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Fundamental to plate tectonics is the subduction of cold and mechanically strong oceanic plates. While the subducted plates are conventionally regarded to be impermeable to mantle flow and separate the mantle wedge and the subslab region, isolated openings have been proposed. By combining new shear wave splitting measurements with results from geodynamic modeling and recent seismic tomography and geochemical observations, we show that the upper ~200 km of the Cocos slab in northern Central America is intensively fractured. The slab there is strong enough to produce typical arc volcanoes and Benioff Zone earthquakes but allows mantle flow to traverse from the subslab region to the mantle wedge. Upwelling of hot subslab mantle flow through the slab provides a viable explanation for the behind-the-volcanic-front volcanoes that are geochemically distinct from typical arc volcanoes, and for the puzzling high heat flow, high elevation, and low Bouguer gravity anomalies observed in northern Central America. 

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. 

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. 

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. 

Abstract Seismic attenuation is an important parameter for characterizing subsurface morphology and thermal structure. In this study, we useP‐wave amplitude spectra from 588 teleseismic events recorded by 477 broadband seismic stations in the southeastern United States to examine the spatial variations of seismic attenuation in the crust and upper mantle. The resulting seismic attenuation parameter (∆t*) measurements obtained using the spectral ratio technique reveal a zone of relatively low attenuation in the Gulf of Mexico Coastal Plain and the southwestern terminus of the Piedmont province. Spatial coherency analysis of the ∆t* observations suggests that the center of the low attenuation layer is located within the uppermost mantle at about 70 km depth. This low attenuation anomaly lies along the suture zone between Laurentia and Gondwana and approximately coincides with the east‐west trending Brunswick magnetic anomaly. The origin of this low attenuation anomaly can be attributed to low attenuation bodies in the form of remnant lithospheric fragments in the deep crust and the uppermost mantle. The contribution of scattering to the observed ∆t* is estimated by calculating the ratio of amplitude on the transverse and vertical components in theP‐wave window. Relative to the rest of the study area, the Gulf of Mexico Coastal Plain demonstrates weaker scattering which is suggestive of a more homogenous crustal and uppermost mantle structure.