Use this URL to cite or link to this record in EThOS: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.683386
Title: Constraining mineralogy and geodynamics at the base of the mantle using seismic anistropy
Author: Walpole, Jack
Awarding Body: University of Bristol
Current Institution: University of Bristol
Date of Award: 2013
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Abstract:
Many new observations of seismic anisotropy in the upper and lower boundary layers of the mantle are presented. These observations (from shear-wave splitting on SKS, receiver corrected S, and receiver/source corrected ScS phases) constrain the mineralogy and geodynamics in these regions, which are crucially important to mantle convection. The main target of this thesis is the lower boundary layer: the lowermost mantle; where the new (ScS) dataset greatly expands on the pre-existing coverage to reveal a global dominance of SH fast anisotropy with strength ~1.4 %. Interesting deviations are detected inside the Pacific large low shear velocity province where SV is fast; and in the 'slab graveyard' region beneath Eurasia where the angle of fast wave polarisation shows coherent regional variations in dip suggesting the presence of large-scale (~1000 km) structural features. A model of flow in the lowermost mantle is tested with the hypothesis that the anisotropy is caused by the lattice preferred orientation of MgSi03 post-perovskite. It is demonstrated that the accuracy of ray theory is inadequate to test general models of anisotropy in the lowermost mantle and that a full waveform finite frequency method is required. Finite frequency waveform results do not match the observations for three candidate post-perovskite plastic deformation models (dislocation glide on (001), (010), and (100)/{11O}). Therefore either the flow model is wrong or anisotropy in the lowermost mantle is not caused by dislocation glide deformation in post-perovskite. Anisotropy in the upper mantle causes on average 0.8 s of splitting beneath seismic stations (determined from the SKS dataset). The fast wave tends to be polarised in line with the direction of absolute plate motion in regions disturbed by orogeny in the last 540 Ma. This suggests that orogenies deform the mantle. Beneath subduction zones, anisotropy causes an average 1.3 s of splitting in events shallower than 300 km (from the S dataset). The fast direction tends to be aligned with the strike of the slab (trench parallel splitting); notable exceptions are identified in South America, the Izu-Bonin arc, a segment of the Sunda arc, and at the Hokkaido corner. Interestingly, in each of these regions the trench is migrating forward in the direction of the subducting plate's motion. With the exception of the Mariana arc, subduction zones experiencing trench roll-back all display trench parallel shear wave splitting. This suggests that trench parallel splitting is caused by mantle deformation associated with trench roll-back. Events deeper than 300 km split by an average 0.9 s; this remains true of events deeper than 520 km. The lack of depth dependence on splitting beyond 300 km hints that anisotropy is confined to the slab or is located in a region deeper than ~660 km.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID: uk.bl.ethos.683386  DOI: Not available
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