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Abstract We invertPg,PmP, andPntraveltimes from an active‐source, multiscale tomography experiment to constrain the three‐dimensional isotropic and anisotropicPwave 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.
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Near-axis crustal structure and thickness of the Endeavour Segment, Juan de Fuca Ridge: Crustal Structure at Endeavour Segment
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Analysis of the time-dependent behavior of the buoyant plume rising above Grotto Vent (Main Endeavour Field, Juan de Fuca Ridge) as imaged by the Cabled Observatory Vent Imaging Sonar (COVIS) between September 2010 and October of 2015 captures long term time-dependent changes in the direction of background bottom currents independent of broader oceanographic processes, indicating a systematic evolution in vent output along the Endeavour Segment of the Juan de Fuca Ridge. The behavior of buoyant plumes can be quantified by describing the volume, velocity, and orientation of the effluent relative to the seafloor, which are a convolved expression of hydrothermal flux from the seafloor and ocean bottom currents in the vicinity of the hydrothermal vent. We looked at the azimuth and inclination of the Grotto plume, which was captured in three-dimensional acoustic images by the COVIS system, at 3-h intervals during October 2010 and between October 2011 and December 2014. The distribution of plume azimuths shifts from bimodal NW and SW to SE in 2010, 2011, and 2012 to single mode NW in 2013 and 2014. Modeling of the distribution of azimuths for each year with a bimodal Gaussian indicates that the prominence of southward bottom currents decreased systematically between 2010 and 2014. Spectral analysis of the azimuthal data showed a strong semi-diurnal peak, a weak or missing diurnal peak, and some energy in the sub-inertial and weather bands. This suggests the dominant current generating processes are either not periodic (such as the entrainment fields generated by the hydrothermal plumes themselves) or are related to tidal processes. This prompted an investigation into the broader oceanographic current patterns. The surface wind patterns in buoy data at two sites in the Northeast Pacific and the incidence of sea-surface height changes related to mesoscale eddies show little systematic change over this time-period. The limited bottom current data for the Main Endeavour Field and other parts of the Endeavour Segment neither confirm nor refute our observation of a change in the bottom currents. We hypothesize that changes in venting either within the Main Endeavour Field or along the Endeavour Segment have resulted in the changes in background currents. Previous numerical simulations (Thomson et al., J. Geophys. Res., 2009, 114 (C9), C09020) showed that background bottom currents were more likely to be controlled by the local (segment-scale) venting than by outside ocean circulation or atmospheric patterns.more » « less
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Abstract 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 invertPgandSgtraveltimes from a seismic refraction experiment conducted at the Endeavour segment usingP‐to‐Scoupling 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/Vshas 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/Vsincreases 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.more » « less
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A multiscale numerical framework has been developed to investigate the dispersion of deep-sea hydrothermal plumes that originate from the Endeavour Segment of the Juan de Fuca Ridge located in the Northeast Pacific. The analysis of simulation outputs presented in this study provides insights into the influences of tidal forcing and the buoyancy flux associated with hydrothermal venting on ocean circulation and plume dispersion in the presence of pronounced seafloor topography. The results indicate that tidal forcing drives anti-cyclonic circulation near the ridge-axis, while hydrothermal venting induces cyclonic circulation around vent fields within the axial rift valley. Tidal forcing has a notable impact on plume dispersion, particularly near the large topographic features to the north of the Endeavour Segment. Furthermore, plume dispersion exhibits notable inter-annual variability, with a northbound trajectory in 2016 and a southbound trajectory in 2021. The study also reveals that both buoyancy fluxes and tidal forcing enhance the mixing of hydrothermal plumes with ambient seawater.more » « less
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1002/2016GL068182
