MIDCRETACEOUS PALEOMAGNETIC RESULTS FROM MARIE BYRD LAND, WEST ANTARCTICA - A TEST OF POST-100 MA RELATIVE MOTION BETWEEN EAST AND WEST ANTARCTICA

As part of the tripartite, United States - United Kingdom - New Zealan d, 1990-1991 South Pacific Rim International Tectonics Expedition, ori ented samples were collected for paleomagnetic analysis from mid-Creta ceous (circa 100 Ma) intrusive rocks at sampling localities across 350 km of the Ruppert and Hobbs Coast area of Marie Byrd Land, West Antar ctica. Paleomagnetic results are presented along with several lines of evidence, including a positive tilt test based on the attitude of cir ca 117 Ma volcanic rocks that the circa 100 Ma rocks intrude, which ar gue that these results are a representative estimate of the mid-Cretac eous magnetic field in Marie Byrd Land (MBL). The new circa 100 Ma mea n south pole (224.1-degrees-E/757-degrees-S, A95 = 3.8-degrees, N = 19 site means) is concordant with other West Antarctic results of simila r age implying that at least Marie Byrd Land, Thurston Island and the Antarctic Peninsula have not experienced any paleomagnetically resolva ble relative motion since the mid-Cretaceous. However, the poles from these Pacific-bordering blocks of West Antarctica are significantly of fset from a synthetic apparent polar wander path that was produced for the East Antarctic craton, implying relative movement between East An tarctica and Pacific West Antarctica since about 100 Ma. Though the pa leomagnetic estimate for east-west Antarctic relative motion may be re conciled with geologic estimates for extension in the Ross Sea at the extremes of the error envelope, the best paleomagnetic estimate of rel ative motion suggests a larger amount of total extension between East and West Antarctica (MBL) than previously suspected. Both estimates ca ll for several hundreds of kilometers of post-100 Ma displacement betw een East Antarctica and the Pacific-bordering blocks of West Antarctic a.