Rare earth element and related chemistry of some drilled southern Indian Ocean basalts and volcanogenic sediments
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Rare-earth element
Incompatible element
Island arc
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We report significant progress in mid-ocean ridge research in China since China joined the InterRidge, an interdisciplinary international science program, as an associate member following the 1st InterRidge workshop in China in October 2003. Two multi-leg expeditions were conducted on the Chinese research vessel “Dayang Yihao” in 2005 and 2007, respectively. Each expedition consists multiple 30-40 day long interdisciplinary legs to investigate ocean ridge hydrothermal vents. The 2005 expedition, which was the first around the world deep sea exploration by China, has surveyed the East Pacific Rise, Mid-Atlantic Ridge, Southwest Indian Ridge, and the Central Indian Ridge. On the other hand, the 2007 expedition has investigated the Southeast Indian Ridge, Southwest Indian Ridge, Central Indian Ridge, and the Lau Basin back-arc spreading center.
Seafloor Spreading
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Linear analytical solutions for bottom-trapped subinertial oscillatory flow over simple ridge topographies in a stratified (two-layer) rotating fluid are presented. Results are compared to moored current meter observations of bottom-intensified motions over the Endeavour Segment of Juan de Fuca Ridge in the northeast Pacific. The solutions reproduce many of the observed features including preferential amplification of the clockwise rotary component of velocity over the ridge and increased velocity amplification with proximity to the ridge crest. For a given internal deformation radius, the degree of current amplification increases with increased bottom slope, ridge height, and oscillation frequency. Amplification decreases with increased width of the ridge relative to the deformation radius.
Crest
Clockwise
Ridge push
Oscillation (cell signaling)
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Mid-Atlantic Ridge
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Global observations of mid‐ocean ridge (MOR) bathymetry demonstrate an asymmetry in axial depth across ridge offsets that is correlated with the direction of ridge migration. Motivated by these observations, we have developed two‐dimensional numerical models of asthenospheric flow and melting beneath a migrating MOR. The modification of the flow pattern produced by ridge migration leads to an asymmetry in melt production rates on either side of the ridge. By coupling a simple parametric model of three dimensional melt focusing to our simulations, we generate predictions of axial depth differences across offsets in the MOR. These predictions are quantitatively consistent with the observed asymmetry.
Ridge push
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Abstract Lithospheric plates diverge at mid-ocean ridges and asthenospheric mantle material rises in response. The rising material decompresses, which can result in partial melting, potentially impacting the driving forces of the system. Yet the geometry and spatial distribution of the melt as it migrates to the ridge axis are debated. Organized melt fabrics can cause strong seismic anisotropy, which can be diagnostic of melt, although this is typically not found at ridges. We present anisotropic constraints from an array of 39 ocean-bottom seismometers deployed on 0–80 Ma lithosphere from March 2016 to March 2017 near the equatorial Mid-Atlantic Ridge (MAR). Local and SKS measurements show anisotropic fast directions away from the ridge axis, which are consistent with strain and associated fabric caused by plate motions with short delay times, δt (<1.1 s). Near the ridge axis, we find several ridge-parallel fast splitting directions, φ, with SKS δt that are much longer (1.7–3.8 s). This is best explained by ridge-parallel sub-vertical orientations of sheet-like melt pockets. This observation is much different than anisotropic patterns observed at other ridges, which typically reflect fabric related to plate motions. One possibility is that thicker sub-ridge lithosphere with steep sub-ridge topography beneath slower spreading centers focuses melt into vertical, ridge-parallel melt bands, which effectively weakens the plate. Associated buoyancy forces elevate the sub-ridge plate, providing greater potential energy and enhancing the driving forces of the plates.
Seafloor Spreading
Ridge push
Mid-Atlantic Ridge
Neutral buoyancy
Seismometer
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Seabed
Heat flow
Anomaly (physics)
Oceanic basin
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The existence of a continuous, rifted, mid-oceanic ridge in the southwestern Indian Ocean, previously predicted by us, has been confirmed by soundings taken by the research vessel Vema during the expedition now in progress.
Rift valley
Rift zone
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[1] Global observations of mid-ocean ridge (MOR) bathymetry demonstrate an asymmetry in axial depth across ridge offsets that is correlated with the direction of ridge migration. Motivated by these observations, we have developed two-dimensional numerical models of asthenospheric flow and melting beneath a migrating MOR. The modification of the flow pattern produced by ridge migration leads to an asymmetry in melt production rates on either side of the ridge. By coupling a simple parametric model of three dimensional melt focusing to our simulations, we generate predictions of axial depth differences across offsets in the MOR. These predictions are quantitatively consistent with the observed asymmetry.
Ridge push
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