Mylonitic mantle peridotites exposed at the Tosa Megamullion in the Shikoku Basin, Philippine Sea, provide direct evidence for amagmatic ductile shear deformation of the upper mantle beneath a back-arc spreading center. Oceanic core complexes (OCCs), or megamullions, are dome-shaped structures formed by detachment faulting and occur locally along slow-spreading mid-ocean ridges and back-arc basins, where they expose fault rocks derived from ductile shear zones in the lower crust and upper mantle. The Shikoku Basin hosts several OCCs, including the Tosa Megamullion, which formed during the early stage of back-arc spreading. In this study, nine ultramafic rocks were collected from the Tosa Megamullion using the submersible Shinkai6500 during cruise YK23-05S. Although all samples were highly serpentinized, several preserved primary peridotitic textures were composed mainly of olivine, orthopyroxene, with subordinate clinopyroxene, plagioclase, and spinel. Seven samples exhibit well-developed foliation and porphyroclastic textures dominated by orthopyroxene porphyroclasts, ranging from rounded to strongly elongated forms, commonly showing microkinks and undulose extinction. Crystallographic preferred orientations (CPOs) of three representative samples, analyzed using SEM-EBSD, reveal E-type-dominant olivine fabrics characterized by the (001)[100] slip system, with a subordinate contribution from C-type (100)[001] slip. These CPOs suggest deformation under non-dry conditions involving moderate hydration and/or elevated differential stress. These results indicate that the ultramafic rocks from the Tosa Megamullion represent mantle-derived mylonitic peridotites formed by ductile shear beneath the spreading axis and subsequently exhumed under strongly magma-poor, amagmatic conditions. The Tosa Megamullion thus represents an amagmatic end-member of the OCC formation in back-arc basins, dominated by tectonic strain localization rather than by magmatic accretion.

The Shikoku Basin, a Miocene back-arc basin in the Philippine Sea, hosts numerous oceanic core complexes (OCCs) that reflect melt-poor lithospheric extension. Based on bathymetric mapping, gravity and magnetic surveys, dredging, and submersible dives conducted between 2007 and 2023, we identified multiple OCCs in its southern part, including the Mado, Sui-Shin, Tosa, Sanuki, and Awa Megamullions. These OCCs expose mantle and lower crustal rocks and are associated with high mantle Bouguer anomalies, indicating thin crust. Geochemical and geochronological data from the Sui-Shin Megamullion suggest its formation immediately after rifting of the proto-Izu-Bonin arc ( 24 Ma). The Tosa Megamullion, dominated by plagioclase-peridotites with only minor gabbros, exemplifies melt-poor lithospheric conditions and may represent Hess-type oceanic crust. The spatial and temporal distribution of OCCs, including the triplet OCCs (Tosa, Sanuki, and Awa), implies multiple ridge jumps during basin evolution. These findings suggest that a significant portion of the Shikoku Basin formed largely through amagmatic or melt-poor processes, contrasting with typical mid-ocean ridge settings. The OCCs serve as tectonic windows into back-arc lithosphere evolution and provide a framework for future investigations into the role of melt supply and slab-derived fluids in shaping oceanic crustal architecture.

Despite its geodynamic significance, the northern Tyrrhenian Back-Arc Basin, characterized by a complex tectono-magmatic evolution, remains poorly investigated. We present the first geophysical characterization of the Flavia Seamount, a previously uninvestigated edifice in the northern Tyrrhenian Sea, integrating new multibeam, seismic, and magnetic data. The new high-resolution bathymetric data reveal a flat, nearly circular summit and strongly asymmetric flanks. Reduced-to-the-pole magnetic anomalies exhibit a north–south polarity pattern, with positive values in the northern sector and negative values in the southern sector. Seismic data, integrated with Sparker profiles collected in 1985, reveal a flat-topped acoustic basement overlain by a ~100 m thick stratified sequence and affected by inactive east-dipping extensional faults, indicating tectonic control on the evolution of the seamount. Widespread landslide scarps and associated mass-transport deposits document recurrent gravitational instability along the flanks and within surrounding basins. Pockmark morphometry suggests distinct formation processes, with summit pockmarks controlled by fluid seepage and gravitational processes, and basin pockmarks mainly related to fluid escape from mass-transport deposits. Magnetic forward modelling constrained by seismic data provides new insights into the distribution of magnetic susceptibility bodies and the crustal architecture beneath the seamount. Results suggest that the present-day morphology of the Flavia Seamount reflects the combined effects of tectonic, sedimentary, and gravitational processes.

