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Hydrothermal vent temperatures fluctuate in response to transient magmatic and tectonic activity at the axis of mid-ocean ridges (MORs) and modulate energy fluxes from the deep Earth to the ocean. Such fluctuations have thus far only been documented on time scales of minutes to years, because of the scarcity of long, continuous observations. Here, we assemble a ~35-year-long time series of exit fluid temperatures from five hydrothermal vents on the East Pacific Rise axis, between 9°46’-51’N. This dataset reveals a steady increase in maximum venting temperatures atop the central part of the axial magma lens (AML), from ~350 °C to ~390 °C between the 1991–92 and 2005–06 eruptions. Temperatures decreased back to ~350 °C shortly after the 2005–06 eruption and have been rising ever since. We interpret the temperature increase as a result of a steady decrease in upflow zone permeability caused by the steady inflation of the AML compressing the oceanic upper crust. Using laboratory-determined pressure–permeability relations, we estimate crustal pressurization rates of 0.38 MPa/y (1992–2005) and 0.33 MPa/y (post-2006), consistent with geodetic observations from 2009–2011. Decadal fluctuations in hydrothermal vent temperatures likely mimic the rate of AML pressurization, yielding valuable new constraints on the dynamics of magmatic replenishment and eruptions at MORs. Notably, this temperature time series underpinned our forecast of the April 2025 eruption at the study site. 

At fast-spreading centers, faults develop within the axial summit trough (AST; 0 to 250 m around the axis) primarily by diking-induced deformation originating from the axial magma lens (AML). The formation of the prominent abyssal-hill-bounding faults beyond the axial high (>2,000 m) is typically associated with the unbending of the lithosphere as it cools and spreads away from the AST. The presence of faults is rarely mapped between these two thermally distinct zones, where the lithosphere is still too hot for the faults to be linked with the process of thermal cooling and outside of the AST where the accretional diking process dominates the ridge axis. Here, we reveal a remarkable vertical alignment between the distinct morphological features of the magma body and the orientation of these faults, by comparison of 3-D seismic imagery and bathymetry data collected at the East Pacific Rise (EPR) 9°50’N. The spatial coincidence and asymmetric nucleation mode of the mapped faults represent the most direct evidence for magmatically induced faulting near the ridge axis, providing pathways for hydrothermalism and magma emplacement, helping to build the crust outside of the AST. The high-resolution seafloor and subsurface images also enable revised tectonic strain estimates, which shows that the near-axis tectonic component of seafloor spreading at the EPR is an order of magnitude smaller than previously thought with close to negligible contribution of lava buried faults to spreading. 

Bathymetric data were collected during a total of 19 AUV Sentry dives conducted from R/V Atlantis and R/V Roger Revelle during 2018 Atlantis cruise AT42-06 (two dives); 2019 Atlantis cruise AT42-21 (ten dives); and, 2021 Revelle cruise RR2102 (seven dives). Track lines were spaced 170 m apart, with Sentry about 65 m above bottom, collecting multibeam data using a 400 kHz Reson 7125 system in 2018, and a 400 kHz Kongsberg EM2040 system in 2019 and 2021. Sentry navigation was obtained using a 300 kHz Teledyne Doppler velocity log (DVL) and a Sonardyne AvTrak2 ultra-short baseline (USBL) acoustic positioning system, combined with an iXblue Phins inertial navigation system (INS), and a Paroscientific 8B7000-I Digiquartz depth sensor. Sonar data from all 19 dives were processed together using the open-source MB-System software, and were gridded at 1 m × 1 m node resolution using a beam footprint calculated with the angular beam widths and weighted by local slope. The grid file is in GMT-compatible netCDF format, in un-projected geographic coordinates. Funding was provided by National Science Foundation awards OCE-1834797, OCE1949485, OCE-1948936, and OCE-1949938. 

This data set presents geological interpretation of lava flows generated during the 2005-2006 eruption, faults, and eruptive fissures for the 9°50'N segment of the East Pacific Rise. Interpretation was obtained based upon the compilation of multibeam bathymetric and sidescan sonar imagery data collected by AUV Sentry in 2018, 2019 and 2021. The data files are in shapefile format, in UTM Zone 9N projection. Funding was provided by National Science Foundation awards OCE-1834797, OCE-1949485, OCE-1948936, and OCE-1949938. 

Comprehensive knowledge of the distribution of active hydrothermal vent fields along midocean ridges is essential to understanding global chemical and heat fluxes and endemic faunal distributions. However, current knowledge is biased by a historical preference for on-axis surveys. A scarcity of high-resolution bathymetric surveys in off-axis regions limits vent identification, which implies that the number of vents may be underestimated. Here, we present the discovery of an active, high-temperature, off-axis hydrothermal field on a fast-spreading ridge. The vent field is located 750 m east of the East Pacific Rise axis and ∼7 km north of on-axis vents at 9° 50′N, which are situated in a 50- to 100-m-wide trough. This site is currently the largest vent field known on the East Pacific Rise between 9 and 10° N. Its proximity to a normal fault suggests that hydrothermal fluid pathways are tectonically controlled. Geochemical evidence reveals deep fluid circulation to depths only 160 m above the axial magma lens. Relative to on-axis vents at 9° 50′N, these off-axis fluids attain higher temperatures and pressures. This tectonically controlled vent field may therefore exhibit greater stability in fluid composition, in contrast to more dynamic, dike-controlled, on-axis vents. The location of this site indicates that high-temperature convective circulation cells extend to greater distances off axis than previously realized. Thorough high-resolution mapping is necessary to understand the distribution, frequency, and physical controls on active off-axis vent fields so that their contribution to global heat and chemical fluxes and role in metacommunity dynamics can be determined. 

