Abstract
The Brazilian Equatorial Margin is a transform passive margin with long fracture zones and several seamounts, including the largest one, the Ceara Plateau. This is a complex and poorly recognized area, with few available geophysical data. Gravity and magnetic data from the Equant I Project, seismic published lines and other previous studies are used to make several models and analyses. We recognized gravity and magnetic anomalies related to a denser, magnetized basement, internally comprised of two magmatic highs. Through gravity and magnetic forward modeling, we estimated the basement surface between 800 and 6000 m and the Mohorovicic discontinuity about 22-23 km below the Ceara Plateau. We also present and discuss the position of the continent-ocean boundary 40 km away from the continental shelf, placing the plateau at the oceanic crust domain, and the area of the transitional crust, with an extension of 40-50 km, formed during the rifting phase of the Atlantic Ocean opening.
Highlights
After the Atlantic rifting the Brazilian margins have been characterized by two different evolutions: (1) the East Brazilian Margin evolved into a passive margin, due to orthogonal crustal extension (Chang et al, 1992), (2) the Brazilian Equatorial Margin (BEM) formed during the strike-slip motion between Brazil and Africa, resulting in complex shear-dominated basins (Gorini, 1977; Zalan, 1984; Azevedo, 1986; Szatmari et al, 1987; Guiraud and Maurin, 1992; Mascle et al, 1998)
They correlate a low in the free air anomaly with a high in the Bouguer anomaly and a magnetic anomaly that separates a smooth signal over the continental shelf in the south, from a noisier region in the north
The profiles cross the Ceará Plateau (CP), terminating 10–20 km away from the edge of the plateau, which prevented us from searching for similar anomaly patterns away from the CP to propose another location for the Continental-Ocean Boundary (COB)
Summary
After the Atlantic rifting the Brazilian margins have been characterized by two different evolutions: (1) the East Brazilian Margin evolved into a passive margin, due to orthogonal crustal extension (Chang et al, 1992), (2) the Brazilian Equatorial Margin (BEM) formed during the strike-slip motion between Brazil and Africa, resulting in complex shear-dominated basins (Gorini, 1977; Zalan, 1984; Azevedo, 1986; Szatmari et al, 1987; Guiraud and Maurin, 1992; Mascle et al, 1998). The transpressional tectonic regime gives rise to fracture zones which dominate the seafloor morphology in the BEM (Attoh et al, 2004). There are several hypotheses for the origin of the BEM seamounts magmatic source: (1) deep hotspot source for magmatism related to the Fernando de Noronha hotspot (Morgan, 1983; Rivalenti et al, 2000; Mizusaki et al, 2002); (2) Edge-Driven Convection (King and Anderson, 1995, 1998); (3) the tectonic formation influenced by the fracture zone, due to a related weakness (Lowrie et al, 1986; Carracedo, 1994)
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