|Résumé||(disponible en anglais seulement)|
About 60 expendable sonobuoys were successfully launched in the Canada Basin of the Arctic Ocean in 2007 and 2008. The sonobuoys recorded wide-angle seismic
energy up to 35km offset along 5800 km of high quality low fold multichannel seismic reflection data. These sonobuoy data, together with existing sonobuoy data in the nearby area, were designed to constrain the velocity/depth information of
coincident reflection profiles, and to provide deeper velocity models that are crucial in understanding the basin's evolutionary history.
Because the sonobuoys drift after deployment, it is necessary to do offset calculation from direct water
waves. After this step, the data are filtered, sometimes trace balanced, and are ready to do modeling. During the modeling, coincident reflection data are first converted to depth using a preliminary velocity model, and then iterated based on forward
wide-angle raytracing (using RAYINVR and SeisWide) until a satisfactory fit to all data is reached. The near-offset information were converted using 2D normal move out corrections to compare with coincident single channel reflection data. The longer
offset information of the sonobuoy data is forward modeled to provide velocity-depth information. Sensitivity analysis is done by iterating the model parameters until phases do not apparently fit. A velocity model is constructed for each sonobuoy
separately, and by combining all sonobuoy in a line, we construct a two dimensional velocity model.
Three zones with distinctive crustal velocity-depth profiles are identified for the Canada Basin. The two southern zones between the Northwind
Ridge, and the Banks (zone 1) and Prince Patrick islands (zone 2) on the Canadian polar margin, are intercepted by a distinct linear gravity low (~10mGals anomaly). Zone 1, west of the Banks Island, has the thickest sedimentary section, where
modeling shows basement velocities of 4.5-4.9 km/sec overlay a sub-basement layer with velocities of 5.5-5.8 km/s at depths of 12-13 km. West of the gravity low (on the Northwind Ridge side), the basement and sub-basement layers are consistently
shallower by >1 km. The velocity/depth profile is compared and contrasted with standard oceanic and continental crustal structures.
Zone 2, west of the Prince Patrick Island, however, shows different velocity characteristics. West of the gravity
low, a layer of 4.2-4.6 km/s velocities directly overlays lower crustal velocities (6.7-7.2km). Upper mantle refraction Pn is observed only here on 2 sonobuoys, suggesting a velocity of 8.0 km/sec below Moho at 16 km depth. In contrast, east of the
gravity low of the center zone, a similar synrift sedimentary layer is also observed, underlain by a ~2-km-thick layer of upper crustal velocities. Beneath this upper crust, refraction modeling indicates a velocity of 6.1km/s (~9 km depth) increasing
quickly downwards to about 6.6 km/s near 13 km depth. At greater depths no seismic energy is recorded. Further northward where the gravity low disappears completely, we see strong basement reflections associated with an unusually low basement
velocity of 4.0-4.1km/s, underlain by 4.2-4.6km/s atop a velocity of >5.6km/s, a feature consistently observed along a ~300-km-line (line 9) across the basin. Combining the new reflection and refraction results with an old refraction line across the
Prince Patrick margin, a 2D crustal velocity model across the entire Canada Basin is constructed for the center zone, with the Moho topography constrained by both the wide-angle and gravity modeling. A tentative interpretation indicates an analogy to
a typical non-volcanic continental stretching and failed rift system, with the simple shear breakup at the linear gravity low, leaving the lower plate to the west and upper plate to the east. The high velocity gradient in the upper plate is possibly
caused by mantle hydration and serpentinization. North of zone 2 where the gravity low disappears, low basement velocities of near 4.0 km/s are observed, possibly due to extension and serpentinization.
In contrast to the above two zones, the
northern zone 3 between Chukchi Plateau and Alpha Ridge, the basement-type velocities of >4.5km/s appear at unusually shallow depth of 5 km, which can be attributed to intrusive rocks related to the Alpha Ridge.