SUMMARY
Atlantic Canada encompasses geological evidence of the orogenic and rifting episodes that inspired the development of the theory of plate tectonics and the fundamental concept of the Wilson ...cycle. To provide a regional crustal-scale view that can complement surface mapping studies and sparse seismological investigations, an onshore–offshore 3-D constrained gravity inversion methodology is proposed involving incorporation of topography and an inversion mesh that is laterally variable in terms of its maximum depth extent. A 3-D density anomaly model for the entirety of Atlantic Canada and its environs is generated, with the inverted density distribution structure and extracted isodensity surfaces showing excellent correspondence with independent and co-located controlled source and passive seismic constraints. The full density model and crustal thicknesses from this work are made freely available so that they may be used for further study, for instance as inputs for deformable plate reconstruction modelling.
Summary
Downward continuation is a very useful technique in the interpretation of potential field data. It would enhance the short wavelength of the gravity anomalies or accentuate the details of the ...source distribution. Taylor series expansion method has been proposed to be one of the best downward continued methods. However, the method using high-order vertical derivatives leads to low accuracy and instability results in many cases. In this paper, we propose a new method using a combination of Taylor series expansion and upward continuation for computing vertical derivatives. This method has been tested on the gravitational anomaly of infinite horizontal cylinder in both cases with and without random noise for higher accurate and stable than Hilbert transform method and Laplace equation method, especially in the case of noise input data. This vertical derivative method is applied successfully to calculate the downward continuation according to Taylor series expansion method. The downward continuation is also tested on both complex synthetic models and real data in the East Vietnam Sea (South China Sea). The results reveal that by calculating this new vertical derivative, the downward continuation method gave higher accurate and stable than the previous downward continuation methods.
SUMMARY
Satellite altimetry provides major data sources for marine gravity recovery, and typical altimetry derived models, for example, DTU21 and SS V32.1, were usually released with 1 arcmin × 1 ...arcmin gridding interval. Their true resolution is much lower than the nominal ∼2 km level. By contrast, the in situ ship-borne measurements are considered to have better short-wavelength resolution. In this paper, we aim to propose a new method to fully utilize satellite altimetry data and ship-borne measurements, namely the frequency-domain fusion method, and give certain analysis of new method along with two spatial-domain fusion methods. Comprehensive analysis is focus on four aspects: gravity signals in fusion images, numerical verifications, power spectra, as well as coherence analysis. Initial evaluation indicates that, first, the frequency-domain fusion method has advantage in flexibility, since it can autonomously select dominant bands to fuse different data sets. Secondly, the new method retains medium-long wavelength signals from altimetry-derived model and effectively incorporate dominant short-wavelength signals of in-situ measurements, while the spatial-domain methods are essentially full-wavelength fusion and inevitably diminish the role of satellite altimetry. To some extent, the new method maximize the positive contribution of satellite altimetry measurements and efficiently exploit the benefits of ship-borne data. Finally, verification experiments were similarly designed in three regions with different amount and ratio of ship-borne data to thoroughly evaluate various methods. In two regions with gridded and dense along-cruise ship-borne data, the average accuracy of this frequency-domain fusion results is improved by 0.346 and 0.613 mGal, respectively. In a region with sparse ship-borne data, we still recommend using spatial-domain fusion methods since the new method is unable to align ship-borne data with model grid. It is concluded from the above analysis that the new method effectively incorporates the short-wavelength signals from ship-borne data into the altimetry-derived gravity field model, and it is significant that the new method maximizes the application of advantageous bands from different data sources.
SUMMARY
Clairaut's theory that relates the Earth's oblate figure and internal ellipticity to its gravity under rotational-hydrostatic equilibrium has reigned classical geodesy over the centuries. In ...this paper, we (i) derive from first principles the classical Clairaut's theory for the polar oblateness of a rotating planet under axi-symmetric rotational-hydrostatic equilibrium and (ii) extend the development to the triaxial case for the equatorial ellipticity of a tidally locked synchronous-rotating moon under rotational-tidal-hydrostatic equilibrium. Typical derivations of the classical Clairaut's theory presented in the literature being rather laborious even to first order, we instead exploit two concise forms of methodology: the gravitational multipole formalism on the physics side, and the Jacobian determinant for the Clairaut coordinate transformation on the mathematics side. The outcome is a logical and straightforward derivation of Clairaut's theory to first order in its entirety, encompassing all the equations and related formulas in geodesy bearing Clairaut's name. That further allows a natural extension to a tidally locked moon. In particular it is demonstrated that the same Clairaut's differential equation applies to both cases governing both the polar oblateness and the equatorial ellipticity.
