1. Cenozoic Tectonics and Mountain Building in Antarctica Jesse F. Lawrence IGPP, Scripps Institution Of Oceanography UCSD Polenet: Seismology in the IPY December 10 th, 2006

2. Transantarctic Mountains East Antarctica West 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]

3. 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

4. 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

5. 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)Topography Magnetic Ice Thickness Geophysical Modeling:Consistent Story

6. 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 interfacesP-waves reflect nearly vertically off interfaces Sensitive to velocity contrasts, velocitySensitive to velocity contrasts, velocity

7. 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 Transantarctic MountainsTransition beneath the Transantarctic Mountains [Lawrence et al., 2006: JGR]

8. Receiver Function and Phase Velocity Surface Moho Surface [Lawrence et al., 2006: G-cubed]

9. West Antarctica West Antarctica: –Thin Crust: 20 km –Slow mantle seismic velocities [Lawrence et al., 2006: G-cubed]

10. 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]

11. 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]

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

13. Differential Attenuation: Attenuation: energy-loss per cycle of a wave. –Inverse correlation suggests thermal anomaly –250  C 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]

14. 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]

15. 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]

16. 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

17. 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]

18. 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]

19. Anisotropy Surface Waves SKS Splitting % anisotropy Depth (km) 0 100 200 50 0.01.02.0 150 [Courtesy of Mitch Barklage] [Lawrence et al., 2006: JGR]

20. 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!