Hot Upper Mantle Beneath the Tristan da Cunha Hotspot From Probabilistic Rayleigh-Wave Inversion and Petrological Modeling

Understanding the enigmatic intraplate volcanism in the Tristan da Cunha region requires knowledge of the temperature of the lithosphere and asthenosphere beneath it. We measured phase-velocity curves of Rayleigh waves using cross-correlation of teleseismic seismograms from an array of ocean-bottom seismometers around Tristan, constrained a region-average, shear-velocity structure, and inferred the temperature of the lithosphere and asthenosphere beneath the hotspot. The ocean-bottom data set presented some challenges, which required data-processing and measurement approaches different from those tuned for land-based arrays of stations. Having derived a robust, phase-velocity curve for the Tristan area, we inverted it for a shear wave velocity profile using a probabilistic (Markov chain Monte Carlo) approach. The model shows a pronounced low-velocity anomaly from 70 to at least 120 km depth. VS in the low velocity zone is 4.1-4.2 km/s, not as low as reported for Hawaii (∼4.0 km/s), which probably indicates a less pronounced thermal anomaly and, possibly, less partial melting. Petrological modeling shows that the seismic and bathymetry data are consistent with a moderately hot mantle (mantle potential temperature of 1,410-1,430°C, an excess of about 50-120°C compared to the global average) and a melt fraction smaller than 1%. Both purely seismic inversions and petrological modeling indicate a lithospheric thickness of 65-70 km, consistent with recent estimates from receiver functions. The presence of warmer-than-average asthenosphere beneath Tristan is consistent with a hot upwelling (plume) from the deep mantle. However, the excess temperature we determine is smaller than that reported for some other major hotspots, in particular Hawaii

Numerical modelling of the continental break-up of the southern South Atlantic

The southern South Atlantic has often been considered a classic example of continental break-up in the presence of a starting mantle plume, evidenced from the Paraná-Etendeka continental flood basalts, the Rio Grande Rise & Walvis Ridge, and wide-spread presence of seaward dipping reflectors & high-velocity lower-crustal bodies along the conjugate margins. However, inconsistencies remain, including a stark contrast of non-volcanic and volcanic passive margins north and south of the aseismic ridges, and evidence from seaward dipping reflector distributions suggesting segmentation influenced magmatism. Within this project, different methods for the formation of the volcanic passive margins in the southern South Atlantic have been explored. To test the main influence on magmatism during break-up, observational constraints of post and syn-rift magmatism originate from 38 wide-angle and multichannel seismic profiles. These measurements show that at 450km south of the Walvis Ridge, oceanic crust has a thickness of 11.7 km, thinning to 6.1km at a distance of 2300km along-strike. Overall, oceanic crustal thickness decreases linearly along-strike with little indication of segmentation. Active mantle upwelling was also tested using the relationship between melt volumes and seismic velocity from syn-rift magmatism, with little evidence for this process. Using a numerical model of continental rifting capable of producing melt thickness and major element oxide compositions through time, the observational constraints from post and syn-rift magmatism required a hot layer present decreasing in temperature relative to the asthenosphere from 250°C in the north to 50°C at a distance of 2300km south along-strike. By reconstructing the lithosphere thickness at break-up, it was suggested that the sublithospheric topography had a strong influence on magmatism throughout the region, including on the asymmetry in magmatism along the margins. Overall however, modelling results suggest that temperature, and in turn possibly a plume, was the primary control of magmatism during the opening of the southern South Atlantic.Open Acces

Asthenosphere and lithosphere structure controls on early onset oceanic crust production in the southern South Atlantic

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

Asthenosphere and lithosphere structure controls on early onset oceanic crust production in the southern South Atlantic

International audienceThe 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 widespread 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