Palaeomagnetic constraints on the age of deformation of the

TOMEZZOLI, R.N.; VILAS, J.F.

GEOPHYSICAL JOURNAL INTERNATIONAL

Royal Astronomical Society

Año: 1999 vol. 128 p. 857 - 870

0956-540X

Summary             A paleomagnetic study of Late Paleozoic rocks exposed in the Sierras Australes thrust and fold belt of Buenos Aires province (Argentina) consisting of early Permian red sandstones and clay-siltstones of the Tunas Formation, has been carried out. The sections sampled are exposed in the eastern parts of the belt, in Sierra de las Tunas (north) and Sierra de Pillahuincó (south). More than three hundred specimens were collected from twenty-five sites in three localities, of differing structural attitude. Demagnetization at high temperatures isolated a characteristic remanence at 20 sites; this is a reverse characteristic remanence at all localities, suggesting that the magnetization was acquired during the Kiaman interval. Stepwise tectonic tilt correction suggests that the Tunas Formation in these localities acquired its magnetization during folding in early Permian times. Paleomagnetic poles were computed for each locality based on partial tilt-corrected remanence directions. Considering that these localities are in the same area close to one another and that the rocks are all of reverse polarity, a group syntectonic paleomagnetic pole called Tunas was calculated: longitude: 13.9° E, latitude: 63.0° S; A95 = 5.4°,  K = 39.7, N =19. This pole is consistent with previous poles from South America assigned to the early Permian. In age it corresponds to the early Permian San Rafaelic tectonic phase of the Sierras Australes. Independent geologic evidence indicates that the Tunas Formation underwent syn-depositional deformation. We conclude that the Tunas Formation was deposited, deformed and remagnetized, all during the early Permian. Key words: paleomagnetism, tectonics, Paleozoic, Gondwana, South America, Argentina, Sierras Australes