Cenozoic Tectonics and Mountain Building in Antarctica
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Transcript of Cenozoic Tectonics and Mountain Building in Antarctica
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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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!