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Copyright of Journal of Geodesy & Geodynamics (1671-5942) is the property of Editorial Board Journal of Geodesy & Geodynamics and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Each point on the earth has a gravity and gravity potential value. Surfaces formed by connecting points with equal gravity potential values are called equipotential surfaces or level surfaces. Determination of gravity field of the earth and the geoid which is one of the earth’s equipotential surfaces is very important for physical geodesy. Gravity values measured on the physical earth are not directly included in studies; firstly, they must be converted into gravity anomalies. For this, in this study precise leveling, gravity and GPS measurements were made in the field. Heights (H) with precise leveling measurements, gravity values (g) with gravity measurements and geographical latitudes (j) with GPS measurements were recorded. Then, gravity reductions (free-air, Bouguer) were calculated at the points. The actual gravity g measured on Earth is not immediately directly comparable to the normal gravity of the ellipsoid surface. Gravity values must be reduced to the geoid. Since there are masses outside the geoid, reduction methods differ according to the way these topographic masses are handled. Bouguer gravity anomalies and gravity disturbances are derived. The gravity anomaly (Dg) is defined as the scalar difference between the Earth’s gravity on the geoid and normal gravity on the surface of the reference ellipsoid. Gravity disturbance (dg) is defined as the difference between the actual gravity magnitude measured on Earth and its equivalent normal gravity in the normal gravity field for the same point. The changes and magnitudes of the calculated quantities are compared. Changes such as the observed gravity and height data, observed gravity changes versus calculated normal gravity changes, normal gravity on ellipsoid versus geographic latitude, observed gravity changes versus latitude changes, Bouguer gravity anomaly and gravity disturbance versus latitude and elevation, free-air reduction and Bouguer gravity reduction versus latitude and elevation have been investigated.

Using the gravity/GNSS data of 318 stations observed in 2020, this paper optimizes the Bouguer and free-air gravity anomalies around the 2021 Yangbi Ms 6.4 Earthquake, inverses the litho-spheric density structure of the focal area, and obtains the distribution of isostatic additional force borne by the lithosphere. The results show that the Bouguer gravity anomaly in western Yunnan varies from -120 to -360 mGal. As a whole the anomalies are large in the north and small in the south, and the value in the source area of the 2021 Yangbi Ms 6.4 Earthquake is about -260 mGal. Significant lateral differences indicates that the crust around the great earthquake does not belong to a solid and stable tectonic unit. The lithosphere in the source area is basically in equilibrium, indicating that the occurrence of the great event is not relative to the lithospheric equilibrium, but to the differential movement of the crust in the horizontal direction. In addition, we obtain the teleseismic SKS phases of 51 stations. As a whole, the polarization direction of fast wave in western Yunnan is approximately vertical to the maximum gradient change direction of regional Bouguer gravity anomaly that reflects the change of Moho. [ABSTRACT FROM AUTHOR]

The Powell Basin is a small oceanic basin bounded by continental blocks that fragmented during break up of Antarctica from South America. This basin bounds the South Orkney Microcontinent to the east, the South Scotia Ridge to the north, and the Antarctic Peninsula to the west. The timing of its opening is poorly constrained due to the low amplitude of the oceanic spreading magnetic anomalies which hampers their identification and interpretation causing large uncertainties in proposed ages that range from the Late Eocene to Early Miocene. This basin has been extensively studied using a variety of geophysical methods including seismic, gravity and magnetics surveys intended to unveil the tectonic domains, the particularities of its magnetic anomalies and the understanding of the thermal regime in this area. Here, we show new magnetic and heat flow data integrated with other geophysical data from international databases (multichannel seismic data, bathymetry and free-air gravity), to analyze the thermal structure of the lithosphere of Powell Basin and the upper mantle dynamics as well as to discuss the probable causes of the abnormally small amplitudes of its magnetic anomalies. Our results show that the low magnetic anomaly amplitudes are not widespread but concentrate in the eastern and southwestern part of the basin. We propose that these small amplitudes result from the thermal dependency of magnetic rocks caused by an asthenospheric branch flow that penetrates the Powell Basin through the northern area.

Gülbahçe Fault Zone (GFZ) is a significant tectonic structure and seismic source for İzmir city and its surroundings. The major earthquakes occurred at the surroundings of GFZ are 2005 Seferihisar, 2020 Samos Island-Aegean Sea and 2021 Seferihisar earthquakes. In this study, the crustal thinning and tectonic stress distribution of GFZ and its surroundings were analyzed by using the new gravity and magnetic data. The correspondence analysis was applied by appraising together the magnetic and free-air anomaly data. Moreover, the average density of the subsurface structure was calculated by applying the Parasnis method. The deviatoric stress calculation was executed and the change of the physical properties that controls the gravity and magnetic anomaly of the crustal structure of the GFZ and its surroundings from north to south was investigated. Therefore, the average density values, which were obtained from gravity anomalies, were computed as 2.59 gr / cm3 in the north of the study region and it decreased to 2.06, 1.8 and 1.49 gr / cm3 towards to the south. The freeair gravity anomaly values were between 0-70 mGal and the magnetic anomalies were between -450-150 nT. The deviatoric stress values were between -0.2-0.1 MPa.

Sichuan-Yunnan and its adjacent area are the main places where the eastward material flow from the Tibet Plateau escapes and spins to SE-SSE with active geological formations and frequent seismic activity. The study of the crustal structure and material transport characteristics of this region is important to reveal the mechanism of earthquake occurrence here. It can also deepen the understanding of the tectonic activity mechanism of the southeastern margin of the Tibet Plateau. This paper summarizes recent achievements and important advances in the study of regional Bouguer gravity anomaly and crustal density structure in Sichuan-Yunnan and its adjacent areas. (1) Bouguer gravity anomaly information of eight gravity profiles was obtained by using gravity and GNSS positioning techniques, and the basic framework of crustal density structure of the southeastern margin of the Qinghai-Tibet Plateau and its adjacent areas was constructed by using gravity inversion techniques constrained by various geophysical observations. (2) New gravitational evidence of clockwise rotation of the Sichuan-Yunnan rhomboid massif was found through the separation and extraction of gravity signals by wavelet decomposition and normalized gradient. (3) Through the scientific research of the Yushu MS7.1, Lushan MS7.0 and Ludian MS6.5 earthquakes, we have deepened our understanding of the seismogenic tectonic environment in the southeastern margin of the Tibet Plateau. There are significant differences in density distribution on sides of the left-slip shear fault zone (Ganzi-Yushu, Xianshuihe and Xiaojiang) and the large thrust nappe structure (Longmen Shan), both of which are regional seismicity-controlled faults. The seismicity of the Ludian earthquake zone may be related to the eastward migration of the Xiaojiang fault zone. The nappe characteristics and detachment structure of the Longmenshan fault zone may be an important factor in the occurrence of the Lushan earthquake at the Houshan fault.

