Further palaeomagnetic results from the Sierras Australes fold and thrust belt, Argentina

Abstract

Summary The southwestern Gondwanaland margin during the late Palaeozoic was located between the South America craton and the Patagonia terrane. The Sierras Australes fold and thrust belt of Buenos Aires province (Argentina) has been interpreted as being the result of a crustal collision between these two blocks. A palaeomagnetic study of the less deformed upper part of the Permian Tunas Formation of the Pillahuinco region within the Sierras Australes belt was carried out in order to obtain reliable palaeomagnetic poles to constrain the palaeogeographic and tectonic evolution of this part of Gondwana. More than 300 specimens were measured from the Tunas Formation in the Sierra de Pillahuinco, from 27 sites in four localities of these red sandstones and clay-siltstones. Demagnetization at high temperatures isolated a reversed characteristic remanent magnetization, consistent with magnetization acquired during the Kiaman reverse superchron. Stepwise tectonic tilt correction suggests that the upper part of the Tunas Formation acquired its magnetization before or early in the deformation. The position of the palaeomagnetic pole on the apparent polar wander path of South America suggests that the magnetization was acquired some time between the late Early Permian and the early Late Permian. From the best grouping of the partially tilt-corrected remanence directions, at 90 per cent unfolding for 24 sites, a mean palaeomagnetic pole Tunas II for the Tunas Formation of the Pillahuinco region was calculated. The position is longitude 025.9°E, latitude 74.1°S; A95 = 5.2°, K = 33.6. This pole is consistent with previous poles from South America assigned to a similar age and allows for a more accurate definition of the apparent polar wander path. An earlier study in the lower part of the Tunas Formation (Tunas I) shows syntectonic magnetization acquired during the Early Permian. Therefore the differences in the type and age of magnetization between the lower and upper parts of the Tunas Formation suggest that the deformation was diachronous and propagated more slowly towards the foreland. Palaeogeographically, these two Tunas Formation poles are interpreted as recording a slight counterclockwise movement of South America during the Permian.