Antarctic crustal modeling from the spectral correlation of free‐air gravity anomalies with the terrain

Abstract

We investigated the use of enhanced spectral correlation theory for modeling the crustal features of the Antarctic from regional observations of gravity and terrain. The analysis considered 1°‐gridded free‐air gravity anomalies and topographic rock, ice, and water components for the region south of 60°S. We modeled terrain gravity effects at 150‐km altitude by Gauss‐Legendre quadrature (GLQ) integration assuming densities of 2800 kg/m3 for rock, 900 kg/m3 for ice, and 1030 kg/m3 for seawater. These effects are substantial relative to the free‐air anomalies and must be compensated by the effects of subsurface density variations. Significant terrain‐correlated free‐air anomalies were revealed by the wavenumber correlation spectrum between the free‐air anomalies and the modeled terrain gravity effects, which we interpreted mostly to reflect possible isostatic imbalances of the crust. Subtracting the terrain‐correlated free‐air anomalies from the total free‐air anomalies and topographic gravity effects yielded terrain‐decorrelated free‐air anomalies and the gravity effects of isostatically compensated terrain features, respectively, which are uncorrelated with each other. The compensating effects that annihilate the latter were attributed to undulations of the Moho, which we estimated by inversion using GLQ integration and a mantle‐to‐crust density contrast of 400 kg/m3. The inversion produced a Moho map with nearly 40 km of total relief that agrees very well with deep seismic refraction soundings. For East Antarctica, a bimodal variation in crustal thickness was found: the crustal wedge between the eastern Weddell Sea (≃330°E) and the eastern flank of the Gamburtsev Subglacial Mountains (≃90°E) has a mean thickness of about 37 km, whereas the mean crustal thickness is near 32 km for the northern half of the rest of East Antarctica up to the western flank of the Transantarctic Mountains (≃150°E). For West Antarctica and the oceanic regions, mean crustal thicknesses of about 30 km and 14 km, respectively, are inferred. The terrain‐decorrelated free‐air anomalies may be related to long‐wavelength, large‐amplitude subcrustal density variations and to much shorter‐wavelength, smaller‐amplitude intracrustal density variations.