Study On Continental Dynamics Of China And Western Mongolia Based On Lithospheric Flexure

Lithospheric flexure is a phenomenon that describes how the lithosphere responds to long-term(>10~5 yr)geological loads.Studies based on lithospheric flexure have successfully explained many geological phenomena,such as the genesis of foreland basins,the geometry of seamounts and subducted plates,and the rebound of continents due to melting of glaciers or unloading of lake water.It has been also indicated that the magnitude and spatial scale of lithospheric flexure are controlled by its strength(or effective elastic thickness),which has a wide range of geological and geodynamic implications.The study of continental lithospheric strength and its controlled flexural processes is of great scientific importance for us to understand the dynamic mechanism of tectonic evolution of continental lithosphere,and the interactions between the dynamic processes of deep mantle and shallow tectonic activities.In this paper,we reviewed the history of the development of the theory of lithospheric flexure,introduced the status quo of research in this field,and systematically elaborates the relevant theories,methods and models of lithospheric flexural isostasy.On this basis,the lateral variations of lithospheric strength and its anisotropy in the mainland of China and its surrounding areas were inverted.Then,to address the current frontier scientific issues of continental dynamics,the characteristics of lithospheric flexure and its geodynamic implications of some typical tectonic domains,including the Qinghai-Tibet Plateau,Sichuan Basin,Longmenshan and western Mongolia,were well studied.To sum up,the paper mainly includes the following aspects:(1)Lithospheric strength and its anisotropy in mainland China and its surrounding areas.The Chinese mainland and its surrounding areas are located in the eastern end of Eurasia.Under the combined influence of the complex dynamic systems,such as the northward migration and collision of the Indian plate,the northwestward subduction of the Pacific plate and the blocking of the Siberian platform,the Chinese mainland and its surrounding areas behave a complex tectonic framework.The information of lithospheric strength and its anisotropy helps us to understand the spatial configuration and deformation mechanism of the lithosphere in these regions.Based on lithospheric flexure analysis,the lithospheric strength and anisotropy were derived from the coherence between Bouguer anomaly and topography.The results show that the lithospheric strength and its anisotropy in the mainland of China and its surrounding areas exhibit a significant lateral variation.The high Te value(Te>40 km)mainly corresponds to the ancient and stable craton basins,and the large-scale low Te value(Te<30 km)appears in the orogenic belts,intracontinental plateaus and other areas with active deformation.There are also lateral variations in Te inside the tectonic unit.The first-order information of results show that the lithospheric strength distribution is roughly consistent with the tectonic zoning,indicating the lithospheric strength is one of the main parameters controlling the lithospheric deformation.By comparing Te anisotropy with SKS,we found that the weak axes of lithospheric strength are parallel to the direction of the mantle asthenosphere flow in the India-Eurasia collision zone,Tien shan,Mongolia-Baikal and Taiwan,indicating that the Te anisotropy and SKS anisotropy are homologous and both are controlled by asthenosphere flow.(2)Lithospheric flexure response characteristics of the Tibet Plateau.The Tibet Plateau is an ideal laboratory for studying the lithospheric flexure since the thick crust and high-elevation characteristics of this region amplifies the response of the lithospheric flexure.By analyzing the Bouguer coherence in the Tibet Plateau,we found an obvious sub-band(100?300 km)high-value(>0.4)in Bouguer coherence.This phenomenon is widespread within the Tibet Plateau.Since the spectral characteristics of the Bouguer anomaly reflect that the isostatic compensation depth roughly corresponds to the upper and lower interfaces of the lower crust,we infer that the development of the weak lower crust,which decouples the crust and mantle,reduces the strength of the lithosphere,and causes a secondary isostatic adjustment,may attribute to the sub-band high-value character of the Bouguer coherence.(3)Paleo-strength of Sichuan Basin and Paleo-elevation of LongmenshanLithospheric strength in Sichuan Basin and the time and mechanism of the uplift of Longmenshan hold some keys to understand the lateral growth of the Qinghai-Tibet Plateau.Based on the flexure analysis,we simulated the flexure characteristics of Mesozoic and Late Triassic strata in Sichuan Basin,and obtained the paleo-strength of Sichuan Basin in the Mesozoic and the paleo-elevation of Longmenshan in Late Triassic.The results show that the lithospheric strength of the Sichuan Basin has been weakening since the Mesozoic,and the Longmenshan may have been elevated to a height of 2000 m in Late Triassic,present-day Longmenshan is inherited from this time.(4)Mantle flow beneath western Mongolia.The origin of the high-elevation and neotectonism of the western Mongolia remains enigmatic.A variety of studies suggested that the mechanisms might,at least in part,lurk in the mantle,but little consensus was reached on its pattern and contributions to the surface topography.In this study,we provided independent evidence for mapping the mantle flow beneath the western Mongolia by lithospheric flexure analysis in terms of the relationship between free-air gravity anomaly and topography.High-value long-wavelength free-air admittance and positive dynamic component of topography lead to a reliable argument for mantle upwelling beneath the western Gobi-Altai.Horizontal flow fed by the upwelling modifies the lithosphere and supports the Hangai Dome uplift.Our results demonstrate the significant role of the mantle flow in shaping the present-day anomalous topography and tectonism in the western Mongolia,central Asia.