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Basaltic samples were collected by the French submersible ``Nautile'' during the “Parisub” cruise (2010, R/V L'Atalante, Ifremer) along a 25 km long sampling profile crossing perpendicularly the current axis of the East Pacific Rise at 15.6°N, as well as the trace of its two former parallel axes located further east. The total length of the profile corresponds to an approximate time interval of ∼ 320,000 years. The corresponding dataset documents the geochemical response of Mid-Ocean Ridge Basalts (MORB) related to the progressive convergence between the ridge segment and a nearby hotspot. It also represents one of the highest-sampling (and in turn geochemical) resolution efforts to date. The major, trace element and isotopic compositions determined through optical and mass spectrometry analysis of 52 samples are presented and compared to other previous data obtained from the same area. The data obtained strictly follow the conventions used in rock geochemistry in terms of data acquisition, reduction, and format, so that they can be compared to similar data from other regions. The different figures present (i) The geological context of study area, (ii) A classification of the samples according to their geochemical composition and geological context for a better legibility of the dataset, (iii) A comparison with data from other oceanic rises, (iv) A detailed method explaining the foundations of the chronology between samples established, and (v) A chronological representation of the geochemical composition of the basalts collected. These data can be useful for anyone interested in marine geosciences and more specifically scientists studying mantle geochemistry, oceanic lithosphere formation, and hotspot-ridge interactions. These data can also be used to model magmatic processes, crust-mantle interactions, and can be integrated in geophysical and geological models of seafloor accretion.

We analyze a large database of abyssal peridotite clinopyroxene compositions using log‐ratio transformation, principal component analysis (PCA) and k‐means clustering, to better understand clinopyroxene compositional systematics in abyssal peridotites. We combine this analysis with open‐system melting models to investigate the potential sources of compositional variation. PCA shows that 84% of the variation in clinopyroxene compositions can be represented using only 2‐dimensional information. We use k‐means clustering to classify clinopyroxene compositions into four clusters. Clusters 1–3, representing 85% of the data, show progressive depletions in LREE/HREE, and are associated with decreases in Na2O in clinopyroxene, and general increases in Cr# of spinel. We interpret peridotites with clinopyroxene compositions from clusters 1–3 to represent residues of partial melt extraction. The degree of melt extraction increases from cluster 1 to 3, and exerts a primary control on compositional variations. The presence or absence of garnet‐field melting prior to spinel‐field melting and the retained melt fraction during partial melting exert secondary controls on clinopyroxene compositions. Cluster 4 clinopyroxenes, representing show less fractionated LREE/HREE with low‐HREE abundances, elevated Sr, and depleted signatures in their host peridotites. Clinopyroxene compositions in cluster 4 cannot be modeled by melt depletion alone. Instead, they are only reconstructed in our models where melt‐rock interaction, suggesting that peridotites with cluster 4 clinopyroxenes have experienced both of these processes. Clusters 1–4 are observed in most ridges, reflecting compositional heterogeneity on each ridge. This variability reflects variations in the degree of partial melting, amount of garnet‐field melting, retained melt fractions, and melt‐rock interaction. Plain Language Summary: "Abyssal peridotites" are rocks that come from the Earth's mantle, beneath the oceanic crust. They are exposed on the seafloor near mid‐ocean ridges, the sites at which new oceanic crust forms by melting the mantle below. We analyzed a large database of chemical analyses of the mineral "clinopyroxene" from these abyssal peridotites using statistical methods, to try and understand what controls their chemical compositions. Our results simplify the complex chemical variations in these clinopyroxenes to just two main variables, and allow us to divide the compositions into 4 groups. The first three groups (85% of the clinopyroxene database) can be explained by forming in mantle rocks that melted by varying amounts, during the melting process that leads to the formation of oceanic crust. The last group (15% of the clinopyroxene database) cannot be explained by simple melting processes. Instead, these mantle rocks were first partially melted, and then reacted with melts that originated deeper in the mantle and passed through them. All clinopyroxene groups are present in most mid‐ocean ridges, showing that the composition of abyssal peridotites is extremely variable even on each ridge. Key Points: We classify database of abyssal peridotite clinopyroxene compositions using statistical methods and compare the data with melting modelsEighty five percent of abyssal peridotites can be residues following melt extraction but 15% experienced both partial melting and melt‐rock interactionAll clinopyroxene compositional clusters are observed in most ridges, reflecting compositional heterogeneities on each ridge [ABSTRACT FROM AUTHOR]

Seismicity along mid‐ocean ridges and oceanic transform faults provides insights into the processes of crustal accretion and strike‐slip deformation. In the equatorial Atlantic ocean, the slow‐spreading Mid‐Atlantic Ridge is offset by some of the longest‐offset transform faults on Earth, which remain relatively poorly understood due to its remote location far from land‐based teleseismic receivers. A catalog of T‐phase events detected by an array of 10 autonomous hydrophones deployed between 2011 and 2015, extending from 20°N to 10°S is presented. The final catalog of 6,843 events has a magnitude of completeness of 3.3, compared to 4.4 for the International Seismic Center teleseismic catalog covering the same region, and allows investigation of the dual processes of crustal accretion and transform fault slip. The seismicity rate observed at asymmetric spreading segments (those hosting detachment faults) is significantly higher than that of symmetric spreading centers, and 74% of known hydrothermal vents along the equatorial Mid‐Atlantic Ridge occur on asymmetric spreading segments. Aseismic patches are present on nearly all equatorial Atlantic transform faults, including on the Romanche transform where regional rotation and transpression could explain both bathymetric uplift and reduction in seismic activity. The observed patterns in seismicity provide insight into the thermal and mechanical structure of the ridge axis and associated transform faults, and potentially provide a method for investigating the distribution of hydrothermal vent systems. Plain Language Summary: New oceanic seafloor is created at mid‐ocean ridges where tectonic plates move apart, causing seafloor volcanism and deep‐sea hydrothermal venting. Meanwhile, changes in the direction of plate motion occur along strike‐slip faults which can be hundreds of kilometers long, called oceanic transforms, and offset the mid‐ocean ridge. Neither of these processes are fully understood, but constantly generate numerous small earthquakes which in places like the remote equatorial Atlantic Ocean, often go undetected by far‐away instruments on land. Using a network of hydrophones spaced thousands of kilometers apart and moored ∼800 m below the sea surface, over 6,000 earthquakes in the equatorial Atlantic ocean were located from 2011 to 2015, roughly 10 times more than would have been detected by land‐based stations alone. Similar to previous studies, we find that transform faults have patches where very few earthquakes occur and hence may be locked or creeping, likely due to spatial variability in the thermal properties of the lithosphere and underlying mantle. More earthquakes occurred in places where the plates spread apart asymmetrically, and hydrothermal vents are also more likely to occur in places where spreading is asymmetric. Key Points: Catalog of 6843 hydroacoustic events recorded by autonomous hydrophones in equatorial Atlantic Ocean between August 2011 and January 2015Asymmetric spreading segments have higher hydroacoustic event rate, and frequently host hydrothermal vent systemsAll equatorial Atlantic transforms host aseismic patches, and precursors are common ∼2–24 hr before mainshocks of normal faulting events on the ridge axis [ABSTRACT FROM AUTHOR]

• Subsidence parameters have statistically significant correlation with spreading rate. • SW Indian and Central Pacific oceans are the end-members in global subsidence trends. • Slow spreading produces narrow oceans with old seafloor age and passive margins. • Fast spreading produces wide oceans with young seafloor age and active margins. • The distribution of hotpots in oceans may be controlled by spreading rate. Ocean age-dependent cooling subsidence with seafloor deepening is traditionally described by models of thermo-chemical buoyancy of oceanic plates with globally constant parameters, that specify a linear correlation between square-root of seafloor age, sqrt(age), and bathymetry. Here I present a worldwide analysis of the ocean floor split into 94 segments, delineated by wide-offset transform faults and mid-ocean ridges, to demonstrate a strong heterogeneity of sediment-corrected isostatic cooling subsidence both between and within normal oceans. Anomalous oceans are identified from bathymetry deviation from age-dependent predictions during data processing. Subsidence parameters for individual ocean segments significantly deviate from global constants in conventional models and show a large variability of subsidence rate (270–535 m/Ma1/2) and zero-age depth (−1.30 to −3.03 km) with plate thickness estimated between 50 and 160 km for cooling models with constant mantle properties. Statistically strong correlations (R2=0.80–0.94) between major characteristics of cooling subsidence and spreading rate indicate that ocean evolution is essentially controlled by spreading rate, despite this factor is not included in conventional models of ocean subsidence. Normal oceans with slower spreading rate have, statistically, higher subsidence rate which implies faster gravitational collapse caused by faster plate cooling with moderate-to-low mantle temperatures at mid-ocean ridges. Fast-spreading oceans have the opposite characteristics. The ultraslow SW Indian and the fast-spreading Central Pacific Oceans are the end-members in ocean cooling subsidence trends, with the Atlantic/NW Indian Oceans tending towards the ultraslow end, and the Pacific/SE Indian Oceans being closer to the fast-spreading end. The Arctic Ocean and the Atlantics north of the Charlie-Gibbs Fracture Zone with an atypical subsidence behavior often deviate from the global trends. Strong correlation between spreading rate, ocean half-width and the type of ocean margins implies that ridge-push dominates tectonic forces in slower-spreading, narrower oceans with passive margins, while slab-pull at active margins is a dominant tectonic force in faster-spreading oceans with half-width exceeding 4250 km. The age of bathymetry departure from cooling subsidence, controlled by distribution of hotspots on ocean floor, correlates (R2=0.76) with spreading rate, and thus is not fully random. Slower-spreading oceans follow normal cooling subsidence to older ages (7.5–9.5 Ma1/2) than faster-spreading oceans (5–7 Ma1/2). Recognition that spreading rate controls ocean evolution with formation of active or passive ocean margins dominated by slab-pull or ridge-push contributes to advances in understanding driving forces in geodynamics. [Display omitted] [ABSTRACT FROM AUTHOR]

