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High-resolution seismic constraints on flow dynamics in the oceanic asthenosphere
Nature
|July 8, 2016
Summary
Seismic wave speed anisotropy reveals that oceanic mantle deformation is dominated by seafloor spreading and asthenospheric flow, not plate motion. This clarifies the processes shaping Earth's interior beneath the ocean floor.
Area of Science:
- Geophysics
- Seismology
- Tectonics
Background:
- Tectonic plate motion and mantle convection deform Earth's interior, creating rock fabrics detectable via seismic wave speed anisotropy.
- Deformation in the oceanic upper mantle is typically inferred near lithospheric boundaries, particularly at seafloor-spreading centers and within the asthenosphere.
- Seismic models present differing views on the relative importance of seafloor spreading versus asthenospheric flow in deforming the oceanic lithosphere.
Purpose of the Study:
- To investigate seismic anisotropy within the oceanic lithosphere-asthenosphere system in the central Pacific using ocean-bottom seismograph data.
- To provide localized constraints on deformation processes in the middle of an oceanic plate.
- To differentiate between seafloor spreading, asthenospheric flow, and plate motion as drivers of mantle deformation.
Main Methods:
- Analysis of Rayleigh waves recorded by the NoMelt Experiment's ocean-bottom seismograph array.
- Mapping seismic anisotropy, specifically azimuthal anisotropy, within the oceanic lithosphere and asthenosphere.
- Comparing the fast seismic wave direction with seafloor spreading direction and plate motion.
Main Results:
- Azimuthal anisotropy is strongest in the high-seismic-velocity lid, aligning with seafloor spreading directions.
- Anisotropy magnitude shows a minimum in the mid-seismic low-velocity zone and increases with depth below the asthenosphere.
- The fast seismic direction does not correlate with plate motion at any depth.
Conclusions:
- The dominant deformation in the shallow oceanic mantle occurs during ridge-axis corner flow and through pressure/buoyancy-driven asthenospheric flow.
- Shear deformation from plate motion over the asthenosphere appears to be a minor process compared to other deformation mechanisms.
- Seismic anisotropy provides crucial insights into the complex deformation processes within the oceanic lithosphere-asthenosphere system.
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