Mantle discontinuity structure beneath China

Abstract

Mantle reflectivity profiles obtained from long‐period recordings of multiple‐ScS reverberations from intermediate and deep‐focus earthquakes surrounding China reveal remarkably sharp images of China deep structure. The Tibetan Plateau and fold belts to the north are underlain by a midlithospheric, low‐velocity zone (LVZ) evidenced by coincident G (lid‐LVZ boundary) and L' (base of the LVZ) reflectors and a thickened lithosphere consistent with uniform thickening as the dominant mode of north‐south deformation. The mantle beneath northeastern China, Mongolia and southeastern Russia has a typically cratonic reflectivity signature with H (Hales) and deep L (Lehmann) discontinuities. The Sino‐Korean Paraplatform of eastern China is a region of active extensional tectonics, as expressed by an “oceanic” reflectivity pattern at shallow depths (a G discontinuity at ∼80 km), but a more continental structure below. The southern Sea of Japan, Korean Peninsula, Yellow Sea and bordering regions of the China mainland are underlain by a ∼5.4% impedance decrease near 330 km depth that we interpret as the upper limit of dense silicate melt atop the 410‐km discontinuity produced at the subduction zones to the east. Regional inversion of the reflection coefficient of the 410‐km discontinuity, R(410), reveals a pronounced high beneath Mongolia and north to northwestern China which is best explained by increased upper mantle olivine content, signifying extensive basalt depletion in a tectosphere that extends below the 410‐km discontinuity. A low value of R(660) associated with topographic depression of the 660‐km discontinuity beneath eastern Asia is consistent with a horizontally inclined slab atop the phase transition marking the seismic discontinuity. Reflection defocusing is an alternative but requires ≥38 km of intermediate‐scale topography within the depressed region, exceeding the amplitude of the depression by a factor of 1.5–2. Either mechanism requires strong heterogeneity far west of seismically active slabs. Two lower mantle reflectors (one beneath southern Honshu, Kyushu, and the Sea of Japan; the other below Shantung province (China) and the Yellow Sea) correlate with horizontally elongated velocity anomalies in western Pacific seismic tomography models. We speculate that they may represent slabs stagnating below the 660‐km discontinuity.