic material and the two-way travel time (TWTT) of first volcanics: first volcanics are observed between 1.2 and 2.2 s TWTT for the highly magmatic Torres High profile, while, in contrast, for the normally magmatic profiles in the south, first volcanics are observed between 4.2 and 6.5 s TWTT. The observed inverse relationship between post-breakup accommodation space and SDR thickness is consistent with predictions by a simple isostatic model of continental lithosphere thinning and magmatic addition melting during breakup. The methodology that we use in this paper provides a new approach for investigating the complex magmatic and sedimentary evolution of rifted continental margins. [ABSTRACT FROM AUTHOR]

The horizontal propagation of slab detachment (slab tearing) is known to control lateral migration of the mountain uplift along the collisional belt. However, along‐strike differential collision due to an oblique passive margin geometry can make the topography response more complex. In this study, we employ 3D thermomechanical modeling to distinguish between the lateral migration of the mountain topography driven by slab tearing and oblique continental collision itself. In our models, slab breakoff is triggered by the transition from oceanic to continental subduction, occurring earlier on one side of the passive margin than on the other due to the initial oblique configuration. However, once slab breakoff has begun, it spreads horizontally in the form of tearing at high velocity (∼38–118 cm yr−1), and associated topographic uplift also propagates with the same velocity. In contrast, the along‐strike migration of subsequent continental collision and related topographic uplift propagation is typically much slower (∼2–34 cm yr−1). Similarly, the vertical magnitude of surface uplift caused by slab tearing is higher (up to 10 mm yr−1) than the following collision phase (<4 mm yr−1). The parametric analysis reveals that slab tearing velocity and the associated horizontal propagation of mountain uplift depends on obliquity angle and slab age, whereas the migration of collision‐induced topographic growth is controlled by the convergence velocity and obliquity angle. Finally, we show that presence of microcontinental block detached from the passive margin leads to spatial and temporal transition from horizontal to vertical slab tearing and more intense syn‐collisional mountain building. Plain Language Summary: Continental collision begins after the complete subduction of the oceanic domain, initially located between the colliding continents. As the oceanic slab has negative buoyancy, it detaches from the positively buoyant continental plate shortly after the start of continental collision. Slab detachment leads to a rapid uplift of the surface topography and usually moves sideways, a process known as slab tearing. Slab tearing in turn results in a lateral propagation of the topographic uplift along the continental margin. However, if the colliding plate margins are initially at an angle to each other, an oblique continental collision occurs, which can also lead to along‐strike migration of the mountain uplift. To differentiate between the topography response to slab tearing and oblique continental collision, we have carried out 3D numerical modeling. Our parametric analysis reveals that the rate of slab tearing and the associated horizontal propagation of mountain uplift primarily depend on the angle of collisional obliquity and the age of the subducting oceanic slab, whereas the velocity of plate convergence controls the migration of collision‐induced topographic growth. Our findings also indicate that the surface uplift associated with slab tearing is more pronounced and spreads laterally much faster than during following oblique continental collision. Key Points: We explored numerically in 3D the topography response to horizontal slab tearing during retreating oblique continental collisionHorizontal slab tearing can initiate before collision due to the transition from oceanic to continental subduction below fore‐ and back‐arc domainSurface uplift associated with slab tearing is more pronounced and spreads laterally much faster than in the subsequent collision phase [ABSTRACT FROM AUTHOR]

Accommodation space governs the spatial and temporal distributions of sediments in continental margins. Mapping the sedimentation patterns, therefore, offers insights into the solid‐Earth processes that shape accommodation space. We assembled an unprecedented amount of seismic and borehole data along the Eastern North American Margin and used it to divide the margin's sedimentary package into eight chronostratigraphic intervals, identifying temporal shifts in depocenters under the continental shelf, slope, and rise. The Jurassic depocenters follow the syn‐rift structure and its thermal subsidence loci. The Long Island Platform is the only margin segment where the early post‐rift sediment thickness matches subsidence predictions from uniform‐stretching models, whereas in Georges Bank Basin (GBB) and Baltimore Canyon Trough (BCT), sediment thickness is 1.5–3 times higher than predicted, pointing to other factors at play. A margin‐wide Jurassic transient shoulder uplift is inferred from the occurrence of stratigraphic onlaps above thinned crust. Unlike the Jurassic, the Cretaceous and Cenozoic depocenters disregard the inherited subsidence pattern. The accommodation space over the shelf and coastal plain during the Cretaceous was affected by regional isostatic compensation of the sedimentary loads accumulated on the shelf and rise. Accommodation space development in the GBB was interrupted during the Cretaceous after the margin crossed the Great Meteor Hotspot track, resulting in a widespread permanent uplift, erosion, and sediment redistribution. The distribution of anomalous Neogene subsidence in the BCT challenges previous suggestions of mantle dynamic control on the accommodation space and favors flexural downwarping of the shelf by sediment accumulation on the rise. Plain Language Summary: The volume available for sediment accumulation at continental margins is key for determining where, when and how much sediments are stored. To explore the processes that determine this volume we assembled a broad geological and geophysical data set that covers the US Atlantic margin from Cape Hatteras to the US‐Canada border. Using these data, the margin's sedimentary cover was divided into eight time‐constrained intervals spanning the last ∼190 Myr. The thickness distribution of these intervals was used to investigate the processes that regulated the space available for sediment accumulation. During the first ∼50 Myr of the margin life span, most of the deposition occurred over areas that were thinned and heated during the margin's formation and later cooled and subsided. During this period, the deepest parts of the margin subsided twice as fast as predicted by conventional models whereas the margin's inland rims experienced uplift. Later, subsidence of the coastal plain by basinward sediment loading allowed sediment accumulation. The passage of the margin over a deep‐seated thermal anomaly resulted in uplift and erosion. An anomalous Neogene sediment accumulation on the continental shelf is explained by subsidence inflicted by seaward sediment loading. Key Points: We present a new stratigraphic framework for the post‐rift period of the US Atlantic marginThe syn‐rift shaping of the lithosphere is key for determining accommodation space during the post‐rift periodObserved deviations from passive thermal subsidence patterns resulted from flexure, hot spot passage, flank uplift or mantle dynamics [ABSTRACT FROM AUTHOR]

Direct geological information in Antarctica is limited to ice free regions along the coast, high mountain ranges, or isolated nunataks. Therefore, indirect methods are required to reveal subglacial geology and heterogeneities in crustal properties, which are critical steps toward interpreting geological history. We present a 3D crustal model of density and susceptibility distribution in the Wilkes Subglacial Basin (WSB) and the Transantarctic Mountains (TAM) based on joint inversion of airborne gravity and magnetic data. The applied "variation of information" technique enforces a coupling between inferred susceptibility and density, relating these quantities to the same gravity and magnetic sources to give an enhanced inversion result. Our model reveals a large body located in the interior of the WSB interpreted as a batholithic intrusive structure, as well as a linear dense body at the margin of the Terre Adélie Craton. Density and susceptibility relationships are used to inform the interpretation of petrophysical properties and the reconstruction of the origin of those crustal bodies. The petrophysical relationship indicates that the postulated batholitic intrusion is granitic, but independent from the Granite Harbor Igneous Complex described previously in the TAM area. Emplacement of a large volume of intrusive granites can potentially elevate local geothermal heat flow significantly. Finally, we present a new conceptual tectonic model based on the inversion results, which includes development of a passive continental margin with seaward dipping basalt horizons and magmatic underplating followed by two distinct intrusive events associated with the protracted Ross Orogen. Plain Language Summary: Most rocks in Antarctica are hidden beneath a thick ice sheet. Therefore, indirect techniques are required to reveal rock provinces within Earth's crust below the ice. Rocks simultaneously influence the gravity and magnetic fields through their physical properties (density and susceptibility). Here we use both the gravity and magnetic fields to reveal rock provinces beneath the ice and use the relationship between density and susceptibility of the rocks to interpret the distribution of granitic rocks of the Transantarctic Mountains and the Wilkes Subglacial Basin region in East Antarctica. Granitic rocks can lead to elevated heat flow due to radiogenic decay of minerals within the rock and thus influence the overlying ice sheet. Based on our subsurface model of rock provinces we speculate on the tectonic evolution of the region. Key Points: We present a new 3D crustal density and susceptibility distribution model based on joint inversion of gravity and magnetic dataDensity and susceptibility data are used to identify crustal level intrusions and the craton marginOur new conceptual tectonic model identifies a earlier intrusive event in the interior of the Wilkes Subglacial Basin, separate from those exposed in the Transantarctic Mountains [ABSTRACT FROM AUTHOR]

