Mapping Anisotropy in the Lowermost Mantle: Towards Understanding Its Cause
Abstract
The lowermost mantle (D″) plays a key role in both core and mantle dynamics yet due to its remoteness (near 3000 km deep) is difficult to study and is hence not well understood. Deformation imposes order in the arrangement of material creating a fabric which is seismically detectable by variations in the wave speed with direction (seismic anisotropy). Recent knowledge linking anisotropy, mineralogy, and deformation potentially enables us to invert seismic observations of anisotropy for flow in D″ and hence to take a major step forward in the understanding of mantle dynamics. However, this requires us to know with confidence the cause of anisotropy in D″.A current barrier preventing us from discovering the cause is a lack of high quality shear wave splitting observations (SWS: when a shear wave separates into two independent and orthogonally polarised waves travelling at different speeds). Such measurements provide the least ambiguous indicator of anisotropy whilst providing detailed information not limited by assumptions about the symmetry or orientation of the anisotropy. Arguably the leading candidate mechanism is that anisotropy is caused by the lattice preferred orientation (LPO) of post-perovskite (ppv: a strongly anisotropic mineral thought to constitute about 70% by volume in "cold" downwelling regions). However, the mineralogical details of how ppv deforms within the dislocation creep regime remain uncertain, though three plausible slip systems have been proposed.Here we introduce a new map of SWS in D″ with unprecedented coverage (from ScS phases corrected for the effects of anisotropy in the upper mantle). We use this to tackle the question of what causes anisotropy, exploring in detail the hypothesis of LPO of ppv. To do this, texture models designed to test the three plausible ppv slip-systems (generated in a broadly realistic flow-field using visco-plastic self-consistent theory) are probed synthetically using finite-frequency wave propagation modelling. We evaluate the success of the hypothetical models based on the similarities and differences between the observed and the synthetic SWS.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2016
- Bibcode:
- 2016AGUFMDI52A..03W
- Keywords:
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- 1025 Composition of the mantle;
- GEOCHEMISTRYDE: 3924 High-pressure behavior;
- MINERAL PHYSICSDE: 7208 Mantle;
- SEISMOLOGY