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All data are provided in supporting information, or in the cited references or associated supplementary material associated with these. K. M. G. and L. A. C. were funded through NSERC Discovery (5098 and 155396) and Accelerator grants.

A new map of the axes of linear magnetic anomalies for eastern regions of the Eurasian Basin has been compiled. Based on the results of Russian geomagnetic surveys, paleoanomaly C25 was distinguished here for the first time, and the transition from rifting to spreading has been dated to the chron interval of С25r–С26n (57.656–59.237 Ma ago). The axis of the zone of splitting of marginal continental fragments of the Lomonosov Ridge from the Siberian shelf has been reconstructed. The Euler poles and angles of rotation that describe the splitting kinematics have been determined for the first time, as well as noncoincidence of conjugate isobaths related to sliding of peripheral continental crust areas along the lithospheric thrust plane.

The ocean level fluctuations relative to continents are caused by both physical processes related to water volume variations and tectonic processes related to changes in the bottom topography. Currently, the main tectonic causes are considered to include the occurrence of midocean ridges and variations in an expansion velocity of the ocean floor with the corresponding rise or fall of the bottom. The specific role of continental drift is not taken into account, or it is given a passive role. This work demonstrates the important role of continents in long-term changes in the ocean level. The numerical model shows the influence of continents on tectonic activity of the mantle and continents “floating” over the mantle with uneven relief, which cause relative variations in the ocean level. While the continent is above the ascending mantle stream, it is raised, and the ocean level relative to the continent falls. After the supercontinent split, the continents diverge, and the ridge previously covered by continents occurs in the ocean. Being close to subduction zones, the continents subside thus increasing the relative ocean level. The model also shows that the continents do not move strictly horizontally along the mantle, but, like floating ships, change their slope depending on the mantle relief.

Hotspots are generated by partial melting due to hot plumes rising within the Earth's mantle, and when tectonic plates move relative to the plume source, hotspot tracks form. Off South America, the oceanic Nazca Plate hosts a large population of hotspot tracks. Examples include seamounts formed far from the Pacific-Nazca spreading center (“off-ridge” seamounts), such as the Juan Fernandez Ridge (Juan Fernandez hotspot), the Taltal Ridge (San Felix hotspot), and the Copiapo Ridge (Caldera hotspot). These hotspot tracks are characterized by a rough and discontinuous topography. Other examples include seamounts formed near the East Pacific Rise (EPR) (“on-ridge” seamounts), such as the Nazca Ridge (Salas y Gomez hotspot) and Easter Seamount Chain (Easter hotspot), and the Iquique Ridge (Foundation hotspot). These oceanic ridges developed a relatively smooth and broad morphology. Here, we present a plate reconstruction of these six oceanic hotspot tracks since the Paleocene, providing a kinematic model of ridge-continental margin collision. For the “off-ridge” seamount group, the plate kinematic reconstruction indicates that the collision point remained quasi-stationary from 40 to 30–25 Ma. Eventually, the southward migration of the collision point of this seamount group accelerated from 23 to 15 Ma (reaching a maxima speed of 300 km/Ma along the trench). From 15 Ma to present the collision point has remained quasi-stationary. The predicted location of the subducted portion of the Taltal, Copiapo and Juan Fernandez Ridges coincides with the southward migrating (relative to South America) flat slab segment. For the “on-ridge” seamount group, the kinematic plate reconstruction indicates a continuous southward migration of the collision point from ~23 Ma, which is related to the fragmentation of the Farallon Plate. The southward migration accelerated until 15 Ma, reaching approximately 150 km/Ma. From 15 Ma to present, the southward migration has been decelerating except an increment of the migration velocity during the Chron 4 due to an increase of the convergence velocity. The migration velocity differences between the on-ridge and off-ridge hotspot tracks are mainly result from the hotspot track azimuth and the margin azimuth on the collision point. Convergence velocity varies along the trench, but it is a minor factor comparing different hotspot tracks migration velocity. Due to the EPR-plume interactions, our reconstruction suggests that the eastern Tuamotu Island Plateau formation occurred 48–27 Ma on the Easter Hotspot, which was located near to the EPR segment between the Marquesas and Austral Fracture Zones. Our model also predicts that the Iquique Ridge seamounts track is consistent with the position of the Foundation hotspot. The Foundation hotspot jumped to the Challenger (Resolution) Fracture Zone from the Farallon plate to the Pacific plate. This process triggered the cessation of the Iquique Ridge volcanic formation, and initiated volcanism at Foundation Chain in the Pacific Plate at ~25 Ma.