At most mid-ocean ridges, a wide region of decompression melting must be reconciled with a narrow neovolcanic zone and the establishment of full oceanic crustal thickness close to the rift axis. Two competing paradigms have been proposed to explain melt focusing: narrow mantle upwelling due to dynamic effects related to in situ melt or wide mantle upwelling with lateral melt transport in inclined channels. Measurements of seismic attenuation provide a tool for identifying and characterizing the presence of melt and thermal heterogeneity in the upper mantle. We use a unique data set of teleseismic body waves recorded on the Cascadia Initiative's Amphibious Array to simultaneously measure seismic attenuation and velocity across an entire oceanic microplate. We observe maximal differential attenuation and the largest delays ([Formula: see text] s and δTS ~ 2 s) in a narrow zone

Abstract We analyze a large database of abyssal peridotite clinopyroxene compositions using log‐ratio transformation, principal component analysis (PCA) and k‐means clustering, to better understand clinopyroxene compositional systematics in abyssal peridotites. We combine this analysis with open‐system melting models to investigate the potential sources of compositional variation. PCA shows that 84% of the variation in clinopyroxene compositions can be represented using only 2‐dimensional information. We use k‐means clustering to classify clinopyroxene compositions into four clusters. Clusters 1–3, representing 85% of the data, show progressive depletions in LREE/HREE, and are associated with decreases in Na2O in clinopyroxene, and general increases in Cr# of spinel. We interpret peridotites with clinopyroxene compositions from clusters 1–3 to represent residues of partial melt extraction. The degree of melt extraction increases from cluster 1 to 3, and exerts a primary control on compositional variations. The presence or absence of garnet‐field melting prior to spinel‐field melting and the retained melt fraction during partial melting exert secondary controls on clinopyroxene compositions. Cluster 4 clinopyroxenes, representing show less fractionated LREE/HREE with low‐HREE abundances, elevated Sr, and depleted signatures in their host peridotites. Clinopyroxene compositions in cluster 4 cannot be modeled by melt depletion alone. Instead, they are only reconstructed in our models where melt‐rock interaction, suggesting that peridotites with cluster 4 clinopyroxenes have experienced both of these processes. Clusters 1–4 are observed in most ridges, reflecting compositional heterogeneity on each ridge. This variability reflects variations in the degree of partial melting, amount of garnet‐field melting, retained melt fractions, and melt‐rock interaction.

Melt extraction from the partially molten mantle is among the fundamental processes shaping the solid Earth today and over geological time. A diversity of properties and mechanisms contribute to the physics of melt extraction. We review progress of the past ∼25 years of research in this area, with a focus on understanding the speed and style of buoyancy-driven melt extraction. Observations of U-series disequilibria in young lavas and the surge of deglacial volcanism in Iceland suggest this speed is rapid compared to that predicted by the null hypothesis of diffuse porous flow. The discrepancy indicates that the style of extraction is channelized. We discuss how channelization is sensitive to mechanical and thermochemical properties and feedbacks, and to asthenospheric heterogeneity. We review the grain-scale physics that underpins these properties and hence determines the physical behavior at much larger scales. We then discuss how the speed of melt extraction is crucial to predicting the magmatic response to glacial and sea-level variations. Finally, we assess the frontier of current research and identify areas where significant advances are expected over the next 25 years. In particular, we highlight the coupling of melt extraction with more realistic models of mantle thermochemistry and rheological properties. This coupling will be crucial in understanding complex settings such as subduction zones. Mantle melt extraction shapes Earth today and over geological time. Observations, lab experiments, and theory indicate that melt ascends through the mantle at speeds ∼30 m/year by reactively channelized porous flow. Variations in sea level and glacial ice loading can cause significant changes in melt supply to submarine and subaerial volcanoes. Fluid-driven fracture is important in the lithosphere and, perhaps, in the mantle wedge of subduction zones, but remains a challenge to model. [ABSTRACT FROM AUTHOR]

Distinct signatures are present in the circum‐Pacific continental margins (e.g., kinematics, magmatism, and basin evolution), possibly influenced by the input of mid‐ocean ridge. It remains enigmatic why the circum‐Pacific continental margins that have experienced trench‐parallel mid‐ocean ridge subduction show diverse geological evolution. Here we present geodynamic modeling results investigating trench‐parallel mid‐ocean ridge subduction and demonstrate two distinct types of model evolution. Type‐Ⅰ model includes a two‐stage steep subduction and is featured by slab detachment preceding the arrival of ridge at the trench. Type‐Ⅱ model is marked by a continuous flat subduction of mid‐ocean ridge with the opening of a slab window beneath intracontinental lithosphere. These two subduction styles produce diverse tectono‐magmatic responses. Our results could explain the magmatic gap and forearc uplift during the Izanagi‐Pacific ridge subduction and the intraplate magmatic flare‐ups and tectonic uplift during the Nazca‐Antarctic ridge subduction, respectively. Plain Language Summary: Mid‐ocean ridge subduction potentially occurs during the evolution of all subduction zones and has significant influence on subduction dynamics and the related geological evolution. The circum‐Pacific domain that has experienced trench‐parallel mid‐ocean ridge subduction shows contrasting tectono‐magmatic evolution. However, it remains poorly understood how the input of mid‐ocean ridge in subduction zones affects the geological evolution of these continental margins. Here, we conduct a series of numerical experiments and reveal two distinct subduction styles. The modeling results show that an interruption of ridge subduction can induce magmatic quiescence, forearc mid‐ocean ridge basalts, and forearc tectonic uplift, whereas the continuous ridge subduction can induce magmatic flare‐ups and tectonic uplift in intraplate region. These two contrasting models are strongly dependent on subducting slab dips. This study provides a new simple model to explain the complex Cenozoic geological evolution observed in the northwest and southeast Pacific continental margins. Key Points: Geodynamic modeling reveals the dynamics of trench‐parallel mid‐ocean ridge subduction affected by the dip angles of the subducting slabSteep subduction promotes slab detachment along the ridge axis, but flat subduction facilitates continuous subduction of the ridgeModel results could explain the contrasting geological features observed in the northwest and southeast Pacific continental margins [ABSTRACT FROM AUTHOR]

Geodynamic simulations underpin our understanding of upper‐mantle processes, but their predictions require validation against observational data. Widely used geophysical datasets provide limited constraints on dynamic processes into the geological past, whereas under‐exploited geochemical observations from volcanic lavas at Earth's surface constitute a valuable record of mantle processes back in time. Here, we describe a new peridotite‐melting parameterization, BDD21, that can predict the incompatible‐element concentrations of melts within geodynamic simulations, thereby providing a means to validate these simulations against geochemical datasets. Here, BDD21's functionality is illustrated using the Fluidity computational modeling framework, although it is designed so that it can be integrated with other geodynamic software. To validate our melting parameterization and coupled geochemical‐geodynamic approach, we develop 2‐D single‐phase flow simulations of melting associated with passive upwelling beneath mid‐oceanic ridges and edge‐driven convection adjacent to lithospheric steps. We find that melt volumes and compositions calculated for mid‐oceanic ridges at a range of mantle temperatures and plate spreading rates closely match those observed at present‐day ridges with the same conditions. Our lithospheric step simulations predict spatial and temporal melting trends that are consistent with those recorded at intraplate volcanic provinces in similar geologic settings. Taken together, these results suggest that our coupled geochemical‐geodynamic approach can accurately predict a suite of present‐day geochemical observations. Since our results are sensitive to small changes in upper‐mantle thermal and compositional structure, this novel approach provides a means to improve our understanding of the mantle's thermo‐chemical structure and flow regime into the geological past. Plain Language Summary: Earth's mantle is a ∼3,000 km thick layer of hot rock that lies between Earth's crust and core. When anomalously hot mantle arrives at shallow depths it begins to melt. These melts are less dense than the surrounding mantle and ascend to the surface to erupt as volcanic lavas. The mantle's slow, creeping, convective motion over billions of years has been integral to Earth's thermal, chemical, tectonic and geological evolution. However, an inability to reproduce observational constraints derived from the composition of volcanic lavas at Earth's surface limits our capacity to validate models of mantle convection back in time. Here, we present a new framework that can predict the volume and composition of melts generated within the mantle. These predictions compare favorably with those recorded by igneous rocks at Earth's surface in two geologic settings: mid‐oceanic ridges, where plates move apart to drive decompression melting, and lithospheric steps, where instabilities associated with changes in the thickness of Earth's rigid outermost shell generate volcanism far from plate boundaries. The approach and tools presented here will allow scientists to better understand the mantle's past structure, dynamics, evolution and impact on Earth's surface. Key Points: We present the BDD21 peridotite‐melt‐chemistry parameterization that can be coupled with geodynamic models to calculate melt compositionBDD21 is applied to simulations of mid‐oceanic ridges to replicate observed patterns of crustal thickness and melt chemistryGeodynamic simulations that incorporate BDD21 can be used to study melting adjacent to lithospheric steps and in other geologic settings [ABSTRACT FROM AUTHOR]

Abstract Geodynamic simulations underpin our understanding of upper‐mantle processes, but their predictions require validation against observational data. Widely used geophysical datasets provide limited constraints on dynamic processes into the geological past, whereas under‐exploited geochemical observations from volcanic lavas at Earth's surface constitute a valuable record of mantle processes back in time. Here, we describe a new peridotite‐melting parameterization, BDD21, that can predict the incompatible‐element concentrations of melts within geodynamic simulations, thereby providing a means to validate these simulations against geochemical datasets. Here, BDD21's functionality is illustrated using the Fluidity computational modeling framework, although it is designed so that it can be integrated with other geodynamic software. To validate our melting parameterization and coupled geochemical‐geodynamic approach, we develop 2‐D single‐phase flow simulations of melting associated with passive upwelling beneath mid‐oceanic ridges and edge‐driven convection adjacent to lithospheric steps. We find that melt volumes and compositions calculated for mid‐oceanic ridges at a range of mantle temperatures and plate spreading rates closely match those observed at present‐day ridges with the same conditions. Our lithospheric step simulations predict spatial and temporal melting trends that are consistent with those recorded at intraplate volcanic provinces in similar geologic settings. Taken together, these results suggest that our coupled geochemical‐geodynamic approach can accurately predict a suite of present‐day geochemical observations. Since our results are sensitive to small changes in upper‐mantle thermal and compositional structure, this novel approach provides a means to improve our understanding of the mantle's thermo‐chemical structure and flow regime into the geological past.

