Causes and consequences of upper-mantle seismic anisotropy: The link between olivine microstructure and the nature of plate tectonics
Prof. Lars Hanen (Associate Professor of Mineralogy and Petrology, University of Oxford)
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 partial melt 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 spatial distribution of
seismic anisotropy may be linked to the evolution of olivine textures
and, therefore, used to place constraints on the effect of partial melts
on lithospheric structure. 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.