Early rifting of Gondwana: Conclusions from the crustal structure of the Falkland Plateau
The reconstruction of continents has been a research field for more than 100 years, since Alfred Wegener proposed his theory of continental drift in 1915. The changing configuration of landmasses and oceans influences the evolution of climate and biosphere and is therefore of great scientific import...
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Format: | Thesis |
Language: | unknown |
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2018
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Online Access: | https://epic.awi.de/id/eprint/51598/ https://epic.awi.de/id/eprint/51598/1/Schimschal-2018-diss-FalklandPlateauCrust.pdf https://hdl.handle.net/10013/epic.b835ebfd-4d02-4c0a-a6ff-5739b819f23d https://hdl.handle.net/ |
Summary: | The reconstruction of continents has been a research field for more than 100 years, since Alfred Wegener proposed his theory of continental drift in 1915. The changing configuration of landmasses and oceans influences the evolution of climate and biosphere and is therefore of great scientific importance. In the southern hemisphere, Gondwana, which comprised the present-day continents of Antarctica, South America, Africa and Australia and smaller landmasses like India, New Zealand, Madagascar and the Arabian Peninsula, was the last supercontinent. In the centre of the continent, the Falkland Plateau was located between the South American, African and Antarctic plates. Today, the Falkland Plateau is a remarkable bathymetric feature in the South Atlantic Ocean offshore southern South America. It stretches for 1500km eastwards from the Falkland Islands into the Georgia Basin, at depths of between one and three kilometres shallower than its surroundings. The central part of the plateau hosts the Falkland Plateau Basin. A bathymetric high, Maurice Ewing Bank (MEB), is located at the eastern termination of the plateau. Although the Falkland Plateau is a key element in Gondwana reconstructions, its crustal structure has long remained speculative. Outcrops on the Falkland Islands prove the presence of Precambrian continental crust at the western end of the plateau. At the eastern end, gneisses recovered at DSDP drill site 330 on MEB, were interpreted to show its continental composition. For the crust underneath the Falkland Plateau Basin, a variety of approaches have led to conflicting conclusions on the presence of continental or oceanic crust. The lack of certainty regarding the crustal composition here has both stimulated and permitted a range of contrasting Gondwana reconstructions. To close this gap, the expedition ANT-XXIX/5 with the research vessel RV Polarstern was conducted in 2013. Wide-angle seismic data were acquired using land stations onshore East Falkland and ocean bottom stations along the 1450 km long transect AWI- 20130010. Magnetic, bathymetric and sediment echosounder data were recorded along the ship track. Additional airborne magnetic profiles were flown with the ship’s helicopter. In this study, I present models of the velocity, density and geology along the transect, derived from ray tracing and amplitude, density and magnetic modelling. By interpreting all of these datasets, I present the first comprehensive crustal model of the Falkland Plateau and the resulting consequences for the earliest rifting of Gondwana within this region. The crustal model shows the presence of 34km thick continental crust beneath East Falkland. Across the eastern margin of East Falkland, which can be classified as a volcanic rifted margin, the basement dips steeply into the Falkland Plateau Basin. Seismic velocities and the magnetic anomaly data indicate the presence of a 90km wide continent-ocean transition zone (COT). Sediments in the Falkland Plateau Basin have an average thickness of 6.5 km. Underneath lies an oceanic crust of up to 20km thickness, with high velocities of 7.4 km/s above Moho. Further east, a second COT of 80km width exists before the basement rises steeply up to MEB. Lower seismic velocities and densities as well as a different velocity structure compared to the crust underneath the Falkland Plateau Basin characterize the MEB as a continental fragment. The continental crust of the central MEB is up to 29km thick and neither tectonically nor magmatically overprinted. The crust thins towards the east and west. High velocities of more than 7.0 km/s and intracrustal reflections in the western part of MEB indicate the presence of magmatic intrusions. In the Georgia Basin, magnetic isochron M10n indicates the onset of oceanic crust. Here, the crust is of average oceanic crustal thickness at 7.1 km. These results are used as constraints for a refined model of the breakup of Gondwana. The presence of oceanic crust beneath the Falkland Plateau Basin allows to position the Falkland Islands and MEB as a continuous feature off the eastern coast of South Africa. To the south, the Weddell Sea region was located and formed a wide rift area. Radiometric dates from onshore magmatic dykes, the age of sediments above basement east of the Falkland Islands and the ages of crustal stretching and rifting in adjacent regions, indicate that rifting and subsequent seafloor spreading on the Falkland Plateau took place between ∼178Ma and ∼154 Ma. This opening of the Falkland Plateau Basin was accommodated by movement of the Patagonian sub-plate along the Gastre Fault. The exceptionally thick oceanic crust underneath the Falkland Plateau Basin likely results from its development in a back-arc regime over a region of anomalously warm or fertile mantle that was also responsible for the extensive onshore Karoo-Ferrar and Chon Aike volcanic provinces. The emplacement of thick oceanic crust was spatially and temporally limited to the Falkland Plateau Basin and the Jurassic Weddell Rift. |
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