We used the geological map and published rock density measurements to compile the digital rock density model for the Hong Kong territories.We then estimated the average density for the whole territory.According to our...We used the geological map and published rock density measurements to compile the digital rock density model for the Hong Kong territories.We then estimated the average density for the whole territory.According to our result,the rock density values in Hong Kong vary from 2101 to 2681 kg·m^(-3).These density values are typically smaller than the average density of 2670 kg·m^(-3),often adopted to represent the average density of the upper continental crust in physical geodesy and gravimetric geophysics applications.This finding reflects that the geological configuration in Hong Kong is mainly formed by light volcanic formations and lava flows with overlying sedimentary deposits at many locations,while the percentage of heavier metamorphic rocks is very low(less than 1%).This product will improve the accuracy of a detailed geoid model and orthometric heights.展开更多
We compile the GOCE-only satellite model GOSG01 S complete to spherical harmonic degree of 220 using Satellite Gravity Gradiometry(SGG) data and the Satellite-to-Satellite Tracking(SST) observations along the GOCE orb...We compile the GOCE-only satellite model GOSG01 S complete to spherical harmonic degree of 220 using Satellite Gravity Gradiometry(SGG) data and the Satellite-to-Satellite Tracking(SST) observations along the GOCE orbit based on applying a least-squares analysis.The diagonal components(V_(xx),V_(yy),V_(zz)) of the gravitational gradient tensor are used to form the system of observation equations with the band-pass ARMA filter.The point-wise acceleration observations(a_x,a_y,a_z) along the orbit are used to form the system of observation equations up to the maximum spherical harmonic degree/order 130.The analysis of spectral accuracy characteristics of the newly derived gravitational model GOSG01 S and the existing models GOTIM04 S,GODIR04S,GOSPW04 S and JYY_GOCE02S based on their comparison with the ultrahigh degree model EIGEN-6C2 reveals a significant consistency at the spectral window approximately between 80 and 190 due to the same period SGG data used to compile these models.The GOCE related satellite gravity models GOSG01 S,GOTIM05S,GODIR05 S,GOTIM04S,GODIR04 S,GOSPW04S,JYY_-GOCE02 S,EIGEN-6C2 and EGM2008 are also validated by using GPS-leveling data in China and USA.According to the truncation at degree 200,the statistic results show that all GGMs have very similar differences at GPS-leveling points in USA,and all GOCE related gravity models have better performance than EGM2008 in China.This suggests that all these models provide much more information on the gravity field than EGM2008 in areas with low terrestrial gravity coverage.And STDs of height anomaly differences in China for the selected truncation degrees show that GOCE has improved the accuracy of the global models beyond degree 90 and the accuracies of the models improve from 24 cm to 16 cm.STDs of geoid height differences in USA show that GOSGOIS model has best consistency comparing with GPSleveling data for the frequency band of the degree between 20 and 160.展开更多
The Gravity Recovery and Climate Experiment(GRACE) satellite mission provides a unique opportunity to quantitatively study terrestrial water storage(TWS) variations. In this paper,the terrestrial water storage variati...The Gravity Recovery and Climate Experiment(GRACE) satellite mission provides a unique opportunity to quantitatively study terrestrial water storage(TWS) variations. In this paper,the terrestrial water storage variations in the Poyang Lake Basin are recovered from the GRACE gravity data from January 2003 to March 2014 and compared with the Global Land Data Assimilation System(GLDAS) hydrological models and satellite altimetry. Furthermore, the impact of soil moisture content from GLDAS and rainfall from the Tropical Rainfall Measuring Mission(TRMM) on TWS variations are investigated. Our results indicate that the TWS variations from GRACE, GLDAS and satellite altimetry have a general consistency. The TWS trends in the Poyang Lake Basin determined from GRACE, GLDAS and satellite altimetry are increasing at 0.0141 km^3/a, 0.0328 km^3/a and 0.0238 km^3/a,respectively during the investigated time period. The TWS is governed mainly by the soil moisture content and dominated primarily by the precipitation but also modulated by the flood season of the Yangtze River as well as the lake and river exchange water.展开更多
