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Evolution of Syn-Rift Crustal Architecture in Foz do Amazonas Basin, North Brazil


The Foz do Amazonas basin located at the northwestern tip of the equatorial Atlantic margin of Brazil has been the focus of exploration activities in recent years since the Zaedyus discovery in nearby French Guyana. Syn-rift hydrocarbon plays in and around relict crustal highs has been the focus of interest along with the post-rift plays. Understanding the regional tectonics and the timing relation between faults and stratigraphic mega sequences are key factors in unlocking the syn-rift plays. This work attempts to describe the crustal evolution of the Foz basin in context with the regional tectonics. 3D and 2D seismic data were used to describe the basin architecture at a sub-regional and regional scale and characterize the associated tectonic evolution based on the depositional history of the syn and post rift mega-sequences. Three main structural domains were mapped from the seismic data. The main objective of the present work is to understand the spatial and temporal relation of these three domains using a simple conceptual model that fits all scales of observations. The structural model conceived from the present study connects the fault evolution from a basin scale to plate-scale. Based on this model the Foz do Amazonas is conceptualized as an initially sigmoidal shaped pull-apart basin above an offset (jog) in the South American plate margin. It was defined by isolated sinuous structural highs and deep grabens evolving during Berriasian-Barremian due to a NW-SE oriented dextral strike slip motion between the South American and African plates. From Barremian to late Albian (breakup) the rotation of the South American and African plates away from each other changed the direction of the dextral slip system to ENE-WSW. The change in the stress direction developed a new system of normal and strike slip faults that cut through the pre-existing structural fabric resulting in relict blocks of crust in the Foz basin. The structural model can be used to explain the crustal evolution of similar basins in the equatorial margins of South America and Africa. Pull-apart basins of similar origin have been reported from areas near the Demerara Plateau (Greenroyd et. Al., 2007, Benkhelil et. al., 1995). The study also reveals that differently oriented structural domains in a basin can originate from a single direction of tectonic transport depending on the geometry of the plate margin and the location of the syn-rift basin on it.