A Large counterclockwise movement of Western Gondwana during the Upper Paleozoic, infered from paleomagnetic data.

TOMEZZOLI, R.N.

Mendoza

Simposio; Gondwana 12.; 2005

Current paleomagnetic results are summarized from the southwest Gondwana boundary, in the western part of an orogenic belt known as the “Samfrau Geosyncline” or Cordón de las “Gondwánides”, exposed along central Argentina. This area may represent the suture zone between Gondwana and the Patagonia terrane. This collision model is still uncertain and there are many controversies related to the evolution of this orogenic belt. Paleomagnetism and the exploration of the magnetic fabric signatures (AMS) are useful to evaluate the regional deformation and a better understanding of the assembly, deformation, and fragmentation of Gondwana / Pangea. A systematic paleomagnetic study on Late Paleozoic rocks along this belt is reported. Those include: a) the Sierras Australes (38°S, 62°W; 55 sites on sedimentary rocks), b) in the Carapacha basin (38°S, 65°5W; 24 sites on sedimentary and volcanic rocks), and c) Cerro Centinela (36ºS, 67ºW; 19 sites on volcanic rocks belonging to the Choiyoi Group). Standard high temperature demagnetization techniques were selected as the demagnetization procedure. A high un­blocking temperature component carried generally by hematite and seldom by magnetite was defined between 500°C and 680°C, showing very good within-site consistency (a95<15° and k>20). All rocks are carriers of reversed characteristic remanent magnetization, suggesting that the magnetization was acquired during the Kiaman reverse superchron. Two groups of remanence directions were isolated in all three localities. Populations 1 and 2 are clearly separated. Population 1 is a syn-tectonic magnetization in the area of the Sierra de la Ventana and post-tectonic in the Carapacha basin (west of Sierra de la Ventana). Population 2 is a younger magnetization and postdates any deformation. In the Cerro El Centinela, Population 1 is at the bottom of the sequence (older), while Population 2 is in the top layers (younger). High quality paleomagnetic poles were computed for each population in each locality on the basis of the best grouping mean directions and allow a more precise tracking of the apparent polar wander path for South America during the late Paleozoic. All poles are consistent with the age of the rocks and with coeval paleomagnetic poles from others regions. The Population 1 paleopole is in the Early Permian position and indicates syntectonic paleopoles for South America. Population 2 paleopole is younger and does not reflect effects of deformation. With these poles it is possible to constrain the age of the deformation along this belt to the early Permian and related to the San Rafaelic orogenic phase. The presence of younger magnetizations imply a migration of the orogenic front to the ENE, with a gradual attenuation of the deformation. Deformation began earlier in the western areas and was contemporaneous with deposition in the eastern areas. The paleogeographic implication of these results are interpreted as a significant counter-clockwise movement of Gondwana between the Early and Late Permian. In accordance with these results, the deformation (probable collision between Patagonia and the South American craton) began during the Devonian in the west and propagated eastward until the Permian and this movement could be the consequence of a late adjustment between the continental blocks before the final assembly of Pangea.