Crustal breakup and continent-ocean transition at South Atlantic conjugate margins

Abstract

[1] Seismic reflection and refraction profiles, and potential field data, complemented by crustal-scale gravity modeling and plate reconstructions are used to study the evolution of the central and south segments of the South Atlantic conjugate margins. The central segment is characterized by a hyperextended continent-ocean transitional domain that shows evidence of rotated fault blocks and a detachment surface active during rifting. A polyphase rifting evolution mode, associated with a complex time-dependent thermal structure of the lithosphere, is substantiated for the central segment that is not a "magma-poor" end-member. Increase of volcanic activity during the late stages of rifting may have "interrupted" the extensional system implying a failed exhumation phase that was replaced instead by continental breakup and emplacement of fully igneous crust. The continent-ocean transitional domain along the "magma-dominated" south segment is characterized by a large volume of flood basalts and high-velocity/high-density lower crust. The northern province of the south segment is characterized by symmetrical seaward-dipping reflections and symmetrical continent-ocean transitional domain. The influence of the Tristan da Cunha plume on this province is very likely. The central province of the south segment is characterized by along-strike tectonomagmatic asymmetry, which can be caused by the initial continental stretching and accompanying magmatism rather than by the subsequent seafloor spreading. The Tristan da Cunha plume on the central province may have influenced the volume of magmatism but did not necessarily alter the process of rifted margin formation, implying that the central province of the south segment may have much in common with "magma-poor" margins.

Architecture of rifted continental margins and break-up evolution: insights from the South Atlantic, North Atlantic and Red Sea–Gulf of Aden conjugate margins

“…14) indicates that the mantle rocks are strongly uplifted during the lithospheric stretching and crustal thinning, but volcanic rocks (including wedges of seaward-dipping reflectors) occur above the crust-mantle transition. The oceanic crust in northern Pelotas Basin is marked by magnetic anomalies indicative of organized spreading (Moulin et al 2010;Blaich et al 2011), similar to the present-day situation of the Gulf of Aden and some segments of the Red Sea.…”

“…The interpretation of regional deep seismic profiles in the Aptian salt basins suggest that evaporites were mobilized from the platform towards the incipient axial troughs and advanced as allochthonous salt tongues towards the continent-ocean boundary Blaich et al 2011). This mechanism resulted in massive salt walls separated by an incipient spreading center, in a process similar to what is observed in the Red Sea margins, where the initial continuous salt layer is ruptured by the spreading center, but is still questionable for the South Atlantic (e.g., Girdler & Whitmarsh 1973;Jackson et al 2000;).…”

“…Regional, deep penetrating seismic images through the SE Brazilian and conjugate West African margin indicate that several segments of these margins are characterized by the presence of seaward-dipping reflectors in the transition from continental to oceanic crust (Gladczenko et al 1997;Franke et al 2007;Blaich et al 2011).…”

“…22) indicates that the salt tectonic domains are controlled by the crustal architecture underlying the sedimentary basins, and the structural styles are highly influenced by subsidence patterns of each domain. On both margins, the conjugate margin seismic transect indicates extensional and compressional salt tectonic domains and igneous addition in the crust near the COB boundary (Mohriak 2003;Mohriak et al 2008;Blaich et al 2011).…”

“…The peak of magmatic activity is associated with the Serra Geral -Etendeka volcanism that pre-dates synrift sedimentation (Mizusaki et al 2002). However, wedges of seaward-dipping reflectors post dating the Serra Geral -Etendeka volcanics are interpreted in the conjugate margins, not only in the Pelotas -Namibe basins, but also in the Santos-Campos-Espírito Santo province and in the African conjugate margin (Chang et al 1992;Gladczenko et al 1997;Jahnert et al 1998;Franke et al 2007;Blaich et al 2011). These reflectors are interpreted to mark the continentocean transition towards an oceanic crust that was developed probably by Aptian time (about 112 Ma).…”