1. Azimuthal anisotropy layering in the Pacific upper mantle
DI21C-07
Azimuthal anisotropy layering in the Pacific upper mantle
Huaiyu Yuan1,2,3 , Scott French4 and
Barbara A. Romanowicz4,5,6
1 CCFS, EPS, Macquarie University, Australia
2 CET, University of Western Australia, Australia
3 Geological Survey of Western Australia, Australia
4 Berkeley Seismological Laboratory, UC Berkeley
5 Collège de France, Paris, France
6 Institut de Physique du Globe, Paris, France 1

2. Lithosphere Layering and the LAB
Lithosphere-asthenosphere-boundary (LAB): one of “Grand Challenges” of modern seismology (Lay 2009)
“Elusive” in continents due to smooth transition in seismic velocity (Eaton 2009; Romanowicz 2009; Fischer et al. 2010)
Most extensively deformed “first-order structural discontinuity” (Eaton 2009) in the global tectonics due to strong plate-asthenosphere coupling
Differential motion accommodated by rock deformation across/at the LAB

3. APM: Absolute Plate Motion direction
Layering and the LAB
Depth dependent azimuthal anisotropy
Craton-wide layering in the upper mantle
Domains of anisotropy possessing distinct fast axis directions
Fast axis directions
Fast axis
directions
Yuan and Romanowicz 2010
- APM
APM: Absolute Plate Motion direction

4. Lithosphere Layering and the LAB
Shallow depleted chemical layer (Layer 1; suture-strike parallel)
Sub-thermal layer (Layer 2; high angle to APM)
Asthenosphere (current APM parallel)
Cooper and Conrad 2009
Yuan and Romanowicz 2010

5. Lithosphere Layering and the LAB
Shallow depleted chemical layer (Layer 1; suture-strike parallel)
Sub-thermal layer (Layer 2; high angle to APM)
Asthenosphere (current APM parallel)
Anisotropic LAB ( km under NA craton)
LAB Depth
Yuan and Romanowicz 2010

6. The Oceanic LAB
Oceanic lithosphere covers much larger surface area than continents
Short life span: e.g., Pacific < 200Ma compared with >3Ga of continents
Simple tectonic history; perturbed by input of deep seated thermal upwellings (e.g., French 2013).

7. Poster: DI21A-2267 this afternoon
The Oceanic LAB
Simple tectonic history; perturbed by input of deep seated thermal upwellings (e.g., French 2013).
See French 2013 for details
Poster: DI21A-2267 this afternoon
Talk S21E-07 now

8. The Oceanic LAB
Thickness of oceanic lithosphere dictated by thermal history
Follows 1 half-space cooling model;
2 plate model;
3 or not (Rychert et al. 2013; Schmerr 2012)
Thickness in the range ~ 100 km in the oldest (Schmerr 2012)
Paleo-APM at shallow and current APM at depth picked up by azimuthal anisotropy (Montagner 2002; Smith et al. 2004; Debayle et al. 2005; Maggi et al. 2006; Debayle and Ricard 2013)

9. The Objectives Can we define a Pacific-wide LAB?
Oceanic upper mantle layering and its tectonic implications.

10. Pacific Inversion
Full waveform inversion using 2D finite-sensitivity kernels (NACT; Li and Romanowicz 1995)
Based on an earlier (2007) Berkeley global model that honors true oceanic Moho (Crust2.0);
Modified linear crustal correction (Lekic and Romanowicz 2009) applied;
5° lateral model spacing; km vertical spacing B-splines down to 300 km, then down to 1000 km.
Isotropic Vs and radial anisotropy ξ; then azimuthal anisotropy strength (G) and fast axis directions

11. Pacific Inversion
60-s global fundamental and overtone waveforms (French et al. 2013); 60- and 40-s Pacific regional network data; OBS waveforms
Overall 1.3 million paths; or ~25 million waveform points handled by NACT for forward and inverse problems
Model error handled by bootstrap resampling (Efron and Tibshitani 1986): standard deviation of the 100 “new” models
No SKS dataset due to the sparse coverage

12. Anisotropy map views
Layered North America confirmed; although weak but AMP-parallel in the asthenosphere
Depth dependent anisotropy domains also found in the Pacific
Modified linear crustal correction (Lekic and Romanowicz 2009) applied;
Full waveform inversion using 2D finite-sensitivity kernels (NACT; Li and Romanowicz 1995)
5° lateral model spacing ; km vertical spacing B-splines down to 300 km, then down to 1000 km.
Isotropic Vs and radial anisotropy ξ; then azimuthal anisotropy strength (G) and fast axis
Lithosphere depth
Asthenosphere depth
Below Asthenosphere

