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FRINATEK-Fri prosj.st. mat.,naturv.,tek

ANIsotropic viscosity in MAntle dynamics

Alternative title: Anisotrop viskositet i manteldynamikk

Awarded: NOK 7.5 mill.

In ANIMA we investigate movements within the Earth’s mantle, which extends to 2900 km depth inside the solid Earth. Beneath the rigid lithospheric plates (~100 km thick), the mantle deforms in a fluid-like (viscous) way under the effects of long-lasting tectonic forces. The rate of deformation is typically very slow (few cm/year) and is determined by the viscosity of the mantle. The viscosity of a material depends on many factors, including temperature, composition, and the forces applied to it. Recent laboratory experiments showed that the viscosity of the mantle also depends on the direction of these forces. We call this behavior anisotropic (i.e. direction-dependent) viscosity. Because the viscosity of the mantle affects many dynamic processes, from the movement of tectonic plates mountain building, fault formation, and volcanism, it follows that anisotropic viscosity can greatly impact the dynamics of our planet. To evaluate the role of anisotropic viscosity for geodynamics, we must first understand its origin. Laboratory experiments on mantle minerals (olivine) showed that anisotropy originates from the structure and the alignment of these minerals. Such alignment usually results from deformation in the mantle due to tectonic forces. Hence, we have to study anisotropic viscosity in combination with texture formation (alignment of olivine crystals) in the mantle. In a preliminary study, we found that the two are interlinked. In ANIMA we will develop a numerical modeling tool that allows us to study the formation of olivine texture in combination with its effect of viscous anisotropy in the mantle. We will use this tool to study the impact of viscosity anisotropy on a variety of geodynamic processes. The results will reform our understanding of the stresses, temperatures, and deformation patterns beneath the ocean basins, and the seismic and volcanic hazards associated with them.

Olivine, the main rock-forming mineral of Earth's mantle, responds to tectonic stress by deforming viscously over millions of years. This deformation creates an anisotropic (direction-dependent) texture that typically aligns with the mantle flow direction. We can observe this anisotropic texture by detecting directional differences in seismic wave propagation speed across the textured mantle. According to laboratory experiments on olivine, we expect this texture to also exhibit anisotropic viscosity (AV), with deformation occurring more easily when it is parallel to, rather than across, the texture. Seismic constraints indicate strong olivine alignment under oceanic plates, in a direction parallel to their movement. AV caused by this alignment may double tectonic plate speeds, but it may also impede changes in plate motion direction. In a subduction zone, where an oceanic plate sinks into the mantle and induces a mantle flux around the sinking slab, olivine alignment varies significantly, complicating the interpretation of seismic anisotropy. The associated AV may result in complex viscosity variations around the slab, which should affect overall mantle flow patterns, and consequently impact the stresses, thermal structure, and volcanism near subduction zones. Although AV fundamentally affects these mantle systems, it has never been considered in its full 3D form within geodynamic models. Recent laboratory measurements on deforming olivine provide us a new deformational framework that allows relating AV to the texture of the olivine crystals in the rock. In ANIMA we will utilize this new framework to create a numerical tool that incorporates coupled olivine texture development and anisotropic viscosity into 3D numerical models of mantle deformation. We will use this tool to constrain the impact of AV on mantle convection, tectonic plate motions, and mantle flow around subduction zones, potentially revolutionizing our understanding of these systems.

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FRINATEK-Fri prosj.st. mat.,naturv.,tek