The Global Lithospheric Flexural Isostatic Characteristics And Geological Significances

As the outermost elastic layer of the solid Earth,the lithosphere deflects in response to topographic and internal Earth’s loads and,in the absence of disturbing forces,eventually tends to isostasy.The mechanical strength of the lithosphere is usually quantified in terms of the effective elastic thickness(T_e),which controls the flexural response of the lithosphere to applied loads.The isostatic gravity anomaly reflects the isostatic state(isostatic or non-isostatic)of the lithosphere.And the residual topography indicates the contribution of various other dynamical motions to lithospheric flexure and deformation.All of these are important in studying the lithosphere’s flexural isostatic characteristics and related scientific issues.However,in the present global studies,these aspects are often discussed singularly and in complex connections with each other,further complicating the study of lithospheric flexure and isostasy.Based on the lithospheric flexural isostatic theory,this thesis first proposes a new method for estimating lithospheric T_e in the spatial domain.By comparing the correlation coefficients between the observed Moho model and different T_e flexural Moho models,the optimal T_e is determined.This method is verified to be capable of recovering T_evariations through synthetic tests,i.e.by placing the random initial loads on a plate model with known T_e.The results of synthetic tests show that the proposed method can efficiently recover the lateral variation of T_e and that the computed correlation coefficients can be used to evaluate the effects of subsurface loads,improving the traditional spatial domain approach.Then,based on the proposed new method for estimating T_e,we calculate the effective elastic thickness of the lithosphere globally using topography/bathymetry and Moho depth data,and discuss the characteristics of T_e distribution.Meanwhile,according to the obtained correlation coefficient results,we initially investigate the flexural characteristics of global lithosphere.Then,based on the obtained global T_e variations,we use the flexural isostatic model with variable T_e to calculate the flexural isostatic gravity anomaly,and discuss the isostatic state of the global lithosphere accordingly.Finally,we calculate the residual topography globally based on the gravity(including bouguer gravity anomaly and flexural isostatic gravity anomaly),topography,Moho,seismic velocity data,and the estimated T_e results,and further discuss the dynamic processes and flexural characteristics of the lithosphere.Through global statistical analysis of the results of T_e,flexural isostatic gravity anomalies and residual topography,as well as regional discussions,and in combination with other data,this thesis obtains the following geological knowledge,(1)The mean value of continental lithospheric T_e is significantly higher than the mean value of oceanic lithospheric T_e,indicating the overall higher mechanical strength of the continental lithosphere.The statistics of oceanic T_e show a clear peak at 10-30 km,while in continents,T_e does not have a clear peak.However,the T_e variations of continental lithosphere demonstrate more complex patterns.The better preserved large cratons show high T_e values,the smaller or broken cratons show a decrease in T_e values,and the orogenic and Meso-Cenozoic tectonic zones have smaller T_e.The statistics of T_eand seismic events show that more than 95%of the earthquakes occur mainly in low T_eregions(<100 km),with a clear peak at 10-30 km(about 40%).To further validate the global T_e results estimated in this study,a comparison with two other published T_e models in the continents shows that the results of this study are in general agreement with those calculated by the other two methods.Furthermore,according to the calculated correlation coefficients,the flexure of the continental lithosphere is mainly caused by surface loads.However,the flexure of the oceanic lithosphere,while significantly affected by subsurface loads,still has 40%of the area where the lithospheric flexure is mainly related to surface loads.(2)The effect of the deep mantle in the flexural isostatic gravity anomaly is subtracted through spherical harmonic analysis.The statistics of filtered flexural isostatic gravity anomaly(12-180 degree and order)show a distinct peak around zero(±10 m Gal)for all global,continental or oceanic regions,and the proportion of low anomalies(±30m Gal)exceeds 85%,which indicate that the global lithosphere is isostatic overall.However,the proportion of±10 m Gal isostatic anomalies in oceanic areas(61.3%)is significantly higher than that in the continental areas(42.8%),indicating a more complex evolution of continental lithosphere.On a regional scale,the amplitude of flexural isostatic gravity anomaly is low in the large Craton(high T_e)and relatively high in areas such as the Meso-Cenozoic orogens(low T_e),reflecting the fact that isostasy is more easily maintained and restored in blocks with high T_e under the action of regional tectonic processes.(3)The medium-to-short wavelength features of the residual topography(also called the isostatic residual topography)reflects the contribution of various lithospheric-scale dynamic processes to the topography and help to understand the characteristics of the lithosphere’s flexure and deformation.The statistics of the isostatic residual topography show a clear peak at±0.1 km,accounting for about 51.1%of the global area,and 84.9%of low amplitude(±0.3 km).These suggest that,on global scale,lithospheric flexure is mainly caused by surface and subsurface loads,with deformation occurring in the vertical direction.In contrast,additional lithospheric flexure and deformation caused by other dynamic processes occurs mainly in areas such as subduction and collision.In this thesis,we calculate the effective elastic thickness,the isostatic gravity anomaly and the residual topography.Based on these results and in combination with other data,we not only validate some of the ideas and conclusions from previous studies,but also present some new insights.These results and insights provide a more comprehensive reference for studying the lithosphere’s flexural isostatic characteristics and contribute to the understanding of related scientific issues such as the mechanical strength,isostatic state and flexure of the lithosphere.