Seismic Evidence for a Paleoslab in the D" Layer Residing Adjacent to the Southeastern Edge of the Perm Anomaly

Abstract

The Perm Anomaly, which seismologically resembles the large low‐shear velocity provinces (LLSVPs) in the lowermost mantle under Africa and the South Pacific, but is considerably smaller, has been identified beneath Eurasia. Numerical simulations have indicated that the LLSVPs dynamically interact with subducted slabs in D", producing strong deformation near the edges of the LLSVPs. The existence of deformation has been corroborated by seismic anisotropic observations in the D" layer nearby the borders of the slow anomalies such as the Perm Anomaly (Long & Lynner, 2015, https://doi.org/10.1002/2015GL065506; Thomas & Kendall, 2002, https://doi.org/10.1046/j.1365-246X.2002.01760.x). The existence of paleoslabs, however, has rarely been reported. Our purpose of this study, therefore, is to detect fast anomalies associated with paleoslabs near the edges of the Perm Anomaly. Here we find evidence of the existence of the D" discontinuity at a height above the core‐mantle boundary of 250 km with a +3% shear wave velocity increase; it is located adjacent to the southeastern edge of the Perm Anomaly, which is most likely due to a phase transition from perovskite to postperovskite in conjunction with the presence of slab remnants, by analyzing the Scd (the lower mantle triplication) phases recorded at stations in southeastern Europe. The existence of fast anomaly in the D" region (likely due to accumulation of a paleoslab) is further confirmed by analyzing differential traveltime residuals of SKS versus SKKS and SKKKS measured at stations in North America. The subducted slab—probably attributed to the Central China slab—at the base of the mantle may play a role in influencing the shape and location of the Perm Anomaly.

Modeling of Seismic Anisotropy Observations Reveals Plausible Lowermost Mantle Flow Directions Beneath Siberia

“…Flow toward the edges of LLSVPs has been inferred in a few other regions from seismic anisotropy observations, most notably along the eastern edge of the African LLSVP (Ford et al., 2015; Reiss et al., 2019). Some studies have argued that flow at the base of the mantle directed toward the Perm Anomaly is a possibility as well based on seismic observations (Hu & He, 2019; Long & Lynner, 2015) that sampled just to the south of our study region.…”

Sharpness of the paired 660-km discontinuity beneath the Izu-Bonin area

“…We analyzed the S‐to‐P converted wave (SdP), which is a seismic phase that descends as an S wave from the earthquake source and then converts to a P wave at the deep velocity interfaces (e.g., Zhou YZ et al, 2012; Li J and Yuen, 2014; Niu FL, 2014; Yang ZT and He XB, 2015; Cui QH et al, 2018; He XB and Zheng YX, 2018). Hence, SdP exhibits a nearly identical (slightly lower) slowness compared with direct P, which helps identify them in the vespagram calculated by an N th ( N = 4) root slant‐stack algorithm (e.g., Rost and Thomas, 2002; Hu JF and He XB, 2019). One‐dimensional waveform modeling based on a propagation matrix algorithm (Wang RJ, 1999) was also conducted to further understand the nature of the mantle discontinuities.…”