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A global study of subduction zone dynamics indicates that the thermal structure of the overriding plate may control arc location. A fast convergence rate and a steep slab dip bring a hotter mantle further into the wedge corner, forming arc volcanoes closer to the trench. Separately, laboratory and numerical experiments showed that the development of a back‐arc spreading center (BASC) is driven by the migration of the subducting hinge, especially following changes in the slab geometry. As both arc location and the deformation regime of the overriding plate depend on slab kinematics and geometry, we investigate the possible correlations between BASC, the position of volcanic arcs, and slab dip at the scale of individual subduction zones. To do this, we compare the distance from trench to arc and trench to BASC at the Mariana, Scotia, Vanuatu, Tonga, and Kermadec subduction zones. In most cases, the arc and BASC are closer to the trench when the slab is dipping steeply. The correlation could result from an interplay between progressive changes in slab geometry and overriding plate deformation. This assumes, on the one hand, that the isotherm at the apex of which the arc forms is tied to a constant slab decoupling depth and, on the other hand, that back‐arc opening accommodates a change in slab dip. As slab dip decreases, both the BASC and the apex of the isotherm controlling the melt focusing move further from the trench. The observed trends are consistent with a slab anchored at 660 km depth. Plain Language Summary At subduction zones, where oceanic plates are recycled into the Earth's interior, water released by the downgoing plate initiates volcanism that forms arc volcanoes. Back‐arc basins sometimes develop beyond the volcanic arc as the upper plate stretches and thins under extensional stress, to the point of forming a back‐arc spreading center (BASC). Subduction parameters such as slab dip and plate velocity remain the dominant control of the arc location globally. The arc and BASC location, expressed as the distance from the trench, are negatively correlated with slab dip in most cases. We show here that the correlation may result from two separate phenomena stemming from variations in slab dip in the upper mantle. Extension in the back‐arc is induced by the retreating motion of the trench as a response to the slowdown of slab dip after reaching the mantle with higher viscosity. As the back‐arc opens, the spreading ridge keeps moving away from the trench, and the slab dip in the upper mantle decreases. As the slab dip decreases, the trenchward limit of the hot mantle flow is located further from the trench, resulting in further distance from the trench to the arc. Key Points The distance from the trench to the arc is negatively correlated with the slab dip within each subduction zone The thermal structure of the upper plate, which is tied to the depth of the slab coupling/decoupling transition, controls arc location Variations in the location of the spreading center in a given subduction zone can be linked to changes in slab dip during trench retreat

This study analyzes up‐to‐date gravity data in the Galapagos triple junction region to understand crustal structure and melt distribution beneath the propagating Cocos‐Nazca spreading center (CNSC). Application of a standard thermal model to the mantle Bouguer gravity anomaly (MBA) does not appear to result in a realistic crustal thickness in this region. The cross‐CNSC MBA profiles flatten and axial values increase from east toward the western end of the CNSC. A simple smoothing filter applied to the standard thermal model with different filter widths can explain the progressive flattening of the MBA and is interpreted as different distribution widths (concentrations) of partial melt in the mantle. The east‐west residual MBA gradient along the CNSC is similar to the east flank of the East Pacific Rise (EPR), suggesting that the along‐CNSC gradient could partly reflect the shallow mantle properties associated with the EPR. Plain Language Summary This study investigates changes in crustal thickness and shallow mantle properties beneath the westward propagating Cocos‐Nazca spreading center (CNSC) by analyzing shipboard gravity data combined with satellite gravity data in the Galapagos triple junction region. By assuming a general 1‐D plate cooling model, we correct the age‐induced thermal effect of the lithosphere with a standard thermal model, which suggest thinner crust near the western tip of the CNSC increasing in thickness toward the east. However, results also indicate that the crust continues to thicken from the CNSC axis to the north and south edges of the gore, which does not seem realistic. Instead, these gravity signals most likely reflect properties within the shallow mantle, specifically a eastward decrease in the width (and hence increase in concentration) of melt within the shallow mantle beneath the CNSC. The residual gravity gradient along the CNSC axis is similar to that on the east flank of the East Pacific Rise (EPR), suggesting that the CNSC axis gravity gradient could in part reflect variation in the mantle density beneath the EPR. Key Points Shipboard and satellite gravity data were used to explore the crustal structure in the Galapagos triple junction region The gravity‐derived crustal thickness with a standard thermal model does not appear to be realistic in this region A simple smoothing filter applied to the standard thermal model could explain gravity variation along the Cocos‐Nazca spreading center

