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Abstract

The Cabo Frio region, offshore Rio de Janeiro, lies between two of the most prolific Brazilian oil provinces, the Campos and Santos basins. Major geologic features have been identified using a multidisciplinary approach integrating seismic, gravity, petrographic, and borehole data. The Cabo Frio frontier region is characterized by marked changes in stratigraphy and structural style and is unique among the Brazilian marginal basins. Major geologic features include the deflection of the coastline and pre-Aptian hinge line from northeast to east; a large east-striking offshore graben related to salt tectonics; a northwest-trending lineament extending from oceanic crust to the continent; basement-involved landward-dipping (antithetic) normal faults in shallow water; a stable platform in the southern Campos Basin; a thick sequence of postbreakup intrusive and extrusive rocks; and, near the Santos Basin, a mobilized sequence of deep-water postrift strata affected by landward-dipping listric normal faults. These faults are unusual in salt-related passive margins in that they dip landward, apparently detach on the Aptian salt, and show large late Tertiary offsets. Locally, the older sequences do not show substantial growth in the downthrown blocks.

South of the Rio de Janeiro coast, a phenomenal landward-dipping fault system detaches blocks of the Albian platform to the north and, to the south, coincides with the depositional limit of the Albian platform. This deep-water fault system controls features that can be mapped for hundreds of kilometers along strike and forms an Albian stratigraphic gap tens of kilometers wide.

Two end-member processes of salt tectonics in the Cabo Frio region result in either synthetic or antithetic basal shear along the fault weld under the overburden: (1) thin-skinned processes, in which the listric faults were caused by salt flow in response to gravity forces related to massive clastic progradation from the continent; and (2) thick-skinned processes, in which faulting was indirectly triggered by diastrophic causes or disequilibrium in the basement topography. The structural styles in the Cabo Frio region are compared with analogs from other basins and with sandbox models.

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