Crustal structure beneath Orphan Basin and implications for nonvolcanic continental rifting

A wide-angle seismic profile was acquired across the northeast Newfoundland margin, seaward to Orphan Knell. The profile is complemented by existing multichannel reflection data, two online deep wells, and other deep seismic reflection and refraction lines. Modeling of data from 15 ocean bott om seismometers provides the seismic velocity structure for a significant portion of the similar to 400-km-wide stretched continental crust on this margin. Prerift metasedimentary rocks have a topographic relief of up to 5 km, with a velocity of similar to5.2 km/s increasing to 5.9 km/s. Unaltered crystalline crust below the metasedimentary section has a velocity range of 6.1-6.5 km/s. Most of the lower crust landward of Orphan Knell has a typical continental lower crustal velocity (6.8-7.0 km/s), and the lower crust shallows significantly beneath Orphan Knell. Our model shows no evidence for the 5-km-thick, 7.35 km/s layer modeled from data of an earlier experiment and interpreted as magmatic underplating but instead indicates that thinned continental crust extends seaward for 360 km without underplating, implying nonvolcanic rifting for the NE Newfoundland margin. We suggest that continental stretching persisted from similar to 180 Ma to Chron 34N time (similar to 101 Ma) when the final breakup between Canada and western Europe took place, leaving a 400-kmwide zone of thinned continental crust underneath the shelf and deep water area of the Orphan Basin. On the landward side of the basin, gravity modeling indicates a zone of very thin (6-8 km) crust, possibly a failed rift center formed as a result of the northward progression of nonvolcanic rifting between Canada and Europe. A possible sub-Moho reflector suggests generally uniform stretching of the crust and upper mantle, and the absence of volcanics and underplating is in contrast to the observations on related rift basins of the Grand Banks.