| Summary: | Single-channel and multichannel marine seismic data off the coast of West Antarctica collected during two Nathaniel B. Palmer cruises (NP92-8 and NP96-2) in the vicinity of 65°S to 71°S, 220°E to 250°E, reveal a NNW trending graben. We interpret this graben to be part of the paleodivergent plate boundary between the Marie Byrd Land and Bellingshausen 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 gravity anomaly. Seismic profiles subparallel to the graben (22 km/Ma half-spreading rate) reveal greater seafloor roughness to the NE, where seafloor spreading was slower, than to the SW (27 km/Ma half-spreading rate). These data 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°W from 68.52°S, 233.65°E to 68.41°S, 233.56°E. The sediment-filled graben has normal separation of sedimentary layers varying from 740±30 m to 580±20 m imaged in seafloor of age A33y (74 Ma). © 1999 American Geophysical Union. Received 10 September 1997; accepted 29 September 1998. This research was supported by NSF grants OPP- 9317318 and OPP-9317872. We thank Bruce Luyendyk, Robert Bird, and Phillip Schmidt for constructive reviews that helped to improve the paper. We thank the personnel of the Nathaniel B. Palmer and of Antarctic Support Associates for their hard work to ensure the success of the data collection during these two cruises. Contribution 8532, Division of Geological and Planetary Sciences, California Institute of Technology. Published - Heinemann1999.pdf
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