We examine the main drivers that may elevate biomass and biodiversity of non-chemosynthetic benthic megafauna of the lower bathyal (800-3500m depth) of the Mid-Atlantic Ridge in the North Atlantic Ocean (MAR). Specifically: 1. Primary production in surface waters (10°-48°N) from remote sensing data 2002-2020 over the MAR was not significantly different from abyssal regions to the east and west. We reject the hypothesis that presence of a mid ocean ridge may enhance surface primary production. 2. The quantity of particulate organic matter reaching the sea floor was estimated as a proportion of surface export production scaled by bathymetry. Flux was 1.3 to 3.0 times greater on the MAR as a function of shorter vertical transport distance from the surface than on adjacent abyssal regions. 3. Depth variation effect on species richness. Demersal fishes living between 41° and 60°N showed a maximum of species richness at 2000 m depth and linear increase in regional (Gamma) diversity of 32 species per 1,000 m elevation of the MAR above the abyss. Elevated topography provides niches for species that cannot otherwise survive. 4. Substrate heterogeneity. The MAR >95% covered with soft sediment with frequent hard rocky patches spaced at a mean nearest neighbour distance of

Plate divergence along mid‐ocean ridges is accommodated through faulting and magmatic accretion, and, at overlapping spreading centers (OSC), is distributed across two curvilinear overlapping ridge axes. One‐meter resolution bathymetry acquired by autonomous underwater vehicles, combined with distribution and ages of lava flows, is used to: (a) analyze the spatial and temporal distribution of flows, faults, and fissures in the OSC between the distal south rift zone of Axial Seamount and the Vance Segment, (b) locate spreading axes, (c) calculate extension, and (d) determine the proportion of extension accommodated at the surface by faults and fissures versus volcanic extrusion over a period of ∼1300–1450 years. Our study reveals that in the recent history of the ridges, extension over a distance of 14 km across the Axial/Vance OSC was asymmetric in proportion and style: faults and fissures across 1–2 km of the Vance axial valley accommodated ∼3/4 of the spreading, whereas dike‐fed eruptions contributed ∼1/4 of the extension and occurred across 4 km of the south rift of Axial Seamount. Plain Language Summary: Along mid‐ocean ridges, oceanic plates separate through the formation and growth of faults and the emplacement of dikes supplying lava flows. Where segments overlap in a zone of separation, these processes are distributed along two spreading axes separated by 2–30 km kilometers. We combine 1‐m resolution bathymetry collected by autonomous underwater vehicles and the age of large lava flows to (a) analyze the distribution of faults and lava flows where Axial Seamount overlaps with the Vance Segment, (b) define the current plate boundary, (c) calculate the speed of plate separation, and (d) determine the proportion and locations of fault extension versus flow emplacement. Our study shows that during the last ∼1300–1450 years, fault formation and growth along the Vance Segment are the main contributor to plate separation. In contrast, the emplacement of dikes and lava flows along Axial Seamount account only for ∼1/4 of the plate separation. Key Points: Autonomous underwater vehicle mapping of an overlapping spreading center reveals the proportion of faulting and eruptions that occurred during the last ∼1300–1450 yearsFaulting at the Vance Segment accommodates ∼3/4 of the spreading and magmatic accretion along Axial Seamount south rift accounts for ∼1/4The spreading axis is <250 m wide along the Vance Segment but ∼4 km wide along the south rift of Axial Seamount [ABSTRACT FROM AUTHOR]

The axial morphology of the Southeast Indian Ridge (SEIR) between Australia and Antarctica changes dramatically along 120oE to 127oE (Fig. 1). At approximately 127oE the ridge changes character along with a difference in water depth of about 1 km. Eastward it changes from a Mid-Atlantic Ridge (MAR) type cross-section to an East-Pacific Rise (EPR) type cross-section (Fig. 2). The MAR profiles a bathymetric low (5- to 20-km wide, 500- to 1500-m-deep rift valley), while the EPR profiles a bathymetric high (10-km-wide, 500-m-high ridge). The MAR type extends westward to approximately 100oE and is called the Australian-Antarctic Discordance (AAD). This geomorphology is unique globally. The MAR is considered a slow-spreading center and like the AAD has similar segmentation characteristics of non-transform (small, oE, and westward propagating rifts are the only non-transform discontinuities.

Abstract Plate divergence along mid‐ocean ridges is accommodated through faulting and magmatic accretion, and, at overlapping spreading centers (OSC), is distributed across two curvilinear overlapping ridge axes. One‐meter resolution bathymetry acquired by autonomous underwater vehicles, combined with distribution and ages of lava flows, is used to: (a) analyze the spatial and temporal distribution of flows, faults, and fissures in the OSC between the distal south rift zone of Axial Seamount and the Vance Segment, (b) locate spreading axes, (c) calculate extension, and (d) determine the proportion of extension accommodated at the surface by faults and fissures versus volcanic extrusion over a period of ∼1300–1450 years. Our study reveals that in the recent history of the ridges, extension over a distance of 14 km across the Axial/Vance OSC was asymmetric in proportion and style: faults and fissures across 1–2 km of the Vance axial valley accommodated ∼3/4 of the spreading, whereas dike‐fed eruptions contributed ∼1/4 of the extension and occurred across 4 km of the south rift of Axial Seamount.

We investigate the spatial distribution of diapycnal mixing and its drivers in the central South Atlantic thermocline between the Rio-Grande Rise to the Mid-Atlantic Ridge. Diapycnal mixing in the ocean interior influences the slowly evolving meridional circulation, yet there are few observations of its variability with space and time or its drivers. To overcome this gap, seismic reflection data are spectrally analyzed to produce a 1,600 km long full-thermocline vertical section of diapycnal diffusivity, that has a vertical and horizontal resolution of O(10) m and spans a period of 4 weeks. We compare seismic-derived diffusivities with CTD-derived diffusivities and direct observations from 1996, 2003, and 2011. In the mean and on decadal scales, we find that thermocline diffusivities have changed little in this region, retaining a background value of 1 × 10–5 m2 s–1. Imprinted upon the background rates, mixing is heterogeneous at mesoscales. Enhanced mixing, exceeding 10 × 10–5 m2 s–1 and spreading between 200 and 700 m depth, is found above the Mid-Atlantic Ridge suggesting the ridge enhances diffusivity by at least one order of magnitude across the entire water column. Rapid decay of diffusivities within 30 km of the ridge implies local dissipation of tidal energy. Above smooth topography, patches of enhanced mixing are possibly caused by a recent storm that injects near-inertial energy into the water column and elevates mixing from 3 × 10–5 m2 s–1 to 50 × 10–5 m2 s–1 down to depths of more than 600 m. The propagation speed of near-inertial energy varies substantially from 17 to 27 m/day. Faster speed, and therefore greater penetration depths of 800 m, are probably facilitated by an eddy. Together, these data extend the observational record of central South Atlantic thermocline mixing and provide insights into drivers of mesoscale variability.