The Mohns and Knipovich mid-ocean ridges in the Arctic Ocean are ideal settings to investigate the dynamics of spreading centers, being the region affected by intricate interplays between oblique plate kinematics and curved plate boundary geometry. Furthermore, significant mantle thermal structure anomalies coexist with basalts having strongly radiogenic Hf isotopes, which suggest a link between mantle heterogeneity and present-day geodynamics. We explore innovative thermo-mechanical numerical models that incorporate the peculiar characteristics of this sector of Arctic Ocean, obtaining along-axis mantle thermal structure variations and across-axis asymmetries that match the observation at Mohns-Knipovich ridge intersection. We show that partial melting processes, magma distribution, and plate reorganization are all influenced by intrinsic parameters such as asymmetric half-spreading rates, non-linear ridge geometry, ridge migration, and pre-existing mantle heterogeneity. Our findings challenge the concept of symmetric oceanic seafloor expansion, highlighting the need for refined models that incorporate regional geodynamics and mantle composition to better understand the evolution of mid-ocean ridges and, ultimately, plate tectonics.

High-resolution bathymetry provides critical information to marine geoscientists. Bathymetric big data help characterise the seafloor and its benthic habitats, understand sedimentary records, and support the development of offshore engineering infrastructures. From 27 September to 20 October 2022, the new CNR research vessel Gaia Blu explored the seafloor of the Naples and Pozzuoli gulfs and the Amalfi coastal area (Tyrrhenian Sea, Italy) from 50 to more than 2000 m water depth, acquiring about 5000 km2 of multi-beam echo sounder data. This area is particularly vulnerable to abrupt changes driven by the dynamics of several volcanic complexes, active in the area, and by human-induced impacts reflecting the proximity to the highly populated and touristic coastal area of Naples and nearby famous islands. For these reasons, the seafloor of the area needs to be known and constantly monitored. The digital bathymetric data previously available are restricted to the shallow highly dynamic area of the Gulf of Naples and appear fragmented as they were acquired in successive years, with different goals thereby using a variety of devices, with markedly different spatial resolutions. In this paper, we present bathymetric maps of the Gulf of Naples and adjacent slope basins at unprecedented resolution using three state-of-the-art multi-beam echo sounders. These high-resolution data highlight the technological advances of geophysical surveys achieved over the last 20 years and contribute to assessing the most dynamic areas where changes in the seafloor over time can be quantified. The new digital multi-resolution bathymetric products are openly accessible via Marine Geosciences Data System MGDS (refer to “Data availability” section, Table 8, for datasets and product DOIs), perfectly matching the FAIR (findable, accessible, interoperable, and reusable) and open science principles.

Mid-ocean ridges (MORs) form as a result of upwelling and partial melting of the underlying mantle, leading to seafloor spreading and new lithosphere formation. They result from an interplay between different geological forces shaping ocean seafloors and offer insights into Earth's mantle convection and lithospheric evolution. Recent advances in numerical models contributed to describe oceanic rift processes, although complex geodynamic settings remain relatively unexplored.Knipovich and Mohns ultraslow spreading ridges are located in the Arctic Ocean, separated from Kolbensey and Gakkel ridges by the Jan Mayen transform and Lena Trough. They do not present any evidence of transform fault along their entire length and are characterized by a high obliquity (~35°-50°) with respect to their spreading direction, constituting some of the most intriguing MORs worldwide. At their intersection, geophysical data revealed a focused mantle upwelling along a narrow, oblique, and strongly asymmetric zone, coinciding with uneven surface uplift. These asymmetrical features have been associated to i) the control on passive upwelling of slow and asymmetric motion of the North America and Eurasia plates, or ii) the results of a major spreading reorganization in the area. However, asymmetries are tipically observed in other geodynamic settings, such as for example subduction zones, where they have been related to the relative motion of lithospheric plates with respect to the asthenosphere. In this work we carried out 3D numerical models reproducing the geodynamic evolution of a ~800-km long segment of the Knipovich and Mohns ridges (extending from ~76°N to ~71°N), including their migration with respect to the asthenosphere. The model uses a visco-plastic rheology which approximate both the asthenospheric and the lithospheric mantle, providing information on the temperature and deformation patterns within the mantle. We also computed the degrees of melting beneath each area of the MOR segment. In agreement with previous geophysical and petrological data, our results suggest that mantle upwelling is focused in a narrow zone, where the MOR makes a sharp bend, providing the inferred asymmetric patterns. On this basis, we propose a mechanism which could have led to the asymmetrical features (e.g., topography, spreading rate, mantle temperature and composition, etc.) characterizing the Knipovich-Mohns segments area.

