3D Structure Inversion Beneath NE China Revealed By Ambient Noise Adjoint Tomography

Northeast China (NE China) is bounded by two distinct cratons, the North China craton in the south and the Siberia craton in the north, and stretches to the Japan Sea in the east. NE China has experienced a complex tectonic evolution,with constituted of several blocks in the Paleozoic under to serial episodes of continental accretion.During and after the closure of the Paleo-Asian Ocean and Mongol-Okhotsk, NE China experiences a series of orogeny, basin formation and volcanism, and now the region has evolved to a mosaic of several distinct terranes, including orogenic belts,basins and volcanoes. The Cenozoic volcanism is widely distributed around NE China and particularly along the valleys and mountain belts on the flanks of the basin.Therefore, this area is a significant region to understand the intraplate volcanism and the interaction of subducted oceanic plate in the mantle transition zone with overlying tectonic structure.By using the ambient noise adjoint tomography method, we construct a new 3-D shear wave speed model of the crust and the uppermost mantle beneath Northeast China. The adjoint tomography, without any intermediate steps of measuring phase dispersion like traditional ambient noise tomography method,can inverts shear wave speeds of the crust and uppermost mantle directly from 6-40 s waveforms of· Empirical Green's functions (EGFs) of Rayleigh waves, which are derived from interferometry of two years of ambient noise data recorded by the 127 Northeast China Extended Seismic Array stations. With an initial 3-D model derived from traditional asymptotic surface wave tomography method, adjoint tomography refines the 3-D model by iteratively minimizing the frequency-dependent traveltime misfits between EGFs and synthetic Green's functions measured in four period bands: 6-15 s,10-20 s, 15-30 s, and 20-40 s.Our new model shows shear wave speed anomalies that are spatially correlated with known tectonic units such as the Great Xing'an range and the Changbaishan mountain range. But the adjoint tomographic images show more vertically continuous velocity anomalies with larger amplitudes due to the consideration of the finite frequency and 3-D effects. The new model also reveals low wave speed conduits in the mid- lower crust and the uppermost mantle with a wave speed reduction indicative of partial melting beneath the Halaha, Xilinhot-Abaga, and Jingpohu volcanic complexes, suggesting that the Cenozoic volcanism in the area has a deep origin.