Brian J. Todd

Two seismic-refraction surveys, providing deep crustal structure information of the continent-ocean boundary across the Southwest Newfoundland Transform Margin and the Flemish Cap Southern margin, were carried out using large air-gun sources and ocean-bottom seismometer receivers.

At the Southwest Newfoundland Transform Margin, continental crust ~30 km thick beneath the southern Grand Banks (P-wave velocity = 6.2-6.5 km s-1) thins seaward to a 25 km wide transition zone. In the transition zone, Paleozoic basement of the Grand Banks (5.5-5.7 km s-1) is replaced by a basement of oceanic volcanics and synrift sediments (4.5-5.5 kms-1). Seaward of the transition zone the crust is oceanic in character, with a velocity gradient from 4.7-6.5 km s-1 and a thickness of 7-8 km. Oceanic layer 3 is absent. No significant thickness of intermediate-velocity (>7 km s-1) material is present at the continent-ocean transition, indicating that no underplating of continental crust has taken place. The continent-ocean transition across the transform margin is narrow (~25 km) suggesting that formation of the transform margin was by shearing between plates with motion occurring along a narrow shear zone.

At the Flemish Cap Southern Margin, seismic-refraction and gravity modelling suggest that thinned continental crust extends 25 km seaward of the shelf break. The transition from continental to oceanic crust occurs over a horizontal distance of 35-45 km. Oceanic crust with a main crustal layer velocity of 7.3 kms extends seaward over 100 km to the south. One refraction profile with thin (~4 km) oceanic crust was probably shot on, or very near, the trace of a fracture zone. Previous plate reconstructions have suggested that Cretaceous-age sea-floor spreading south of Flemish Cap occurred as a series of short spreading segments offset by transform faults, or by asymmetric rifting between Iberia and Flemish Cap. This study suggests that an oblique shear margin formed south of Flemish Cap, possibly as a result of transucrrent motion between Flemish Cap and Iberia.

A thermal model of transform margin evolution, including both shear heating and lateral conduction of heat from hot oceanic to colder continental lithosphere, was developed to gain insight into crustal structure. Results indicate that over 2 km of crustal uplift occurs at the fault trace for modelled transform faults of 250 km and 500 km in length. This uplift decreases away from the fault to a distance of 60-80 km. Thermal model predictions are applied to the geology of the Grand Banks, the crustal structure of the Southwest Newfoundland Transform Margin, and global transform margins. The age and distribution of unconformities on the Grand Banks is supported by the modelling of migration to the southeast of the oceanic spreading ridge, and the associated continental lithospheric uplift along the transform margin. The calculated uplift and erosion can account for the ~9 km of crustal thinning across the transform margin. The viscosity of the continental crust adjacent to the margin is reduced by a factor of more than 100. Flow of this thermally-weakened material in response to plate motion and asthenospheric upwelling at the spreading ridge may also play a role in crustal thinning. Subsidence of oceanic lithosphere occurs adjacent to the transform margin, similar to oceanic transforms.

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Supervisors:  Charlotte Keen, Ian Reid, P. Ryall