|
||||
Research of Geert Strik
Archaean continents: to wander or not to wander ? |
||||
|
The nature of the tectonic regime of Archaean times (> 2.5 Ga) remains controversial, and centers on the question of ‘plumes versus plates’. In recent years, a wealth of new structural and radiometric data has become available, indicating that there are still Archaean rocks in (near) pristine condition. Therefore, a study of igneous and sedimentary sequences from the well-preserved Pilbara (Australia) and Kaapvaal (South Africa) Cratons has started. This study concentrates on the undeformed basins of late Archaean age (< 2.8 Ga) that unconformably overly the older granite-greenstone belts to determine (rates of) displacement, and to construct a detailed apparent polar wander path (APWP). This must also answer the question: did the two Cratons once form a larger ‘Vaalbara supercontinent’?
Research in the Pilbara, Western Australia The Nullagine and Mount Jope Supersequences of the Pilbara Craton, Western Australia comprise relatively unmetamorphosed volcanic and sedimentary rocks. The stratigraphy covers about 60 million years, with ages ranging from approximately 2772 to 2715 Ma. The sequence overlies the older, metamorphosed and deformed rocks of the Archaean granite-greenstone terrain of the Pilbara Craton. The sequence has been studied throughout the northern Pilbara. In addition, three dykes have been sampled, which have formed within the time constraints of the studied stratigraphy.
Positive field tests (conglomerate tests, reversal tests and fold tests) indicate that the primary magnetization is still preserved. The sequence is dominantly normal, but so far, it has revealed two intervals of reversed polarity, which are records of the oldest known well-documented and stratabound reversals of the geomagnetic field, that even pass the reversal test. The palaeomagnetic data show a rapid phase of rifting across an unconformity that represents a significant change in basalt geochemistry. Combined data indicate a shift of at least 1600 km in ca. 3 Myr, which is a speed of 53.5 cm yr-1. The tectonic model for the evolution of Pilbara is in agreement with and supported by the palaeomagnetic data. Substantial horizontal movement is the essence of plate tectonics, and this process probably best explains the latitudinal shift of the Pilbara Craton between 2721 and 2718 Ma.
Samples from underground mines are free of lightning induced magnetization, but unfortunately appear to be affected by a chaotic remagnetisation, which is possibly shock induced. 
Looking at cores with Maarten de Wit ... A paper about this work on the Kaapvaal Craton can be summarised as follows: The lack of proper field tests inhibits solid proof that the rocks of the Kaapvaal Craton have preserved their original magnetization. Field tests of our study were inconclusive, and the only publication that has a positive conglomerate test leaves room for doubt about the primary nature of the studied magnetization. We conclude that the APWP of the Kaapvaal Craton is not well determined between ca. 3.0 and 2.3 Ga.  |
||||
|
|
||||
2009
|