Devastating earthquakes continue to surprise scientists, especially when they exhibit unexpected characteristics, such as the 2023 doublet of Mw>7.5 earthquakes in a day along the same fault system in eastern Türkiye. These earthquakes struck the East Anatolian Fault, a major >600 km long tectonic boundary, separating the Anatolian, Arabian, and Eurasian plates, resulting in approximately 60,000 fatalities in Türkiye and Syria and causing more slip than expected. Occurrences of temporally and spatially close earthquakes are hence rare and unmissable opportunities to advance our understanding of active fault mechanics and regional hazard. Such superevents could be part of a supercycle, wherein the likelihood of a large earthquake is determined by accumulated strain rather than time since past earthquakes. To advance our understanding of multiple earthquakes along fault systems and hence of seismic supercycles, we compare tectonic and seismological features of the two 2023 earthquake sequences near Pazarcik and Elbistan with those of the two previous Mw≥6.1 sequences, which occurred in 2010 and 2020, respectively, near Elâzığ along the northeastern East Anatolian Fault. We examined the four strong sequences along the East Anatolian Fault within a multimillennial context of historical seismicity and discovered progressively younger and nonuniform earthquakes moving southwestward. This pattern corresponds to a general progression and dispersion of seismic ruptures southwestward and we use it as a proxy to understand the mechanism of at least two major supercycles identified over the last two millennia. The supercycles evolved from the northeast spreading southwestward with an increasing number of earthquakes. Earthquakes to the northeast are spatially and kinematically well channelized along the main fault, efficiently translating slip toward the southwest, where dispersed and kinematically nonuniform earthquakes are triggered by the push from the northeast, until a new supercycle restarts from the northeast. Insights from recent events offer a crucial framework for interpreting past supercycles and enhancing seismic hazard assessment, providing essential guidance for future mitigation strategies.

In the western Mediterranean, the subduction of the Tethyan ocean has progressively come to an end, following the intervening continent-continent collision. Compressional deformation connected with the ongoing Africa (AF) – Eurasia (EU) convergence has therefore progressively resumed mostly along the southern passive margins of the Mediterranean back-arc basins. The use of geodetic, seismological, and pre-existing tectonic data recorded between the Gulf of Cadiz and the Ionian Sea helps to trace this nascent AF-EU boundary and constrain its kinematics. Based on these data, this plate boundary is detected, kinematically defined, and compared with the previously identified boundaries in the same region. The nascent boundary is articulated and formed by variably oriented inherited structures. It is characterized by a discrepancy between the general motion of Africa with respect to Eurasia and the local contractional/compressive axes deduced from geodetic and seismic data. The oblique convergence along the nascent boundary matches that recorded in other instances of subduction initiation elsewhere, but the average convergence rate equal to 5 mm/yr in the Mediterranean seems currently too small for such a subduction initiation. Based on the assumption of a future northward tectonic vergence (i.e., Eurasian foreland), the Tyrrhenian, Algerian, and Betic salients, the Oran and Fès recesses, and the Ionian, Trans-Alboran, and Gibraltar transfer zones are identified along the nascent boundary. The latter zones connect salients and recesses through strike-slip displacements. The Algerian offshore hosts a long segment of the boundary characterized by locally increased seismic rate and actual northward vergence that would suggest this area being the first nucleus of subduction initiation in the western Mediterranean.

