OCEANIC CRUST OF THE CANADA BASIN, ARCTIC OCEAN

Velocity measurements from 70 sonobuoys have been used to classify crustal types in the Canada Basin of the Arctic Ocean. Velocity categories were interpreted as continental (≤6.6 km/s), oceanic (6.7 – 7.2 km/s), and transitional (7.2-7.7 km/s). Moho was also identified on many of the sonobuoys from either direct refracted or wide-angle reflected arrivals. Oceanic crust occupies a polygon in the middle of the basin ~340 km (east-west) by ~590 km (north-south). On seismic reflection profiles, the interpreted top of oceanic crust is irregular and rough, with high relief and contrasts with the smooth, bright reflection outside the polygon. Other geophysical data support this interpretation of oceanic crust in that paired magnetic anomalies are limited to this polygon, and a linear negative gravity anomaly (Canada Basin Gravity Low, CBGL), interpreted as a fossil spreading center by others, roughly bisects it. Seismic reflection profiles confirm the presence of a prominent, deep basement valley beneath the gravity anomaly. The presence of this buried valley, together with crustal thicknesses of 4-7 km, or thinner-than-normal oceanic crust, suggests slow-spreading opening is likely. The transitional crust that surrounds the oceanic polygon is of variable width (20-300 km), and is characterized by velocities typically 7.4-7.5 km/s. Beneath the Mackenzie Fan, this wide zone of transitional crust is interpreted as serpentinized mantle because of the lack of volcanic material in rift-age material known from drilling. The southern arm of the CBGL bisects this zone of transitional crust. Near Northwind Ridge, the velocities in transitional crust are slightly lower (7.2-7.3 km/s) and probably represent underplating or magmatic intrusion associated with emplacement of the High-Arctic Large Igneous Province that encompasses the Alpha-Mendeleev Ridge System. A block of thinned continental crust extends more than 300 km northward from Alaska with an extremely narrow band of transitional crust between it and oceanic crust. Although the data are broadly consistent with a rotational opening of the Canada Basin in the early Cretaceous, the asymmetry of continental and transitional crustal bodies suggests a more complicated history and geometry of opening.