Asthenospheric Temperature and Lithospheric Thickness Beneath the Tristan da Cunha Hotspot From Probabilistic Inversion of Surface-Wave Dispersion Data and Petrological Modeling

Tristan da Cunha is a primary hotspot in the South Atlantic, located at the south-western end of the aseismic Walvis Ridge and connected to the Cretaceous Etendeka Flood Basalt province in Namibia via age-progressing seamounts and submarine volcanic plateaus. The origin of the hotspot is debated; more observational evidence on the structure of the lithosphere-asthenosphere system beneath it is required. In this study, we use Rayleigh-wave, phase-velocity data obtained by cross-correlation of teleseismic seismograms from a recent ocean bottom seismic experiment and constrain the shear-wave velocity structure and temperature of the lithosphere and asthenosphere beneath Tristan and surroundings. The ocean-bottom dataset presented a number of challenges (low signal to noise ratios; low data redundancy due to the short term of the deployment and its remoteness from areas of abundant seismicity; 60 s period limit of the wide-band instruments). This required development of data-processing and measurement approaches different from those tuned for land-based arrays of stations. We were then able to derive a robust, phase-velocity curve that averages across the Tristan area. We inverted the curve for the shear-wave velocity distribution with depth using a probabilistic, Bayesian (McMC) approach. Our shear-velocity models show a 70-km thick lithosphere and a pronounced low-velocity anomaly from 70 to at least 120 km depth. S-velocity in the LVZ is around 4.2 km/s, not as low as reported for beneath Hawaii ( 4.0 km/s). Petrological modeling shows that the seismic and bathymetry data can be fit by models with a moderately warm mantle (mantle potential temperature of 1715K, an excess of 120K compared to the global average), a melt fraction smaller than 1% and a 65-70 km lithospheric thickness (in agreement with the purely seismic inversions). The lithospheric thickness constrained by our inversions matches independent estimates from receiver functions. Recent thermobarometry estimates of the Tristan island mantle potential temperature show only a small warm anomaly, in agreement with our results. Although our observations are consistent with a hot upwelling from the mantle, the excess temperature we determine is considerably smaller than that reported for other major hotspots, in particular Iceland and Hawaii.