Subduction of the Izanagi-Pacific Ridge–transform intersection at the northeastern end of the Eurasian plate

Recent reconstructions of global plate motions suggest that the Izanagi-Pacific Ridge was subducted along the eastern margin of Eurasia at ca. 50 Ma. In the Hidaka magmatic zone (HMZ), which was located at the northeastern end of the Eurasian plate, three magmatic pulses occurred (46–45, 40–36, and 19–18 Ma). We report whole-rock geochemical and Sr-Nd-Pb isotopic data for 36 Ma high-Sr/Y (adakitic) rocks from the HMZ and show that these rocks formed by partial melting of oceanic crust and were emplaced as near-trench intrusions during ridge subduction. We reevaluate the nature of plutonic rocks in the HMZ and show that both the 46–45 and 40–36 Ma granitoids have essentially identical geochemical features. The distribution of plutons and magmatic cessation between 45 and 40 Ma are best explained by subduction of a ridge-transform intersection with a large offset of the ridge axis. The boundary between the Eocene granitoids corresponds to the position of a paleo–transform fault, and adakitic magmatism was caused by partial melting triggered by slab tearing at an overlapping spreading center. The paleoridge-transform configuration coincides with the locations of later large faults and a peridotite body.

Magmatic channelisation by reactive and shear-driven instabilities at mid-ocean ridges: a combined analysis

Summary It is generally accepted that melt extraction from the mantle at mid-ocean ridges is concentrated in narrow regions of elevated melt fraction called channels. Two feedback mechanisms have been proposed to explain why these channels grow by linear instability: shear flow of partially molten mantle and reactive flow of the ascending magma. These two mechanisms have been studied extensively, in isolation from each other, through theory and laboratory experiments as well as field and geophysical observations. Here, we develop a consistent theory that accounts for both proposed mechanisms and allows us to weigh their relative contributions. We show that interaction of the two feedback mechanisms is insignificant and that the total linear growth rate of channels is well-approximated by summing their independent growth rates. Furthermore, we explain how their competition is governed by the orientation of channels with respect to gravity and mantle shear. By itself, analysis of the reaction-infiltration instability predicts the formation of tube-shaped channels. We show that with the addition of even a small amount of extension in the horizontal, the combined instability favours tabular channels, consistent with the observed morphology of dunite bodies in ophiolites. We apply the new theory to mid-ocean ridges by calculating the accumulated growth and rotation of channels along streamlines of the solid flow. We show that reactive flow is the dominant instability mechanism deep beneath the ridge axis, where the most unstable orientation of high-porosity channels is sub-vertical. Channels are then rotated by the solid flow away from the vertical. The contribution of the shear-driven instability is confined to the margins of the melting region. Within the limitations of our study, the shear-driven feedback does not appear to be responsible for significant melt focusing or for the shallowly dipping seismic anisotropy that has been obtained by seismic inversions.

Geomorphological and Spatial Characteristics of Underwater Volcanoes in the Easternmost Australian-Antarctic Ridge

Underwater volcanoes and their linear distribution on the flanks of mid-ocean ridges are common submarine topographic structures at intermediate- and fast-spreading systems, where sufficient melt supplies are often available. Such magma sources beneath the seafloor located within a few kilometers of the corresponding ridge-axis tend to concentrate toward the axis during the upwelling process and contribute to seafloor formation. As a result, seamounts on the flanks of the ridge axis are formed at a distance from the spreading axis and distributed asymmetrically about the axis. In this study, we examined three linearly aligned seamount chains on the flanks of the KR1 ridge, which is the easternmost and longest Australian-Antarctic Ridge (AAR) segment. The AAR is an intermediate-spreading rate system located between the Southeast Indian Ridge and Macquarie Triple Junction of the Australian-Antarctic-Pacific plates. By inspecting the high-resolution shipboard multi-beam bathymetric data newly acquired in the study area, we detected 20 individual seamounts. The volcanic lineament runs parallel to the spreading direction of the KR1 segment. The geomorphologic parameters of height, basal area, volume, and summit types of the identified seamounts were individually measured. We also investigated the spatial distribution of the seamounts along the KR1 segment, which exhibits large variations in axial morphology with depth along the ridge axis. Based on the geomorphology and spatial distribution, all the KR1 seamounts can be divided into two groups: the subset seamounts of volcanic chains distributed along the KR1 segment characterized by high elevation and large volume, and the small seamounts distributed mostly on the western KR1. The differences in the volumetric magnitude of volcanic eruptions on the seafloor and the distance from the given axis between these two groups indicate the presence of magma sources with different origins.