Despite significant study, when and how plate tectonics initiated on Earth remains contentious. Geologic evidence from some of Earth's earliest cratons has been interpreted as reflecting the formation of initial continental blocks by non‐subduction processes, which then trigger subduction initiation at their margins. Numerical models of mantle convection with a plastic yield stress rheology have shown this scenario is plausible. However, whether continents can trigger subduction initiation has not been tested with other rheologies. We, therefore, use numerical models of mantle convection with an imposed continental block to test whether continents facilitate subduction initiation with a grain‐damage mechanism, where weak shear zones form by grain size reduction. Our results show that continents modestly enhance stresses in the lithosphere, but not enough to significantly impact lithospheric damage or subduction initiation: continents have minimal influence on lithospheric damage or plate speed, nor does subduction preferentially initiate at the continental margin. A new regime diagram that includes continental blocks shows only a small shift in the boundary between the mobile‐lid and stagnant‐lid regimes when continents are added. However, as we do find that stresses are modestly enhanced at the continental margin in our models, we develop a scaling law for this stress enhancement to more fully test whether continents could trigger subduction initiation on early Earth. We find that lithospheric stresses supplied by continents are not sufficient to initiate subduction on the early Earth on their own with grain‐damage rheology; instead, additional factors would be required. Plain Language Summary: How plate tectonics initiates on Earth is controversial. One possibility is that primitive continents trigger subduction initiation at their margins; an idea our study tests with computer models. We specifically use a mechanism called grain‐damage in our model, where weak zones that allow for subduction develop due to grain size reduction. The results showed only modest effects on lithospheric damage from the addition of continents. Continents do not appear to dictate the subduction initiation location, as subduction initiates readily both at continent margins and far from margins. In addition, plate boundaries do not get weaker nor do plates move faster with continents included. We observe slightly higher stress at continental margins, which leads to plate tectonics developing for a few models where it otherwise would not have. However, scaling our results to early Earth conditions, we find continents do not impart enough stress in the lithosphere on their own for subduction to initiate by grain size reduction. Other factors may have been important during this critical phase in our planet's history. Key Points: We test whether continents can trigger subduction initiation on the early Earth using convection models with grain‐damageModel results show continents do not necessarily lead to subduction initiation, imparting only a minor increase in lithospheric stressScaling analysis shows that continents are not capable of triggering subduction on the early Earth [ABSTRACT FROM AUTHOR]

The study of sedimentary mélanges holds pivotal importance in understanding orogenic processes and unveiling geodynamic mechanisms. In this study, we present findings on zircon U‐Pb isotopes and whole‐rock elemental data concerning the recently uncovered Zongzhuo Formation sedimentary mélanges within the Dingri area. Field observations reveal the predominant composition of the Zongzhuo Formation, characterized by a matrix of sandstone‐mudstone mixed with sand‐conglomerates within native blocks exhibiting soft sediment deformation. Moreover, exotic blocks originating from littoral–neritic seas display evidence of landslide deformation. Our study identifies the depositional environment of the Zongzhuo Formation in Dingri as a slope turbidite fan, with its provenance traced back to the passive continental margin. Notably, this contrasts with the Zongzhuo Formation found in the Jiangzi–Langkazi area. Based on existing data, we conclude that the Zongzhuo Formation in the Dingri area was influenced by the Dingri–Gamba fault and emerged within a fault basin of the passive continental margin due to Neo‐Tethys oceanic subduction during the Late Cretaceous period. Its provenance can be attributed to the littoral–neritic sea of the northern Tethys Himalaya region. This study holds significant implications for understanding the tectonic evolution of Tethys Himalaya and for reevaluating the activity of the Dingri–Gamba fault, as it controls the active deposition of the Zongzhuo Formation. [ABSTRACT FROM AUTHOR]

The diversity of the tholeiitic, alkali or calc-alkali, high-silica, and adakitic rocks (I-, FG- and А-types) in the northeastern margin of Asia often leads to controversial conclusions about the character of magmatism and the active margin evolution in the West Pacific during the Late Mesozoic–Cenozoic. In addition, paleogeodynamic reconstructions of the Asia-Pacific region can only be made based on the synthesis of patterns identified in compositions of magma sources, evolution and character of magmatism, and strain and kinematic analysis of fault tectonics of all parts of the eastern margin of the paleo-Asian continent. The paper presents new isotopic, geochemical, and structural data on one of the largest early Paleogene structures located at the continental margin of the southern Sikhote-Alin. Based on the new and previously published data we have concluded that the tectonic and magmatic settings of the region were shaped by oblique interaction of continental and oceanic plates during the Late Cretaceous-early Paleogene. The igneous products of the frontal (FG-type), in respect to the trench, and intracontinental (А-type) parts of the area are characterized by isotopic-geochemical variations caused by thermal changes in upwelling asthenosphere and by interaction with lithospheric components. These processes accounted for the formation of initial magmas similar to the IAB- or OIB-type mantle source, respectively. [ABSTRACT FROM AUTHOR]

We present significant findings of the Kausani Granite Gneiss within the Inner Lesser Himalayan Sedimentary Zone (iLHSZ) just north of the North Almora Thrust (NAT). The Kausani Granite Gneiss body lies within the quartzite of the Someshwar Formation and has a tectonized contact and a discordant relationship on the north side. Detailed seismic profiling across the body also confirms a similar result. Cathodoluminescence images of zircon from the Kausani body show no inheritance of older cores. The U–Pb ages from the zircon populations separated from the Kausani body give a crystallization age of 1866 ± 3 Ma. Along with the Upalda granite gneiss and Toneta granite gneiss near the Alakhnanda Thrust in the Garhwal Himalaya and the Dungeshwari granite gneiss near the Dailekh Thrust in Nepal, the Kausani Granite Gneiss north of NAT Kumaun Himalaya forms a major terrain boundary. These gneissic bodies mark the southernmost extent of felsic magmatism at NAT, rather than the Main Central Thrust. An about 1.8 billion years‐old magmatic event in the LHSZ suggests that it is a currently active continental margin inside the 'Greater India' region, now situated in the Himalayan domain. However, the Pb‐loss modelling of the U–Pb zircon data reveals thermal events during the Himalayan Orogeny (~45 Ma). [ABSTRACT FROM AUTHOR]

The amalgamation and breakup of the West Gondwana shaped the South American platform. The dynamics during the processes can be reflected by crust anisotropy of the platform, but there are no specialized crustal anisotropic measurements yet. Splitting analysis of Moho-converted shear waves in P -wave receiver functions (Pms) can reveal crustal-scale anisotropy, which is important for understanding the dynamic evolution of the crust and for the interpretation of mantle anisotropy from splitting analysis of core–mantle refracted shear waves (XKS phases). This study measured crustal anisotropy for the old and stable South American platform by Pms splitting analysis. The splitting times vary mainly in the range of 0–0.5 s, with a regional mean of 0.2 s, slightly lower than that observed in tectonically active regions. The detected crustal anisotropy shows distinct characteristics and spatial zoning, providing insights into tectonic processes. (1) Fast polarization directions at stations close to the Transbrasiliano Lineament (TBL) are oriented NNE–SSW, generally consistent with the strike of the TBL but inconsistent with the maximum horizontal compressive stress, implying that they might be formed by dynamic metamorphism during the formation of the TBL. (2) Crustal anisotropy along the passive continental margin in the east and northeast is weak. Still, the fast polarization directions tend to be oriented along the margin, implying the existence of fossil extensional crustal fabrics formed during the continental rifting of West Gondwana. (3) The Paraná Basin, one of the world's largest Large Igneous Provinces (LIP) covered by continental flood basalts, shows distinctively strong anisotropy, with fast polarization directions highly aligned with mantle anisotropy, implying that synchronous crust–mantle deformation occurred in these regions as a result of magmatism during the breakup of West Gondwana. [ABSTRACT FROM AUTHOR]

The subduction and rollback of the paleo-Pacific plate during Mesozoic time was the key engine for the evolution of the continental margin in eastern China. It led to lateral accretion of continental crust in Northeast China, lithospheric destruction beneath the North China Craton, and the generation of huge volumes of felsic magmatic rocks in South China. This had a profound influence on deep material cycles and the evolution of epigenetic environmental systems along the continental margin of East Asia. To fully understand the transformation of the dynamic mechanism during the subduction and rollback of the paleo-Pacific plate, we have attempted to trace the remnants and fragments of the subducted paleo-Pacific plate at great depths. Such remnants in both temporal and spatial dimensions can be tracked by using geochemical and geophysical approaches. Studies of the trace elements, Mg-Zn isotopes and Os-Nd-Hf-Pb-O isotopes in continental basalts from eastern China reveal a significant number of the remnants of subduction of the paleo-Pacific plate, and the initial subduction can be traced back to the Early Jurassic. Large-scale geophysical imaging unveils a multitude of high-velocity anomalies in the lower mantle of East Asia. Notably, many high-velocity bodies, aptly referred to as "slab graveyards", are nestled at the base of the lower mantle. Numerous isolated high-velocity anomalies are also present in the upper part of the lower mantle, creating conduits for the descent of the subducted slabs into the lower mantle. However, a resolution of the remnants for the subducted slabs within the lower mantle are quite low. Consequently, their impact on the lower mantle's dynamics is yet to be thoroughly investigated. Finally, the presently observed big mantle wedge (BMW) in East Asia has developed through subduction of the Pacific plate in the Cenozoic. However, following the rollback of the paleo-Pacific plate (began at ∼145 Ma), a Cretaceous BMW system would also form above the mantle transition zone in East Asia. There are significant differences in tectonic-magmatic processes and basin-forming and hydrocarbon-accumulation processes among different regions along the East Asian continental margin. Such differences may be controlled by variations in the speed and angle of rollback of the paleo-Pacific plate. [ABSTRACT FROM AUTHOR]

