CONTRIBUTIONS OF CRETACEOUS QUIET ZONE NATURAL REMANENT MAGNETIZATIONTO MAGSAT ANOMALIES IN THE SOUTHWEST INDIAN-OCEAN

The Magsat magnetic anomalies over the Southwest Indian Ocean are mode led using a combination of induced plus viscous remanent magnetization (IM/VRM) and natural remanent magnetization (NRM). Two broad, roughly parallel, SW to NE trending triple-peaked positive anomalies dominate the region, one lying south of Africa and the other north of Antarcti ca. Although these anomaly peaks generally correspond with the Agulhas Plateau/Maud Rise, Mozambique Plateau/Astrid Ridge, and Madagascar Ri dge/Conrad Rise conjugate pairs, the IM/VRM contribution from structur al characteristics (i.e., crustal thickness) accounts for only about 2 0% of the anomaly amplitudes. A spatially variable but observationally constrained NRM contribution in Cretaceous Quiet Zone (KQZ) crust is required to account for the location, shape, and amplitude contrast of these anomalies. Many crustal features in the Southwest Indian Ocean near Antarctica have little geophysical data to constrain their struct ure but do have tectonic conjugates near Africa for which much more ge ophysical data are generally available. Using geophysical and geologic al constraints from one member to model the magnetization structure of its conjugate reproduces the observed Magsat reduced-to-pole anomalie s over both structures very well. This suggests that no significant al teration in their magnetization structure has occurred since the featu res split. Models of these conjugate structures show that IM/VRM repro duces the Magsat anomalies associated with non-KQZ crust but that both IM/VRM and a dominant NRM component are required to explain the anoma lies associated with KQZ crust.