The Structure of the Continental

Lithosphere: Constraints from Receiver Functions

Erin Cunningham - Grad Talks May 2014

Structure of Continental Lithosphere

The Continental Lithosphere – layer stable over 2.5 Ga

From mantle xenoliths the base of the continental lithospheric mantle (CLM) ~200-250 km in depth

Seismic tomography suggests 200-250 km thick CLM

Converted seismic waves indicate low velocity discontinuity at 80-120 km globally within continents

Too shallow to be the base of the CLM

mid-lithospheric discontinuity (MLD)

Lekic & Romanowicz EPSL 2011

Mid-lithospheric seismic discontinuity

Chemical discontinuity MLD

Anisotropy – Chemical layer at MLD and base of CLM much deeper (Yuan and Romanowicz 2010) – North America

Seismic Tomography – Chemical MLD with Melt near solidus, slab accretion (Artemiva 2009)

Other Discontinuity MLDPs Receiver Functions – Thermal Boundary at MLD and Chemical Boundary at base of CLM (Rychert and Shearer 2009) – Global

Seismic Reflection, Seismic Tomography, Magnetotelluircs – Chemical boundary with magmatic intrusions, presence of fluids , or phase transformation at at the MLD (Thybo 2006) – Global

Sp Receiver Functions – Remnants from slab accretion (Miller and Eaton 2010)- Canadian Shield

Though this 80-120 km is found globally – no clear explanation exists

Guiding Questions

1. Is the MLD a single sharp discontinuity or a region where velocity changes gradually with depth?

2. Can we constrain how sharp or gradient the discontinuity is?

3. Ultimately, what is the origin of the MLD and what does it tell us about CLM formation ?

Goals

Improve converted wave techniques for noisy sediment dominated areas

Determine the gradient of the low velocity MLD from converted wave observations

Analyze converted waves for all available TA stations across the US

Map variations in MLD depth and gradient across the US

Receiver Functions

Receiver Functions tell us about velocity contrasts in Earth’s structure

Free Air Surface

Crust

Lithosphere

Lithosphere

Time (S to P)

Relative Velocity Structure

Receiver Functions

Receiver Functions tell us about velocity contrasts in earth’s structure

Free Air Surface

Crust

Lithosphere

Lithosphere

S wave

Time ( S to P )

Receiver Functions

Receiver Functions tell us about velocity contrasts in earth’s structure

Free Air Surface

Crust

Lithosphere

Lithosphere

S wave

S wave

Time (S to P)

Receiver Functions

Receiver Functions tell us about velocity contrasts in earth’s structure

Free Air Surface

Crust

Lithosphere

Lithosphere

S wave

Time (S to P)

Velocity Decrease with depth

S wave

Receiver Functions

Receiver Functions tell us about velocity contrasts in earth’s structure

Free Air Surface

Crust

Lithosphere

Lithosphere

S wave

Time (S to P)

Velocity Increase with depth

S wave

Receiver Functions

Receiver Functions tell us about velocity contrasts in earth’s structure

Free Air Surface

Crust

Lithosphere

Lithosphere

S wave

Time (S to P)

S wave

Receiver Functions

Receiver Functions tell us about velocity contrasts in earth’s structure

Free Air Surface

Crust

Lithosphere

Lithosphere

S wave

Time (S to P)

S wave

Receiver Functions

The Amplitude of the S to p conversion is due to the :

1. Sharpness of the Velocity change with depth

2. Total Change in Velocity

Time ( S to P )

Sp Station Stacking

Sp RFs are very noisy require stacking

Sp RF have poor lateral resolution if stacked by station

Free Air Surface

Crust

Lithosphere

Lithosphere

S wave

v

v

v

v

Map View – average depth for each station

v

Dense Seismic Arrays

Earth scope USArray database – Enhance “pixels” of earth structure

Prior Station Coverage

Common Conversion Point Stacking

Sp Receiver Functions have better lateral resolution if stacked from all station

Free Air Surface

Crust

Lithosphere

Lithosphere

S waves

Map View – average depth all stations that sample the same area each station

Frequency Dependence of RFs

Consider a sharp vs a gradient velocity structure

Frequency Dependence of RFs

At low frequencies, seismic waves cannot “see” the difference between a sharp and gradational MLD

Frequency Dependence of RFs

At high frequencies, the gradational MLD produces weaker conversions

Sharp: Predicted Sp RFs

Low Frequencies Medium Frequencies High Frequencies

Gradational: Predicted RFs

Low Frequencies Medium Frequencies High Frequencies

Predicted Amplitude Ratio Moho to MLD (Positive to

Negative)

Preliminary Results- Amplitude Ratio Moho to MLD

Data (F31A P01C)

Station F31A – Hecla, SD

Station P01C – Willits, CA

Preliminary Results- Amplitude Ratio Moho to MLD

Data (F31A P01C)

Station F31A – Hecla, SD

Station P01C – Willits, CA

Preliminary Results- Amplitude Ratio Moho to MLD

Data (F31A P01C)

Station F31A – Hecla, SD

Station P01C – Willits, CA

Guiding Questions

1. Is the MLD a single sharp discontinuity or a region where velocity changes gradually with depth? – the nature of the MLD seems to change with location. Age? Geologic structures?

2. Can we constrain how sharp or gradient the discontinuity is? – Yes, using the frequency dependence of gradient features. More work will be focused on quantifying the gradational structure

3. Ultimately, what is the origin of the MLD and what does it tell us about CLM formation ?

References

Artemieva, I.M., (2006) Global 1°x1° model for the continental lithosphere: age, temperatures, and implications for lithosphere secular evolution. Tectonophysics 416, 245-277. C.A.

H. Thybo, Tectonophysics 416, 1-4 (2006)

M.Miller, D.Eaton, Geophys. Res. Lett. 37,18 (2010)

Rychert, C.A, and Shearer, P.M., (2009) A Global View of the Lithosphere- Asthenosphere Boundary . Science. 324, 495-498.

Yuan, H., and Romanowicz, B. (2010) Lithospheric Layering in the North American Craton. Nature. 466, 1063-1068.

Preliminary Results- Moho depth mapped across the US

For Bill

Preliminary Results Expected Sp RF