Characterization of Seaward‐Dipping Reflectors Along the South American Atlantic Margin and Implications for Continental Breakup

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.

Seismic velocity structure of seaward-dipping reflectors on the South American continental margin

“… The two classes of SDR identified from their reflectivity and magnetic properties (McDermott et al, 2018;Collier et al, 2017) correlate with distinct seismic velocity patterns. The velocities of the Type I SDRs are the most variable, suggesting that there is much greater heterogeneity within these SDRs.…”

“…Type I are both shorter and straighter reflection packages that form fault-bounded wedges. The Type II reflectors form concave downward, off-lapping reflection packages, with dips increasing at their distal ends (McDermott et al, 2018).…”

Lithospheric Thickness of Volcanic Rifting Margins: Constraints From Seaward Dipping Reflectors

“…However, if the strike of the data profile is not parallel to the spreading direction, the apparent dip measured is smaller than the true dip. But the effect is small for profiles away from fracture zones according to McDermott et al ().…”

“…SDR wedges generally feature downdip thickening and their dip angle increases with depth (Jackson et al, ; Mutter et al, ; Paton et al, ; Figure ). Globally, SDRs appear to be several to tens of kilometers thick, up to hundreds of kilometers wide (across margin) and several thousand kilometers long (along margin; McDermott et al, ).…”

“…However, as we will show, the shape of magmatic loading formed SDRs is a direct expression of the plate strength during rifting and should change little following their formation. Recent workers (McDermott et al, ; Paton et al, ) argue that near‐shore (or type 1) SDRs result from tectonic processes while offshore (or type 2) SDRs are formed by magmatic processes. The numerical models we develop can treat both tectonic and magmatic processes, but here we focus on the effects of magmatic loading and leave the complexities of faulting for future investigations.…”