The isostatic gravity anomalies have been traditionally used to solve the inverse problems of isostasy. Since gravity measurements are nowadays carried out together with GPS positioning, the utilization of gravity dis...The isostatic gravity anomalies have been traditionally used to solve the inverse problems of isostasy. Since gravity measurements are nowadays carried out together with GPS positioning, the utilization of gravity disturbances in various regional gravimetric applications becomes possible. In global studies, the gravity disturbances can be computed using global geopotential models which are currently available to a relatively high accuracy and resolution. In this study we facilitate the definition of the isostatic gravity disturbances in the Vening-Meinesz Moritz inverse problem of isostasy for finding the Moho depths. We further utilize uniform mathematical formalism in the gravimetric forward modelling based on methods for a spherical harmonic analysis and synthesis of gravity field. We then apply both mathematical procedures to determine globally the Moho depths using the isostatic gravity disturbances. The results of gravimetric inversion are finally compared with the global crustal seismic model CRUST2.0;the RMS fit of the gravimetric Moho model with CRUST2.0 is 5.3 km. This is considerably better than the RMS fit of 7.0 km obtained after using the isostatic gravity anomalies.展开更多
The Atlas region in northwest Africa is characterized by the Quaternary volcanism and elevated topography with past complex tectonic mitigation between the African and European plates.Geodynamics of this atypical regi...The Atlas region in northwest Africa is characterized by the Quaternary volcanism and elevated topography with past complex tectonic mitigation between the African and European plates.Geodynamics of this atypical region has left indubitably imprints in crustal architectonics,mainly regarding the crustal thickness as well as the crustal density structure.The knowledge of crustal thickness variations is of a significant interest,since it provides a crucial constraint to geodynamic and geophysical modelling of this region.In this study,we use gravity,topographic,bathymetric and sediment data together with results of seismic surveys to image the Moho topography beneath the Atlas region.The Bouguer gravity anomalies used for a gravimetric Moho recovery are obtained from the free-air gravity anomalies after subtracting the gravitational contributions of topography,bathymetry and sediments.The regional gravimetric Moho inversion constrained on seismic data is carried out by applying a regularized inversion technique based on Gauss-Newton’s formulation of improved Bott’s method,while adopting Earth’s spherical approximation.The numerical result reveals relatively significant Moho depth variations in the Moroccan Atlas,with minima of approximately 24 km along continental margins of the Mediterranean Sea and maxima exceeding 51 km beneath the Rif Cordillera.The Moho depth beneath the West African Craton varies from 32 km in its southern margin to 45 km beneath the Middle Atlas.The Tell Atlas is characterized by the shallow Moho depth of approximately 22 km and further deepening to 42 km towards the northern edge of the Aures Mountains.Our findings indicate a limited tectonic shortening of the High Atlas with the crustal thickness mostly within 36-42 km.Topographic discrepancies between the Rif Cordillera and the Atlas Mountains suggest that the hypothesis of isostatic compensation cannot be fully established.展开更多
The determination of accurate orthometric or normal heights remains one of the main challenges for the geodetic community in Ethiopia.These heights are required for geodetic and geodynamic scientific research as well ...The determination of accurate orthometric or normal heights remains one of the main challenges for the geodetic community in Ethiopia.These heights are required for geodetic and geodynamic scientific research as well as for extensive engineering applications.The main objective of this study is to estimate the geoid-to-quasi geoid separation(GQS)in Ethiopia(ETH-GQS).Such separation would be required for the conversion between geoid and quasigeoid models,which is mandatory for the determination of accurate geodetic heights in mountain regions.The airborne free-air gravity anomalies and the topo-graphic information retrieved from the SRTM3(Shuttle Radar Topography Mission of a spatial resolution 3 arc-second)digital elevation model were used to compute the ETH-GQS model according to the Sjoberg's strict formula for the geoid-to-quasigeoid separation.The ETH-GQS was then validated using GNSS-levelling data as well as geoid heights determined from different Global Geopotential Models(GGMs),namely the EGM2008,EIGEN-6C4 and GECO.The results reveal that the standard deviation of differences between the geoid heights obtained from the EIGEN-6C4 model and the geometric geoid heights obtained from GNSS-levelling data were improved by~75%(i.e.from~24 to~6 cm)when considering GQS values obtained from the ETH-GQS.展开更多