A high‐resolution sampling profile constituted of 52 basalt samples from across the East Pacific Rise (EPR) was investigated. These samples provide a unique opportunity to study the coeval recording of isotopic signals derived from sub‐marine eruptions at a fast spreading ridge over a time interval of ∼320 kyrs. Additionally, the study area is marked by the presence of a neighboring seamount chain that has recently caused the ridge to jump twice toward it. Combining previous geochemical studies and bathymetry, we established a first‐order chronology between analyzed samples, and have reconstructed the evolution of basalt compositions as the ridge and seamounts advance and finally merge. Our data reveal the existence of two distinct types of isotopic variability within the samples. One that has a low amplitude and frequency and is accounted for by the continuous melting of the ambient mantle, indicating a process with a ∼125 kyr periodicity. The other, of higher amplitude, is discontinuous in time, and likely reflects the seamounts source influence on the ridge. Our results support a two‐step hotspot‐ridge interaction including a first stage (≥600 ka) of regional enrichment of the depleted ridge mantle by hotspot material; and a second, more recent (at least 300 ka) even wherein ambient mantle melts mixed with proximal melts from heterogeneous seamounts sourced nearby. We also propose that the ancient gabbroic component previously identified in this region appeared very recently (300 kyrs. We have established a relative chronology between these samples, and propose a paleo‐reconstruction of the area accounting for both the isotopic variations observed and previously reported geophysical results. Our new results also suggest rapid magma transfers within the crust independent of melting of seamounts and/or ridge sources, including periodic injections of enriched mantle material into the ridge‐melting region. Key Points: Ridge‐hotspot interactions were tracked via the isotopic composition of basalts sampled across the East Pacific Rise axisChronological reconstructions of collected mid‐ocean ridge basalts reveal a periodicity of 125 kyrs in the recorded isotopic signalHigh‐resolution sampling allows the integration of geochemical information within the framework of geophysical and geological models [ABSTRACT FROM AUTHOR]

Abstract The 4.5 km deep IDDP‐2 drill hole was drilled at Reykjanes, Iceland, in an active seawater‐recharged hydrothermal system on the landward extension the Mid‐Atlantic Ridge. Drilling targeted a well‐defined hydrothermal up‐flow zone feeding the Reykjanes geothermal reservoir, which produces fluids compositionally equivalent to basalt‐hosted deep sea hydrothermal vents. Spot cores recovered from depths between 3,648 to 4,659 m depths consist of pervasively altered sheeted diabase dikes. The shallowest core has an alteration mineral assemblage similar to the producing geothermal reservoir but is being overprinted by the underlying Hornblende Zone assemblage dominated by labradorite and hornblende. The deepest cored section (4,634–4,659 m) retains a diabasic texture, but all primary minerals are replaced or have changed composition. Plagioclase ranges from An30 to An99 and igneous augite is replaced by intergrown hornblende, clinopyroxene, and orthopyroxene. Hydrothermal biotite and potassium feldspar formed locally due to reaction with a phase‐separated brine, indicated by co‐existing vapor‐rich and salt + vapor‐rich fluid inclusions. Seawater‐derived supercritical hydrothermal fluid entering the bottom of the bore hole actively phase separates due pressure drop controlled by the fluid levels in the drill hole. Felsic veins, present in trace amounts, record a continuous transition from magmatic to hydrothermal conditions, including incipient hydrous melting. The vertical changes observed in mineralogy and mineral chemistry indicate that fluids from the deep high temperature reaction zone can undergo significant cooling and reaction with host rocks along their path to the seafloor.

Trench-parallel subduction of mid-ocean ridges occurs frequently in plate motion history, such as along the western boundary of the Pacific plate in the early Cenozoic and along the eastern boundary of the Pacific plate at present. Such subduction may strongly alter the surface topography, volcanic activity and slab morphology in the mantle, whereas few studies have been conducted to investigate its evolutionary process. Here, we construct a 2-D viscoelastoplastic numerical model to study the modes and key parameters controlling trench-parallel subduction of mid-ocean ridges. Our model results show that the subduction modes of mid-ocean ridges can be primarily categorized into three types: the fast spreading mode, the slow spreading mode, and the extinction mode. The key factor controlling these subduction modes is the relative motion between the foregoing and the following oceanic plates, which are separated by the mid-ocean ridge. Different subduction modes exert different surface geological expressions, which may explain specific evolutionary processes related to mid-ocean ridge subduction, such as topographic deformation and the eruption gap of volcanic rocks in East Asia within 55–45 Ma and in the western North American plate during the late Cenozoic.

Abstract A high‐resolution sampling profile constituted of 52 basalt samples from across the East Pacific Rise (EPR) was investigated. These samples provide a unique opportunity to study the coeval recording of isotopic signals derived from sub‐marine eruptions at a fast spreading ridge over a time interval of ∼320 kyrs. Additionally, the study area is marked by the presence of a neighboring seamount chain that has recently caused the ridge to jump twice toward it. Combining previous geochemical studies and bathymetry, we established a first‐order chronology between analyzed samples, and have reconstructed the evolution of basalt compositions as the ridge and seamounts advance and finally merge. Our data reveal the existence of two distinct types of isotopic variability within the samples. One that has a low amplitude and frequency and is accounted for by the continuous melting of the ambient mantle, indicating a process with a ∼125 kyr periodicity. The other, of higher amplitude, is discontinuous in time, and likely reflects the seamounts source influence on the ridge. Our results support a two‐step hotspot‐ridge interaction including a first stage (≥600 ka) of regional enrichment of the depleted ridge mantle by hotspot material; and a second, more recent (at least 300 ka) even wherein ambient mantle melts mixed with proximal melts from heterogeneous seamounts sourced nearby. We also propose that the ancient gabbroic component previously identified in this region appeared very recently (

The Shatsky Rise ridge-ridge-ridge triple junction is an ancient triple junction in the Western Pacific Ocean whose initial geodynamic process is poorly understood and can only be inferred based on indirect geological and geophysical constraints. In this paper, we present three-dimensional numerical models that simulate the Shatsky Rise triple junction and calculate its coupled mantle flow and temperature structure. The mantle flow velocity field shows several distinctive features: 1) stronger mantle upwelling closer to the ridge axis and triple junction; 2) greater upwelling velocity at the faster-spreading ridges; and 3) the most significant increase in upwelling velocity for the slowest-spreading ridge toward the triple junction. The calculated mantle temperature field also reveals distinctive characteristics: 1) sharp increases in the mantle temperature with depth and increases toward the spreading ridges and triple junction; 2) the faster-spreading ridges are associated with higher temperatures at depth and identical distances from the triple junction; and 3) the slowest-spreading ridge shows the greatest increase in the along-ridge-axis temperature toward the triple junction. Compared to many present-day triple junctions with slower spreading rates, the along-ridge-axis velocity and thermal fields of the Shatsky Rise are more altered due to the presence of the triple junction. [ABSTRACT FROM AUTHOR]

Recent multi‐channel seismic studies of fast spreading and hot‐spot influenced mid‐ocean ridges reveal magma bodies located beneath the mid‐crustal Axial Magma Lens (AML), embedded within the underlying crustal mush zone. We here present new seismic images from the Juan de Fuca Ridge that show reflections interpreted to be from vertically stacked magma lenses in a number of locations beneath this intermediate‐spreading ridge. The brightest reflections are beneath Northern Symmetric segment, from ∼46°42′‐52′N and Split Seamount, where a small magma body at local Moho depths is also detected, inferred to be a source reservoir for the stacked magma lenses in the crust above. The imaged magma bodies are sub‐horizontal, extend continuously for along‐axis lengths of ∼1–8 km, with the shallowest located at depths of ∼100–1,200 m below the AML, and are similar to sub‐AML bodies found at the East Pacific Rise. At both ridges, stacked sill‐like lenses are detected beneath only a small fraction of the ridge length examined and are inferred to mark local sites of higher melt flux and active replenishment from depth. The imaged magma lenses are focused in the upper part of the lower crust, which coincides with the most melt rich part of the crystal mush zone detected in other geophysical studies and where sub‐vertical fabrics are observed in geologic exposures of oceanic crust. We infer that the multi‐level magma accumulations are ephemeral and may result from porous flow and mush compaction, and that they can be tapped and drained during dike intrusion and eruption events. Key Points: Multi‐channel seismic data reveal stacked magma lenses within the crystal mush zone at the Juan de Fuca RidgeStacked lenses underlie the Axial Magma Lens within the melt‐rich mid crust and span depths of the foliated gabbros in crustal exposuresMid‐crust magma lenses are interpreted to be ephemeral, formed by mush compaction and contributing magmas during eruption events [ABSTRACT FROM AUTHOR]

We quantified the systematic variations in global transform fault morphology, revealing a first-order dependence on the spreading rate. (1) The average age offset of both the full transform and transform sub-segments decrease with increasing spreading rate. (2) The average depth of both the transform valley and adjacent ridges are smaller in the fast compared to the slow systems, reflecting possibly density anomalies associated with warmer mantle at the fast systems and rifting at the slow ridges. However, the average depth difference between the transform valley and adjacent ridges is relatively constant from the fast to slow systems. (3) The nodal basin at a ridge-transform intersection is deeper and dominant at the ultraslow and slow systems, possibly reflecting a lower magma supply and stronger viscous resistance to mantle upwelling near a colder transform wall. In contrast, the nodal high, is most prominent in the fast, intermediate, and hotspot-influenced systems, where robust axial volcanic ridges extend toward the ridge-transform intersection. (4) Statistically, the average transform valley is wider at a transform system of larger age offset, reflecting thicker deforming plates flanking the transform fault. (5) The maximum magnitude of the transform earthquakes increases with age offset owing to an increase in the seismogenic area. Individual transform faults also exhibit significant anomalies owing to the complex local tectonic and magmatic processes. [ABSTRACT FROM AUTHOR]

Abstract Ultraslow spreading ridges form the slowest divergent plate boundaries and exhibit distinct spreading processes in volcanically active magmatic sections and intervening amagmatic sections. Local seismicity studies of ultraslow spreading ridges until now cover only parts of segments and give insight into spreading processes at confined locations. Here, we present a microseismicity data set that allows to study spreading processes on the scale of entire segments. Our network of 26 ocean bottom seismometers covered around 160 km along axis of the ultraslow spreading Knipovich Ridge in the Greenland Sea and recorded earthquakes for a period of about 1 year. We find seismicity varying distinctly along‐axis. The maximum earthquake depths shallow over distances of 70 km toward the Logachev volcanic center. Here, swarm activity occurs in an otherwise aseismic zone. Melts may thus be guided along the subparallel topography of the lithosphere‐asthenosphere boundary toward major volcanic centers explaining the uneven along‐axis melt distribution typical for ultraslow ridges. Absence of shallow seismicity in the upper 8 km of the lithosphere with a band of deep seismicity underneath offsets presumably melt‐poor regions from magma richer sections. Aseismic deformation in these regions may indicate weakening of mantle rocks by alteration. We do not find obvious indications for major detachment faulting that characterizes magma‐poor spreading at some ultraslow spreading segments. The highly oblique spreading of Knipovich Ridge may be the reason for a fine‐scale segmentation of the seismic activity with zones of weak seismicity possibly indicating transform motion on short obliquely oriented faults.

