Abstract The Weddell Sea embayment is central to reconstructions of the West Antarctic region of Gondwana. Some reconstructions represent this area as a pre-break-up component, the Filchner block, defined by the present 2000 m isobath and coastline but take no account of crustal extension and sedimentary progradation of the continental margin since break-up. Seismic refraction shows a wide, deep sedimentary basin beneath the central part of the embayment, indicating extensive rifting and stretching of continental crust during the break-up of Gondwana. This may have increased the width of the block by about 300 km. Potential field data suggest extensive progradation of sediments on the Weddell Sea continental margin. This may have moved the margin seaward between 200 and 400 km. Taken together, these modifications in the dimensions of the pre-break-up Filchner block represent between one half and one third the area of the present Weddell Sea embayment. The presence of extensive rifting is indicative that hot, thin crust was present during break-up providing further evidence that several closely spaced mantle plumes (or a single ‘megaplume’) impinged on the Weddell Sea sector of Gondwana in the early Jurassic. The lithospheric stretching in the embayment provided a mechanism for the rotation of the Ellsworth Mountains region during the early stages of Gondwana break-up commensurate with the paleomagnetic data.