We investigate the evolution of stress fields during the supercontinent cycle using the 2D Cartesian geometry model of thermochemical convection with the non-Newtonian rheology in the presence of floating deformable c...We investigate the evolution of stress fields during the supercontinent cycle using the 2D Cartesian geometry model of thermochemical convection with the non-Newtonian rheology in the presence of floating deformable continents.In the course of the simulation,the supercontinent cycle is implemented several times.The number of continents considered in our model as a function of time oscillates around 3.The lifetime of a supercontinent depends on its dimension.Our results suggest that immediately before a supercontinent breakup,the over-lithostatic horizontal stresses in it(referring to the mean value by the computational area)are tensile and can reach-250 MPa.At the same time,a vast area beneath a supercontinent with an upward flow exhibits clearly the over-lithostatic compressive horizontal stresses of 50-100 МРа.The reason for the difference in stresses in the supercontinent and the underlying mantle is a sharp difference in their viscosity.In large parts of the mantle,the over-lithostatic horizontal stresses are in the range of±25 MPa,while the horizontal stresses along subduction zones and continental margins are significantly larger.During the process of continent-to-continent collisions,the compressive stresses can approximately reach 130 MPa,while within the subcontinental mantle,the tensile over-lithostatic stresses are about-50 MPa.The dynamic topography reflects the main features of the su-percontinent cycle and correlates with real ones.Before the breakup and immediately after the disin-tegration of the supercontinent,continents experience maximum uplift.During the supercontinent cycle,topographic heights of continents typically vary within the interval of about±1.5 km,relatively to a mean value.Topographic maxima of orogenic formations to about 2-4 km are detected along continent-to-continent collisions as well as when adjacent subduction zones interact with continental margins.展开更多
It is a well-known fact that the long-wavelength terrestrial geoid undulations are mainly attributed to deep mantle density heterogeneities,while more detailed features in the geoid geometry are associated with the to...It is a well-known fact that the long-wavelength terrestrial geoid undulations are mainly attributed to deep mantle density heterogeneities,while more detailed features in the geoid geometry are associated with the topography and the lithospheric density structure.To enhance a gravitational signature of mantle density heterogeneities below the lithosphere,the gravitational contributions of topography and lithospheric density heterogeneities should be modelled and subsequently removed from Earth’s gravity field.The refined gravity field obtained after this numerical procedure is more suitable for a recovery of a mantle density structure(below the lithosphere).Following this idea,methods for a spherical harmonic analysis and synthesis of gravity field and lithospheric density structures are presented,and a theoretical relation between gravity field and mass density structure is formulated.Since a gravimetric recovery of inner density structure has a non-unique solution,we propose an alternative method based on a conversion of seismic velocities to mass densities.A forward modelling approach is then employed to find the mantle density configuration that generates the gravitational field that best approximates the corresponding refined gravitational field obtained from observed gravity field after subtracting the gravitational signal of the lithosphere.展开更多
The average density of 3300 kg m-3 is often attributed for the asthenosphere. In this study, we inspect this value by estimating the average value of the(upper) asthenosphere based on applying the gravimetric forward ...The average density of 3300 kg m-3 is often attributed for the asthenosphere. In this study, we inspect this value by estimating the average value of the(upper) asthenosphere based on applying the gravimetric forward modelling of major known lithospheric density structures. The LITHO1.0 global seismic model of the lithospheric density structure is used for this purpose, while considering that the lithosphere-asthenosphere boundary(LAB) is rheological, conventionally taken at the 1300C isotherm,above which the mantle behaves in a rigid fashion and below which it behaves in a ductile fashion.According to our result, the average density of the upper asthenosphere is roughly 3400 kg m-3. This density value closely agrees with the corresponding average value 3371 kg m-3 computed based on an empirical density model provided in the Preliminary Reference Earth Model(PREM), while using the LITHO1.0 LAB depth data. We also demonstrate that the sub-lithospheric mantle gravity map exhibits mainly a thermal signature. The most prominent features in this gravity map are mid-oceanic spearing ridges marked by gravity lows, while oceanic subductions in the West Pacific are characterized by the most pronounced gravity highs.展开更多