Residual Bouguer gravity anomaly inversion can be used to imaging for local density structures or to interpret near-surface anomalous mass distribution. The reasonable prior information is the crucial recipe for obtaining a realistic geological inversion result, especially for the ill-posed geophysical inversion problem. The conventional strategies introduce the prior constraints or joint multidisciplinary information in object function as regularization, and then use some optimization algorithm to minimize the object function. This process is called model-driven approach and is usually time-consuming. In recent years, the rapid development of machine learning technology has provided new solutions for solving geophysical inversion problems. Machine learning methods can reduce the dependence on prior information in the inversion process through setting special training datasets, and the time consumption of an inversion process executed by the trained model can be shortened by several orders of magnitude, which is conducive to fast inversion for the same type of application scenarios. In this study, we were inspired by the U-net model and develops the GV-Net (Gravity voxels inversion network) model using the convolutional neural network for the inversion of residual gravity anomalies. We first discussed the effects of different loss functions on the convergence speed of model training and prediction accuracy. Then, we analyzed the robustness of our model by changing noise levels of the datasets. At last, we employed this model in a real scenario. The results have demonstrated that the GV-Net model has the ability to deal with specific inverse problems by predefined training datasets.

The Indian Ocean witnessed numerous seismic and tectonic activities, making the region an intricate seismotectonic and seismogenesis potential zone. The high-resolution global satellite-derived Bouguer Gravity Anomaly (BGA) data was used for comparative analysis of the structural patterns of six Indian Ocean basins. THETA (TA) edge enhancement and upward continuation techniques were used to study the Indian Ocean crustal structure by mapping both short and longwave lineaments. Rayleigh wave group velocity (RWGV) tomography at different time periods indicates pseudo depths of ~30 km and ~50 km are suitable for the upward continuation (UC) of BGA that perhaps lead to understanding different contrasting boundaries in their TA map. The UC of the BGA was generated at 60 km and 100 km for highlighting different lineaments/structures/contrasting geological boundaries at the depths of about 30 km and 50 km. The orientation of TA lineaments shows NNE-SSW, NE-SW, and EES-WWN structural trends. The statistical analysis of the lineaments shows that the Mid-Indian Basin constitutes the highest values of lineament number, lineament density (LD), circular variance (CV) and circular standard deviation (CSD), indicating a slow tectonic deformation over a long period. The Somalian Basin shows the minimum lineament number and lowest LD with high values of CV and CSD, indicating a relatively stable tectonic set-up compared to other Indian Ocean basins. The Arabian and Bay of Bengal basins offer relatively low lineament statistics than the Mid-Indian, Wharton and Madagascar basins, indicating frequent tectonic activity. [ABSTRACT FROM AUTHOR]

The Ying-Qiong Basin is located on the northwestern margin of the South China Sea and at the junction of the South China Block and the Indochina Block. It is characterized by complex geological structures. The existing seismic data in the study area is sparse due to the lack of earthquake activities. Because of the limited source energy and poor coverage of seismic data, the knowledge of deep structures in the area, including the spatial distribution of deep faults, is incomplete. Contrarily, satellite gravity data cover the entire study area and can reveal the spatial distribution of faults. Based on the wavelet multi-scale decomposition method, the Bouguer gravity field in the Ying-Qiong Basin was decomposed and reconstructed to obtain the detailed images of the first- to sixth-order gravitational fields. By incorporating the known geological features, the gravitational field responses of the main faults in the Ying-Qiong Basin were identified in the detailed fields, and the power spectrum analysis yielded the depths of 1.4, 8, 15, 26.5, and 39 km for the average burial depths of the bottom surfaces from the first- to fifth-order detailed fields, respectively. The four main faults in the Yinggehai Basin all have a large active depth range: fault A (No.1) is between 5 and 39 km, fault B is between 26.5 and 39 km, and faults C and D are between 15 and 39 km. However, the depth of active faults in the Qiongdongnan Basin is relatively shallow, mainly between 8 and 26.5 km. [ABSTRACT FROM AUTHOR]

Calculated Bouguer gravity anomalies from the Andean orogenic belt interpreted as derived from regional gravity data to aid understanding of the lithospheric structure and tectonic evolution of the belt. These anomalies reveal lithospheric structures distributed throughout the belt, including linear and circular structures. NE‐trending structures reflect sinistral transpression across the northern part of the belt, and NW‐trending structures represent dextral transtension in the southern part. These results are supported by gravity‐anomaly patterns that demonstrate mantle flow in a trench‐parallel direction both northward and southward away from the stagnation band that is beneath the subducting Nazca slab. This mantle flow has served as an important driving force in the evolution of the Andean orogenic belt. Features of the modified tectonic model of the Andean orogenic belt are consistent with the spatial variation in and interpretation of Bouguer gravity anomalies. [ABSTRACT FROM AUTHOR]

We introduce a novel approach to three-dimensional gravity inversion in the presence of the sediment-basement interface with a strong density contrast. This approach makes it possible to incorporate the known information about the basement depth in the inversion. It also allows the user to determine the depth-to-basement in the initial inversion phase. One can then use this interface to constrain the final inversion phase. First, the inversion generates the depth-to-basement model based on the 3D Cauchy-type integral representation of the gravity field. Then, in the second phase, full 3D voxel-type inversion applies the depth-to-basement model determined in the first phase as an a priori constraint. We use this approach to the 3D inversion of the Bouguer gravity anomaly data observed in Utah, USA. The results of inversion generated a 3D density model of the top layers of the earth's crust, including unconsolidated sediments and the top of the crystalline basement. [ABSTRACT FROM AUTHOR]

