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Causes and Consequences of Upper-mantle Seismic Anisotropy: The Link Between Olivine Microstructure and the Nature of Plate Tectonics

The link between plate tectonics and the dynamics of Earth's upper mantle is heavily dependent on the material properties of mantle rocks. Seismic anisotropy (the dependence of seismic wave speed on propagation direction) has been a valuable tool in determining the behavior of tectonic plates through our understanding of the microphysics of olivine deformation. However, major questions remain. For instance, can we precisely map seismic anisotropy to flow and kinematics? What is the best interpretation of the seismic structures that are used to define the lithosphere? Can our current understanding of upper-mantle mechanics explain the major features of plate tectonics?

Here I present the results of a campaign of laboratory experiments on olivine aggregates that yield new insight into the mechanisms of upper-mantle deformation. This set of experiments provides a link between seismic properties, deformation mechanism, rheological behavior, and composition. Four primary results are presented. First, olivine crystallographic textures follow a protracted evolution that includes several distinct changes in the symmetry of seismic anisotropy. Second, samples that develop strong textures also exhibit viscosities that are both grain-size sensitive and non-Newtonian. Third, changing the deformation geometry results in different measured viscosities, implying that the viscosity (in addition to the elasticity) is anisotropic. Fourth, the addition of a melt phase measurably changes the evolution of olivine textures.

These results have three important implications. First, we now have a constraint on how fast seismic anisotropy responds to changing directions of flow, potentially allowing us to map the magnitude and time scale of upper-mantle deformation using seismic tomography. Second, the regions of the upper mantle that present strong seismic anisotropy have mechanical properties that are likely anisotropic and depend on the grain size, characteristics that may be essential in the formation of new tectonic plate boundaries. Third, the nature of the coupling between the plates and convecting mantle can be related to olivine deformation through the spatial distribution of seismic anisotropy. The importance of each of these implications will be explored through comparison with measured geophysical signals with special emphasis on the nature of the lithosphere and its boundaries.

By: Lars Hansen, Associate Professor, University of Oxford

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Host: Thorsten Becker, UTIG

When: Fri Apr 7, 2017 11:30am - 12:30pm Central Time