Download The Structure of the Continental Lithosphere

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
Though this 80-120 km is found globally – no clear explanation exists
Chemical discontinuity MLD
Anisotropy – Chemical layer at
MLD and base of CLM much
deeper (Yuan and Romanowicz 2010) –
Other Discontinuity MLD
Ps Receiver Functions – Thermal
Boundary at MLD and Chemical
Boundary at base of CLM (Rychert
and Shearer 2009) – Global
North America
Seismic Tomography – Chemical
MLD with Melt near solidus, slab
accretion (Artemiva 2009)
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
Guiding Questions
1. Is the MLD a single sharp discontinuity or a region
where velocity changes gradually with depth?
1. Can we constrain how sharp or gradient the
discontinuity is?
2. 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
Relative Velocity
Structure
Time (S to P)
Receiver Functions
Receiver Functions tell us about velocity contrasts in earth’s
structure
Free Air
Surface
Time ( S to P )
Crust
Lithosphere
Lithosphere
S
wave
Receiver Functions
Receiver Functions tell us about velocity contrasts in earth’s
structure
Free Air
Surface
Time (S to P)
Crust
Lithosphere
Lithosphere
S
wave
S
wave
Receiver Functions
Receiver Functions tell us about velocity contrasts in earth’s
structure
Free Air
Surface
Time (S to P)
Crust
Velocity Decrease
with depth
Lithosphere
Lithosphere
S
wave
S
wave
Receiver Functions
Receiver Functions tell us about velocity contrasts in earth’s
structure
Free Air
Surface
Time (S to P)
Crust
Velocity Increase
with depth
Lithosphere
Lithosphere
S
wave
S
wave
Receiver Functions
Receiver Functions tell us about velocity contrasts in earth’s
structure
Free Air
Surface
Time (S to P)
Crust
Lithosphere
Lithosphere
S
wave
S
wave
Receiver Functions
Receiver Functions tell us about velocity contrasts in earth’s
structure
Free Air
Surface
Time (S to P)
Crust
Lithosphere
Lithosphere
S
wave
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
v
Free Air
Surface
Map View –
average depth
for each
station
Crust
v
v
Lithosphere
v
Lithosphere
S
wave
v
Dense Seismic Arrays
Earth scope USArray database – Enhance “pixels” of earth
structure
Prior Station
Coverage
Common Conversion Point
Stacking
Map View –
average depth
all stations
that sample
the same area
each station
Sp Receiver Functions have better
lateral resolution if stacked from all
station
Free Air
Surface
Crust
Lithosphere
Lithosphere
S
waves
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?
1. 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
1. 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