Buoyancy-driven flow beneath mid-ocean ridges: the role of chemical heterogeneity

<p>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). </p><p>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. </p><p>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. </p><p>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. </p><p>Balay et al. (2020), PETSc Users Manual, ANL-95/11-Revision 3.13.</p><p>Katz (2010), G-cubed, 11(Q0AC07), 1-29, https://doi.org/10.1029/2010GC003282</p><p>Melt Seismic Team (1998), Science, 280(5367), 1215–1218, https://doi.org/10.1126/science.280.5367.1215 </p><p>Pusok et al. (2020), EGU General Assembly 2020, EGU2020-18690 https://doi.org/10.5194/egusphere-egu2020-18690 </p><p>Rychert et al. (2020), JGR Solid Earth, 125, e2018JB016463. https://doi. org/10.1029/2018JB016463  </p><p>Toomey et al. (2002), EPSL, 200(3-4), 287-295, https://doi.org/10.1016/S0012-821X(02)00655-6</p>

Formation and evolution of a magma reservoir at a slow-spreading center (Atlantis Bank, Southwest Indian Ridge)

<p>The heterogeneous presence of ephemeral magmatic systems below the ridge axis and their complexity mostly account for the heterogeneous character of the oceanic crust accreted at (ultra) slow-spreading ridges. In order to better understand the magmatic processes involved in slow-spreading lower oceanic crust formation, we studied a drilled section of an oceanic core complex (OCC) interpreted as an exhumed portion of lower crust close to the ridge axis. We focused on ODP Hole 735B which presents the most primitive lithologies sampled at Atlantis Bank OCC (Southwest Indian Ridge) in a ~250 m thick section previously interpreted as a single crustal intrusion.</p><p>We combined detailed structural and petrographic data with whole-rock and <em>in situ</em> mineral analyses to determine the processes of emplacement and differentiation of melts within this section. The lower half of the unit is comprised of alternating troctolites and olivine gabbros showing intrusive contacts, and both magmatic and crystal-plastic fabrics. Such features are lacking in the upper half, rather uniform, gabbroic sequence. Whole-rock compositions highlight the cumulative character of both lower and upper units, and a great compositional variability in the lower sequence, whereas the upper sequence is rather homogeneous and differentiates up-section. <em>In situ</em> analyses of mineral phases document magma emplacement processes and provide evidence for ubiquitous reactive porous flow during differentiation. Comparison between both units' geochemistry also led us to strongly favor a model of formation of the reservoir that genetically links melts from the lower and the upper unit.</p><p>We show that the whole section, and related geochemical units, likely constitutes a single magmatic reservoir, in which the lower unit formed by emplacement of primitive sills related to the continuous recharge of primitive melts. Recharge led to partial assimilation of the crystallizing primitive mush, and related hybridization with interstitial melts. Hybrid melts were progressively collected in the overlying mushy part of the reservoir (upper unit), whereas the sills' residual melt differentiated by reactive porous flow processes under a predominantly crystallization regime. Similarly, hybrid melts’ evolution in the upper unit was governed by upward reactive porous flow and progressive differentiation and accumulation of evolved melts at the top of the reservoir. Our results provide the first integrated model for magma reservoir formation in the lower slow-spreading oceanic crust, and have potential implications regarding the lower crust structure and the composition of MORBs.</p>

Ridge Dynamics in the expanding Artemis Coronae: axis sinuosity, transform faults, and microplates.

<p>Venus today presents no large-scale network of subduction and accretion ridges, which is the signature of plate tectonics on Earth. On the other hand, Venus relatively young surface points towards either a quite recent catastrophic renewal of the whole planet surface (« episodic subduction regime »), or the continuous renewal of small areas of the planet for exemple by volcanism.</p> <p>Unique to Venus, coronae are circular features from 50 to 2600 km in diameter. The largest ones have been attributed to mantle plumes. Close inspection of Magellan’s data revealed that subduction features are also encountered on part of their rim (McKenzie et al, 1992 ; Sandwell and Schubert, 1992, 1995).  Recent modeling has shown that plumes could indeed induce roll-back subduction around segments of an expanding coronae. Artemis coronae is the largest coronae on Venus and shows both plume and subduction features that are well explained by the plume-induced subduction mechanism (Davaille et al, 2017). Scaling laws then predict a slab roll-back (and therefore a coronae expansion) velocity between 1 and 10 cm/yr.  If the coronae has been expanding, then we should expect the existence of an accreting ridge system  inside the coronae, equivalent to the Earth’s mid-ocean ridges developing in back-arc basins. Artemis interior indeed also presents a prominent ridge system (Sandwell and Schubert, 1992 ; Brown and Grimm, 1996 ; Spencer, 2001 ; Hansen, 2002), but its lateral tortuosity is much more prononced than on Earth (fig.1).   </p> <p>Using laboratory experiments, we recently showed that the shape of an accretion ridge is governed primarily by the axial failure parameter Π<sub>F</sub>, which depends on the spreading velocity, the mechanical strength of the lithospheric material and the axial elastic lithosphere thickness (Sibrant et al, 2018). Experiments with the largest Π<sub>F</sub> presented quite unstable ridge axis with a large lateral sinuosity,  long transform faults, and the formation of numerous microplates. These microplates rotate along the transforms before getting incorporated in the main plate on one side of the ridge axis or the other. There, they appear as blocks whose main fabric is either concentric or rotated compared to the main plate’s. </p> <p>On a planet, this regime occurs for high spreading velocity and/or low axial elastic thickness. For the Earth, it would require spreading velocities greater  than 30 cm/yr. But on Venus, where the surface temperature is about 500°C higher, and therefore the elastic thickness on the ridge axis is smaller than on Earth, spreading velocities between 1 and 10 cm/yr would suffice. The scaling laws derived from the laboratory experiments further predict a tortuosity of the ridge axis comparable to what is observed inside Artemis coronae (fig.1). Furthermore, guided by the experiments, we are tempted to identify two long transform faults on each side of Britomartis, as well as a number of rotated blocks or microplates. However, the resolution of Magellan data is not sufficient to be sure of our interpretation. There is an urgent need for better resolution and better coverage of Venus topography, that a mission such as VERITAS could provide.</p> <p> </p>

