Ocean-continent transition along the Northeast Brazilian rifted margin

The aim of this thesis was (i) to develop analytical methodologies and a workflow to identify the ocean-continent transition (OCT) and to locate the continent-ocean crustal and lithosphere boundaries (COCB and COLB) at continental rifted margins and (ii) to apply these methodologies and workflow to six cross-sections of the Northeast Brazilian rifted margin. The six cross-sections were taken from the margin segments Almada-Jequitinhonha, Jacuipe, Sergipe, Alagoas and Pernambuco and were constructed using seismic reflection sections from the borders of sedimentary basins to the oceanic crust. The OCT of the Northeast Brazilian rifted margin is complex due to variable extrusive and intrusive magmatism and antithetic tectonics. COCB locations interpreted on seismic reflection data have been tested with those determined using the analytical methodologies and workflow developed in this study. The workflow comprises: 1) the determination of sediment-corrected basement depth profiles, 2) the analysis of the residual depth anomalies of the oceanic crust and its extrapolation into the rifted margin, 3) the gravity inversion of the Moho with crustal thinning determination and 4) the inversion of lithosphere thinning using subsidence analysis. The calibration of the gravity inversion through the fit of the residual depth anomaly in unequivocal oceanic crust is a new approach to determine the reference Moho depth in areas without seismic refraction data. The Almada-Jequitinhonha OCT corresponds to a region of antithetic faulting and low free-air gravity anomaly. The Jacuipe OCT corresponds to a region of development of normal magmatic addition, expressed in the seismic sections as seaward-dipping reflectors, and has a relatively high free-air gravity anomaly. Although the Jacuipe margin represents a narrow rift, this margin seems to be wider and offshore syn-rift sediments are either not imaged or have been eroded. The Sergipe margin is interpreted as being magma-poor during rifting and evolved to slightly magma rich in the early post-rift. The OCT corresponds to a region of continental crust thinner than the adjacent oceanic crust, with antithetic faulting and a stable free-air gravity anomaly. The Alagoas margin is interpreted as being magma-poor during rifting and evolved to normal magmatic addition in the early post-rift, while the Pernambuco margin is interpreted as being slightly magma-rich. The OCT of both margins corresponds to a region of synthetic and antithetic faulting with a low free-air gravity anomaly. The predicted COCB and COLB locations for each cross-section tend to be similar to each other. Only minor differences between the crustal and lithosphere thinning profiles have been observed. The rifted margin width, measured along the cross-sections, does not vary significantly in the South between Almada-Jequitinhonha and Sergipe North, ranging from 110 to 130 km. While, in the North between Alagoas and Pernambuco it is wider, around 170 km. The Northeastern Brazil margin varies from magma-poor in the Almada-Jequitinhonha region to slightly magma-rich in Sergipe and Pernambuco, both probably affected by the Santa Helena Hot Spot. The workflow developed here can be considered successful in locating the COCB and COLB positions and in identifying the OCT in the cross-sections across the Northeast Brazil margin and can be applied to other rifted margins without seismic refraction data.