Alteration of the Lithosphere beneath Northeast China Inferred from Seismic Structure

The East North China Craton (ENCC) with Archean cores has undergone significant reactivation since the Mesozoic. However, there has been no direct geophysical evidence illustrating the specific alteration processes in ENCC. Northeast China (NEC), which is adjacent to ENCC with less seismicity, might shed some lights on understanding the alteration process of the ENCC lithosphere. Employing a two-station method, we first build isotropic and azimuthally anisotropic Rayleigh wave phase velocity maps of 16-160 s from 5-year teleseismic data recorded at 48 CEA permanent stations in NEC. Employing these data as well as 6-40 s phase velocity maps constructed from 2-year ambient noise data in the same region, we then construct a high-resolution shear velocity structure from the surface to 350 km depth beneath NEC through a nonlinear Bayesian Monte-Carlo inversion. Our inferred velocity structure shows distinct features beneath the center of the NEC, Songliao Basin (SLB). The northwestern SLB has continuous, thick high VS layer just beneath the Moho, with relatively low VS at greater depths. In contrast, the southern SLB displays fragmented high VS zones underneath the Moho with significant high VS zone at greater depths. At the same time, the northwestern SLB exhibits high speed lower crust, while its southern counterpart does not. Moreover, Cenozoic volcanos of NEC are all located on a semicircle surrounding SLB. They have low velocity roots which are linked to an interconnected low velocity zone under 100 km, but no high speed lower crust. Among them, the lowest velocity of the crust is observed beneath Great Khingan, whereas the lowest velocity in the mantle is beneath the Changbai Mountain. These features offer new insight into the on-going alteration process of the lithosphere beneath the NEC. Specifically, the ascent of melt from the mantle may not only lead to underplating, but also induce lithospheric delamination, while the weak zones with different formation mechanisms which include asthenosphere, lower crust, old magma chamber, and faults, could provide needed magma conduits.