Crustal structure, seismic stratigraphy, and rift processes of the continental margin off eastern Canada: ocean bottom seismic refraction results off Nova Scotia
Two crustal seismic refraction profiles were obtained on the continental margin of eastern Canada. These lines were located on the upper continental rise and on the outer continental shelf off Nova Scotia. A large air gun and explosive charges provided the sound sources, and ocean bottom seismometers were used to receive the signals. The travel time data were analyzed using the tau–p extremal inversion method and ray tracing techniques. This analysis allowed velocity–depth models to be constructed that included low-velocity zones and velocity gradients, as well as abrupt velocity contrasts at layer interfaces.The results show that about 7 km of sediments lies beneath the upper continental rise, beneath which oceanic layers 2 and 3 have been identified. Layer interfaces at depths of 0.39, 1.79, and 2.80 km below the sea floor correspond to strong, regional seismic reflectors, perhaps correlative with horizons Au, A*, and β. The top of oceanic layer 2 is associated with a velocity of 5.3 km s−1 and there is no difficulty in distinguishing between basement and high-velocity sediments in this region.The crust beneath the outer shelf includes 9–16 km of sediments. A layer with compressional wave velocity of 6.3 km s−1 constitutes the main crustal layer, below which the mantle is estimated to lie at total depths of 26–30 km. Within the sediments the velocity–depth model for the upper 5 km agrees well with sonic log velocities measured in deep wells. A seismic stratigraphy for the sediments can be deduced by comparing the refraction result with sonic log velocities and with the regional stratigraphy. The total thickness of the mid-Jurassic and older sediments, not sampled in the wells, is at least 4.5 km. The total thickness of crustal rocks of pre-rift age is between 14 and 21 km, which is significantly thinner than the 35–38 km values measured beneath mainland Nova Scotia in earlier studies. Both the thin crust and the high rate of subsidence during the Early Jurassic are consistent with extention of the lithosphere during initial rifting of this margin in Late Triassic time.