1. Constraints on flow in the deepest mantle from seismic anisotropy (and other observations)
Maureen D. Long1
With contributions from: Heather Ford1,2, Neala Creasy1, Jie Deng1, Colton Lynner1,3
1Yale University; 2now at University of California - Riverside; 3now at University of Arizona;
CIDER Pre-AGU workshop, 10/11/16

2. Garnero & McNamara, Science, 2008
Processes at the base of the mantle play a critical role in Earth’s thermal evolution, pattern of mantle convection, generation of upwellings, chemical evolution of mantle, and tectonic features we see at the surface. If we knew the pattern of flow at the base of the mantle, we would understand these processes better. Can we constrain this pattern of flow using seismic anisotropy?

3. Measuring and interpreting D” anisotropy: The challenges
Substantially more challenging than upper mantle anisotropy, for several reasons:
Measurements difficult (need to correct for upper mantle)
Limited source-receiver geometries (for body wave studies)
Mechanism poorly known (CPO vs. SPO? What phases(s) contribute?)
Dominant slip systems relevant minerals poorly known
Nowacki et al., J. Geodyn., 2011
Panning & Romanowicz, Science, 2004

4. Part I: Large Low Shear Velocity Provinces (LLSVPs)
Lekic et al., EPSL, 2012
Garnero et al., Nat. Geosci., 2016
Although volumetrically small (<5% of mantle), these are first-order structures in the lowermost mantle, with a number of anomalous properties.
Perm Anomaly: much smaller structure with low Vs.

5. Study of the lowermost mantle beneath Africa using SKS-SKKS splitting discrepancies
Lynner & Long, GRL, 2014

6. An interesting observation: Strong anisotropy concentrated at the edges of LLSVPs
Lynner & Long, GRL, 2014
Follows on work by Wang and Wen (2007) and Cottaar and Romanowicz (2013)

7. What about the Perm Anomaly
What about the Perm Anomaly? Is it associated with lowermost mantle anisotropy?
Long and Lynner, GRL, 2015
Yes! Perm Anomaly is very well-sampled using SKS-SKKS measured at stations in Europe; there is evidence for anisotropy associated with Perm Anomaly.

8. Strong anisotropy at LSVP boundaries: Implications?
Torsvik et al., GJI, 2006
Suggests concentration of deformation at (L)LSVP edges, while interior remains undeformed (?). Could LLSVPs represent strong, highly viscous structures that are resistant to deformation? Are there unique geodynamical processes at LLSVP edges that produce strong anisotropy?

9. Part II: How do we move towards inferences on flow pattern?
Nowacki et al.,
J. Geodyn., 2011
Observations of D” anisotropy are robust, but:
WE STILL DON’T HAVE THE MECHANISM NAILED DOWN, SO CANNOT RELATE TO FLOW GEOMETRY. How to move forward?

10. What candidate mechanisms may play a role in generating D” anisotropy?
CPO due to dislocation creep
Candidate minerals:
SPO due to melt/slab remnants
Perovskite; Wookey
et al., Nature, 2005
Post-perovskite
Stackhouse et al.,
EPSL, 2005
Nowacki et al., JG, 2011
MgO; Karki et al., Science, 1999

11. Our approach: a detailed dataset and mineral physics-based forward modeling
Combination of 2 types of data: SKS-SKKS and ScS-S
Ford et al., EPSL, 2015

12. The approach: a detailed dataset and mineral physics based forward modeling
Ford et al., EPSL 2015

13. Testing plausible anisotropic scenarios against our splitting observations
The approach: use single- crystal elastic constants (as a start!) for different minerals and carry out forward modeling for all possible orientations.
Check: which minerals/ orientations are consistent with the splitting data?
Ford et al.,
EPSL 2015

14. Testing plausible anisotropic scenarios against our splitting observations
Ford et al., EPSL 2015
Our measurements are able to dramatically narrow down the possibilities. Specifically: can rule out any melt SPO scenario. MgO and perovskite also very poor fits to the data. Most likely: CPO of post-perovskite, with a few (~4) plausible orientations.

15. Testing plausible mantle flow scenarios: Which (simple) models are consistent with the data?
Ford et al., EPSL, 2015
IF [100] is most likely slip direction for dislocation creep of post-perovskite, THEN:
- We can (mostly) rule out horizontal flow.
- Most likely simple flow scenario involves some component of vertical flow at LLSVP edge

16. Ongoing work: Application of this approach to other regions of D”
Legend:
SKS
SKKS
ScS
Creasy, Long, Ford, in prep for JGR
Target region beneath Australia, located between the African and Pacific LLSVP boundaries. Goal: identify sub-regions where we can get crossing paths for multiple phases.

17. Lowermost mantle anisotropy beneath Australia: observations
Creasy, Long, Ford, in prep for JGR
Observations of SKS-SKKS and S-ScS splitting discrepancies are plentiful, yielding evidence for lowermost mantle anisotropy in our study region. Observations appear heterogeneous, however – can we focus on smaller sub-regions?

18. Outlook: what is needed for progress?
Seismic observations:
Crossing ray paths, multiple data types, global + regional
Ford et al., EPSL 2015
Geodynamic models: Lots of models of lowermost mantle out there = scenarios to test!
Cottaar et al., GJI 2014
Mineral physics: Better constraints on dominant slip systems needed, but rapid ongoing progress. Is [100](010) the answer?
Goryaeva et al., Sci. Rep. 2016

19. Testing global models of flow and elasticity
Underlying models from Walker et al., G^3, 2011
Evaluate global models for lowermost mantle flow: measurements most consistent with post-perovskite CPO with slip on (010) plane
Ford & Long,
PEPI, 2015