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Gondwana Breakup: The South American, African and Indian plate movements and remaining problems.

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Jokat, W. and König, M. (2007): Gondwana Breakup: The South American, African and Indian plate movements and remaining problems. , Proceedings of the 10th ISAES X, edited by A. K. Cooper and C. R. Raymond et al., USGS Open-File Report 2007. .
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Abstract:

Geological mapping and geophysical data indicate that the geological history of the EastAntarctic between Berkner Island in the west and Enderby Land in the east faced severalgeological events, which are not fully understood. The main information on the sub-icegeology is provided by aerogeophysical investigations. However, these surveys are somehowlogistically limited to a region, which is parallel to the coast line. Thus, large parts of the innerAntarctic continent are only sparsely surveyed due to the lack of long-range aircrafts, whichcan operate also in these remote areas. As a consequence geological units, which areidentified in the coastal or near coastal mountain ranges, cannot be followed into the interior.Thus, most of the models, which predict the continuation of the geological features from thesurrounding continents into the Antarctic craton are still highly speculative. Thus, weconcentrate in this contribution on the aero/marine magnetic investigations, which aim tofurther constrain the plate movements since 160 Myr, and thus provide a good fit. This mightfacilitate at a later stage the correlation of geological units across the continents, if there is anycontinuation. The areas and remaining problems from our perspective are described(Abbreviations: SAM-South America, ANT-Antarctica, FRS-Filchner-Ronne Shelf, EANTEastAntarctica, WANT-West Antarctica, AFR-Africa, IND-India):South America-AntarcticaAccording to the latest models SAM was situated deep in the southern Weddell Sea. Thebreak-up with the formation of oceanic crust started some 148 Ma (König and Jokat, 2006)with spreading rates around 1-2 cm/yr. Slow to ultra-slow spreading rates dominated thedocumented drift history of ANT and SAM. The magnetic anomalies in the southwesternWeddell Sea are for unknown reasons- not well developed. A short high resolution magneticsurvey along the Larsen rifted margin did not show any continuous magnetic anomalies,which would allow to better describe the evolution of this area. It is questionable, if a furtherdensification of the existing data will provide better constraints than the existing ones.Filchner-Ronne Shelf-EastANT/WestANTWe consider the FRS to consist mainly of highly stretched continental crust. However, in thecentre of this huge sedimentary basin, some indications for the existence of oceanic crust canbe deduced from deep seismic sounding profiles. This is supported by aeromagnetic data,which show at least four more or less continuous magnetic anomalies. They can be interpretedas edge or spreading anomalies. In our current geodynamic model we have introduced somemoderate extension of the FRS to account for this observation.Falkland Plateau (FP)The distance between the Falkland islands and the easternmost part of the FP, the MauriceEwing Bank, is almost 800 km. The nature of the underlying crust is speculative. It mightconsist either of stretched continental or oceanic crust or a mixture of both. Since the earlygeophysical investigations no critical new geophysical data were gathered to constrain thesize of the FP in a pre-breakup configuration.Mozambique RidgeThis ridge in the Indian Ocean is almost 700 km long, and is located almost parallel to theEast African coast. Recent magnetic and seismic investigations show that the ridge might beof volcanic origin, and might have been formed during the separation of AFR and ANT.Magnetic anomalies in the strike of the ridge show high amplitudes, and are partly continuouswithin the survey area. The current model on the origin of the ridge is not unique due to thelimited geophysical data sets. In any case, evidence for a large amount of volcanic materialerupted between 130-100 Ma exists. If true, this region would have faced much more postbreak-up volcanism than previously expected.AfricaThe spreading system between AFR and ANT moved twice as fast as the SAM-ANT platesystem. This might be an indication for separate spreading cells, which receivedindependently largely different volumes of mantle material to drive the plate drift. Recentconjugate magnetic investigations in the Riiser Larsen Sea and the Mozambique Basinconfirmed models, which predicted a 155 Myr old spreading system. According to König andJokat (2006) this area together with the Somali Basin was the first true oceanic basin, whichformed within Gondwana.India and MadagascarNumerous models are published on the base of a growing geophysical data set. However, dueto the line spacing of the magnetic data their interpretation is not unique. Furthermore, therole of the Kerguelen Plateau as a large oceanic plateau is questioned. Deep drilling hasrecovered continental rocks.Currently a Japanese/German project tries to derive additional constraints on the separation ofIND and ANT. The first results indicate that at least in the area close to the Gunnerus Ridgethe oceanic crust has a different age than predicted in published models. A denseaeromagnetic grid indicates a lack of Mesozoic magnetic anomalies north of 67°S. Oneprominent magnetic anomaly is could be mapped close to the rifted margin of ANT.Therefore, the following interpretations are possible:--This anomaly represents M0. This would shift the break-up of IND and ANT to much younger times than in published models.--The magnetic anomaly is of continental origin-- It is an anomaly of the M-series, but the other anomalies are beyond the survey area. In this case the ANT/IND plates must have moved significantly faster than any model predicted.--The survey is still located within the continent-ocean transition, and M-anomalies are located more to the north.

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