We report 3He/4He, He and CO2 concentrations for the dissolved (glass) and vapor (vesicle) phase of basalts, for 23 lavas from the 2005–2006 eruption and 4 lavas from 1991–92 at 9°50′N on the East Pacific Rise. We also determined δ 13 C and δ 18 O of CO2 in vesicles in 18 of these lavas. These sample suites provide a rare opportunity to study volatile systematics related to magma recharge at a mid-ocean ridge volcano and to quantify degassing prior to and during the eruptions. Our study covers the spatial and temporal extent of the 2005–06 eruption and is complementary to previous studies of variations in basaltic volatiles with distance from the ridge axis during part of this multi-stage eruption ( Soule et al., 2012 , Gardner et al., 2016 ). 3He/4He shows minor variation with a mean value of 8.51 ± 0.03 RA. All but 1 sample lies in a narrow range of 280–420 ppm for total (vesicles + glass) CO2 indicating volatile saturation at 1.2–2.3 km depth in the crust, similar to depths of the seismically imaged magma lenses in the region. Vesicle He and CO2 concentrations vary throughout the lava flow field by factors of 17 and 250, respectively. The vapor phase CO2/He ratio co-varies positively with the fraction of CO2 contained in vesicles due to kinetic fractionation between He and CO2 during vesiculation. Vesicle δ 13 CPDB co-varies with the fraction of CO2 in vesicles ( r 2 = 0.83 ) , ranging from −2.6 to −5.0‰, and straddles the isotope composition of EPR vent fluids. The vesicle 13C-enrichment ( δ vapor − δ melt ) is +3.3‰ and comparable to the experimentally determined fractionation factor for basaltic systems. Collectively the observations indicate closed-system degassing during rapid ascent of melt from the axial magma lens through the conduit to its eruption and emplacement on the seafloor, with minimal bubble loss. In contrast, CO2/Ba systematics and a comparison of δ 13 C for lavas and vent fluids indicate an earlier period of open-system degassing during which 50% or more of the initial (parental magma) CO2 inventory was lost prior to magma storage in the shallow crust.

Highlights: • Lake Asal (Afar Rift) is fed by seafloor-type hydrothermal fluids. • An oceanic “embryo” in arid climate is mildly acidic and metal rich. • It has heavy C, O and Ca, and light Zn isotope composition. • Lake chemistry is controlled by hydrothermal discharge and aeolian input. Abstract: At the place where the submarine Aden Ridge encroaches on the African continent and interacts with the East African Rift system, two small basins form: Ghoubbet-al-Kharab and Lake Asal. Whereas Ghoubbet-al-Kharab is connected to the open ocean, Lake Asal is a typical example of oceanic “embryo”, which is defined as a system that is detached from the ocean, but has features of a marine basin with an oceanic type crust and a seawater-based water body. In order to shed light on the source of water, type of hydrothermal activity and hydrothermal deposits, and controls on the water chemistry in an oceanic “embryo”, we undertook a mineralogical-geochemical study of the lake water, hydrothermal fluids and hydrothermal carbonate deposits of Lake Asal. The geochemical analyses of lake water and hydrothermal fluids show that Lake Asal (located in an arid zone with strong evaporation and with no riverine input) is fed by seafloor-type hydrothermal fluids according to the following scenario: percolation of seawater along faults and cracks of extension in the rift, reaction of seawater with the hot basaltic rocks and hydrothermal fluid generation, discharge of the hydrothermal fluid in the Asal depression and accumulation of the Lake Asal water body. The fluid venting at the Lake Asal bottom is a mixture of 97% end-member hydrothermal fluid and 3% lake water. The calculated end-member hydrothermal fluid of this oceanic “embryo” is poorer in metals than the seafloor hydrothermal fluids of an open and evolved ocean. In addition to the seawater/rock interaction, the chemistry of Lake Asal is also controlled by evaporation leading to hyper salinity. In a hyper saline water body a number of hydrothermally supplied metals are stabilized as chloride complexes and accumulate. This results in a metal rich and mildly acidic “embryonic” ocean. Unlike an open and evolved modern ocean, the “embryonic” ocean located in an arid zone has heavy C and O isotope composition and light Zn and Fe isotope composition. Calcium isotope compositions of both types of ocean are similarly heavy. There are two genetically different sources of elements to the Lake Asal that are vertically separated: hydrothermal (lower, or bottom) and aeolian (upper, or surficial). Another important control on the lake water chemistry is the formation of carbonate spires at the lake bottom. Ca‑carbonate precipitation immobilizes substantial amount of hydrothermally supplied Ca and drives up the (Mg/Ca)mol of the lake water. Increasing (Mg/Ca)mol of the evolving lake water leads to changes in the mineralogy of spires: from low-Mg calcite to aragonite. Thus, the spire formation exerts a self-control on its mineralogy. Carbonate spire deposition affects also the Ca, Zn and Fe isotope composition of the lake water through adsorption or/and co-precipitation induced isotope fractionation.

