Structural architecture and nature of the continent-ocean transitional domain at the Camamu and Almada Basins (NE Brazil) within a conjugate margin setting

Abstract

Abstract Regional seismic reflection profiles and potential field data across the conjugate magma-poor Camamu/Almada–Gabon margins, complemented by crustal-scale gravity modelling and plate reconstructions, are used to reveal and illustrate the relationship of crustal structure to along-margin variation of potential field anomalies, to refine and constrain the continent–ocean boundary, as well as to study the structural architecture and nature of the continent–ocean transitional domain. The analysis reveals that the prominent conjugate Salvador–N'Komi transfer system appears to be a first-order structural element, governing the margin segmentation and evolution, and may have acted as an intraplate decoupling zone. The continent–ocean transitional domain, offshore northeastern Brazil, is characterized by rotated fault-blocks and wedge-shaped syn-rift sedimentary sequences overlying a prominent and undulated reflector (‘M-reflector’), which in turn characterizes the boundary between an extremely thinned, possibly magmatically intruded, continental crust and normal lithospheric mantle. The ‘M-reflector’ in the northeastern Brazilian margin shows remarkable similarities to the S-reflector at the West Iberia margin. In the same way, the ‘M-reflector’ is interpreted as a detachment surface that was active during rifting. Unlike the well studied central and northern segments of the West Iberia margin, however, the present study of the northeastern Brazilian margin does not clearly reveal evidence of an exhumation phase. The latter predicts exhumation of middle and lower crust followed by mantle exhumation. Increase in volcanic activity during the late stages of rifting may have ‘interrupted’ the extensional system, implying a failed exhumation phase. In this setting, the break-up and drift phase may have replaced the exhumation phase. Nevertheless, the available observations cannot discount the possibility that the ‘M-reflector’ is underlain by partially serpentinized mantle. Our study further leads to the development of a detailed conceptual model, accounting for the complex tectonomagmatic evolution of the conjugate northeastern Brazilian–Gabon margins. This model substantiates a polyphase rifting evolution mode, which is associated with a complex time-dependent thermal structure of the lithosphere. In the conjugate margin setting, asymmetrical lithospheric extension resulted in the formation of the thinned continental crust domain prior to the formation of the approximately symmetrical transitional domain.