We report the mineralogy and geochemistry of hydrothermal sulphide from the crater of a volcanic high near 18°36.4′S of the Central Lau Spreading Center. During 1990s, that volcanic structure was reported active and sulphide samples were collected by MIR submersible. A section of a chimney-like structure from the crater-floor was studied here. The Fe-depleted sphalerites, and Co-depleted pyrites in that chimney were similar to those commonly found in low to moderate temperature (<300°C) sulphides from sediment-starved hydrothermal systems. Bulk analyses of three parts of that chimney section showed substantial enrichment of Zn (18%–20%) and Fe (14%-27%) but depletion of Cu (0.8%–1.3%). In chondrite-normalized rare earth element-patterns, the significant negative Ce-anomalies (Ce/Ce*=0.27–0.39) and weakly positive Eu-anomalies (Eu/Eu*=1.60–1.68) suggested sulphide mineralisation took place from reduced low-temperature fluid. The depleted concentration of lithophiles in this sulphide indicates restricted contribution of sub-ducting plate in genesis of source fluid as compared to those from other parts of Lau Spreading Centre. Uniform mineralogy and bulk composition of subsamples across the chimney section suggests barely any alteration of fluid composition and/or mode of mineralisation occurred during its growth.

Major and trace element and Sr, Nd, and Pb isotope data for lavas from 12 seamounts along the western (older) 1500 km section of the Louisville Seamount Chain in the southwest Pacific show remarkably uniform compositions over a ∼30–40 Myr period of volcanism. All 56 samples analyzed are alkalic to transitional in composition. Unlike Hawaiian volcanoes, Louisville volcanoes appear not to pass through a sequence of evolutionary stages characterized by older tholeiitic basalts overlain by incompatible element enriched alkalic and silica‐undersaturated lavas. The youngest lavas from a given Louisville seamount tend to have the least enriched incompatible element compositions. This unusual chemical evolution may be the result of re‐melting of heterogeneous hot spot mantle that was partially depleted during the earlier, age progressive stages. The oldest Louisville seamounts were constructed close to the extinct Osbourn Trough spreading center, located north of the chain, but age‐progressive lavas from these older seamounts are not significantly different to lavas from younger seamounts. This may indicate that spreading at this fossil ridge ceased several tens of millions of years before the oldest Louisville seamounts were constructed. Large fracture zones apparently had no significant effects on the composition of Louisville magmatism. However, lavas from the central part of the Louisville Seamount Chain, where volcanoes are smaller and more widely spaced, tend to have more variable and more enriched compositions. We suggest this may reflect smaller degrees of melting resulting from greater lithosphere thickness, and hence a shorter melting column for this section of the Louisville Seamounts. Key Points Composition of magmas was remarkably uniform over a  ∼30–40 Myr period Smaller melting degrees are the result of increased lithosphere thickness No evolutionary stages of the Louisville Seamount Chain volcanoes

Volcanic eruptions are important events in Earth's cycle of magma generation and crustal construction. Over durations of hours to years, eruptions produce new deposits of lava and/or fragmentary ejecta, transfer heat and magmatic volatiles from Earth's interior to the overlying air or seawater, and significantly modify the landscape and perturb local ecosystems. Today and through most of geological history, the greatest number and volume of volcanic eruptions on Earth have occurred in the deep ocean along mid-ocean ridges, near subduction zones, on oceanic plateaus, and on thousands of mid-plate seamounts. However, deepsea eruptions (> 500 m depth) are much more difficult to detect and observe than subaerial eruptions, so comparatively little is known about them. Great strides have been made in eruption detection, response speed, and observational detail since the first recognition of a deep submarine eruption at a mid-ocean ridge 25 years ago. Studies of ongoing or recent deep submarine eruptions reveal information about their sizes, durations, frequencies, styles, and environmental impacts. Ultimately, magma formation and accumulation in the upper mantle and crust, plus local tectonic stress fields, dictate when, where, and how often submarine eruptions occur, whereas eruption depth, magma composition, conditions of volatile segregation, and tectonic setting determine submarine eruption style.