The Messina Strait and surrounding areas are one of the most interesting regions of the western Mediterranean Sea, characterized by the complex interplay between the Mesozoic-Paleogene Ionian basin, where the Calabrian Arc accretionary prism extends towards the southeast, and the Neogene Tyrrhenian back-arc basin to the northwest. Complex fault networks with different kinematics, running from the inner side of the Calabrian arc through the Messina Strait and the Ionian coast of Sicily, as far as the Hyblean Plateau, result from the coexistence of different geodynamic settings in the area. Some of these faults are responsible for several of the largest earthquakes occurred in southern Italy and the Mediterranean Sea in recent times. Different works aimed at establishing a relationship between seismogenic sources and mapped faults, defining the location and rupture mechanism of some of these fault lineaments. Even tough, many uncertainties still exist for earthquakes occurred in offshore areas, where the fault kinematics and geometry are in some cases still poorly constrained. In this work, we focus on a group of offshore faults located between the northern sector of the Messina Strait and the Gioia Basin (southern Tyrrhenian Sea). We aim at understanding the kinematics and the style of deformation in this area, and to investigate the role played by the main fault networks in the framework of the regional complex geodynamic setting of the Ionian-Tyrrhenian transition zone. This study is based on the interpretation of a multichannel seismic dataset (TIR10 survey), combined with the analysis of morpho-bathymetric and geodetic data, and with numerical modeling. This multidisciplinary and multiscale approach can contribute to unravel the particular role of this region in the context of a stepwise migrating subduction system and provides new constrains for the study of this highly populated area characterized by severe seismic and tsunamigenic hazard.

The region offshore the Ionian Islands (Kefalonia or Cephalonia, Lefkada, Zakynthos, and Ithaca), western Greece, is characterized by very high seismogenic and tsunamigenic potential. Despite numerous studies, many questions remain regarding the structures and kinematics in this area. Here, the right‐lateral Cephalonia Transform Fault divides the Hellenic Arc from the Albanian compressional system. Historically, this area has been hit by several destructive earthquakes, such as the 1953 sequence (Mw >6) that destroyed Cephalonia. To improve the understanding of shallow sediment deformation, fault geometry and potential hazard, we carried out two oceanographic cruises (May 2022 and June 2023) during which a significant multiscale and multidisciplinary geophysical and geological data set has been collected. These data allowed us to define the deformation style of sediments at the intersection between the Cephalonia Fault and the thrust fronts of the Hellenic System, and the geometry of the Cephalonia Fault. The Fault, south of Cephalonia, opens in a 25 km‐wide fan composed of elongated, sigmoidal, positive flower structures ‐ features indicative of transpressive tectonics regime. In the area to east of Cephalonia we mapped for the first time a series of seaward‐verging anticlines with strike‐slip component and uplifted blocks. While to southeast of Cephalonia the deformation regime changes, becoming dominated by extensional and large landslides, affecting mainly superficial sediments. These new data allow us to map the transition between the transcurrent and compressional domains, unveiling the geometry of the faults laying the foundation for future studies on slip rates and seismic hazard.