A seawater throttle on H2production in Precambrian serpentinizing systems

Since the initial discovery of low-temperature alkaline hydrothermal vents off the Mid-Atlantic Ridge axis nearly 20 y ago, the observation that serpentinizing systems produce abundant H2has strongly influenced models of atmospheric evolution and geological scenarios for the origin of life. Nevertheless, the principal mechanisms that generate H2in these systems, and how secular changes in seawater composition may have modified serpentinization-driven H2fluxes, remain poorly constrained. Here, we demonstrate that the dominant mechanism for H2production during low-temperature serpentinization is directly related to a Si deficiency in the serpentine structure, which itself is caused by low SiO2(aq) concentrations in serpentinizing fluids derived from modern seawater. Geochemical calculations explicitly incorporating this mechanism illustrate that H2production is directly proportional to both the SiO2(aq) concentration and temperature of serpentinization. These results imply that, before the emergence of silica-secreting organisms, elevated SiO2(aq) concentrations in Precambrian seawater would have generated serpentinites that produced up to two orders of magnitude less H2than their modern counterparts, consistent with Fe-oxidation states measured on ancient igneous rocks. A mechanistic link between the marine Si cycle and off-axis H2production requires a reevaluation of the processes that supplied H2to prebiotic and early microbial systems, as well as those that balanced ocean–atmosphere redox through time.

Magmatic and tectonic segmentation of the intermediate-spreading Costa Rica Rift—a fine balance between magma supply rate, faulting and hydrothermal circulation

SUMMARY 3-D tomographic modelling of wide-angle seismic data, recorded at the intermediate-spreading Costa Rica Rift, has revealed a P-wave seismic velocity anomaly low located beneath a small overlapping spreading centre that forms a non-transform discontinuity at the ridge axis. This low velocity zone displays a maximum velocity anomaly relative to the ‘background’ ridge axis crustal structure of ∼0.5 km s−1, has lateral dimensions of ∼10 × 5 km, and extends to depths ≥2.5 km below the seabed, placing it within layer 2 of the oceanic crust. We interpret these observations as representing increased fracturing under enhanced tectonic stress associated with the opening of the overlapping spreading centre, that results in higher upper crustal bulk porosity and permeability. Evidence for ongoing magmatic accretion at the Costa Rica Rift ridge axis takes the form of an axial magma lens beneath the western ridge segment, and observations of hydrothermal plume activity and microearthquakes support the presence of an active fluid circulation system. We propose that fracture pathways associated with the low velocity zone may provide the system through which hydrothermal fluids circulate. These fluids cause rapid cooling of the adjacent ridge axis and any magma accumulations which may be present. The Costa Rica Rift exists at a tipping point between episodic phases of magmatic and tectonically enhanced spreading. The characteristics inherited from each spreading mode have been preserved in the crustal morphology off-axis for the past 7 Myr. Using potential field data, we contextualize our seismic observations of the axial ridge structure at the whole segment scale, and find that the proposed balance between magmatic and tectonically dominated spreading processes observed off-axis may also be apparent along-axis, and that the current larger-scale magma supply system at the Costa Rica Rift may be relatively weak. Based on all available geophysical observations, we suggest a model for the inter-relationships between magmatism, faulting and fluid circulation at the Costa Rica Rift across a range of scales, which may also be influenced by large lithosphere scale structural and/or thermal heterogeneity.

