Abstract
SUMMARY We analyse broad-bandSS waveform data recorded by several networks in Europe with sources mainly in the west Pacific to study the underside reflections of teleseismic SS waves in the lithosphere and the upper mantle beneath eastern Asia and the NW Pacific ocean. SS bounce points sample a corridor from the Aleutian, Kamchatka and Japan subduction zones through the North China Craton and Central Asian Orogenic Belt to the Tibetan plateau. The corridor passes through different tectonic units such as subduction zones, an old continental shield, a fold belt and a high plateau. We investigate the seismic structure of the lithosphere and the mantle transition zone beneath the different geotectonic units along the profile and infer the correlation of geodynamic processes at different depths. We explore the short period frequency content in the SS waveform data and use moveout correction and common midpoint stack to acquire profiles with high lateral and depth resolution from the crust to the mantle transition zone. Clear SS precursors of the 410 and 660 km discontinuities show the effects of the interaction between the subducted oceanic lithosphere and the mantle transition zone beneath the NW Pacific subduction zones. A low-velocity layer has also been detected beneath the 410 km discontinuity and can be traced along the entire profile. Due to the improved resolution acquired by the method presented here we have been able to study the shallower structures such as the Moho and the lithosphere–asthenosphere boundary by SS precursors. The continental Moho can be clearly seen along this corridor. The depth variation agrees well with earlier receiver function results. We also see negative reflectors along the profile at varying depths, which can be interpreted as the lithosphere–asthenosphere boundary.
Highlights
The Earth’s mantle is divided into layers according to seismic velocity variations detected by the propagation of body waves
Unlike the crust–mantle boundary (Moho), which is widely accepted as compositional change, the lithosphere–asthenosphere boundary (LAB) was originally defined with regards to rheology, with the lithosphere behaving essentially as elastic solid and the asthenosphere deforming as a viscous fluid (Barrell 1914)
For a better understanding of the interpreted data we separate the observations according to the major phases in the lithosphere and the mantle detected in this study
Summary
The Earth’s mantle is divided into layers according to seismic velocity variations detected by the propagation of body waves. We preserve the high-frequency content of the SS waveform data and improve the lateral and depth resolution for the upper-mantle discontinuities and create new possibilities to study the shallower structures such as the LAB and the continental Moho by using SS precursors. The distribution of the selected SS bounce points facilitate the analysis of the three profiles from 75◦E to 180◦E between 25◦N and 60◦N (Fig. 2b) These profiles are designed to sample the lithosphere and upper mantle beneath different tectonic units from the western Tibetan plateau, throughout a series of old continental blocks to the Pacific subduction zone on the east. Receiver function studies (Li et al 2000; Li & Yuan 2003) found no deepening of the 660 to the northwest of the Pacific subduction zone and concluded that deepening of the 660 likely occurs locally where the plate penetrates the MTZ
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