Download Region B

Model Validation
• RegSEM waveform fit:
model 3 + Baffin Bay 6.0 event
Model Validation
• RegSEM waveform fit:
model 3 + Oklahoma 5.6 event
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EGAK Z
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BORG
SFJD
HOPS
FFC
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Waveform Correlation
Z-component
MTDJ
1500
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FFC Z
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FFC T
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EGAK T
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EGAK
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HOPS T
200 400 600 800 1000
Isotropic Vs
• Craton and WUS separated by the Rocky Mountain Front (RMF)
• Transition zone depth: slow craton; fast WUS; subducted JdF slab
Radial Anisotropy (ξ)
• Sutures: VSv>VSh
Shear Wave Splitting
• Shear wave splits to the fast and slow
symmetry axis in the anisotropic medium
http://garnero.asu.edu/research_images/images_all.html
• Integrated effect of the medium (δt and ψ)
• Spread sensitivity along ray path
ψ
http://garnero.asu.edu/research_images/images_all.html
Region
Long and Becker, EPSL, 2010
Shear Wave Splitting
Shear Wave Splitting
Laminated Mantle
Don L. Anderson’s nomenclature (2011)
Region B:
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Seismic Lid & Low Velocity Layer
G-discontinuity
L-discontinuity
“the most heterogeneous &
anisotropic region of the mantel”
Anderson, J. of Petrology 2011
Laminated Mantle
Local Tomography
Crust + Shallow upper Mantle A+B
Regional Tomography
Upper mantle B+C
Global Tomography
Full mantle B+C+D and core
Anderson, J. of Petrology 2011
Laminated Mantle
Mantle discontinuities
Region B:
Moho;
G-discon. (ocean);
Hales discon.(continents);
L- discon. (220 km);
Lithosphere-asthenosphere-boundary
Region C:
410-km discon.; 520-km discon.; 660-km discon.
Region D:
D’’-discon.
Regional Tomography
Anderson, J. of Petrology 2011
“2-Layer” SKS Model
• Lithosphere or
asthenosphere origin of
the SKS
Silver and Chan 1996;
Vinnik et al 1989
• Apparent SKS fitting
from 3D model
• 2-layer model prediction:
lithosphere layer and
asthenosphere layer
“2-Layer” Model at HRV
Surface wave and local SKS modeling
results (Levin et al. 1999) agree
well:
Top: east-west direction (N100E)
Bottom: plate motion direction (N50E)
Western US upper mantle (2010 Model)
• Sharp transition from craton to WUS along the RMF
• Subducted JdF in the transition zone
• Depth dependent anisotropy field explains the “swirl” SKS
splitting pattern
• Deep anisotropy associated with the stagnant slab?
Depth Dependent Anisotropy in WUS
• Excellent ray azimuthal
coverage from the TA
Isotropic Vs
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Fast Craton and its Boundary wrt. the western US
Fast Vs 100-200 km under Great Plains
Slow Ridges (<100 km)
“Neutral” CP in Vs
“Neutral” CA Coast
Ranges
• Slow B&R (~250 km)
Isotropic Vs
• Fast Vs (250-400 km) under Oregon:
Correlating with the
subducted JdF slab:
van der Lee and Nolet 1997;
Burdick et al. 2009;
Obrebski et al. 2010;
Schmandt and Humphreys 2010
Shallow depth azimuthal anisotropy
• Rapid anisotropy pattern change across the RMF
• Large amplitude, shallow depth (<150 km), NA/JdF Absolute
Plate Motion (APM) parallel
Intermediate Depth Azimuthal Anisotropy
• Intermediate depth (~150), E-W under B&R, N-S along the
RMF, E-W under SRP
Deep Azimuthal Anisotropy
• Large amplitude, Pacific APM parallel
• Deep (>250 km) E-W direction under Washington and Oregon
Anisotropy and SKS in WUS
•
Circular pattern of the SKS splitting (Savage and Sheehan, 2000)
Circular pattern of the SKS splitting
Schutt and Humphreys, Pure and Applied Geophysics, 1998
Savage and Sheehan, JGR, 2000
Circular pattern of the SKS splitting
NA-SWS-1.1, Liu, G-Cubed 2009
Eakin et al., EPSL, 2010
Proposed Interpretations from others
• Savage and Sheehan (2000): Active upwelling
• Zandt and Humphreys (2008): Passive
edge/toroidal flow
• West et al (2009): Lithospheric drip +
associated mantle flow
Savage and Sheehan Models
Plume model
Model 3
Mantle Upwelling
Savage and Sheehan 2000
Zandt and Humphreys model
Zandt and Humphreys, Geology, 2008
Slab Rollback + Slab window
Passive edge/toroidal Flow
West et al. model
Center of the circular pattern
Low Volcanism
Low Heat flow
Large Vp variation
Small Splitting Time
Downward Drip + mantle
flow 
West et al., Nature Geo., 2009
Berkeley Model
• Circular Pattern Predicted by 3D model
Anisotropy and SKS in WUS
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•
More than “one-layer” of anisotropic domain is needed.