Despite practical limitations of isostatic theories to model the Moho geometry are well-known,gravimetric methods are often used in terrestrial studies of crustal structure in regions with a low seismic data coverage....Despite practical limitations of isostatic theories to model the Moho geometry are well-known,gravimetric methods are often used in terrestrial studies of crustal structure in regions with a low seismic data coverage.Moreover,these methods are indispensable in planetary studies.Various gravimetric methods have been proposed.The Airy and Pratt theories are defined based on adopting a local compensation mechanism.The Vening Meinesz theory assumes a regional isostatic flexural model.The Vening Meinesz regional isostatic model generally describes a respond of the lithosphere to a load more realistically than the Airy model over continents.The Pratt method,on the other hand,better describes a compensation mechanism of the oceanic lithosphere.The application of a particular isostatic model also depends on applied numerical procedures,parameters for inversion,input data specifications,and many other aspects.In this study,we address some basic aspects by applying local and regional isostatic models for a Moho recovery.We also conduct a spectral analysis to assess a spectral resolution of gravity data that is optimal for a Moho recovery.Furthermore,we inspect the influence of low-degree spherical harmonics of gravity field on a Moho geometry.Gravimetric results are validated using seismic data at the European plate.Our results confirm a better performance of a regional compensation principle.We also demonstrate that a different thickness of the oceanic and continental crustal thickness should be taken into account as a priori information.Spectral analysis indicates that gravity data used for a Moho inversion should optimally have a spatial resolution between degrees 60 to 180.Results also show that low-degree spherical harmonics do not modify significantly the Moho geometry,particularly over regions with a relatively homogenous structure of deep mantle.展开更多
The coefficients of the Global Gravitational Models(GGMs) define the external gravitational field of the Earth.In many geoscience applications and gravity interpretations these coefficients are routinely used to repre...The coefficients of the Global Gravitational Models(GGMs) define the external gravitational field of the Earth.In many geoscience applications and gravity interpretations these coefficients are routinely used to represent the geoid surface and related gravitational field quantities without taking into consideration the internal convergence domain for computing the gravitational field inside the Earth's masses.In this study we discuss this issue and present the numerical approach for computing the gravitational field quantities on the geoid.The proposed numerical approach utilizes spectral expressions for the gravimetric forward modelling of topographic mass density distribution and the indirect gravimetric modelling of mass density heterogeneities inside the geoid.In the numerical realization we demonstrate that for precise applications the differences between the potential values computed at the topographic surface and on the geoid should be taken into consideration especially in mountainous regions with complex geology as well as in polar regions,where these differences in absolute values reach 40m^2 s^(-2)(or more),which translates into vertical displacements between the geoid and the quasigeoid of about 4m.展开更多
The results of global and regional studies often show significant disagreement between the Moho depths determined using seismic and isostatic models. In this study, we estimate the differences between these two models...The results of global and regional studies often show significant disagreement between the Moho depths determined using seismic and isostatic models. In this study, we estimate the differences between these two models in central Eurasia. The Vening Meinesz-Moritz(VMM) inverse problem of isostasy is utilized to determine the isostatic Moho depths. The estimated VMM Moho depths are then corrected for the sediment density contrast. The application of this correction improves the agreement between the isostatic and seismic Moho models. The existing discrepancies between the isostatic and seismic models are finally modeled by applying the non-isostatic correction, which accounts for the unmodelled mantle density heterogeneities and other geodynamic processes, which are not taken into account in classical isostatic models. Our results reveal that the non-isostatic correction still cannot fully describe mechanisms affecting the Moho geometry along the convergent continent-tocontinent tectonic plate boundaries occurring beneath Himalayas despite an overall good performance of the applied method.展开更多
基金supported by the Hong Kong GRF RGC project 15217222:“Modernization of the leveling network in the Hong Kong territories.”。
文摘We used the geological map and published rock density measurements to compile the digital rock density model for the Hong Kong territories.We then estimated the average density for the whole territory.According to our result,the rock density values in Hong Kong vary from 2101 to 2681 kg·m^(-3).These density values are typically smaller than the average density of 2670 kg·m^(-3),often adopted to represent the average density of the upper continental crust in physical geodesy and gravimetric geophysics applications.This finding reflects that the geological configuration in Hong Kong is mainly formed by light volcanic formations and lava flows with overlying sedimentary deposits at many locations,while the percentage of heavier metamorphic rocks is very low(less than 1%).This product will improve the accuracy of a detailed geoid model and orthometric heights.
