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Abstract Seismic azimuthal anisotropy beneath Australia is investigated using splitting of the teleseismic PKS, SKKS, and SKS phases to delineate asthenospheric flow and lithospheric deformation beneath one of the oldest and fast‐moving continents on Earth. In total 511 pairs of high‐quality splitting parameters were observed at 116 seismic stations. Unlike other stable continental areas in Africa, East Asia, and North America, where spatially consistent splitting parameters dominate, the fast orientations and splitting times observed in Australia show a complex pattern, with a slightly smaller than normal average splitting time of 0.85 ± 0.33 s. On the North Australian Craton, the fast orientations are mostly N‐S, which is parallel to the absolute plate motion (APM) direction in the hotspot frame. Those observed in the South Australian Craton are mostly NE‐SW and E‐W, which are perpendicular to the maximum lithospheric horizontal shortening direction. In east Australia, the observed azimuthal anisotropy can be attributed to either APM induced simple shear or lithospheric fabric parallel to the strike of the orogenic belts. The observed spatial variations of the seismic azimuthal anisotropy, when combined with results from depth estimation utilizing the spatial coherency of the splitting parameters and seismic tomography studies, suggest that the azimuthal anisotropy in Australia can mostly be related to simple shear in the rheologically transition layer between the lithosphere and asthenosphere. Non‐APM parallel anisotropy is attributable to modulations of the mantle flow system by undulations of the bottom of the lithosphere, with a spatially variable degree of contribution from lithospheric fabric. 

As a consequence of the growth of accretionary orogenic belts in central Asia, the high elevation of the East Sayan Mountains concomitant with the plutonic activities in Jom-Bolok and Azas Plateau volcanic fields provide a rare opportunity to unravel lithospheric deformation induced by large-scale tectonic processes such as the passage of thermal plumes. Here we use receiver functions (RFs) to obtain high-resolution images of the 410 km (d410) and 660 km (d660) discontinuities and to measure mantle transition zone (MTZ) thickness. The average apparent depression of the d410 and d660 for a circular area under northern Mongolia and East Sayan are 14 km and 51 km respectively, leading to a significant thicker-than-normal MTZ with a mean value of 37 km. Our results, when incorporated with previous geochemical characteristics, suggest heterogeneous deep mantle materials highlighted by the great depression of the d660, revealing that possible foundered lithospheric remnants have dripped into the MTZ beneath the East Sayan Mountains. Negative thermal anomalies generated by the recycled lithosphere in the MTZ elucidate the prominent lateral undulation of the MTZ discontinuities, and a MTZ thinning beneath the southwest part of the study area is ascribed to the upward small-scale mantle convection initiated by the foundered lithospheric materials. We suggest that the descending lithosphere is due to the hot mantle plumes interacting with base of the mantle lithosphere which provided a viable perspective for the origin of the widespread magmatisms with distinct geochemical signatures in the region. 

Approximately two-thirds of Earth’s outermost shell is composed of oceanic plates that form at spreading ridges and recycle back to Earth’s interior in subduction zones. A series of physical and chemical changes occur in the subducting lithospheric slab as the temperature and pressure increase with depth. In particular, olivine, the most abundant mineral in the upper mantle, progressively transforms to its high-pressure polymorphs near the mantle transition zone, which is bounded by the 410 km and 660 km discontinuities. However, whether olivine still exists in the core of slabs once they penetrate the 660 km discontinuity remains debated. Based on SKS and SKKS shear-wave differential splitting times, we report new evidence that reveals the presence of metastable olivine in the uppermost lower mantle within the ancient Farallon plate beneath the eastern United States. We estimate that the low-density olivine layer in the subducted Farallon slab may compensate the high density of the rest of the slab associated with the low temperature, leading to neutral buoyancy and preventing further sinking of the slab into the deeper part of the lower mantle.