Characterization of Seaward‐Dipping Reflectors Along the South American Atlantic Margin and Implications for Continental Breakup. Issue 9 (28th September 2018)

• Record Type: Journal Article
• Authors: McDermott, Carl; Lonergan, Lidia; Collier, Jenny S.; McDermott, Kenneth G.; Bellingham, Paul
• Is Part Of: Tectonics. Volume 37:Issue 9(2018)
• Page Start: 3303
• Page End: 3327
• Publication Date: 2018-09-28
• Subjects: SDRs -- continental breakup -- South America -- seismic reflection data -- Tristan plume; Geology, Structural -- Periodicals; 551.8
• DOI: 10.1029/2017TC004923
• Languages: English
• ISSNs: 0278-7407

Abstract

Abstract: 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. Key Points: Two types of seaward‐dipping reflectors occur along the South American Atlantic margin with different geometries dependent on whether they formed in response to tectonic or magmatic breakup Type I lavas erupted into synrift grabens during the stretching of the crust, while Type II formed in response to plate separation at a subaerial spreading center, fed by underlying dikes The Type II lavas are substantially more voluminous offshore of the Paraná LIP and most likely represent part of the Cretaceous subaerial spreading center above a mantle plume