Wenjin Chen

Harmonic coefficients of the Earth’s Spectral Crustal Model 180 – ESCM180

We compile the harmonic coefficients, which describe the Earth’s crustal density structure with a spectral resolution complete to degree/order 180. These coefficients can be used in gravimetric studies of the Earth’s lithosphere structure, isostasy, crustal loading, sedimentary basins and related topics. The crustal structure of the Earth’s Spectral Crustal Model 180 (ESCM180) is separated into 9 individual layers of the topography, bathymetry, polar ice sheets, sediments (3-layers) and consolidated crust (3-layers). The harmonic coefficients describe uniformly the geometry and density (or density contrast) distribution within each individual crustal component. The topographic and bathymetric coefficients are generated from the topographic/bathymetric model ETOPO1 and the global geoid model GOCO03s. A uniform density model is adopted for the topography. The ocean density distribution is approximated by the depth-dependent seawater density model. The ETOPO1 topographic and the DTM2006.0 ice thickness data are used to generate the ice coefficients, while assuming a uniform density of the glacial ice. The geometry and density distribution within sediments is described by the 3 stratigraphic layers of a laterally varying density model, and the same structure is used to describe the density distribution within the consolidated crust down to the Moho interface. The sediment and consolidated crust coefficients are generated from the global crustal model CRUST1.0. The density contrasts of the ocean, ice, sediments and remaining crustal structures are taken relative to the reference crustal density.

Sediment Stripping Correction to Marine Gravity Data

The knowledge of the bedrock topography (instead of the ocean-floor relief) is required in various geoscience studies investigating the evolution and structure of the oceanic lithosphere. The gross density structure and thickness of marine sediments were obtained from ocean drilling data or seismic surveys. Alternatively, marine gravity data corrected for the ocean and sediment density contrasts can be used for a detailed mapping of the bedrock topography. In this study, we compute and apply the sediment stripping correction to marine gravity data. The sediment density distribution is approximated by a 3-D density model derived based on the analysis of density samples from the Deep Sea Drilling Project. Methods for a spherical harmonic analysis and synthesis are utilized in computing the sediment stripping correction. Results show that this correction varies between 0 and 32 mGal. We also demonstrate that the approximation of heterogeneous sediment structures by a uniform density model yields large errors. The spectral analysis reveals a high correlation (>0.75) between the sediment-stripped marine gravity data and the bedrock topography. The application of the sediment stripping correction to marine gravity data enhanced the gravitational signature of the sediment-bedrock interface.

Gravity Maps of Antarctic Lithospheric Structure from Remote-Sensing and Seismic Data

Remote-sensing data from altimetry and gravity satellite missions combined with seismic information have been used to investigate the Earth’s interior, particularly focusing on the lithospheric structure. In this study, we use the subglacial bedrock relief BEDMAP2, the global gravitational model GOCO05S, and the ETOPO1 topographic/bathymetric data, together with a newly developed (continental-scale) seismic crustal model for Antarctica to compile the free-air, Bouguer, and mantle gravity maps over this continent and surrounding oceanic areas. We then use these gravity maps to interpret the Antarctic crustal and uppermost mantle structure. We demonstrate that most of the gravity features seen in gravity maps could be explained by known lithospheric structures. The Bouguer gravity map reveals a contrast between the oceanic and continental crust which marks the extension of the Antarctic continental margins. The isostatic signature in this gravity map confirms deep and compact orogenic roots under the Gamburtsev Subglacial Mountains and more complex orogenic structures under Dronning Maud Land in East Antarctica. Whereas the Bouguer gravity map exhibits features which are closely spatially correlated with the crustal thickness, the mantle gravity map reveals mainly the gravitational signature of the uppermost mantle, which is superposed over a weaker (long-wavelength) signature of density heterogeneities distributed deeper in the mantle. In contrast to a relatively complex and segmented uppermost mantle structure of West Antarctica, the mantle gravity map confirmed a more uniform structure of the East Antarctic Craton. The most pronounced features in this gravity map are divergent tectonic margins along mid-oceanic ridges and continental rifts. Gravity lows at these locations indicate that a broad region of the West Antarctic Rift System continuously extends between the Atlantic–Indian and Pacific–Antarctic mid-oceanic ridges and it is possibly formed by two major fault segments. Gravity lows over the Transantarctic Mountains confirms their non-collisional origin. Additionally, more localized gravity lows closely coincide with known locations of hotspots and volcanic regions (Marie Byrd Land, Balleny Islands, Mt. Erebus). Gravity lows also suggest a possible hotspot under the South Orkney Islands. However, this finding has to be further verified.

A regional gravimetric Moho recovery under Tibet using gravitational potential data from a satellite global model

The Moho information under Tibet Plateau is important for a better understanding of the geodynamic processes associated with the continental collision of the Indian and Eurasian tectonic plates and subsequent formation of Himalayan and Tibetan orogens. However, under the central and western parts of Tibet, the existing Moho models are still relatively inaccurate due to a sparse and irregular distribution of seismic surveys. To overcome this problem, the gravimetric data could be used to interpolate the Moho information, where seismic data are missing. In this study, we apply the gravimetric method for a regional Moho recovery under Tibet. Compared to existing methods that use either the gravity or gravity-gradient data, the method presented here utilizes a more generic definition based on a functional relation between the Moho depth and the gravitational potential. Since the gravity and gravity-gradient data have more regional support than the potential field, a numerical test is conducted to find an optimal data area extension that is needed to solve a regional inversion problem in order to reduce errors caused by disregarding the far-zone contribution. Our analysis shows that for the potential field such extension should be at least 25°, while 5° for the gravity and only about 1° for the gravity gradient. The comparison of our gravimetric result with the CRUST1.0 seismic model shows differences at the level of expected accuracy of the gravimetric method of about 5 km and without the presence of significant bias.

Gravimetric Forward and Inverse Modeling Methods of the Crustal Density Structures and the Crust-Mantle Interface

The numerical models and results of the gravimetric interpretation of the crustal density structures and the Moho geometry are presented. The numerical scheme applied utilizes the gravimetric forward and inverse modeling derived in a frequency domain. Methods for a spectral analysis and synthesis of the gravity and crustal structure models are applied in the gravimetric forward modeling of the gravity field generated by the major known crustal density structures. The gravimetric inversion scheme is formulated by means of a linearized Fredholm integral equation of the first kind. In numerical results we show the gravitational contributions of crustal density structures and the refined gravity field quantities, which have a minimum as well as maximum correlation with the Moho geometry. The resulting gravimetric Moho model is finally presented.