The Gysian ophiolite of NW Iran is located at the intersection of the ophiolite belts of SE Turkey, NE Iraq, and Iran, and provides the opportunity to investigate the preserved subduction and obduction history of an important tectonic site that has not previously been studied. The serpentinized peridotites of the Gysian ophiolite contain the assemblage lizardite + chrysotile + spinel/Mg-spinel with relict clinopyroxene (diopside) and very rare relict orthopyroxene and olivine. The compositions of clinopyroxenes and spinels are more consistent with the formation of the inferred protolith harzburgites in a fore-arc or supra-subduction zone instead of an abyssal environment. The Gysian ophiolite is a remnant of the Neo-Tethyan arc-ophiolitic system and records shallow subduction (< 50 km, indicated by the absence of antigorite) in the Late Cretaceous to Paleocene before obduction along thrust faults over the continental margin. We review the spatial trends of the metamorphic grade of the Neo-Tethyan ophiolites in this region and provide detailed information about the petrology and mineral chemistry of the Gysian ophiolite. [ABSTRACT FROM AUTHOR]

The petrographic and geochemical attributes of the Oligocene Barail Group of rocks are used to decipher the likely source area(s) or tectonic domains, as this sequence of rocks was deposited in a foreland basin governed by orogenic domain, namely the North-east Arunachal Himalayas. The river system that gave rise to the Brahmaputra River (Yarlung-Tsangpo), which flowed through several tectonic domains of the Himalayan ranges, primarily from Bomi-Chayu, Gangadese Granitoid, Higher Himalayan Leucogranites, and Namche Barwa into the proto Bengal Basin now a part of Assam Arakan Basin and Naga Schuppen Belt, was the main source of the sandstone formation of the Barail Group. The purpose of sandstone petrography, which combines modal analysis with XRF (Major Oxides) and HR-ICP-MS (Trace & Rare Earth Elements) research, is to identify the type of source rock(s), their weathering pattern, and its paleo-environmental circumstances. These sandstones were formed from recycled orogen and include lithic and sublithic arenite variants with advanced texture and chemical maturity. The sediments were felsic (Th/Co: 1.38, Cr/Th: 9.78, La/Lu: 11.58, Th/Sc: 0.99, Eu/Eu*: 0.66, La/Sc: 3.05, La/Co: 4.18), with contributions from intermediate source rocks and low-rank metamorphics deposited in an active continental margin to a continental island arc setting. Climatic conditions impacted the sediments of Barails, characterised by being warm and semi-humid to humid which resulted in moderate to a high degree of chemical weathering, as shown by weathering indices like CIA (79.14), PIA (85.47), CIW (86.9), WIP (32.50), ICV (0.71), and Th/U (6.03), which were further additionally supported by C-Value (1.01), PF (1.20), Sr/Cu (2.04), and Rb/Sr (0.97). [ABSTRACT FROM AUTHOR]

The results of geochemical and Nd isotopic studies of rocks and U–Th–Pb (LA-ICP-MS) geochronological and Hf isotopic studies of detrital zircon from metaterrigenous rocks of the Kodar Group of the Udokan Complex (Aldan Shield) are presented. It is established that the age of rocks of the Kodar Group is 1.99–1.91 Ga, whereas the age of rocks of the Chinei and Kemen groups of the Udokan Complex is 1.90–1.87 Ga (Kovach et al., 2018, 2023a). This allows us to raise the issue of recognition of the Kodar Group as an independent stratigraphic subdivision. The terrigenous rocks of the group were sourced from the Archean igneous and metamorphic rock of the Chara–Olekma Geoblock and probably the Kalar and Kurulta blocks of the Stanovoi suture zone, as well as the Paleoproterozoic (2.04–1.99, 2.08, 2.20, and 2.30 Ga) complexes of active continental margins or ensialic island arcs in the western–northwestern and southern (in the present-day coordinates) frame of the Chara–Olekma Geoblock, which are unidentified in the region at the current erosion level. Erosion of rocks of magmatic arcs and the continental slope led to the deposition of rocks of the Kodar Group in a retroarc foreland basin, whereas further collapse of the orogen and the formation of an intracontinental extension basin were responsible for the deposition of terrigenous rocks of the Chinei and Kemen groups. The obtained data indicate widespread previously unidentified Paleoproterozoic continental crust formation at about 2.04–1.97 Ga in the western part of the Aldan Shield. [ABSTRACT FROM AUTHOR]

This study focuses on the post-Miocene neotectonic evolution of the slope-perched sedimentary basins around the Eastern Korean Continental Margin and which category these basins could be classified into. The Hupo Basin, the largest neotectonic sedimentary basin on the Eastern Korean Continental Margin, comprises three key geological components: 1) Neotectonic Sequence, which has divergently filled the Hupo Basin since the Early Pliocene, 2) Hupo Fault, a N-S striking, high-angle thrust (or reverse) fault that bounds the Hupo Basin to the east, and 3) Hupo Bank, a N-S elongated submarine wave-cut platform that sits on top of the uplifted eastern Hupo Fault block. On the basis of stratigraphic and structural restoration, we conclude that the Hupo Basin was formed by the compressional uplift of the Hupo Bank during the incipient subduction of the Ulleung Basin crust after the Miocene. Given the morphostructural features of the Hupo Basin and its host incipient subduction system, we categorize it as a sort of piggyback basins. The basin-forming process shown in this study is commonly found in numerical models that simulate modern cases of subduction initiation around the world. [ABSTRACT FROM AUTHOR]

Mineral-rich hardgrounds, such as ferromanganese (FeMn) crusts and phosphorites, occur on seamounts and continental margins, gaining attention for their resource potential due to their enrichment in valuable metals in some regions. This study focuses on the Southern California Borderland (SCB), an area characterized by uneven and heterogeneous topography featuring FeMn crusts, phosphorites, basalt, and sedimentary rocks that occur at varying depths and are exposed to a range of oxygen concentrations. Due to its heterogeneity, this region serves as an optimal setting for investigating the relationship between mineral-rich hardgrounds and benthic fauna. This study characterizes the density, diversity, and community composition of macrofauna (>300 μm) on hardgrounds as a function of substrate type and environment (depth and oxygen ranges). Rocks and their macrofauna were sampled quantitatively using remotely operated vehicles (ROVs) during expeditions in 2020 and 2021 at depths above, within, and below the oxygen minimum zone (OMZ). A total of 3,555 macrofauna individuals were counted and 416 different morphospecies (excluding encrusting bryozoans and hydrozoans) were identified from 82 rocks at depths between 231 and 2,688 m. Average density for SCB macrofauna was 11.08 ± 0.87 ind. 200 cm−2 and mean Shannon-Wiener diversity per rock (H′[loge]) was 2.22 ± 0.07. A relationship was found between substrate type and macrofaunal communities. Phosphorite rocks had the highest H′ of the four substrates compared on a per-rock basis. However, when samples were pooled by substrate, FeMn crusts had the highest H′ and rarefaction diversity. Of all the environmental variables examined, water depth explained the largest variance in macrofaunal community composition. Macrofaunal density and diversity values were similar at sites within and outside the OMZ. This study is the first to analyze the macrofaunal communities of mineral-rich hardgrounds in the SCB, which support deep-ocean biodiversity by acting as specialized substrates for macrofaunal communities. Understanding the intricate relationships between macrofaunal assemblages and mineral-rich substrates may inform effects from environmental disruptions associated with deep-seabed mining or climate change. The findings contribute baseline information useful for effective conservation and management of the SCB and will support scientists in monitoring changes in these communities due to environmental disturbance or human impact in the future. [ABSTRACT FROM AUTHOR]

Traditionally, 3D modeling of marine controlled-source electromagnetic (CSEM) data (in the frequency domain) involves high-memory demand, requiring solving a large linear system for each frequency. To address this problem, we propose to solve Maxwell's equations in a fictitious dielectric medium with time-domain finite-difference methods, with the support of the correspondence principle. As an advantage of this approach, we highlight the possibility of its implementation for execution with GPU accelerators, in addition to multi-frequency data modeling with a single simulation. Furthermore, we explore using the correspondence principle to the inversion of CSEM data by calculating the gradient of the least-squares objective function employing the adjoint-state method to establish the relationship between adjoint fields in a conductive medium and their counterparts in the fictitious dielectric medium, similar to the approach used in forward modeling. We validate this method through 2D inversions of three synthetic CSEM datasets, computed for a simple model consisting of two resistors in a conductive medium, a model adapted from a CSEM modeling and inversion package, and the last one based on a reference model of turbidite reservoirs on the Brazilian continental margin. We also evaluate the differences between the results of inversions using the steepest descent method and our proposed momentum method, comparing them with the limited-memory BFGS (Broyden–Fletcher–Goldfarb–Shanno) algorithm (L-BFGS-B). In all experiments, we use smoothing by model reparameterization as a strategy for regularizing and stabilizing the iterations throughout the inversions. The results indicate that, although it requires more iterations, our modified momentum method produces the best models, which are consistent with results from the L-BFGS-B algorithm and require less storage per iteration. [ABSTRACT FROM AUTHOR]