Detachment faults that exhume mantle peridotites to the seafloor play a major role in the accommodation of plate divergence at slow‐spreading ridges. Using 99 samples of partially serpentinized peridotites dredged from a nearly amagmatic segment of the eastern part of the Southwest Indian Ridge, we characterize the deformation processes active in the root zone of detachment fault systems. The deformation is heterogeneous even at the sample scale and combines both brittle and crystal‐plastic mechanisms. Strain localization is initially controlled by strength contrasts at the grain scale between olivine and orthopyroxene and between variably oriented olivine crystals. Orthopyroxene deformation is primarily brittle (microfractures), but kink bands and dynamic recrystallization are locally observed. In contrast, olivine deforms primarily by dislocation creep with dynamic recrystallization under high deviatoric stresses (80‐270 MPa). Olivine grains poorly oriented to deform by dislocation glide display kink bands and localized microfractures. Dynamic recrystallization controlled by strain and stress concentrations produce anastomosing zones of grain size reduction (GSR). GSR zones contain limited late to post‐kinematic amphibole, suggesting the presence of small volumes of hydrous fluids. Plagioclase, when present, is post‐kinematic. This heterogeneous high‐stress deformation is observed, with variable intensity, in every sample investigated, suggesting that it was pervasively distributed in the root region of axial detachments. Abyssal peridotite samples from more magmatically robust slow mid‐ocean ridges do not show this pervasive high stress deformation microstructure, implying magma, when present, tends to localize most of the strain at the root of axial detachment systems.

The iron isotope contrast between mid-ocean ridge basalts and abyssal peridotites is far greater than can be explained by mantle melting alone. Here we investigate a suite of mid-ocean ridge magma chamber rocks sampled by the Ocean Drilling Project Hole 735B in the Atlantis Bank of the Indian Ocean. We report major and trace element geochemistry from these rocks and measure their iron isotope compositions to investigate the potential role of fractional crystallization during melt evolution. We observe a large range of δ56Fe that defines a significant inverse curvilinear correlation with bulk rock MgO/FeOT. These data confirm that δ56Fe in the melt increases as fractional crystallization proceeds but, contrary to expectation, δ56Fe continues to increase even when oxides begin to crystallize. We conclude that iron isotope fractionation through fractional crystallization during the evolution of mid-ocean ridge basalts from abyssal peridotites reconciles the disparity in isotopic compositions between these two lithologies. Fractional crystallization during the evolution of mid-ocean ridge basalts can explain why they are so different in iron isotopic composition from abyssal peridotites, according to chemical analyses of drill core samples from the Atlantis Bank.

Offshore the emissions of dihydrogen are highlighted by the smokers along the oceanic ridges. Onshore in situ measurements in ophiolitic contexts and in old cratons have also proven the existence of numerous H2 emissive areas. When H2 emanations affect the soils, small depressions and vegetation gaps are observed. These depressions, called fairy circles, have similarities with the pockmark and vent structures recognized for long time in the sea floor when natural gas escapes but also differences. In this paper we present a statistic approach of the density, size, and shape of the fairy circles in various basins. New data from Brazil and Australia are compared to the existing database already gathered in Russia, USA, and again Brazil. The comparison suggests that Australia could be one of the most promising areas for H2 exploration, de facto a couple of wells already found H2, whereas they were drilled to look for hydrocarbons. The sum of areas from where H2 is seeping overpasses 45 km2 in Kangaroo Island as in the Yorke Peninsula. The size of the emitting structures, expressed in average diameter, varies from few meters to kilometers and the footprint expressed in % of the ground within the structures varies from 1 to 17%. However, globally the sets of fairy circles in the various basins are rather similar and one may consider that their characteristics are homogeneous and may help to characterize these H2 emitting zones. Two kinds of size repartitions are observed, one with two maxima (25 m and between 220 m ± 25%) one with a simple Gaussian shape with a single maximum around 175 m ± 20%. Various geomorphological characteristics allow us to differentiate depressions of the ground due to gas emissions from karstic dolines. The more relevant ones are their slope and the ratio diameter vs. depth. At the opposite of the pockmark structures observed on the seafloor for which exclusion zones have been described, the H2 emitting structures may intersect and they often growth by coalescence. These H2 emitting structures are always observed, up to now, above Archean or Neoproterozoic cratons, it suggests that anoxia at the time the sedimentation and iron content play a key role in the H2 sourcing.

Oceanic transform faults are seismically and tectonically active plate boundaries1 that leave scars-known as fracture zones-on oceanic plates that can cross entire ocean basins2. Current descriptions of plate tectonics assume transform faults to be conservative two-dimensional strike-slip boundaries1,3, at which lithosphere is neither created nor destroyed and along which the lithosphere cools and deepens as a function of the age of the plate4. However, a recent compilation of high-resolution multibeam bathymetric data from 41 oceanic transform faults and their associated fracture zones that covers all possible spreading rates shows that this assumption is incorrect. Here we show that the seafloor along transform faults is systemically deeper (by up to 1.6 kilometres) than their associated fracture zones, in contrast to expectations based on plate-cooling arguments. Accretion at intersections between oceanic ridges and transform faults seems to be strongly asymmetric: the outside corners of the intersections show shallower relief and more extensive magmatism, whereas the inside corners have deep nodal basins and seem to be magmatically starved. Three-dimensional viscoplastic numerical models show that plastic-shear failure within the deformation zone around the transform fault results in the plate boundary experiencing increasingly oblique shear at increasing depths below the seafloor. This results in extension around the inside corner, which thins the crust and lithosphere at the transform fault and is linked to deepening of the seafloor along the transform fault. Bathymetric data suggest that the thinned transform-fault crust is augmented by a second stage of magmatism as the transform fault intersects the opposing ridge axis. This makes accretion at transform-fault systems a two-stage process, fundamentally different from accretion elsewhere along mid-ocean ridges.

