Master´s thesis defense by Line Pinna – Niels Bohr Institute - University of Copenhagen

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Master´s thesis defense by Line Pinna

Title: Upper mantle discontinuities under West Greenland from seismic receiver functions - Can a track of the Iceland mantle plume be observed?

Abstract: In this thesis seismological receiver functions (RFs) are used to study the crust and upper mantle discontinuities beneath seismic stations in West Greenland. The method exploits what happens when a seismic P-wave crosses an interface inside the Earth, such as Moho or a mantle discontinuity. Part of its energy is converted into an S-wave, and the S-wave is thus the result of the local structure beneath the station. The information it is carrying is isolated in the RF, which can be interpreted as a fingerprint of the subsurface. The RFs can therefore be utilized to map the depths of strong interfaces in the Earth.

The data collected are upper mantle and teleseismic earthquakes registered on 26 seismological stations on Greenland’s west coast and on the ice sheet. The data coverage is sufficient for studying the most significant features of Moho, the 410 km discontinuity and the 660 km discontinuity along the west coast. Three methods are used to study the RFs in this thesis. Two of these methods provide single value estimates for the depths of the discontinuities at each station. The third method results in detailed images of the subsurface structures from 0 to 900 km depth. Particularly one image giving of the structures along the entire west coast of Greenland is successful in mapping both the 410 and 660 km discontinuity.

At most stations Moho estimates agree with previous results, and at stations where Moho has not previous been studied, the results give good first approximations. As the methods used are rather simple, the depths are cursory estimations. The results suggest that the depths of the 410 and

the 660 km discontinuities are close to global average observations. For most parts the discontinuities are both strong and coherent. The presence of the 660 km discontinuity beneath some parts of the ice sheet can furthermore be perceived from the results. Some scientists have suggested that the Iceland mantle plume has passed beneath the west coast of Greenland, while others do not agree on these

theories. An estimate of the thickness of the mantle transition zone shows that its thickness is typically 240-250 km. As the mantle transition zone is assumed to be thinned in the presence of anomalous hot temperatures, this would have been the case, if the Iceland plume had traversed below the west coast. Since the thickness here is close to the global mean, this study shows no trace of the Iceland plume having passed beneath the west coast of Greenland.