The accretionary history of Southern South America from the latest Proterozoic to the Late Paleozoic: Some Paleomagnetic Constraints

RAPALINI, AUGUSTO E.

Terrane Processes at the Margins of Gondwana, Geological Society of London, Special Publication, 246

Geological Society of London

Lugar: Londres, UK; Año: 2005; p. 305 - 328

Abstract             It is now accepted that southern South America have been formed from several terranes of diverse origin and evolution. However, a detailed history of the accretionary processes has not been unraveled yet. Paleomagnetism can play an important role in such  an endeavour. Paleomagnetic constraints on the tectonic evolution of this region in the Proterozoic and Paleozoic are reviewed and discussed. Data from the Rio de la Plata craton suggest that this block was already assembled to most major Gondwana blocks by the end of the Proterozoic and that may have formed a single continental mass with the Congo-Sao Francisco, West Nile and Arabia through out most of the Vendian. A large ocean separating these cratons from Amazonia and West Africa, prior to Gondwana assembly, is supported by available paleomagnetic data. To the west of the Rio de la Plata craton is the Pampia terrane. Despite lack of paleomagnetic data, geological evidence supports a model of Early Cambrian collision between these blocks. An Early Ordovician magmatic arc, the Famatina-Eastern Puna belt, developed on the western margin of the already accreted Pampia terrane, shows a systematic pattern of large clockwise rotation that has been interpreted as representative of the whole terrane. A tectonic model that portrays a continental magmatic arc with a back-arc basin to the east that was closed when the terrane rotated is favoured. There is little doubt of a Laurentian origin for the Cuyania (Precordillera) terrane given the amount and diversity of evidence, including paleomagnetism. The tectonic mechanism for accretion and its timing are still controversial. New paleomagnetic data from Late Ordovician rocks of Cuyania supports the “Laurentian plateau” hypothesis which suggests that Cuyania was still linked to Laurentia well into the Ordovician. Nevertheless, these new data do not rule out the more generally favoured  “microcontinent model”. To the west of Cuyania is the Chilenia terrane, separated by a belt of ophiolites of Late Ordovician age. Very little is known about this terrane, although some U-Pb ages and Nd model ages point to a Laurentian origin for its basement. Lack of paleomagnetic data precludes determining its kinematic evolution. The Arequipa-Antofalla block may actually be a composite terrane. Paleomagnetic data obtained so far come exclusively from the southern Antofalla block. Recently acquired data in the western Puna of Argentina confirms the originally proposed distribution of Early Paleozoic paleomagnetic poles, which despite several uncertainties delineate a pattern of significant counterclockwise rotations with a possible anomaly in paleolatitude for the late Cambrian. The data suggest a major tectonic discontinuity between the Eastern and Western Puna of Argentina in the Early Paleozoic. Four paleomagnetic poles of Devonian to Permian age from the North-Patagonian massif are consistent in position and age with the Gondwana APWP, suggesting that both continental masses did not experience major relative displacement since the Devonian. The data do not, however, rule out a restricted separation of Patagonia orthogonal to its northern boundary in the Early or Middle Paleozoic and subsequent collision in the Late Paleozoic.