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The Uniqueness of Santos Basin Among all Other South Atlantic Marginal Basins

Abstract

Observing the continental limits of the South Atlantic conjugate margins, one region where the asymmetry is extreme, compared to its African counterpart, is the region of the Santos Basin, SE Brazil. In order to resolve a tectonic problem of overlapping spreading centers, Santos Basin must have worked as an accommodation zone. This phenomenon caused a resistant crustal block to rotate acting as a transcurrent margin inducing an extreme asymmetry with its African counterpart. A complete geophysical data set was used for this study including wide angle seismic, deep reflection seismic, gravity and magnetic. The geological model shows a proximal domain with small scale normal faults corresponding to the initial stages of rifting undergone to different degrees of erosion; a well defined necking zone where the continental crust thins from 40 km to 20 km thick; a 690 km wide distal domain. The central portion of the distal domain includes a hyperextended crust with interpreted serpentinized upper mantle underlying the continental crust. This structure is consequence of large scale detachment faults, responsible for accommodating a huge amount of horizontal movement. In the outer distal domain, a thinned resistant block was identified. This block is due to a late rotation rather than continuous rifting, that caused the asymmetry and created the transcurrent margin with its African counterpart. The continent-ocean-transition is marked by an allochthonous crust with two possible interpretations in terms of composition: serpentinized upper mantle or gabbroic crust. True oceanic crust is interpreted to lay 740 km away from the Cretaceous hinge line of Santos Basin. A reconstruction of the continental crust was performed with the objective to show the importance of detachment faults to accommodate all the horizontal movement and also to show how the rotation of a resistant block was responsible for creating the asymmetry and a transcurrent. The particularity of Santos Basin concerns not only the latest giant oil discoveries in the pre-salt region, which changed the scenario of hydrocarbon production in Brazil, but also the geological complexity. How continents break apart is still an opened question and depicting the formation processes of Santos Basin is a key point for understanding all other South Atlantic marginal basins.