基金financially supported by the National Key Basic Research Program of China(973 program,grant no.:2013CB733302,2013CB733301)the Major International(Regional) Joint Research Project(grant no.:41210006)+1 种基金DAAD Thematic Network Project(grant no.:57173947)the National Natural Science Foundation of China(grant No.41374022)
文摘We compile the GOCE-only satellite model GOSG01 S complete to spherical harmonic degree of 220 using Satellite Gravity Gradiometry(SGG) data and the Satellite-to-Satellite Tracking(SST) observations along the GOCE orbit based on applying a least-squares analysis.The diagonal components(V_(xx),V_(yy),V_(zz)) of the gravitational gradient tensor are used to form the system of observation equations with the band-pass ARMA filter.The point-wise acceleration observations(a_x,a_y,a_z) along the orbit are used to form the system of observation equations up to the maximum spherical harmonic degree/order 130.The analysis of spectral accuracy characteristics of the newly derived gravitational model GOSG01 S and the existing models GOTIM04 S,GODIR04S,GOSPW04 S and JYY_GOCE02S based on their comparison with the ultrahigh degree model EIGEN-6C2 reveals a significant consistency at the spectral window approximately between 80 and 190 due to the same period SGG data used to compile these models.The GOCE related satellite gravity models GOSG01 S,GOTIM05S,GODIR05 S,GOTIM04S,GODIR04 S,GOSPW04S,JYY_-GOCE02 S,EIGEN-6C2 and EGM2008 are also validated by using GPS-leveling data in China and USA.According to the truncation at degree 200,the statistic results show that all GGMs have very similar differences at GPS-leveling points in USA,and all GOCE related gravity models have better performance than EGM2008 in China.This suggests that all these models provide much more information on the gravity field than EGM2008 in areas with low terrestrial gravity coverage.And STDs of height anomaly differences in China for the selected truncation degrees show that GOCE has improved the accuracy of the global models beyond degree 90 and the accuracies of the models improve from 24 cm to 16 cm.STDs of geoid height differences in USA show that GOSGOIS model has best consistency comparing with GPSleveling data for the frequency band of the degree between 20 and 160.
基金supported by the National Natural Science Foundation of China(NSFC)Projects(11173050 and 11373059)
文摘The Gravity Recovery and Climate Experiment(GRACE) satellite mission provides a unique opportunity to quantitatively study terrestrial water storage(TWS) variations. In this paper,the terrestrial water storage variations in the Poyang Lake Basin are recovered from the GRACE gravity data from January 2003 to March 2014 and compared with the Global Land Data Assimilation System(GLDAS) hydrological models and satellite altimetry. Furthermore, the impact of soil moisture content from GLDAS and rainfall from the Tropical Rainfall Measuring Mission(TRMM) on TWS variations are investigated. Our results indicate that the TWS variations from GRACE, GLDAS and satellite altimetry have a general consistency. The TWS trends in the Poyang Lake Basin determined from GRACE, GLDAS and satellite altimetry are increasing at 0.0141 km^3/a, 0.0328 km^3/a and 0.0238 km^3/a,respectively during the investigated time period. The TWS is governed mainly by the soil moisture content and dominated primarily by the precipitation but also modulated by the flood season of the Yangtze River as well as the lake and river exchange water.