Based on the linear shallow-water equations, new analytical solutions are derived for trapped waves over a ridge with a hyperbolic-cosine squared cross-sectional profile which may be used to idealize many real-world ocean ridges. In the new analytical formulation, the free surface of the trapped waves is described using the combination of the first and second kinds of the associated Legendre functions, which is further analysed to reveal the existence of both symmetrical and anti-symmetrical trapped waves on the ridge under consideration. New algebraic equations are also derived to depict the wave dispersion relationships, allowing explicit quantification of their sensitivity to the topographic profile. Furthermore, a ray-tracing method is applied to interpret the propagation paths of trapped waves over the ridge and better understand the excitation mechanisms. Finally, an extensively validated Boussinesq wave model is used to conduct numerical experiments for trapped waves induced by tsunamis. The numerical predictions are consistent with the new analytical solutions, which effectively confirms the validity of the new analytical framework for trapped waves over a more general type of oceanic ridges.

The availability of magma is a key to understand mid-ocean ridge tectonics, and specifically the distribution of the two contrasted spreading modes displayed at slow and ultraslow ridges (volcanically-dominated, and detachment fault-dominated). The part of the plate divergence that is not accommodated by magma emplaced as gabbros or basaltic dikes is taken up by normal faults that exhume upper mantle rocks, in many instances all the way to the seafloor. Magma is, however, more than just a material that is, or is not, available to fill the gap between two diverging plates. It is the principal carrier of heat into the axial region and as such it may contribute to thin the axial lithosphere, hence diminishing the volume of new plate material formed at each increment of plate separation. Magma as a heat carrier may also, however, if emplaced in the more permeable upper lithosphere, attract and fuel vigorous hydrothermal circulation and contribute instead to overcooling the newly formed upper plate (Cochran and Buck, JGR 2001). Magma is also a powerful agent for strain localization in the axial region: magma and melt-crystal mushes are weak; gabbros that crystallize from these melts are weaker than peridotites because they contain abundant plagioclase; and hydrothermally-altered gabbros, and gabbro-peridotite mixtures, are weaker than serpentinites because of minerals such as chlorite and talc. As a result, detachment-dominated ridge regions that receive very little magma probably have a stronger axial lithosphere than detachment-dominated ridge regions that receive a little more magma. Because magma has this triple role (building material, heat carrier, and strain localization agent), and because it is highly mobile (through porosity, along permeability barriers, in fractures and dikes), it is likely that variations in magma supply to the ridge, in time and space, and variations in where this magma gets emplaced in the axial plate, cause a greater diversity of spreading modes, and of the resulting slow and ultraslow lithosphere composition and structure, than suggested by the first order dichotomy between volcanically-dominated and detachment-dominated spreading. In this talk I illustrate these points using results of recent studies at the Mid-Atlantic and Southwest Indian ridges.

In the classical model, mid-ocean ridges (MOR) sit above an asthenospheric corner flow that is symmetrical about a vertical plane aligned with the ridge axis. However, geophysical observations of MORs indicate strong asymmetry in melt production and upwelling across the axis (e.g., Melt Seismic Team, 1998, Rychert et al., 2020). In order to reproduce the observed asymmetry, models of plate-driven (passive) flow require unrealistically large forcing, such as rapid asthenospheric cross-axis flow (~30 cm/yr) at high asthenospheric viscosities (~10^21 Pa.s), or temperature anomalies of >100 K beneath the MELT region in the East Pacific Rise (Toomey et al, 2002). Buoyancy-driven flows are known to produce symmetry-breaking behaviour in fluid systems. A small contribution from buoyancy-driven (active) flow promotes asymmetry of upwelling and melting beneath MORs (Katz, 2010). Previously, buoyancy has been modelled as a consequence of the retained melt fraction, but depletion of the residue (and heterogeneity) should be involved at a similar level. Here, we present new 2-D mid-ocean ridge models that incorporate density variations within the partial-melt zone due to the low density of the liquid relative to the solid (porous buoyancy), and the Fe/Mg partitioning between melt and residue (compositional buoyancy). The model is built after Katz (2010) using a new finite difference staggered grid framework for solving partial differential equations (FD-PDE) for single-/two-phase flow magma dynamics (Pusok et al., 2020). The framework uses PETSc (Balay et al., 2020) and aims to separate the user input from the discretisation of governing equations, thus allowing for extensible development and a robust framework for testing. Results show that compositional buoyancy beneath the ridge is negative and can partially balance porous buoyancy. Despite this, models with both chemical and porous buoyancy are susceptible to asymmetric forcing. Asymmetrical upwelling in this context is obtained for forcing that is entirely plausible. A scaling analysis is performed to determine the relative importance of the contribution of compositional and porous buoyancy to upwelling, which is followed by predictions on the crustal thickness production and asymmetry beneath the ridge axis. Balay et al. (2020), PETSc Users Manual, ANL-95/11-Revision 3.13.Katz (2010), G-cubed, 11(Q0AC07), 1-29, https://doi.org/10.1029/2010GC003282Melt Seismic Team (1998), Science, 280(5367), 1215–1218, https://doi.org/10.1126/science.280.5367.1215 Pusok et al. (2020), EGU General Assembly 2020, EGU2020-18690 https://doi.org/10.5194/egusphere-egu2020-18690 Rychert et al. (2020), JGR Solid Earth, 125, e2018JB016463. https://doi. org/10.1029/2018JB016463 Toomey et al. (2002), EPSL, 200(3-4), 287-295, https://doi.org/10.1016/S0012-821X(02)00655-6

