Asymmetry of high-velocity lower crust on the South Atlantic rifted margins and implications for the interplay of magmatism and tectonics in continental breakup

Abstract

Abstract. High-velocity lower crust (HVLC) and seawarddipping reflector (SDR) sequences are typical features of volcanic rifted margins. However, the nature and origin of HVLC is under discussion. Here we provide a comprehensive analysis of deep crustal structures in the southern segment of the South Atlantic and an assessment of HVLC along the margins. Two new seismic refraction lines off South America fill a gap in the data coverage and together with five existing velocity models allow for a detailed investigation of the lower crustal properties on both margins. An important finding is the major asymmetry in volumes of HVLC on the conjugate margins. The seismic refraction lines across the South African margin reveal cross-sectional areas of HVLC 4 times larger than at the South American margin, a finding that is opposite to the asymmetric distribution of the flood basalts in the Paraná-Etendeka Large Igneous Province. Also, the position of the HVLC with respect to the SDR sequences varies consistently along both margins. Close to the Falkland-Agulhas Fracture Zone in the south, a small body of HVLC is not accompanied by SDRs. In the central portion of both margins, the HVLC is below the inner SDR wedges while in the northern area, closer to the Rio Grande Rise-Walvis Ridge, large volumes of HVLC extend far seaward of the inner SDRs.This challenges the concept of a simple extrusive/intrusive relationship between SDR sequences and HVLC, and it provides evidence for formation of the HVLC at different times during the rifting and breakup process. We suggest that the drastically different HVLC volumes are caused by asymmetric rifting in a simple-shear-dominated extension.

Causes and consequences of asymmetric lateral plume flow during South Atlantic rifting

“…Instead, the combination of a preexisting plume and rifting could form SDRs, as seems clear for the cases of the Iceland (27) and Reunion/ Deccan plumes (28). This scenario is consistent with the observation that rifting along the southern South Atlantic margins appears to start with an early magma-poor rift stage, and evolves to a magma-rich breakup stage (29), an observation inconsistent with straightforward interpretations of a plume-head source for the creation of SDRs (e.g., ref. 5).…”

Fault‐controlled lithospheric detachment of the volcanic southern South Atlantic rift

“…The break‐up unconformity (dark green in Figures b and c), merges with the top of the SDR wedges to suggest that the SDR formed before seafloor spreading. The earliest oceanic crust is interpreted immediately seaward of the SDRs [ Koopmann et al ., ; Franke et al ., ; Becker et al ., ]. The postrift sediments on the South American margin reveal a thickness of 3.5 km at the seaward end of the SDRs, which thins to 2.5 km at the feather edge.…”

“…The HVLC body is asymmetric, with a much greater cross‐sectional area on the South African margin, as discussed by Becker et al . []. It remains an open question, to what an amount of continental crust is found below the SDRs.…”

“…After the most-landward SDR wedge was emplaced, a hiatus occurred, which possibly included weathering and subsidence before the next wedge was emplaced [Franke et al, 2007;Koopmann et al, 2014a]. Indication of regional simple-shear extension is not only provided by the irregular distribution and volumes of the magmatic extrusives [Koopmann et al, 2014b] but also by the major asymmetry in the across-margin volumes of the HVLC [Becker et al, 2014]. In addition, the wide and deeply subsided rift grabens on the African margin, e.g., the Orange Basin, are in sharp contrast to the narrow and scattered synrift grabens on the conjugate margin [Franke et al, 2007] and may be explained by simple-shear extension.…”

Conjugate margins — An oversimplification of the complex southern North Atlantic rift and spreading system?

“…Through such studies globally, the large-scale structure of a rift system has been studied by numerous previous workers (Chian et al, 1995;Pichot et al, 2014;Kvarven et al, 2015;Peace et al, 2016). Such studies are useful because they reveal broad characteristics of rifting (Becker et al, 2014), such as rift asymmetry (Lister et al, 1986). These studies have shown that asymmetric rifts are common, with implications for exploration since structure, heat flow and thus hydrocarbon prospectivity vary between conjugate margins (Peace et al, 2016).…”