Genesis and Evolution of the Punta del Este Basin, Offshore Uruguay: Complex Interplay Between Structural Inheritance and Multi-Phase South Atlantic Rifting

Abstract

The crustal structure along the Uruguayan margin is highly complex as a direct result of the pre-rift structuration and subsequent multi-phase rifting. This complexity has major implications for understanding rifted margins and their subsequent hydrocarbon exploration. Long-offset 2D and regional-scale 3D seismic data from offshore Uruguay show how initial rifting developed normal to the margin becoming progressively more oblique over time. This multi-phase rifting created regionally thinned oceanic crust and a major oceanic depression (Rio Grande Depression) extending over 500km from offshore Uruguay up to the Florianopolis High in Brazil. The southern end of this depression, directly offshore Uruguay, is marked by a sigmoidal pull-apart basin (Punta del Este Basin) covering over 20,000km². Limited age data along the margin suggest these structural features formed from the Hauterivian to mid-Aptian. The configuration of the Punta del Este Basin is highly complex and is fault-bounded to the north and south against packages of seaward dipping reflectors and oceanic crust; to the east the basin is fault-bounded with oceanic crust. The western margin is marked by the Polonio High, a large granitic body that is part of the Neoproterozoic Dom Feliciano Belt. Regional mapping suggests that this granitic body formed a major terrain boundary prior to South Atlantic rifting, influencing the location of the La Plata rift, which extended south of the Polonio High offshore Uruguay to the south-west. The lack of rift basin development within the Polonio High, except at the margins, suggests that regional strain was not uniformly distributed across the margin but accommodated offshore. The offshore crust is highly complex, comprising stretched/fragmented continental crust, intrusive volcanics and a spectacularly imaged failed spreading centre underneath which is an elevated Moho with a crustal thickness of 3km locally. As South Atlantic rifting continued various oceanic fracture zones were created, including the Meteor Fracture Zone which is a direct consequence of the location of the Punta del Este Basin and the Polonio High. The Meteor Fracture Zone/ Transform offsets the present day Atlantic mid-ocean ridge by c. 250km. However, there is no evidence for this level of displacement along the continental Uruguayan margin. As such, we interpret that the majority of this ‘offset’ is accommodated within thinned oceanic crust manifested as the Rio Grande Depression.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016