Shipborne ex-situ oxygen measurements in mid-ocean ridge flank sediment cores from the eastern low-latitude North Pacific (Clarion-Clipperton Zone) revealed a downward increase of pore-water oxygen above the sediment-crust interface (Mewes et al., 2016, Kuhn et al., 2017). This inverse redox zonation is caused by an upward diffusion of oxygen (and other solutes) from fluids circulating through the underlying 20 Mio. Year old and still cooling ocean crust. In consequence, these sediments experience a cyclic change in redox-conditions from oxic seafloor conditions at the top through mostly suboxic conditions throughout the sediment column back to oxygen-rich pore water in the last few sediment meters above the rock basement. We studied paleomagnetic records and bulk magnetic properties of three gravity cores from such settings that were collected during RV Sonne expedition SO-240 in 2015 and obtained high-quality magnetostratigraphic records covering the past 3.2 Ma. The generally very good preservation and interpretability of our reversal and RPI records, however, conflicts with a well-defined, but irregular ‘ghost event’ of normal polarity within the upper Gilbert reversed C2Ar section. This magnetic polarity and intensity artifact cannot be explained by sediment tectonics, but coincides with the present depth of the lower suboxic-to-oxic redox boundary. Although chemical overprinting could be considered as an obvious explanation of such findings, bulk magnetic analyses (FORCs, thermomagnetics) infer no diagenetic alteration of the magnetic minerals. Over the entire paleomagnetic record, bacterial magnetite appears to be the predominant NRM carrier. We therefore introduce a novel conceptual model of secondary biogenic magnetite formation at crustal depth, hypothesizing that microaerophilic magnetotactic bacteria live and biomineralize not only in the shallow subsurface, but also near the deep oxygen above the sediment-crust interface. References Mewes, K., Mogollón, J.M., Picard, A., Rühlemann, C., Eisenhauer, A., Kuhn, T., Ziebis, W., Kasten, S., 2016. Diffusive transfer of oxygen from seamount basaltic crust into overlying sediments: An example from the Clarion-Clipperton Fracture Zone. Earth and Planetary Science Letters 433, 215-225.Kuhn, T., Versteegh, G.J.M., Villinger, H., Dohrmann, I., Heller, C., Koschinsky, A., Kaul, N., Ritter, S., Wegorzewski, A.V., Kasten, S., 2017. Widespread seawater circulation in 18-22 Ma oceanic crust: Impact on heat flow and sediment geochemistry. Geology 45, 799-802.

Melt transport beneath the lithosphere is elusive. With a distinct viscosity and density from the surrounding mantle, magmatic melt moves on a different time scale as the surrounding mantle. To resolve the temporal scale necessary to accurately capture melt transport in the mantle, the model simulations become numerically expensive quickly. Recent computational advances make possible two-phase numerical explorations to understand magma transport in the mantle. We review results from a suite of two-phase models applied to the mid-ocean ridges, where we varied half-spreading rate and intrinsic mantle permeability using new openly available models, with the goal of understanding melt focusing beneath mid-ocean ridges and its relevance to the lithosphere-asthenosphere boundary (LAB). Here, we highlight the importance of viscosities for the melt focusing mechanisms. In addition, magmatic porosity waves that are a natural consequence of these two-phase flow formulations. We show that these waves could explain long-period temporal variations in the seafloor bathymetry at the Southeast Indian Ridge.

