| dc.description | New structural observations across the Grand Banks/Newfoundland Basin continental margin are presented in this thesis to define how continental crust is modified under large extension and its transition to seafloor spreading. A P-wave velocity model from wide-angle seismic reflection/refraction data collected along a 565-km 2-D profile and coincident multi-channel seismic reflection data of the seaward 350-km are presented. Three major crustal zones are identified: (1) continental crust; (2) transitional basement; (3) unequivocal oceanic crust. Continental crust beneath the Grand Banks is modeled with upper (5.8-6.25 km/s), middle (6.3-6.53 km/s) and lower layers (6.77-6.9 km/s), consistent with Avalon Terrane Appalachian crust. A zone of rifted-continental crust ∼170km wide is characterized by abrupt thinning across a large rift basin (Carson Basin) and seaward fault block, and farther seaward by a more gradual thinning with formation of a series of smaller fault blocks beneath the Salar/Newfoundland Basin. Within this zone, lower crust thins preferentially until it is completely removed, leaving a ∼60km wide section of extremely thinned upper crust (<3km thick). Seaward of the continental crust, high velocity gradients (0.5-1.5s-1) characterize an 80-km wide zone of transitional basement defined landward by a basement high that may be serpentinized peridotite and seaward by a pair of basement highs of unknown crustal origin. The flat and unreflective part of the transitional basement may either be exhumed serpentinized mantle or ultra-thin oceanic crust. Further seaward, oceanic crust 2-6km thick with layer 2 (4.5-6.3km/s) and layer 3 (6.3-7.2km/s) velocities is observed. Reconstruction of this profile with a near conjugate profile in the Iberia Abyssal Plain suggests that crustal thinning is symmetrical but that continental breakup is asymmetrically displaced towards the Iberia margin. Farther north, the breakup point shifted toward the Newfoundland side. Comparison with other North Atlantic non-volcanic margins indicates that the low velocity mantle layer occurs underneath crust that is <6-8 km thick and always terminates seaward by the formation of oceanic crust ∼6km thick. Asymmetry in breakup is also observed on these margins. | en_US |
