Multiple Phase Changes in the Mantle Transition Zone Beneath Northeast Asia: Constraints From Teleseismic Reflected and Converted Body Waves

Abstract

We reassess the mantle transition zone structure below the northeast Asia margin in the context of subduction of the Pacific plate below the Eurasian continent. We use two independent approaches of teleseismic imaging, namely, compressional‐to‐shear converted waves (receiver functions) and shear wave underside reflections (SS precursors), and compare them within their statistical uncertainties. We find localized complexity in the interfaces marking solid phase changes in mantle minerals, in terms of both apparent topography and reflectivity. The 660‐km discontinuity is doubled, with ∼80‐km maximum vertical distance between the interfaces, over an 890 × 350 km2 region between 36–44°N and 130–133°E at the tip of the subducted Pacific plate. A similar complexity exists on the 410‐km discontinuity, coinciding with the presence of a deep cluster of seismicity below the Japan Sea. Both methods suggest the presence of low‐velocity zones atop the 410, within the mantle transition zone, and below the 660. This complex seismic signature is related to the Pacific plate and interpreted in light of the subduction thermal regime and phase equilibria for a pyrolitic mantle composition. Phase changes manifest themselves as broad zones of velocity gradients with localized doubled or multiple first‐order discontinuities, associated with transitions in the olivine, pyroxene, and garnet systems. An average pyrolitic composition and local temperatures of 1000–1300 K can explain the observed velocity gradients and multiple discontinuities. We show that the dissolution of stishovite, a high‐pressure polymorph of SiO2, into the higher‐pressure perovskite mineral, is a possible explanation for the low‐velocity zones at the top of the lower mantle.

Stochastic Inversion of P‐to‐S Converted Waves for Mantle Composition and Thermal Structure: Methodology and Application

“…Distinct 660 RF peaks have been reported in a number of regional and global studies (e.g., Ai et al, ; Andrews & Deuss, ; Maguire et al, ; Niu & Kawakatsu, ; Simmons & Gurrola, ; Tauzin et al, ). Additionally, differences between EA and MM models are also reflected in P410s and P660s waveforms.…”

Evidence of Subduction‐Related Thermal and Compositional Heterogeneity Below the United States From Transition Zone Receiver Functions

“…As is well documented (e.g., Deuss et al, ; Shearer, ), phase transitions in the olivine system produce sharp gradients in seismic velocity observed near depths of 410, 520, and 660 km. Additional seismic discontinuities have been observed near the base of the transition zone between about 660 and 750 km depth (e.g., Deuss et al, ; Simmons & Gurrola, ; X. Wang & Niu, ), which are commonly interpreted as phase transitions in the garnet system, although a poststishovite phase transition may be important in subduction settings (Tauzin et al, ). Since garnet is a primary constituent in basaltic material at transition zone pressures, the detection of seismic signals produced just below 660‐km depth may provide a useful estimate of the basalt content at the base of the MTZ.…”

Effects of Near‐Surface Complexities on Differential Travel Times and Amplitude Ratios Between PP and Its Precursors

“…Over the past decades, a large number of seismic techniques have been applied to probe the 410 and 660 km discontinuities, including the teleseismic receiver function (Ai et al, 2008; Tauzin et al, 2018; Thompson et al, 2011; Zhang & Schmandt, 2019), body wave triplication (Chu et al, 2012; Song et al, 2004; B. Wang & Niu, 2010), source side scattered waveforms (p d P, s d P, s d S, etc. ; Flanagan & Shearer, 1998; Li & Yuen, 2014), and asymmetric backscattering waves (Wu et al, 2019).…”