Fault geometry, kinematics, geophysics, the tectonic stress field and tectonic evolution of the Zhouwang fault in the southern Jiangnan tectonic transition zone of the Lower Yangtze region, eastern China are examined. Field observations show the fault is composed of a series of nearly E–W trending, N–S dipping faults, and four stages of tectonism (sinistral strike‐slip, thrust nappe, normal fault, and dextral strike‐slip) developed in turn. Geophysical data show that the fault trends almost linearly E–W along a flat, steep gravity gradient at shallow depth, with distinct gravity anomalies to the north and south and different in the north and south. Also, the deep part is characterized by northward dip and a gradual slowing down. Tectonic stress field analysis indicates that the fault experienced four tectonic movements: NNE–SSW compression, NNW–SSE compression, NEE–SWW extension, and E–W compression. Combined with regional tectonic background and previous research results, this indicates that: (1) the Zhouwang fault experienced sinistral strike‐slip movement during the Indosinian Period (260–200 Ma); (2) thrust nappes developed during the early Yanshanian Period (163–145 Ma); (3) a normal fault occurred in the late Yanshanian Period (125–65 Ma); and (4) dextral strike‐slip movement occurred in the Himalayan Period (ca. 50–37 Ma). The results reveal the tectonic evolution of the fault during Mesozoic deformation in the area, and also reveal the geological evolution and tectonic transformation of the Lower Yangtze region, which is key to our understanding of intracontinental deformation in eastern China. [ABSTRACT FROM AUTHOR]

To investigate the crustal structure of the Bitlis–Pötürge Suture Zone and its vicinity, gravity complete Bouguer anomaly data obtained from the World Gravity Map (WGM2012) were analyzed using Spectral Depth Estimation, Frequency Filtering, Total Horizontal Derivative (THD) and Parker-Oldenburg inversion techniques. The THD method was applied to the data after band-pass filtering of Bouguer gravity data in order to image the discontinuities in the basement levels which are determined by the critical wave numbers determined from the amplitude spectrum of gravity complete Bouguer anomaly data. The maximum amplitude values of the THD were used to reveal the discontinuities caused by the density difference in the BPSZ and its vicinity. In addition, the basement upper surface topography of the region was calculated and mapped with the inverse solution. The presence of an uplift area that is an antiroot in the south of the study area, as well as the subsidence that is a root of the BPSZ was determined. As a result of the inverse solution, it was determined that the depth of the sedimentary basement in the south approximately reaches 6 km and was detected the presence of approximately 7 km thick sedimentary deposit on the BPSZ. Basement upper surface depth is calculated as 8 km in average under the Eastern Anatolian High Plateau. [ABSTRACT FROM AUTHOR]

China is located in the southeast of the Eurasian Plate and is subject to the effects of subducting, squeezing and collision by the Pacific Plate, Philippine Plate and Indian Ocean Plate. It has exceptional geotectonic structure. Based on the satellite gravity data with high precision, high resolution and ample geophysical information, combined with geological data, by using satellite gravity potential field and its full tensor gradient, this paper studies the distribution characteristics of gravity anomalies and the identification of tectonic boundaries in East Asia. Results suggest that the Bouguer gravity anomaly in eastern China reduces gradually from east to west, mostly in the direction of NNE; in the western, it reduces gradually in a wave mode from north to south, mainly in the directions of NW and NWW. In general, the stress field reduces gradually from west to east, and the tectonic of stress field in western China is complex. The change in eastern China is relatively simple. In addition to the above study results, we update the extension route of Red River fault zone and deduce the tectonic unit boundary between the North China and South China active tectonic block regions. This paper identifies in East Asia 6 primary active tectonic blocks, 22 secondary active tectonic blocks, 7 tertiary active tectonic blocks, and the 20 active tectonic block boundary zones. The results of this study can improve the understanding of gravity anomalies and boundary structures in China and surrounding regions, and provide certain geophysical supports for geological structure analysis and crustal dynamic process. [ABSTRACT FROM AUTHOR]

Abstract Dense gravity/GPS measurements collected around the southwest margin of the Ordos Block in 2014 and 2017 were used to obtain three gravity anomaly profiles across the Liupan Shan Mountains. The Liupan Shan is located in a Bouguer gravity anomaly (BGA) transitional zone; however, low BGAs to the west do not correspond to the topography of the Liupan Shan in that region, with a maximum offset of approximately 34 km. This offset is also found in inverted crustal density structures whereby the interface between the upper and lower crust is notably concave to the west of the Liupan Shan. Flexural analysis indicates that the effective elastic thickness is 5 km and the uplift is caused by oblique subduction of surface materials. According to this uplift mechanism, the offset suggests that Liupan Shan migrated eastward following partial isostatic compensation. Therefore, we suggest that Liupan Shan has experienced an uplift–compensation–uplift tectonic process.

This study focuses on interpreting Bouguer gravity anomalies by two-sided fault structures. Faults have prime concerns for hazardous zones, mineralized areas, and hydrocarbon systems. The proposed scheme is done through the following steps: first, it utilizes the residual moving average anomalies estimated from the Bouguer gravity anomalies using several window lengths. Second, each residual anomaly is interpreted using the particle swarm. Third, calculate the average value for all interpreted anomalies. Fourth, the average values for the fault parameters are utilized to build the forward gravity model, which is compared with the true ones. The efficiency of this method has been studied by applying it to a synthetic example with different levels of impeded noise (0%, 5%, and 10%). Gravity data for fault structures were investigated from Egypt. It was found that the obtained results are in good agreement with the previously published studies. [ABSTRACT FROM AUTHOR]

We constructed a 3-D coherent velocity model by extracting the common features from recent tomographic models and utilized the forward gravity modeling and wavelength filtering method to explore the depth origin of the gravity anomalies, such as the long-puzzling Puli Bouguer gravity low in west central Taiwan. The similarity analysis among the tomographic models shows the velocity structure most reliable in the depth range from 5 to 35 km. Using empirical velocity-to-density relationships, we show that the calculated Bouguer gravity anomalies from the 3-D coherent velocity model could well reproduce the gravity observations in general. Through filtering process, we separated the Bouguer gravity anomalies into shallow (0 - 17 km) and deep gravity effects (17 - 60 km) for structural exploration. The results show that the shallow gravity effect mainly reflects the surface geological features with low gravity values for unconsolidated sediments in plain and foothill areas to the west and high values for exhumed metamorphic rocks in the mountainous areas to the east. The deep gravity effect reveals the lowest gravity anomalies located in central Taiwan beneath the western Central Range. The Puli Bouguer gravity low is likely a coincident feature arising from the mismatch of the shallow and deep gravity effects due to the N-S trending undulation of crustal thickness beneath the western plain/foothill. This crustal thickness undulation may be related to the rifting activity of Taihsi and Tainan basins in the past. Better discriminating the gravity effect from shallow and deep crust could help better understand the architecture and tectonic process of the Taiwan orogenic belt. [ABSTRACT FROM AUTHOR]

We introduce a novel approach to three-dimensional gravity inversion in the presence of the sediment-basement interface with a strong density contrast. This approach makes it possible to incorporate the known information about the basement depth in the inversion. It also allows the user to determine the depth-to-basement in the initial inversion phase. One can then use this interface to constrain the final inversion phase. First, the inversion generates the depth-to-basement model based on the 3D Cauchy-type integral representation of the gravity field. Then, in the second phase, full 3D voxel-type inversion applies the depth-to-basement model determined in the first phase as an a priori constraint. We use this approach to the 3D inversion of the Bouguer gravity anomaly data observed in Utah, USA. The results of inversion generated a 3D density model of the top layers of the earth’s crust, including unconsolidated sediments and the top of the crystalline basement.

