SEISMIC VELOCITY MODEL ACROSS THE EASTERN NEWFOUNDLAND (GRAND BANKS) CONTINENTAL MARGIN

We conducted a large-scale, deep seismic survey across the Eastern Grand Banks and Newfoundland Basin in July-August, 2000. The objective was to determine the structure of this non-volcanic rifted margin, reaching from full thickness continental crust seaward to unequivocal oceanic crust along three transects. Both wide-angle and multi-channel seismic (MCS) data were acquired. Transect 3, reported here, is 565km long and over this distance we recorded wide-angle reflection/refraction data on 23 ocean bottom seismometers. Shot spacing was 50m over the shelf and 200m seaward of the shelf break. Results of velocity modelling show full thickness (35km) continental crust of typical Newfoundland Appalachian structure from the beginning of profile eastward to the landward limit of the Carson Basin, with local thinning underneath the Jeanne d’Arc Basin. Beneath the Carson Basin, the crust thins abruptly to ~7km (extension factor=5) over a distance of 50km, and over the next 60km it thins gradually to < 3km thickness beneath the Newfoundland Basin. The abrupt thinning is predominantly within the lower continental crust (extension factor=~ 8.0). Seaward beneath the extremely thin (< 5km) crust of the Newfoundland basin, a wide layer (~200km) of low velocity mantle (7.5-8.0km/s) is observed up to 15km below sea level; these velocities are interpreted to represent serpentinized mantle. Seaward of the extremely thin continental crust (~150km from the shelf break) is a transition zone ~80km wide where the serpentinized mantle is observed very close to the basement. The “crust” over this zone is extremely thin (~ 2km) with very high velocity gradient possibly composed of exhumed mantle with serpentinization decreasing rapidly with depth. The transition zone ends abruptly 230km seaward of the shelf break. Farther seaward, oceanic crust is present, exhibiting velocities characteristic of oceanic layer 2 (4.7-6.3km/s) and layer 3 (6.3-7.4km/s), and the serpentinized-mantle layer pinches out. The MCS reflection data also show changes in reflection character of basement that match the changes in velocity structure outlined above. Taken together, the data suggest that a wide zone of transitional crust, in which mantle exhumation and serpentinization are important factors, characterizes this non-volcanic margin.