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Upper Crustal Vp/Vs Ratios at the Endeavour Segment, Juan de Fuca Ridge, From Joint Inversion of P and S Traveltimes: Implications for Hydrothermal Circulation

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...

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Published in:Geochemistry, geophysics, geosystems : G3 geophysics, geosystems : G3, 2019-01, Vol.20 (1), p.208-229
Main Authors: Kim, Eunyoung, Toomey, Douglas R., Hooft, Emilie E. E., Wilcock, William S. D., Weekly, Robert T., Lee, Sang‐Mook, Kim, Young Hee
Format: Article
Language:English
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Summary: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 Pg and Sg traveltimes from a seismic refraction experiment conducted at the Endeavour segment using P‐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, average Vp ~5 km off axis increases by ~0.4 km/s compared to the ridge axis. The average Vp/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 that Vp/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. Key Points A 3‐D tomographic model of Vp, Vs, and Vp/Vs informs crack aspect ratios in the upper crust at the Endeavour segment, Juan de Fuca Ridge Seafloor spreading produces thick cracks with low Vp/Vs, which fill within a few kilometers of the ridge axis by hydrothermal circulation Thick cracks on the axis may provide long‐lived conduits for hydrothermal upflow beneath the high‐temperature vent fields
ISSN:1525-2027
1525-2027
DOI:10.1029/2018GC007921