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Título

STRUCTURAL GEOLOGY OF THE RIO GRANDE FAN – PELOTAS BASIN, OFFSHORE BRAZIL

Texto do resumo

Intraplate continental margins produced by rifting and breakup tectonic processes have been interpreted according to the passive basement paradigm, which emphasizes gravitational processes and withholds far-field tectonic activity. However, detailed structural analysis in adjacent onshore margins, geomechanic studies, and advances in seismic imaging technology in offshore basins have been revealing evidence of tectonic and particularly neotectonic activity during the evolution of intraplate continental margins.
This work focuses on the structural geology of fault patterns observed during seismic reprocessing of vintage 2D data from the Rio Grande Fan (RGF), a striking Neogene feature in Pelotas Basin, in southern Brazil. The faults are associated with a detached wedge with overall proximal extension and distal compression. Such structures are often defined as fully decoupled gravitational systems. We integrated the interpretation of seismic, magnetic, and gravimetric data in the RGF to study the fault patterns.
Deformation analysis in seismic data showed an evident obliquity of the mapped fault trends, with proximal NNE-SSW normal faults, and a distal deepwater fold and thrust belt (DWFTB) trending N60E. The misaligned structural pattern, and the gross rhombohedral geometry of the main depocenter of the RGF suggest that it was probably formed by strike-slip flexural processes, a recently proposed subsidence mechanism. In this way, it is proposed that the detached gravitational wedge of the RGF evolved under the influence of regional fracture zones (FZs) trending E-W. The FZs follow the projection towards the continent of the oceanic lineaments Chuí and Jaguarão, well defined in magnetic, gravimetric and bathymetric data, at the southern and northern limits of the RGF. The strike of the mapped DWFTB fits quite well with right-lateral kinematics along the Chuí FZ. The observed control of Neogene deformation by plate-scale active structures at the boundary between continental and oceanic crust, in turn suggests basement coupling, and neotectonic reactivations in Pelotas Basin, at Brazil´s intraplate continental margin in the South-Atlantic.

Palavras Chave

Rio Grande Fan; Pelotas Basin; Intraplate Margins; Neotectonics; Flexural Strike-Slip Processes