Understanding the tectonic evolution of the Weddell Sea embayment provides valuable insights on the breakup of Gondwana. Existing geodynamic models for this region differ mainly in the nature of the crust beneath the Filchner-Ronne shelf and the movement of the Ellsworth-Whitmore Mountains (EWM) crustal block. New gravity data presented here constrain the area and timing of this EWM movement from its prebreakup position to its present-day position. The crustal thickness estimates of 29¿5 km and 27¿4 km derived from power spectral analysis as well as forward gravity modeling make the existence of oceanic crust beneath the Filchner-Ronne shelf unlikely. A new crustal unit consisting of Precambrian crust has been identified with gravity and magnetic data and extends ~175 km off the coast of Coats Land. This crustal unit conflicts with the late Mesozoic movement of the EWM in this area. The effective elastic thickness of Te=35¿5 km has been estimated from coherence spectra for the Ronne shelf. Together with the crustal thickness of 27 km from refraction seismic studies and comparison with theoretical models, the thermal age is estimated to be between 165 and 230 Ma for the Ronne shelf. This age marks the last significant tectonic event in this area and is in good agreement with the breakup of Gondwana. It further makes later movements of microplates like the EWM in this region unlikely. Thus a Filchner block consisting of continental crust should be considered in any geodynamic model of the early Weddell Sea evolution. ¿ 1999 American Geophysical Union