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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 - PowerPoint PPT Presentation
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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