Paleo-digital elevation models (paleoDEM) based on plate tectonic and paleogeographic reconstructions use age grids of ocean floor to determine ocean bathymetry. In recent years, such age grids have also been developed for now-subducted oceans from the far geological past, as far back as the Neoproterozoic, using geology and paleomagnetism-based estimates of ocean opening. In such reconstructions, mid ocean ridges are drawn based on estimated Euler poles and rotations, and conceptual knowledge on the geometry consisting of spreading ridges and transform faults.Current procedures to draw mid ocean ridges in plate tectonic reconstructions are laborious, as new ridges are drawn every time the Euler pole location changes. Fortunately this is also a task that can be automated. We have written an algorithm using pyGPlates that takes as input a smooth curve at the approximate position of the reconstructed mid ocean ridge at the moment of its formation, and then calculates spreading and transform segments according to their typical geometries in modern oceans, assuming symmetric spreading. The algorithm allows gradual readjustment of ridge orientations upon Euler pole changes comparable to documented cases in the modern oceans (e.g., in the Weddell Sea). The algorithm also contains modules that can convert the calculated mid ocean ridges with other plate boundaries to boundary topologies – which can be used as input for the recently published TracerTectonics algorithm, produce isochrons which can be converted to age grids, check for subduction of isochrons and subsequently create bathymetry grids. We illustrate the use of the MORGEN algorithm with recently published reconstructions of subducted, as well as future oceans.

Mid-ocean ridges (MOR) axes are not straight, but segmented over scales of 10s to 100s of kilometers by several types of offsets including transform faults (TF), overlapping spreading centers (OSC) and non-transform, non-overlapping offsets (NTNOO). Variations in axial morphology and segmentation have been attributed to changes in magma supply, axial thermal structure (which depends on mantle temperature and spreading rate), and axial mechanical properties. To isolate the effect of each of these processes is difficult with field data alone. We therefore present a series of analogue experiments using colloidal silica dispersions as an Earth analogue. Diffusion of salt from saline solutions placed in contact with these fluids, causes formation of a skin, whose rheology evolves from viscous to elastic and brittle with increasing salinity. Applying a fixed spreading rate to this pre-formed, brittle plate results in cracks, faults, and ridge segments. Lithospheric thickness is varied independently by changing the surface water layer salinity. Experimental results depend on the axial failure parameter Π F , the ratio of a mechanical length scale ( Z m ) and the axial elastic thickness ( Z a x i s ), which depends on mantle temperature and spreading velocity. Slow-spreading fault-dominated, and fast-spreading fluid intrusion-dominated, ridges on Earth and in the laboratory are separated by the same critical value Π F c ± 0.024, suggesting that the axial failure mode governs ridge geometry. Here, we examine ridge axis segmentation. Measurements of >4000 experimental ridge segments and offsets yield an average segment length L m that is quasi-constant at all spreading velocities. Scaled to the Earth, L m ∼ 55 km, in agreement with the natural data. Experiments with low Π F show offset size varying as d l = c s t e L m Z a x i s regardless of offset type, a correlation well explained by fracture mechanics. Finally, as on Earth, experimental ridge segments are separated by transform and non-transform discontinuities, and their nature and occurrence vary with Π F . NTNOOs develop when Π F Π F c , while OSCs develop when Π F > Π F c . In contrast, TF may form at any Π F , but the proportion of TFs relative to OSCs or NTNOOs decreases when Π F / Π F c > > 1 or

Over 95% of volcanism on Earth occurs at mid-ocean ridges (MORs) where tectonic plates diverge. How melt is produced depends on the mineral composition and water content. However, it has proven cha...

Two-thirds of the Earth is covered by mid-ocean ridge basalts, which form along a network of divergent plate margins. Basalts along these margins display a chemical diversity, which is consequent to a complex interplay of partial mantle melting in the upper mantle and magmatic differentiation processes in lower crustal levels. Igneous differentiation (crystal fractionation, partial melting) and source heterogeneity, in general, are key drivers creating variable chemistry in mid-ocean ridge basalts. This variability is reflected in iron isotope systematics (expressed as δ57Fe), showing a total range of 0.2 ‰ from δ57Fe = + 0.05 to + 0.25 ‰. Respective contributions of source heterogeneity and magma differentiation leading to this diversity, however, remain elusive. This study investigates the iron isotope systematics in basalts from the ultraslow spreading Gakkel Ridge in the Arctic Ocean and compares them to existing data from the fast spreading East Pacific Rise ridge. Results indicate that Gakkel lavas are driven to heavier iron isotope compositions through partial melting processes, whereas effects of igneous differentiation are minor. This is in stark contrast to fast spreading ridges showing reversed effects of near negligible partial melting effects followed by large isotope fractionation along the liquid line of descent. Gakkel lavas further reveal mantle heterogeneity that is superimposed on the igneous differentiation effects, showing that upper mantle Fe isotope heterogeneity can be transmitted into erupting basalts in the absence of homogenisation processes in sub-oceanic magma chambers.