Rifting of continental lithosphere leading to oceanic basins is a complex process conditioned by different factors such as the rheology and thermal structure of the underlying lithosphere, as well as underlying asthenospheric dynamics. All these processes, which finally lead to oceanic domains, can better be recognized in small oceanic basins. Powell Basin is a small oceanic basin bounded to the north by the South Scotia Ridge, to the east by the South Orkney Microcontinent, and to the west by the Antarctic Peninsula. It was formed between the Oligocene and Miocene, however, its age is not well defined, among other reasons due to the small amplitude of its spreading magnetic anomalies. This basin is an ideal framework to analyze the different rifting and spreading phases, which leads from continental crust to the formation of an oceanic domain through different extensional regimes. To identify the different boundaries during the formation of Powell Basin from the beginning of the rifting until the end of the spreading, we use different data sources: magnetic, gravity, multichannel seismic profiles and bathymetry data. We use seismic and bathymetry data to estimate the Total Tectonic Subsidence. Total Tectonic Subsidence has proven to be useful to delineate the different tectonic regimes present from early rifting to the formation of oceanic seafloor. This result together with magnetic data has been used to delimit the oceanic domain and compare with previous authors’ proposals. This method could be applied in any other basin or margin to help delimiting its boundaries. Finally, we analyze the role that an asthenospheric branch intruding from the Scotia Sea played in the evolution of the magnetic anomaly signature on an oceanic basin.

Nowadays, satellite technology has developed significantly. Geodesy satellites such as Grace and Grace-FO can be used for subsurface mapping. The mapping is in the form of detection of the plate details, faults, and regional geodynamic conditions. This study aims to detect plate and faults from space geodesy using the gravity disturbance and Bouguer gravity anomaly parameter. The study area is in the Sunda Strait. Gravity disturbance is one of the gravity model parameters. Gravity disturbance is the gravitational potential of the topography expressed by the spherical harmonic model and the topographic effect by Barthelmes's calculations. Gravity disturbance can visualize subsurface conditions. Bouguer gravity anomaly is needed to get the condition on subsurface objects. This parameter visualizes subsurface conditions in the form of rocks and non-rocks. These conditions can distinguish oceanic crust and continental crust. Gravity contours are needed to obtain plate and faults predictions. The results obtained are validated patterns and shapes with plate and faults secondary data. The tolerance used in this validation is 80%. The gravity disturbance parameter obtained a value of 83% in verifying the accuracy of assessment in plate and faults detection. The Bouguer gravity disturbance parameter obtained a verification value of accuracy assessment in plate detection but 65% in faults detection. This accuracy assessment uses pattern and texture parameters in detecting the similarity of two or more images. This plate and faults detection results are more detailed and can be used for geophysical, geological, earthquake, and earth dynamics applications.

The Gulf of Cadiz has been affected by a long and complex geodynamic evolution. The lithospheric structure is poorly understood in this region, and it also shows a diffuse seismicity, spanning over a broad area. The Canary Archipelago has been extensively studied. Nevertheless, there are fundamental topics that are still under debate. No studies have addressed or suggested the possibility of a plausible geodynamic connection between both remarkable locations. In this study we integrate total tectonic subsidence (TTS), Curie point depth (CPD), Bouguer gravity anomaly, and seismic information. TTS shows the existence of a basement bulge in the area of the Canary Archipelago that extends to the north, and in the area of Madeira Island, which extends in a more subtle way to the north, too. Likewise, the CPD reaches the shallowest values in the same location at the Canary Archipelago. These two aspects suggest a cause-effect relationship between TTS and CPD at this specific area. Gravity data and CPD show a linear feature, which links the NW of the Canary Archipelago and the Gulf of Cadiz. The data we manage in this work show remarkable clues as: (a) the absence of a similar signal in the TTS, (b) the fact that CPD it is rather constant along this track, (c) CPD amplitude also almost doubles the values obtained on Canary Archipelago, and (d) the existence of a clear correlation between seismogenic depths and CPD, which points to the existence of a correlation between seismicity and the thermal architecture of the lithosphere. All of these evidences support the presence of a lithospheric thinning between Canary Islands and Gulf of Cádiz area, and in turn, the presence of an asthenospheric channel which feeds and alter locally the Eurasia–Africa Plate Boundary. [ABSTRACT FROM AUTHOR]

To determine the lateral and vertical variations in crustal structure and their influence on the seismicity of the Western North China Craton, the Trans -North China Orogen, and the surrounding regions, the wavelet multi-scale structures, Moho depth, crustal density structures, and isostatic state are modelled using Bouguer gravity anomaly data, topography, and earthquake focal mechanisms. We obtained homogeneous crustal densities and deviations of <1 ​km between the crustal thicknesses estimated from the isostatic model and those inverted from the Bouguer gravity anomalies in the Ordos Block, the Inner Mongolia Suture Zone, the Sichuan Basin, and the Jizhong Depression. These results provide new evidence for relatively simple and stable continental crustal structures, and indicate that these regions will remain stable in both the vertical and lateral directions. The Hetao Graben, Yinchuan Graben, Weihe Basin, and Shanxi graben system have heterogeneous crustal densities and are isostatically over-compensated. In contrast, the crust beneath the Yinshan Uplift, Lvliang Uplift, and northern and central Taihang Uplift is thin and under-compensated. The heterogeneous crustal densities and non-isostatic state beneath the Tibetan Plateau and Qinling Central China Orogen indicate that these two blocks are unstable in the vertical and lateral directions. Although Cenozoic deformation of the North China Craton is thought to be driven by lithospheric stresses related to the India-Eurasia collision and Pacific slab retreat in South East Asia, we suggest that gravitational potential energy created by the heterogeneous crustal structure modulates these first-order forces. The results of this study could constrain the causes of seismicity in systems surrounding the Ordos Block. Image 1 • We have determined the density for four crustal layers and of the upper mantle of the Ordos Block. • The Ordos Block is rigid, isostatically stable, whereas the surrounding regions are not in isostatic equilibrium. • The heterogeneous isostatic state of the smaller regions modulates the regional stresses causing the seismicity. [ABSTRACT FROM AUTHOR]

