A comprehensive model of the Antarctic lithosphere based on geophysical data integration

Abstract

The Antarctic continent is almost entirely ~99% covered by a thick ice layer impeding classical in-situ measurements. It hence remains one of the least geophysically known areas on Earth. Little is known about the structure and the thermal and rheological properties of its lithosphere. Since the state of the lithosphere is strongly linked to near-surface processes such as ice dynamics or glacial isostatic adjustment (GIA) as well as the deeper, convecting mantle, knowledge of those properties is crucially important when modeling the coupled systems. This cumulative thesis consists of three published scientific papers that together characterize the lithosphere of Antarctica in terms of strength, temperature, density, composition and upper crustal properties. As a measure of strength, the effective elastic thickness Te was derived by cross-spectral analysis of the gravity field with the adjusted topography. The fan wavelet technique was employed to, for the first time, calculate variations of Te over the entire continent by means of admittance and coherence analysis. The same gravity and topography data was then combined with tomography models constrained by mineral physics equations in an iterative inversion scheme to develop a 3D density, thermal and compositional model of the Antarctic lithosphere and upper mantle. Seismic data on crustal structures was further employed to create a new Moho and crustal density model. In order to investigate upper crustal structures and properties, corrections of the gravity effect of isostatic compensation of geological loads were further applied to the isostatic gravity anomalies. The resulting so-called decompensative gravity anomalies were translated into sediment distributions previously hidden below the ice sheet. A general division of the Antarctic lithosphere is confirmed by all parameters under study. A transition is visible along the Transantarctic Mountains. Whether the mountain chain belongs to West Antarctica (WANT) or East Antarctica (EANT) has been under question, but especially the estimates of Te indicate a closer connection to WANT. Apart from this general division, lithospheric fragmentation was discovered within EANT. Cratonic fragments of Precambrian origin exhibiting high depletion, low temperatures and high Te were detected in Dronning Maud Land, in Wilkes Land and close to the South Pole. The latter two are likely part of the Mawson craton. Lithospheric weakening combined with an almost primitive upper mantle exists in the Lambert Graben and was probably the result of rifting in the East Antarctic Rift System. The obtained decompensative gravity anomalies correspond well to known sedimentary basins such as the Lambert Graben and the Filchner-Ronne Ice shelf. They also suggest the presence of large sedimentary deposits that were not ore only sparsely mapped previously. Therefore, this thesis provides a comprehensive model of the lithosphere of Antarctica and a basis for further analysis of its coupling with the deep mantle and surface processes. As such, the resulting model facilitates surface heat flux modeling and estimates of upper mantle viscosity crucial for GIA modeling.