Magmatism and Continental Breakup in the South Atlantic

gplates

Our current work aims to  test the predictions of melt volumes and seismic velocity from our numerical models against a unique oil-industry seismic reflection dataset from the Central and South Atlantic provided by ION-GXT. The 2D seismic lines are spectacular- routinely imaging to beneath the Moho allowing insight into the deep structure of the continental margins. Volcanic rocks, including sea-ward dipping reflectors and igneous, crustal, underplated material are routinely imaged on these data. In the South Atlantic the along-strike pattern of volcanism seems far less systematic than the better studied North Atlantic margins. This project aims to investigate why this is the case and will lead to a significant advance in our understanding of the thermal conditions during continental break-up

 

IonExample seismic reflection line from Argentine continental margin, with seaward dipping reflections. Image dimensions: length ~ 50 km; depth ~10km. Fom ION website.

 

Publications

Collier JS, McDermott C, Warner G, Gyori N, Schnabel M, McDermott K, Horn BW, 2017, New constraints on the age and style of continental breakup in the South Atlantic from magnetic anomaly data, Earth and Planetary Science Letters, Vol: 477, Pages: 27-40

We present new constraints on the opening of the South Atlantic Ocean from a joint interpretation of marine magnetic anomaly grids and forward modelling of conjugate profiles. We use 45,000 km of recently collected commercial ship track data combined with 561,000 km of publically available data. The new data cover the critical ocean–continental transition zones and allow us to identify and downgrade some poorly navigated older ship tracks relied upon in earlier compilations. Within the final grids the mean cross-over error is 14 nT computed from 8,227 ship track intersections. The forward modelling used uniformly magnetised bodies whose shapes were constrained from coincident deep-seismic reflection data. We find the oldest magnetic anomalies to date from M10r (134.2 Ma, late Valanginian) north of the Falkland-Agulhas Fracture Zone and M3 (129.3 Ma, Barremian) south of the Rio Grande Fracture Zone. Hence, assuming the GPTS used is correct, continental breakup was contemporaneous with the Parana and Etendeka continental flood basalts. Many of the landward linear anomalies overlap seismically mapped Seaward Dipping Reflectors (SDRs). We interpret this to mean that a significant portion of the SDRs overlay crust formed by subaerial seafloor spreading. Here crustal accretion is envisaged to be similar to that at mid-ocean ridges, but sheet lava flows (that later form the SDRs) rather than pillow basalts form the extrusive component. Segmentation of the linear anomalies generated implies that this stage of continental breakup is organised and parallels the seafloor spreading centre that follows. Our results call into question the common assumption that at volcanic continental margins the first linear magnetic anomalies represent the start of conventional (submarine) oceanic crustal generation.

Taposeea CA, Armitage JJ, Collier JS, 2016, Asthenosphere and lithosphere structure controls on early onset oceanic crust production in the southern South Atlantic, Tectonophysics

The southern South Atlantic has often been considered a classic example of continental break-up in the presence of a starting mantle plume. Evidence for a mantle plume includes the Paranà-Etendeka continental flood basalts, which are associated with the Rio Grande Rise and Walvis Ridge, and the wide-spread presence of seaward dipping reflectors and high-velocity lower-crustal bodies along the conjugate margins. Observations from seaward dipping reflector distributions suggested that lithospheric segmentation played a major role in the pattern of volcanism during break-up in this region, and consequent numerical modelling was used to test this. We tested this hypothesis ourselves by measuring the thickness of the earliest oceanic crust generated. This was done through the use of 37 measurements of initial oceanic crustal thickness from wide-angle and multichannel seismic profiles collected along the conjugate margins. These measurements show that at 450. km. south of the Paranà-Etendeka flood basalts the oceanic crust is thicker than the global average at 11.7. km. Farther south the oceanic crust thins, reaching 6.1. km at a distance of 2300. km along-strike. Overall, the along-strike trend of oceanic crustal thickness is linear with a regression coefficient of 0.7 and little indication of segmentation. From numerical models representing extension of the lithosphere, we find that observed melt volumes are matched with the presence of a hot layer. If we assume this region of hot mantle has a thickness of 100. km, its excess temperature relative to the asthenosphere has to decrease from 200 to 50. C, north to south. This decrease in temperature, also seen in published thermobarometry results, suggests that temperature was the primary control of volcanism during the opening of the southern South Atlantic.

Team members