AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Synrift Tectonic Domains in South Atlantic Salt Basins: Observations from Long-Offset Deep Seismic Data Combined with Plate Reconstructions Allow for Comparisons of Regional Extension Along West Africa and Brazil Continental Margins
(1) Jackson School of Geosciences, Univ. Texas, Austin, TX.
(2) ION Geophysical - GX Technology/ISS, Houston, TX.
The Brazil and West Africa salt basins were formed during the final stages of the rift to drift transition in the South Atlantic. Study of these salt basins has recently been greatly enhanced by ION/GXT’s acquisition of nearly 40,000 km of high-quality, deep penetration (25 km) 2-D seismic data. These data were processed using pre-stack depth migration (PSDM) resulting in depth sections that yield a unique view of the crust in this highly extended domain. In this preliminary study, we interpreted about 8,999 km of this seismic data to define extensional domains. Extension can be quantified using plate reconstructions of the South Atlantic that incorporate all potential field, geologic and regional data. Combining constraints on the amount and timing of extension from the plate reconstructions with meso-crustal imaging from seismic data allows identification of a number of structural provinces with differing expressions of how extension was accommodated. Total extension varies along the conjugate margins, ranging from 200 km between northern Gabon and Sergipe Alagoas to more than 500 km between southern Angola and the Santos Basin. This implies, for instance, that there was 500 km of motion between South America and Africa before the Santos Basin was completely formed, i.e. before onset of sea floor spreading. Comparison with other areas of large-magnitude extension such as the Basin and Range in North America and the Iberia-Newfoundland rift in the North Atlantic are useful for understanding how such vast areas of extension could have been accommodated on the South Atlantic margins. Structural provinces include, 1) areas of significant volcanism where crust is entirely igneous, 2) domains with large-magnitude extension accommodated by low-angle normal faults, 3) areas of continental thinning resulting from faulting that cut through the entire crust, and 4) areas where extension has possibly resulted in exhumation of lower crust or, in places mantle. The areas of low-angle normal faulting are significant because they also include thick pre-salt sedimentary accumulations that are proving to be potentially significant hydrocarbon provinces.