Korrelation von Gravimetrie und Bathymetrie zur geologischen Interpretation der Eltanin-Impaktstruktur im Südpazifik

Abstract

Meteorite impacts are subject of extensive scientific research because of their immense environmental influence. Most detected impacts are localised on continental or sea areas with low water depth. Oceans cover major part of the earth's surface (71%) which have an average depth of approximately 3800 meter. Therefore the first and only found deep sea impact, the Eltanin Impact, is regarded to be a special one.In 1981 Frank T. Kyte discovered iridium traces in sediment cores, which were sampled during 'USNS Eltanin' cruise in 1960. To explore the causing meteorite impact in more detail, the Alfred Wegener Institute for Polar and Marine Research exerted two expeditions to the assumed impact area in 1995 und 2001. On bord the research vessel 'Polarstern' numerous geophysical and geological samplings were accomplished. Gravity corer, multi corer, sediment echo sounder and multibeam echo sounder provided results about the seabed, seismic, magnetic and gravimetric measurements delivered insight into the lower crust. Using geologic sampling and stratigraphic analysis the impact event could be dated to 2.15 million years and the diameter of the impact estimated to be larger than one kilometer. More detailed predications concerning elevation angle, impact velocity, and other parameters particularly the spezific position of the impact center could be determined only approximately by now. To verify these topics geophysical data are used in this thesis to study if the meteorite impact has affected the seabed and left deformations and structures in the crust. In the introductory chapter the basics of crater forming caused by meteorite impacts are briefly explained and the peculiarities of the Eltanin deep sea impact emphasised. The relating geological boundary conditions and settings of the South Pazific and the impact area are addressed in chapter two. Additional geophysical background knowledge concerning potential field theory, correlation of bathymetry and gravity as well as their geological relation are explained in chapter three. Considering that no morphologic structures caused by the Eltanin impact could be detected by now, the processing of shipborne measured data is executed in high attention. Therefore all measurements and other data consulted and processed in this work are presented in terms of their origin and accuracy in chapter four. In chapter five the processing steps to achieve grids of bathymetry, free air anomaly, bouguer anomaly, and geoid undulations in best quality are explained. Additionally precision and accuracy information are derived to guarantee anomalies, structures and resulting interpretations. To detect subsurface attracting bodies an Euler-deconvolution is applied to bouguer anomalies whereat the maximum depth of source bodies is calculated to be situated at less than 1200 meter. No impact related structures could be identified with this methode. A ring structure, which was identified within bouguer anomalies, is modelled by the hydrodynamic theory, achieving a geometric description and first-time a precise position of the impact centre. The results of data processing and modelling are centralised and emphasised in chapter six. Similarities, differences and correlations between marine gravity and bathymetry are pointed out likewise the relations to and between the corresponding altimeter satellite derived datasets (so called predictions). All shipborne sampled raw and processed data are described in metadata and published in the 'PANGAEA' Publishing Network for Geoscientific&Environmental Data of the Alfred Wegener Institute. Chapter seven proceeds with discussion and interpretation of processed datasets. Position, form, distribution, and source of bouguer anomalies is discussed as well as the differences between measured and predicted bathymetry. The difference bathymetry is meant to be the anomaly between the real situation and the geologic model, which was utilised to compute the predicted bathymetry. Therefore this anomaly is considered to reflect a subsurface density contrast.The origin of the observed ring structure is compared with other structures which are not caused by a meteorite impact. Finally, the outlook of this paper gives an proposal about expansion and completion of existing datasets.