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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

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.

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.

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

Black smokers are the dramatic result of seawater being heated to high temperatures (generally 250° to 350°C) by magmatic systems, then discharging at the seafloor. However, not all seawater that circulates through the oceanic crust is heated to high temperatures. \"Diffuse flow\" is a catchall term to describe lowtemperature (< 0.2° to ~ 100°C) fluids that slowly discharge through sulfide mounds, fractured lava flows, and assemblages of bacterial mats and macrofauna. Diffuseflow fluids are generally mixtures of cold seawater and either magmatically heated fluids, conductively heated seawater, or both. Although the limited data indicate that 50–90% of the hydrothermal heat loss occurs as diffuse flow at the seafloor, modeling results coupled with geochemical data suggest that nearly 90% of the heat loss ultimately stems from magmatically heated fluids. There is a critical need to obtain more diffuse-flow measurements to improve models of heat and geochemical fluxes, better understand subsurface fluid flow dynamics, and determine the extent of the subsurface biosphere as well as the spatial and temporal variability of diffuse flow. New measurement techniques and diffuse-flow models provide insight into the characteristics of these subsurface fluids and their manifestation at the seafloor.

Visually striking faunal communities of high abundance and biomass cluster around hydrothermal vents, but these animals don't spend all of their lives on the seafloor. Instead, they spend a portion of their lives as tiny larvae in the overlying water column. Dispersal of larvae among vent sites is critical for population maintenance, colonization of new vents, and recolonization of disturbed vents. Historically, studying larvae has been challenging, especially in the deep sea. Advances in the last decade in larval culturing technologies and more integrated, interdisciplinary time-series observations are providing new insights into how hydrothermal vent animals use the water column to maintain their populations across ephemeral and disjunct habitats. Larval physiology and development are often constrained by evolutionary history, resulting in larvae using a diverse set of dispersal strategies to interact with the surrounding currents at different depths. These complex biological and oceanographic interactions translate the reproductive output of adults in vent communities into a dynamic supply of settling larvae from sources near and far.

P and S wave data from the L-SCAN active-source wide-angle reflection/refraction experiment are modelled to investigate upper crustal structure in the Lau backarc basin. A combination of ray tracing and finite difference numerical wavefield simulation is used to identify P and P-to-S converted seismic phases. The phases primarily arise from two shallow interfaces, one at ~ 80 m depth or less, and the other at 500–650 m depth. The shallower interface is deeper than the sediment base, is observed across the study area, and is interpreted as a ‘layer 2Aa’ boundary, proposed to result from a rapid change in crack density. The deeper interface is interpreted as the layer 2A–2B boundary, corresponding to a transition from lavas to sheeted dykes. Layer 2A, on average, is 150 m thicker in crust that formed at the spreading center when spreading was located near the arc (< 50 km away), as compared to when spreading was located farther away from the arc (> 70 km away). Layer 2A thickness and Vp/Vs values indicate that a thicker and more porous lava layer, dominated by basalts to basalt-andesites, cap near-arc crust, while a thinner and less-porous, mostly basaltic, volcanic layer caps the far-arc crust. These results are consistent with the waning influence of slab-derived volatiles on crustal formation as seafloor spreading moves away from the active arc.

Endeavour Segment of the Juan de Fuca Ridge is one of three Integrated Study Sites for the Ridge 2000 Program. It is a remarkable, dynamic environment hosting five major hydrothermal fields, numerous smaller fields, and myriad diffuse-flow sites; magma chambers underlie all fields. Over 800 individual extinct and active chimneys have been documented within the central ~ 15 km portion of the ridge, with some edifices reaching 50 m across and up to 45 m tall. Fluid flow is focused along faults within the rift zone, and seismically active faults along the western axial valley wall have been used by both magmas and upwelling hydrothermal fluids. There is significant chemical heterogeneity in basalt compositions within the axial rift valley, with the greatest diversity occurring near the base of the western axial valley wall where normal, transitional, and enriched type mid-ocean ridge basalts occur within tens of meters of each other. Endeavour is the only site where seismic intensity has been linked directly to heat flux at the individual vent field scale. Installation of the world's first high-power and high-bandwidth cabled observatory at Endeavour via NEPTUNE Canada ensures that new discoveries along the Juan de Fuca Ridge will continue into the future.