The Tibet Plateau was generated from a long-term, multiple and complex convergence of many micro-blocks and the final Indian-Eurasian collision, which records the evolution of the eastern Neo-Tethys ...Ocean. In order to clarify the evolution of the eastern Neo-Tethys, we reviewed the Bangong-Nujiang Suture Zone (BNSZ) according to the geological and geophysical evidence. We analyzed the BNSZ deep structure by geophysical data, and subduction polarity and the closure process of the Bangong-Nujiang Ocean by combining geophysical and geochronological data. Bouguer gravity anomaly is used to identify tectonic units on both sides of the BNSZ. We used multi-scale wavelet decomposition to separate the gravity anomaly at different depths for comparative analysis. Subsequently, the Moho discontinuity was estimated by gravity inversion. The inversion results show that the Moho depth varies from south to north beneath the Tibet Plateau, indicating a compression environment in a NS direction during the collision of micro-blocks. The collision caused uplift of the Tibet Plateau and an increase in crustal thickness. There is a clear Moho gradient zone along the BNSZ, and Moho depth changes more than 5 km between the South Qiangtang Terrane (SQT) and North Lhasa Terrane (NLT). The relatively shallow Moho surface of SQT indicates that the SQT experienced crustal thinning, which probably can be attributed to the different delamination of the lithosphere beneath the NLT and SQT. The nearly E–W-trending gravity gradient zone correlates well with the BNSZ. There are several gravity lows south of the BNSZ, that correspond well with outcrops of volcanic rocks, which developed from the Late Jurassic to Early Cretaceous, indicating that the Bangong-Nujiang Ocean may have subducted southward at that time. The BNSZ is a gravity gradient zone in the central Tibet Plateau, and the gravity field is very different on both sides of the BNSZ, which may be an important tectonic boundary. A combined analysis of gravity anomaly and ophiolite characteristics reveals different evolution of the eastern, central and western BNSZ. The evolution of the eastern Neo-Tethys consists of Carboniferous-Permian breakup, Triassic-Early Jurassic extension, Middle-Late Jurassic subduction and Cretaceous extinction. In particular, the closure of the Bangong-Nujiang Ocean represents the extinction of the eastern Neo-Tethys, which closed in the Yanshanian Period (Jurassic to Cretaceous), with scissor-style movement from east to west. The eastern BNSZ closed in the Middle Jurassic, while the central and western BNSZ closed in the late Early Cretaceous, and the collision-related deformation occurred mainly in the Cretaceous.
•The nature of the Bangong-Nujiang Suture Zone (BNSZ), the subduction polarity and closing time of the Bangong-Nujiang Ocean are revealed from a geophysical perspective.•There are clear offsets on the Moho discontinuity of both sides of the BNSZ which obtained by gravity inversion.•A scissor-like closure from east to west of the Bangong-Nujiang Ocean during the Yanshanian Period is proposed.
Three approaches are used to reduce the error in the satellite‐derived marine gravity anomalies. First, we have retracked the raw waveforms from the ERS‐1 and Geosat/GM missions resulting in ...improvements in range precision of 40% and 27%, respectively. Second, we have used the recently published EGM2008 global gravity model as a reference field to provide a seamless gravity transition from land to ocean. Third, we have used a biharmonic spline interpolation method to construct residual vertical deflection grids. Comparisons between shipboard gravity and the global gravity grid show errors ranging from 2.0 mGal in the Gulf of Mexico to 4.0 mGal in areas with rugged seafloor topography. The largest errors of up to 20 mGal occur on the crests of narrow large seamounts. The global spreading ridges are well resolved and show variations in ridge axis morphology and segmentation with spreading rate. For rates less than about 60 mm/a the typical ridge segment is 50–80 km long while it increases dramatically at higher rates (100–1000 km). This transition spreading rate of 60 mm/a also marks the transition from axial valley to axial high. We speculate that a single mechanism controls both transitions; candidates include both lithospheric and asthenospheric processes.
Lateral viscosity variations (LVVs) in the mantle influence geodynamic processes and their surface expressions. With the observed long-wavelength geoid, free-air anomaly, gravity gradient in three ...directions and discrete, high-accuracy residual topography, we invert for depth- and temperature-dependent and tectonically regionalized mantle viscosity with a mantle flow model. The inversions suggest that long-wavelength gravitational and topographic signals are mainly controlled by the radial viscosity profile; the pre-Cambrian lithosphere viscosity is slightly (∼ one order of magnitude) higher than that of oceanic and Phanerozoic lithosphere; plate margins are substantially weaker than plate interiors; and viscosity has only a weak apparent, dependence on temperature, suggesting either a balancing between factors or a smoothing of actual higher amplitude, but short wavelength, LVVs. The predicted large-scale lithospheric stress regime (compression or extension) is consistent with the world stress map (thrust or normal faulting). Both recent compiled high-accuracy residual topography and the predicted dynamic topography yield ∼1 km amplitude long-wavelength dynamic topography, inconsistent with recent studies suggesting amplitudes of ∼100 to ∼500 m. Such studies use a constant, positive admittance (transfer function between topography and gravity), in contrast to the evidence which shows that the earth has a spatially and wavelength-dependent admittance, with large, negative admittances between ∼4000 and ∼104 km wavelengths.
