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Abstract

Within the scope of our ongoing seismic reflection interpretations of basement at magmatic continental margins, and in particular those of the South Atlantic, we report on our current views concerning the São Paulo Plateau offshore Brazil, which involves the Campos, Santos, and Pelotas basins. In addition, much can be gleaned by integrating the African conjugate margin, which we also consider to a lesser extent. Our broad-brush view for this segment of South Atlantic rifting is that:

  1. A broad zone of mixed magmatic/thinned continental crust formed between opposing “hinge zones” of the two larger plates during an initial period of intracontinental extension, thereby generating the synrift section in numerous grabens and half-grabens;

  2. This basinal area of thin, mixed crust began to subside without much further faulting for an interval, thereby generating “sag sections” along both continental margins;

  3. Plate divergence was renewed or accelerated and the thin crustal region and sag section broke up in what effectively could be viewed as a second rifting episode, and which created an irregular zone of deeper basement comprising faulted subsag crust, exhumed mantle, and magmatic build ups outboard of the respective sag sections; and

  4. Plate divergence was finally taken up by more normal styles of seafloor spreading.

Definition of crustal type is hindered in seismic reflection interpretation by whether or not there should be much observable difference in the crustal structure resulting from rifting of a pre-existing oceanic plateau versus rifting of already-thinned continental crust. After all, Iceland is covered by normal faults, and if extension continued as magmatic supply waned, we might expect the resulting surface to have many geometries of rifted continental crust. However, the 080° trending Atlantic fracture zone fabric east of the São Paulo Plateau appears to us to dominate the crustal fabric under the Plateau as well, and thus we strongly suspect spreadingrelated processes in the development of the Plateau east of the sag section, at azimuths similar to the well-mapped parts of the South Atlantic Ocean.

Within this 3-staged context, outlined as a general overview of “basin zones” and basin opening kinematics, our main objective here is to propose the idea of the generation of “lateral tectonic accommodation space” for salt accumulation in the central South Atlantic. We envision that salt flowed seaward at both conjugate margins as tectonic extension and/or seafloor spreading continued, and that the “lateral accommodation space” created by the extension was continuously filled with migrating salt while new salt was deposited.

Further, the hypothetical instantaneous salt deflation at the depositional surface as a result of the tectonic extension and salt migration was constantly replaced by the deposition of new salt across the entire basin. This mechanism allows all the salt to be deposited effectively at global sea level, and it entirely avoids poorly supported models, in our view, involving progressive seawater seepage through semi-permeable marine barriers into enormous, deep (>1 km below global sea level), air-filled depressions. While we acknowledge that the basin’s depositional surface needn’t have remained immediately at global sea level at all times during salt deposition, we present the hypothesis as though it were, simply to provide an end member view that opposes the air-filled hole hypothesis.

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