文摘The isostatic gravity anomalies have been traditionally used to solve the inverse problems of isostasy. Since gravity measurements are nowadays carried out together with GPS positioning, the utilization of gravity disturbances in various regional gravimetric applications becomes possible. In global studies, the gravity disturbances can be computed using global geopotential models which are currently available to a relatively high accuracy and resolution. In this study we facilitate the definition of the isostatic gravity disturbances in the Vening-Meinesz Moritz inverse problem of isostasy for finding the Moho depths. We further utilize uniform mathematical formalism in the gravimetric forward modelling based on methods for a spherical harmonic analysis and synthesis of gravity field. We then apply both mathematical procedures to determine globally the Moho depths using the isostatic gravity disturbances. The results of gravimetric inversion are finally compared with the global crustal seismic model CRUST2.0;the RMS fit of the gravimetric Moho model with CRUST2.0 is 5.3 km. This is considerably better than the RMS fit of 7.0 km obtained after using the isostatic gravity anomalies.
基金conducted under the HK science project 1-ZE8F:Remote-sensing data for studding the Earth’s and planetary inner structure.
文摘The Atlas region in northwest Africa is characterized by the Quaternary volcanism and elevated topography with past complex tectonic mitigation between the African and European plates.Geodynamics of this atypical region has left indubitably imprints in crustal architectonics,mainly regarding the crustal thickness as well as the crustal density structure.The knowledge of crustal thickness variations is of a significant interest,since it provides a crucial constraint to geodynamic and geophysical modelling of this region.In this study,we use gravity,topographic,bathymetric and sediment data together with results of seismic surveys to image the Moho topography beneath the Atlas region.The Bouguer gravity anomalies used for a gravimetric Moho recovery are obtained from the free-air gravity anomalies after subtracting the gravitational contributions of topography,bathymetry and sediments.The regional gravimetric Moho inversion constrained on seismic data is carried out by applying a regularized inversion technique based on Gauss-Newton’s formulation of improved Bott’s method,while adopting Earth’s spherical approximation.The numerical result reveals relatively significant Moho depth variations in the Moroccan Atlas,with minima of approximately 24 km along continental margins of the Mediterranean Sea and maxima exceeding 51 km beneath the Rif Cordillera.The Moho depth beneath the West African Craton varies from 32 km in its southern margin to 45 km beneath the Middle Atlas.The Tell Atlas is characterized by the shallow Moho depth of approximately 22 km and further deepening to 42 km towards the northern edge of the Aures Mountains.Our findings indicate a limited tectonic shortening of the High Atlas with the crustal thickness mostly within 36-42 km.Topographic discrepancies between the Rif Cordillera and the Atlas Mountains suggest that the hypothesis of isostatic compensation cannot be fully established.