Ultraslow spreading ridges form the slowest divergent plate boundaries and exhibit distinct spreading processes in volcanically active magmatic sections and intervening amagmatic sections. Local seismicity studies of ultraslow spreading ridges until now cover only parts of segments and give insight into spreading processes at confined locations. Here, we present a microseismicity data set that allows to study spreading processes on the scale of entire segments. Our network of 26 ocean bottom seismometers covered around 160 km along axis of the ultraslow spreading Knipovich Ridge in the Greenland Sea and recorded earthquakes for a period of about 1 year. We find seismicity varying distinctly along‐axis. The maximum earthquake depths shallow over distances of 70 km toward the Logachev volcanic center. Here, swarm activity occurs in an otherwise aseismic zone. Melts may thus be guided along the subparallel topography of the lithosphere‐asthenosphere boundary toward major volcanic centers explaining the uneven along‐axis melt distribution typical for ultraslow ridges. Absence of shallow seismicity in the upper 8 km of the lithosphere with a band of deep seismicity underneath offsets presumably melt‐poor regions from magma richer sections. Aseismic deformation in these regions may indicate weakening of mantle rocks by alteration. We do not find obvious indications for major detachment faulting that characterizes magma‐poor spreading at some ultraslow spreading segments. The highly oblique spreading of Knipovich Ridge may be the reason for a fine‐scale segmentation of the seismic activity with zones of weak seismicity possibly indicating transform motion on short obliquely oriented faults. Plain Language Summary: At mid‐ocean spreading ridges, tectonic plates drift apart and new seafloor is built by upwelling magma. The slowest spreading ridges do not receive enough magma to build new seafloor along the entire ridge. Rather, they show widely spaced volcanic centers with magma‐poor areas in‐between. The study of small earthquakes with seismometers placed on the seafloor has greatly helped to understand how new seafloor forms. Since such studies require substantial logistic effort, only confined ridge sections have been studied and spreading processes operating at segment‐scale remain poorly understood. In this study, we present for the first time observations of earthquakes covering several segments and one major volcanic center along the Knipovich Ridge in the Greenland Sea. Underneath the volcano, earthquake swarms and a gap in seismicity indicate recent magmatic activity. The maximum depth of earthquakes marks the thickness of the mechanically strong lithosphere. It shallows over 70 km toward the volcano such that melts can be channeled over large distances to the prominent volcanoes. Magma‐poor regions have deep earthquakes but do not show earthquake activity in the upper 8 km. We suppose that water reacts with the mantle rocks that become too weak to break in earthquakes. Key Points: Magma‐poor sections are distinguished from magma‐rich sections by deeper hypocenters and an absence of shallow seismicityShallowing maximum earthquake depths over distances of 70 km suggest along‐axis melt focusing toward major volcanic centersMajor detachment faults on the highly oblique spreading Knipovich Ridge were not obvious in the observed seismicity [ABSTRACT FROM AUTHOR]

• MOR vent fluids show the largest range of δ138/134Ba seen so far in marine systems. • Endmember vent fluid δ138/134Ba values are the same as those of the source rocks. • Barite precipitation leads to high δ138/134Ba values in vent fluids as they evolve. • Hydrothermal Ba input may explain non-conservative δ138/134Ba seen in deep waters. • Hydrothermal input contributes 3–9% of the Ba in some Atlantic deep waters. Mid-ocean ridge (MOR) hydrothermal vent fluids are enriched with dissolved barium, but due to barite (BaSO 4) precipitation during mixing between Ba-bearing vent fluids and SO 4 -bearing seawater, the magnitude of hydrothermal Ba input to the ocean remains uncertain. Deep-ocean Ba isotopes show evidence for non-conservative behavior, which might be explained by input of isotopically heavy hydrothermal Ba. In this study we present the first Ba isotope data in mid-ocean ridge hydrothermal vent fluids and particles from systems on the Mid-Atlantic Ridge (Rainbow 36°N and TAG 26°N), the East Pacific Rise (EPR9–10°N and 13°N) and the Juan de Fuca Ridge (MEF and ASHES). The vent fluids display a wide range of dissolved Ba concentrations from 0.43 to 97.9 μmol/kg and δ138/134Ba values from −0.26 to +0.91‰, but are modified relative to initial composition due to precipitation of barite. Calculated endmember vent fluid δ138/134Ba values, prior to barite precipitation, are between −0.17 and +0.09‰, consistent with the values observed in oceanic basalts and pelagic sediments. Water-rock interaction at depth in the oceanic crust appears to occur without Ba isotope fractionation. During subsequent venting and mixing with seawater, barite precipitation preferentially removes isotopically light Ba from vent fluids with a fractionation factor of Δ138/134Ba hyd-barite-fluid = −0.35 ± 0.10‰ (2SE, n = 2). Based on knowledge of barite saturation and isotope fractionation during precipitation, the effective hydrothermal Ba component that mixes with seawater after barite precipitation has completed can be calculated: δ138/134Ba hyd = +1.7 ± 0.7‰ (2SD). This value is isotopically heavier than deep ocean waters and may explain the observed non-conservative of Ba isotopes in deep waters. These new constraints on hydrothermal Ba compositions enable the hydrothermal input of Ba to Atlantic deep waters to be assessed at ≈3–9% of the observed Ba. Barium isotopes might be used as a tracer to reconstruct the history of hydrothermal Ba inputs and seawater SO 4 concentrations in the past. [ABSTRACT FROM AUTHOR]

Abstract In 2016, temperature recorders were recovered, temperatures were measured, and fluid samples were collected from Vent 1, a high temperature (338°C) hydrothermal discharge site on the southern Cleft Segment of the Juan de Fuca Ridge. Coupled with previous sampling efforts, this collection represents a 32‐year record of discharge from a single chimney structure, the longest record to date. Remarkably, the fluid has remained brine‐dominated for more than three decades. This brine formed during phase separation and segregation prior to initial observations in 1984. Although the chloride concentration of the discharging fluid has decreased with time, the fluid temperature has remained nearly constant for at least 3.3 years and probably for 15 or even 22 years. Compositions of the discharging fluids are consistent with inputs from a deep‐sourced brine, which was last equilibrated at >400°C at a depth consistent with the base of the sheeted dikes and the brittle‐ductile transition. This brine mixed (diffusion or dispersion) with a likely non‐phase‐separated, hydrothermal fluid prior to discharge. A survey of hydrothermal endmember fluids with chlorinities in excess of 700 mmol/kg shows, with the exception of Fe, a single trend between major ion concentrations and chlorinity even though data are from a range of crustal compositions, spreading rates, and water and magma depths. Calculated deep‐sourced brines from hydrothermal fluid data are similar to data based on fluid inclusions and estimates of brine assimilation in magmas. A better understanding of brines is required given their potential duration of discharge and capacity for mobilizing metals.

AbstractThe existence, or not, of periodicities in abyssal hill morphology has been vigorously debated in recent publications, and some have hypothesized that such periodicities are evidence of the impact of Milankovitch cycle‐caused sea level fluctuations on the volcanic construction process at mid‐ocean ridges. Periodicities are detected by the presence of spectral peaks that rise significantly above the random variations of sample power spectra associated with an aperiodic, continuous spectrum process, typically modeled as a band‐limited fractal (von Kármán model). Here, I formulate and test a new algorithm to “empirically prewhiten” the sample power spectrum which, without needing to model the continuous spectrum, flattens it to a zero‐mean process. This greatly simplifies definition of the null hypothesis, and additional modeling approximates standard deviation levels that provide a conservative basis for detecting peaks that may be indicative of periodicity. The algorithm is applied to extensive bathymetric data flanking the southern East Pacific Rise. Significant periodicities are detected on many profiles analyzed, but the periods vary widely, and do not cluster at Milankovitch periods. The most substantial harmonic signals detected exhibit periods ∼0.082–0.216 my, with root‐mean square (RMS) heights approximately a quarter to a third of the RMS height for the aperiodic signal. It is hypothesized that the dominant aperiodic component of abyssal hills corresponds to morphology constructed by faults that follow a random distribution governed by scaling laws, whereas longer‐scale periodic signals are associated with crustal thickness variations controlled internally by variations in melt supply.

