Wide and narrow margins of the Brazilian South Atlantic

Abstract

Two contrasting end-members of passive continental margins, here referred to as wide and narrow margins, are present in the South Atlantic, although some margin segments are intermediate between these two styles. Narrow margins are characterized by a large bounding fault (>4 km throw) near the shelf edge; a sharp transition zone (10–20 km wide) from normal thickness to substantially thinned continental crust or oceanic crust; where the total width of extended continental crust is usually less than 100 km. Wide continental margins are characterized by a broad continental shelf and a wide zone (50–600 km) of thinned crust with an even distribution of faults. Wide margins usually have a more gentle continental shelf and slope. Narrow–narrow, wide–wide, and wide–narrow conjugate margin pairs exist in the South Atlantic and are suggested to be controlled mainly by the opening kinematics and the thermal structure of the lithosphere during rifting. Basement structure and strain rate appear to have exerted little control on the style of margin extension. Although basement architecture appears to control the trend of the initial rifts, the positions of fracture zones which separate narrow and wide segments, and the rate of lateral rift propagation.