Mapping Crustal Thickness & the Ocean-Continent-Transition in the Santos & Campos Basins, Brazilian South Atlantic

Alan Roberts¹, Nick J. Kusznir², Kevin Boyd³, and Mark Thompson^3
1Badley Geoscience Ltd, Hundelby, Lincolnshire, United Kingdom
²Liverpool University, Liverpool, United Kingdom
³BP Exploration, Sunbury, United Kingdom

We have applied 3D gravity inversion and 3D flexural backstripping to a large segment of the Brazilian continental margin. 3D gravity inversion predicts depth to Moho and crustal thickness. 3D flexural backstripping is used to map stretching factors, derive maps of crustal thickness, and produce a 3D palaeobathymetric history for the Brazilian margin.

A key question at the Brazilian margin is the age of the Aptian salt relative to the age of breakup. Seismic data alone do not distinguish whether the salt is part of the syn-breakup sequence or whether it is the basal part of the post-breakup. Our analysis of subsidence history shows that the salt almost certainly cannot be part of the post-breakup sequence. We believe the salt was deposited during the breakup process itself. A syn-breakup age for the salt allows most of the Santos/Campos margin to be floored by thinned continental crust (rather than oceanic crust). A post-breakup origin for the salt would require all but the coastal strip of the margin to be floored by oceanic crust. A post-breakup age for the salt also means that the results of the gravity inversion and the flexural backstripping cannot be reconciled with each other.

A key sensitivity when predicting the crustal structure of the Santos/Campos margin is the amount of volcanic addition assumed to have occurred during continental breakup. We believe the Campos margin is best modelled as a “non-volcanic margin”, while the Santos margin is almost certainly a “volcanic margin”. This has considerable implications for heat-flow history.
Both the gravity inversion and backstripping indicate that in the SW Santos there exists a segment of highly-stretched (oceanic?) crust. This is probably a failed breakup basin, indicating that continental separation originally attempted to occur much closer to the present-day Brazilian coast than was ultimately the case.

AAPG International Conference and Exhibition, Cape Town, South Africa 2008 © AAPG Search and Discovery