The northwestern continental margin of the South China Sea, extending from the broad shelf and canyon-growth slope across the Xisha Trough, represents a relic crustal rift that eventually connects to the abyssal plain. However, the provenance and source-to-sink process of sediments in this special topography remains unclear. In this study, we present the detrital zircon U-Pb geochronology of surface sediments from the northwestern margin of the South China Sea to identify the sediment provenance, track the transport pathway, and understand the source-to-sink process in this margin. The results showed that detrital zircons exhibit a wide range of U-Pb ages from 3,062.9 to 41.5 Ma and are characterized by peaks centered on 140–154 Ma, 240–258 Ma, 425–452 Ma, and 738–991 Ma. Statistical analysis indicated that the clastic sediments on the shelf closely resemble those sourced from the Pearl River system. Conversely, sediments on the slope and abyssal plain demonstrated characteristics indicative of multiple sources, primarily originating from the Pearl River, with a minor contribution from the Red River. Further quantitative study revealed a progressive increase (from 8.2% to 43.2%) in the prevalence of Red River-derived sediments with ascending water depth on the shelf and slope, whereas sediment influx in the deepest sector remains predominantly sourced from the Pearl River. This study emphasizes the controls of topography and currents in modulating the transportation of coarse-grained and fine-grained sediments within the sourceto- sink system and provides a favorable solution for reconstructing the sedimentary process in continental margins. [ABSTRACT FROM AUTHOR]

The East Pisco Basin, occupying the coastal plain of Peru between 13°S and 16°S, is widely known for its extensive Eocene to Quaternary biosiliceous deposits and excellent preservation of fossil marine vertebrates. Biochronologic studies published over the past 35 years record a hiatus of about 13 million years (*32-19 Ma) separating the youngest Paleogene deposits (Otuma Formation) from the oldest Neogene deposits (Chilcatay Formation). We describe a newly identified Lower Miocene depositional sequence that lies below documented Chilcatay strata, rich in diatoms and benthic foraminifers, which we name the Tunga Formation. A low-latitude diatom assemblage from the Tunga Formation indicates an age of 21.6 to 20.5 Ma age, whereas an ash within basal Tunga strata yields an 206Pb/238U weighted mean age of 20.58 ± 0.13 Ma. Benthic foraminifer paleodepth analysis and the diatom assemblage of the Tunga Formation indicate deposition took place along the continental margin at upper bathyal depths under hypoxic conditions beneath highly productive waters, a scenario also supported by the presence of abundant clupeoid fish scales and dolomitized horizons. The Tunga Formation is characterized by a profound scarcity of cetacean fossils, unlike the cetacean-rich neritic sediments of the overlying Chilcatay Formation. [ABSTRACT FROM AUTHOR]

Unravelling source‐to‐sink relationships of sediment in coastal regions can be particularly challenging due to a variety of transport directions and mixing within varying local environments in response to sea level fluctuations. Post‐glacial sea level rise in the Holocene has resulted in the flooding of former continental margins, locally leading to the separation of islands such as Rottnest in southwest Australia. Rottnest lies approximately 20 km offshore from the mouth of the Swan River, one of the largest permanent river systems across thousands of kilometres of west Australian coastline. In this contribution, we investigate the size, U–Pb age distribution and α‐dose values of detrital zircon grains within 13 sand samples collected from three upstream tributaries that drain the Archean Yilgarn Craton, the Swan River estuary, offshore waters surrounding Rottnest Island and modern beaches. We explore sediment derivation, storage and mixing on this passive margin. Carbonate–silicate sands of the region contain detrital zircon with Archean, Mesoproterozoic and Cambro‐Neoproterozoic age modes, reflecting regional crystalline basement. Eo‐ to Paleoarchean zircon grains, including a previously enigmatic >3500 Ma component, are traced from offshore into the estuary, and specifically the Avon River tributary. Detrital mixing models imply an overall fluvial contribution to the estuary and offshore systems of up to 50–65%. By contrast, modern beach samples are dominated by Swan Coastal Plain recycled sediment of up to 96%. The α‐dose values of the prominent 3300–3150 Ma age component suggest more efficient fluvial discharge in the Paleo‐Swan River than in more recent times. Modern estuary samples have lower average and progressively lower downstream zircon α‐dose values, consistent with prolonged chemical and physical reworking and loss of metamict grains with transport distance in the river. We conclude that fluvial drainage networks distribute a locally persistent catchment signal whilst coastal plains in tectonically quiescent settings appear characterized by sediment reprocessing and mixed provenance. [ABSTRACT FROM AUTHOR]

The ca. 150–135 Ma magmatic belts in the Middle–Lower Yangtze River Valley and its southern adjacent parallel northeastern Jiangnan Orogen in China experienced largescale Cu and W metallogenesis, respectively. The magmatic belts interrupted contemporaneous magmatism along the east China continental margin and are an ideal locality to verify if Cu and W metallogenesis could be linked to a slab window. Oceanic plate subduction, intracontinental extension, lower crustal delamination, or ridge subduction models have been debated for the Cu and W belts. However, these models have weaknesses that cannot explain unique features of the belts such as the belts being coeval and intersecting with the trend of the paleo-trench, or the asymmetric distribution of the magmatic-metallogenic belts along both sides of the Middle–Lower Yangtze River Valley. Based on the configurative, compositional, and isotopic evidences, the magmatic belts record the fingerprints of the slab window generated by disassembly of the paleo-ridge between the Paleo-Pacific and Izanagi plates at ca. 150–135 Ma. During this time, the Paleo-Pacific plate rolled back beneath the eastern margin of the South China block and the Izanagi plate experienced flatslab subduction that reached beneath the intracontinental regions of the North China block. The distinct and contrasting behaviors of the slabs produced a slab window that caused and promoted upwelling mantle to flow farther southward, inducing the asymmetric magmatic-metallogenic belts along the Middle–Lower Yangtze River Valley on both sides. The Middle–Lower Yangtze River Valley Cu-enriched magmatic belt was formed by melting of the lower crust and oceanic slab edge along the ridge by upwelling asthenospheric mantle. However, the Jiangnan Orogen W-enriched magmatic belt was induced by melting of the accretionary belt and mixing the melting of accretionary belt and the same hybrid mantle materials, during which the upwelling asthenospheric mantle tongue infiltrated southward due to the asymmetric structure of the slab window. [ABSTRACT FROM AUTHOR]

The effects of Cd-rich venting fluids on the Cd contents and isotopic compositions of continental margin sediments remain poorly understood. Characterization of the Cd isotopic composition of sediment over-printed by venting fluids is critical, as this may provide insights into how mixing between the venting fluids and bottom waters affects the Cd isotopic composition. Our study determined the Cd isotopic compositions of Lower Cambrian metalliferous sediment deposited on the passive continental margin of the southeastern Yangtze Block, South China. The Cd is predominantly hosted in sphalerite of sulfide-bearing barite sediments, V deposits, and Ni-Mo–platinum group element (PGE) sulfide layers. The Cd isotopic compositions of samples from these three units indicate that the distribution of Cd at this type of continental margin was controlled by submarine hydrothermal activity. Following the venting of hydrothermal fluids onto the seafloor, Cd2+ reacted with a limited amount of H2S generated mainly by thermochemical sulfate reduction near the vents. This resulted in isotopically lighter Cd in the sulfide-bearing barite sediments and V deposits, which have δ114/110Cd values of 0.00‰ ± 0.13‰ and 0.00‰ ± 0.14‰, respectively. The residual dissolved Cd in the fluids migrated with the upwelling seawater and ultimately precipitated almost entirely as sphalerite in the sulfidic wedge, which resulted in Ni-Mo–PGE sulfide layers with a narrow range of high δ114/110Cd values (0.35‰ ± 0.03‰). The presence of Cd-rich venting fluids can alter the Cd contents and isotopic compositions of passive margin sediments, particularly those located within long-term stratified seawater and fault depressions. Previous studies may have underestimated the migration distance and affected areas of hydrothermal activity involving Cd-rich fluids. [ABSTRACT FROM AUTHOR]

