Print this pageSeismic Imaging of South Atlantic Continent-Ocean Boundary Through Deep, Long Offset, Prestack Depth Migrated (PSDM), 2D Seismic Data
During the last five years, ION Geophysical (GX Technology) has acquired more than 50,000 km of deep (18 sec recording), long offset (10 km), Prestack Depth Migrated (PSDM) 2D seismic data in South Atlantic, from Nigeria to Angola in the east and from Brazil to Argentina in the west. The South Atlantic data have been processed mostly to 25 km (40 km off Argentina) depth and most lines traverse the transition zone from the continental to the “true” oceanic crust. In addition, strike lines, some >1,000 km, and at least one, almost 3,000 km long, have also been acquired.
Defining the geometry of continent-ocean boundary (COB) is critical for establishing the deep-water exploration potential of the continental margins in the area. Existing models for COB in South Atlantic suggest that the margins are primarily extensional, volcanic margins. A zone of “transitional crust”, varying in width from less than 100 km to almost 500 km, has been defined between the craton and the mid-oceanic ridge basalts. The new PSDM data have allowed us to image the continent-ocean transition in a uniformly acquired and processed seismic data set and thus refine existing models for continent-ocean transition which are based primarily on gravity, magnetic, and thermal modeling and refraction data.
By juxtaposing seismic lines from the South Atlantic conjugate margins, we can reconstruct the basin just before the initiation of the mid-ocean ridge spreading. The PSDM data image reflector packages underneath the evaporite layer on both sides of the South Atlantic which we have interpreted as sedimentary and volcanic sequences overlying a thinned continental crust. This crust is 10 to 20 km thick. Just landward of the COB, the extended crust often includes up to 5 km of presumed volcanic material. Seaward of the COB, the oceanic crust is generally less than 10 km thick but displays local variation.
The MOHO is not always imaged as a distinct reflector, but mostly as a band of coherent amplitudes overlying a zone lacking coherency. Depth to MOHO has been iteratively derived in conjunction with modeling the gravity/magnetic data acquired during seismic acquisition, and ranges from ~12 km under oceanic crust to >25 km approaching the cratons.
The data clearly show that while processes in the lithosphere control the rifting and subsequent drifting processes, local variations in the continental fabric impact the final geometry of the transitional zone and location of the COB.
AAPG Search and Discover Article #90100©2009 AAPG International Conference and Exhibition 15-18 November 2009, Rio de Janeiro, Brazil