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 years Faulting at the Vance Segment accommodates ∼3/4 of the spreading and magmatic accretion along Axial Seamount south rift accounts for ∼1/4 The spreading axis is <250 m wide along the Vance Segment but ∼4 km wide along the south rift of Axial Seamount

Chemolithoautotrophic microorganisms are at the nexus of hydrothermal systems by effectively transferring the energy from the geothermal source to the higher trophic levels. While the validity of this conceptual framework is well established at this point, there are still significant gaps in our understanding of the microbiology and biogeochemistry of deep-sea hydrothermal systems. Important questions in this regard are: (1) How much, at what rates, and where in the system is organic carbon being produced? (2) What are the dominant autotrophs, where do they reside, and what is the relative importance of free-swimming, biofilm-forming, and symbiotic microbes? (3) Which metabolic pathways are they using to conserve energy and to fix carbon? (4) How does community-wide gene expression in fluid and biofilm communities compare? and (5) How efficiently is the energy being utilized, transformed into biomass, and transferred to higher trophic levels? In particular, there is currently a notable lack of process-oriented studies that would allow an assessment of the larger role of these ecosystems in global biogeochemical cycles. By combining the presently available powerful \"omic\" and single-cell tools with thermodynamic modeling, experimental approaches, and new in situ instrumentation to measure rates and concentrations, it is now possible to bring our understanding of these truly fascinating ecosystems to a new level and to place them in a quantitative framework and thus a larger global context.

The East Pacific Rise from ~ 9–10°N is an archetype for a fastspreading mid-ocean ridge. In particular, the segment near 9°50'N has been the focus of multidisciplinary research for over two decades, making it one of the best-studied areas of the global ridge system. It is also one of only two sites along the global ridge where two historical volcanic eruptions have been observed. This volcanically active segment has thus offered unparalleled opportunities to investigate a range of complex interactions among magmatic, volcanic, hydrothermal, and biological processes associated with crustal accretion over a full magmatic cycle. At this 9°50'N site, comprehensive physical oceanographic measurements and modeling have also shed light on linkages between hydrodynamic transport of larvae and other materials and biological dynamics influenced by magmatic processes. Integrated results of highresolution mapping, and both in situ and laboratory-based geophysical, oceanographic, geochemical, and biological observations and sampling, reveal how magmatic events perturb the hydrothermal system and the biological communities it hosts.

Experiments were carried out to observe the variation in propagation and linkage of parallel en-echelon cracks with varying orientation of the crack array and different relative position of the cracks within the array in an extensional regime. Two-layered analogue model, with a basal layer of pitch overlain by a layer of kaolin paste was used in the experiments. En-echelon cracks were pre-cut within the kaolin layer maintaining specific geometrical parameters of the cracks (e.g., length, centre spacing, separation) in such a manner that there was a weak (though not negligible) local tip-induced stress favouring curvature of adjacent crack tips towards one another. The results obtained were matched with natural pattern of linkage of veins, rift basins and spreading ridges, as described in the relevant literature. The experimental results showed that the final pattern of linkage between the cracks was a result of initial deflection of crack tip from its plane due to combined effect of local and far-field stress. When the deflection of tip from the crack plane was between 0 ∘ to 45 ∘ , a ‘tip to wall’ linkage took place between adjacent cracks isolating a rhombohedral area in the interaction zone. The resultant structure could be geometrically comparable to a micro-plate-like structure isolated due to linkage of ridge segments initially forming an overlapping spreading centre (OSC). When the deflection of tip from the crack plane was greater than 45 ∘ , a ‘tip to tip’ linkage between adjacent cracks took place resulting in a structure similar to a transform fault between spreading ridges and or rift basins. When effect of the remote stress opposed the tip induced stress, no linkage took place between the adjacent cracks, and finally the tips propagated straight along a plane perpendicular to the remote extension direction.

In the classical concept, a hotspot track is a line of volcanics formed as a plate moves over a stationary mantle plume. Defying this concept, intraplate volcanism in Greenland and the North Atlantic region occurred simultaneously over a wide area, particularly around 60 million years ago, showing no resemblance to a hotspot track. Here, we show that most of this volcanism can nonetheless be explained solely by the Iceland plume interacting with seafloor spreading ridges, global mantle flow and a lithosphere (the outermost rigid layer of the Earth) with strongly variable thickness. An east–west corridor of thinned lithosphere across central Greenland, as inferred from new, highly resolved tomographic images, could have formed as Greenland moved westward over the Iceland plume between 90 and 60 million years ago. Our numerical geodynamic model demonstrates how plume material may have accumulated in this corridor and in areas east and west of Greenland. Simultaneous plume-related volcanic activities starting about 62 million years ago on either side of Greenland could occur where and when the lithosphere was thin enough due to continental rifting and seafloor spreading, possibly long after the plume reached the base of the lithosphere.