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The Brazilian Equatorial Margin: A Snapshot in Time of an Oblique Rifted Margin


We investigated rift-system faults along the Brazilian equatorial margin to better understand the nature of the Continent-To-Ocean Transition Zone (COTZ). The COTZ is that region of attenuated continental crust which lies between the outboard edge of full-thickness continental crust, and the inboard edge of oceanic crust. We mapped the COTZ using a seismic and potential field data. Along the margin definitive continental basement is less than 3 seconds deep under the continental shelf, while oceanic crust is more than 6 seconds deep at the toe of slope. Our mapping shows that basement faults in the COTZ form a fault system consisting of W-striking segments, NW-striking segments and N-striking segments. Faults along the Amapá Basin are N-striking; have small displacements (∼1 second) and are as long as 180 km. Faults along the Barreirinhas and Pará-Maranhão basins strike NW, have lengths ranging from 5-50 km, and have moderate displacement (1.5-3 seconds). Faults along the Piauí-Ceará Basin strike E-W, are long (80 to 210 km), and have large displacements (∼4 seconds). By comparing the strike of the fault segment to the spreading direction in the South Atlantic we infer N-striking segments to be dominantly extensional while E-W striking segments would primarily accommodate strike-slip motion and NW-striking segments accommodate oblique motion. The continental margin is narrowest (20 km) at the strike-slip segments and widest at the oblique segments (∼220 km). Since rift-related faults are active only during a geologically brief episode of rifting, and become dormant with emplacement of oceanic crust, they provide a snapshot of the evolution of a complex rifted continental margin.