麻豆传媒

The eastern North American continental margin features a shift from volcanic style rifting off the eastern USA, to nonvolcanic style off Newfoundland; the Nova Scotia margin has been suggested to be a transitional style margin between the two. The composition of a layer in the lower crust (the High Velocity Lower Crust or HVLC) in the Ocean Continent Transition has been debated. The composition of this layer has a large impact on the classification of the margin, as it has been proposed to be either gabbro (suggesting volcanic style rifting) or serpentinized mantle (suggesting nonvolcanic) These show similar seismic velocities; however, gabbro is isotropic and serpentinite is anisotropic. Previously created P-wave velocity model of margin-perpendicular, wide-angle seismic line SMART1 shows a layer of partially serpentinized mantle above the mantle. Using forward modeling of the refracted arrivals of one OBS to create a 1D model, the newly collected margin-parallel, wide-angle seismic line OCTOPUS also shows this layer, however, with slower velocities than those observed in the SMART1 model (6.8 to 6.9 km/s as compared to 7.2 to 7.6 km/s). These differing velocities suggest anisotropy, therefore suggesting the HVLC is composed of serpentinized mantle. The degree of anisotropy observed was 6-9%; this is within a similar range to values observed along the west Iberia margin and also along the Mid Ocean Ridge where the fast direction is parallel to the direction spreading, (given that olivine crystals are likely to align their fast axis with the direction of stress (rifting)). This gives further evidence towards the conclusion that the HVLC is composed of serpentinized olivine-rich mantle rather than volcanics. Along strike marginal variations in the HLVC were also observed along the OCTOPUS line; these variations are consistent with the variations observed in previous work showing the HVLC thinning towards the southwest.

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Pages: 95
Supervisor: Keith Louden