Dense gravity/GPS measurements collected around the southwest margin of the Ordos Block in 2014 and 2017 were used to obtain three gravity anomaly profiles across the Liupan Shan Mountains. The Liupan Shan is located in a Bouguer gravity anomaly (BGA) transitional zone; however, low BGAs to the west do not correspond to the topography of the Liupan Shan in that region, with a maximum offset of approximately 34 km. This offset is also found in inverted crustal density structures whereby the interface between the upper and lower crust is notably concave to the west of the Liupan Shan. Flexural analysis indicates that the effective elastic thickness is 5 km and the uplift is caused by oblique subduction of surface materials. According to this uplift mechanism, the offset suggests that Liupan Shan migrated eastward following partial isostatic compensation. Therefore, we suggest that Liupan Shan has experienced an uplift–compensation–uplift tectonic process. [ABSTRACT FROM AUTHOR]

The study area comprises the NE–SW trending Burdur Basin situated at the tectonically active northeastern part of the Fethiye–Burdur Fault Zone (FBFZ), SW Turkey. The basin demonstrates a half graben geometry hosting lacustrine sedimentary deposits from the Late Miocene onward and is bounded by normal faults on its southern side namely the Burdur Fault Zone. In this study, gravity anomalies over the Burdur sedimentary basin were inverted for the first time in terms of mapping its basement relief. The algorithm used for inverting the gravity anomalies provides accuracy depth estimates by incorporating an exponential increase in density with depth at its inversion procedure. Thus the obtained depth configuration yields also a major improvement on the results of depth content of the sedimentary infill reported previously by other studies that used a constant density contrast in their interpretation. Along the east of Burdur Fault from south to north, the basement depth to the southern end of Burdur Basin is ca 1.8 km and gets shallower to ca 0.6 km towards the north around the Burdur city. The deepest section of the basin is ca 3.2 km to the western side of the Burdur Fault close the southern end of the Burdur Lake. Towards north, out of the depression area of the Burdur Basin, the sedimentary infill is about in range of 0.4–1.2 km. The lateral limits of the basin structure have also been outlined by a recent edge detection method based on the logistic function of the total horizontal gradient (LTHG). The LTHG map related to the Burdur Basin shows maximal amplitudes trending NE–SW as two major lines that clearly delineates the segments of the Burdur Fault Zone to the S-SE of Burdur Lake. The inverted basin depth model by a cross-section perpendicular to the regional strike of the basin represents two-step depositional area of the sedimentary fills confirming a geometry of a half graben structure. [ABSTRACT FROM AUTHOR]

The Baihetan hydropower station is the second largest hydropower station worldwide. It began to store water in April 2021. We conducted a dense hybrid gravity and GNSS survey at 223 stations, obtained the free-air and Bouguer gravity anomalies, inversed the lithospheric density structure, and calculated the isostatic additional force (IAF) borne by lithosphere in the reservoir area. Moreover, we studied the gravity change and Coulomb stress change around the Baihetan reservoir due to impoundment. The main findings are the following. (1) Hybrid gravity and GNSS observations significantly improved the spatial resolution of the gravity field, and the maximum improvement reached up to 150 mGal. (2) A new method for risk assessment of reservoir-induced earthquakes is proposed from the perspective of lithospheric equilibrium. It was found that there is an IAF of −30 MPa at approximately 20 km upstream of the Baihetan dam, and the risk of a reservoir-induced earthquake in this area warrants attention. (3) It was found that the Coulomb stress variation on the Xiaojiang fault near Qiaojia at a depth of 10 km exceeded the threshold for inducing an earthquake (0.1 bar).

Integral and differentiation are two mathematical operations in modern calculus and analysis which have been commonly applied in many fields of science. Integration and differentiation are associated and linked as inverse operation by the fundamental theorem of calculus. Both integral and differentiation are defined based on the concept of additive Lebesgue measure although various generations have been developed with different forms and notations. Fractals can be considered as geometry with fractal dimension (e.g., non-integer) which no longer possesses Lebesgue additive property. Accordingly, the ordinary integral and differentiation operations are no longer applicable to the fractal geometry with singularity. This paper introduces a recently developed concept of fractal differentiation and integral operations. These operations are expressed using the similar notations of the ordinary operations except the measures are defined in fractal space or measures with fractal dimension. The calculus operations can be used to describe the new concept of fractal density, the density with fractal dimension or density of matter with fractal dimension. The concept and methods are also applied to interpret the Bouguer anomaly over the mid-ocean ridges. The results show that the Bouguer gravity anomaly depicts singularity over the mid-ocean ridges. The development of new calculus operations can significantly improve the accuracy of geodynamic models. [ABSTRACT FROM AUTHOR]

• Potential field inversion and 3-D visualization are revealled from Bouguer gravity data. • There is a low-density body in the lower crust of the Longmen Shan fault zone. • The density structure of the fault zone is complex and the Moho depth is an abrupt change. • A counter-clockwise rotation of the Sichuan Basin is shown by source edge detection. • The rotation is one of the important dynamic backgrounds for the 2008 Wenchuan earthquake. In this paper, we adopt the methods of Euler deconvolution, source parameter imaging (SPI), source edge detection (SED), inversion of the Moho interface, and three-dimensional (3-D) visualization of the crust for the inversion of the Bouguer gravity anomaly of the Longmen Shan and adjacent regions, analyse the 3-D structure, deep structure, and deformation characteristics of the crust, and investigate the cause of the deep seismogenic environment and the Wenchuan earthquake. The research indicates that the Longmen Shan fault zone is characterized by a NE-trending large gravity gradient zone with a gradient value that exceeds 3.7 mGal/km. The Euler diagram reveals that there is a low-density anomaly body in the lower crust of the Longmen Shan fault zone that is a NE-trending narrow strip approximately 100 km long, 65 km wide and 35–50 km deep. The Songpan-Garze block on the northwest side does not have the material source and pathway of the lower crustal flow in the Euler diagram. Therefore, the low-density anomaly body is related to the upwelling of the mantle heat flow below it. In comparison with the Songpan-Garze and Chuan-Dian blocks, the strength of the field source of the Sichuan Basin is high (−175 to −96 mGal), indicating that the average density of the crust is high, and the depth of the field source is stable at 15–25 km. The SED map reveals that there has been counter-clockwise rotation of the Sichuan Basin since the Cenozoic. The SPI shows that the Longmen Shan fault zone is narrowly located in the beaded linear "shallow source anomaly" belt and that its two sides are stable areas of deep source anomalies, indicating characteristics of crustal hardness on both sides, so that it is prone to accumulate stress and strain. The Moho depth across the Longmen Shan descends from 41.8 km in the east to 61.6 km in the west. The complex intersection of high-density and low-density anomaly bodies appears in the deep part of the Longmen Shan tectonic belt, and the crustal structure is fragmented. Moreover, there is a strong abrupt change in the Moho topography, and therefore, this region has a deep environment for the genesis of strong earthquakes. The dextral strike-slip movement of the Longmen Shan fault zone is caused by the movement of the Songpan-Garze block and the counter-clockwise rotation of the Sichuan Basin; therefore, the counter-clockwise rotation of the Sichuan Basin is one of the important dynamic backgrounds for the 2008 Wenchuan earthquake. [ABSTRACT FROM AUTHOR]