Abstract Quantifying eruption dynamics in submarine environments is challenging. During the 2015 eruption of Axial Seamount, the formation of hummocky mounds along the north rift was accompanied by tens of thousands of impulsive acoustic signals generated by the interaction of lava and seawater. A catalog of these sounds was integrated with detailed seafloor mapping to better understand eruptive processes in time and space. Mounds grew over a period of 28 days with average extrusion rates of 22 to 45 m3 s−1. The most distant mounds, ~9.5 to 15.5 km down rift from the caldera, grew primarily over the first few days of the eruption. The focus of eruptive activity then retreated ~5 km toward the caldera where it was sustained. Mounds are constructed as a series of superimposed lobes formed through alternating periods of flow inflation, generating up to 30‐m‐thick hummocks, and periods of flow advancement, with

Abstract Four new hydrothermal vent fields were discovered on the slow spreading Central Indian Ridge (8–12°S; Segments 1–3), all located off‐axis on abyssal hill structures or Ocean Core Complexes (OCCs). Each site was characterized using seafloor observation (towed camera system), plume chemistry (Fe, Mn, and CH4; Conductivity, Temperature, and Depth sensor [CTD]/Miniature Autonomous Plume Recorder [MAPR]), and rock sampling (TVgrab/dredges). Different styles of venting on each segment reflect different geological settings, rock types, likely heat sources, and fluid pathways. The segment 1 field was located on the western flank of the axial valley at the base of OCC‐1‐1. High‐temperature venting was inferred from plume characteristics and extensive seafloor sulfide mineralization, but only diffuse venting was observed. This site appears to be a magmatic‐influenced basaltic‐hosted system despite its off‐axis location. Two low‐temperature diffusely venting sites were located on abyssal hills 6 and 9 km off‐axis on Segment 2. Plume particle, metal, and CH4 concentrations were all very low, suggesting dilution of hydrothermal fluids by intrusion of seawater into the highly permeable flank area fault zone. The “Onnuri Vent Field” (OVF), located at the summit of OCC‐3‐2, vented clear, low‐temperature fluids supporting abundant vent organisms (21 macrofaunal taxa). The plume particle signal was low to absent, but strong ORP anomalies correlated with high CH4 and low metal concentrations. Sulfide mineralization was present, which suggests both serpentinization and magmatic/lithospheric influence on fluid composition. The detachment fault is the likely pathway for hydrothermal fluid circulation at this off‐axis location. These new vent field discoveries, especially the OVF, contribute valuable information toward understanding Indian Ocean hydrothermal systems and their ecology/biogeography.

The existence, or not, of periodicities in abyssal hill morphology has been vigorously debated in recent publications, and some have hypothesized that such periodicities are evidence of the impact of Milankovitch cycle‐caused sea level fluctuations on the volcanic construction process at mid‐ocean ridges. Periodicities are detected by the presence of spectral peaks that rise significantly above the random variations of sample power spectra associated with an aperiodic, continuous spectrum process, typically modeled as a band‐limited fractal (von Kármán model). Here, I formulate and test a new algorithm to "empirically prewhiten" the sample power spectrum which, without needing to model the continuous spectrum, flattens it to a zero‐mean process. This greatly simplifies definition of the null hypothesis, and additional modeling approximates standard deviation levels that provide a conservative basis for detecting peaks that may be indicative of periodicity. The algorithm is applied to extensive bathymetric data flanking the southern East Pacific Rise. Significant periodicities are detected on many profiles analyzed, but the periods vary widely, and do not cluster at Milankovitch periods. The most substantial harmonic signals detected exhibit periods ∼0.082–0.216 my, with root‐mean square (RMS) heights approximately a quarter to a third of the RMS height for the aperiodic signal. It is hypothesized that the dominant aperiodic component of abyssal hills corresponds to morphology constructed by faults that follow a random distribution governed by scaling laws, whereas longer‐scale periodic signals are associated with crustal thickness variations controlled internally by variations in melt supply. Key Points: A new algorithm is formulated to detect periodicities embedded in an aperiodic process using "empirical prewhitening" spectral analysisPeriodicities are detected at the southern East Pacific Rise, but are spatially and temporally inconsistentThe dominant aperiodic signal is likely fault‐constructed, while ephemeral periodic signals may relate to internal melt supply variations [ABSTRACT FROM AUTHOR]

In 2016, temperature recorders were recovered, temperatures were measured, and fluid samples were collected from Vent 1, a high temperature (338°C) hydrothermal discharge site on the southern Cleft Segment of the Juan de Fuca Ridge. Coupled with previous sampling efforts, this collection represents a 32‐year record of discharge from a single chimney structure, the longest record to date. Remarkably, the fluid has remained brine‐dominated for more than three decades. This brine formed during phase separation and segregation prior to initial observations in 1984. Although the chloride concentration of the discharging fluid has decreased with time, the fluid temperature has remained nearly constant for at least 3.3 years and probably for 15 or even 22 years. Compositions of the discharging fluids are consistent with inputs from a deep‐sourced brine, which was last equilibrated at >400°C at a depth consistent with the base of the sheeted dikes and the brittle‐ductile transition. This brine mixed (diffusion or dispersion) with a likely non‐phase‐separated, hydrothermal fluid prior to discharge. A survey of hydrothermal endmember fluids with chlorinities in excess of 700 mmol/kg shows, with the exception of Fe, a single trend between major ion concentrations and chlorinity even though data are from a range of crustal compositions, spreading rates, and water and magma depths. Calculated deep‐sourced brines from hydrothermal fluid data are similar to data based on fluid inclusions and estimates of brine assimilation in magmas. A better understanding of brines is required given their potential duration of discharge and capacity for mobilizing metals. Plain Language Summary: Magma from the mantle ascends forming new oceanic crust that is cooled by hydrothermal processes. Seawater within the permeable crust is heated and reacts with rock before discharging at the seafloor, forming high temperature (>300°C) hydrothermal fluids and metal‐rich deposits. In some cases, circulating seawater is heated to form vapor‐and brine‐dominated phases. The latter, because of its greater density, largely remains within the crust, resulting in a segregation of the two phases. This study focuses on a brine‐dominated hydrothermal system on the Juan de Fuca Ridge (northeastern Pacific Ocean) that has remained relatively uniform for 32 years. During this period, the amount of brine input to a likely non‐phase‐separated hydrothermal fluid has decreased while the temperature (338°C) has remained constant, possibly for the past 15–22 years. We propose that the brine resides at depth and is incorporated into the circulating hydrothermal fluid. We use new data and a compilation of data from many different seafloor hydrothermal systems to constrain the compositions of circulating hydrothermal fluid and brine, which is globally consistent. Brine‐dominated hydrothermal systems have been largely overlooked, but are capable of mobilizing metals from new oceanic crust and can persist for decades or longer. Key Points: Thermal and chemical data provide a 32‐year record of brine‐dominated hydrothermal discharge from the Cleft Segment, Juan de Fuca RidgeThe brine component is a deep‐sourced (>2 km below the seafloor), hot (>400°C) fluid that mixes with hydrothermal fluid before dischargeCompilation of mid‐ocean ridge endmember compositions with >700 mmol Cl/kg shows uniform trends, implying brine composition is globally consistent [ABSTRACT FROM AUTHOR]

Four new hydrothermal vent fields were discovered on the slow spreading Central Indian Ridge (8–12°S; Segments 1–3), all located off‐axis on abyssal hill structures or Ocean Core Complexes (OCCs). Each site was characterized using seafloor observation (towed camera system), plume chemistry (Fe, Mn, and CH4; Conductivity, Temperature, and Depth sensor [CTD]/Miniature Autonomous Plume Recorder [MAPR]), and rock sampling (TVgrab/dredges). Different styles of venting on each segment reflect different geological settings, rock types, likely heat sources, and fluid pathways. The segment 1 field was located on the western flank of the axial valley at the base of OCC‐1‐1. High‐temperature venting was inferred from plume characteristics and extensive seafloor sulfide mineralization, but only diffuse venting was observed. This site appears to be a magmatic‐influenced basaltic‐hosted system despite its off‐axis location. Two low‐temperature diffusely venting sites were located on abyssal hills 6 and 9 km off‐axis on Segment 2. Plume particle, metal, and CH4 concentrations were all very low, suggesting dilution of hydrothermal fluids by intrusion of seawater into the highly permeable flank area fault zone. The "Onnuri Vent Field" (OVF), located at the summit of OCC‐3‐2, vented clear, low‐temperature fluids supporting abundant vent organisms (21 macrofaunal taxa). The plume particle signal was low to absent, but strong ORP anomalies correlated with high CH4 and low metal concentrations. Sulfide mineralization was present, which suggests both serpentinization and magmatic/lithospheric influence on fluid composition. The detachment fault is the likely pathway for hydrothermal fluid circulation at this off‐axis location. These new vent field discoveries, especially the OVF, contribute valuable information toward understanding Indian Ocean hydrothermal systems and their ecology/biogeography. Plain Language Summary: We discovered four new hydrothermal vent fields along the slow spreading Central Indian Ridge. The different styles of hydrothermal venting discovered at each site reflect the different geological settings, types of surrounding rocks, and likely heat sources driving the hydrothermal circulation. Among the new vent fields, the "Onnuri Vent Field" is particularly interesting because the isolated location, unique style of venting, and abundant vent biology provide valuable new information for understanding vent ecology and biogeography of the Indian Ocean. Key Points: Four new hydrothermal vent fields were discovered along the Central Indian Ridge, 8–12°S, all off‐axisA particle‐poor, diffuse vent field with abundant vent fauna was located on an OCC at a nontransform discontinuityChemical compositions of the plumes reveal both basaltic‐ and ultramafic‐hosted hydrothermal systems [ABSTRACT FROM AUTHOR]

Our understanding of oceanic crustal formation is mostly limited to observations of young crust formed in the past several million years, due to the thick sediments on older crust and the remote location of many spreading centers. Here we use 40 m‐resolution bathymetric data collected over hundreds of square kilometers during the search for Malaysia Airlines Flight 370 on the flank of the Southeast Indian Ridge, which provides a record of crustal accretion from 11–23 Ma. Spectra calculated from the data show a characteristic timescale of 300–400 kyr and no evidence for periodicity coinciding with glacial cycles. This characteristic timescale could be explained by fluctuations in melt supply and the amount of faulting, leading to variations in crustal thickness. We show that this timescale of variation is consistent with porosity waves observed in a two‐phase flow model, which persist over millions of years. Plain Language Summary: A 12‐million‐year‐long record of crustal formation is contained within the seafloor mapping data collected during the search for Malaysia Airlines Flight 370, at a resolution that is 15 times higher than previous maps. These data illuminate the structure of a vast area of crust formed on the Southeast Indian Ridge and show that crustal production, rather than being a constant process, has varied in cycles that last hundreds of thousands of years. This pattern can be explained by the varying amount of molten rock that rises from deep in Earth's mantle, arriving in episodic waves. This behavior could be a general feature of mid‐ocean ridges spreading at similar rates, which has not been previously recognized due to a lack of available data. Key Points: Multibeam bathymetric data from MH370 search reveal off‐axis crustal structure of Southeast Indian RidgeCrustal accretion fluctuates at a characteristic timescale of 300–400 kyr, with no evidence for periodicity at glacial cycle intervalsCrustal accretion variability could be explained by the combined effects of melt‐rich porosity waves and mantle heterogeneities [ABSTRACT FROM AUTHOR]

