1. Anisotropy layering in the craton
Fast axis direction with respect to NA APM
Layered upper mantle

2. Continuous lines: % Fo (Mg) from Griffin et al. 2004 Grey: Fo%93
All over the craton
Continuous lines: % Fo (Mg) from
Griffin et al. 2004
Grey: Fo%93
black: Fo%92

3. LAB and MLD in the craton
Laying shown in both fast axis directions
and anisotropy strength
Lithosphere-asthenosphere-boundary (LAB)
Intra-lithospheric layer boundary: related to the Mid-Lithospheric Discontinuity (MLD) from the receiver functions (Fischer et al., 2010)
Change of velocity associated with the gradient change of anisotropy strength

4. LAB and MLD in the craton
Craton LAB km; consistent with numerous local geochemistry, electric conductivity and other tomographic studies
MLD thickness km; thickest beneath the Trans-Hudson Orogen and Proterozoic sutures, analog to modern Himalayas
In the following I will focus more on the Layer 1 and 2 boundary and try to draw some conclusions from its correlation with other geodynamic observations.

5. Lithospheric layering
Craton wide anisotropic layering correlates with:
Ultra-depleted chemical layer
Strong elastic layer
Receiver function MLD
The 8° discontinuity
In the following I will focus more on the Layer 1 and 2 boundary and try to draw some conclusions from its correlation with other geodynamic observations.

6. Craton is chemically layered
Layer 1- layer 2 boundary & Mg# contours from xenocryst samples (Griffin et al. 2004)
Layer 1: chemically depleted layer
Magnesium content from thermo-barometric analysis.
Indicate Layer 1 is old, chemically deplete stuff.

7. Craton is elastically layered
Layer 1/layer 2 & chemical/sub-thermal layers from geodynamic modeling (Cooper et al. 2004; King 2005; Lee 2006)
Layer 1: chemically depleted strong layer

8. Receiver function MLD Layer 1- layer 2 boundary & the
negative velocity
gradient zone (red)

9. Receiver function Low Velocity Zone (LVZ)
Layer 1- layer 2
boundary & the
negative velocity
gradient zone (red)
Range of negative velocity gradient (red)

10. The 8° Discontinuity and LVZ
Scattered traveltime arrival due to presence of a low velocity zone (LVZ)
Starting at 8° horizontal distance, or ~100 km depth

11. LVZ in the shallow mantle
Thybo & Perchuc (1998)
Water &CO2 introduce a kink in the
solidus curve
Occurs around ~100 km
Sleep (2009)
Kimberlites bring up CO2 to beneath
Layer 1
Consistent with diamonds mostly seen in
mobile belts around craton boundaries
Depth (km)
~100 km
dykes

12. Layered Cratonic upper mantle
Deep layer
associated with the current plate shear (fast axis direction; increased strength)
Layer 1:
old, strong and depleted chemical layer
Relatively strong anisotropy
Frozen-in anisotropy in early craton amalgamation
Quick summary for this session: (Take home message) layered lithosphere; old, depleted, strong shallower layer.

13. Viscous, metasomatized (less depleted) sub-thermal layer
North-south direction
Layer 2
Viscous, metasomatized
(less depleted) sub-thermal layer
North-south anisotropy: paleo-plate shear; fossil east-west subduction; or craton-wide aligned dykes in the lower lithosphere? (can’t be a plume)
Take home message: layered lithosphere; old, depleted, strong shallower layer.
Introduce the WUS

14. Anisotropy layering in the craton
Instabilities in Vs
Negative ξ under sutures