High Resolution Paleomagnetism and Magnetostratigraphy of Ferrar Volcanic rocks from the Southern Mesa Range (North Victoria Land, Antarctica)
Final Report Abstract
The Transantarctic mountains predominantly consist of Jurassic continental flood basalts (Kirkpatrick) and sills (Ferrar) emplaced during the earliest phase of the break-up of Pangea. Published ages, based on a variety of geochronological methods all agree rather well and cluster around 180 Ma suggesting emplacement during a rather short time interval of not more than 3 Myr, probably significantly shorter. Paleomagnetic studies, mostly carried out between the 60s and 90s of the last century, however, yield two well defined but significantly different groups of paleomagnetic pole positions. Whereas one of these two clusters (A) is defined by intermediate latitudes of the virtual geomagnetic poles at around 55° S, group B pole are characterized by significantly higher latitudes. The significance of cluster A and B have been discussed in great detail, and a general agreement has been reached that cluster A is representative for the geomagnetic field during Jurassic times. The interpretation of cluster B, however, has been less straightforward and several lines of arguments have been brought forward reaching from complete remagnetization during a Cretaceous thermal event to unsuccessful averaging of secular variation. Here we report new data from the Kirkpatrick basalts of Northern Victoria Land, Antarctica, where about 157 oriented paleomagnetic samples were taken, covering about 800 meters of stratigraphy and 23 volcanic flows in Gair Mesa (73.4666° S; 162.8666° E). Maximum unblocking temperatures do not exceed 580° C, maximum coercivities rarely 60mT. After removal of a steep magnetic overprint with rather low coercivities and unblocking temperatures of around 150° C, straight linear segments trending toward the origin of projection are identified in 151 samples from 22 sites. All characteristic directions are of normal polarity. The resulting mean virtual geomagnetic pole (VGP) for the Gair Mesa Range plots at 66.4° S; 227.7° E (α95 of 6.2° , k = 25.7) very similar to group B poles. However, reflecting light microscopy reveals that the magnetic inventory of all samples analyzed is dominated by magnetite showing irregularly curved shrinking cracks and broad ilmenite lamellae, the latter being diagnostic for high temperature oxidation. Shrinking cracks, in turn, are caused by low temperature oxidation that does not alter the original directions of the natural remanent magnetization. Additional material from coeval volcanic rocks from elsewhere in Antarctica (Litell), interpreted earlier to carry secondary magnetizations of Cretaceous age show similar characteristics. Analysis of the distribution of the 22 pole positions obtained here strongly suggests that secular variation has successfully been averaged (Sb = 15.9) leading strong support to our interpretation that the paleomagnetic results from Gair Mesa have recorded the time averaged geomagnetic field during early mid Jurassic times. Based on these new results and including additional evidence from previously published paleomagnetic data from elsewhere in the Karoo-Ferrar magmatic province, we postulate that both clusters A and B of pole positions reflect primary magnetizations and that the Kirkpatrick basalts of Gair Mesa have been emplaced in a rather short time interval of 3 Myr. If our reasoning is correct, the Apparent Polar Wander Path for the latest early Jurassic might be more complex than previously thought. Whether this complexity is evidence for massif True Polar Wander remains speculative.
Publications
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Paleomagnetism of the Jurassic Transantarctic Mountains revisited - evidence for large dispersion of APW within less than 3 Myr. Gondwana Research, Volume 31, March 2016, Pages 124-134
O.S. Lemna, V. Bachtadse, U. Kirscher, C. Rolf, and N. Petersen
