Characterization of seaward-dipping reflectors along the South American Atlantic Margin and implications for continental breakup

• Published in: Tectonics
• Main Authors: McDermott, C, Lonergan, L, Collier, JS, McDermott, KG, Bellingham, P
• Format: Article in Journal/Newspaper
• Language: unknown
• Published: European Geosciences Union 2018
• Subjects: Science & Technology; Physical Sciences; Geochemistry & Geophysics; SDRs; continental breakup; South America; seismic reflection data; Tristan plume; NORTH-ATLANTIC; LAVA FLOWS; OCEANIC-CRUST; SEISMIC VOLCANOSTRATIGRAPHY; NORWEGIAN MARGIN; FLOOD VOLCANISM; MANTLE PLUME; EVOLUTION; CONSTRAINTS; EMPLACEMENT; 0403 Geology; 0404 Geophysics; Tristan; North Atlantic
• Online Access: http://hdl.handle.net/10044/1/64298; https://doi.org/10.1029/2017TC004923

Thick packages of lavas forming seaward‐dipping reflectors (SDRs) are diagnostic features of volcanic passive margins. Despite their significance to continental breakup studies, their formation mechanism is still debated. We use ~22,000 km of high‐quality, depth‐migrated, seismic data to document the three‐dimensional geometry of SDRs offshore South America. We find two types: Type I are planar and occur as fault‐bounded wedges. Type II are characterized by reflections that become more convex‐upward in the downdip direction and terminate against a subhorizontal base. We interpret the transition from Type I to Type II SDRs to represent a continuum from continental rifting to full plate separation with formation of new, subaerially generated, magmatic crust. Type I SDRs formed in half grabens during the stretching of continental crust, while Type II lavas infill the space produced by flexing of the crust due to the solidification of the underlying feeder dikes as the magmatic crust moved away from the spreading center. Type II SDRs vary in length and thickness along the margin. In the north, close to the Paraná flood basalts, they are long (tens of kilometers), reach thicknesses of up to 15 km, and have an across margin width of up to 600 km. To the south the Type II SDRs are thinner with lava lengths of <10 km. We propose that Type II lavas in the north erupted from a subaerial, plate spreading center above the Tristan mantle plume and that the shorter lava flows to the south indicates eruption into water, consistent with a cooler, off‐plume mantle.