Seismic Anisotropy in the Lower Mantle Transition Zone Induced by Lattice Preferred Orientation of Akimotoite

Guan, Longli; Tsujino, Noriyoshi; Tange, Yoshinori; Higo, Yuji

doi:10.1029/2022gl098549

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Geophysical Research Letters

2022

DOI: 10.1029/2022gl098549

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Seismic Anisotropy in the Lower Mantle Transition Zone Induced by Lattice Preferred Orientation of Akimotoite

Longli Guan

Noriyoshi Tsujino

Yoshinori Tange

et al.

Abstract: Seismic anisotropy has been widely observed near the subducting slabs in the lower mantle transition zone (MTZ) and is often interpreted by the lattice preferred orientation (LPO) of constituent minerals. Akimotoite is one of the dominant minerals near the cold subducting slabs. Therefore, we conducted the well‐controlled uniaxial and shear deformation experiments on the MgSiO3 akimotoite aggregates at 21–23 GPa and 900–1300°C by using the D111‐type Kawai‐type multianvil apparatus. We observed strong LPOs and … Show more

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2024

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Cited by 3 publications

References 39 publications

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Constraining seismic anisotropy on the mantle transition zone boundaries beneath the subducting Nazca slab

Zhang,

Yu,

Huang

2024

Physics of the Earth and Planetary Interiors

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Constraining seismic anisotropy on the mantle transition zone boundaries beneath the subducting Nazca slab

Zhang,

Yu,

Huang

2024

Physics of the Earth and Planetary Interiors

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Pressure-dependent large-scale seismic anisotropy induced by non-Newtonian mantle flow

Magali,

Ledoux,

Thomas

et al. 2024

Geophysical Journal International

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Summary Observations of large-scale seismic anisotropy can be used as a marker for past and current deformation in the Earth’s mantle. Nonetheless, global features such as the decrease of the strength of anisotropy between ∼150–410 km in the upper mantle and weaker anisotropy observations in the transition zone remain ill-understood. Here, we report a proof of concept method that can help understand anisotropy observations by integrating pressure-dependent microscopic flow properties in mantle minerals particularly olivine and wadsleyite into geodynamic simulations. The model is built against a plate-driven semi-analytical corner flow solution underneath the oceanic plate in a subduction setting spanning down to 660 km depth with a non-Newtonian n = 3 rheology. We then compute the crystallographic preferred orientation (CPO) of olivine aggregates in the upper mantle (UM), and wadsleyite aggregates in the upper transition zone (UTZ) using a viscoplastic self-consistent (VPSC) method, with the lower transition zone (LTZ, below 520 km) assumed isotropic. Finally, we apply a tomographic filter that accounts for finite-frequency seismic data using a fast-Fourier homogenization algorithm, with the aim of providing mantle models comparable with seismic tomography observations. Our results show that anisotropy observations in the UM can be well understood by introducing gradual shifts in strain accommodation mechanism with increasing depths induced by a pressure-dependent plasticity model in olivine, in contrast with simple A-type olivine fabric that fails to reproduce the decrease in anisotropy strength observed in the UM. Across the UTZ, recent mineral physics studies highlight the strong effect of water content on both wadsleyite plastic and elastic properties. Both dry and hydrous wadsleyite models predict reasonably low anisotropy in the UTZ, in agreement with observations, with a slightly better match for the dry wadsleyite models. Our calculations show that, despite the relatively primitive geodynamic setup, models of plate-driven corner flows can be sufficient in explaining first-order observations of mantle seismic anisotropy. This requires, however, incorporating the effect of pressure on mineralogy and mineral plasticity models.

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Toward Tectonic-Type and Global 1D Seismic Models of the Upper Mantle Constrained by Broadband Surface Waves

Civiero,

Lebedev,

et al. 2024

Bulletin of the Seismological Society of America

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The unprecedentedly dense current sampling of the upper mantle with seismic data offers an opportunity for determining representative seismic velocity models for the Earth’s main tectonic environments. Here, we use over 1.17 million Rayleigh- and 300,000 Love-wave, fundamental-mode, phase-velocity curves measured with multimode waveform inversion of data available since the 1990s, and compute phase-velocity maps in a 17–310 s period range. We then compute phase-velocity curves averaged over the globe and eight tectonic environments, and invert them for 1D seismic velocity profiles of the upper mantle. The averaged curves are smooth and fit by VS models with very small misfits, under 0.1%, at most periods. For phase-velocity curves extending up to 310 s, Rayleigh waves resolve VSV structure down to the shallow lower mantle. Love-wave sampling is shallower, and VSH and, thus, radial anisotropy profiles are resolved down to 375–400 km depth. The uncertainty of the VS models is dominated by the trade-offs of VS at neighboring depths. Using the model-space-projection approach, we quantify the uncertainty of VS in layers of different thickness and at different depths, and show how it decreases with the increasing thickness of the layers. Example 1D VS models that fit the data display the expected increase of the lithospheric seismic velocity with the age of the oceanic lithosphere and with the average age of the continental tectonic type. Radial anisotropy in the global and most tectonic-type models show a flip of the sign from positive (VSH>VSV) to negative at 200–300 km depth. Negative anisotropy is also observed in the shallow mantle lithosphere beneath oceans down to 45–55 km depth. We also compute a global model with the minimal structural complexity, which fits the data worse than the best-fitting one but does not include a sublithospheric low-velocity zone, providing a simple reference for seismic studies.

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Seismic Anisotropy in the Lower Mantle Transition Zone Induced by Lattice Preferred Orientation of Akimotoite

Cited by 3 publications

References 39 publications

Constraining seismic anisotropy on the mantle transition zone boundaries beneath the subducting Nazca slab

Constraining seismic anisotropy on the mantle transition zone boundaries beneath the subducting Nazca slab

Pressure-dependent large-scale seismic anisotropy induced by non-Newtonian mantle flow

Toward Tectonic-Type and Global 1D Seismic Models of the Upper Mantle Constrained by Broadband Surface Waves

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