The Endeavour segment of the Juan de Fuca Ridge is one of the most active and long‐lived hydrothermal areas of the mid‐ocean ridge system. However, the permeability structure that gives rise to long‐term venting at well‐established fields, such as the High Rise, Main Endeavour, and Mothra fields, is not fully understood. Here we jointly invert
- Award ID(s):
- 1736702
- NSF-PAR ID:
- 10430459
- Date Published:
- Journal Name:
- Frontiers in Earth Science
- Volume:
- 10
- ISSN:
- 2296-6463
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract Pg andSg traveltimes from a seismic refraction experiment conducted at the Endeavour segment usingP ‐to‐S coupling constraints. We then calculate porosity and crack density as a function of crack aspect ratio by applying the differential effective medium theory to the seismic velocities. At 1.4‐km depth, averageVp ~5 km off axis increases by ~0.4 km/s compared to the ridge axis. The averageVp /Vs has a minimum of ~1.75 on the ridge axis and increases to a maximum of ~1.84 off axis. The inferred porosity and crack density distributions show that the proportion of thick versus thin cracks decreases from the ridge axis to the flanks, since theoretical models indicate thatVp /Vs increases going from thick to thin cracks (aspect ratio decreasing from 0.1 to 0.01). The dominant presence of thick cracks on the axis may provide long‐term conduits for upflow in high‐temperature hydrothermal circulation potentially forming the vent fields. The increased proportion of thin cracks on the flanks, coupled with the increased seismic velocity, indicates a decrease in permeability caused by progressive clogging of thick cracks due to mineral precipitation likely in the downflow zone of hydrothermal circulation. -
Abstract Mid‐ocean ridge eruptions, initiating or revitalizing hydrothermal discharge and disrupting seafloor ecosystems, occur regularly as a consequence of plate spreading. Evaluating their impact on long‐term hydrothermal discharge requires information on the scale and duration of any posteruption enhancement. Here we describe a unique hydrothermal plume time series of annual (or more frequent) observations at Axial Seamount vent fields from 1985 through 2017, missing only 7 years. Axial, a hot spot volcano astride the Juan de Fuca Ridge, experienced eruptions in 1998, 2011, and 2015. In 1998 and 2011 lava flooded the SE caldera and south rift zone, but in 2015 most lava was extruded in a series of flows extending ~20 km down the north rift zone. Response cruises occurred within 18 days (1998) to about 4 months, followed by regular posteruption observations. All 30 cruises measured plume rise height (a proxy for heat flux) and turbidity (indicative of chemical changes in vent discharge) at several vent sites, yielding an integrated view of vent field activity. Venting in the SE caldera area persisted throughout the time series, consistent with the imaged location of the shallowest portion of the melt‐rich magma reservoir. Eruptions produced substantial and diagnostic increases in plume rise and turbidity, and posteruption enhancements lasted 2–5 years, totaling ~10 years over the course of the time series. Estimates of the relative heat flux indicate a sixfold increase during eruption‐enhanced periods, implying that generalizations about mid‐ocean ridge hydrothermal fluxes may be underestimates if based on non–eruption‐enhanced hydrothermal activity alone.
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Abstract We use ocean bottom seismometer data from the Endeavour segment of the Juan de Fuca ridge to construct a long‐term earthquake catalog for an intermediate spreading rate mid‐ocean ridge. We present >50,000 new earthquake locations for 2016–2021 from the Ocean Networks Canada NEPTUNE cabled observatory and relocate earthquakes from two autonomous networks in 1995 and 2003–2006. The catalog comprises >85,000 earthquakes located using three‐dimensional segment‐scale
P andS wave velocity models from a prior tomography experiment. Despite the small footprints of networks near the segment center, locations show good agreement with geologic features at segment ends. The improved locations show that the northern Endeavour segment ruptured southwards from 48.3°N to 48.05°N during two diking events in early 2005, possibly accompanied by diking on the West Valley (WV) propagator. Persistent off‐axis seismicity near the segment center appears to be related to the WV and Cobb propagating rifts which we infer extend ∼10 km closer to the Endeavour segment center than is apparent in bathymetry. We suggest that the proximity of the propagators to the Endeavour vent fields (VFs) contributes to the localization, vigor, and longevity of the fields by focusing permeability through ongoing fracturing and by limiting extrusive magmatism through degassing of the axial magma lens. Increasing rates of seismicity beneath the VFs beginning in late 2018 and a deepening of earthquakes in 2020 indicate that the central portion of the segment may be entering the later stages of the eruptive cycle. -
Abstract We invert
Pg ,PmP , andPn traveltimes from an active‐source, multiscale tomography experiment to constrain the three‐dimensional isotropic and anisotropicP wave velocity structure of the topmost oceanic mantle and crust and crustal thickness variations beneath the entire Endeavour segment of the Juan de Fuca Ridge. The isotropic velocity structure is characterized by a semicontinuous, narrow (5‐km‐wide) crustal low‐velocity volume that tracks the sinuous ridge axis. Across the Moho, the low‐velocity volume abruptly broadens to approximately 20 km in width and displays a north‐south linear trend that connects the two overlapping spreading centers bounding the segment. From the seismic results, we estimate the thermal structure and melt distribution beneath the Endeavour segment. The thermal structure indicates that the observed skew, or lateral offset, between the crustal and mantle magmatic systems is a consequence of differences in mechanisms of heat transfer at crustal and mantle depths, with the crust and mantle dominated by advection and conduction, respectively. Melt volume estimates exhibit significant along‐axis variations that coincide with the observed skew between the mantle and crustal magmatic systems, with sites of enhanced crustal melt volumes and vigorous hydrothermal activity corresponding to regions where the mantle and crustal magmatic systems are vertically aligned. These results contradict models of ridge segmentation that predict enhanced and reduced melt supply beneath the segment center and ends, respectively. Our results instead support a model in which segment‐scale skew between the crustal and mantle magmatic systems governs magmatic and hydrothermal processes at mid‐ocean ridges. -
Abstract The Cabled Observatory Vent Imaging Sonar (COVIS) was installed on the Ocean Observatories Initiative's Regional Cabled Array observatory at ASHES hydrothermal vent field on Axial Seamount in July 2018. The acoustic backscatter data recorded by COVIS in August–September 2018, in conjunction with in situ temperature measurements, are used to showcase and verify the use of COVIS for long‐term, quantitative monitoring of hydrothermal discharge. Specifically, sonar data processing generates three‐dimensional backscatter images of the buoyant plumes above major sulfide structures and two‐dimensional maps of diffuse flows within COVIS's field‐of‐view. The backscatter images show substantial changes of plume appearance and orientation that mostly reflect plume bending in the presence of ambient currents and potentially the variations of outflow fluxes. The intensity of acoustic backscatter decreases significantly for highly bent plumes as compared to nearly vertical plumes, reflecting enhanced mixing of plume fluids with seawater driven by ambient currents. A forward model of acoustic backscatter from a buoyancy‐driven plume developed in this study yields a reasonable match with the observation, which paves the way for inversely estimating the source heat flux of a hydrothermal plume from acoustic backscatter measurements. The acoustic observations of diffuse flows show large temporal variations on time scales of hours to days, especially at tidal frequencies, but no apparent long‐term trend. These findings demonstrate COVIS's ability to quantitatively monitor hydrothermal discharge from both focused and diffuse sources to provide the research community with key observational data for studying the linkage of hydrothermal activity with oceanic and geological processes.