Deep east-west direction beneath Oregon
Deep anisotropy & Subducted slab
• Fast Vs (250-400 km) under Oregon:
Origin of the deep anisotropy
Isotropic Vs
Origin of the deep anisotropy
Anisotropy direction
Isotropic Vs
Origin of the deep anisotropy
Anisotropy direction
Isotropic Vs
Slab
Stagnant Slab
• Frozen-in/structural anisotropy
in the stagnant/flattened slab
Schmid et al., EPSL, 2002;
Fukao, Annu. Rev. Earth Planet. Sci. 2009
Depth dependent anisotropy in the
WUS
1. Shallower than 150 km NE-SW plate shear
2. At 150 km circular flow due to slab rollback
3. At > 350 km east-west frozen-in/structural anisotropy in the
stagnant/flattened slab
Pacific Plate
Plate shear
NA Plate
Plate shear
Slab Rollback
Frozen-in fabric
660 km
Stagnant
slab
Modified from Fukao, Annu. Rev. Earth Planet. Sci. 2009
Global Stagnant Slab in Transition Zone
• SEMum2.2 Vs model (French et al 2011 AGU)
Future Directions
• Higher frequencies: better vertical resolution in the
lithosphere
• Numerical approach: Spectral Element Method and Adjoint
methods
• Other regions: East Asia and Middle East
• Global detection of the LAB and MLD
• Plunging symmetry axis: geodynamic implications
• Anisotropy detecting with SKS and receiver functions
Laminated Mantle
Mantle discontinuities
Region B:
Moho;
G-discon.;
Hales discon.;
L- discon. (or 220-km);
Lithosphere-asthenosphere-boundary (LAB)
Region C:
410-km discon.; 520-km discon.; 660-km discon.
Region D:
D’’-discon.
Anderson, J. of Petrology 2011
Region B in the continents
Xenolith data suggest depleted
(i.e. high velocity) layer goes
down to 150-175 km range:
Lee et al. 2011;
Fischer et al. 2010;
Eaton et al. 2008
Fischer et al., Annu. Rev. Earth Planet. Sci. 2010;
Lee et al., Annu. Rev. Earth Planet. Sci. 2011
Romanowicz, Science 2009
North American Regional Inversion
• Rich tectonic history of the continent
• USArray Transportable Array (TA) of
EarthScope
• Better inversion technique inherited from
global inversion
NA Continent Formation
Hoffman, P. F. (1988)
United Plates of America, The Birth of a
Craton: Early Proterozoic Assembly and
Growth of Laurentia, Annu. Rev. Earth
Planet. Sci., 16(1), 543-603
Karlstrom 1988
Condie et al. 1992
Thomas 2006
…
Whitmeyer & Karlstrom 2007
http://csmres.jmu.edu/Geollab/Whitmeyer/web/documents/WK2007A2.ppt
NA Assembly Processes
Whitmeyer & Karlstrom 2007
• Collision of
Archean blocks
• Accretion of
juvenile terranes
• Rifting along
margins
http://csmres.jmu.edu/Geollab/Whitmeyer/web/documents/WK2007A2.ppt
NA Continent Assembly & Evolution
Whitmeyer & Karlstrom 2007
• Collision of
Archean blocks
• Accretion of
juvenile terranes
• Rifting along
margins
http://csmres.jmu.edu/Geollab/Whitmeyer/web/documents/WK2007A2.ppt
NA Continent Assembly & Evolution
Whitmeyer & Karlstrom 2007
• Collision of
Archean blocks
• Accretion of
juvenile terranes
• Rifting along
margins
http://csmres.jmu.edu/Geollab/Whitmeyer/web/documents/WK2007A2.ppt
USArray of EarthScope:
Ubiquitous Coverage of the Continent
“A network of seismometers deployed across the U.S. to record earthquakes and provide high-resolution
images of the continent's structure and the Earth's deep interior.”
http://www.earthscope.org/observatories/usarray
NA Station Coverage
• IRIS DMC
• Canadian Geological
Survey; GEOSCOPE
• >20years
• >2000 Stations
• Most dense
broadband coverage
Anisotropy layering: craton wide feature
Continuous lines: % Fo (Mg)
from
Griffin et al. 2004
Grey: Fo%93
black: Fo%92
Surface wave and receiver function
MLD
LAB
MLD
• LAB in the WUS
• MLD in the craton
• Nearly same depth (gray bar)!
LAB or MLD
• Kumar et al SRL in press
http://www.solid-earth-discuss.net/4/1/2012/sed-4-1-2012-print.pdf
VelocityLABmatters
MLD
Surface wave model Yuan et al 2011
• In WUS: LAB on top of asthenosphere
• In Craton: MLD in the middle of high Vs lid
• Need to consider velocity!