Aerogeophysical constraints for the geodynamic evolution of Dronning Maud Land, East Antarctica

Wilfried.Jokat [ at ]


The central question of this thesis is: what is hidden under the ice sheet of Antarctica? To unravel the tectonic structure and to study the geodynamic evolution of Antarctica are not only of fundamental geological interest. Knowledge of the amalgamation and break-up of supercontinents provides basic information for studying the evolution of the climate and the biosphere during Earth’s history. Furthermore, knowledge about the tectonic structure of Antarctica is essential for estimating its crustal heat flow, a key parameter when modeling ice flow and future changes of ice sheets. The Antarctic ice sheet is mostly several kilometers thick, and rocks crop out of it in only a few locations to allow direct geological sampling. None of these locations is in the innermost parts of the continent. Hence, regional geophysical reconnaissance and a correlation of its data with geological findings are required for unraveling the tectonic structure of Antarctica. In recent decades, the Jurassic break-up process of Gondwana has been reconstructed based on analyses of the magnetic striping pattern of the oceanic crust, making it apparent that East Antarctica was positioned centrally in this supercontinent between Africa, India, Australia, and New Zealand throughout Paleozoic and Early Mesozoic times. However, fundamental data about the tectonic structure of East Antarctica that would be needed to better understand the assembly of Gondwana in Late Proterozoic and Early Paleozoic times, as well as about earlier collision and break-up processes, are still missing. This study pursues the question of whether the interior of East Antarctica is composed of one or many crustal fragments. Its method objective is to search for the courses of supraregional shear zones and the positions of crustal blocks and their boundaries. For this purpose, the Alfred-Wegener-Institut Helmholtz-Zentrum für Polar- und Meeresforschung (AWI) has undertaken systematic airborne geophysical surveying in the region of Dronning Maud Land (DML) and Coats Land over the last decades. The data set includes ice-penetrating radar, aeromagnetic and areogravity measurements. Interpretation of the magnetic and gravity anomaly pattern caused by the rocks below the ice and the resulting conclusions are based on geological findings in DML and adjacent regions in East Antarctica and southern Africa. One substantial result of this thesis for understanding of the tectonic structure of East Antarctica is the discovery of a distinct aerogeophysical province in southeastern DML. It is concluded that the magnetic and gravity anomaly data of this province can be regarded as the signature of a discrete crustal fragment. This southeastern DML province is interpreted to have collided with southern Africa as part of the Pan-African amalgamation of Gondwana, during which the Grenvillian Namaqua-Natal-Maud belt was deformed within central DML. The prominent “Forster Magnetic Anomaly” might represent the suture zone of this collision. Aeromagnetic data of the “Geodynamic Evolution of East Antarctica” (GEA) project reveal an eastward extent of the southeastern DML province into the southern Sør Rondane region, while several smaller terranes, shear zones, and tectonic boundaries are mapped farther north. It is shown how the airborne geophysical data invite an interpretation of the central Sør Rondane region as a distinct tectonic unit with a presumably increased crustal thickness. Furthermore, the eastern boundary of the BLM craton in Coats Land is mapped and its prominent magnetic anomaly pattern is shown to be unique within DML and southern Africa, raising the question of the provenance of this crustal fragment, which appears not to have been high-grade reworked during the Pan-African orogeny. In contrast, the Kohnen lineament is interpreted as a Pan-African shear zone with similarities to the outcropping Heimfront shear zone. In addition, a detailed study and modeling of the “Giæver Magnetic Anomaly” show that it is likely to be the signal of a shallow subcrop of banded iron formation. With the multiplicity of these findings, results, and conclusions, this thesis contributes significant new knowledge and future prospects for unraveling the tectonic structure and the geodynamic evolution of East Antarctica.

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Mieth, M. (2014): Aerogeophysical constraints for the geodynamic evolution of Dronning Maud Land, East Antarctica , PhD thesis, University Bremen.

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