The Earth has witnessed the emergence of continental-sized ice sheets, starting with Antarctica and gradually extending to both hemispheres over the past 40 million years. These ice accumulations have had a dramatic impact on both paleoclimate and sea level, substantially influencing sediment deposition in the continental margins. However, understanding sediment accumulation on an orbital scale in continental margins remains limited because of the scarcity of high-resolution, chronologically constrained sedimentary records. Here, we conducted a highly resolved cyclostratigraphic analysis based on natural gamma radiation (GR) series in depth domain at the continental margin of the South China Sea. We established a 22.8 m.y.-long high-resolution astronomical time scale spanning from the Miocene to the Quaternary by tuning the GR records to the global deep-sea benthic foraminifera carbon isotope curves and the 405 k.y. eccentricity cycles. The m.y.-scale sea-level changes since the Miocene were reconstructed through the sedimentary noise modeling of the 405-k.y.- tuned GR series. These reconstructions aligned with regional and global sea-level changes. The phase correlation between the filtered 1.2 m.y. cycles of sea-level change curves (dynamic noise after orbital tuning and ρ1 median models) from δ13Cbenthic and tuned GR series and the 1.2 m.y. obliquity amplitude modulation cycles revealed a shift from an anti-phase to an in-phase relationship across the middle Miocene climate transition (ca. 13.8 Ma), suggesting extensive expansion of the Antarctic ice sheet played a key role. In addition, a shift from an in-phase to an anti-phase relationship during the late Miocene (ca. 8 Ma and 5.3 Ma), indicating ephemeral expansion of the Arctic ice sheets or the changes in carbon cycle involving the terrestrial and deep ocean carbon reservoirs, might be the primary driver of eustatic changes. Furthermore, obliquity forcing and changes in meridional gradients in insolation that transported poleward flux of heat, moisture, and precipitation increased ice accumulation in both pole ice sheets and nonlinearly transferred high-latitude signals to low-latitude regions. This phenomenon is supported by the observation of strong obliquity signals in low latitude during global climate cooling interval. Our results suggest that m.y.-scale sea-level variations respond to astronomically induced climate change and ice sheet dynamics of both poles. This work contributes a highly resolved low-latitude geological archive to the future reconstruction of paleoclimate evolution on a global scale. [ABSTRACT FROM AUTHOR]

Sedimentological and geochronological data from late Paleozoic strata located between the East Junggar and Chinese Altai regions in NW China were examined, aiming to decipher the tectono-sedimentary evolution of this important tectonic boundary. Carboniferous sediments on the East Junggar side show arc-proximal depositional characteristics of the proximal Heishantou and Nanmingshui Formations and distal Beitashan and Yundukala Formations, while the Erqis complex on the Chinese Altai side is characterized by continental margin affinity. Lithological analysis revealed the dominant input of arc-related detritus for all these sequences and a uniform transition from volcaniclastic to siliciclastic components in their respective upper sections. The investigated East Junggar strata are dominated by Carboniferous zircons with positive εHf(t) values, sourced exclusively from the southerly Yemaquan-Jiangjunmiao arc domain, whereas the Erqis complex received detritus from the same arc domain but also evolved components from the northerly Chinese Altai. Combined with regional data, the examined strata are interpreted to have developed in a back-arc basin with regard to an arc that developed above the north-dipping Kalamaili subduction system. In contrast, the unmetamorphosed Lower Permian Tesibahan Formation, unconformably overlying the Erqis complex, received detritus mainly from the Chinese Altai. These sediments were deposited in an intracontinental piggyback or synformal basin following closure of the back-arc basin. The late Paleozoic sedimentation records support the interpretation that the Chinese Altai and East Junggar domains evolved from the same suprasubduction system prior to the Carboniferous rather than as independent terranes mutually juxtaposed during Permian lateral translation, as previously proposed. [ABSTRACT FROM AUTHOR]

Changes in lithospheric composition and strength associated with rifting of the Rodinian supercontinent (ca. 780–485 Ma) served to localize Mesozoic tectonic activity in the McCall region of west-central Idaho, USA. Although discrimination diagrams support slab failure melting, ca. 150–121 Ma calcic magmatism in the Hazard Creek complex and chemically similar intrusive rocks of northeastern Oregon, USA (e.g., Cornucopia stock), is best explained by progressive loading of oceanic crust. Westward-directed thrusting across the Laurentian continental margin and adjacent island-arc terranes (Olds Ferry/Wallowa; Blue Mountains province) led to partial melting of metabasaltic source rocks at pressures and depths sufficient for garnet stability (>10 kbar, ∼35 km). Early pluton emplacement during right-oblique arc–continent collision overlapped in time with subduction zone reorganization, which involved a change in dip direction (westward to eastward) under the accreting Blue Mountains block. Ensuing magmatic activity predated and was concurrent with ca. 116 Ma thrust reactivation of the Wallowa/Olds Ferry boundary, which temporally overlapped with contractional deformation along the terrane–continent accretionary boundary. Later synorogenic magmatism was focused along the accretionary boundary, where rising anatectic melts were caught between the eastwar-dimpinging/northward-migrating (obliquely colliding) terrane block and continental margin backstop (Rodinian rift architecture). Middle Cretaceous dextral transpression viewed in this manner precludes the need for an outboard “hit-and-run” collider (Insular superterrane) and thus, a post-accretion shear zone in western Idaho. [ABSTRACT FROM AUTHOR]

Sedimentary deposits along convergent margins contain a record of sediment transfer and coupled tectonic processes. Deciphering the evolution of ancient convergent margins, both spatially and temporally, is challenging as their stratigraphic successions are often locally deformed, which makes it difficult to correlate stratigraphic units over large distances, and they may have limited age constraints. Here, we construct a novel Bayesian chronostratigraphic framework for Late Cretaceous–Paleocene units of the Nanaimo forearc basin in western British Columbia, Canada, which reveals unparalleled detail into long-term sedimentation processes along an active deep-water margin. The Upper Nanaimo Group outcrop belt features ∼2000 m of forearc basin fill that includes the deposits of multiple submarine channel systems along a 160-km-long depositional strike-oriented cross section of the ancient continental margin. The age and longevity of individual slope-channel systems was determined by constructing a Bayesian Monte Carlo numerical model in which biostratigraphic and magnetostratigraphic measurements were used to further constrain 37 detrital zircon maximum depositional ages. Important context for the refined maximum depositional ages is provided by a detailed stratigraphic dataset composed of 2199 m of measured stratigraphic section and 4207 paleoflow measurements, which demonstrate the facies, architecture, distribution, and orientation of 12 slope-channel systems. In combination, our results reconstruct the spatio-temporal evolution of coarse-grained, deep-water sediment routing along the paleo-margin and enable the timing of sedimentation to be compared with hinterland and forearc processes. Our integrative approach demonstrates that submarine channel-system deposits of the upper Nanaimo Group cluster into three long-lived fairways (8–18 m.y.), each of which has a unique depositional history. Along-strike variations in the timing of sediment routing, channel-system architecture, and channel-system orientation are interpreted to be driven by local subsidence, magmatism, and subduction-related processes. We show, for the first time, how Bayesian age models can be applied at a basin-scale to produce robust chronostratigraphic frameworks for deciphering basin evolution that provide valuable insight into long-term geodynamic processes. [ABSTRACT FROM AUTHOR]

Magmatic arcs are the main environment where continental crust is created on the post-Archean Earth; however, how juvenile arc crust evolves into mature continental crust is still controversial. In this study, we report new bulk-rock major and trace elements, SrNd isotopes, and zircon U-Pb ages and Hf isotopes from a large suite of granites collected from the eastern segment of the Gangdese arc, southern Tibetan Plateau, which record a complete history of arc crust evolution from Mesozoic subduction to Cenozoic collision. These new data show that Gangdese crustderived granites generated during the subduction to collisional stages record significant geochemical changes with age, indicating that the bulk composition, lithological makeup, and thicknesses of the arc crust evolved over time. Here, we propose that the Gangdese arc had a thick juvenile crust with a small volume of ancient crustal components during late-stage subduction of the Neo-Tethys Ocean, a thin juvenile crust with heterogeneously distributed ancient crustal materials during early collision, and a thick juvenile crust with minor proportions of ancient rocks during late collision. This implies that the arc experienced episodes of crustal thickening during the Late Cretaceous and Eocene, interspersed by periods of thinning during the Paleocene and Miocene, and several discrete episodes of partial melting in the lower arc crust, and cycling or recycling of juvenile and ancient crustal materials within the arc crust and between the crust and mantle. We suggest that shallow subduction of the Neo-Tethys during the Late Cretaceous promoted tectonic thickening of the arc crust, partial melting of lower crust, and formation of high Sr/Y granites. After the onset of the Indo-Asian collision, breakoff of the subducted Neo-Tethyan oceanic slab during the Paleocene/early Eocene allowed thinning of the overlying arc crust and generation of granites derived from juvenile and ancient crustal sources. Continued underthrusting of the Indian continental crust and subsequent delamination of thickened lithospheric mantle led to thickening and thinning of the arc crust, respectively, and partial melting of thickened lower crust and generation of high Sr/Y granites during the Oligocene and Miocene. Using the Gangdese as an analogue for post-Archean continental margins, we suggest that the repeated thickening and thinning of arc crust, and associated multistage remelting of the lower arc crust, and material cycling or recycling within the crust and between the crust and mantle from subduction to collision are common processes that drive maturation of juvenile arc crust. [ABSTRACT FROM AUTHOR]

