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Tectonic Influence of the Pre-break-up Basement on the Mesozoic Half Grabens: The Atlantic Margin (33 and 41 S), Offshore Uruguay and Argentina)

Rossello, Eduardo A.1; Lopez-Gamundi, Oscar; Guillocheau, Francois; and de Santa Ana, Hector
1[email protected]

The opening of the South Atlantic is characterized by a diachronous extensional phase dominated by Mesozoic half-grabens followed by sag and passive margin phases during the Tertiary. These three phases are fully developed along the wide continental platform developed in the southern segment of the South American plate.

This records have been deposited on heterogeneous nature of units principally made up of: a) Precambrian crystalline and high-grade metamorphic rocks of the La Plata craton, b) Relatively thin calcareous and siliciclastic shelfal sequences of the Neoproterozoic La Tinta Group, c) Lower Paleozoic sedimentary rocks prograding toward SW and, d) Late Paleozoic Gondwana sequences developed in a broad foreland basin prograding toward NE. The last units are equivalent to those exposed onshore in the Paraná basin (Brazil, Uruguay and Argentina), the Sierras Australes (Argentina) and Sierras Septentrionales of Buenos Aires (Argentina). These Paleozoic sequences were considered as the western part of the former du Toit's (1927) "Samfrau Geosyncline" related to the Karoo basin-Cape foldbelt (South Africa) and Malvinas (Falkland) Islands based on similar paleoclimatic, paleogeographic and paleotectonic features.

The structural framework of the Pelotas, Punta del Este, Santa Lucia, Salado and Colorado basins is largely controlled by the different rheologies and anisotropies present in compositionally heterogeneus basement. This heterogeneity led to differential tectonically-induced subsidence. The Punta del Este, Santa Lucia, Salado and Colorado basins exhibit half-graben geometries at the rift stage followed by laterally extensive Tertiary sag sediments deposited by the proto La Plata and Colorado rivers. In contrast, fully developed half grabens have not been identified in the Pelotas basin. The rifts present in the Punta del Este basin have a Jurassic-Cretaceous fill and show a dominant westward vergence, opposite to the present deepening of the continental margin. Volcanism coeval with sedimentation is rare in these basins with the exception of Jurassic to Cretaceous alkaline volcanics in the Santa Lucia basin.


AAPG Search and Discovery Article #90166©2013 AAPG International Conference & Exhibition, Cartagena, Colombia, 8-11 September 2013