Seismic velocity structure of the Newfoundland rifted margin: Evidence for thinned continental crust, unroofed mantle and slow-spreading oceanic crust

The rifted margins of western Iberia and the Grand Banks of Newfoundland have been studied extensively with seismic profiling and drilling in recent years. Both margins show a seaward development of extended and exhumed continental lithosphere towards oceanic spreading. However, recent studies show that the width of these zones varies along the strike of the continental margins and between rifted margin pairs. We present a seismic refraction tomography study of SCREECH Line 2, a seismic transect across the Newfoundland margin. The results show that the continental crust thins seaward from ~27 km thick to less than 5 km over just 50 km distance. A 70 km wide zone of very thin continental crust is followed by a 30 km wide zone that appears to be serpentinized upper mantle. Further seaward we image crust of variable thickness that could represent the first oceanic crust formed after continental breakup. In our interpretation, IODP Sites 1276 and 1277, which are located on this seismic line, lie on opposite sides of the breakup point. One observation on SCREECH Line 2 that is puzzling is that the thinned continental crust in this area appears to be underlain by pristine mantle with a seismic velocity of ~8.1 km/s. On other seismic transects, such as SCREECH Line 1 and 3, lower mantle seismic velocities were imaged in the distal margin. In the late stages of the rift, the mantle in the continent-ocean transition zone of SCREECH Line 2 may have experienced relatively little extension and faulting.