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

Abstract

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. VnormalS 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.

Seafloor evidence for pre-shield volcanism above the Tristan da Cunha mantle plume

“…The projection of this conduit to the seafloor intersects the area of numerous small volcanic edifices. Slower average S-wave seismic velocities in the upper mantle were also observed in this area by Bonadio et al 33 by analyzing teleseismic surface waves. Among the range of possible causes for relatively recent tectono-magmatic activity between TdC and the MAR, these seismic anomalies independently support the presence of warm and melt-rich (<1% 33 ) upper mantle.…”

“…Slower average S-wave seismic velocities in the upper mantle were also observed in this area by Bonadio et al 33 by analyzing teleseismic surface waves. Among the range of possible causes for relatively recent tectono-magmatic activity between TdC and the MAR, these seismic anomalies independently support the presence of warm and melt-rich (<1% 33 ) upper mantle. According to Baba et al 29 and Geissler et al 21 , the regional thickness of the oceanic lithosphere is about 60–70 km.…”

“…The westwards regional extent of anomalous upper mantle is not clearly resolvable with the seismological data 31 , 33 . We mapped volcanic cones in our bathymetric dataset, noting a westward decrease in the frequency of their occurrence (Fig.…”

“…1 ) around the volcanically active islands of TdC was the focus of a major scientific project, ISOLDE (2010–2018). ISOLDE’s main objective was to characterize the upper mantle in the search for evidence of a mantle plume beneath the volcanic archipelago by means of geophysical and petrological studies 13 , 21 , 29 – 33 . The geophysical program saw ocean-bottom seismometers and magnetotelluric instruments deployed over a period of 11 months between 2012 and 2013.…”

Shear‐Wave Velocity Structure of Southern Africa's Lithosphere: Variations in the Thickness and Composition of Cratons and Their Effect on Topography

“…We also vary the values of the parameters at the 410 and 660 km discontinuities, and interpolate linearly between them; below 660 km the model is the isotropic AK135. The a priori information on the model parameters is expressed in terms of Gaussian prior probability distributions centered at reference velocity models with standard deviations of ∼400 m/s (Bonadio et al, ; Ravenna & Lebedev, ). The reference model for V s is AK135 in the mantle and CRUST 2.0 (Bassin et al, ) in the crust (Figure , black lines).…”