The CORSUB project aims to explore and investigate unidentified morphological features located between 75 and 100 meters depth off the Punta Licosa Promontory (Tyrrhenian Sea, Campania, Italy), on submerged terraces. These features were firstly observed during a survey in 2004,where a biogenic origin was hypothesized, but no further research had been conducted. The CORSUB project adopts an interdisciplinary, integrated approach that combines geophysical, stratigraphical, sedimentological and palaeontological analyses to investigate the formation, evolution, and ecological significance of these submerged morphologies.As part of the “TREMOR” oceanographic cruise, organized by the Italian National Research Council (CNR) aboard the CNR research vessel Gaia Blu in December 2024, the CORSUB team collected high-resolution multibeam bathymetry data, chirp profiles, and box-corer sediment samples (n=4) from the project areas.The preliminary results indicate that the anomalous morphologies are located between 75 and 85 meters depth and consist of clusters of subcircular features, with sub-metric diameters. Interestingly, the edge is sunken, while the central area is gently raised. Chirp profiles revealed that the sedimentary cover over these features is relatively thin, with a rocky substrate likely corresponding to the Cilento Flysch Unit identified beneath. Box-corer samples revealed a composition of coarse detrital sand and gravel at the top, predominantly biogenic in origin, transitioning to muddy-sandy sediment at the base. Notably, all samples contained dead, centimeter-sized boxwork rhodoliths, ranging from 8 to 20 cm above the top of the box-corer. Live rhodoliths were found in only one sample, and these showed clear evidence of ongoing mudding.These preliminary findings suggest several potential interpretations. The observed structures may have a biogenic origin, possibly linked to the development of rhodolith beds in the past. Alternatively, their location on the flanks of the submerged terraces may indicate a strong correlation with glacial and post-glacial sea-level changes. The morphologies could have originated as erosional features during the Last Glacial Maximum, when sea levels were as much as 120 meters lower than today, subsequently providing a substrate for biological colonization as sea levels rose during the deglaciation and into the Holocene.The ongoing analyses of both remote sensing data and collected samples, which also include dating, will allow for a more accurate determination of the nature and evolutionary history of these structures.CORSUB is funded as part of the PRIN 2022 program under Mission 4 of the Italian Piano Nazionale di Ripresa e Resilienza (PNRR). Principal Investigator: Professor Valentina Alice Bracchi. A special thank to the scientific crew of the TREMOR survey.

This research presents the discovery of a kink in the frequency-size distribution of plate angular velocities in different reference frames, a feature not predicted by current physical models of plate tectonics. We analyze global plate motions to investigate the origin of this scaling break and find that larger plates systematically exhibit slower angular velocities than smaller ones. Moreover, we explore the relationship between plate velocities and the physical properties of the lithospheric basal layers, revealing that faster-moving plates correlate with more homogeneous lid rheology and structure, as inferred from reduced P-wave velocity variability. A significant negative correlation between angular velocity and latitude is observed with a persistent west-ward residual drift suggesting the long-term influence of large-scale forcings determining asymmetric mantle flows with respect to the outer layers of the planet. Based on these findings, we propose two possible reasons to explain the observed plate velocity scaling break. The first one suggests different dominant driving mechanisms for large and small plates resulting in a two-tier plate tectonics with a threshold in the range 3-5× 10 ^6 km ^2 ; an alternative explanation is the presence of a substantial bias in currently assumed hotspot reference frames with respect to the true mantle-reference plate motions to be recovered through a net westward rotation of ω≈ 0.5-0.8 °/Myr.

The La Gomera-Tenerife Channel is a narrow passage between La Gomera and Tenerife Islands, i.e., two volcanic edifices of the Canary Archipelago (Atlantic Ocean). A geophysical study was conducted to identify the main geomorphic processes affecting the seabed and their interplay. In particular, submetric resolution bathymetric and side scan sonar backscatter data were collected in the southern sector of the Channel, down to 1200 m water depth. Their integrated analysis revealed a complex seabed morphology and a variety of morpho-sedimentary features, resulting from three main geomorphic processes: submarine volcanic activity, mass wasting (e.g., turbidity currents, small landslides and exotic blocks emplaced by a massive landslide event), and bottom currents activity. Bottom currents strongly reshaped the seabed into bedforms, confined drifts, and moats. Although the flanks of volcanic islands are typically dominated by mass wasting and volcanic features, our results indicate that bottom current activity can be predominant in confined settings and around topographic features due to modification of flow patterns and enhancement of current flows. This study is the first to document volcanic, mass wasting and bottom current features within the La GomeraTenerife Channel. Furthermore, it provides insights on: i) morpho-sedimentary reconstructions of narrow passages between volcanic islands; ii) interplay among different geomorphic processes; iii) oceanographic reconstructions. The variety of geomorphic processes shaping the La Gomera-Tenerife Channel makes this area significant for high-resolution studies. Moreover, it provides new insights on poorly known processes, such as: the interaction of bottom currents with complex topography and bottom current morpho-dynamic in curved moats.

