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LIGHT, MALCOLM P. R., Intera, United Kingdom, CARLOS M. URIEN, Urien & Associates, B.A., M. P. MASLANYJ and M. L. KEELEY, Intera, United Kingdom, and S. L. HOGGS, Cullen Valdez Rojas y Asociados, B.A.

ABSTRACT: The Tectonographic Development of Patagonia and its Relevance to Hydrocarbon Exploration

Patagonia accreted successively from the southwest onto the southern margin of the Proterozoic Plata Craton and Brazilian Guapore Shield between the Late Proterozoic and Early Devonian. The thrust-like stacking of terranes onto the southern termination of the Pelotas Terrane is considered to have developed a pervasive northwest to north-trending fabric.

During the Permo-Triassic the northwest to north-trending fabric of the Patagonian Plate was re-activated by dextral strike-slip movement causing extension. The deformation was caused by oblique subduction and accretion of the Madre Dos Dios to Pichidangui Terranes along its western margin. To the northeast the more competent shield underwent compression (Ventania-Gondwanide Folding) and extension occurred parallel to the axis of the embryo South Atlantic, where a shallow sea transgressed.

Ridge on its western side, now preserved on the facing shelf margins of Argentina and Namibia. In the Late Triassic-Lower Jurassic, the Malvinas Microplate was situated south of the Transkei (South Africa) and an intracratonic basin separated it from two sutures formed at the margin of the Argentine Shelf and along the axis of the West Malvinas Basin. Subduction/arc activity on the west flank of this intracratonic basin, in association with trench pull is believed to have initiated Late Triassic-Early Jurassic strike slip extension and volcanicity in Patagonia. This exploited the pervasive northwest and north-trending Paleozoic fabric.

By the Mid-Jurassic the Malvinas Microplate had docked with the eastern margin of the Patagonian Shelf and was undergoing clockwise rotation between the Malvinas-Agulhas and Burwood Bank-Scotia Ridge dextral strike-slip systems. Rifting had now progressed southwestwards to the Pacific and north eastwards to the Colorado and Outeniqua Basins.

The opening of the South Atlantic was heralded in the Mid to Late Jurassic by the Tobifera volcanics in Patagonia. Oceanic spreading began in the Neocomian along the northward propagating axis of the South Atlantic. Continental marginal basins west of the Outer Basement Ridge (San Jorge, Colorado and Salado) continued to develop, as did new detrital strike slip systems to the north, in association with the northward propagation of the rifting. During the Maastrichtian, the Atlantic Ocean transgressed furthest west onto Patagonia. Both the Outer Basement Ridge and continental margin basins were flooded. This transgression occurred after emplacement of the Andean batholith and eastward tilting of the South American continent. Most basinal areas in Argentina underwent thermal subsidence n the Jurassic, Cretaceous and Tertiary? following fault induced subsidence in Triassic-Jurassic times. The tectonostratigraphic model for the development of the South Atlantic places new constraints on the distribution of source rocks, traps and petroleum generation and migration and this has invited a new perspective to be placed on the petroleum potential of Patagonia.

AAPG Search and Discovery Article #90988©1993 AAPG/SVG International Congress and Exhibition, Caracas, Venezuela, March 14-17, 1993.