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

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

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

LAB and MLD in the craton Craton LAB 180-240 km; consistent with numerous local geochemistry, electric conductivity and other tomographic studies MLD thickness 50-150 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.

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.

Craton is chemically layered Layer 1- layer 2 boundary & Mg# 92-93 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.

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

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

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

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

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

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.

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

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