13. Depth cross-section Isotropic Vs Radial Anisotropy Azimuthal
Strength (%)
Fast-axis
direction

14. Resolution Tests Azimuthal Anisotropy Strength (%) Fast-axis Input
direction
Input
Azimuthal
Anisotropy
Strength (%)
Fast-axis
direction
Output

15. Age Profiles: Vs and LAB
Plate-model like
Punctuated between Ma
Radial Anisotropy
Ritzwoller et al Vs Profile
Isotropic Vs Age Profile
Priestley & McKenzie 2006 Vs Profile

16. Age Profiles: Vs and LAB
Defined by maximum Vs depth gradient
Seismic LAB: km depth range
Consistent with S-receiver functions and SS-precursor studies
LAB from
negative Vs gradient

17. Age Profiles: Azimuthal Anisotropy
Current APM direction > km
High angle to the APM at shallow depth
Two domains of anisotropy
Paleo-plate motion
Fast axis directions
with APM removed
Current Plate Motion
Azimuthal anisotropy
Strength (%)

18. Age Profiles: Azimuthal Anisotropy
Oceanic lithosphere LAB: km
Defined by azimuthal anisotropy
LAB from
Azimuthal anisotorpy

19. Azimuthal anisotropy LAB
Two LAB horizons
Shallower Vs LAB
Deeper anisotropy LAB
Velocity LAB
Azimuthal anisotropy LAB

20. LAB Age Profiles Shallower Vs LAB Deeper Anisotropy LAB LAB from Vs

21. Chemical oceanic layer?
Resembles the continent layer model
LAB from Vs
Cooper and Conrad 2009
LAB from
anisotropy

22. Depleted chemical oceanic layer?
Depleted Layer in the oceanic lithosphere (Hirth & Kohlstedt 1996; Lee et al. 2005);
thickness dependent on ambient mantle temperature
Growth of thermal sub-layer when thermal effect out-weights chemical effect
LAB from Vs
LAB from
anisotropy
Lee et al. 2005

23. Green area: Large Igneous Provinces (LIPs) Azimuthal anisotropy LAB
Role of LIPs
Correlation of deep Anisotropic LAB with Large Igneous Provinces?
Green area: Large Igneous Provinces (LIPs)
Velocity LAB
Azimuthal anisotropy LAB

24. Conclusions Layered oceanic upper mantle
Uniform fossil lithosphere structure
APM parallel asthenosphere
Vs LAB shallower than anisotropy LAB
Chemical depleted oceanic layer

27. Age Profiles and Error Estimates
Based on an early Berkeley global model that honors true oceanic Moho (Crust2.0);
Modified linear crustal correction (Lekic and Romanowicz 2009) applied;
Full waveform inversion using 2D finite-sensitivity kernels (NACT; Li and Romanowicz 1995)
5° lateral model spacing ; km vertical spacing B-splines down to 300 km, then down to 1000 km.
Isotropic Vs and radial anisotropy ξ; then azimuthal anisotropy strength (G) and fast axis

28. Pacific Inversion
Model error handled by bootstrap resampling (Efron and Tibshitani 1986): standard deviation of the 100 “new” models
60-s global fundamental and overtone waveforms (French et al. 2013) and 60- and 40-s Pacific regional network data
Overall 1.3 million paths (~30 million waveform points) handled by NACT forward and inversion
Good azimuthal coverage for Pacific

32. Age Profiles: Vs & ζ
Model points sorted by age and averaged for model parameters
Isotropic Vs
Radial Anisotropy

33. Age Profiles: Azimuthal Anisotropy
Model points sorted by age and averaged for model parameters
Isotropic Vs
Radial Anisotropy

34. Resolution Tests
Model error handled by bootstrap resampling (Efron and Tibshitani 1986): standard deviation of the 100 “new” models
60-s global fundamental and overtone waveforms (French et al. 2013) and 60- and 40-s Pacific regional network data
Overall 1.3 million paths (~30 million waveform points) handled by NACT forward and inversion
Good azimuthal coverage for Pacific

35. Resolution Tests
Model error handled by bootstrap resampling (Efron and Tibshitani 1986): standard deviation of the 100 “new” models
60-s global fundamental and overtone waveforms (French et al. 2013) and 60- and 40-s Pacific regional network data
Overall 1.3 million paths (~30 million waveform points) handled by NACT forward and inversion
Good azimuthal coverage for Pacific