As part of the tripartite, United States - United Kingdom - New Zealand, 1990-1991 South Pacific Rim International Tectonics Expedition, oriented samples were collected for paleomagnetic analysis from mid-Cretaceous (circa 100 Ma) intrusive rocks at sampling localities across 350 km of the Ruppert and Hobbs Coast area of Marie Byrd Land, West Antarctica. Paleomagnetic results are presented along with several lines of evidence, including a positive tilt test based on the attitude of circa 117 Ma volcanic rocks that the circa 100 Ma rocks intrude, which argue that these results are a representative estimate of the mid-Cretaceous magnetic field in Marie Byrd Land (MBL). The new circa 100 Ma mean south pole (224.1¿E/75.7¿S, A95=3.8¿, N=19 site means) is concordant with other West Antarctic results of similar age implying that at least Marie Byrd Land, Thurston Island and the Antarctic Peninsula have not experienced any paleomagnetically resolvable relative motion since the mid-Cretaceous. However, the poles from these Pacific-bordering blocks of West Antarctica are significantly offset from a synthetic apparent polar wander path that was produced for the East Antarctic craton, implying relative movement between East Antarctica and Pacific West Antarctica since about 100 Ma. Though the paleomagnetic estimate for east-west Antarctic relative motion may be reconciled with geologic estimates for extension in the Ross Sea at the extremes of the error envelope, the best paleomagnetic estimate of relative motion suggests a larger amount of total extension between East and West Antarctica (MBL) than previously suspected. Both estimates call for several hundreds of kilometers of post-100 Ma displacement between East Antarctica and the Pacific-bordering blocks of West Antarctica. ¿ American Geophysical Union 1994