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

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

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

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

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 (180-240 km under NA craton) LAB Depth Yuan and Romanowicz 2010

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).

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

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)

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

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; 20-30 km vertical spacing B-splines down to 300 km, then 50-150 down to 1000 km. Isotropic Vs and radial anisotropy ξ; then azimuthal anisotropy strength (G) and fast axis directions

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

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 ; 20-30 km vertical spacing B-splines down to 300 km, then 50-150 down to 1000 km. Isotropic Vs and radial anisotropy ξ; then azimuthal anisotropy strength (G) and fast axis Lithosphere depth Asthenosphere depth Below Asthenosphere

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

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

Age Profiles: Vs and LAB Plate-model like Punctuated between 70-100 Ma Radial Anisotropy Ritzwoller et al. 2004 Vs Profile Isotropic Vs Age Profile Priestley & McKenzie 2006 Vs Profile

Age Profiles: Vs and LAB Defined by maximum Vs depth gradient Seismic LAB: 30-100 km depth range Consistent with S-receiver functions and SS-precursor studies LAB from negative Vs gradient

Age Profiles: Azimuthal Anisotropy Current APM direction > 100-140 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 (%)

Age Profiles: Azimuthal Anisotropy Oceanic lithosphere LAB: 50-140 km Defined by azimuthal anisotropy LAB from Azimuthal anisotorpy

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

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

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

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

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

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

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 ; 20-30 km vertical spacing B-splines down to 300 km, then 50-150 down to 1000 km. Isotropic Vs and radial anisotropy ξ; then azimuthal anisotropy strength (G) and fast axis

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

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

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

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

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