A Study Of Gravity-Magnetic Features And Deep Tectonics Of The NE China Basin Groups

The NE China Basin Groups are located between the Siberian Plate and the North China Plate,forming a united plate from many mosaic micro-plates in the pre-Mesozoic period.Study of the groups' deep tectonics,including the basement,crustal density,Moho and Curie,will be conducive to the solution of fundamental problems in geodynamics.This research is based on 1:100000 Bouguer gravity and aeromagnetic anomalies data,as well as seismic,geothermal and petrophysical data.Forward and inversion modeling approaches,such as Wavelet Multiscale Decomposition(WMD),Power Spectrum Analysis(PSA),Parker method and Simulated Annealing(SA),are utilized in this research to deduce deep tectonics of the NE China Basin Groups.Gravity and magnetic anomalies of the NE China Basin Groups mainly strike NE and NNE,followed by WE,which indicates the pre-existing WE Paleo-Asian Ocean tectonics domain superposed and modified by the NE Pacific domain.Zoning characteristics of gravity and magnetic fields prove a significant mosaic tectonic background of the study area.Gravity and magnetic fields of NE China Basin Groups are separated in this research using WMD method with the best separation order(order 4),anomalies' apparent depth of each order is determined using Logarithmic PSA,and then shallow high-frequency anomalies are removed to obtain gravity and magnetic anomalies of the basement and crustal deep interfaces.Five typical features of gravity and magnetic anomalies in the study area are summarized,the first derivative of gravity and magnetic anomalies of basement faults is calculated,changing characteristics of the potential field anomaly gradient zones are strengthened,and linear basement faults are extracted.A total of 31 major basement faults are finally identified,and gravity and magnetic zoning is carried out in the study area.In addition,details of order 2-3 wavelet gravity and magnetic anomalies and correlation coefficients between gravity and magnetic anomalies,combined with drilling and petrophysical data in the NE China Basin Groups,are employed to forecast the basement lithology and to draw the basement lithology forecast map of the entire NE Basin Groups.Deep seismic sounding data as constraint information,the improved Parker method is used to inverse Moho depth of the NE Basin Groups and the Moho is discovered striking mainly NE and NNE,reflecting newer Pacific tectonic domain characteristics.The Moho depth shows a feature of shallow east and deep west.Moho depth of the Sanjiang Basin is the shallowest,only about 31 km.The Songliao Basin lies at a major Moho uplift area,extending to the Bohai Bay Basin with a depth between 32km and 34km.The Xiaoxing'anling and Zhangguangcailing areas sandwiched between the Sanjiang Basin and the Songliao Basin hold a relatively large Moho depth,from 35km to 36km.From the east slope of the Daxinganling to west,the Moho depth increases obviously,from 33 km to 40 km or more,and the Moho depth of southwest of the study area has the deepest,44 km or more.The central and eastern basin groups typically correspond to Moho uplift areas,while the western ones do not have this characteristic,indicating that there are some differences in the genetic model between the central and eastern basin groups and the western ones.Inversed Moho as a constraint,the SA algorithm is employed to inverse the exponential distribution function of crust density changing with the depth,to calculate the density contrast(between the crust surface and the mantle)and the attenuation coefficient,and to obtain a crustal density structure model.Compared with the crustal density structure converted from seismic P-wave velocity,the model can accurately reflect the vertical density changes of the crust.Based on WMD and PSA results,the Curie depth map of the study area is inversed using order 4 approximation of aeromagnetic anomalies.The Curie depth of the study area ranges between 18km and 34km,that of the central and eastern basin groups is shallow while that of the north and southwest is deep.Major faults of the study area are located at the steep slope zones.Volcanic active areas are often situated on the edge of Curie uplift areas.Magma and deep earthquakes at Changbai Mountain volcanic active area may be caused by retention and deep dehydration of the Pacific subduction plate in the mantle transition zone.The above analysis suggests distinct characteristics of the eastern,central and western basin groups in deep tectonics in the study area,which is believed a result of the front end of the Pacific plate blocked at the Daxinganling line in a Eurasia eastern stretching background.The eastern and central basin groups east of the Daxinganling line are not only influenced by Eurasia northeastern stretching,but also by subducting plate retention.Upwelling asthenosphere hot material further exacerbates tensional rifts and faults,which contributes to the formation of large-scale extensional basins in the study area.Finally,a new potential field quantitative separation method is used to study the basement,faults and deep tectonics of the Hailar Basin based on geophysical information,such as gravity,magnetic,seismic and geothermic data.In addition,the 2D WMD is introduced to the density structure research to explore geo-interfaces,which can serve as reference for the study of single basins of the NE Basin Groups in the future.