We present general closed-form solutions for the vertical gravitational anomaly caused by a polyhedral prism with mass density contrast varying with depth. Our equations are the first ones to ...implement a polynomial vertical mass density contrast of arbitrary order. Singularities in the gravity field which arise when the observation site is close to or in the anomalous polyhedral prism are removed in our analytic expressions. Therefore, the observation site can be located outside, on the faces of or inside the anomalous mass bodies. A simple prismatic body of anomalous density is adopted to test the accuracy of our newly developed closed-form solution. Cases of constant, linear, quadratic, cubic and quartic polynomial orders of mass density contrast are tested. For cases of constant, linear, quadratic and cubic polynomial orders, the relative errors between our results and other published exact solutions are less than 10(-11)%. For the case of quartic polynomial order, relative errors less than 10(-10)% are obtained between our solutions and those computed by a high-order Gaussian quadrature rule (512 x 512 x 512 = 134 217 728 quadrature points), where our new analytic solution needs significantly less computational time (0.0009 versus 31.106 s). These numerical experiments not only verified the accuracy of our new formula but also demonstrated their potential in computing exact gravity anomalies for complicated mass density distributions in the Earth.
SUMMARY
To estimate the tectonic displacement history of the Yangsan Fault (YSF) in the Cretaceous Gyeongsang Basin (GB), SE-Korea, gravity field measurements were implemented and interpreted. A ...total of 8000 ground-measured gravity data were compiled and deployed to calculate the Bouguer and residual gravity field. The residual field shows that the mean value in the southern segment of the YSF is very similar to that of the northern segment, meanwhile that of the central segmentation is significantly lower than those of the other segmentations. This implies that the crustal structure and petrophysical properties of the central segment differ significantly from those of the other segments. Applying dip curvature analysis to enhance the image of the residual gravity field, we find that (1) the Gaum fault system (GF) in the western and middle part of the GB extends up to the YSF, where A-type granite outcrops are observed. (2) The middle- and eastern segments of the GF that formed before tectonic motion of the YSF are revealed in turn in areas east of the YSF, which were now covered by the younger sedimentary basin. (3) The middle segment of the GF which occupies between the YSF and the Yeonil tectonic line (YTL) has been displaced approximately 25 km southward from the main GF trace. (4) The easternmost segment is revealed buried in the eastern side of the YTL, which is displaced southward about 18 km from the main GF trace. Comparing all the available geoscientific data sets, we conclude that the right-lateral tectonic movement of the YSF, and the deeply buried middle segment of the GF remained stationary, while the most eastern segment of the GF is reversely moved to the north through left-lateral strike movement of the YTL.
SUMMARY
Many significant geophysical anomalies are generated by fault systems, and effectively estimating fault parameters is a key step to understand the characteristics of these field sources. In ...general, the 2-D dipping fault is frequently utilized to simplify the representation of fault geometry. However, this approach could lead to larger errors when comparing realistic scenarios with simplifications. In this study, we use a parametrized 3-D dipping model to represent fault systems and compute gravity anomalies using an analytic method in the wavenumber domain. To estimate the marginal distribution of the 3-D dipping fault parameters, we utilize the Bayesian theory that integrates the likelihood function and prior information. Subsequently, we use the Markov chain Monte Carlo sampling method to assess the marginal distribution of the parameters. Compared with other deterministic inversion approaches, our method provides an effective means of quantifying parameter uncertainty. We first use synthetic data to validate the effectiveness of the proposed method. The results demonstrate the efficiency of our method in revealing 3-D dipping fault parameters and evaluating their uncertainty. We subsequently apply our method to the Pudu River fault, located in Yunnan Province, China, to demonstrate its practical applicability. Two alternative realistic fault models referred the prior seismic profile have been proposed. The optimal parameters and uncertainty with the specific model have been estimated. The results show that the fault characteristics retrieved by our method deviate slightly from existing knowledge. Specifically, the estimated depth of the bottom surface is greater than that reported in previous studies. This could potentially offer us new perspectives on the geological evolution and structural attributes of this fault.
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