The Impact of the Deformation History on the Microstructural Evolution
of Polycrystalline Olivine and How It Affects the Interpretation of
Mantle Seismic Anisotropy
Abstract
Relating seismic anisotropy with mantle flow requires a good
understanding of the rock’s microstructural evolution and the
development of crystal preferred orientations (CPO), because plastic
deformation of olivine is interpreted as the main cause for mantle
seismic anisotropy. In this contribution, the influence of deformation
history in the microstructure evolution and resulting seismic anisotropy
is investigated by means of full-field numerical simulations at the
microscale. We explicitly simulate the microstructural evolution of
olivine polycrystalline aggregates during dynamic recrystallization up
to high strain using the code VPFFT/ELLE (e.g. Griera et al., 2011;
2013; Llorens et al., 2016). Modeling results indicate that the
evolution of a CPO is highly sensitive to the initial olivine fabric.
When the initial fabric is formed by a random distribution of
crystallographic orientations, there is a rapid alignment of the a-axes
(or [100]) with the stretching direction of flow. However, when
there is an initial CPO inherited from previous deformation, larger
strains are required for the a-axes to become re-aligned with the
stretching direction. Our numerical results agree with field and
experimental observations (e.g. Boneh and Skemer, 2014; Skemer et al.,
2012). The analysis of the seismic properties reveals that an increase
of the strength of the initial inherited CPO produces a reduction of the
azimuthal seismic anisotropy, compared to the case with an initial
random fabric. It is concluded that the deformation history
significantly influences the development of fabrics. Accordingly,
seismic anisotropy interpretations must be carried out with caution in
regions with complex deformation histories. References Boneh Y., Skemer
P. 2014. EPSL, 406. Griera A, et al. 2011. Geology 39, 275-278. Griera
A, et al. 2013. Tectonophysics 587, 4-29. Llorens, M.-G. et al. 2016
EPSL, 450, 233-242. Skemer P. et al. 2012. Geochemistry Geophysics
Geosystems, 13:3.