The Eocene Rasht-Abad volcanic rocks are located in the Alborz-Azerbaijan magmatic belt (including the Tarom-Hashtjin province) of NW Iran. Those are mainly mafic to intermediate with calc-alkaline affinities, comprising andesite, andesite-basalt, trachy-andesite, and dacite. Clinopyroxene ranging in composition from diopside to augite is the most significant mafic mineral of the basic rocks. Aluminum partitioning between tetrahedral and octahedral sites shows that those crystalized at low pressure. Ferric iron of clinopyroxene also indicates high oxygen fugacity for formation of crystal. Geothermobarometry using clinopyroxene-melt equilibrium calculations constrains the crystallization temperature and pressure of this mineral as 1100–1200 °C and 2–6 kbar. Ce/Pb values of the mafic lavas are lower than values expected for mantle-derived melts but do not support crustal contamination. Co-existing basalt to trachyandesite lavas display parallel and tight REE patterns that suggest these rocks originated from a common mantle source or parental magma. Tectonomagmatic discrimination diagrams and mineral compositions are consistent with the host volcanic rocks having the characteristics of continental margin arcs. Geochemical data are consistent with the parental magma of mafic-intermediate rocks of Rasht-Abad area being derived from a typical subcontinental lithospheric mantle which was enriched by subducted slab-derived fluids and melts during tectonic events in the active continental margin. The data support a model of Eocene flare-up magmatism associated with rollback of a flattened slab. [ABSTRACT FROM AUTHOR]

The undrained shear strength of marine sediment is of vital importance because of its critical role in seafloor slope stability, seafloor infrastructure, and influencing sediment dynamics that can lead to underwater landslides. Therefore, understanding the undrained shear strength of marine sediments and its influencing factors is a fundamental requirement for both offshore engineering and geoscience studies. Core data obtained from 198 sites across 46 legs of the Ocean Drilling Program/International Ocean Discovery Program (ODP/IODP) were used to analyze the undrained shear strength of marine sediments and their influencing factors. The undrained shear strength of marine sediments is significantly influenced by the depositional environment. Sediments deposited in active continental margins exhibit a higher undrained shear strength than those deposited in deep-sea and carbonate platform environments due to seismic strengthening and over-consolidation. It was found that fine-grained siliciclastic lithofacies with less than 50% carbonate content exhibited high variability and a rapid increase in the undrained shear strength with depth. In contrast, fine-grained carbonate lithofacies with more than 50% carbonate, as well as reef-facies carbonates, showed low variability and only a gradual increase in undrained shear strength with depth. Additionally, we showed a positive association between the undrained shear strength and physical characteristics including bulk density and P-wave velocity, as well as an inverse correlation with porosity. An exponential relationship was found between these physical properties and the undrained shear strength, with coefficients of determination (R²) values of 0.71, 0.74, and 0.69, respectively. [ABSTRACT FROM AUTHOR]

Glacio-eustatic cycles lead to changes in sedimentation on all types of continental margins. There is, however, a paucity of sedimentation rate data over eustatic sea-level cycles in active subduction zones. During International Ocean Discovery Program Expedition 375, coring of the upper ∼110 m of the northern Hikurangi Trough Site U1520 recovered a turbidite-dominated succession deposited during the last ∼45 kyrs (Marine Isotope Stages (MIS) 1–3). We present an age model integrating radiocarbon dates, tephrochronology, and δ18O stratigraphy, to evaluate the bed recurrence interval (RI) and sediment accumulation rate (SAR). Our analyses indicate mean bed RI varies from ∼322 yrs in MIS1, ∼49 yrs in MIS2, and ∼231 yrs in MIS3. Large (6-fold) and abrupt variations in SAR are recorded across MIS transitions, with rates of up to ∼10 m/kyr occurring during the Last Glacial Maximum (LGM), and <1 m/kyr during MIS1 and 3. The pronounced variability in SAR, with extremely high rates during the LGM, even for a subduction zone, are the result of changes in regional sediment supply associated with climate-driven changes in terrestrial catchment erosion, and critical thresholds of eustatic sea-level change altering the degree of sediment bypassing the continental shelf and slope via submarine canyon systems. [ABSTRACT FROM AUTHOR]

The geochemistry of Paleogene strata (sandstones and shales) of the N'kapa Formation in part of the western Douala Basin has been studied through major, trace, and rare earth elements (REEs) in order to understand their provenance and tectonic settings. The NF sandstones and shale are clustered around granodiorite with minor contributions from granitic source. The samples show high and fairly constant Th/Sc ratios 0.64–6.9 for sandstone and 1.01–8.7 for shale that strongly indicate provenance from a relatively evolved igneous source (mafic) also different geochemical signatures of Eu/Eu*, La/Sc, La/Co, Th/Sc, Th/Co, Zr/Sc, Cr/V, and Y/Ni ratios, and their plots suggest a mafic igneous provenance with substantial sediment recycling. Geochemical signatures from plots of The La/Th–Hf diagram, La/Sc vs Ti/Zr, and Fe2O3 + MgO vs TiO2 tectonic discrimination diagrams show that the studied rocks were deposited mainly in a passive continental margin setting. The large quantities of alkalis classify the rocks as wackes, Fe-sand, shale, and Fe-shale. The studied lithofacies have been classified based on Al–Fe-Mn enrichment as follows: terrigenous and metalliferous rocks. [ABSTRACT FROM AUTHOR]

Continental margins receive, process and sequester most of the terrestrial organic carbon (terrOC) released into the ocean. In the Arctic, increasing fluvial discharge and collapsing permafrost are expected to enhance terrOC release and degradation, leading to ocean acidification and translocated CO2 release to the atmosphere. However, the processes controlling terrOC transport beyond the continental shelf, and the amount of terrOC that reaches the slope and the rise are poorly described. Here we study terrOC transport to the Laptev Sea continental slope and rise by probing surface sediments with dual‐isotope (δ13C/Δ14C) source apportionment, degradation‐diagnostic terrestrial biomarkers (n‐alkanes, n‐alkanoic acids, lignin phenols) and 210Pbxs‐based mass accumulation rates (MAR). The MAR‐terrOC (g m−2 yr−1) decrease from 14.7 ± 12.2 on the shelf, to 7.0 ± 5.8 over the slope, to 2.3 ± 0.3 for the rise. Scaling this to the respective regimes yields that 80% of the terrOC accumulates on the shelf, while 11% and 9% of the accumulation occurs in slope and rise sediments, respectively. TerrOC remineralization is evidenced by biomarker degradation proxies (CPI of n‐alkanes and 3,5Bd/V) indicating 40% and 60% more terrOC degradation from slope to rise, consistent with a decline in terrOC concentrations by 57%. TerrOC degradation only partially explains this decline. An updated Laptev Sea terrOC budget suggests that sediment transport dynamics such as turbidity currents may drive terrOC shelf‐basin export, contributing to the observed accumulation pattern. This study quantitatively demonstrates that Arctic shelf seas are key receptor systems for remobilized terrOC, emphasizing their importance in the carbon cycle of the rapidly changing Arctic. Key Points: Terrestrial carbon export from the Laptev Sea shelf to the slope and rise is studied using δ13C/Δ14C, biomarkers and 210Pb mass accumulationThe accumulation of terrestrial carbon declines by 52% at the shelf edge and by 68% from slope to rise due to transport and degradation dynamicsA terrestrial carbon budget for the Laptev Sea suggests 80%–90% of the input is retained on the shelf via accumulation and re‐mineralization [ABSTRACT FROM AUTHOR]

The Lomagundi-Jatuli Event (LJE) is a significant positive δ13Ccarb anomaly in Paleoproterozoic carbonates globally. In the northeast São Francisco Craton (SFC), this event is documented in phosphorous (P)-rich marbles, along with subordinate graphite schists and iron formations, located in the Tanque Novo-Ipirá Complex (TNIC), Jacurici Valley (JV) and Rio Salitre Complex (RSC). Although the TNIC, JV, and RSC share comparable geological, geochronological, and mineralization characteristics, their connection and phosphogenic ties to the LJE are still uncertain. By combining new and reviewed C and O isotopes, cerium (Ce) anomalies relative to an array of rare earth elements, and yttrium (REY), we aim to constrain the paleoenvironmental conditions governing the phosphogenesis in the northeast SFC and its cause-and-effect relationship with the LJE. The Ce anomalies between 0.12 and 0.67 reveal a connection between the TNIC, JV, and RSC marbles and an oxic-suboxic shallow ocean influenced by detritus inflow. The remaining dataset links the carbonate protoliths to a continental margin of a paleocean and continental rift. Additionally, The positive δ13C excursions in the marbles, ranging from 5. 38 to 9.69‰, in conjunction with available geochronological data, indicate a maximum deposition age within 2.20 and 2.02 Ga. Our results indicate that the carbonate protoliths in the TNIC, JV, and RSC were deposited in similar paleoenvironmental conditions during the Paleoproterozoic. During this time, factors such as elevated oxygen levels, oceanic primary paleoproductivity, and organic matter burial played a crucial role in determining P supply, precipitation, and accumulation. These factors ultimately led to the positive δ13C excursions observed in the SFC, which are consistent with marine carbonates found around the world towards the end of the LJE. [ABSTRACT FROM AUTHOR]