Urban faults in Shenzhen are potential threat to city security and sustainable development. To improve the knowledge of the Shenzhen fault zone, interpretation and inversion of gravity data were carried out. Bouguer gravity covering the whole Shenzhen City was calculated with a 1-km resolution. Wavelet multi-scale analysis (MSA) was applied to the Bouguer gravity data to obtain the multilayer residual anomalies corresponding to different depths. In addition, 2D gravity models were constructed along three profiles. The Bouguer gravity anomaly shows an NE-striking high-low-high pattern from northwest to southeast, strongly related to the main faults. According to the results of MSA, the correlation between gravity anomaly and faults is particularly significant from 4 to 12 km depth. The residual gravity with small amplitude in each layer indicates weak tectonic activity in the crust. In the upper layers, positive anomalies along most of faults reveal the upwelling of high-density materials during the past tectonic movements. The multilayer residual anomalies also yield important information about the faults, such as the vertical extension and the dip direction. The maximum depth of the faults is about 20 km. In general, NE-striking faults extend deeper than NW-striking faults and have a larger dip angle. [ABSTRACT FROM AUTHOR]

The North–South Seismic Belt (NSSB) is a Chinese tectonic boundary with a very complex structure, showing a sharp change in several geophysical field characteristics. To study these characteristics and their relationship with the distribution of earthquakes and faults in the study area, we first analyze the spatial gravity anomaly to achieve the Bouguer gravity anomaly (EGM2008 BGA) and the regional gravity survey Bouguer gravity anomaly. Next, we ascertain the Moho depth and crustal thickness of the study area using interface inversion with the control points derived from the seismic and magnetotelluric sounding profiles achieved in recent years. In this paper, we summarize the relief, trend, Moho gradient, and crustal nature, in addition to their relationship with the distribution of earthquakes and faults in the study area. The findings show that earthquakes with magnitudes greater than Ms7.0 are mainly distributed in the Moho Bouguer anomaly variation belt and faults. The results of the study are important for future research on tectonic characteristics, geological and geophysical surveys, and seismicity patterns.

Abstract: Curie depths beneath Greenland are revealed by spectral analysis of data from the World Digital Magnetic Anomaly Map 2. A thermal model of the lithosphere then provides a corresponding geothermal heat flux map. This new map exhibits significantly higher frequency but lower amplitude variation than earlier heat flux maps and provides an important boundary condition for numerical ice‐sheet models and interpretation of borehole temperature profiles. In addition, it reveals new geologically significant features. Notably, we identify a prominent quasi‐linear elevated geothermal heat flux anomaly running northwest–southeast across Greenland. We interpret this feature to be the relic of the passage of the Iceland hotspot from 80 to 50 Ma. The expected partial melting of the lithosphere and magmatic underplating or intrusion into the lower crust is compatible with models of observed satellite gravity data and recent seismic observations. Our geological interpretation has potentially significant implications for the geodynamic evolution of Greenland. [ABSTRACT FROM AUTHOR]

b - value is one of the most significant seismic parameters for describing the seismicity of a given region at a definite time window. In this study, high-resolution map of the Gutenberg-Richter b-value, seismic moment-release, Bouguer gravity anomaly and fault-plane solutions containing faulting styles are analyzed in the Indo-Burma ranges of northeast India using the unified and homogeneous part of the seismicity record in the region (January 1964–December 2016). The study region is subdivided into few square grids of geographical window size 1° × 1° and b-values are calculated in each square grid. Our goal is to explore the spatial correlations and anomalous patterns between the b-value and parameters like seismic moment release, Bouguer gravity anomaly and faulting styles that can help us to better understand the seismotectonics and the state of present-day crustal stress within the Indo-Burma region. Most of the areas show an inverse correlation between b-value and seismic moment release as well as convergence rates. While estimating the b-value as a function of depth, a sudden increase of b-value at a depth of 50–60 km was found out and the receiver function modeling confirms that this depth corresponds to the crust-mantle transition beneath the study region. The region is also associated with negative Bouguer gravity anomalies and an inverse relation is found between Gravity anomaly and b-value. Comparing b-values with different faulting styles, reveal that the areas containing low b-values show thrust mechanism, while the areas associated with intermediate b-values show strike-slip mechanism. Those areas, where the events show thrust mechanism but containing a strike-slip component has the highest b-value. [ABSTRACT FROM AUTHOR]

Variation in spatial density of Quaternary volcanic vents, and the occurrence of vent clusters, correlates with boundaries in Proterozoic crust in the Springerville volcanic field (SVF), Arizona, USA. Inverse modeling using 538 gravity measurements shows that vent clusters correlate with gradients in the gravity field due to lateral variation in crustal density. These lateral discontinuities in the crustal density can be explained by boundaries in the North American crust formed during Proterozoic accretion. Spatial density of volcanic vents is low in regions of high-density Proterozoic crust, high in areas of relatively low density Proterozoic crust, and is greatest adjacent to crustal boundaries. Vent alignments parallel these boundaries. We have developed 2-D and 3-D numerical models of magma ascent through the crust to simulate long-term, average magma migration that led to the development of vent clusters in the SVF, assuming that a viscous fluid flow through a porous media is statistically equivalent to magma migration averaged over geological time in the full field scale. The location and flux from the uniform magma source region are boundary conditions of the model. Changes in model diffusivity, associated with changes in the bulk properties of the lithosphere, can simulate preferential magma migration paths and alter estimated magma flux at the surface, implying that large-scale crustal structures, such as inherited tectonic block boundaries, influence magma ascent and clustering of volcanic vents. Probabilistic models of volcanic hazard for distributed volcanic fields can be improved by identifying crustal structures and assessing their impact on volcano distribution with the use of numerical models. [ABSTRACT FROM AUTHOR]