基金conducted in the framework of the statutory project “Problems of Geodesy and Geodynamics” of the Institute of Geodesy and Cartography (IGiK), Warsaw, financially supported by the Polish Ministry of Science and Higher Educationsupported by the Hong Kong GRF RGC project 15218819: “The modernization of height datum in the Hong Kong territories”
文摘The determination of accurate orthometric or normal heights remains one of the main challenges for the geodetic community in Ethiopia.These heights are required for geodetic and geodynamic scientific research as well as for extensive engineering applications.The main objective of this study is to estimate the geoid-to-quasi geoid separation(GQS)in Ethiopia(ETH-GQS).Such separation would be required for the conversion between geoid and quasigeoid models,which is mandatory for the determination of accurate geodetic heights in mountain regions.The airborne free-air gravity anomalies and the topo-graphic information retrieved from the SRTM3(Shuttle Radar Topography Mission of a spatial resolution 3 arc-second)digital elevation model were used to compute the ETH-GQS model according to the Sjoberg's strict formula for the geoid-to-quasigeoid separation.The ETH-GQS was then validated using GNSS-levelling data as well as geoid heights determined from different Global Geopotential Models(GGMs),namely the EGM2008,EIGEN-6C4 and GECO.The results reveal that the standard deviation of differences between the geoid heights obtained from the EIGEN-6C4 model and the geometric geoid heights obtained from GNSS-levelling data were improved by~75%(i.e.from~24 to~6 cm)when considering GQS values obtained from the ETH-GQS.
文摘We investigate the evolution of stress fields during the supercontinent cycle using the 2D Cartesian geometry model of thermochemical convection with the non-Newtonian rheology in the presence of floating deformable continents.In the course of the simulation,the supercontinent cycle is implemented several times.The number of continents considered in our model as a function of time oscillates around 3.The lifetime of a supercontinent depends on its dimension.Our results suggest that immediately before a supercontinent breakup,the over-lithostatic horizontal stresses in it(referring to the mean value by the computational area)are tensile and can reach-250 MPa.At the same time,a vast area beneath a supercontinent with an upward flow exhibits clearly the over-lithostatic compressive horizontal stresses of 50-100 МРа.The reason for the difference in stresses in the supercontinent and the underlying mantle is a sharp difference in their viscosity.In large parts of the mantle,the over-lithostatic horizontal stresses are in the range of±25 MPa,while the horizontal stresses along subduction zones and continental margins are significantly larger.During the process of continent-to-continent collisions,the compressive stresses can approximately reach 130 MPa,while within the subcontinental mantle,the tensile over-lithostatic stresses are about-50 MPa.The dynamic topography reflects the main features of the su-percontinent cycle and correlates with real ones.Before the breakup and immediately after the disin-tegration of the supercontinent,continents experience maximum uplift.During the supercontinent cycle,topographic heights of continents typically vary within the interval of about±1.5 km,relatively to a mean value.Topographic maxima of orogenic formations to about 2-4 km are detected along continent-to-continent collisions as well as when adjacent subduction zones interact with continental margins.
文摘It is a well-known fact that the long-wavelength terrestrial geoid undulations are mainly attributed to deep mantle density heterogeneities,while more detailed features in the geoid geometry are associated with the topography and the lithospheric density structure.To enhance a gravitational signature of mantle density heterogeneities below the lithosphere,the gravitational contributions of topography and lithospheric density heterogeneities should be modelled and subsequently removed from Earth’s gravity field.The refined gravity field obtained after this numerical procedure is more suitable for a recovery of a mantle density structure(below the lithosphere).Following this idea,methods for a spherical harmonic analysis and synthesis of gravity field and lithospheric density structures are presented,and a theoretical relation between gravity field and mass density structure is formulated.Since a gravimetric recovery of inner density structure has a non-unique solution,we propose an alternative method based on a conversion of seismic velocities to mass densities.A forward modelling approach is then employed to find the mantle density configuration that generates the gravitational field that best approximates the corresponding refined gravitational field obtained from observed gravity field after subtracting the gravitational signal of the lithosphere.
基金the HK science project 1ZE8F: Remote-sensing data for studding the Earth’s and planetary inner structure
文摘The average density of 3300 kg m-3 is often attributed for the asthenosphere. In this study, we inspect this value by estimating the average value of the(upper) asthenosphere based on applying the gravimetric forward modelling of major known lithospheric density structures. The LITHO1.0 global seismic model of the lithospheric density structure is used for this purpose, while considering that the lithosphere-asthenosphere boundary(LAB) is rheological, conventionally taken at the 1300C isotherm,above which the mantle behaves in a rigid fashion and below which it behaves in a ductile fashion.According to our result, the average density of the upper asthenosphere is roughly 3400 kg m-3. This density value closely agrees with the corresponding average value 3371 kg m-3 computed based on an empirical density model provided in the Preliminary Reference Earth Model(PREM), while using the LITHO1.0 LAB depth data. We also demonstrate that the sub-lithospheric mantle gravity map exhibits mainly a thermal signature. The most prominent features in this gravity map are mid-oceanic spearing ridges marked by gravity lows, while oceanic subductions in the West Pacific are characterized by the most pronounced gravity highs.
