Journal of South American Earth Sciences, Vol. 4, No. 1/2, pp. 2541, 1991 Printed in Great Britain 0895-9811/91 $3.00+ 0.00 © 1991 PergamonPress plc & Earth Sciences& ResourcesInstitute Preliminary paleomagnetic data from the Sierra Grande Formation: Tectonic consequences of the first mid-Paleozoic paleopoles from Patagonia A. E. RAPALINI 1. and J. F. VILAS2~ Dept. Geol. ScL, Polytechmc South West, Drake Circus Plymouth, Devon, PL4 8AA England UK; Lab. de Paleomag. Darnel A. Valenclo," Depto. Clen. Geol., Fac. Cmn. Exactas Nat., Umvermdad Buenos Aires, Ciudad Universitaria, Pabell6n 2, 1428 Buenos Aires, Reptlblica Argentina (received January 1990; acceptedNovember 1990) Abstract--The first two mid-Paleozoic paleomagnetic poles from Patagonia were obtained from a preliminary paleomagnetic study of the Sierra Grande Formation, exposed at 65.4°W, 41.6°S in Argentina. The firstpole,SG1, is located at 238.0°E, 3.4°N (N= 14, D1 = 14.3°,D2 =8.5 °)and corresponds to a ferrfferous horizon intercalated in the lower member of the formation, which isassigned to the Wenlockian. The second pole, SG2, located at 283.5°E, 42.0°S (N=10, DI =20.4 °,D2=20.2°), belongs to the upper member of the for- mation and is assigned to the Late Silurian-Early Devonian. These pole positions are interpreted according to differenttectonic hypotheses. The position of SG1 is more easily understood ifPatagonia had an alloch- thonous origin, for itdoes not agree with the presently most accepted Silurian pole position of Gondwana. Nevertheless, the position of SG2, consistent with the Late Silurian-Early Devonian Gondwana poles, suggests that Patagonia was next to collidewith South America in the Early Devonian. On the other hand, the hypothesis of an autochthonous Patagonia issupported by the consistent position of SG2 with respect to the coetaneous poles of Gondwana, while SG1 would indicate that the reversed polarity path of Schmidt and Morris (1977) should be taken as the most probable early-middle Paleozoic apparent polar wander path of Gondwana. Resumen--Un estudio paleomagndtico preliminar de la F. Sierra Grande, aflorante a los 65.4°0, 41.6°S, Argentina, dio como resultado los dos primeros polos paleomagn~ticos del Paleozoico Medio de Patagonia. SG1, el polo paleomagn~tico del horizonte ferrifero Rosales, intercalado en el miembro inferior de la formaci6n y asignado al Wenlockiano, esta ubicado en 238.0°E, 3.4°N (N = 14, D1 = 14.3°,D2 =8.50), mientras que SG2, correspondiente al miembro superior de la formaci6n, de edad Siltirico tardio-Devbnico temprano est~ situado en 283.5°E, 42.0°S (N=10, DI=20.4 °, D2=20.2°). Estas posiciones polares ban sido inter- pretadas de acuerdo a dos hip6tesis dfferentes sobre la evolucibn tect6nica de la Patagonia. La posici6n de SG1 es facilmente interpretada si se supone un origen al6ctono para la Patagonia, ya que no coincide con la posici6n polar siltlrica del Gondwana aparentemente m~is aceptada actualmente. La posici6n de SG2, con- cordante con los polos coethneos gondwhnicos, indica en este caso que para el Dev6nico temprano la Pata- gonia se encontraba muy cerca de Amdrica del Sur y pr6xima a su colisi6ncon la misma. Por el contrario, la hip6tosis de un origen aut6ctono para la Patagonia es avalada por la posicion de SG2; mientras que SG1 indicarla que la curva de desplazamiento polar aparente, propuesta primeramente por Schmidt y Morris (1977) y basada en adoptar una polaridad opuesta para los polos del Paleozoico Inferior y Medio del Gond- wana es la m~s probable, ya que solo es consistento con la misma. INTRODUCTION IN THE FIRST PART of this century, some geologists proposed that the extreme southern part of South America, called "the Patagonian Block," had a geo- logic and tectonic evolution apart from the South American continent, the Brazilian Craton, during part of the Phanerozoic (Keidel, 1925; Windhausen, 1931). In the past decade, other authors have sug- gested models for the tectonic evolution of southern South America that include an allochthonous origin for Patagonia. These tectonic models can be divided into two different types. The first proposes a north- western (present--day coordinates) provenance for Patagonia, with docking mainly by sinistral trans- currence, probably during the late Paleozoic to early Mesozoic (Dalmayrac et al., 1980; Martinez, 1980; *For reprints: Tel:44-752-233100; Fax: 44-752-232293 rE-mail: postmaster%lpgfcg.edu.ar@uunet.uu.net Sell6s Martinez, 1988). The second suggests a frontal collision of the Patagonian Block with the South American continent (Ramos, 1984; 1986; 1988); this hypothetical collision would have ocurred between the Late Devonian and Late Permian. Nonetheless, many geologists have preferred an "autochthonous hypothesis" -- i.e., that Patagonia and the Brazilian Craton have not undergone important relative dis- placements since at least the mid- or late Paleozoic (Halpern, 1972; Forsythe, 1982; Uliana and Biddle, 1987; among others). Some paleomagnetic data from late Paleozoic rocks in the Southern Andes outside Patagonia (Vilas and Valencio, 1982) were initially interpreted as possible evidence of a late Paleozoic aecretional origin of that region and Patagonia. However, new paleo- magnetic data (Vilas and Rapalini, 1989; Bobbio et aI., 1990) have ruled out this interpretation. Unex- pected warm-climate Permian fossils found in Pata- SAES--4:I:2--C 25