In this study, the GNSS and gravity data were processed and compared together for examining the continental structures of the Western Anatolia region which has very complicated tectonism. The GNSS data of three national projects were processed and GNSS velocities were found as approximately 25 mm per year towards southwest with respect to the Eurasia fixed frame. In order to investigate the interplate motions of the region, the Anatolian and Aegean block solutions were calculated and the differences in directions and amplitudes of velocities were observed particularly in the Anatolian block solution. Due to the Anatolian block solutions, the study area was grouped into three regions and compared with the tectonic structures as the first time for Western Anatolia by this study. Additionally, W-E and N-S relative GNSS solutions were obtained for observing the possible tectonic borders of the study area. Besides, 2nd order horizontal derivative and low-pass filter methods were applied to Bouguer gravity anomalies and the results of the gravity applications and the changes on crustal-mantle interface were compared with the GNSS horizontal velocities. [ABSTRACT FROM AUTHOR]

Yushu Ms7.1 earthquake occurred on the Ganzi-Yushu fault zone, across which we carried out a joint relative-gravity and seismic-reflection survey, and then performed a gravity inversion constrained by the seismic-reflection result. Based on the data of complete Bouguer gravity anomaly and seismic reflection, we obtained a layered interface structure in deep crust down to Moho. Our study showed that the inversion could reveal the interfaces of strata along the survey profile and the directions of regional faults in two-dimension. From the characteristics of the observed topography of the Moho basement, we tentatively confirmed that the uplift of eastern edge of Qinghai-Tibet plateau was caused by the subduction of the Indian plate.

Traditionally, geoid determination is applied by Stokes’ formula with gravity anomalies after removal of the attraction of the topography by a simple or refined Bouguer correction, and restoration of topography by the primary indirect topographic effect (PITE) after integration. This technique leads to an error of the order of the quasigeoid-to-geoid separation, which is mainly due to an incomplete downward continuation of gravity from the surface to the geoid. Alternatively, one may start from the modern surface gravity anomaly and apply the direct topographic effect on the anomaly, yielding the no-topography gravity anomaly. After downward continuation of this anomaly to sea-level and Stokes integration, a theoretically correct geoid height is obtained after the restoration of the topography by the PITE. The difference between the Bouguer and no-topography gravity anomalies (on the geoid or in space) is the “secondary indirect topographic effect”, which is a necessary correction in removing all topographic signals. In modern applications of an Earth gravitational model (EGM) in geoid determination a topographic correction is also needed in continental regions. Without the correction the error can range to a few metres in the highest mountains. The remove-compute-restore and Royal Institute of Technology (KTH) techniques for geoid determinations usually employ a combination of Stokes’ formula and an EGM. Both techniques require direct and indirect topographic corrections, but in the latter method these corrections are merged as a combined topographic effect on the geoid height. Finally, we consider that any uncertainty in the topographic density distribution leads to the same error in gravimetric and geometric geoid estimates, deteriorating GNSS-levelling as a tool for validating the topographic mass distribution correction in a gravimetric geoid model.

This paper deals with the interpretation of Bouguer gravity anomalies measured along a 250 km long Suhaitu-Etuokeqi gravity profile located at the transitional zone of the Alxa and Ordos blocks where geophysical characteristics are very complex. The analysis is carried out in terms of the ratio of elevation and Bouguer gravity anomaly, the normalized full gradient of a section of the Bouguer gravity anomaly (Gh) and the crustal density structure reveal that (1) the ratio of highs and lows of elevation and Bouguer gravity anomaly is large between Zhengyiguan fault (F4) and Helandonglu fault (F6), which can be explained due to crustal inhomogeneities related to the uplift of the Qinghai-Tibet block in the northeast; (2) the main active faults correspond to the Gh contour strip or cut the local region, and generally show strong deformation characteristics, for example the Bayanwulashan mountain front fault (F1) or the southeast boundary of Alxa block is in accord with the western change belt of Gh, a belt about 10 km wide that extends to about 30 km; (3) Yinchuan- Pingluo fault (F8) is the seismogenic structure of the Pingluo M earthquake, and its focal depth is about 15 km; (4) the Moho depth trend and Bouguer gravity anomaly variation indicates that the regional gravity field is strongly correlated with the Moho discontinuity. [ABSTRACT FROM AUTHOR]

Understanding the crustal architecture of Kutch rift region, India is important due to the presence of thick Mesozoic sediments and also due to occurrence of earthquakes. We present here three-dimensional gravity models. The major features of our shallow crustal model are: (1) thickness of Deccan Traps is almost negligible towards northern side of Vigodi Fault and Katrol Hill Fault (2) Mesozoic sediment thickness is about 3 km on the southern side and (3) several undulations in the basement on the Wagad Uplift. The model is consistent with the geological observation that the basin slopes towards southwest. The observed Bouguer gravity anomalies are explained by ∼10 km thick Granitic upper crust of 2.7 g/cm 3 ; ∼10 km thick intermediate mid crust of 2.82 g/cm 3 ; ∼15 km thick mafic lower crust of 2.91 g/cm 3 density. Crustal thickness is about 40–45 km on Wagad Uplift and near Katrol Hill Fault. The modelled gravity data do not require presence of anomalous amount of fluids, melts and/or magma chambers in the deep crust of Kutch region. [ABSTRACT FROM AUTHOR]

Traditional gravimetric geoid determination relies on Stokes’ formula with removal and restoration of the topographic effects. It is shown that this solution is in error of the order of the quasigeoid-to-geoid difference, which is mainly due to incomplete downward continuation (dwc) of gravity from the Earth’s surface to the geoid. A slightly improved estimator, based on the surface Bouguer gravity anomaly, is also biased due to the imperfect harmonic dwc the Bouguer anomaly. Only the third estimator,which uses the (harmonic) surface no-topography gravity anomaly, is consistent with the boundary condition and Stokes’ formula, providing a theoretically correct geoid height. The difference between the Bouguer and no-topography gravity anomalies (on the geoid or in space) is the “secondary indirect topographic effect”, which is a necessary correction in removing all topographic signals.

