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Zusammenfassung:
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 phasevelocity
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
1,410–1,4308C, an excess of about 50–1208C 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.
