Palaeomagmetism of Permian rocks from Sardinia - Testing Pangaea B
Final Report Abstract
Although the Wegenerian model of Pangaea, generally known as Pangaea A, is almost universally accepted for the time just prior to the opening of the Atlantic, a massif and sometimes very controversial debate exists on Pangaeas pre-Jurassic configuration which has been triggered by the fact that a significant overlap of the northern (Laurasia) and southern continents (Gondwana) exists in the Permian if these are positioned according to their palaeomagnetically derived palaeolatitudes. One of several ways to solve this problem is to introduce Pangaea B during the early Permian. Pangaea B differs from Pangaea A in so far as the southern continents have been moved to the East by three thousand kilometres. In order, however, to arrive at the Pangaea A configuration which was the starting point for the opening of the Atlantic, a dextral megashear zone is required between northern and southern continents along which the southern continents have been transferred into their pre Atlantic-opening position. If this model is valid, Sardinia should have been caught within this shear zone and suffered massive internal deformation, which in turn can be detected applying palaeomagnetic methods. In the course about 200 oriented samples, covering 23 sites and a variety of rock types (volcanic rocks and continental redbeds) have been taken from the Nurra, Gallura, Iglesiente and Escalaplano regions, representing major basins of Permian age. The overwhelming majority of the specimens studied displayed rather simple directional behaviour. A positive intraformational conglomerate test as well as the general improvement of the between-site scatter after correcting for minor tilt supports the primary character of the magnetisation. The resulting mean direction from the Nurra basin agrees very well with data from coeval rocks from southern Corsica implying crustal coherence between these regions. Furthermore, the resulting palaeopoles plot on the Permian part of the European Apparent Polar Wander Path, after restoring Corsica and Sardinia into their pre-Miocene position. However, the poles from the Nurra basin and those from Central and Southern Corsica are displaced counterclockwise by 10° and 70° respectively, from the reference polar wander curve. The leads to the conclusion that Sardinia cannot be consiedered as a coherent block. What is puzzling and hasn't been solved yet, however, is the fact that the rotations observed in Central and Southern Sardinia are counterclockwise (when minimizing the amount of rotation) which is counterintuitive to the model outlined above. However, before a final interpretation can be given, the age of these rotations has to be firmly established by palaeomagnetic studies concentrating on overlying rock series.
Publications
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2004, Permian Sardinia Megapuzzle?, Rocky Mountain (56th Annual) and Cordilleran (100th Annual) Joint Meeting (May 3-5, 2004), Paper No. 34-6
Muttoni, G., Ronchi, A., Kent, D.V., Bachtadse, V., Moser, E., Garzanti, E.
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2005, Palaeomagnetism of Permian sediments and volcanic rocks from Sardinia. American Geophysical Union, Fall Meeting San Francisco
Moser, E., Emmer, B., Bachtadse, V., Kent, D.V., Muttoni, G., Ronchi, A.
