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

TOMEZZOLI, R.N.,

Geophysical Journal International,

Royal Astronomical Society

Año: 2001 vol. 147 p. 356 - 366

The southwestern Gondwanaland margin during the late Palaeozoic was locatedbetween the South America craton and the Patagonia terrane. The Sierras Australesfold and thrust belt of Buenos Aires province (Argentina) has been interpreted asbeing the result of a crustal collision between these two blocks. A palaeomagnetic studyof 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 reliablepalaeomagnetic poles to constrain the palaeogeographic and tectonic evolution of thispart of Gondwana. More than 300 specimens were measured from the Tunas Formationin the Sierra de Pillahuinco´ , from 27 sites in four localities of these red sandstones andclay-siltstones. Demagnetization at high temperatures isolated a reversed characteristicremanent magnetization, consistent with magnetization acquired during the Kiamanreverse superchron. Stepwise tectonic tilt correction suggests that the upper part ofthe Tunas Formation acquired its magnetization before or early in the deformation.The position of the palaeomagnetic pole on the apparent polar wander path of SouthAmerica suggests that the magnetization was acquired some time between the lateEarly Permian and the early Late Permian. From the best grouping of the partially tiltcorrectedremanence directions, at 90 per cent unfolding for 24 sites, a mean palaeomagneticpole Tunas II for the Tunas Formation of the Pillahuinco´ region wascalculated. The position is longitude 025.9uE, latitude 74.1uS; A95=5.2u, K=33.6. Thispole is consistent with previous poles from South America assigned to a similar ageand allows for a more accurate definition of the apparent polar wander path. An earlierstudy in the lower part of the Tunas Formation (Tunas I) shows syntectonic magnetizationacquired during the Early Permian. Therefore the differences in the type andage of magnetization between the lower and upper parts of the Tunas Formation suggestthat the deformation was diachronous and propagated more slowly towards theforeland. Palaeogeographically, these two Tunas Formation poles are interpreted asrecording a slight counterclockwise movement of South America during the Permian.