SYNTECTONIC MAGNETIZATION OF THE MIDPALEOZOIC SIERRA-GRANDE FORMATION- FURTHER CONSTRAINTS ON THE TECTONIC EVOLUTION OF PATAGONIA

A palaeomagnetic study was carried out in the Silurian-Devonian clasti c sedimentary rocks of the Sierra Grande Formation, exposed in northea stern Patagonia (41.6 degrees S, 65.3 degrees W). Thirteen sites (n=88 ) were located on opposite limbs of a syncline-anticline structure. St epwise thermal demagnetization permitted the identification of a very stable magnetic component of reversed polarity carried by hematite. St epwise performance of the fold test yielded negative results both in s itu and after 100% bedding correction, but positive after partial unfo lding (19%). This indicates a syntectonic origin for the isolated magn etization. A pole position was computed for the partially (19%) correc ted remanence: SG3: 77.3 degrees S, 310.7 degrees E, delta p=7.7 degre es, delta m=6.6 degrees, N=13. Its position is coincident with late Ea rly to Late Permian palaeomagnetic poles from South America, suggestin g that age for the previously undated folding of the Sierra Grande seq uence and therefore for the main tectonic event that affected the nort hern boundary of Patagonia. Palaeozoic palaeomagnetic poles from Patag onia obtained to dale agree with those from Gondwana of Devonian or yo unger age, suggesting that Patagonia did not undergo important displac ements relative to South America since those times. This and the Permi an age of deformation determined in this study invalidates tectonic mo dels involving collision of a Ear-travelled Patagonia with Gondwana in the mid-or Late Palaeozoic.