HR: 08:30h
AN: T41A-01    [Abstracts]
TI: Attempts to reconcile plate reconstructions with interpreted positions of the continent-ocean boundary in the South Atlantic.
AU: * Norton, I O
EM: ian.o.norton@exxonmobil.com
AF: ExxonMobil Upstream Research, P.O. Box 2189, Houston, TX 77252 United States
AB: South Atlantic continental margins are separated into two major domains, volcanic margins south of the Walvis and Rio Grande Ridges and non-volcanic margins north of the ridges. In addition, variable age of the continent-ocean boundary (COB) in the South Atlantic reflects diachronous separation between Africa and South America. South of the Walvis Ridge, along the volcanic margin, the COB is progressively younger to the north, as evidenced by oceanic spreading magnetic anomalies adjacent to the COB that are about 5 million years older in the far south than near the Walvis Ridge. This age progression has generally been interpreted to continue to the north as far as the Niger Delta, with first oceanic crust at the delta younger still than at the Walvis Ridge. Age of this more northerly crust cannot be dated by oceanic spreading anomalies as these margins formed during the Cretaceous normal-polarity interval. Along the non-volcanic margins north of the ridges there are extensive salt basins developed on highly extended continental crust. Seismic and potential field evidence suggests that the COBs in these basins approximately (i.e. within 20-30 km) coincide with the outer edge of salt. It is generally assumed that the edge of salt is an isochron, i.e. salt deposition stopped at the same time along the entire 2,000 km strike length of the salt basins. Plate reconstructions that attempt to match the COBs from South America and Africa along the entire South Atlantic demonstrate, however, that it is not possible to match the COBs along the length of the salt basin margins. This means that the edges of salt and thus the COBs are not isochrons. Possible explanations for this observation include 1) that the edge of salt is not the COB or 2) that the COB and end of salt deposition are diachronous. We tend to favor the diachronous model as, even if the edge of salt is not the COB, it would mean that salt would have had to move out onto oceanic crust by as much as 200 km to account for the mismatch. This talk discusses our tectonic observations and implications of the diachronous model for our understanding of salt deposition in evolving rift basins and the transition from continental to oceanic crust.
DE: 8109 Continental tectonics--extensional (0905)
DE: 8150 Plate boundary--general (3040)
DE: 9325 Atlantic Ocean
SC: Tectonophysics [T]
MN: 2005 Joint Assembly