Free hydrogen (H2) is a basal energy source underlying chemosynthetic activity within igneous ocean crust. In an attempt to systematically account for all H2 within young oceanic lithosphere (<10 Ma) near the Mid-Ocean Ridge (MOR), we construct a box model of this environment. Within this control volume, we assess abiotic H2 sources (~6 x 1012 mol H2/y) and sinks (~4 x 1012 mol H2/y) and then attribute the net difference (~2 x 1012 mol H2/y) to microbial consumption in order to balance the H2 budget. Despite poorly constrained details and large uncertainties, our analytical framework allows us to synthesize a vast body of pertinent but currently disparate information in order to propose an initial global estimate for microbial H2 consumption within young ocean crust that is tractable and can be iteratively improved upon as new data and studies become available. Our preliminary investigation suggests that microbes beneath the MOR may be consuming a sizeable portion (at least ~30%) of all produced H2, supporting the widely held notion that subseafloor microbes voraciously consume H2 and play a fundamental role in the geochemistry of Earth's ocean-atmosphere system. [ABSTRACT FROM AUTHOR]

We present results of a study of the morphology, internal structure, and chemical composition of oceanic zircon, which show that zircon is a sensitive indicator of tectonic and physicochemical processes occurring in the lower oceanic crust. Crystallization of magmatic zircon in gabbroids is not an instantaneous process; it proceeds in the course of differentiation of parental melts in the tectonically active mid-ocean ridge (MOR) setting. The main geochemical indicator of crystallization differentiation during magma cooling is an increase in Hf content toward the zircon grain edge. Zoning is also observed in magmatic zircons from oceanic plagiogranites (OPG), but it is weaker, apparently because of the narrower temperature range of zircon crystallization in these rocks. The OPG zircons are depleted in REE as compared with the least altered magmatic zircons of gabbro, which is probably due to the formation of OPG during the partial melting of gabbro with the participation of concentrated water-salt fluid, a derivate of seawater, and due to the co-crystallization of zircon and apatite. High-temperature hydrothermal processes within slow-spreading MORs lead to a partial or complete recrystallization of zircon as a result of dissolution/redeposition. A significantly reduced cerium anomaly and the presence of microinclusions of xenotime, uranium and thorium oxides or silicates, and, sometimes, baddeleyite in zircon alteration zones indicate a reducing type and high alkalinity of the hydrothermal fluid. The fluid, a derivate of seawater, acquires these features during circulation near the axial zone of ridges as a result of phase separation in the system H2O-NaCl and interaction of the fluid with abyssal peridotites of oceanic core complexes. The estimated solubility of zircon in basic melts indicates that even near-solidus crystallization of zircon is highly unlikely in anhydrous basaltic melts but is possible in differentiates of deep-seated hydrous basic magmas. The Ti-in-Zrn geothermometer must be used with caution, because variations in the Ti content in zircon might be controlled not only by temperature but also by other factors, especially when mineral inclusions in zircon testify to a drastic change in its growth (dissolution) conditions. A geothermometer based on the distribution of Zr and Hf between zircon and the host rock has several advantages over indicators of the crystallization temperature of magmatic zircon that are based on the zircon saturation index and the content of Ti in zircon. It does not depend on the composition of melt and on the correct estimates of the SiO2 and TiO2 activity. In addition, reconstruction of the Zr and Hf fractionation trends during crystallization of zircon from granitoid melts makes it possible to evaluate the temperature of separation of more differentiated melt fractions. [ABSTRACT FROM AUTHOR]

Hydrothermal circulation is a process fundamental to all types of mid‐ocean ridges that largely impacts the chemical and physical balance of the World Ocean. However, diversity of geological settings hosting hydrothermal fields complicates the exploration and requires thorough investigation of each individual case study before effective criteria can be established. Analysis of high‐resolution bathymetric and magnetic data, coupled with video and rock samples material, furthers our knowledge about mid‐ocean‐ridge‐hosted venting sites and aid in the interpretation of the interplay between magmatic and tectonic processes along the axial volcanic ridges. The rock‐magnetic data provide constraints on the interpretation of the observed contrasts in crustal magnetization. We map the areal extent of the previously discovered active basalt‐hosted Loki's Castle and inactive sediment‐hosted Mohn's Treasure massive sulfide deposits and infer their subsurface extent. Remarkably, extinct hydrothermal sites have enhanced magnetizations and display clear magnetic signatures allowing their confident identification and delineation. Identified magnetic signatures exert two new fossil hydrothermal deposits, MT‐2 and MT‐3. The Loki's Castle site coincides with negative magnetic anomaly observed in the 2‐D magnetic profile data crossing the deposit. First geophysical investigations in this area reveal the complexity of the geological setting and the variation of the physical properties in the subsurface. Key Points: We present first geophysical observations from the basalt‐hosted active hydrothermal site Loki's Castle, situated at the axial volcanic ridgeSediment‐hosted extinct hydrothermal site Mohn's Treasure, located at the ridge flank, is associated with magnetization highMagnetic data analysis reveals two new fossil hydrothermal sites in the vicinity of the Mohn's Treasure [ABSTRACT FROM AUTHOR]

The Troodos complex, Cyprus, provides an opportunity to study the structural configuration along a fossil intersection of a spreading axis and a transform fault. We complement studies at Troodos that have reconstructed the brittle deformation of the upper crust by new paleomagnetic data from the gabbro suite. The gabbro suite is exposed at the extinct spreading axis continuing the Solea graben toward the intersection with the fossil Arakapas oceanic transform. This is a unique exposure of deep crustal rocks formed at both an inside-corner and an outside-corner of a ridge-transform intersection. Remanence directions from gabbros (23 sites) were used as indicators for rigid body rotation. The spatial distribution of rotation axes allow recognition of three regions to which deformation is partitioned: 1) a western region (outside corner) that experienced primarily tilt about horizontal axis 2) a central region with minor rotation and, 3) an eastern area (inside corner) where vertical axis rotations are dominant. The absence of significant rotation in the 6 km-wide central domain together with its location between the inside- and the outside corner uncover the root of a fossil axial volcanic zone, a zone sufficiently hot so the upper crust can decouple from the substrate. Clockwise rotation in the gabbro increases from the axial zone eastward, similar to that in the overlying dikes, indicating coupling of the lower crust with the brittle upper oceanic crust. The transition from the decoupled layers of sheeted dikes and gabbro in the axial zone to the dikes-gabbro coupling in the inside corner is in keeping with deepening of the brittle-ductile transition from the dike-gabbro boundary into the lower crust away from the axial zone. Our conclusions are consistent with one of the previous reconstructions in which the Solea spreading axis was orthogonal to the Arakapas trunsform fault, and with recent studies of the present-day lower oceanic crust. However, the newly inferred surface trace of the Solea spreading axis is further to the east, probably reflecting the tilt of axial upper crust rotated blocks. (c) 2006 Elsevier B.V. All rights reserved.

In 2018, two earthquake swarms occurred along spreading ridge segments of the ultra-slow Southwest Indian Ridge (SWIR). The first swarm was located at the spreading-ridge intersection with the Novara Fracture Zone, comprising 231 events (ISC catalogue) and spanning over 6 days (10 July to 15 July). The second swarm was more of a cluster of events focusing near a discontinuity, 220 km west of the Rodrigues Triple Junction, composed of 92 events and spanning over 31 days (27 September to 27 October). We examined these two swarms using hydroacoustic records from the OHASISBIO network with seven to nine autonomous hydrophones moored on either side of the SWIR. We detected 1109 hydroacoustic events spanning over 13 days (6 July to 18 July) in the first swarm and 4880 events spanning over 33 days in the second swarm (25 September to 27 October). The number of events per day was larger, and the hydroacoustic magnitude (source level) was, on average, smaller during the second swarm than the first. The spatio-temporal distribution of events from both swarms indicates a magmatic origin initiated by dike intrusions and followed by a readjustment of stresses in the surrounding crust.

