Deep Electrical Structure And Rheological Analysis Of Cenozoic Volcanism In Northeast China

Northeast(NE)China is characterized by widespread intraplate volcanism spanning from the Cenozoic to present.However,the exact origin and dynamic that is responsible for the intraplate volcanism is still controversial.Electrical resistivity is strongly affected by fluid or melt.In order to study the deep electrical structure beneath the Cenozoic volcanoes in Northeast China from electrical perspective,the NE China magnetotelluric(MT)data were acquired using long period MT instruments from 2016 to 2018.This can visually reveal the internal thermal state and fluid state of the Earth,and study the depth of these volcanoes' structure that may open the veil of volcanic activity in Northeast China.In order to obtain a fine electrical structure above the mantle transition zone in northeastern China,long-period magnetotelluric survey covers a total of 82 effective physical sites across 40°N-48°N and 116° N-135°N.The measuring station spacing is about 1° within the Songliao basin to ensure that it constitutes a constraint on the lithospheric scale around the Songliao basin;the distance for the outer region is about 2° so that the sufficient aperture reflects the conductivity of the upper mantle in the entire area.After a series of standards data processing,the effective period range 10-30,000 s of full-impedance information is obtained.The full-impedance components of all stations are used for the 3D inversion.Before the 3D inversion,the data was edited using magnetotelluric Nyquist plot.In order to obtain a stable conductivity model,different inversion methods were used in this paper,and we conducted many inversions runs to assess the stable and sensitivity of inversion results by changing the inversion parameters.Then we obtained a fine 3D resistivity structure of the upper mantle in the Northeast China.The model with the following characteristics:(1)We found that there are a number of locally low-resistance anomalies C1-C8 and EC1-EC3 which are consistent with the Cenozoic volcanoes in spatially at the shallow region from 30-50 km of the Northeast.It is speculated that the lava beneath these volcanoes are still in hot and well status.The elliptical ring low-resistance anomalies are distributed around the Songliao basin,which are estimated for hydration and partial melting.(2)In this study,three relatively obvious high-conductivity anomalies were found at the bottom of the upper mantle in the northeastern region,which were around the Songliao basin.They are DC1 on the northeastern corner,DC2 on the southwestern corner,and DC3 on the southeastern corner.The high-conductivity anomalies are elliptical but not completely closed annular low resistance anomaly.Two of these deep high-conductance DC1 and DC3 are almost interconnected at a depth of 300 km.Because these anomalies are very deep,we have validated the reliability of deep anomalies by constraining the inversion of deep anomalies.The anomalous distribution of the conductivity model in this paper are consistent with the seismic velocity model,especially the low velocity in the deep of Changbai Mountain is corresponding to DC3,which indicates the existence of deep melting.(3)The ring-like anomalies at the deep upper mantle developed upwards as a tree are thought as the common root of the independent low-resistance anomalies in the crust.We believe that the Cenozoic volcanoes in northeast China were originally born with multiple births.In addition,thanks to the anomalous importance of DC1,DC2,and DC3,we calculated the water content and the melting ratio for them according to the thermal structure of northeast China.The results reveal that the deeper parts of the northeast China have been fully hydrated.At a depth of about 200 km-350 km,the water content of DC1 and DC3 is more than 855 ppm,especially the DC1 resistivity is lower,which exceeds 1000 ppm,and partial melting will occur.Water content of DC2 is close to 855 ppm at 220 km.It is further inferred that these deep hydrous conductive abnormalities are rooted in the mantle transition zone,which is the source of the Cenozoic volcanoes in Northeast China.Finally,considering the combination of geochemistry and natural earthquakes,we believe that the volcanic mechanism in the northeastern region is due to the deep subduction of the Pacific plate,which has brought in plenty of water in the mantle transition zone.Because the Pacifi plate retreats during Cenozoic,the mantle material will undergo partial melting with the decrease of pressure and deep heat.In this condition,the fluid and the molten material migrate upward due to buoyancy.During the transfer to the top of the asthenosphere,the upwelling is stopped by the hard and dry continent lithosphere,but the weak parts of the structure such as the plate suture zone were pierced and erupted in the form of sodic and potassic continental basalt magma eventually.