The Lesser Xing'an—Zhangguangcai Range of northeast China is located in the eastern segment of the Central Asian Orogenic Belt (CAOB), which records intense magmatism during the Mesozoic. The petrogenesis and geodynamic setting of the Early Jurassic intrusive rocks in this region are unclear. In this paper, we present new zircon U–Pb age and whole-rock geochemical data for these intrusive rocks to investigate their origins and tectonic setting. Zircon U–Pb dating suggests these intrusive rocks were emplaced during the Early Jurassic (197–187 Ma). The granites are enriched in silica and alkali, and depleted in MgO and CaO. They are metaluminous to weakly peraluminous, and have high A/CNK values and low zircon saturation temperatures (TZr ~ 779°C), suggesting they are highly fractionated I-type granites derived by partial melting of lower crustal materials. The granites exhibit negative Nb, Ta, P, Eu, and Ti anomalies due to fractional crystallization. The diorites and gabbros have low SiO2 contents and high Mg# values, and are enriched in light rare earth and large-ion lithophile (Ba, K, and Sr) elements, and depleted in heavy rare earth and high field strength (Nb, Ta, and Ti) elements. The geochemical characteristics show that the mafic magmas were derived by partial melting of mantle that had been metasomatized by subduction-related fluids. Based on the geochemical characteristics of coeval intrusive rocks and the regional geological setting, we suggest the Early Jurassic intrusive rocks in the Lesser Xing'an—Zhangguangcai Range were formed along an active continental margin, possibly as a result of bidirectional subduction of the Mudanjiang Oceanic plate between the Jiamusi and Songnen—Zhangguangcai Range massifs. [ABSTRACT FROM AUTHOR]

The closure of an ancient ocean basin via oceanic arc‐continent collision has two subduction styles with opposite polarities, which may proceed via subduction polarity reversal (SPR) or a subduction zone jump (SZJ). Interpreting the geometry or kinematic evolution of ancient collisional zones, especially the original subduction polarity, can be challenging. Here we used 2D thermo‐mechanical modeling to investigate the dynamic evolution process of SPR versus SZJ. Our modeling predicts different structural, topographic, magmatic, and basin histories for SPR and SZJ, which can be compared against, and help interpret, the geologic record past sites of oceanic closure during collisional orogens. Our results match geologic observations of past collisions in Kamchatka, eastern Russia, and the Banda Arc, eastern Indonesia, and thus our results can help effectively decode the evolutionary history of past arc‐continent collisions. Plain Language Summary: Determining the geometry and kinematic evolution of ancient subduction zones that experienced collision with an oceanic island arc can be challenging based on the surface geology along. Such collisions usually result in different dynamical evolution processes, namely subduction polarity reversal (SPR) or a subduction zone jump (SZJ). Here we conducted numerical modeling of oceanic island arcs that collide with a continental margin to explore the dynamic evolution process of different subduction styles. Our results reveal geologic indicators to decipher SPR versus SZJ in natural oceanic arc‐continent collisions, such as the distribution of thrust faults, metamorphic rocks, magmatism, crustal thickness, and topography. The numerical simulations help explain the geologic history of Kamchatka in eastern Russia and Banda Arc in eastern Indonesia. This study provides provide new insights and implications diagnosing the polarity of vanished subduction in arc‐continental systems. Key Points: New numerical models with convergent velocity boundary condition for deciphering subduction polarity during arc‐continent collisionsThrust faults, metamorphic rocks, magmatism, topography and Moho morphology can be used as indicators to diagnose subduction polarityThe evolution of subduction polarity reversal well explains the tectonic activities of the Kamchatka and Banda Arc in the Cenozoic [ABSTRACT FROM AUTHOR]

Structural fracture distribution is essential in oil and gas transportation and development in passive continental margin basins. In this paper, taking as an example the clastic reservoirs in the A-Basin, a passive continental margin in northeastern South America, the paleotectonic stress field of the Late Cretaceous Maastrichtian formation in Basin A was numerically simulated by finite element technique through the integrated interpretation of seismic total data, logging data and core data, and the distribution of tectonic fractures was later predicted based on rock fracture criterion. The results of the study show that: (1) The distribution of tectonic stress and fractures during the Late Cretaceous Maastrichtian formation of Basin A is affected by the fracture zone, mechanical properties of rocks and tectonic stress, regions with extensive fracture development are susceptible to stress concentrations, resulting in significant stress gradients. (2) The development of structural fractures in the study area was predicted using the Griffiths criterion, and the tensile rupture coefficient T was introduced to quantitatively characterise the intensity of fracture development, with larger values reflecting a higher degree of fracture development. The well-developed and relatively well-developed fractures are mainly located in the fracture zones and the interior of submarine fans. (3) Fracture zones and sedimentary phases mainly control structural fractures in Basin A; within 5 km outside the fracture zones, the development of fractures is controlled by the fracture zones, beyond which the regional tectonic stress field controls them; inside the sedimentary fan, the development of fractures is controlled by the sedimentary subphase, which decreases in the order of the upper fan, the middle fan, and the lower fan; inside the subphase, they are controlled by the regional tectonic stress field, and the fractures show the increasing trend in the direction of NW-NE. [ABSTRACT FROM AUTHOR]

During the formation and evolution of the South China Sea, a series of multiscale submarine geomorphologies have been produced in the continental margin. The obvious submarine canyons and channels are widely distributed in the continental shelf and slope of the South China Sea. In the Taixinan Basin, several submarine canyons and channels termed as the Taixinan canyon‐channel system (TCCS) are distributed between the active and passive continental margins. Based on acquired ship‐borne multi‐beam bathymetry data in this study and the global GEBCO 2023 bathymetric dataset, we identify and define nine submarine canyons and seven submarine channels in the Taixinan Basin. The TCCS consists of the Dongsha, Taiwan, Jiulong, West Penghu, Penghu, Kaoping, Shoushan, Kaohsiung and the Fangliao canyons and submarine channels. The detailed geomorphological features of different submarine canyons and channels within the TCCS are analysed and summarized using multi‐beam bathymetry data and seismic reflection profiles across canyons. Based on the slope variations of the continental margin and the effects of turbidity currents and bottom currents on canyon, we propose a three‐stage evolutionary model of the TCCS. In the initial formation stage of canyon, the initial erosional grooves were created by tectonic activity on the continental slope and it represents the foundation of submarine canyons. During the growth and development stage, the submarine canyons are further evolved and the canyons began to deepen and widen from the continental slope to the deep‐water areas. It shows the weak erosion and sediment infilling within the canyons in this stage. On the northern continental slope of the South China Sea, continuous transportation and erosion of sediments led to the initial formation of grooves and it becomes the embryonic stage of submarine channels. The present stage of the TCCS was formed when the initial grooves on the continental slope have further developed and rebuilt under the erosion by the turbidity current and the scouring by the bottom current. In the last stage, the intense erosion by the turbidity current is supported by sediment waves around the submarine canyons and the migration of canyons is suggested by the cyclic steps formed within some canyons. [ABSTRACT FROM AUTHOR]

Whether the Yangtze Block was involved in the global late Mesoproterozoic orogeny remains contentious. The Mesoproterozoic Huodiya Group exposed in the Hannan‐Micangshan Massif on the northwestern margin of the Yangtze Block could provide sufficient evidence to further clarify the regional tectonic affinity. However, quantitative insights into the depositional age, provenance and basin tectonic setting of the Huodiya Group still lack comprehensively constraining. This study presents a combined analysis of zircon U–Pb dating for meta‐sedimentary rocks of the Shangliang Formation of the upper Huodiya Group and intruded gabbroic dykes. Intergraded dating results constrain the late Mesoproterozoic maximum depositional age at ca. 1050 Ma. Ca. 900 Ma formation age for the intruded gabbroic dyke, plus the minimum formation age of the overlying Xixiang Group at ca. 950 Ma, further suggests its deposition should be prior to ca. 950–900 Ma. Concordant detrital zircon ages define pronounced age clusters of ca. 2229–1741 and 2950–2388 Ma and few age populations of ca. 3232–3082 and 1069–1033 Ma, consistent with the tectonothermal events in proximal domains at the northwestern and northern Yangtze Block. Detrital zircon age distribution patterns and cumulative curves, coupled with the stable carbonate platform and shallow‐marine facies sedimentation, suggest a passive continental margin setting for the Huodiya Group on the Yangtze Block margin at the late Mesoproterozoic. Comparative insights into the contemporaneous passive continental margin from north to southwest argue against the existence of a late Mesoproterozoic orogeny along the exterior periphery of the Yangtze Block. [ABSTRACT FROM AUTHOR]