基金funded by the Hong Kong Research Grants Council, Project 1-ZE8F: Remote-sensing data for studying Earth’s and planetary inner structure
文摘Despite practical limitations of isostatic theories to model the Moho geometry are well-known,gravimetric methods are often used in terrestrial studies of crustal structure in regions with a low seismic data coverage.Moreover,these methods are indispensable in planetary studies.Various gravimetric methods have been proposed.The Airy and Pratt theories are defined based on adopting a local compensation mechanism.The Vening Meinesz theory assumes a regional isostatic flexural model.The Vening Meinesz regional isostatic model generally describes a respond of the lithosphere to a load more realistically than the Airy model over continents.The Pratt method,on the other hand,better describes a compensation mechanism of the oceanic lithosphere.The application of a particular isostatic model also depends on applied numerical procedures,parameters for inversion,input data specifications,and many other aspects.In this study,we address some basic aspects by applying local and regional isostatic models for a Moho recovery.We also conduct a spectral analysis to assess a spectral resolution of gravity data that is optimal for a Moho recovery.Furthermore,we inspect the influence of low-degree spherical harmonics of gravity field on a Moho geometry.Gravimetric results are validated using seismic data at the European plate.Our results confirm a better performance of a regional compensation principle.We also demonstrate that a different thickness of the oceanic and continental crustal thickness should be taken into account as a priori information.Spectral analysis indicates that gravity data used for a Moho inversion should optimally have a spatial resolution between degrees 60 to 180.Results also show that low-degree spherical harmonics do not modify significantly the Moho geometry,particularly over regions with a relatively homogenous structure of deep mantle.
文摘The coefficients of the Global Gravitational Models(GGMs) define the external gravitational field of the Earth.In many geoscience applications and gravity interpretations these coefficients are routinely used to represent the geoid surface and related gravitational field quantities without taking into consideration the internal convergence domain for computing the gravitational field inside the Earth's masses.In this study we discuss this issue and present the numerical approach for computing the gravitational field quantities on the geoid.The proposed numerical approach utilizes spectral expressions for the gravimetric forward modelling of topographic mass density distribution and the indirect gravimetric modelling of mass density heterogeneities inside the geoid.In the numerical realization we demonstrate that for precise applications the differences between the potential values computed at the topographic surface and on the geoid should be taken into consideration especially in mountainous regions with complex geology as well as in polar regions,where these differences in absolute values reach 40m^2 s^(-2)(or more),which translates into vertical displacements between the geoid and the quasigeoid of about 4m.
基金financial support (No.214273812)supported by the Swedish National Space Board (SNSB) (No.76/10:1)
文摘The results of global and regional studies often show significant disagreement between the Moho depths determined using seismic and isostatic models. In this study, we estimate the differences between these two models in central Eurasia. The Vening Meinesz-Moritz(VMM) inverse problem of isostasy is utilized to determine the isostatic Moho depths. The estimated VMM Moho depths are then corrected for the sediment density contrast. The application of this correction improves the agreement between the isostatic and seismic Moho models. The existing discrepancies between the isostatic and seismic models are finally modeled by applying the non-isostatic correction, which accounts for the unmodelled mantle density heterogeneities and other geodynamic processes, which are not taken into account in classical isostatic models. Our results reveal that the non-isostatic correction still cannot fully describe mechanisms affecting the Moho geometry along the convergent continent-tocontinent tectonic plate boundaries occurring beneath Himalayas despite an overall good performance of the applied method.
