Crustal structure beneath Orphan Basin and implications for nonvolcanic continental rifting

Abstract

A wide‐angle seismic profile was acquired across the northeast Newfoundland margin, seaward to Orphan Knoll. 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 bottom seismometers provides the seismic velocity structure for a significant portion of the ∼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 ∼5.2 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 Knoll has a typical continental lower crustal velocity (6.8–7.0 km/s), and the lower crust shallows significantly beneath Orphan Knoll. 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 ∼180 Ma to Chron 34N time (∼101 Ma) when the final breakup between Canada and western Europe took place, leaving a 400‐km‐wide 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.