The Eastern Romanche ridge-transform intersection (Equatorial Atlantic): slow spreading under extreme low mantle temperatures. Preliminary results of the SMARTIES cruise.

<p>A strong edge effect is predicted at the intersections between long-offset transforms and mid ocean ridge segments. The Equatorial Atlantic hosts several megatransforms, where the connections of potentially low mantle temperatures due to the large lithospheric age contrast with melt production are poorly understood. The SMARTIES cruise focused on the Romanche transform that offsets the Mid Atlantic Ridge (MAR) laterally by 900 km with an age offset of 55 Ma. The eastern Ridge-Transform Intersection (RTI) markedly shows the effects of the lateral cooling of the ridge segment. To better understand the thermal regime at these complex domains, we acquired surface geophysical data and bathymetry of the area, and geological observations and sampling during 25 HOV Nautile dives. The integrated study of rock characteristics and of geophysical surveys allows tackling the connections between magmatism and tectonics. A network of 19 OBS was also deployed to study the seismic activity during the cruise in collaboration with the ILAB project.</p><p>There is a striking change in deformation patterns along the ridge axis moving away from the transform southwards. The bathymetry is extremely complex, with several structural directions, partly resulting from transtension. A low melt supply is focused at the ridge axis resulting in a long oblique axial domain, that forms a relay zone between the roughly north-south ridge axis in the south and the area close to the transform fault, while the transform fault domain is highly complex. Trends oblique to both the main spreading axis direction and the transform fault direction are widespread. A clear Principal Transform Displacement Zone (PTDZ) can be followed as a long, near continuous alignment, on the seafloor of the wide Romanche valley. However, the valley morphology suggests a migration of the PTDZ and intense deformation within the transform domain. The RTI is complex and the position of the spreading axis clearly evolved with time, through at least two and possibly three eastward ridge jumps.</p><p>Six Nautile dives explored the northern wall of the Romanche, the damaged zone of the transform fault, and the exceptionally deep nodal basin. The north wall exposes a very thick basalt unit covered with a thick layer of sediments. Eight dives explored the southern flank of the Romanche identifying fragments of old Oceanic Core Complexes (OCCs) formed by highly deformed peridotites, and a large OCC located at the RTI that exposes mylonitized peridotites and is dissected by several normal faults. The magmatic zones of the axial domain (nine dives) are formed by volcanic ridges affected by important tectonic activity. The dives show pillow and tube volcanic flows with intersecting faults. An oblique elongated faulted and sedimented ridge (2 dives) parallel to the oblique relay zone was shown to be of peridotitic nature Recent faults have been observed, as well as traces of high-T hydrothermal activity consistent with black-smoker type venting, recently overprinted by low temperature diffuse venting related to active faulting.</p>

Distribution, morphology and magnetic characteristics of near-axis seamounts along the KR1, the easternmost segment of the Australian-Antarctic Ridge

<p>The Australian-Antarctic Ridge (AAR) is the spreading boundary between the Australian and the Antarctic plates, and extended from the Southeast Indian Ridge (SEIR) to the Macquarie Triple Junction (MTJ) of Australian-Antarctic-Pacific plates. The KR1 is the easternmost segment of AAR, with a quite large variation in axial morphology. In this study, we identified 3-volcanic seamount chains aligned linearly parallel to the spreading direction of the KR1. The spatial distribution, morphology and summit types for the isolated volcanic structures composing the seamount chains were determined. Eastern seamount chain has the 3-isolated volcanoes which are significantly steep and located at a considerable distance away from the ridge-axis. The central seamount chain is morphologically connected to the ridge-axis, and relatively small and less isolated compared to the other seamount chains. Western seamount chain shows a massive volcanic eruption with significantly large volcanic structures. In usual, a seamount is formed on pre-existing seafloor, and the observed magnetic signal of the seamount is correspond to that of the underlying seafloor when the seamount formation occurs nearly simultaneously with the formation of the underlying seafloor. However, if the observed magnetic anomalies of the seamount have a large misfit or/and reversed geomagnetic polarities with respect to the modeled magnetic anomalies of the underlying seafloor, it implies that there is a sufficiently large temporal gap between the formations of the seamount and the underlying seafloor. Applying this assumption, we forced the relatively younger geomagnetic history to move into the seamount having such misfits, and finally reduced the misfits. As a result, our magnetic model for the seamount chains shows that the isolated volcanoes were mostly originated from off-axis volcanisms of 7~20 km, and have a time span of less than ~600 kyrs to build. In particular, it is assumed that the seamount formations were intensively active during four-periods of 0.3~0.8 Ma, 0.9~1.1 Ma, 1.6~2.1 Ma and 2.2~2.7 Ma.</p>