The southeastern Australian margin hosts a series of submarine canyons. Although the origin and evolution of canyons within the northwestern segment of the margin is relatively well studied, their quantitative morphology, interaction with longshore drift currents and slope failure remain poorly understood in the southeastern region. In this study, high-resolution bathymetry and 3D seismic reflection datasets revealed five main submarine canyons present in the central offshore Otway Basin. The canyons have V-shaped, evolving to U-shaped morphology downslope with sedimentary infill characterized by medium–high amplitudes, recognizable stratal pattern and localized chaotic seismic reflections. Analysis reveals these canyons were initiated by retrogressive slope failure on the continental slope, that once the shelf edge was reached and subsequently incised, development of the canyon heads was influenced by the shelf parallel Zeehan Current, active on the continental shelf of the region. The heads of the shelf-incised canyons preferentially migrate northwestward and were infilled by laterally accreting sedimentary packages characterized by southeast dipping seismic reflections. These indicate an early erosive phase followed by a period of deposition, a result of the prevalent eastward flowing Zeehan Current. These results have important implications for understanding of the mechanisms that control initiation and development of submarine canyons, and their morphology, both offshore southeastern Australia, and similar settings on continental margins worldwide. [ABSTRACT FROM AUTHOR]

Offshore freshened groundwater (OFG) represents a significant, yet underexplored, component of the global freshwater system as it is found across various continental margins. In this study, a 3-D geological and groundwater model was developed for the Canterbury Plains and Bight (New Zealand) to assess the role of present-day onshore recharge in the emplacement of OFG. Topographic and bathymetric, seismic reflection and well data have been integrated to construct an onshore-offshore geological model. The facies property distribution and associated hydrodynamic parameters were determined using stochastic modelling techniques. The geological reconstruction was imported within a variable density groundwater model. Simulations, under transient regime, were run until equilibrium conditions between the involved aquifer portion and the adjacent ocean have been reached. Our results show that the geological model accurately captures the stratigraphic and sedimentary features of the area, and that onshore recharge at present contributes to OFG emplacement up to 15 km from the coast, which is equivalent to 25% of its maximum extent. Onshore recharge during sea-level lowstands is therefore the dominant OFG emplacement mechanism in the Canterbury Bight. [ABSTRACT FROM AUTHOR]

Devonian successions in the Calliope and Yarrol provinces of the northern New England Orogen in central to northern Queensland have a lithological assemblage and depositional environments indicative of a close association with a subduction-related volcanic arc. These rocks have been interpreted as related to island arc and backarc settings based on geochemical characteristics of magmatic rocks. We present new detrital zircon ages from the Devonian to lower Carboniferous successions of the Calliope and Yarrol province successions. Most samples have no or relatively few zircon grains extracted except for samples of quartz-rich sandstone in the Middle Devonian Erebus beds of Hunter Island, located 160 km north-northwest of Rockhampton and another sample in the Rockhampton district. The samples with abundant zircon, and some with less common zircon, have prominent peaks of mid Cambrian to Ordovician ages and are consistent with derivation from the northern Thomson Orogen where igneous rocks of this age are documented in the Charters Towers Province. These data indicate a connection between the Devonian arc-flank deposits of the northern New England Orogen and the Thomson Orogen, which had previously been well established for Carboniferous deep-marine sandstones of the subduction complex (e.g. Shoalwater Formation). These results favour an east-facing continental margin arc setting in the Devonian contra to intra-oceanic island arc settings previously proposed. Detrital zircon age spectra from sampled early Carboniferous units of northern New England Orogen association indicate a pulse of felsic magmatism of that age. KEY POINTS: Devonian quartz-rich sandstones from the northern New England Orogen have abundant detrital zircon ages of mid Cambrian to Ordovician indicating a provenance connection with the Charters Towers Province of the northern Thomson Orogen. These results confirm a provenance linkage to Gondwana and imply that the Devonian tectonic setting was a continental margin arc and associated forearc basinal infill. Geochemical comparisons suggesting an island arc setting have overly influenced previous tectonic interpretations. [ABSTRACT FROM AUTHOR]

The structure of the post-Caledonian sedimentary cover in the transition from the Precambrian to the Paleozoic platforms in the Polish sector of the Baltic Sea is a matter of ongoing debate due to the sparsity of quality seismic data and insufficient well data. The new high-resolution BalTec seismic data acquired in 2016 contributed greatly to deciphering the regional geology of the area. Here we show an optimal seismic data-processing workflow for the selected new BalTec seismic profiles offshore Poland, as well as legacy PGI97 regional seismic data. Due to the acquisition in a shallow-water environment, the processing strategy focused on suppressing multiple reflections and guided waves through a cascaded application of modern multiple elimination approaches. We illustrate the potential of the new and re-processed data for focusing seismic interpretation on the area of the Koszalin Fault. In light of the available data, the Koszalin Fault was the main structure controlling Mesozoic subsidence and Late Cretaceous–Paleocene inversion of the eastern portion of the Mid-Polish Trough offshore Poland. The inversion changed its character from thin- to thick-skinned towards the north, away from the Polish coast. The Koszalin Fault reactivated older structural grain inherited from the time of Devonian continental rifting at the margin of Laurussia. The fault runs obliquely to the Caledonian Deformation Front, the feature that remained inactive since its formation at the Silurian–Devonian transition. [ABSTRACT FROM AUTHOR]

The Early Cambrian represents a critical time period characterized by extraordinary biological innovations and dynamic redox conditions in seawaters. Nitrogen isotopic signatures of ancient sediments have the potential to elucidate the evolutionary path of marine redox states and the biogeochemical nitrogen cycle within the water column of the Early Cambrian ocean. While existing research on this topic has predominantly focused on South China, the exploration of other continental margins has been limited, leaving contradictory hypotheses untested. In this study, paired δ15N and δ13Corg analyses were performed on the Lower Cambrian successions from the Shiairike section (inner ramp) and Well Tadong 2 (deep shelf/basin) in the northwestern and eastern Tarim Basin, respectively. Our data from the Shiairike section reveal a discernible shift in the operation of different nitrogen cycles for the black chert-shale unit, also referred to as the black rock series in Chinese literature, of the Yurtus Formation (Fortunian stage to lower Stage 3). Oscillating δ15N values for its lower part are suggestive of alternating anaerobic assimilation of NH4+ and denitrification/anammox. This is likely attributed to a shallow, unstable chemocline consistent with the upwelling and incursion of deep, anoxic waters during a major transgression. In contrast, aerobic nitrogen cycling, indicated by positive δ15N values of >2‰, dominated the upper part alongside a reduction in upwelling intensity. On the other hand, the δ15N signatures of Xishanbulake and Xidashan Formations of Well Tadong 2, which encompass a time interval from the Cambrian Fortunian Age to Age 4, are indicative of N2 fixation by diazotrophs as the major nitrogen source. The two studied intervals, although not time-equivalent, exhibit separated states of nitrogen cycling at least during the deposition of the Yurtus black rock series. The spatially different nitrogen cycling of the studied sections is compatible with a redox-stratified ocean during the deposition of the Yurtus black rock series. The build-up of a NO3− reservoir and aerobic nitrogen cycling in seawater was largely restricted to near-shore settings whereas anaerobic nitrogen cycling dominated by N2 fixation served as the main nitrogen uptake pathway in off-shore settings. [ABSTRACT FROM AUTHOR]

This article presents the results of U–Pb (ID-TIMS) geochronological, geochemical, and Sm–Nd isotopic-geochemical studies of metavolcanic rocks of the Djagdagle Formation, which belong to the key elements of the Bureya continental massif. It was established that the age of metavolcanic rocks of the Djagdagle Formation is 217 ± 7 Ma and corresponds to the Late Triassic. This fact contradicts the traditional ideas, according to which the Djagdagle formation is the Early Precambrian in age. The results of Sm–Nd isotope studies of the metavolcanic rocks indicate the melting of rocks of continental crust with Paleoproterozoic model ages during the formation of initial melts. The new geochronological data and previously published data allow us to distinguish at least two stages of magmatic activity in the Triassic within the northwestern part of the Bureya massif: ~243 and 219–201 Ma. On the basis of synchronous of Neoproterozoic, Early Paleozoic, Late Paleozoic, and Early Mesozoic magmatic events in the geologic history of the Bureya and Songnen–Zhangguangcai Range massifs, an assumption about their common geological history at least since the Late Neoproterozoic has been proposed. The close spatial position of metavolcanic rocks of the Djagdagle Formation and the Mongol–Okhotsk Orogenic Belt, their Late Triassic age (217 ± 7 Ma), and geochemical features allow us to link the formation of initial melts of metavolcanic rocks of this formation with within-plate magmatism in the rear part of subduction zone of the Mongol–Okhotsk Ocean beneath the northern (in modern coordinates) margin of the Bureya continental massif. [ABSTRACT FROM AUTHOR]