A low-velocity layer atop the mantle transition zone beneath the western Central Asian Orogenic Belt: Upper mantle melting induced by ancient slab subduction

Increasing evidence shows that the low-velocity layer (LVL) atop the mantle transition zone (MTZ) may play a critical role in Earth's evolution. The existence and formation of the LVL in continental interiors remain ambiguous. This study investigates the P-wave velocity structure atop the MTZ by matching P and sP-wave triplicated waveforms simultaneously in the western Central Asian Orogenic Belt far away from continental margins. Two groups of nearly orthogonal triplications are allocated using records of four earthquakes at stations from the China National Seismic Network and the Incorporated Research Institutions for Seismology. Our observations resolve an LVL with 21-23 km thickness and a P-wave velocity drop of approximately -5.7% to -5.8% atop the MTZ. The two nearly orthogonal directions show little variation in the velocity structure of the LVL. It is inferred that the LVL results from partial melting induced by water and/or other volatiles released from the subducted Paleo-Asian oceanic slab and, hence, has existed since the Paleozoic, with possible variations in melt fraction. The long-lived partially molten LVL may play an important role in the intraplate magmatism and mantle dynamics in the Central Asian Orogenic Belt. Our results provide strong seismic evidence for the LVL existing in the continental interior, indicating that the LVL may be pervasive in Eurasia because of vast subductions of ancient plates; this provides new insights into the material circulation in the ancient oceanic subduction zones. (c) 2021 Elsevier B.V. All rights reserved.