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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. 

Summary To provide constraints on a number of significant controversial issues related to the structure and dynamics of the Australian continent, we utilize P-to-S receiver functions (RFs) recorded by 182 stations to map the 410 and 660 km discontinuities (d410 and d660, respectively) bordering the mantle transition zone (MTZ). The RFs are stacked in successive circular bins with a radius of 1o under a non-plane wavefront assumption. The d410 and d660 depths obtained using the 1-D IASP91 Earth model show a systematic apparent uplifting of about 15 km for both discontinuities in central and western Australia relative to eastern Australia, as the result of higher seismic wavespeeds in the upper mantle beneath the former area. After correcting the apparent depths using the Australian Seismological Reference Model, the d410 depths beneath the West Australia Craton are depressed by ∼10 km on average relative to the normal depth of 410 km, indicating a positive thermal anomaly of 100 K at the top of the MTZ which could represent a transition from a thinner than normal MTZ beneath the Indian ocean and the normal MTZ beneath central Australia. The abnormally thick MTZ beneath eastern Australia can be adequately explained by subducted cold slabs in the MTZ. A localized normal thickness of the MTZ beneath the Newer Volcanics Province provides supporting evidence of non-mantle-plume mechanism for intraplate volcanic activities in the Australian continent.