Abstract Convergent plate boundaries are often characterized by widely deformed zones, where coexisting tectonic processes and variable fault kinematics can occur. Here, we quantify this variability along the Africa‐Eurasia deformed boundary in southern Italy, based on the evaluation of geodetic strain rate by recent space geodesy observations and plate motions, which are integrated by main geometric properties of detected faults in the area. We propose a compilation of 160 known faults. We use numerical methods to predict fault kinematics and net slip rate, due to the geodetic deformation field with the inclusion of fault strain accommodation. The obtained tectonic setting is compared with the observable, showing a fault rake agreement of the 73%, which allows us to consider this approach potentially favorable to improve the knowledge of fault kinematics along diffuse plate boundaries, when fault properties are not directly available.

Oceanic geodiversity provides essential information on the dynamics of the Earth. Here, we focus on the geodiversity of three oceanic back-arc spreading centers: the Mariana Spreading Center, the Central-Southern Lau Basin spreading centers, and the East Scotia Ridge. We defined a method to identify their axial zones, obtaining spreading center depths along the basins. Results improve global plate boundary models and morphology variations, revealing that the average depths along the Mariana, East Scotia, and Lau Basin spreading ridges are 4.5, 3.5, and 2 km, respectively. We also measured new spreading rates based on five magnetic profiles crossing the three back-arc spreading centers, contributing to plate kinematic models. Furthermore, we computed subduction rates, including hinge velocities along the Mariana, South Sandwich, and Tonga Subductions, to understand the existing interactions between the subducting slab hinge motion and the kinematics of their related back-arc spreading centers. Our bathymetric, magnetic, and kinematic data show several differences among the Mariana, the East Scotia, and the Lau spreading centers, stressing the oceanic geodiversity in a similar geodynamic context. Our results also suggest a strong correlation between axial depth and full spreading rates along the back-arc spreading centers, a geological correspondence that allows a similar description of these divergent plate boundaries within the mid-ocean ridge classification. Finally, we show how hinge kinematics affects the relationship between convergence along subduction zones and back-arc spreading rates. All our findings contribute to understand how the oceanic geodiversity is directly related to geodynamic processes, increasing the knowledge of global tectonics.

AbstractThe stretching of the lithosphere leading to back-arc basins formation generally develops behind arc-trench systems and is considered the consequence of slab retreat relative to the upper plate. Here, we examine the deformation regime evolution within the overriding plate due to subduction processes, using thermo-mechanical numerical simulations. We explore the north-eastern Eurasia plate boundary and the mechanisms of subducting Pacific plate since 57 Ma. During this time interval, several extensional basins formed along the Eurasia margin, such as the East China Sea, the Japan Sea, and the Kuril basin. Here, we increased the simulation complexity, with the inclusion of (i) the kinematic variability of the Pacific plate over the geological past with respect to a fixed Eurasia, incorporating time-dependent (i.e., temporally evolving) velocities computed from plate motion reconstructions; (ii) a Low-Velocity Zone within the asthenosphere, and (iii) a horizontal eastward mantle flow. Our results show a crucial role of the mantle flow for the development of lithospheric extension and back-arc basin opening, and a main kinematic control of the subduction trench position, which advances and retreats, into distance intervals in the order of $$\sim$$ ∼ 100 km, and providing stages of compression and extension in a back-arc basin.

The two Mw > 7.5 earthquakes that struck the East Anatolian Fault (EAF), Türkiye, in 2023 caused more slip than expected, indicating that they were potentially part of a supercycle, in which the occurrence probability of a large earthquake is determined by accumulated strain rather than time since the last large earthquake. Here, we show two potential supercycles along the EAF, analyzing earthquakes from the last two millennia. Within each supercycle, seismic ruptures originated in the northeast and progressively spread southwestward with an increasing number of earthquakes until a new supercycle began with another large earthquake in the northeast. To understand the supercycle behavior, we analyze the aftershock sequences of the four most recent Mw≥6.1 mainshocks (2010-2023). This series of earthquakes progressed southwestward, characterized by an increasing diversity of focal mechanisms and a heightened dispersion of epicenters across a branched seismotectonic environment. Earthquakes in the northeast exhibit spatial and kinematic channeling along the master fault surface, effectively transferring slip southwestward and there potentially triggering dispersed and heterogeneous earthquakes. This spatiotemporal pattern seems connected with varying levels of a presumably-innate property of fault sections or regions, ruling the process of seismic slip channeling, which could also explain the behavior of long-term supercycles.

