Cenozoic Tectonics and Mountain Building in Antarctica

Jesse F. LawrenceIGPP, Scripps Institution Of Oceanography

UCSD

Polenet: Seismology in the IPYDecember 10th, 2006

TransantarcticMountains

East AntarcticaWest Antarctica

TAMSEIS: Transantarctic Mountain Seismic Experiment

• Transantarctic Mountains– ~4000 km long– Peaks 4 km above Sea Level– 200-300 km wide

• East Antarctica– Thick Precambrian block that held

central position in Gondwana

• West Antarctica– Thin group of younger crustal blocks

• 42 Broadband Seismic Stations from 2000-2003

[Bedmap: Lythe et al., 2001]

Participants:• Washington University:

– Doug Wiens, Rigobert Tibi, Patrick Shore, Brian Shiro, Moira Pyle, Sara Pozgay, Bob Osburn, James Conder, Mitch Barklage

• Penn State:– Paul Winberry, Tim Watson, Don Voigt, Andy Nyblade, Audrey Huerta,

Juliette Florentin, Sridhar Anandakrishnan, Maggie Benoit• IRIS PASSCAL:

– Tim Parker, Bruce Beaudoin• SOAR:

– John Holt, Don Blankenship• Others:

– John Pollack, Bruce Long, Jennifer Curtis, Jerry Bowling, Ted Voigt

• USAP/NSF

The Seismic Stations:• Seismometer & Data Acquisition System• 100 Ahr batteries charged by ~180W solar panels• 4Gb Disks (solid state: low energy & more stable)• Heating element: excess energy warms system• Low temp & energy shutdown

TAMSEIS: Seismic Studies

• Receiver functions: Crustal thickness• Surface Wave Dispersion: Velocity variation

Anisotropy• Body Wave Tomography: Seismic Velocity• SKS Splitting: Anisotropy• Differential Attenuation: Anelasticity• Airborne Geophysics: Gravity

(SOAR) TopographyMagnetic Ice Thickness

Geophysical Modeling: Consistent Story

Surface Waves and Receiver Functions

• Surface Waves • Travel horizontally at depth ~ period• Sensitive to average velocities• Obtain velocity for each period

Surface

Moho

• Receiver FunctionsReceiver Functions• P-waves reflect nearly vertically off P-waves reflect nearly vertically off

interfacesinterfaces• Sensitive to velocity contrasts, Sensitive to velocity contrasts,

velocityvelocity

Phase Velocities• 16-25 Seconds (20-35 km)

• East Antarctica - slow• West Antarctica - fast• Transition beneath the

Transantarctic Mountains

• 120-170 Seconds (160-260 km)120-170 Seconds (160-260 km)• East Antarctica - fastEast Antarctica - fast• West Antarctica - slowWest Antarctica - slow• Transition beneath the Transition beneath the

Transantarctic MountainsTransantarctic Mountains

[Lawrence et al., 2006: JGR]

Receiver Function and Phase Velocity

Surface

Moho

Moho

Surface

[Lawrence et al., 2006: G-cubed]

West Antarctica• West Antarctica:

– Thin Crust: 20 km– Slow mantle seismic velocities

[Lawrence et al., 2006: G-cubed]

East Antarctica• West Antarctica:

– Thin Crust: 20 km– Slow mantle seismic velocities

• East Antarctica:– Thick Crust: 35– Fast mantle seismic velocities

[Lawrence et al., 2006: G-cubed]

Across the The Transantarctic Mountains• West Antarctica:

– Thin Crust: 20 km– Slow mantle seismic velocities

• East Antarctica:– Thick Crust: 35– Fast mantle seismic velocities

• Transantarctic Mountains:– 5 2 km crustal root– Thins toward WA

[Lawrence et al., 2006: G-cubed]

Seismic Tomography

[Watson et al., 2006: G-cubed]

Differential Attenuation:

• Attenuation: energy-loss per cycle of a wave.– Inverse correlation

suggests thermal anomaly

– 250C difference between East and West Antarctica inferred from both velocity & attenuation

– Thermal expansion indicates ~1% more dense beneath East Antarctica

[Lawrence et al., 2006: GRL]

Modeling Attenuation

• East Antarctica:– Little asthenosphere (~0 km)– Thick lithosphere (>300 km)

• West Antarctica:– Thick or very “mushy”

asthenosphere– Little lithosphere (< 80 km)

• Transantarctic Mountains– Transition between the two– Thickening of lithosphere– Thinning of asthenosphere

[Lawrence et al., 2006: GRL]

The Geophysical Model

• Bedrock Topography:– Ice-penetrating radar

• Moho:– Receiver Functions

• Mantle Density:– Tomography & Attenuation

• Compare with Gravity:- Good fit to gravity, especially

when mantle density anomaly is accounted for.

[Lawrence et al., 2006: G-Cubed]

Conductive Heating Model

• East Antarctica is an old craton.– Likely has a cold, deep lithospheric root.

• West Antarctica experienced extension during the Cenozoic.

– Stretching factor: ~2– Thinned the lithosphere– Increase mantle temperatures

• East Antarctica’s deep keel will heat up at its edge.

– This will reduce seismic velocities– Thin the lithosphere– Decrease density

Conductive Heating Model

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are needed to see this picture.

Thermal History

• East Antarctica is an old craton.– Likely has a cold, deep lithospheric

root.

• West Antarctica experienced extension during the Cenozoic.– Increase mantle temperatures

• East Antarctica’s deep keel will heat up at its edge.– This will reduce seismic velocities– Thin the lithosphere– Decrease density

Shear Velocity at 100 km

[Lawrence et al., 2006: G-cubed]

Flexure Model

• Constrained Parameters:– Surface & bedrock topography– Moho topography– Mantle Density Anomaly– Thinning lithosphere– Up to 6 km erosion

• Flexure model – agrees with ten Brink and Stern model – accounts for current topography that is not

currently compensated isostatically– Requires thinning of lithosphere toward

the Ross Sea

• Seismic, gravity, & Topography data agree!!!

[Lawrence et al., 2006: G-cubed]

AnisotropySurface Waves SKS Splitting

% anisotropy

Dep

th (k

m)

0

100

200

50

0.0 1.0 2.0

150

[Courtesy of Mitch Barklage][Lawrence et al., 2006: JGR]

Conclusions• TAMSEIS was a success!

– Stations located on the ice operate well with low noise– Reasonable data recovery given the harsh environment

• TAMSEIS taught us a great deal about large-scale broadband seismic deployments in polar regions– We can use this knowledge for future broadband deployments

• There is a great deal to be learned from broadband seismic studies in Antarctica!