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  1. Tectonic Evolution of the Pacific Margin of Antarctica Since 100 MaRobert D. Larter1, Alex P. Cunningham1,3, Karsten Gohl2, Graeme Eagles2, Peter F. Barker1, and Frank O. Nitsche2,41. British Antarctic Survey, Cambridge, UK2. Alfred Wegener Institute for Polar and Marine Science, Bremerhaven, Germany3. Now at ARK CLS Ltd, Milton Keynes, UK4. Now at Lamont-Doherty Earth Observatory of Columbia University, USA

  2. Talk Outline • Introduction • Paleomagnetic constraints • Schematic reconstructions • New Late Cretaceous Reconstructions of the Pacific margin • Summary: sequence of events • 4. Suggestions for seismological studies

  3. South Pacific and West Antarctic Topography Topographic data north of 72°S courtesy of W.H.F. Smith (NOAA). Antarctic topographic data from SCAR Antarctic Digital Database.

  4. Summary of Antarctic paleomagnetic results

  5. SCHEMATIC REGIONAL RECONSTRUCTIONS

  6. CONTINENTAL MARGIN, FRACTURE ZONE AND MAGNETIC ANOMALY PICKS

  7. C30r (67.7 Ma) Reconstruction

  8. C33y (73.6 Ma) Reconstruction

  9. Reconstruction of Campbell Plateau to Marie Byrd Land (~C34, 83 Ma)

  10. C34 (83 Ma) Reconstruction(assuming West Antarctic Margin unaffected by Bellingshausen Plate motion)

  11. Cretaceous Normal Superchron (90 Ma) Reconstruction(assuming West Antarctic margin unaffected by Bellingshausen Plate motion)

  12. C34 (83 Ma) Reconstruction(assuming part of West Antarctic Margin moves with Bellingshausen Plate)

  13. Creteceous Normal Superchron (90 Ma) Reconstruction (assuming part of West Antarctic Margin moves with Bellingshausen Plate)

  14. Summary: Sequence of Events PAC – ANT ridge propagation forms Henry and Hudson Troughs 47 Ma C21 Capture of large area of Pacific plate by Antarctic plate 55 Ma C24 Major decrease in Pacific–Antarctic spreading rate Development of right-stepping offsets at Pacific–Antarctic ridge Antarctic–Phoenix spreading established at DGGA Bellingshausen plate incorporated into Antarctic plate 61 Ma C27 South Pacific tectonic reorganisation: 74 Ma C33y Start of independent Bellingshausen plate motion? Bounty Trough extension stops 83 Ma C34 Campbell Plateau separates from Marie Byrd Land Bounty Trough extension starts 90 Ma CNS Chatham Rise separates from Marie Byrd Land Distributed PAC–ANT extension starts in Ross Sea and MBL 105 Ma CNS Proto-Pacific subduction stops at NZ sector of Gondwana margin

  15. What can seismologists do? • Receiver functions: Reconstructions predict that crust and lithosphere beneath Amundsen Sea shelf has been thinned by multiple episodes of extension: diffuse PAC–ANT extension (105 – 90 Ma) rifting during separation of Chatham Rise (~90 Ma) extension related to Bellingshausen plate motion (74 – 61 Ma) In contrast, reconstructions predict that continental lithosphere east of the Bellingshausen Gravity Anomaly was unaffected by these events. • Shear wave splitting: Reconstructions predict different extension directions in Amundsen Sea during Chatham Rise rifting and Bellingshausen plate motion, e.g. >20° difference near Thurston Island. If splitting can resolve this, then can use it to test whether Bellingshausen motion did affect Amundsen Sea shelf. • Wide-angle recording of controlled sources: Determine detailed crustal structure to compare with that predicted from tectonic history (see above), and with Campbell Plateau (presently being studied). N.B. R/V Polarstern Amundsen Sea geophysical cruise in early 2006.

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