Seafloor anomalies along mid-ocean ridges with exceptionally thick and compositionally distinct basaltic crust, for example, at Iceland, suggest that the underlying mantle is hotter and chemically different from the adjacent subridge mantle. Here we present hafnium and neodymium isotope ratios of peridotites from the Charlie Gibbs Transform Zone, which is located at the southern end of the Reykjanes Ridge south-west of Iceland. These peridotites are strongly depleted in incompatible elements with extremely high hafnium isotope ratios, suggesting that they had already melted to a large extent before being incorporated into the plume, at least 1 billion years ago, and thereby also became less dense. We argue that seismic velocity anomalies, geodynamic models and geochemical affinities of ridge basalts connect the peridotites from the Charlie Gibbs Transform Zone to the ‘Iceland plume’. The thermochemical buoyancy of the moderately hot Iceland plume, but also that of other plumes worldwide, may therefore be strongly influenced by composition. Variable peridotite depletion along the rising Iceland plume could also cause the transient, density-driven pulses in plume flux, which have formed the V-shaped Reykjanes Ridge south of Iceland. Overall, expansion of a ridge-centred plume along adjacent ridges and melting of heterogeneous plume material explains the topographic swell, the seismic anomaly and the formation of V-shaped ridges, as well as the regional distribution of basalts with Icelandic affinity. The upwelling mantle beneath Iceland underwent melt depletion at least 1 billion years ago and is therefore compositionally buoyant, according to a study of neodymium and hafnium isotope ratios in peridotites from the Charlie Gibbs Transform Zone.

Extension at back-arc basins generally occurs behind arc-trench systems and the mechanisms which act at its origin, as well as the deformation regime developed, are strongly related to the subduction of oceanic lithosphere. Here, we examine the Japan Sea back-arc basin evolution using numerical simulations along the western margin of the Pacific plate, where the subduction processes have been responsible for the deformation style during the last 57 Ma. We carried out 2D high-resolution thermo-mechanical numerical models of subduction dynamics in this area, increasing the simulation complexity integrating into the computations i) the kinematic variability of the Pacific plate over the geological past with respect to a fixed Eurasia, ii) a Low-Viscosity Zone within the asthenosphere, iii) a horizontal eastward mantle flow. Our results show a main kinematic control of the subduction trench position, which advances and retreats in time, providing stages of compression and extension in the Japan Sea back-arc basin. The obtained deformation regime is comparable with the tectonic evolution history occurred along the Eastern Eurasian margin and with analyses on paleo-volcanic front position and paleo-stress reconstructions in the Japan Sea area.

The Pacific, Antarctic, and Macquarie lithospheric plates diverge from the Macquarie Triple Junction (MTJ) in the southwestern Pacific Ocean, south of Macquarie Island. Morphobathymetric, magnetic, and gravity data have been used to understand the evolution of the three accretionary/transform boundaries that meet at the MTJ. Plate velocities, estimated near the MTJ and averaged over the past 3 m.y., indicate an unstable ridge–fault–fault triple junction. The long life (>6 m.y.) of this configuration can be attributed to a rapid increase in spreading asymmetry along the Southeast Indian Ridge segment as it approaches the MTJ, and to transtension along the southernmost strand of the Macquarie–Pacific transform boundary. A major change in plate motion triggered the development of the Macquarie plate at ca. 6 Ma and makes clear the recent evolution of the MTJ, including (1) shortening of the Southeast Indian Ridge segment; (2) formation of the westernmost Pacific-Antarctic Ridge, which increased its length over time; and (3) lengthening of the two transform boundaries converging in the MTJ. The clockwise change of the Pacific-Antarctic motion (ca. 12–10 Ma) led to complex geodynamic evolution of the plate boundary to the east of the triple junction, with fragmentation of the long-offset Emerald transform fault and its replacement over a short time interval (1–2 m.y.) with closely spaced, highly variable transform offsets that were joined by short ridge segments with time-varying asymmetries in the spreading rates.