Isostatic Features Of The Crust In The Southwest Ordos Block And Northeast Asia

The crustal isostatic state and lithosphere flexure based on gravity data can provide vital information for dynamic process and uplift mechanism.By use of least square collocation prediction algorithm,2-dimensional bodies method,and free-air gravity anomaly admittance method,this research studied the crustal density structures,crustal isostatic state and lithosphere flexure mechanism in Southwest Ordos Block basing on in-situ gravity/GPS data,EIGEN-6C4 gravity model,ETOPO1 topography model,and SRTM15_PLUS topography model.The main study findings are summarized below.1.The crustal density structures,isostatic state and lithosphere flexure mechanism in Southwest Ordos Block.This research conducted gravity and GPS hybrid measurements at 385 stations throughout the Qinling Orogen of China to update the Bouguer Gravity Anomalies(BGAs)and free-air gravity anomalies of the area.Based on the in-situ observations,this research studied in detail the crustal density structure,the lithosphere flexure mechanism,and the isostatic state throughout the Qinglin Orogen.The BGAs in the study area are negative as a whole,ranging from-410 m Gal in the Tibetan Plateau to-100 m Gal in the eastern Qinglin Orogen.The estimated crustal thicknesses range from 56 – 35 km,thinning eastward along the Qinling Orogen.The optimal effective elastic thickness(Te)of the study area is 8 km,and the corresponding loading ratios are F1 = 0.3,F2 = 0.35,and F3 = 0.35.The vertical tectonic stress borne by the lithosphere varies observably in the study area.The downward stress comes to a peak of-14 MPa in the Liupanshan Mountain,whereas upward stress reaches the highest value of 8 MPa in the middle Qinling Orogen.Allowing for the distributions of the crustal density structures and the vertical tectonic stress of the lithosphere,as well as the distinct loading ratios of the eastern and western Qinling Orogen,a piecewise combination model was developed to interpret the uplift of the Qinling Orogen.This model indicates that the eastern part of the Orogen is thickened by the upward migration of mantle materials,under the thrusting of the South and North China Blocks,but that the uplift of the western part is resulted from the lower crustal flow that comes from the Tibetan Plateau.The uplift of Liupanshan Mountain is resulted from the Tibet Plateau's outward thrusting.2.The crustal density structures,isostatic state and lithosphere flexure mechanism in Northeast Asia.Considering the lateral density variation of Crust,this research investigated the isostatic state of Northeast Asia with the utilization of EIGEN-6C4 gravity data and ETOPO1 topography data,and analyzed the uplifting mechanism of the Changbai Mountain.Two profiles in roughly east-west direction which stretch over Changbai Mountain and Sakhalin Island were studied in detail.Specifically,taking CRUST1.0 model as initial conditions,this research inversed the density structures of crust along the two profiles with Bouguer gravity anomalies,under the considerations of uniform density and lateral density in each layer,respectively.The results show that,to some degree,there is lateral inhomogeneity in the crust density structure along the profiles.According to the inversed density structures and topography data,this research calculated the depth of isostasy along the 2 profiles.Basing on the comparison of the depth of isostasy with the one of Moho,the vertical tectonic stress of the two profiles were calculated.Then,with the method illustrated above,this research calculated the vertical tectonic stress along 21 profiles in exactly east-west direction within our study area,with sampling interval of 1 degree.By interpolating the above results the distribution of the vertical tectonic stress of the whole Northeast Asia was obtained.In total,the Crust of Northeast Asia,whose vertical tectonic stress is about 0,is in isostatic state.Among which,the vertical tectonic stress at the Changbai Mountain is-(15~25)MPa,at Japan Islands is-(40~50)MPa,and at the Pacific Trench is 15~25MPa.This research estimated the effective elastic thickness(Te)and the load ratio of the area around the Changbai Mountain,using the method of Free-air gravity anomaly admittance.The result shows that the Te of the Changbai Mountain is about 10 km,which mean that the lithosphere of the area is relatively soft.The initial load is mainly from the Moho,about 78% of total load,which suggest that the uplifting of the Changbai Mountain was mainly resulted from the uplifting of material of the Mantle.