Abstract Fissures and faults provide insight into how plate separation is accommodated by magmatism and brittle deformation during crustal accretion. Although fissure and fault geometry can be used to quantify the spreading process at mid‐ocean ridges, accurate measurements are rare due to insufficiently detailed mapping data. Here, fissures and faults at the fast‐spreading 9°50′N segment of the East Pacific Rise were mapped using bathymetric data collected at 1‐m horizontal resolution by autonomous underwater vehicleSentry. Fault dip estimates from the bathymetric data were calibrated using co‐registered near‐bottom imagery and depth transects acquired by remotely operated vehicleJason. Fissures are classified as either eruptive or non‐eruptive (i.e., cracks). Tectonic strain estimated from corrected fault heaves suggests that faulting plays a negligible role in the plate separation on crust younger than 72 kyr (<4 km from the ridge axis). Pre‐ and post‐eruption surveys show that most fissures were reactivated during the eruptions in 2005–2006. Variable eruptive fissure geometry could be explained by the frequency with which each fissure is reactivated and partially infilled. Fissure swarms and lava plateaus in low‐relief areas >2 km from the ridge are spatially associated with off‐axis lower‐crustal magma lenses identified in multichannel seismic data. Deep, closely spaced fissures overlie a relatively shallow portion of the axial magma lens. The width of on‐axis fissures and inferred subsurface dike geometry imply a ∼9‐year long diking recurrence interval to fully accommodate plate spreading, which is broadly consistent with cycle intervals obtained from estimates of melt extraction rates, eruption volumes, and spreading rate. 

Abstract Magnetic anomaly variations near mid‐ocean ridge spreading centers are sensitive to a variety of crustal accretionary processes as well as geomagnetic field variations when the crust forms. We collected near‐bottom vector magnetic anomaly data during a series of 21 autonomous underwater vehicleSentrydives near 9°50′N on the East Pacific Rise (EPR) covering ∼26 km along‐axis. These data document the 2–3 km wide axial anomaly high that is commonly observed at fast‐spreading ridges but also reveal the presence of a superimposed ∼800 m full wavelength anomaly low. The anomaly low is continuous for ≥13 km along axis and may extend over the entire survey region. A more detailed survey of hydrothermal vents near 9°50.3′N reveals ∼100 m diameter magnetic lows, which are misaligned relative to active vents and therefore cannot explain the continuous axial low. The axial magnetization low persists in magnetic inversions with variable extrusive source thickness, indicating that to the extent to which layer 2A constitutes the sole magnetic source, variations in its thickness alone cannot account for the axial low. Lava accumulation models illustrate that high geomagnetic intensity over the past ∼2.5 kyr, and decreasing intensity over the past ∼900 years, are both consistent with the broad axial anomaly high and the superimposed shorter wavelength low. The continuity of this axial low, and similar features elsewhere on the EPR suggests, that either crustal accretionary processes responsible for this anomaly are common among fast‐spread ridges, or that the observed magnetization low may partially reflect global geomagnetic intensity fluctuations. 

Abstract Seafloor volcanic eruptions are difficult to directly observe due to lengthy eruption cycles and the remote location of mid‐ocean ridges. Volcanic eruptions in 2005–2006 at 9°50′N on the East Pacific Rise have been well documented, but the lava volume and flow extent remain uncertain because of the limited near‐bottom bathymetric data. We present near‐bottom data collected during 19 autonomous underwater vehicle (AUV)Sentrydives at 9°50′N in 2018, 2019, and 2021. The resulting 1 m‐resolution bathymetric grid and 20 cm‐resolution sidescan sonar images cover 115 km², and span the entire area of the 2005–2006 eruptions, including an 8 km²pre‐eruption survey collected with AUVABEin 2001. Pre‐ and post‐eruption surveys, combined with sidescan sonar images and seismo‐acoustic impulsive events recorded during the eruptions, are used to quantify the lava flow extent and to estimate changes in seafloor depth caused by lava emplacement. During the 2005–2006 eruptions, lava flowed up to ∼3 km away from the axial summit trough, covering an area of ∼20.8 km²; ∼50% larger than previously thought. Where pre‐ and post‐eruption surveys overlap, individual flow lobes can be resolved, confirming that lava thickness varies from ∼1 to 10 m, and increases with distance from eruptive fissures. The resulting lava volume estimate indicates that ∼57% of the melt extracted from the axial melt lens probably remained in the subsurface as dikes. These observations provide insights into recharge cycles in the subsurface magma system, and are a baseline for studying future eruptions at the 9°50′N area.