Progressive crystallisation of Earth's inner core over geological times drives convection in the outer core and the generation of the Earth's magnetic field. Resolving the rate and pattern of inner core growth is thus crucial to understanding the evolution of the geodynamo. The growth history of Earth's inner core is likely recorded in the distribution and strength of seismic anisotropy arising from 19 deformation texturing constrained by boundary conditions at the inner-core solid-20 fluid boundary. Travel times of seismic body waves indicate that seismic anisotropy 21 increases with depth. Here we find that the strongest anisotropy is offset from Earth's rotation axis. Using geodynamic growth models and mineral physics calculations, we simulate the development of inner core anisotropy in a self-consistent manner. We show for the first time that an inner core model composed of hexagonally closepacked iron-nickel alloy, deformed by a combination of preferential equatorial growth and slow translation can match the seismic observations without requiring the introduction of hemispheres with sharp boundaries. We find a model of the inner core 28 growth history compatible with external constraints from outer core dynamics, supporting arguments for a relatively young inner core (~0.5-1.5 Ga) and a viscosity >10 18 Pa-s.The presence of seismic anisotropy -the dependence of seismic wavespeed on direction of propagation -in the inner core (IC) was proposed over 30 years ago to explain the early arrival times of IC sensitive seismic body waves (PKPdf) travelling on paths parallel to the Earth's rotation axis 1,2 and anomalous splitting of core-sensitive free oscillations 3 . This anisotropy is thought to result from alignment of iron crystals caused by deformation in a flow field induced by the evolution of the core, i.e. deformation texturing. In previous work, different geodynamic 4 and plastic deformation mechanisms 5 were explored to explain the variation of PKPdf travel times with angle of the ray path with respect to the rotation axis.Here, for the first time, we combine geodynamic modelling of the evolution of flow in the IC, allowing for slow lateral translation, with presently available knowledge on the mineralogy and deformation mechanisms proposed for the IC to explain spatially varying patterns of observed seismic travel times in an updated global dataset.

SUMMARY If a crystal lattice is subjected to a stress, it becomes distorted and no longer represents the ideal crystal symmetry, and if the stress introduces defects such as dislocations, some of this distortion is preserved after the applied stress is removed. In this study, we investigate lattice distortion in quartz at the micron scale with synchrotron X-ray Laue diffraction. From Laue images the local deviatoric strain tensor is derived and corresponding stresses are calculated based on elastic properties. The method is applied to metasedimentary quartzites from the Bergell Alps that were deformed at conditions of greenschist facies metamorphism. The residual palaeostrain is represented in maps of the deviatoric strain tensor components and with deviatoric strain axis pole figures. Data suggest overall shortening perpendicular to the schistosity plane but with considerable asymmetry relative to foliation and lineation, probably attributed to simple shear. Crystallographic pole figures from Laue diffraction agree with neutron diffraction and EBSD measurements and display quartz c-axes girdle distributions with maxima also perpendicular to schistosity. The method shows promise to be used as a palaeo-piezometer to unravel the stress field during tectonic deformation.

Summary The presence of seismic anisotropy at the base of the Earth's mantle is well established, but there is no consensus on the deformation mechanisms in lower mantle minerals that could explain it. Strong anisotropy in magnesium post-perovskite (pPv) has been invoked, but different studies disagree on the dominant slip systems at play. Here, we aim to further constrain this by implementing the most recent results from atomistic models and high-pressure deformation experiments, coupled with a realistic composition and a 3-dimensional geodynamic model, to compare the resulting deformation-induced anisotropy with seismic observations of the lowermost mantle. We account for forward and reverse phase transitions from bridgmanite (Pv) to pPv. We find that pPv with either dominant (001) or (010) slip can both explain the seismically observed anisotropy in colder regions where downwellings turn to horizontal flow, but only a model with dominant (001) slip matches seismic observations at the root of hotter large-scale upwellings. Allowing for partial melt does not change these conclusions, while it significantly increases the strength of anisotropy and reduces shear and compressional velocities at the base of upwellings.