The circular 625 km2 alkaline Pilanesberg Complex, South Africa, contains coeval eruptive and several distinctive intrusive syenitic and foyaitic components, concentrically arranged at the surface. However, owing to poor outcrop the relationships between the different intrusive rocks, and their shape in the third dimension cannot be convincingly determined in the field. The original interpretation was a laccolith, whereas later models suggested a funnel shape, and appealed to ring-dyke and cone-sheet emplacement mechanisms. However, the radial widths of these coarse-grained bodies are over 1 km and so cannot have been emplaced as ring dykes or cone sheets, which are usually quite thin and fine grained. Creating the space for emplacement and removal of pre-existing country rocks for each postulated subsequent intrusive event presents a major challenge to this latter hypothesis. Extensive previously published and new field relationships are re-evaluated here to suggest that the body is a gently inward-dipping sheet and that subsequent injections of magma merely pumped up an existing and evolving magma chamber rather than intruded into solid rocks. A Bouguer gravity anomaly model is presented that supports the concept of a shallow, flat-bottomed body rather than one that continues to significant depth. There are many analogies with the Kangerlussuaq Intrusion, Greenland. [ABSTRACT FROM PUBLISHER]

We constructed a new free-air gravity anomaly map of the Philippine Sea Plate (PSP) using ship-tracked gravity data from the National Geophysical Data Center (NGDC). Our results show that the isogals trend correlates well with the tectonic structures in the PSP. After removing the gravity induced by sea water from the free-air gravity data, we obtained the regional Bouguer gravity anomaly, which is later used to compute the Moho geometry in the PSP by applying the Parker-Oldenburg iterative method. Our results indicate that in the southern part of the West Philippine Basin (WPB) the crustal thickness is nearly homogeneous with a value of about 5 km, which implies that the WPB is quite stable. The low-amplitude and near-zero free-air gravity anomalies clearly indicate that the whole WPB, except at trenches and island arcs, is nearly in a state of isostatic equilibrium. The average crustal thickness of the Palau Kyushu Ridge (PKR) is more than 10 km. In the eastern PSP the crustal thickness gradually increases eastward. Our results also imply that a relatively thin and low density mantle exists beneath the Parece Vela Basin (PVB) as a consequence of back-arc spreading and serpentinized upwells of the thin crustal thickness. [ABSTRACT FROM AUTHOR]

There are a large number of simplifying assumptions in the external gravity correction methods in order to reduce the amount of calculation and improve the efficiency of operation. However, there are some problems caused by the simplifying assumptions, for example, errors in normal gravity caused by the confusion of ellipsoids, the indirect effect caused by the difference between the geoid surface and the ellipsoid surface, errors in the Bouguer correction caused by the improper selection of the density and destruction of structure caused by the Bouguer correction. In order to overcome this problem, we proposed a new method of gravity external correction for high precision gravity measurement. There are two steps in the correction method. The first one is to calculate the normal gravity of stations in any position with arbitrary latitude and geodetic height and then gravity anomaly is calculated by subtracting normal gravity from absolute gravity. The gravity anomaly to the position of station can be calculated instead of the gravity anomaly relative to the datum. Using some theoretical models and actual data in western China, we verified the gravity correction method proposed in the paper, and the result showed that the method can improve the accuracy and exploration effect of gravity exploration. [ABSTRACT FROM AUTHOR]

[Display omitted] • The crustal structure beneath the Yinchuan Basin is apparently heterogeneous. • The Yellow River fault is divided into two segments, with the boundary at Yongning. • The primary causative structure of the 1739 Pingluo earthquake is the Yinchuan fault. The Yinchuan Basin is located along the transition belt between the Alxa Massif and Ordos Basin, and is characterised by a complex geological structure. However, several important scientific problems remain unresolved, such as, the causative fault of the 1739 Pingluo Ms 8.0 earthquake. In order to provide new insight into the geodynamic processes, the normalized full gradient (NFG) method and 3-D gravity inversion are utilised to image the crustal structure beneath the Yinchuan Basin in the study. The NFG results indicate that the upper crustal structure is complex, particularly at depths shallower than 20 km. The inversion results infer that the locations of large faults are consistent with the high–low gradient zones in horizontal direction. The integrating NFG and density contrasts imaging results reveal that the south-western boundary of the Ordos Basin is the Yellow River Fault which is divided into two segments with the boundary at Yongning; the East Helanshan and Qingtongxia–Guyuan faults are the south-eastern boundary of the Alxa Massif; the Yinchuan fault is the causative fault of the 1739 event and the focal depth of the event is ∼15 km. These findings provide a scientific basis for studying the crustal dynamics and seismogenic mechanisms of strong earthquakes. [ABSTRACT FROM AUTHOR]

The joint inversion of different geophysical datasets is an effective means to reduce the non-uniqueness and improve the reliability of geophysical inversion. In this study, seismic arrival time and Bouguer gravity datasets are jointly inverted to obtain an image of 3-D velocity structures in the Taiwan orogen. The model obtained from joint inversion fits the arrival time observations at least as well as when inverted individually, and the gravity observations are much better fit when included in the inversion, implying a reduction in ambiguity by simultaneously modeling the disparate datasets. Moho depth variations estimated by the 3-D P wave velocity model suggest a maximum Moho depth of 56 km located beneath the Backbone Central Range, and the trend of the Moho is largely consistent with the topography of the Central Range with eastward, asymmetric crustal thickening. The root beneath the Central Range appears to be smaller in lateral extent than previously imaged, and the velocity gradients into the uppermost mantle are significantly higher. The lack of evidence for a significant amount of Eurasian crust in the mantle supports geodynamical models of accreted, rather than consumed, continental crust in a collisional environment. [ABSTRACT FROM AUTHOR]

This paper will help to further constrain geodynamic models for the evolution of northern Tibet through two techniques that employ Bouguer gravity anomaly data: the first is isostatic correction of Bouguer gravity anomalies and the second is 2-D density modeling along a profile at 88.5°E from 30°N to 37°N within the Tibetan Plateau. Areas dominated by volcanic rocks exhibit ultra-low Bouguer gravity anomalies and low isostatic residual anomalies. The 2-D density structure beneath Profile 88.5°E shows a giant "upward dome" distributed body of low density in the middle-lower crust and mantle beneath the high-to-ultra-potassic areas, suggesting that the original middle and lower crusts were reformed and replaced extensively in the area. Based on existing regional geology and tectonics, geochemical data from magmatic rocks, and geophysical information, this paper proposes that the sustained northward subduction of the Indian plate has resulted in breaking off of the frontal margin of the Asian lithospheric mantle. Subsequently, the original lithosphere was reactivated by thermal-tectonic changes, high-to-ultra potassic volcanoes erupted extensively in the northern Tibetan Plateau during Neogene, and the plateau experienced rapid NE-trending uplift. [ABSTRACT FROM AUTHOR]