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 seismicity. Shallowing maximum earthquake depths over distances of 70 km suggest along‐axis melt focusing toward major volcanic centers. Major detachment faults on the highly oblique spreading Knipovich Ridge were not obvious in the observed seismicity., Deutsche Forschungsgemeinschaft (DFG) http://dx.doi.org/10.13039/501100001659, Helmholtz Excellence Network POSY at the Alfred Wegener Institute, Ministry of Science and Higher Education of Poland

The crust and mantle both in ophiolites (fossil ocean lithosphere) and in modern oceans are enormously diverse. Along-axis morphology and lower crustal accretion at ultraslow-spreading ocean ridges are fundamentally different from those at faster-spreading ridges, and are critical to understanding how crustal accretion varies with spreading rate and magma supply. Ultraslow-spreading ridges provide analogues for ophiolites, to identify those that may have formed under similar conditions. Parallel studies of modern ocean lithosphere and ophiolites therefore can uniquely inform the origin and genesis of both. Here we report the results of structural and petrological studies on the Xigaze ophiolite in the Tibetan Plateau, and compare it with the morphology and deep drilling results at the ultraslow-spreading Southwest Indian Ridge. The Xigaze ophiolite has a complete but laterally discontinuous crust, with discrete diabase dykes and sills cutting both mantle and lower crust. The gabbro units are thin (c. 350 m) and show upward cyclic chemical variations, supporting for an episodic and intermittent magma supply. These features are comparable with the highly focused magmatism and low magma budget at modern ultraslow-spreading ridges. Thus we suggest that the Xigaze ophiolite represents an on-land analogue of ultraslow-spreading ocean lithosphere. Supplementary material: Single-spot analysis and average compositions for mineral major element compositions (wt.%) of the Jiding gabbro section, Xigaze ophiolite are available at https://doi.org/10.17632/dtzpmwkscp.1

The plate tectonic cycle produces chemically distinct mid-ocean ridge basalts (MORB) and arc volcanics, with the latter enriched in fluid-mobile elements and depleted in Nb owing to fluxes from the subducted slab. Basalts from back-arc basins (BABB), with intermediate compositions, show that the subduction flux can escape the arc. Hence it is puzzling why arc signatures have rarely been recognized in MORB. Here we report the first MORB samples with distinct arc signatures, akin to BABB, from the Arctic Gakkel Ridge. A new high precision dataset for 576 Gakkel samples suggests a pervasive subduction influence. This influence can also be identified in Atlantic and Indian MORB with a “BABB filter”, but is nearly absent in Pacific MORB. This global distribution reflects the control of a “subduction shield” that has surrounded the Pacific Ocean for 180Myr. Statistics suggest that a flux equivalent to ~ 13% of output at arcs is incorporated into the convecting upper mantle.

Lithosphere extension, which plays an essential role in plate tectonics, occurs both in continents (as rift systems) and oceans (spreading along mid-oceanic ridges). The northern Red Sea area is a unique natural geodynamic laboratory, where the ongoing transition from continental rifting to oceanic spreading can be observed. Here, we analyze travel time data from a merged catalogue provided by the Egyptian and Saudi Arabian seismic networks to build a three-dimensional model of seismic velocities in the crust and uppermost mantle beneath the northern Red Sea and surroundings. The derived structures clearly reveal a high-velocity anomaly coinciding with the Red Sea basin and a narrow low-velocity anomaly centered along the rift axis. We interpret these structures as a transition of lithospheric extension from continental rifting to oceanic spreading. The transitional lithosphere is manifested by a dominantly positive seismic anomaly indicating the presence of a 50–70-km-thick and 200–300-km-wide cold lithosphere. Along the forming oceanic ridge axis, an elongated low-velocity anomaly marks a narrow localized nascent spreading zone that disrupts the transitional lithosphere. Along the eastern margins of the Red Sea, several low-velocity anomalies may represent crustal zone of massive Cenozoic basaltic magmatism., Scientific Reports, 11 (1), ISSN:2045-2322

Serpentinites are a powerful tool to evaluate mantle composition and subsequent alteration processes during their tectonic emplacement. Exposures of this type of rocks can be found in the Argentine Precordillera (Cuyania terrane) and Frontal Cordillera, both located in central-western Argentina, within the Central Andes. In these regions a Neoproterozoic to Devonian mafic-ultramafic belt composed of serpentinites, metabasaltic dikes/sills, pillow lavas (with an Enriched to Normal Mid-Ocean Ridge Basalts (E- to N-MORB) geochemical signature) and mafic granulites crop out, spatially associated with marine metasedimentary rocks. The serpentinite bodies consist of lizardite/chrysotile+brucite+magnetite, with scarce pentlandite and anhedral reddish-brown Cr-spinel (picotite, pleonaste and spinel sensu stricto) as relict magmatic phases. The original peridotites were moderately-depleted harzburgites (ultramafic cumulates) with an intermediate chemical signature between a mid-ocean ridge and an arc-related ophiolite. Whole-rock Rare Earth Elements (REE) patterns of serpentinites exhibit enriched REE patterns ((La/Yb)CN=13-59) regarding CI chondrite with positive Eu anomalies. These features are the result of an interaction between hydrothermal fluid and serpentinites, in which moderate temperature (350º-400ºC), CO2-rich, mildly basic hydrothermal fluid was involved and was responsible for the addition of Ca, Sr and REE to serpentinites. The presence of listvenites (silica-carbonate rocks) in the serpentinite margins allow us to infer another fluid metasomatism, where lowtemperatures (

