Strain localization in plate tectonics: the role of olivine crystal preferred orientations in the upper mantle

L. MAMERI; L. HANSEN; A. TOMMASI; J.W. SIGNORELLI

PAris

Congreso; Goldschmidt 2017; 2018

Geochemical society

Textured olivine aggregates are mechanically anisotropic.Laboratory data [1,2] show that aggregates first deformed intorsion and then in extension have tensional strengths up to1.9 times higher than the torsional ones. The mechanicalanisotropy increases with texture (crystal preferredorientation or CPO) intensity. Yet these experiments onlytested the contrast in mechanical behavior between simpleshear and axial tension; this is not sufficient to fullycharacterize the mechanical anisotropy of olivinepolycrystals. Numerical models allow the prediction of theviscoplastic behaviour of a polycrystal as a function of thefull range of orientation relations between the mechanicalsolicitation and the texture. Viscoplastic self-consistent(VPSC) models predict mechanical anisotropy similar to thatobserved in experiments: polycrystals in which the CPO isoriented oblique to extensional stresses deform faster thanthose where the CPO is parallel or normal to the imposedextension [3]. Low strengths also characterize polycrystalssheared parallel or normal to the main orientation of thedominant olivine glide plane (010). Here we present a directcomparison between the mechanical predictions of VPSCmodels and the experimental data [1,2], which shows that themodels predict higher anisotropy magnitudes than observedin the laboratory. VPSC models reproduce the strengthsmeasured in torsion. However, for tension the stressespredicted by the models are on average 2.5 times higher thanthe experimental ones, leading to tensional strengths up to 4.5times higher than in torsional ones. The difference in modelpredictions and experimental results is consistent withactivation, in the fine-grained experimental polycrystals, ofother deformation mechanisms in addition to dislocationcreep. VPSC models including additional isotropicdeformation processes do reproduce the experimental data.[1] Hansen et al. (2012) Nature, 492, 415-418. [2] Hansen et al.(2016) EPSL 445, 92-103. [3] Knoll et al. (2009), G-Cubed, 10:Q08009. This project has received funding from the EuropeanUnion?s Horizon 2020 research and innovation program under theMarie Sklodowska-Curie grant agreement No 642029 - ITN CREEP