Constraints on the proposed Marie Byrd Land-Bellingshausen plate boundary from seismic reflection data

Single-channel and multichannel marine seismic data off the coast of West A ntarctica collected during two Nathaniel B. Palmer cruises (NP92-8 and NP96 -2) in the vicinity of 65 degrees S to 71 degrees S, 220 degrees E to 250 d egrees E, reveal a NNW trending graben. We interpret this graben to be part of the paleodivergent plate boundary between the Marie Byrd Land and Belli ngshausen plates. This graben coincides with a -520 nT magnetic anomaly to the NNW and a -720 nT anomaly to the SSE, as well as a 20 mGal negative gra vity anomaly. Seismic profiles subparallel to the graben (22 km/Ma half-spr eading rate) reveal greater seafloor roughness to the NE, where seafloor sp reading was slower, than to the SW (27 km/Ma half-spreading rate). These da ta allow the position of the Marie Byrd Land-Bellingshausen plate boundary to be constrained more precisely than has previously been possible, with a trend of N17 degrees W from 68.52 degrees S, 233.65 degrees E to 68.41 degr ees S, 233.56 degrees E. The sediment-filled graben has normal separation o f sedimentary layers varying from 740 +/- 30 m to 580 +/- 20 m imaged in se afloor of age A33y (74 Ma).