Tectonics investigates the vertical and horizontal movements of the lithosphere that leads to mountain building, the opening and closing of the oceans, and other such first-order geological processes. Plate tectonics is a revolutionary theory that has been developed during the 1960s. According to plate tectonics, the Earth’s lithosphere is broken into multiple adjacent slabs or plates of various sizes that float on the underlying asthenospheric mantle and move horizontally at a speed of a few centimetres per year. Plates can move apart, converge, or slip past one another. Plate margins are the places where the most important geological processes occur. Divergent margins are characterised by extensional faulting, ascent and melting of the asthenosphere, and formation of vast quantities of basaltic magmas. The divergent margins passing through continents generate rift valleys, whereas they form mid-ocean ridges in the oceanic environments. The mid-ocean ridge is a chain of submarine volcanoes some 65,000 km in length; it stretches across all the Earth’s oceanic basins, rarely emerging to the surface. Converging margins are the places where two adjacent plates move against each other, with the denser oceanic plate slipping under the edge of a less dense oceanic or continental plate and sinking into the upper mantle (lithospheric subduction). Converging margins are marked by deep oceanic trenches, intense seismic activity extending from a depth of a few km to about 650-700 km, metamorphism, magmatism, and formation of mountain ranges (orogenesis). Presently active subduction zones are located along the borders of the Pacific Ocean, the Sunda Islands, the South Sandwich, the Lesser Antilles, and the centraleastern Mediterranean Sea. Finally, transform margins separate adjacent plates that slide sideways past each other. The San Andreas Fault in California is the best-known transform margin. Plate tectonic theory envisages the Earth as a dynamic system in which there is continuous competition between global phenomena operating in opposing directions. Extension along the ocean ridges creates new lithosphere, which subsequently is destroyed in subduction zones. Horizontal plate mobility is promoted by the gravitational sinking of heavy oceanic lithosphere into the mantle. The weight of the subducting lithosphere drags the entire plate and is the very reason for plate mobility. Probably, lunar attraction also plays a role in the lithospheric slab mobility.

The Scotia Plate is interacting between the South American and the Antarctic plates. The original plate has been subdivided in relation to new studies that recognized an active oceanic ridge that separate it from the Sandwich Plate. At the same time, the Shackleton Fracture Zone separates it into two other small plates: the Drake and the South Shetland plates. All these plates are mostly of oceanic composition with a small portion of continental crust emerging at the northwestern corner, at the south of the Fuegian Archipelago. Most of the emergent areas of these plates have been repeatedly glaciated during the Quaternary, although the last glaciation was not so extended as in the Northern Hemisphere. In order to get light about rock provenance, beach and coastal sands were collected and analyzed in their mineral composition, assuming that they are indicating the geotectonic setting. Quartz, lithic fragments and plagioclases are dominant at the Atlantic coast of the Isla Grande de Tierra del Fuego. These sediments have been transported by piedmont glaciers from the Darwin Cordillera. On the other hand, and due to their volcanic origin, opaque minerals are dominant at the beaches of the South Shetland Islands, and less common at the Antarctic Peninsula. To the north and south of the Scotia Plate, there is an important contribution of metamorphic minerals (garnets). These contributions are related to the Patagonian Batholith at Beagle Channel, and to those rocks outcropping at the South Orkney Islands and the Antarctic Peninsula.

Earthquakes are mostly generated inside the crust by the brittle failure of rigid rocks. The Earth’s seismicity is concentrated along the edges of the Pacific Ocean, Indonesia, the Caribbean, South Sandwich and the Himalayan-Alpine belt, where most of the global seismic energy is released. Some seismicity also occurs along the mid-ocean ridges and in continental rifts such as in East Africa. Almost all seismic zones are associated with active volcanism.Most earthquakes occur at a depth of a few kilometres. However, along the border of the Pacific Ocean, the Lesser Antilles, the South Sandwich Islands, and in some sectors of the Mediterranean area earthquake hypocentres extend from a few kilometres to several hundred kilometres depths. In these zones, earthquakes distribute along inclined planes known as Benioff-Wadati planes. The Wadati-Benioff planes highlight the occurrence of large blocks of rigid oceanic crust and lithosphere sinking inside the mantle. These are called subduction zones and are the places where the lithosphere, which is formed at the ocean ridges, sinks deep into the Earth and is ultimately destroyed by melting within the mantle.