GPS Evidence for a Coherent Antarctic plate and for Postglacial Rebound in Marie Byrd Land and the Northern Transantarctics Andrea Donnellan, JPL Bruce Luyendyk, UCSB Carol Raymond, JPL Tom James, GSC, Canada GPS measurements collected in Marie Byrd Land, West Antarctica indicate no significant motion between East and west Antarctica greater than 1-2 mm/yr. This result indicates that the two subcontinents are likely joined as a single coherent lithosphere plate. They could have been joined since the end of Adare Trough spreading in Oligocene time. The volcanic activity in the Ross Sea sector at present such as at Mount Erebus is most likely related to mantle upwelling not associated with continental rifting. Low strain rates imply that the Ross Sea rift is inactive and also that the Transantarctic Mountains are stable. GPS measurements of vertical rates indicate postglacial rebound of up to 12.6±4.1 mm/yr in western Marie Byrd Land and 4.6 ± 3.8 mm/yr in the northern TAM. These vertical rates deviate significantly from uplift predictions based on deglaciation models ICE-3G and 4G; they are consistent with uplift rates on the order of 6.7 ± 2.3 mm/yr at the O'Higgins continuous GPS station on the Antarctic Peninsula. The measured uplift rates support an interpretation of ice mass changes larger than predicted by global deglaciation models such as ICE-3G, or mantle viscosity beneath West Antarctica significantly less than the 10 21 Pa-s (as assumed in ICE-3G), coupled with mid-late Holocene neoglacial fluctuations. The large uplift rate in wMBL indicates significantly lower mantle viscosity there, assuming that the ice mass change at Roosevelt Dome began 4000 years ago. The inferred low mantle viscosity in Marie Byrd Land is to be expected due to the past subduction history, and is consistent with recent extensive volcanism there. The wMBL rock uplift rates are consistent with postglacial rebound models for an ice sheet center located in the eastern Ross Ice Shelf, which thinned between 4000, and 2000 years B.P.