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Understanding accretion and deformation processes at mid-ocean ridges is crucial as they control the resulting oceanic crustal structure, which covers two-thirds of Earth’s surface. The most common tool for observing such dynamic processes within the Earth is seismic anisotropy. Iceland, which is uplifted by a convective mantle plume and has an active spreading ridge system exposed above sea level, offers a unique opportunity for studying this phenomenon. Here we use a high-resolution dataset of Love and Rayleigh wave speeds to constrain the seismic anisotropy in the Icelandic crust. We show that seismic anisotropy in the lower crust is controlled by crystal preferred orientation, providing a direct observation of lower crustal flow. Furthermore, since shear is needed to align the crystals, our results reveal that crustal flow cannot be a simple translation of mass and requires internal deformation. This finding suggests that crustal flow plays an important role in oceanic crustal accretion and deformation where thick, hot oceanic crust is formed, such as at volcanic rifted margins and where there are mantle plume–ridge interactions.