Thermodynamic predictions of fluid-mineral equilibria significantly expand our ability to test hypotheses regarding mass and heat transfer in geological systems. In many cases, successful development of thermodynamic models relies heavily on experimental measurements. A dearth of experimental data in low-density hydrothermal fluids has prevented calculation of fluid-mineral equilibria at the most extreme temperature-pressure conditions associated with mid-ocean ridge hydrothermal systems. This study compares newly attained anhydrite solubility measurements in low-density NaCl-H 2 O fluids (410 °C, 25–32 MPa and 450 °C, 36.3–50 MPa) with novel thermodynamic calculations. To address the lack of equilibrium constants at the precise temperature-pressure conditions of the experiments, data are extrapolated from reference conditions as a linear function of the logarithm of water density. Several solubility calculations are performed that vary in the choice of aqueous speciation and activity coefficient model. There is overall good agreement between experimental observations and thermodynamic predictions, indicating the potential for accurate assessment of fluid-mineral equilibria in low-density fluids. However, speciation calculations are non-unique, highlighting uncertainties in aqueous speciation and calculation of activity coefficients that will require further examination. This study represents an important first step in increasing the temperature-pressure conditions over which thermodynamic calculations of fluid-mineral equilibria are possible, with important implications for understanding mass and heat transfer in submarine hydrothermal systems. [ABSTRACT FROM AUTHOR]

We investigated the short-period variations in vertical fluxes of hydrothermal plumes at mid-ocean ridges through quantitative analysis of digital video images of plumes using the particle image velocimetry method. The analyzed digital video images of hydrothermal plumes were selected from the V Vent and Biotransect Vent at the fast-spreading East Pacific Rise, the Grotto Mound of the Endeavour Segment at the intermediate fast-spreading Juan de Fuca Ridge, and the TAG vent at the slow-spreading Northern Mid-Atlantic Ridge. The PIV was able to track the motion of turbulent parcels instead of individual particles within a hydrothermal plume. The mean plume vertical flux was shown to increase with increasing height above the vent orifice, revealing significant turbulent disperse of the rising hydrothermal plumes. The dominant periods of oscillation in the mean vertical flux were observed to be within a relatively narrow band width of 0.5–5 s despite diverse geological settings. Such pervasive short-period variations in plume flux are hypothesized to be caused by the pressure-drop oscillation within a network of compressible two-phase fluid within a sub-seafloor hydrothermal source zone. It is further argued that both the period and amplitude of plume flux oscillation might increase with decreasing plume vertical flux. The relatively narrow band width of the observed oscillation periods might reflect common characteristics in the plume dynamics despite diverse geological settings. [ABSTRACT FROM AUTHOR]

Mid‐ocean ridge axes are marked by segmentation of the axes and underlying magmatic systems. Fine‐scale segmentation has mainly been studied along fast‐spreading ridges. Here we offer insight into the third‐ and fourth‐order segmentation of intermediate‐spreading ridges and their temporal evolution. The Alarcón Rise and the Endeavour Segment have similar spreading rates (49 and 52.5 mm/year, respectively) but contrasting morphologies that vary from an axial high with a relatively narrow axial summit trough to an axial valley. One‐meter resolution bathymetry acquired by autonomous underwater vehicles, lava geochemistry, and ages from sediment cores is combined with available seismic reflection profiles to analyze variations in (1) geometry and orientation of the axial summit trough or valley, (2) seafloor depth near the axis, and (3) distribution of hydrothermal vents, (4) lava chemistry, and (5) flow ages between contiguous axes. Along both intermediate‐spreading segments, third‐ and fourth‐order discontinuities and associated segments are similar in dimension to what has been observed along fast‐spreading ridges. The Alarcón Rise and the Endeavour Segment also allow the study of the evolution of fine‐scale segmentation over periods of 300 to 4,000 years. Comparison between old and young axes reveals that the evolution of fine‐scale segmentation depends on the intensity of the magmatic activity. High magmatic periods are associated with rapid evolution of third‐order segments, while low magmatic activity periods, dominated by tectonic deformation and/or hydrothermal activity, are associated with little to no change in segmentation. Plain Language Summary: Structure and morphology of two intermediate‐rate spreading ridge axes were analyzed in 1‐m resolution bathymetry collected using autonomous underwater vehicles. The observations are combined with information on the distribution of hydrothermal vents, lava flow ages, and geochemistry, and, where available, the geometry of the magma lens. The aim is to determine the third‐ and fourth‐order segmentation of the volcanic axis along intermediate‐spreading ridges and their evolution related to cycles in magma supply. The study focuses on two ridge segments, the Alarcón Rise and the Endeavour Segment, that have similar spreading rates but are in magmatic and tectonic phases, respectively, which has resulted in contrasting morphology. Our analysis shows that along both segments the third‐ and fourth‐order segments and their associated discontinuities are similar in geometry to those observed along fast‐spreading ridges, independent of the magmatic activity. However, the intensity of magmatic activity impacts the evolution of segmentation with segments that remain at the same position during tectonic or low magmatic periods and propagate, decrease, or disappear during high magmatic periods. Key Points: Intermediate‐spreading rate Alarcón Rise and Endeavour Segment illustrate past and current fine‐scale segmentationThird‐order segmentation evolves faster throughout the high magmatic phase than through the tectonic phase during which it remains steadyThird‐ and fourth‐order segmentation have similar characteristics along intermediate‐ and fast‐spreading ridges [ABSTRACT FROM AUTHOR]

Abstract Mid-ocean ridges are important geological features that cover around 33% of the global ocean floor, increase environmental heterogeneity on a regional scale and influence benthic community ecology. Benthic communities at the Mid-Atlantic Ridge (MAR) were studied at four contrasting sites, located east and west of the ridge, which were further separated into northern (54°N) and southern (48°N) sites by the Charlie-Gibbs Fracture Zone (CGFZ) and the Sub-Polar Front (SPF). The MAR in the CCFZ region area had flat areas surrounded by gentle slopes between rocky cliffs. A total of 32 remotely operated vehicle video transects (32,000 m2 of seafloor) were surveyed on the flat areas and sedimented slopes (10°). In total, 154 distinct taxonomic units were identified (from 9 phyla) across all sites. The sediments of the flat and sloping sites were generally similar, but differences were seen in the community composition and faunal abundance (~ 4 times higher in the flat sites, except at the northwestern site). Significant differences in abundance were observed between sites (highest in the northern sites). The two northern sites had distinct community compositions, while the two southern sites were similar. This suggests that the MAR acts as a stronger barrier between communities north of the CGFZ than it does to the south. There was high heterogeneity between transects and it was not possible to identify general drivers for the benthic megafauna at the MAR. Our results emphasize the limited knowledge of this vast system with its unique benthic megafauna. Highlights • High-resolution large-area quantitative study of Mid-Atlantic Ridge (MAR) megafauna. • MAR appears to be important in structuring the Atlantic benthos. • Distinct megafaunal communities north and south of the Sub-Polar Front (SPF). • Communities differed either side of the MAR, particularly north of the SPF. • Generally higher abundances on flat terraced areas than gentle (10°) slopes. [ABSTRACT FROM AUTHOR]

Abstract Iceland is a natural laboratory where plate separation and mid-ocean ridge formation can be studied directly in the field. In this setting, it is not fully understood how magma interacts with faulting and fissuring, and how rift zones propagate. In order to improve the understanding of how these processes operate, we conducted a detailed field study about the kinematics and propagation of 33 Holocene faults along the N–S Theistareykir rift (North Volcanic Zone) and mapped all tension fractures. This rift hosts the Holocene Theistareykir central volcano, for which ground deformation data (InSAR and GPS) indicate an inflating shallow magma chamber throughout 2006–2008. Analyses of the cumulative fault slip distribution at 696 sites show two opposite directions of fault/rift propagation: The first, more developed, is directed northwards and affects the area located north of Theistareykir volcano, whereas south of the volcano, the rift propagates southwards. Tension fractures increase in frequency, with respect to faults, outwards from the volcano, along a N–S to NNE–SSW direction. The presence of aligned vents, dykes and topographic bulging along some of the N–S structures, and the active magma chamber below Theistareykir volcano, suggest that faults and tension fractures propagate following repeated dyke intrusions from the magma chamber outward along the plate margin. The fact that the rift is more developed north of the volcano than south of it, is interpreted as the effect of buttressing from another, coterminous magma chamber sitting below a neighbouring central volcano. Other causes for the general observed pattern such as tectonic stresses, mechanical interaction of faults, and changes in the rheological characteristics of rocks, are discussed as well. Highlights • We studied kinematics and propagation of Holocene faults in the North Iceland Rift. • Faults and fissures propagated outwards respect to a magma chamber following dyking. • At the side of the chamber, faults are more symmetric without preferred propagation. • Rift progress in one direction may be hindered by buttressing from another chamber. • Rift evolution is complicated by the presence of faults guided by tectonic stresses. [ABSTRACT FROM AUTHOR]

Abstract Multibeam bathymetric data, acquired along mid-ocean ridges (MORs), provide critical information for the modeling of seabed terrains and the understanding of related geology. An automated detection of geological features, such as fault structures, helps to elucidate the structural characteristics of MORs and quantify e.g., the magnitude and spatial variability of geological phenomena such as faulting. For this purpose, this research presents a developed cross-sectional methodology where continuous elevation data are (1) collected across a MOR from individual transect lines at various spatial resolutions (50–150 m), and then (2) analyzed with a supervised learning algorithm to discriminate fault structures. An artificial neural network (ANN) is applied for the detection and classification of fault scarps which have either an east or west tilt orientation; the classification uses attributes of elevation data calculated from surface derivation, simulated relief shading and statistical analyses of transects. Results indicate an average detection accuracy of 92%, which is dependent on the data sampling resolution, the terrain complexity and the predictor variables considered. Both the variance and regression slope variables played a key role in the training phase for identifying and classifying the tectonic features. The cross-sectional learning method presented in this research finally evidences the possibility to achieve an automated quantification system for different landform types and emphasizes the need for complementary classification methods to deepen the interpretation of landform complexities and related geological processes at MORs. Graphical abstract Image 1 Highlights • Seabed fault offsets are analyzed automatically via a Neural Network model. • The detection accuracy of geomorphic features is 92%. • The data resolution, terrain complexity and predictors affect this accuracy. • Both the variance and regression slope variables played a key role in the detection. [ABSTRACT FROM AUTHOR]