Untersuchung der Lithosphäre des Dronning Maud Lands, Antarktis, mit geophysikalischen Methoden

Abstract

The area of investigation, Dronning Maud Land (DML), comprises with the Weddell Sea and the Lazarev Sea approximately the region between 15 degrees W and 15 degrees E longitude and 68 degrees and 75 degrees latitude - a surface area of more than 1.500.000 qkm. Three major events formed the present-day geology and tectonic settings of DML: (1) the Grenvillian Orogeny, ca. 1.1 Ga ago, caused by the formation of the supercontinent Rhodinia, (2) the Panafrican Orogeny 500 Ma ago, forming the supercontinent Gondwana due to the collison of West and East Gondwana, and (3) the break-up of Gondwana 180 Ma ago, which started in the present-day Lazarev Sea. Seismological, refraction seismic and aerogravity data-sets form the basis of this thesis. The combination of these data-sets allows to investigate the structural composition and the spatial variation of the lithospheric thickness. The latter is essential for the determination of the regional geoid, which is the main objective of the VISA-project. Within the framework of this project airborne measurements of the potential field as well as ice-penetrating radar and GPS-measurements have been carried out. Moreover, seismographic stations were temporarily deployed in various parts of DML. Despite their very short operation time, applications of seismological methods to the recorded teleseismic events yielded conclusions about the structure and dynamic of the deeper subsurface. By analyses of seismic anisotropy, which investigates the splitting of shear waves traversing an anisotropic medium (so-called shear wave splitting) conclusions were drawn about past and recent regional deformation processes in the upper mantle. The results of this study differ widely across the area of investigation, but share in common that observed anisotropy originates in past deformation processes rather than in recent plate motion. In particular, the abrupt change of the direction of the fast axis in the vicinity of the Heimefront shear zone (western DML) refers to a suture separating the Mesoproterozoic Maudheim province from the more southern juxtaposed East Antarctic craton. The observed shear wave splitting from recordings of the Russian base Novolazarevskaja (Novo, central DML) can be explained with a double layered anisotropic model of the upper mantle. Inconsistent results for the South African base Sanae IV (SNAA) refer to complicated deeper structures. By calculations of receiver functions, which are based on the conversion of teleseismic P- to S-waves at seismic discontinuities, crustal depths and vp/vs-ratios could be estimated. The latter are not uniform within the study area and classify the crustal composition of the central DML as felsic and that of the western DML as mafic. Basaltic intrusions, probably originating from the jurassic Gondwana break-up, explain the higher vp/vs-ratio for station SNAA. The estimated crustal depths show a similar behaviour as formerly published refraction seismic experiments of the study area: a crust-mantle-transition (Moho) that decreases stepless from the coast towards the south. Underneath the mountain ranges Heimefrontfjella and Wohlthat Massif, the crustal depth approaches a maximum of 50 km, indicative for an orogenic root. However, such an orogenic root cannot be identified unambiguously due to the lack of boundary conditions especially south of the mountain ranges. A comparison with the Moho depths from other fragments of Gondwana, e.g. the southern Africa, shows high similarity.In the Antarctic summer season 1989/90 a seismic refraction experiment was conducted at the Kottas mountains (western DML). The profile spanned the northern foreland, the entire escarpment of the Heimefrontfjella and the adjacent southern plateau. The Moho topography shows an offset of several kilometers, which is interpreted as a suture. It separates the Kibarian Kottas mountains from the adjacent southern East Antarctic craton.These local information served as boundary conditions for a 3D-gravity model, which finally allowed the spatial mapping of the Moho.