Oceanic lithosphere descends into Earth's mantle as subducting slabs. Boundaries between the subducting slab and the surrounding mantle are defined as slab interfaces, whose seismic imaging is the key to understanding slab dynamics in the mantle. However, the existence of slab interfaces below 200 km remains elusive.

Prof. CHEN Qifu's group from the Institute of Geology and Geophysics, Chinese Academy of Sciences (IGGCAS) and their collaborators observed two distinct seismic discontinuities within the mantle transition zone (~410 km to 660 km) beneath the western Pacific subduction zone.

The two discontinuities represented the upper and lower boundaries of the subducted Pacific high-velocity slab, corresponding to the slab Moho and the surface of partially molten sub-slab asthenosphere, respectively.

This work was published in Nature Geoscience on Nov. 9.

The subduction process transports chemically differentiated and hydrated rocks into Earth's mantle. At shallow depths (<200 km), the upper and/or lower interfaces of subducting slabs have been identified, all of which are characterized by sharp seismic velocity discontinuities.

However, how deep the seismic velocity discontinuities at slab interfaces can extend remains unclear, mainly due to the lack of high-resolution imaging of slab interfaces at depths below 200 km.

To understand the existence and origin of deep slab interfaces, the researchers took advantages of the dense seismic arrays in northeast China to study the upper mantle structures in the region.

They found sharp-dipping, double seismic velocity discontinuities within the mantle transition zone (~410 km to 660 km) beneath the western Pacific that coincide spatially with the upper and lower bounds of the high-velocity slab.

"Based on detail seismological analyses, the upper discontinuity was interpreted to be the Moho discontinuity of the subducted slab," said Prof. CHEN. "The lower discontinuity is likely caused by partial melting of sub-slab asthenosphere under hydrous conditions in the seaward portion of the slab."

The imaged distinct slab-mantle boundaries at depths between 410 and 660 km, deeper than previously observed, suggest a compositionally layered slab and high-water contents beneath the slab.

The study was done in collaboration with California Institute of Technology, Rice University, China University of Petroleum (Beijing), Earth Observatory of Singapore, Nanyang Technological University, Peking University, Institute of Earthquake Forecasting, China Earthquake Administration, and University of Illinois Urbana-Champaign.

The work was supported by the Strategic Priority Research Program (B) of Chinese Academy of Sciences and the National Natural Science Foundation of China.