Paleomagnetic constraints on the origin of Patagonia

Abstract

After four decades of investigations and disputes, whether Patagonia is an autochthonous or an allochthonous block to Gondwana and if collided against this megacontinent margin at some time during the Paleozoic or Early Mesozoic is still a matter of controversy. Since the early 1990′s paleomagnetic studies have been carried out on Paleozoic rocks exposed mainly in the North Patagonian Massif (NPM) to test these opposite models. However, progress has been slow due to paucity of exposures of suitable rocks. Seven paleomagnetic poles with ages ranging from the Late Ordovician to the Late Permian –Early Triassic have been published. Some of them do not fulfill several of the accepted reliability criteria, while others appear more robust. A comparison of these pole positions with the reference apparent polar wander path for Gondwana in the Paleozoic suggests that the NPM was at paleolatitudes compatible with those corresponding to SW Gondwana since the Late Ordovician. This can be taken as evidence in favor of an autochthonous or para-autochthonous origin of NPM with respect to Gondwana. The latter is however favored due to compelling evidence of a major orogenic event between NPM and Gondwana in Permo-Triassic times that led to the Gondwanides orogen, the development of the huge syn-orogenic Hesperides Basin and a widespread regional remagnetization that affected many geological units in the Rio de la Plata craton. Furthermore, available paleomagnetic poles suggest a rigid-body counter-clockwise rotation of NPM (and the whole of Patagonia?) during final accretion to the SW Gondwana margin. Prior to the Late Ordovician there is no paleomagnetic control on the paleoposition of Patagonia.