WIDE AND NARROW MARGINS OF THE BRAZILIAN SOUTH-ATLANTIC

Two contrasting end-members of passive continental margins, here refer red to as wide and narrow margins, are present in the South Atlantic, although some margin segments are intermediate between these two style s. Narrow margins are characterized by a large bounding fault (>4 km t hrow) near the shelf edge; a sharp transition zone (10-20 km wide) fro m normal thickness to substantially thinned continental crust or ocean ic crust; where the total width of extended continental crust is usual ly less than 100 km. Wide continental margins are characterized by a b road continental shelf and a wide zone (50-600 km) of thinned crust wi th an even distribution of faults. Wide margins usually have a more ge ntle continental shelf and slope. Narrow-narrow, wide-wide, and wide-n arrow conjugate margin pairs exist in the South Atlantic and are sugge sted to be controlled mainly by the opening kinematics and the thermal structure of the lithosphere during rifting. Basement structure and s train rate appear to have exerted little control on the style of margi n extension. Although basement architecture appears to control the tre nd of the initial rifts, the positions of fracture zones which separat e narrow and wide segments, and the rate of lateral rift propagation.